Posted on by Tammy Plotner

The Corona Borealis Constellation

Alphecca is the brightest star in a C-shaped pattern of stars: the constellation Corona Borealis. It’s near the bright star Arcturus on the sky’s dome. Credit: EarthSky

Welcome to another edition of Constellation Friday! Today, in honor of the late and great Tammy Plotner, we take a look at the “Northern Crown” – the Corona Borealis constellation. Enjoy!

In the 2nd century CE, Greek-Egyptian astronomer Claudius Ptolemaeus (aka. Ptolemy) compiled a list of all the then-known 48 constellations. This treatise, known as the Almagest, would be used by medieval European and Islamic scholars for over a thousand years to come, effectively becoming astrological and astronomical canon until the early Modern Age.

One of these constellations was Corona Borealis, otherwise known as the “Northern Crown”. This small, faint constellation is the counterpart to Corona Australis – aka. the “Southern Crown”. It is bordered by the constellations of Hercules, Boötes and Serpens Caput, and has gone on to become one of the 88 modern constellations recognized by the International Astronomical Union.

Name and Meaning:

In mythology, Corona Borealis was supposed to represent the crown worn by Ariadne – a present from Dionysus. In Celtic lore, it was known as Caer Arianrhod, or the “Castle of the Silver Circle”, home to the Lady Arianrhod. Oddly enough, it was also known to the Native Americans as well, who referred to it as the “Camp Circle” – a heavenly rendition of their celestial ancestors.

Hercules and Corona Borealis, as depicted in Urania’s Mirror (c.?1825). Credit: Library of Congress

History of Observation:

Corona Borealis was one of the original 48 constellations mentioned in the Almagest by Ptolemy. To the medieval Arab astronomers, the constellation was known as al-Fakkah,  which means “separated” or “broken up”– a reference to the resemblance of the constellation’s stars to a loose string of jewels (sometimes portrayed as a broken dish). The name was later Latinized as Alphecca, which was later given to Alpha Coronae Borealis. In 1920, it was adopted by the International Astronomical Union (IAU) as one of the 88 modern constellations.

Notable Objects:

Corona Borealis has no bright stars, 6 main stars and 24 stellar members with Bayer/Flamsteed designations. It’s brightest star – Alpha Coronae Borealis (Alphecca) – is an eclipsing binary located about 75 light years away. The primary components is a white main sequence star that is believed to have a large disc around it (as evidenced by the amount of infrared radiation it emits), and may even have a planetary or proto-planetary system.

The second brightest star, Beta Coronae Borealis (Nusakan), is a spectroscopic binary that is located 114 light years away. It is an Alpha-2 Canum Venaticorum (ACV) type star, a class of variable (named after a star in the constellation Canes Venatici) that are main sequence stars that are chemically peculiar and have strong magnetic fields. Its traditional name, Nusakan, comes from the Arabic an-nasaqan which means “the (two) series.”

Corona Borealis Galaxy Cluster – Abell 2065. Credit: NASA (Wikisky)

Corona Borealis contains few Deep Sky Objects that would be visible to amateur astronomers. The most notable is the Corona Borealis Galaxy Cluster (aka. Abell 2065), a densely-populated cluster located between 1 and 1.5 billion years from Earth. It lies about one degree southwest of Beta Coronae Borealis, in the southwest corner of the constellation. The cluster contains more than 400 galaxies in an area spanning about one degree in the sky.

Corona Borealis also has five stars that have confirmed exoplanets orbiting them, most of which were detected using the radial velocity method. These include the the orange giant Epsilon Coronae Borealis, which has a Super-Jupiter (6.7 Jupiter masses) that orbits it at a distance of 1.3 AU and with a period of 418 days.

There’s also Kappa Coronae Borealis, an orange subgiant that is orbited by both a debris disk and a gas giant. This planet is 2.5 times as massive as Jupiter and orbits the star with a period of 3.4 years. Omicron Coronae Borealis is a clump giant (a type of red giant) with one confirmed exoplanet – a gas giant with 0.83 Jupiter masses that orbits its star every 187 days.

HD 145457 is an orange giant that has one confirmed planet of 2.9 Jupiter masses that takes 176 days to complete an orbit. XO-1 is a yellow main-sequence star located approximately 560 light-years away with a hot Jupiter (roughly the same size as Jupiter) exoplanet. This planet was discovered using the transit method and completes an orbit around its star every three days.

Artist’s concept of “hot Jupiter” orbiting a distant star. Credit: NASA/JPL-Caltech

Finding Corona Borealis:

Corona Borealis is visible at latitudes between +90° and -50° and is best seen at culmination during the month of July. Using binoculars, let’s start with Alpha Coronae Borealis. It’s name is Gemma, or on some star charts – Alphecca. At 75 light years away, we have a nice binary star system whose companion star produces a very faint eclipse every 17.3599 days. Even though Gemma is quite some distance in relative sky terms from Ursa Major, you might be surprised to know that it’s actually part of the Ursa Major moving star group!

Shift your attention to Beta Coronae Borealis. It’s traditional name Nusakan. Again, it looks like one star, but it’s actually two. Nusakan is a double star that’s about 114 light-years and the primary is a variable star that changes every so slightly about every 41 days. The two components are separated by about 0.25 arc seconds – way too close for amateur telescopes – but that’s not all. In 1944 F.J. Neubauer found a small variation in the radial velocity of Nusakan which may lead to a third orbiting body about 10 times the size of Jupiter.

Now have a look at Gamma. Again, we have a binary star that’s just too darn close to split with anything but a large telescope. Struve 1967 is a close binary with an orbit of 91 years. The position angle is 265º and separation about 0.2″. Instead, try focusing your attention on Zeta 1 and Zeta 2. Known as Struve 1965, this pair is a pretty blue white and they are well spaced at 7.03″ and about one stellar magnitude in difference. Nu1 and Nu2 are also very pretty in binoculars. Here we have an optic double star. Although they aren’t physically related, this widely seperated pair of orange giant stars is a pleasing sight in binoculars!

The location of the Corona Borealis Constellation. Credit: IAU/Sky&Telescope magazine

Out of all the singular stars here, you definitely have to take a look at R Coronae Borelis – known as R Cor Bor. Discovered nearly 200 years ago by English amateur, Edward Pigot, R Coronae Borealis is the prototype star of the R Coronae Borealis (RCB) type variables. They are very unusual type of variable star – one where the variability is caused by the formation of a cloud of carbon dust in the line of sight. Near the stellar photosphere, a cloud is formed – dimming the star’s visual brightness by several magnitudes.

Then the cloud dissipates as it moves away from the star. All RCB types are hydrogen-poor, carbon- and helium-rich, and high-luminosity. They are simultaneously eruptive and pulsating. They could fade anywhere from 1 to 9 magnitudes in a month… Or in a hundred days. It’s normally magnitude 6… But it could be magnitude 14. No wonder it has the nickname “Fade-Out star,” or “Reverse Nova”!

Unfortunately, Corona Borealis contains no bright deep sky objects, but it does have one claim to fame – the highly concentrated galaxy cluster, Abell 2065. For observers with larger telescope, many members of this fascinating 1-1.5 billion light years distant group are visible. This rich cluster of galaxies is located slightly more than a degree southwest of Beta Cor Bor and covers about a full degree of sky! Not for the faint of heart… Some of these galaxies list at magnitude 18….

We have written many interesting articles about the constellation here at Universe Today. Here is What Are The Constellations?, What Is The Zodiac?, and Zodiac Signs And Their Dates.

Be sure to check out The Messier Catalog while you’re at it!

For more information, check out the IAUs list of Constellations, and the Students for the Exploration and Development of Space page on Canes Venatici and Constellation Families.

Sources:

Posted on by Tammy Plotner

The Corona Australis Constellation

The southern constellation of Corona Astralis (aka. the "Southern Crown"). Credit: Torsten Bronger.

Welcome back to Constellation Friday! Today, in honor of the late and great Tammy Plotner, we will be dealing with the “Southern Crown” – the Corona Australis constellation!

In the 2nd century CE, Greek-Egyptian astronomer Claudius Ptolemaeus (aka. Ptolemy) compiled a list of all the then-known 48 constellations. This treatise, known as the Almagest, would be used by medieval European and Islamic scholars for over a thousand years to come, effectively becoming astrological and astronomical canon until the early Modern Age.

One of these was the Coronoa Australis constellation, otherwise known as the “Southern Crown”.  This small, southern constellation is one of the faintest in the night sky, where it is bordered by the constellations of Sagittarius, Scorpius, Ara and Telescopium. Today, it is one of the 88 modern constellations recognized by the International Astronomical Union.

Name and Meaning:

Corona Australis – the “Southern Crown” – is the counterpart to Corona Borealis – the “Northern Crown”. To the ancient Greeks, this constellation wasn’t seen as a crown, but a laurel wreath. According to some myths, Dionysus was supposed to have placed a wreath of myrtle as a gift to his dead mother into the underworld as well. Either way, this small circlet of dim stars definitely has the appearance of a wreath – or crown – and belongs to legend!

False-colour image from the ESO’s Very Large Telescope of the star-forming region NGC 6729. Credit: ESO

History of Observation:

Like many of the Greek constellations, it is believed that Corona Australis was recorded by the ancient Mesopotamian in the MUL.APIN – where it may have been called MA.GUR (“The Bark”). While recorded by the Greeks as early as the 3rd century BCE, it was not until Ptolemy’s time (2nd century CE) that it was recorded as the “Southern Wreath”, a name that has stuck ever since.

In Chinese astronomy, the stars of Corona Australis are located within the Black Tortoise of the North and were known as ti’en pieh (“Heavenly Turtle”). During the Western Zhou period, the constellation marked the beginning of winter. To medieval Islamic astronomers, Corona Australis was known alternately as Al Kubbah (“the Tortoise”), Al Hiba (“the Tent”) or Al Udha al Na’am (“the Ostrich Nest”).

In 1920, the constellation was included in the list of 88 constellations formally recognized by the IAU.

Notable Objects:

Corona Australis is a small, faint constellation that has no bright stars, consists of 6 primary stars and contains 14 stellar members with Bayer/Flamsteed designations. There is one meteor shower associated with Corona Australis – the Corona-Australids which peak on or about March 16 each year and are active between March 14th through the 18th. The fall rate is minimal, with an average of about 5 to 7 per hour.

It’s brightest star, Alpha Coronae Australis (Alphekka Meridiana), is a class A2V star located about 130 light years from Earth. It is also the only properly-named star in the constellation. It’s second brightest star, Beta Coronae Australis, is a K-type bright giant located approximately 510 light years distant.

And then there’s R Coronae Australis, a well-known variable star that is located approximately 26.8 light years from Earth. This relatively young star is still in the process of formation – accreting material onto its surface from a circumstellar disk – and is located within a star forming region of dust and gas known as NGC 6726/27/29.

Corona Australis is also home to several Deep Sky Objects, such as the Corona Australis Nebula. This bright reflection nebula, which is located about 420 light years away, was formed when several bright stars became entangled with a dark cloud of dust. The cloud is a star-forming region, with clusters of young stars embedded inside, and consists of three nebulous regions – NGC 6726, NGC 6727, and NGC 6729.

Other reflection nebulas include NGC 6726/6727 and the fan-shaped NGC 6729. Corona Australis also boasts many star clusters, such as the large, bright globular cluster known as NGC 6541. There’s also the Coronet cluster, a small open star cluster that is located approximately 420 light years from Earth. The cluster lies at the heart of the constellation and is one of the nearest known regions that experiences ongoing star formation.

Color image of the Coronet Australis Nebula, taken by NASA’s WISE (Wide-field Infrared Survey Explorer). Credit: NASA/Caltech

Finding Corona Australis:

Corona Australis is visible at latitudes between +40° and -90° and is best seen at culmination during the month of August. It can be explored using both binoculars and small telescopes. Let’s start with binoculars and a look at Alpha Coronae Australis – the only star in the constellation to have a proper name.

Called Alfecca Meridiana – or “the sixth star in the river Turtle” – Alpha is a spectral class A2V star which is located about 160 light years from Earth. Alfecca Meridiana is a fast rotator, spinning at least at 180 kilometers per second at its equator, 90 times faster than our Sun and making a full rotation in about 18 hours.

Even more interesting is the fact that Alpha is a Vega-like star, pouring out excess infrared radiation that appears to be coming from a surrounding disk of cool dust. Just what does that mean? It means that Alfecca Meridiana could possibly have a planetary system!

Now have a look at Beta. Although this orange class K (K0) giant star is rather ordinary, where it’s at is not. It’s sitting on the edge of the Corona Australis Molecular Cloud, a dusty, dark star-forming region which contains huge amounts of nebulae. While Beta does seem pretty plain, it is almost 5 times larger than our Sun and 730 times brighter. Not bad for a star that’s about a hundred million years old!

Image of the globular cluster NGC 6541 in Corona Australis, based on observations made with the NASA/ESA Hubble Space Telescope. Credit: STScI/NASA/ST-ECF/ESA/CADC/NRC/CSA.

Now, take a look at a really bizarre star – Epsilon Coronae Australis. At a distance of 98 light years, there doesn’t seem to be much going on with this fifth magnitude, faint stellar point, but there is. That’s because Epsilon isn’t one star – but two. Epsilon is an eclipsing binary with two very similar eclipses that take place within an orbital period of 0.5914264 days, as first a faint star passes in front of the bright one that gives us 95 or so percent of the light, and then the bright one passes in front of the fainter.

So what does that mean? It means that if you sit right there at watch, you can see the changes in less than 7 hours. While watching for hours for a half magnitude drop might not seem like your cup of tea, think about what you’re watching…. These two stars are actually contacting each other as they go by! Can you imagine stars spinning so fast that they produce huge amounts of magnetic activity and dark starspots that also add to the variation as they swing in and out of view? Sharing mass and pulling at each other in just a matter of hours? Now that’s a show worth watching…

Now try variable star R Coronae Borealis (RA 19 53 65 Dec -36 57 97). Here we have another unusual one – a “Herbig Ae/Be” pre-main sequence star. The star is an irregular variable with more frequent outbursts during times of greater average brightness, but it also has a long-term periodic variation of about 1,500 days and about 1/2 magnitude that may be linked to changes in its circumstellar shell, rather than to stellar pulsations. Although R Coronae Australis is 40 times brighter than Sol, and about 2 to 10 times larger, most of its stellar luminosity is obscured because the star is still accreting matter. Protoplanetary bodies? Maybe!

Keep your binoculars handy and get out the telescope as we start deep sky first with NGC 6541. Also known as Caldwell 78 and Bennett 104, this beautiful 6th magnitude globular cluster was first discovered by N. Cacciatore on March 19, 1826. It belongs in our Milky Way galaxy’s inner halo structure and it is rather metal poor in structure – but beautifully resolved in a telescope. In binoculars, this splendid southern sky study will appear as a large faint globular with a bright star to the northeast.

The location of the southern constellation of Corona Astralis. Credit: IAU/ Sky&Telescope magazine

Now head for the telescope and NGC 6496 (RA 17 59 0 Dec -44 16). At right around magnitude 9, this globular cluster also has a bonus nebula attached to it. Collectively known as Bennett 100, Dreyer described it as a “nebula plus cluster” but it will take dark skies to make out both. Look for 5th magnitude star SAO 228562 that accompanies it. In a small telescope, only a hazy, faint patch can be seen, but larger aperture does get some resolution.

Try emission/reflection nebula NGC 6729 (RA 19 01 55 Dec -36 57 30) next. In a wide field, you can place NGC 6726, NGC 6727, NGC 6729 and the double star BSO 14 in the same eyepiece. The three nebulae NGC 6726-27, and NGC 6729 were discovered by Johann Friedrich Julius Schmidt, during his observations at Athen Observatory in 1861. The nebula are very faint and almost comet-like in appearance and the double star is easily split. Don’t forget to mark your notes as having captured Caldwell 68!

We have written many interesting articles about the constellation here at Universe Today. Here is What Are The Constellations?, What Is The Zodiac?, and Zodiac Signs And Their Dates.

Be sure to check out The Messier Catalog while you’re at it!

For more information, check out the IAUs list of Constellations, and the Students for the Exploration and Development of Space page on Canes Venatici and Constellation Families.

Sources:

Posted on by Tammy Plotner

The Columba Constellation

The southern constellation Columba. Credit: Torsten Bronger

Welcome back to Constellation Friday! Today, in honor of the late and great Tammy Plotner, we will be dealing with the dove – the Columba constellation!

In the 2nd century CE, Greek-Egyptian astronomer Claudius Ptolemaeus (aka. Ptolemy) compiled a list of all the then-known 48 constellations. This treatise, known as the Almagest, would be used by medieval European and Islamic scholars for over a thousand years to come, effectively becoming astrological and astronomical canon until the early Modern Age.

Since then, thanks to the efforts of astronomer and explorers, many more constellations have come to be recognized. One of these is the constellation Columba (also known as “the dove”), which was discovered in the 16th century. Located in the southern hemisphere, this small constellation is bordered by the constellations of Caelum, Canis Major, Lepus, Pictor, and Puppis.

Name and Meaning:

Since Columba was unknown to the ancient Greeks and Romans, no mythlogy is associated with it, but its original name was Columba Noachi, which refers to the Torah’s and Bible’s Dove of Noah that was the first bird to find land after the Deluge.

It could also belong to the story of Argo, where a dove was sent out to lead the Argonauts to safety between the clashing rocks. The legend of the dove is supported by the brightest star in the constellation – Alpha – whose name is Phact, Arabic for “ring dove”.

The constellation seen as “Columba Noachi” in Urania’s Mirror (1825). Credit: US Library of Congress/Wikipedia Commons

History of Observation:

Columba first appeared on the constellation charts of Petrus Plancius – a sixteen century Dutch astronomer and cartographer. In 1589, he created a celestial globe using what little information he could gather from the times explorers to help “fill in” the blank area around the south celestial pole.

Columba was then introduced into a large wall map of the constellations in 1592 and later included in Johann Bayer’s Uranometria sky atlas. In 1920, it was included among the 88 constellations recognized by the IAU, where it has remained to this day.

Notable Objects:

Columba has several major stars associated with it. The brightest is Alpha Columbae (aka. Phact), which is located approximately 270 light years from Earth. Phact is a double star that belongs to the spectral class B7IVe, and is omposed of a Be-type subgiant and a faint companion star. Its name is derived from the Arabic world Al-Fakhita, which means “the dove”.

Beta Columbae (aka. Wezn) is the second brightest star in the constellation, a giant K1-type star located 86 light years from Earth. It’s name is derived from the Arabic word Al-Wazen, which means “the weight”. Third is Delta Columbae (aka. Ghusn al Zaitun), a spectroscopic binary that is located approximately 237 light years away. Its name is derived from the Arabic phrase al-ghasn alzzaytun, which means “olive branch.”

The barred spiral galaxy NGC 1808. Credit: Jim Flood (Amateur Astronomers Inc., Sperry Observatory), Max Mutchler (STScI)

Columba is also home to several Deep Sky Objects. There’s NGC 1808, a barred spiral galaxy that is located approximately 40 million light years from Earth. Similar in many ways to the Milky Way, this galaxy has an unusual nuclear which is shaped like a warped disk and is believed to have a lot of star-forming activity within it.

There’s also NGC 1851 (aka. Caldwell 73), is a globular cluster located approximately 39,500 light years away, and NGC 1792, a starburst spiral galaxy that also goes by the name Bulliens Columbae (or the “bubbling galaxy”). This is due to its appearance, which is characterized by the patchy distribution of dust throughout the galaxy and the way this dust is heated by young stars.

Last, there’s ESO 306-17, a fossil group giant elliptical galaxy that is located at a distance of about 493 million light years from Earth. The galaxy spans about 1 million light years in diameter and is believed to have cannibalized smaller galaxies in its neighbourhood. Hence why it is designated as a fossil group, which refers to the fact that it is believed to be the end-result of a galaxy colliding and merging with a regular galaxy group.

Finding Columba:

Columba consists of 1 bright star and 5 primary stars, with 18 Bayer/Flamsteed designated stellar members. It is bordered by the constellations of Lepus, Caelum, Pictor, Puppis and Canis Major. Columba is easily visible to viewers at latitudes between +45° and -90° and is best seen at culmination during the month of February.

The globular cluster NGC 1851. Credit: NASA, JPL-Caltech, SSC

Get out your telescope and take a look at Alpha Columbae – the A symbol on the map. Here we have a a subgiant star – a star that has just stopped fusing hydrogen to helium – with an an apparent magnitude of approximately 2.6. Located about 268 light years from Earth, Phact is spinning rapidly… at a speed of at least 180 kilometers per second at its equator.

That’s over 90 times faster than our Sun! This rapid rotation causes Phact to flatten at its poles and to spin off a low density envelope about twice its radius. Now, look closely you’ll see that Phact is actually a binary star system. Its faint companion has an apparent magnitude of 12.3 and is 13.5″ distant from the main star.

Now aim binoculars at Beta Columbae – the B symbol on the map. Its proper name is Wazn the “Weight”. If you don’t think there is anything particularly interesting about this 86 light-year distant, spectral class K1IIICN+1, 3.12 magnitude star, then you better think again. This calm looking, core helium fusing giant star might be a little on the small side as giant stars go, but it is about 12 times the size of our own Sun and shines 53 times brighter.

Of course, that’s not all that unusual either. Nor is the fact that Wazn is about 2 billion years old. What is really strange is that Beta Columbae is scooting along through space at a speed of 103 kilometers per second. That’s about six to seven time faster than what’s considered “normal”! Why? It’s a runaway star, just like Mu Columbae.

Turn your binoculars toward the U symbol on the map and have a look. At 1,300 light years from our solar system, Mu is one of the few O-class stars that is visible to the unaided eye. Like Phact, Mu is a relatively fast rotating star that completes a full revolution approximately every 1.5 days.

Colour composite image of the starburst spiral galaxy NGC 1792. Credit: ESO

But Mu is also like Wazn – speeding along at relative velocity of over 200 km/s. Just where did these these two “runaways” come from? Chances are Wazn came from the other side of the Milky Way, while Mu may have originated from a binary star collision in Orion. Catch them while they’re still there!

Now aim your binoculars or telescopes at 7th magnitude globular cluster, NGC 1851 (RA 5 14 6.7 Dec -40 2 48). This Class II beauty was discovered by James Dunlop on May 29, 1826 and cataloged as Dunlop 508. What you’ll find is a very rich, almost impenetrable core surrounded by a nice halo of resolvable stars in a delightful field.

NGC 1851 has two distinct stellar populations with very different initial metal mixtures: a normal alpha-enhanced component, and one characterized by strong anti correlations among the CNONa abundances. Known in the Caldwell Catalog as Object 73, this fine object does well in all aperture sizes – even to Dunlop who almost 200 years ago wrote:

“An exceedingly bright, round, well-defined nebula, about 1.5′ diameter, exceedingly condensed, almost to the very margin. This is the brightest small nebula that I have seen. I tried several magnifying powers on this beautiful globe; a considerable portion round the margin is resolvable, but the compression to the centre is so great that I cannot reasonably expect to separate the stars. I compared this with the 68 Conn. des Temps, and this nebula greatly exceeds the 68 in condensation and brightness.”

Image of ESO 306-17, taken by the Advanced Camera for Surveys aboard the NASA/ESA Hubble Space Telescope. Credit: NASA/ESA/Michael West (ESO)

For a telescope challenge, try NGC 1792 (RA 05 05.2 Dec -37 59). Despite being billed at slightly fainter than magnitude 10, you’ll find the surface brightness of this spiral galaxy a little more in need of larger aperture. Noted as a starburst galaxy, NGC 1792 has a patchy distribution of dust throughout the galactic disc. The galaxy itself is abundant in neutral hydrogen gas and is in the star formation process.

The galaxy is characterized by unusually luminous far-infrared radiation from the young stars heating the dust with their intense activity. This activity could be caused by gravitational interaction with galaxy NGC 1808 (RA 5 7 42.3 Dec -37 30 47) – also a Seyfert galaxy. Easily seen in larger telescopes as an elongated glow, with a bright, round central core. There’s a reason for that…

The barred spiral galaxy NGC 1808 is undergoing an episode of intense star formation near its very center, perhaps triggered by rotation of the bar or by material transported inward along the bar. This new star formation is somehow being organized into clusters of between 10 and 100 light years in diameter, and filaments of dark, obscuring dust are mixed in with the gas and stars.

Thanks to studies done with the XMM-Newton and Chandra observatories, they have directly proved the co-existence of thermal diffuse plasma and non-nuclear unresolved point-like sources associated with the starburst activity, along with a Low Luminosity Active Galactic Nucleus (LLAGN) or an Ultra Luminous X-ray source (ULX). What a show!

Now try your luck with galactic star cluster NGC 1963 (RA 05 32.2 Dec -36 23). While it is not a very rich and populous star cluster, it is an interesting stellar association of perhaps two dozen stars arranged in chains over a wide field with a size of 10.0′. Look for an asterism that appears like the number 3!

We have written many interesting articles about the constellation here at Universe Today. Here is What Are The Constellations?, What Is The Zodiac?, and Zodiac Signs And Their Dates.

Be sure to check out The Messier Catalog while you’re at it!

For more information, check out the IAUs list of Constellations, and the Students for the Exploration and Development of Space page on Canes Venatici and Constellation Families.

Source:

Posted on by Tammy Plotner

The Circinus Constellation

Celestial map of the constellation Circinus, the Pair of Compasses. Credit: Torsten Bronger

Welcome back to Constellation Friday! Today, in honor of the late and great Tammy Plotner, we will be dealing with the compass – the Circinus constellation!

In the 2nd century CE, Greek-Egyptian astronomer Claudius Ptolemaeus (aka. Ptolemy) compiled a list of all the then-known 48 constellations. This treatise, known as the Almagest, would be used by medieval European and Islamic scholars for over a thousand years to come, effectively becoming astrological and astronomical canon until the early Modern Age.

Over time, the number of recognized constellations has grown as astronomers and explorers became aware of other stars visible from other location around the world. By the 20th century, the IAU adopted a modern catalog of 88 Constellations. One of these is the Circinus constellation, a small, faint constellation located in the southern skies. It is bordered by the constellations Apus, Centaurus, Lupus, Musca, Norma, Triangulum Australe.

Name and Meaning:

Because Circinus was unknown to the ancient Greeks and Romans, it has no mythology associated with it. The three brightest stars form a narrow triangle. The shape is reminiscent of a drawing (or drafting) compass of the sort used to plot sea and sky charts. Nicolas Louis de Lacaille had a fascination with secular science and the thought of naming a constellation after a science tool fascinated him.

Lacaille’s table, showing his representations of the constellations. Credit: gallica.bnf.fr

In this case, Circinus represents a drafting tool used in navigation, mathematics, technical drawing, engineering drawing, in cartography (drawing maps) – and which many elementary school age children use to learn to draw circles and in geometry to bi-sect lines, draw arcs and so forth. In this case, the device should not be confused with Pyxis, a constellation associated with a ship’s compass… despite the similarity in names with the Latin language!

History of Observation:

The small, faint southern constellation Circinus was created by Nicholas de Lacaille during his stay at the Cape of Good Hope in the mid-18th century. Circinus was given its current name in 1763, when Lacaille published an updated sky map with Latin names for the constellations he introduced.

On the map he created, Lacaille portrayed the constellations of Norma, Circinus, and Triangulum Australe as a set of draughtsman’s instruments – as a ruler, compass, and a surveyor’s level, respectively. This constellation has endured and became one of the 88 modern constellation recognized by the IAU in 1920.

Notable Features:

Circinus has no bright stars and consists of only 3 main stars and 9 Bayer/Flamsteed designated stars. However, the constellation does have several Deep Sky Objects associated with it. For instance, there’s the Circinus Galaxy, a spiral galaxy located approximately 13 million light years distant that was discovered in 1975. The galaxy is notable for the gas rings inside it, one of which is a massive star-forming region, and its black hole-powered core.

Composite image of the central regions of the nearby Circinus galaxy, located about 12 million light years away. Credit: NASA/Chandra/HST

Then there’s the X-ray double star known as Circinus X-1, which is located approximately 30,700 light years away and was discovered in 1969. This system is composed of a neutron star orbiting a main sequence star. Circinus is also home to the bright planetary nebula known as NGC 5315, which was created when a star went supernova and cast off its outer layers into space.

Then there’s NGC 5823 (aka. Caldwell 88), an open cluster located on the border between Circinus and Lupus. Located about 3,500 light years away, this cluster is about 800 million years old and spans about 12 light years.

Finding Circinus:

Circinus is visible at latitudes between +10° and -90° and is best seen at culmination during the month of June. Start by taking out your binoculars for a look at Alpha Circini – a great visual double star. Located about 53.5 light years from Earth, this stellar pair isn’t physically related but does make a unique target. The brighter of the two, Alpha, is a F1 Bright Yellow Dwarf that is a slight variable star. This contrasts very nicely with the fainter, red companion.

For the telescope, take a look at Gamma Circini – a faint star a little over five hundred light years from the Solar System. In the sky, it lies in the Milky Way, between bright Alpha Centauri and the Southern Triangle. Gamma Circini is a binary system, containing a blue giant star with a yellow, F-type, companion. Gamma is unique because it possess a stellar magnetic buoyancy!

Location of the Circinus constellation. Credit: IAU

For larger binoculars and telescopes, have a look at galactic star cluster NGC 5823 (RA 15 : 05.7 Dec -55 : 36). This dim cluster will appear to have several brighter members which are actually foreground stars, but does include Mira-type variable Y Circini. While it will be hard to distinguish from the rich, Milky Way star fields, you will notice an elliptical shaped compression of stars with an asterism which resembles and open umbrella.

For large telescopes, check out ESO 97-G13 – the “Circinus Galaxy”. Located only 4 degrees below the Galactic plane, and 13 million light-years away (RA 14h 13m 9.9s Dec 65° 20? 21?), this Seyfert Galaxy is undergoing tumultuous changes, as rings of gas are being ejected from the galactic core. While it can be spotted in a small telescope, science didn’t notice it until 25 years ago!

We have written many interesting articles about the constellation here at Universe Today. Here is What Are The Constellations?, What Is The Zodiac?, and Zodiac Signs And Their Dates.

Be sure to check out The Messier Catalog while you’re at it!

For more information, check out the IAUs list of Constellations, and the Students for the Exploration and Development of Space page on Canes Venatici and Constellation Families.

Sources:

Posted on by Tammy Plotner

The Chamaeleon Constellation

The Constellation Chamaeleon. Credit: Till Credner/AlltheSky.com

Welcome back to Constellation Friday! Today, in honor of the late and great Tammy Plotner, we will be dealing with that famous lizard that specializes at blending in – the Chamaeleon constellation!

In the 2nd century CE, Greek-Egyptian astronomer Claudius Ptolemaeus (aka. Ptolemy) compiled a list of all the then-known 48 constellations. This treatise, known as the Almagest, would be used by medieval European and Islamic scholars for over a thousand years to come, effectively becoming astrological and astronomical canon until the early Modern Age.

In time, this list would come to be expanded as astronomers became aware of more asterisms in the night sky. One of these is Chamaeleon, a small constellation located in the southern sky that was first defined in the 16th century. This constellation was appropriately named, given its ability to blend into the background! Today, it is one of the 88 constellations recognized by the IAU.

Name and Meaning:

Since Chamaeleon was unknown to the ancient Greeks and Romans, it has no mythology associated with it, but it’s not hard to understand how it came about its fanciful name. As exploration of the southern hemisphere began, what biological wonders were discovered! Can you imagine how odd a creature that could change its skin color to match its surroundings would be to someone who wasn’t familiar with lizards?

Map of the dark molecular clouds associated with the Chamaeleon constellation. Credit: Roberto Mura

Small wonder that a constellation that blended right in with the background stars could be considered a “chamaeleon” or that it might be pictured sticking its long tongue out to capture its insectile constellation neighbor – Musca the “fly”!

History of Observation:

Chamaeleon was one of twelve constellations created by Pieter Dirkszoon Keyser and Frederick de Houtman between 1595 and 1597. Both were Dutch navigators and early astronomical explorers who made attempts to chart southern hemisphere skies. Their work was added to Johann Bayer’s “Uranometeria” catalog in 1603, where Chamaeleon was first introduced as one of the 12 new southern constellations and its stars given Bayer designations.

To this day, Chamaeleon remain as one of the 88 modern constellations recognized by the IAU and it is bordered by Musca, Carina, Volans, Mensa, Octans and Apus. It contains only 3 main stars, the brightest of which is 4th magnitude Alpha – but it also has 16 Bayer/Flamsteed designated stars within its boundaries.

Notable Features:

The Chamaeleon constellation is home to several notable stars. These include Alpha Chamaeleontis, a spectral type F5III star located approximately 63.5 light years from Earth. Beta Chamaeleontis is a main sequence star that is approximately 270 light years distant. This star is the third brightest in the constellation, after Alpha and Gamma Chamaeleontis.

Artist’s concept of “hot Jupiter”, a Jupiter-sized planet orbiting closely to its star. Credit: NASA/JPL-Caltech

And then there’s HD 63454, a K-type main sequence star located approximately 116.7 light years away. It lies near the south celestial pole and is slightly cooler and less luminous than the Sun. In February of 2005, a hot Jupiter-like planet (HD 63454 b) was discovered orbiting the star.

The “Chamaeleon” also disguises itself with a huge number of dark molecular clouds that are often referred to as the “Chamaeleon Cloud Complex”. Situation about 15 degrees below the galactic plane, it is accepted is one of the closest low mass star forming regions to the Sun with a distance of about 400 to 600 light years.

Within these clouds are pre-main sequence star candidates, and low-mass T Tauri stars. The southern region of the Chamaeleon Cloud is a complex pattern of dark knots connected by elongated, dark, wavy filaments, with a serpentine-like shape. Bright rims with finger-like extensions are apparent, and a web of very faint, extremely thin but very long and straight shining filaments.

These feeble structures, reflecting stellar light, extend over the entire Chamaeleon complex and are considered very young – not yet capable of the type of collapse needed to introduce major star formation. Thanks to Gemini Near Infrared Spectrograph (GNIRS) on Gemini South Telescope, a very faint infrared object confirmed – a very low-mass, newborn brown dwarf star and the lowest mass brown dwarf star found to date in the Chamaeleon I cloud complex.

A newly formed star lights up the surrounding cosmic clouds in this image from ESO’s La Silla Observatory in Chile. Credit: ESO

Chamaeleon is also home to the Eta Chamaeleontis Cluster (aka. Mamajek 1). This open star cluster, which is centered on the star Eta Chamaeleontis, is approximately 316 light years distant and believed to be around eight million years old. The cluster was discovered in 1999 and consists of 12 or so relatively young stars. It was also the first open cluster discovered because of its X-ray emissions its member stars emit.

Finding Chamaeleon:

Chamaeleon is visible at latitudes between +0° and -90° and is best seen at culmination during the month of April. Now take out your telescope and aim it towards Eta for a look at newly discovered galactic star cluster – the Eta Chamaeleontis cluster – Mamajek 1. In 1999, a cluster of young, X-ray-emitting stars was found in the vicinity of eta Chamaeleontis from a deep ROSAT high-resolution imager observation.

They are believed to be pre-main-sequence weak-lined T Tauri stars, with an age of up to 12 million years old. The cluster itself is far from any significant molecular cloud and thus it has mysterious origins – not sharing proper motions with other young stars in the Chamaeleon region. There’s every possibility it could be a moving star cluster that’s a part of the Scorpius/Centaurus OB star association!

For binoculars, take a look at fourth magnitude Alpha Chamaeleontis. It is a rare class F white giant star that is about 63.5 light years from Earth. It is estimated to be about 1.5 billion years old. Its spectrum shows it to be a older giant with a dead helium core, yet its luminosity and temperature show it to be a younger dwarf.

The location of the Chamaeleon Constellation. Credit: IAU /Sky&Telescope magazine

Now point your binoculars or telescope towards Delta Chamaeleontis. While these two stars aren’t physically connect to one another, the visual double star is exceptionally pleasing with one orange component and one blue.

Last, but not least, take a look at Gamma Chamaeleontis. Although the south celestial pole currently lacks a bright star like Polaris to mark its position, the precession of the equinoxes will change that. One day – in the next 7500 years – the south celestial pole will pass close to the stars Gamma Chamaeleontis. But don’t wait up…

We have written many interesting articles about the constellation here at Universe Today. Here is What Are The Constellations?, What Is The Zodiac?, and Zodiac Signs And Their Dates.

Be sure to check out The Messier Catalog while you’re at it!

For more information, check out the IAUs list of Constellations, and the Students for the Exploration and Development of Space page on Canes Venatici and Constellation Families.

Sources:

Posted on by Tammy Plotner

The Cetus Constellation

The Cetus Constellation. Credit and Copyright ©: Torsten Bronger/Wikipedia Commons

Welcome back to Constellation Friday! Today, in honor of the late and great Tammy Plotner, we will be dealing with the sea monster – the Cetus constellation!

In the 2nd century CE, Greek-Egyptian astronomer Claudius Ptolemaeus (aka. Ptolemy) compiled a list of all the then-known 48 constellations. This treatise, known as the Almagest, would be used by medieval European and Islamic scholars for over a thousand years to come, effectively becoming astrological and astronomical canon until the early Modern Age.

One of these constellations is Cetus, which was named in honor of the sea monster from Greek mythology.  Cetus is the fourth largest constellation in the sky, the majority of which resides just below the ecliptic plane. Here, it is bordered by many “watery” constellations – including Aquarius, Pices, Eridanus, Piscis Austrinus, Capricornus – as well as Aries, Sculptor, Fornax and Taurus. Today, it is one of the 88 modern constellations recognized by the IAU.

Name and Meaning:

In mythology, Cetus ties in with the legendary Cepheus,Cassiopeia, Andromeda, Perseus tale – for Cetus is the monster to which poor Andromeda was to be sacrificed. (This whole tale is quite wonderful when studied, for we can also tie in Pegasus as Perseus’ horse, Algol and the whom he slew to get to Andromeda and much, much more!)

Cetus, as represented by Sidney Hall in this card from Urania’s Mirror (1825). Credit: Library of Congress/Sidney Hall

As for poor, ugly Cetus. He also represents the gates to the underworld thanks to his position just under the ecliptic plane. Arab legend has it that Cetus carries two pearl necklaces – one broken and the other intact – which oddly enough, you can see among its faint stars in the circular patterns when nights are dark. No matter what the legends are, Cetus is an rather dim, but interesting constellation!

History of Observation:

Cetus was one of many Mesopotamian constellations that passed down to the Greeks. Originally, Cetus may have been associated with a whale, and is often referred to as the Whale. However, its most common representation is that of the sea monster that was slain by Perseus.

In the 17th century, Cetus was depicted variously as a “dragon fish” (by Johann Bayer), and as a whale-like creature by famed 17th-century cartographers Willem Blaeu and Andreas Cellarius. However, Cetus has also been variously depicted with animal heads attached to an aquatic animal body.

The constellation is also represented in many non-Western astrological systems.In Chinese astronomy, the stars of Cetus are found among the Black Tortoise of the North (B?i F?ng Xuán W?) and the White Tiger of the West (X? F?ng Bái H?).

Cetus, as depicted by famed 17th century cartographer Willem Blaeu, 1602. Credit: WIkipedia Commons/Erik Lernestål

Notable Features:

Cetus sprawls across 1231 square degrees of sky and contains 15 main stars, highlighted by 3 bright stars and 88 Bayer/Flamsteed designations. It’s brightest star is Beta Ceti, otherwise known as Deneb Kaitos (Diphda), a type K0III orange giant which is located approximately 96.3 light years away. This star has left its main sequence and is on its way to becoming a red giant.

The name Deneb Kaitos is derived from the Arabic “Al Dhanab al Kaitos al Janubiyy”, which translates as “the southern tail of Cetus”. The name Diphda comes “ad-dafda at-tani“, which is Arabic for “the second frog” – the star Fomalhaut in neighboring Piscis Austrinus is usually referred to as the first frog.)

Then there’s Alpha Ceti, a very old red giant star located approximately 249 light years from Earth. It’s traditional name (Menkar), is derived from the Arabic word for “nostril”. Then comes Omicron Ceti, also known as Mira, binary star consisting located approximately 420 light years away. This binary system consists of an oscillating variable red giant (Mira A).

After being recorded for the first time by David Fabricius (on August 3, 1596), Mira has since gone on to become the prototype for the Mira class of variables (of which there are six or seven thousand known examples). These stars are red giants whose surfaces oscillate in such a way as to cause variations in brightness over periods ranging from 80 to more than 1,000 days.

Composite image of Messier 77 (NGC 1068), showing it in the visible, X-ray, and radio spectrums. Credit: NASA/CXC/MIT/C.Canizares/D.Evans et al/STScI/NSF/NRAO/VLA

Cetus is also home to many Deep Sky Objects. A notable examples is the barred spiral galaxy known as Messier 77, which is located approximately 47 million light years away and is 170,000 light years in diameter, making it one of the largest galaxies listed in Messier’s catalogue. It has an Active Galactic Nucleus (AGN) which is obscured from view by intergalactic dust, but remains an active radio source.

Then there’s NGC 1055, a spiral galaxy that lies just 0.5 north by northeast of Messier 77. It is located approximately 52 million light years away and is seen edge-on from Earth. Next to Messier 77, NGC 1055 is a largest member of a galaxy group – measuring 115,800 light years in diameter – that also includes NGC 1073 and several smaller irregular galaxies. It has a diameter of about 115,800 light years. The galaxy is a known radio source.

Finding Cetus:

Cetus is the fourth largest constellation in the sky, is visible at latitudes between +70° and -90° and is best seen at culmination during the month of November. Of all the stars in Cetus, the very first you must look for in binoculars is Mira. Omicron Ceti was the very first variable star discovered and was perhaps known as far back as ancient China, Babylon or Greece. The variability was first recorded by the astronomer David Fabricius while observing Mercury.

Now aim your binoculars at Alpha Ceti. It’s name is Menkar and we do know something about it. Menkar is an old and dying star, long past the hydrogen and perhaps even past the helium stage of its stellar evolution. Right now it’s a red giant star but as it begins to burn its carbon core it will likely become highly unstable before finally shedding its outer layers and forming a planetary nebula, leaving a relatively large white dwarf remnant.

Location of Mira and Tau Ceti. Credit: Constellation Guide/Torsten Bronger

Hop down to Beta Ceti – Diphda. Oddly enough, Diphda is actually the brightest star in Cetus, despite its beta designation. It is a giant star with a stellar corona that’s brightening with age – exerting about 2000 times more x-ray power than our Sun! For some reason, it has gone into an advanced stage if stellar evolution called core helium burning – where it is converting helium directly to carbon.

Are you ready to get out your telescope now? Then aim at Diphda and drop south a couple of degrees for NGC 247. This is a very definite spiral galaxy with an intense “stellar” nucleus! Sitting right up in the eyepiece as a delightful oval, the NGC 247 is has a very proper galaxy structure with a defined core area and a concentration that slowly disperses toward its boundaries with one well-defined dark dust lane helping to enhance a spiral arm. Most entertaining! Continuing “down” we move on to the NGC 253. Talk about bright!

Very few galactic studies come in this magnitude (small telescopes will pick it up very well, but it requires large aperture to study structure.) Very elongated and hazy, it reminds me sharply of the “Andromeda Galaxy”. The center is very concentrated and the spiral arms wrap their way around it beautifully! Dust lanes and bright hints of concentration are most evident. and its most endearing feature is that it seems to be set within a mini “Trapezium” of stars. A very worthy study…

Now, let’s hop off to Delta Ceti, shall we? I want to rock your world – because spiral galaxy M77 rocked mine! Once again, easily achieved in the small telescope, Messier 77 comes “alive” with aperture. This one has an incredible nucleus and very pronounced spiral arms – three big, fat ones! Underscored by dark dust lanes, the arms swirl away from the center in a galactic display that takes your breath away!

The location of the Cetus Constellation. Credit: IAU/Sky&Telescope magazine

The “mottling” inside the structure is not just a hint in this ovalish galaxy. I guarantee you won’t find this one “ho hum”… how could you when you know you’re looking at something that’s 47.0 million light-years away! Messier 77 is an active galaxy with an Active Galactic Nucleus (AGN) and one of the brightest Seyfert galaxies known.

Now, return to Delta and the “fall line” runs west to east on the north side. First up is galaxy NGC 1073, a very pretty little spiral galaxy with a very “stretched” appearing nucleus that seems to be “ringed” by its arms! Continuing along the same trajectory, we find the NGC 1055. Oh, yes… Edge-on, lenticular galaxy! This soft streak of light is accompanied by a trio of stars. The galaxy itself is cut through by a dark dust lane, but what appears so unusual is the core is to one side!

Now we’ve made it to back to the incredible M77, but let’s keep on the path and pick up the NGC 1087 – a nice, even-looking spiral galaxy with a bright nucleus and one curved arm. Ready to head for the beautiful variable Mira again? Then let her be the guide star, because halfway between there and Delta is the NGC 936 – a soft spiral galaxy with a “saturn” shaped nucleus. Nice starhoppin’!

We have written many interesting articles about the constellation here at Universe Today. Here is What Are The Constellations?, What Is The Zodiac?, and Zodiac Signs And Their Dates.

Be sure to check out The Messier Catalog while you’re at it!

For more information, check out the IAUs list of Constellations, and the Students for the Exploration and Development of Space page on Canes Venatici and Constellation Families.

Sources:

Posted on by Matt Williams

What Constellation is the Sun in?

The constellations, distant stars that appear close in the night sky, have been organized for millennia based on the shapes they appear to form. Credits: NASA

Since ancient times, astronomers have organized the stars into various constellations. We have the Big Dipper (Ursa Major), Orion the Hunter, and his “Greater Dog” and “Lesser Dog”(Canis Major and Canis Minor). And those are just some of the better-known ones. But have you ever wondered if the Sun belongs to one of these collections of stars?

The simple answer is that – in accordance with both ancient astrological tradition and modern astronomy – the Sun technically has no constellation. But if you were to change locations and travel to a new star system, you would then be able to view the Sun as we do other distant collection of stars. Unfortunately, depending on where you are, the answer would change.

The Zodiac:

First, let us consider the astrological answer to this question. Unless you were born prior to the Scientific Revolution – during which time Nicolaus Copernicus proposed the heliocentric model of the Solar System – you know that the Earth revolves around the Sun. Over the course of a year, the position of the stars changes as the Earth’s position relative to the Sun changes.

A chart of the constellations and signs that make up the zodiac. Credit: NASA

During the year, the Sun passes through each of the constellations of the Zodiac. For example, in August, the Sun is in Leo, and then in September, the Sun is in Virgo. Your astrological sign is based on this. What this means is that the Sun is part of each constellation of the Zodiac over the course of a single year, so it can’t be said to be in any single constellation.

However, astrology is an obsolete and entirely unscientific practice. And if someone were to ask which constellation the Sun is in, surely they are seeking an answer that was astronomical (and not astrological) in nature. For that, we must consider what the constellations are in scientific terms.

The 88 Constellations:

Since ancient times, astronomers and scholars have been keeping track of “asterisms” (aka. constellations) in the night sky. By definition, these are collections of stars that, when viewed from Earth, appear in the same general area as each other night after night. In reality, they are actually located in very different locations, and can sometimes be up to thousands of light-years away from each other.

During the 2nd century CE, Hellenistic astronomer Claudius Ptolemaeus (Ptolemy) organized the constellations into a single treatise. This treatise, known as the Almagest, was the definitive source on Greek astronomy, and contained the names and meanings of the then-known 48 constellations. For over a thousand years, this work would remain canon for European and Islamic Astronomers.

The modern constellations. color-coded by family, with a dotted line denoting the ecliptic. Credit: NASA/Scientific Visualization Studio

Thanks to the Scientific Revolution and “Age of Exploration” – ca. 15th to 18th centuries CE – astronomers became aware of many more constellations. This was due to extensive overseas exploration, which brought European traders, explorers and waves of colonization to the Southern Hemisphere, East Asia and the Americas.

By 1922, the International Astronomical Union (IAU) officially divided the celestial sphere into 88 constellations. Of these, 36 lie predominantly in the northern sky while the other 52 lie predominantly in the southern. While it would take years to work out the exact delineation between these constellations, and many corresponded to their Greco-Roman predecessors, these 88 modern constellations would remain in use until this day.

However, these constellations divide up the night sky based on how it is viewed from Earth. Once again, our Sun cannot be considered to lie in any one of them because – relative to the Earth-bound observer – it passes through them. Alas, the only way to answer this question is to change our perspective.

From Other Star Systems:

If you could move away to another star, then our Sun would indeed appear to be part of the background stars. For example, if you were to travel to a planet orbiting the nearest star to the Solar System – Alpha Centauri (aka. Rigil Kentaurus) – then the Sun would indeed appear to be part of a constellation.

Artist’s impression of the Earth-like exoplanet orbiting Alpha Centauri B Credit: ESO

To be scientifically accurate, let us consider a planet that we actually know of. This would be the rocky extrasolar planet recently discovered around Proxima Centauri, which is known as Proxima b. Viewed from the surface of this planet, the Sun would appear to be part of the Cassiopeia constellation. However, rather than forming a W shape, our Sun would form a sixth point on its “western” end, making it look like a mountain chain (or a scribbled line).

But if you went to a different star system, the Sun’s position would change, depending on the direction. As such, the Sun really isn’t in any constellation per se. But then again, none of the other stars that make up the Milky Way are either. Much like what Einstein’s Theory of Relativity teaches us about space and time, the constellations themselves are relative to the observer.

We have written many interesting articles about the Sun and the constellations here at Universe Today. Here’s What are the Constellations?, Zodiac Signs and their Dates?, Where is the Sun?, and Earth’s Orbit Around the Sun.

For more information on how our Sun looks from Alpha Centauri, be sure to check out this page from Learn Astronomy. SAnd here’s an article about all 88 recognized constellations.

Astronomy Cast also has episodes on the subject. Here’s Episode 30: The Sun, Spots and All and Episode 157: Constellations.

Sources:

Posted on by Tammy Plotner

The Centaurus Constellation

The Centaurus A galaxy (NGC 5128), a luminous galaxy located in the Centaurus constellation. Credit: ESO

Welcome back to Constellation Friday! Today, in honor of the late and great Tammy Plotner, we will be dealing with the “Centaur”, the Centaurus constellation!

In the 2nd century CE, Greek-Egyptian astronomer Claudius Ptolemaeus (aka. Ptolemy) compiled a list of all the then-known 48 constellations. This treatise, known as the Almagest, would be used by medieval European and Islamic scholars for over a thousand years to come, effectively becoming astrological and astronomical canon until the early Modern Age.

One of these is the famous Centaur of classical antiquity, otherwise known as the constellation Centaurus. As one of the 48 constellation included in the Almagest, it is now one of the 88 modern constellations recognized by the IAU. Located in the southern sky, this constellation is bordered by the Antlia, Carina, Circinus, Crux, Hydra, Libra, Lupus, Musca, and Vela constellations.

Name and Meaning:

In classic Greco-Roman mythology, Centaurus is often associated with Chiron the Centaur – the wise half-man, half-horse who was a teacher to both Hercules and Jason and the son of the Titan king Cronus and the sea nymph Philyra. According to legend, Cronus seduced the nymph, but they were interrupted by Cronus’ wife Rhea. To evade being caught in the act, Cronus turned himself into a horse.

Centaurus, as depicted on a globe created by Gullielmus Janssonius Blaeu (1602), photographed at Skokloster Castle in Stockholm, Sweden. Credit: Wikipedia Commons/Erik Lernestål

As a result, Philyra gave birth to a hybrid son. He died a tragic death in the end, having been accidentally struck by one of Heracles’ poisoned arrows. As an immortal god, he suffered terrible pains but could not die. Zeus eventually took pity on the centaur and released him from immortality and suffering, allowing him to die, and placed him among the stars.

It is believed that the constellation of Sagitta is the arrow which Chiron fired towards Aquila the Eagle to release the tortured Prometheus. The nearby constellation of Lupus the Wolf may also signify an offering of Hercules to Chiron – whom he accidentally poisoned. Just as Virgo above represents the maid placed in the sky as a sign of pity for the Centaur’s plight.

History of Observation:

The first recorded examples of Centaurus date back to ancient Sumeria, where the constellation was depicted as the Bison-man (MUL.GUD.ALIM). This being was depicted in one of two ways – either as a four-legged bison with a human head, or as a creature with a human head and torso attached to the rear legs of a bison or bull. In the Babylonian pantheon, he was closely associated with the Sun god Utu-Shamash.

The Greek depiction of the constellation as a centaur is where its current name comes from. Centaurus is usually depicted as sacrificing an animal, represented by the constellation Lupus, to the gods on the altar represented by the Ara constellation. The centaur’s front legs are marked by two of the brightest stars in the sky, Alpha and Beta Centauri (aka. Rigil Kentaurus and Hadar), which also serve as pointers to the Southern Cross.

Johannes Hevelius’ depiction of Centaurus, taken from Uranographia (1690). Credit: NASA/Chandra

In the 2nd century AD, Ptolemy catalogued 37 stars in the constellation and included it as one of the 48 constellations listed in the Almagest. In 1922, it was included in the 88 modern constellations recognized by the International Astronomical Union (IAU).

Notable Features:

Centaurus contains 11 main stars, 9 bright stars and 69 stars with Bayer/Flamsteed designations. Its brightest star – Alpha Centauri (Rigel Kentaurus) – is the Solar System’s closest neighbor. Located just 4.365 light years from Earth, this multiple star system consists of a yellow-white main sequence star that belongs to the spectral type G2V (Alpha Centauri A), and a spectral type K1V star (Alpha Centauri B).

Alpha Centauri A, the brightest component in the system, is the fourth brightest individual star (behind Arcturus) in the night sky, B is the 21st individual brightest star in the sky. Taken together, however, they are brighter than Arcturus, and rank third among the brightest star system (behind Sirius and Canopus). The two stars are believed to be roughly the same age – ~4.85 billion years old – and are close in mass to our Sun.

Proxima Centauri, a red dwarf system (spectral class M5Ve or M5Vie), if often considered to be a third member of this star system. Located about 0.24 light years from the binary pair (and 4.2 light years from Earth), this star system was confirmed in 2016 to be home to the closest exoplanet to Earth (Proxima b).

The two brightest stars of the Centaurus constellation – (left) Alpha Centauri and (right) Beta Centauri. The faint red star in the center of the red circle is Proxima Centauri. Credit: Wikipedia Commons/Skatebiker

Then there’s Beta Centauri, a blue-white giant star (spectral class B1III) located 348.83 light years from Earth that is the tenth brightest star in the sky. The star’s traditional names (Hadar or Agena), are derived from the Arabic words for “ground” and “the knee”, respectively. This multiple star system consists of Hadar A, a spectroscopic binary of two identical stars, while Hadar B orbits the primary pair with a period of at least 250 days.

Next up is Theta Centauri (aka. Menkent), an orange K-type giant (spectral class K0IIIb) that is located approximately 60.9 light years from Earth. Its traditional name, which comes from its location in the constellation, translates to “shoulder of the Centaur” in Arabic.

And then there’s Gamma Centauri (Muhlifain), a binary star system located 130 light years from Earth which is composed of two stars belonging to the spectral type A0. It’s name is translated from Arabic and means “two things”, or the “swearing of an oath”, which appears to be a case of name-transfer from Muliphein, a star located in the Canis Majoris constellation.

The constellation is also home to many Deep Sky Objects. For instance, there is the Centaurus A galaxy, the fifth brightest galaxy in the sky and one of the closest radio galaxies to the Solar System (between 10 and 16 million light years distant). The galaxy has an apparent visual magnitude of 6.84 and is believed to contain a supermassive black hole at its center.

Image of the Centaurus A galaxy, combining optical, x-ray and infrared data. Credit: X-ray: NASA/CXC/SAO/Rolf Olsen/JPL-Caltech

Centaurus A’s brightness is attributed to the intense burst of star formation going on inside it, which is believed to be the result of it undergoing a collision with a spiral galaxy. Centaurus A is located at the center of the Centaurus A subgroup of the Centaurus A/M83 Group of galaxies, which includes the Southern Pinwheel Galaxy (aka. Messier 83, M83).

Then there’s the famous Omega Centauri globular cluster, one of the brightest globular clusters in the Milky Way. Located approximately 15,800 light years distant, this cluster is bright enough to be visible to the naked eye. Originally listed as a star by Ptolemy in the Almagest, the cluster’s true nature was not discovered until John Herschel studied it in the early 19th century.

Next up is NGC 4945, one of the brightest galaxies in the Centaurus A/M83 group, and the second brightest galaxy in the Centaurus A subgroup. The spiral galaxy is approximately 11.7 million light years distant and has an active Seyfert II nucleus, which could be due to the presence of a supermassive black hole at its center.

The galaxy NGC 4650A is also located in Centaurus, some 130 million light years from Earth. This galaxy is one of only 100 polar-ring galaxies known to exist, which are so-named because their outer ring of stars and gas rotate over the poles of the galaxy. These rings are believed to have formed from the gravitational interaction of two galaxies, or from a collision with a smaller galaxy in the past.

The Blue Planetary (NGC 3918), as imaged by the Hubble telescope. Credit: ESA/Hubbl/e NASA

The Blue Planetary nebula (aka. the Southerner), is a bright planetary nebula in Centauru, approximately 4,900 light years distant. With an apparent visual magnitude of 8.5, it is the brightest planetary nebula in the far southern region of the sky and and can be observed in a small telescope.

Finding Centaurus:

Centaurus is one of the largest constellations in the night sky – covering over 1000 square degrees – and the brightest in the southern hemisphere.  For observers located at latitudes between +30° and -90°, the entire constellation is visible and the northern portion of the constellation can be spotted easily from the northern hemisphere during the month of May.

For the unaided southern skies observer, the constellation of Centaurus holds a gem within its grasp – Omega Centauri (NGC 5139). But of course, this object isn’t a star – despite being listed on the catalogs as its Omega star. It’s a globular cluster, and the biggest and brightest of its kind known to the Milky Way Galaxy. Though visible to the naked eye, it is best observed through a telescope or with binoculars.

This 18,300 light-year beauty contains literally millions of stars with a density so great at its center the stars are less than 0.1 light year apart. It is possible Omega Centauri may be the remains of a galaxy cannibalized by our own. Even to this present day, something continues to pull at NGC 5139’s stars… tidal force? Or an unseen black hole?

Omega Centauri (NGC 5139), a massive globular cluster that is part of the Centaurus constellation. Credit: Jose Mtanous

Now, hop down to Alpha. Known as Rigil Kentaurus, Rigil Kent, or Toliman, is the third brightest star in the entire night sky and the closest star system to our own solar system. To the unaided eye it appears a single star, but it’s actually a binary star system. Alpha Centauri A and Alpha Centauri B are the individual stars and a distant, fainter companion is called Proxima Centauri – a red dwarf that is the nearest known star to the Sun.

Oddly enough, Proxima Centauri is also a visual double, which is assumed to be associated with Centaurus AB pair. Resolution of the binary star Alpha Cen AB is too close to be seen by the naked eye, as the angular separation varies between 2 and 22 arc seconds, but during most of the orbital period, both are easily resolved in binoculars or small telescopes.

Then stop for a moment to take a look at Beta Centauri. Beta Centauri is well-known in the Southern Hemisphere as the inner of the two “Pointers” to the Southern Cross. A line made from the other pointer, Alpha Centauri, through Beta Centauri leads to within a few degrees of Gacrux, the star at the top of the cross. Using Gacrux, a navigator can draw a line with Acrux to effectively determine south.

But, that’s not all! Hadar is also a very nice double star, too. The blue-white giant star primary is also a spectroscopic binary, accompanied by a widely spaced companion separated from the primary by 1.3″. Or try Gamma Centauri! Muhlifain has an optical companion nearby, but check it out in the telescope… it’s really two spectral type A0 stars each of apparent magnitude +2.9!

The location of the Centaurus constellation in the southern sky. Credit: IAU/Sky & Telescope magazine/Roger Sinnott & Rick Fienberg

For binoculars or telescopes, hop on over to Centaurus A. This incredible radio source galaxy is one of the closest to Earth and also the fifth brightest in the sky. When seen through an average telescope, this galaxy looks like a lenticular or elliptical galaxy with a superimposed dust lane, and oddity first noted in 1847 by John Herschel.

The galaxy’s strange morphology is generally recognized as the result of a merger between two smaller galaxies and photographs reveal a jet of material streaming from the galactic core. Although we cannot see it, there may be a supermassive black hole at the center of the galaxy is responsible for emissions in the X-ray and radio wavelengths!

For binoculars and rich field telescopes, head towards the Crux border and center on Lambda Centauri for open cluster, IC2944. Also known on some observing lists as Caldwell 100, this scattered star cluster contains about 30 stellar members and some faint nebulosity. About 2 degrees southwest of Beta you’ll find another pair of open clusters, NGCs 5281 and 5316. Or try your hand just about a degree west of Alpha for open cluster, NGC5617. These last three are far more rich in stars and photon satisfying!

Centaurus has been known to human astronomers since the Bronze Age and has gone through some changes since that time. But even after thousands of years’ time, the Centaur is still hunting in the night sky! And for those who love viewing classic constellations and bright objects, it still provides viewing opportunities that are bound to dazzle the eyes and inspire the mind!

We have written many interesting articles about the constellation here at Universe Today. Here is What Are The Constellations?, What Is The Zodiac?, and Zodiac Signs And Their Dates.

Be sure to check out The Messier Catalog while you’re at it!

For more information, check out the IAUs list of Constellations, and the Students for the Exploration and Development of Space page on Canes Venatici and Constellation Families.

Sources:

Posted on by Tammy Plotner

The Cassiopeia Constellation

The familiar W patterns of the Cassiopeia constellation. Credit & Copyright: Rogelio Bernal Andreo (Deep Sky Colors)

Welcome back to Constellation Friday! Today, in honor of the late and great Tammy Plotner, we will be dealing with the “keel of the ship”, the Carina constellation!

In the 2nd century CE, Greek-Egyptian astronomer Claudius Ptolemaeus (aka. Ptolemy) compiled a list of all the then-known 48 constellations. This treatise, known as the Almagest, would be used by medieval European and Islamic scholars for over a thousand years to come, effectively becoming astrological and astronomical canon until the early Modern Age.

One of the most famous of these constellations is Cassiopeia, which is easily recognized by its W-shape in the sky. As one of the 48 constellation included in the Almagest, it is now one of the 88 modern constellations recognized by the IAU. Located in the norther sky opposite of the Big Dipper (Ursa Major), it is bordered by Camelopardalis, Cepheus, Lacerta, Andromeda and Perseus.

Name and Meaning:

In mythology, Cassiopeia the wife of King Cepheus and the queen of the mythological Phoenician realm of Ethiopia. Her name in Greek means “she whose words excel”, and she was renowned for her beauty but also her arrogance. This led to her downfall, as she boasted that both she and her daughter Andromeda were more beautiful than all the Nereids – the nymph-daughters of the sea god Nereus.

Cassiopeia in her chair, as depicted in Urania’s Mirror. Credit: Sidney Hall/United States Library of Congress

This led the Nerieds to unleash the wrath of Poseidon upon the kingdom of Ethiopia.Accounts differ as to whether Poseidon decided to flood the whole country or direct the sea monster Cetus to destroy it. In either case, trying to save their kingdom, Cepheus and Cassiopeia consulted a wise oracle, who told them that the only way to appease the sea gods was to sacrifice their daughter.

Accordingly, Andromeda was chained to a rock at the sea’s edge and left there to helplessly await her fate at the hands of Cetus. But the hero Perseus arrived in time, saved Andromeda, and ultimately became her husband. Since Poseidon thought that Cassiopeia should not escape punishment, he placed her in the heavens in such a position that, as she circles the celestial pole, she is upside-down for half the time.

History of Observation:

Cassiopeia was one of the traditional constellations included by Ptolemy in his 2nd century CE tract, the Almagest.  It also figures prominently in the astronomical and astrological traditions of the Polynesian, Indian, Chinese and Arab cultures. In Chinese astronomy, the stars forming the constellation Cassiopeia are found among the areas of the Purple Forbidden enclosure, the Black Tortoise of the North, and the White Tiger of the West.

Chinese astronomers also identified various figures in its major stars. While Kappa, Eta, and Mu Cassopeiae formed a constellation called the Bridge of the Kings, when combined with  Alpha and Beta Cassiopeiae – they formed the great chariot Wang-Liang. In Indian astronomy, Cassiopeia was associated with the mythological figure Sharmishtha – the daughter of the great Devil (Daitya) King Vrishparva and a friend to Devavani (Andromeda).

Kappa Cassiopeiae and its bow shock. Spitzer infrared image (NASA/JPL-Caltech)

Arab astronomers also associated Cassiopeia’s stars with various figures from their mythology. For instance, the stars of Alpha, Beta, Gamma, Delta, Epsilon and Eta Cassiopeiae were often depicted as the “Tinted Hand” in Arab atlases – a woman’s hand dyed red with henna, or the bloodied hand of Muhammad’s daughter Fatima. The arm was made up of stars from the neighboring Perseus constellation.

Another Arab constellation that incorporated the stars of Cassiopeia was the Camel. Its head was composed of Lambda, Kappa, Iota, and Phi Andromedae; its hump was Beta Cassiopeiae; its body was the rest of Cassiopeia, and the legs were composed of stars in Perseus and Andromeda.

In November of 1572, astronomers were stunned by the appearance of a new star in the constellation – which was later named Tycho’s Supernova (SN 1572), after astronomer Tycho Brahe who recorded its discovery. At the time of its discovery, SN1572 was a Type Ia supernova that actually rivaled Venus in brightness. The supernova remained visible to the naked eye into 1574, gradually fading until it disappeared from view.

The “new star” helped to shatter stale, ancient models of the heavens by demonstrating that the heavens were not “unchanging”. It helped speed the the revolution that was already underway in astronomy and also led to the production of better astrometric star catalogues (and thus the need for more precise astronomical observing instruments).

Star map of the constellation Cassiopeia showing the position (labelled I) of the supernova of 1572. Credit: Wikipedia Commons

To be fair, Tycho was not even close to being the first to observe the 1572 supernova, as his contemporaries Wolfgang Schuler, Thomas Digges, John Dee and Francesco Maurolico produced their own accounts of its appearance. But he was apparently the most accurate observer of the object and did extensive work in both observing the new star and in analyzing the observations of many other astronomers.

Notable Features:

This zig-zag shaped circumpolar asterism consists of 5 primary stars (2 of which are the most luminous in the Milky Way Galaxy) and 53 Bayer/Flamsteed designated stars. It’s brightest star – Beta Cassiopeiae, otherwise known by its traditional name Caph – is a yellow-white F-type giant with a mean apparent magnitude of +2.28. It is classified as a Delta Scuti type variable star and its brightness varies from magnitude +2.25 to +2.31 with a period of 2.5 hours.

Now move along the line to the next bright star – Alpha. Its name is Schedar and its an orange giant (spectral type K0 IIIa), a type of star cooler but much brighter than our Sun. In visible light only, it is well over 500 times brighter than the Sun. According to the Hipparcos astrometrical satellite, distance to the star is about 230 light years (or 70 parsecs).

Continue up the line for Eta, marked by the N shape and take a look in a telescope. Eta Cassiopeiae’s name is Achird and its a multiple is a star system 19.4 light years away from Earth. The primary star in the Eta Cassiopeiae system is a yellow dwarf (main sequence star) of spectral type G0V, putting it in the same spectral class as our Sun, which is of spectral type G2V. It therefore resembles what our Sun might look like if we were to observe it from Eta Cassiopeiae.

Mosaic image of Cassiopeia A, a supernova remnant, taken by the Hubble and Spitzer Space Telescopes. Credit: NASA/JPL-Caltech/STScI/CXC/SAO

The star is of apparent magnitude 3.45. The star has a cooler and dimmer (magnitude 7.51) orange dwarf companion of spectral type K7V. Based on an estimated semi major axis of 12″ and a parallax of 0.168 mas, the two stars are separated by an average distance of 71 AU. However, the large orbital eccentricity of 0.497 means that their periapsis, or closest approach, is as small as 36 AU.

The next star in line towards the pole is Gamma, marked by the Y shape. Gamma Cassiopeiae doesn’t have a proper name, but American astronaut Gus Grissom nicknamed it “Navi” since it was an easily identifiable navigational reference point during space missions. The apparent magnitude of this star was +2.2 in 1937, +3.4 in 1940, +2.9 in 1949, +2.7 in 1965 and now it is +2.15. This is a rapidly spinning star that bulges outward along the equator. When combined with the high luminosity, the result is mass loss that forms a disk around the star.

Gamma Cassiopeiae is a spectroscopic binary with an orbital period of about 204 days and an eccentricity alternately reported as 0.26 and “near zero.” The mass of the companion is believed to be comparable to our Sun (Harmanec et al. 2000, Miroschnichenko et al. 2002). Gamma Cas is also the prototype of a small group of stellar sources of X-ray radiation that is about 10 times higher that emitted from other B or Be stars, which shows very short term and long-term cycles.

Now move over to Delta Cassiopeiae, the figure 8. It’s traditional name is Ruchbah, the “knee”. Delta Cassiopeiae is an eclipsing binary with a period of 759 days. Its apparent magnitude varies between +2.68 mag and +2.74 with a period of 759 days. It is of spectral class A3, and is approximately 99 light years from Earth.

Gamma Cassiopeiae. Credit & Copyright: Noel Carboni/Greg Parker, New Forest Observatory

Last in line on the end is Epsilon, marked with the backward 3. Epsilon Cassiopeiae’s tradition name is Segin. It is approximately 441 light years from Earth. It has an apparent magnitude of +3.38 and is a single, blue-white B-type giant with a luminosity 720 times that of the Sun.

Finding Cassiopeia:

Cassiopeia constellation is located in the first quadrant of the northern hemisphere (NQ1) and is visible at latitudes between +90° and -20°. It is the 25th largest constellation in the night sky and is best seen during the month of November. Due to its distinctive shape and proximity to the Big Dipper, it is very easy to find. And the constellation has plenty of stars and Deep Sky Objects that can be spotted using a telescope or binoculars.

First, let’s begin by observing Messier 52. This one’s easiest found first in binoculars by starting at Beta, hopping to Alpha as one step and continuing the same distance and trajectory as the next step. M52 (NGC 7654) is a fine open cluster located in a rich Milky Way field. The brightest main sequence star of this cluster is of mag 11.0 and spectral type B7.

Two yellow giants are brighter: The brightest is of spectral type F9 and mag 7.77, the other of type G8 and mag 8.22. Amateurs can see M52 as a nebulous patch in good binoculars or finder scopes. In 4-inch telescopes, it appears as a fine, rich compressed cluster of faint stars, often described as of fan or “V” shape; the bright yellow star is to the SW edge. John Mallas noted “a needle-shaped inner region inside a half-circle.” M52 is one of the original discoveries of Charles Messier, who cataloged it on September 7, 1774 when the comet of that year came close to it.

The location of the Cassiopeia constellation in the northern sky. Credit: IAU/Sky&Telescope magazine

For larger telescopes, situated about 35′ southwest of M52 is the Bubble Nebula NGC 7635, a diffuse nebula which appears as a large, faint and diffuse oval, about 3.5×3′ around the 7th-mag star HD 220057 of spectral type B2 IV. It is difficult to see because of its low surface brightness. Just immediately south of M52 is the little conspicuous open cluster Czernik 43 (Cz 43).

Now let’s find Messier 103 by returning to Delta Cassiopeiae. In binoculars, M103 is easy to find and identify, and well visible as a nebulous fan-shaped patch. Mallas states that a 10×40 finder resolves the cluster into stars; however, this is so only under very good viewing conditions. The object is not so easy to identify in telescopes because it is quite loose and poor, and may be confused with star groups or clusters in the vicinity.

But telescopes show many fainter member stars. M103 is one of the more remote open clusters in Messier’s catalog, at about 8,000 light years. While you are there, enjoy the other small open clusters that are equally outstanding in a telescope, such as NGC 659, NGC 663 and NGC 654. But, for a real star party treat, take the time to go back south and look up galactic star cluster NGC 457.

It contains nearly one hundred stars and lies over 9,000 light years away from the Sun. The cluster is sometimes referred by amateur astronomers as the Owl Cluster, or the ET Cluster, due to its resemblance to the movie character. Those looking for a more spectacular treat should check out NGC 7789 –  a rich galactic star cluster that was discovered by Caroline Herschel in 1783. Her brother William Herschel included it in his catalog as H VI.30.

Chandra image of the Supernova remnant of Tycho’s Nova. Credit: NASA/CXC/Rutgers/J.Warren & J.Hughes et al.

This cluster is also known as “The White Rose” Cluster or “Caroline’s Rose” Cluster because when seen visually, the loops of stars and dark lanes look like the swirling pattern of rose petals as seen from above. At 1.6 billion years old, this cluster of stars is beginning to show its age. All the stars in the cluster were likely born at the same time but the brighter and more massive ones have more rapidly exhausted the hydrogen fuel in their cores.

Are you interested in faint nebulae? Then try your luck with IC 59. One of two arc-shaped nebulae (the other is IC 63) that are associated with the extremely luminous star Gamma Cassiopeiae. IC 59 lies about 20′ to the north of Gamma Cas and is primarily a reflection nebula. Other faint emission nebulae include the “Heart and Soul” (LBN 667 and IC 1805) which includes wide open star clusters Collider 34 and IC 1848.

Of course, no trip through Cassiopeia would be complete without mentioning Tycho’s Star! Given the role this “new star” played in the history of astronomy (and as one of only 8 recorded supernovas that was visible with the naked eye), it is something no amateur astronomer or stargazer should pass up!

While there is no actual meteoroid stream associated with the constellation of Cassiopeia, there is a meteor shower which seems to emanate near it. On August 31st the Andromedid meteor shower peaks and its radiant is nearest to Cassiopeia. Occasionally this meteor shower will produce some spectacular activity but usually the fall rate only averages about 20 per hour. There can be some red fireballs with trails. Biela’s Comet is the associated parent with the meteor stream.

We have written many interesting articles about the constellation here at Universe Today. Here is What Are The Constellations?, What Is The Zodiac?, and Zodiac Signs And Their Dates.

Be sure to check out The Messier Catalog while you’re at it!

For more information, check out the IAUs list of Constellations, and the Students for the Exploration and Development of Space page on Canes Venatici and Constellation Families.

Sources:

Posted on by Tammy Plotner

The Constellation Capricornus

The constellation Capricornus as it can be seen with the naked eye. Credit: AlltheSKy/Till Credner

Welcome back to Constellation Friday! Today, in honor of the late and great Tammy Plotner, we will be dealing with the “Sea Goat” – aka. Capricornus!

In the 2nd century CE, Greek-Egyptian astronomer Claudius Ptolemaeus (aka. Ptolemy) compiled a list of all the then-known 48 constellations. This treatise, known as the Almagest, would be used by medieval European and Islamic scholars for over a thousand years to come, effectively becoming astrological and astronomical canon until the early Modern Age.

One of these constellations is Capricornus, otherwise known as the “Sea Goat” (or simply as Capricorn). Positioned on the ecliptic plane, this constellation is one of the 12 constellations of the Zodiac, and is bordered by Aquarius, Aquila, Sagittarius, Microscopium and Piscis Austrinus. Today, it is one of the 88 modern constellations recognized by the International Astronomical Union.

Name and Meaning:

The name Capricornus is derived from Latin, which translates to “goat horn” or “horns of the goat”.  This arises from the fact that representations dating back to the Middle Bronze Age consistently depict the constellation as a hybrid of a goat and fish. This may be due to the fact that at that time, the northern hemisphere’s Winter Solstice occurred while the sun was in Capricorn.

Mesopotamian low relief depicting Sumerian sun-god Shamash rising in the center. From left to right, he is flanked by Ninurta (thunderstorms),  Ishtar (morning star), Enki (water) and Usmu (Enki’s vizier). Credit: britannica.com

The concern for the Sun’s rebirth might have rendered astronomical and astrological observation of this region of space very important. For the same reason, the Sun’s most southerly position, which is attained at the northern hemisphere’s winter solstice, is now called the Tropic of Capricorn, a term which also applies to the line on Earth where the Sun is directly overhead at noon on that solstice.

The earliest recorded evidence of this constellation is dated to the 21st century BCE, where the “Sea Goat” was depicted on a Sumerian cylinder-seal. In the Babylonian star catalogues, which are dated to ca. 1000 BCE, Capricornus was named suhurmašu (“The Goat Fish”). The constellation would later become the symbol of Ea (Enki) and was associated with the winter solstice.

In Greek mythology, the constellation was sometimes identified as Amalthea, the goat that suckled Zeus after Rhea saved him from Cronos. The goat’s broken horn was transformed into the cornucopia or horn of plenty, and ancient sources claim that this derives from the sun “taking nourishment” while in the constellation, in preparation for its climb back northward.

However, the constellation is often depicted as a sea-goat (i.e. a goat with a fish’s tail). One myth that deals with this says that when the goat-god Pan was attacked by the monster Typhon, he dived into the Nile. The parts of him that were above the water remained a goat, but those under the water transformed into a fish.

Johannes Hevelius’ depiction of Capricornus, from Uranographia (1690). Credit: chandra.harvard.edu

The Greeks regarded the constellation area with an alternative interpretation, namely the Augean Stable – a stable full uncleanliness – representing the concept of sin accumulated during the year. The Aquarius constellation, who was said to have poured out a river, then represent the yearly cleaning rains, associating to one of The Twelve Labors of Hercules.

History of Observation:

Despite being a faint constellation, Capricornus is one of the oldest recognized constellations. As with the other constellations associated with the Zodiac, Capricornus was catalogued by Ptolemy in the 2nd century CE and included in his treatise the Almagest. Despite its faintness, the constellation has also been recognized by other cultures around the world.

For example, in Chinese astronomy, Capriconus lies in The Black Tortoise of the North, one of the four symbols of the Chinese constellations. In 1922, Capricornus included in the list of 88 modern constellations recognized by the International Astronomical Union.

Capricornus as a sea-goat, from Urania’s Mirror (1825). Credit: US Library of Congress/
Sidney Hall

Notable Features:

In terms of stars few bright stars or Deep Sky Objects. It’s brightest star is also not its primary, but Delta Capricorni. Also known as its traditional names Deneb Algedi and Sheddi (from the Arabic danab al-jady, “the tail of the goat”), this magnitude 2.85 star is actually a four-star system located approximately 39 light years from Earth. Its brightest star (Delta Capricorni A) being a white giant with a luminosity 8.5 times that of the Sun.

It’s second brightest star, Beta Capricorni, is also known by the traditional name Dabih – which comes from the Arabic al-dhibii (which means “the butcher”). Located 328 light years way, this star system consists of Dabih Major (Beta-1) and Dabih Minor (Beta-2); both of which is actually composed of multiple stars – Beta-1 is composed of a three stars while Beta-2 is a double star.

It’s primary star, Alpha Capricorni, is also known as Algiedi (or Algedi), which is derived from the Arabic al-jady (“the billy goat”.) It is composed of two star systems, Prima Giedi (Alpha-2 Capricorni) and Secunda Giedi (Alpha-2 Capricorni); the former being a double star located 690 light-years away, and the latter is a G-type yellow giant 109 light years away.

The only Deep Sky Object associated with this constellation is Messier 30, a globular cluster located approximately 28,000 light years from Earth. This cluster is currently approaching us at a speed of about 180 km per second, and was one of the first Deep Sky Objects discovered by Charles Messier in 1764 (and included in The Messier Catalog).

Messier 30, imaged by the Hubble Telescope. Credit: NASA/Wikisky

Finding Capricornus:

The constellation is located in an area of sky called the Sea or Water, consisting of many watery constellations such as Aquarius, Pisces, and Eridanus. For binocular observers, the best place to start is to the northwestern corner first to find Alpha Capricorni. This is an absolutely beautiful optical double star that goes by the traditional name of Algiedi. The more western of the pair is Alpha¹ Capricorni, or Prima Giedi.

Put a telescope on it, because Prima Giedi is a true binary star. Located 690 light years from Earth, Alpha¹ Capricorni A, is a yellow G-type supergiant with an apparent magnitude of +4.30. Its companion, Alpha¹ Capricorni B, is an eighth magnitude star, separated by 0.65 arcseconds from the primary. Now go back and look at Alpha² Capricorni, aka. Secunda Giedi. Alpha² Capricorni is a yellow G-type giant with an apparent magnitude of +3.58.

For even more fun, aim your telescope all the way across the constellation at the northeastern corner for Delta Capricorni. Now you’re in for a real treat because Deneb Algedi is a a quaternary star system. Located 39 light years away, Delta Capricorni A, is classified a white giant star of the spectral type “A”. The system is a spectroscopic binary whose two components are of magnitude +3.2 and +5.2, and separated by 0.0018 arc seconds.

Similar to Algol, Delta Capricorni A is an eclipsing binary. Its unresolved companion orbits with Capricorni A around their common centre of mass every 1.022768 days, causing the brightness to drop 0.2 magnitudes during eclipses. Two other stars are thought to orbit further out in the system. The sixteenth magnitude Delta Capricorni C is one arc minute away, while the thirteenth magnitude Delta Capricorni D is two arc minutes away from the primary.

Location of the constellation Capricornus. Credit: IAU/Sky&Telescope magazine

Now go back to binoculars and hop one bright star west to take a look at Gamma Capricorni. Nashira, or “the bearer of good news” is one of those really cool stars right on the ecliptic that’s often occulted by the Moon. Gamma Capricorni is also a blue-white A-type (A7III) giant star with a mean apparent magnitude of +3.69. It is approximately 139 light years from Earth.

It is classified as an Alpha2 Canum Venaticorum type variable star and its brightness varies by 0.03 magnitudes. Now, go right in the center for Theta. It’s name is Dorsum – the Latin word for “Back”. Theta Capricorni is a white A-type main sequence dwarf with an apparent magnitude of +4.08. It is approximately 158 light years from our solar system. Want more viewing opportunities? Then go back west with binoculars and look at Beta.

Now, keep your binoculars handy and use the chart to help you located Messier 30. This one is rather hard to see in binoculars. But with a telescope, its stars can be resolved. It’s brightest red giant stars are about of apparent visual magnitude 12.1, its horizontal branch giants at magnitude 15.1. Only about 12 variable stars have been found in this globular cluster.

The core of M30 exhibits an extremely dense stellar population, and has undergone a core collapse. Despite its compressed core, close encounters of the member stars of globular cluster M30 seem to have occurred comparatively rare, as it appears to contain only few X-ray binary stars.

The NGC 6907 spiral galaxy, located in the direction of the Capricornus constellation. Credit: NOAO/KPNO

For more advanced telescope observing, try the NGC 7103 galaxy group (RA 21 39 51 Dec -22 28 24). Averaging about 15th magnitude elliptical is extremely faint and a definite big scope challenge. It pairs with NGC 7104, which is also 15th magnitude and has no classification. More realistically, try NGC 6907 (RA 20 25 1 Dec -24 49).

At slightly fainter than magnitude 11, this classy spiral galaxy shows some nice arm structure to even mid-sized telescopes. Why? Because it is doing a little galaxy interaction with background lenticular galaxy NGC 6908. This pair of spirals is engaging in some galaxy cannibalism! This act has caused some nice supernovae events within recent history and makes for some great observing – as well as astro-imaging opportunities!

The constellation of Capricornus also has a meteor shower associated with it. The Capricornid meteor stream peaks on or about July 30 and is active about a week before and after that date. The average fall rate is about 10 to 30 per hour and it is know to produce bolides.

We have written many interesting articles about the constellation here at Universe Today. Here is What Are The Constellations?, What Is The Zodiac?, and Zodiac Signs And Their Dates.

Be sure to check out The Messier Catalog while you’re at it!

For more information, check out the IAUs list of Constellations, and the Students for the Exploration and Development of Space page on Canes Venatici and Constellation Families.

Sources: