Posted on by Tammy Plotner

Messier 36 – The Pinwheel Cluster

The open star cluster Messier 38, in proximity to Messier 36 and Messier 37. Credit: Wikisky

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the Pinweel Cluster, otherwise known as Messier 36. Enjoy!

During the 18th century, famed French astronomer Charles Messier noted the presence of several “nebulous objects” in the night sky. Having originally mistaken them for comets, he began compiling a list of them so that others would not make the same mistake he did. In time, this list (known as the Messier Catalog) would come to include 100 of the most fabulous objects in the night sky.

Included in this list is the open star cluster Messier 36, also known as the Pinwheel Cluster. This cluster is so-named because of its association with the Auriga constellation (aka. “the Charioteer”). Though similar in size and make-up to the Pleiades Cluster (Messier 45), the Pinwheel Cluster is actually ten times farther away from Earth – and one of the most distant of any clusters catalogued by Messier.

What You Are Looking At:

Located a little more than 4000 light years from our solar system, this group of about 60 stars spans across about 14 light years of space. As you are studying it, you’ll notice one star which seems brighter than the rest… With good reason! Its a spectral type B2 and about 360 more luminous than our Sun. Many of the cluster members here are also B-type stars and rapid rotators.

Close-up of the central region of Messier 36. Credit: Wikisky

This means that 25 million year old Messier 36 shares a lot in common with another nearby star cluster, the Pleiades. By taking a deep look at young clusters with stars of varying ages, astronomers are able to how long circumstellar disks may last – giving us a clue as to whether or not planet-forming stars may lay within them.

As Karl E. Haisch, Jr. (et al), wrote in a 2001 study “Disk Frequencies and Lifetimes in Young Clusters“:

“We have completed the first systematic and homogeneous survey for circumstellar disks in a sample of young clusters that both span a significant range in age and contain statistically significant numbers of stars whose masses span nearly the entire stellar mass spectrum. Analysis of the combined survey indicates that the cluster disk fraction is initially very high and rapidly decreases with increasing cluster age, such that one-half the stars within the clusters lose their disks in 3 million years. Moreover, these observations yield an overall disk lifetime of ~6 million years in the surveyed cluster sample. This is the timescale for essentially all the stars in a cluster to lose their disks. This should set a meaningful constraint for the planet-building timescale in stellar clusters.”

ut, can M36 hold surprises? You betcha’. As Bo Reipurth stated in a 2008 study titled “Star Formation and Molecular Clouds towards the Galactic Anti-Center“:

“The open cluster M36 (NGC 1960), which apparently forms the center of the Aur OB1 association, has been the subject of numerous analyses, and of these the earliest studies are today of historical interest only. NGC 1960 has recently attracted attention as the most likely origin of a massive OB star that exploded about 40,000 yr ago, creating the supernova remnant Simeis 147, an old supernova remnant listed in the catalog compiled at Simeiz by Gaze & Shajn (1952). A pulsar, PSR J0538+2817, has been found near the center of Simeis 147.”

2MASS Atlas Image Mosaic of the open star cluster Messier 36. Credit: NASA/IPAC/Caltech/University of Massachusetts

And the search for planet-building stars within M36 hasn’t stopped yet. The Spitzer Space telescope will also be investigating it, thanks to a proposal made by George Rieke:

“We propose a deep IRAC/MIPS survey of NGC 1960, a ~20 Myr-old massive cluster unexplored in the mid infrared. This cluster is at a key stage in terrestrial planet formation. Our survey will likely detect infrared excess emission from debris disks and transition disks from ~ 100 intermediate-mass (1-3 solar mass) stars. Together with ground-based photometry/spectroscopy of this cluster, proposed observations of 10 Myr-old NGC 6871, scheduled cycle 4 observations of the massive 13 Myr old clusters h and chi Persei, and existing data on NGC 2547 at 30 Myr, this survey will yield robust constraints on the frequency of debris/transition disks as a function of spectral type, age, and cluster environment at a critical age range for planet formation. This survey will provide a benchmark study of the observable signatures of terrestrial planet formation that will inform James Webb Space Telescope observations of planet-forming disks a decade from now.”

History of Observation:

The presence of this awesome star cluster was first recorded by Giovanni Batista Hodierna before 1654 and re-discovered by Le Gentil in 1749. However, it was Charles Messier who took the time to carefully record its position for future generations:

“In the night of September 2 to 3, 1764, I have determined the position of a star cluster in Auriga, near the star Phi of that constellation. With an ordinary refractor of 3 feet and a half, one has difficulty to distinguish these small stars; but when employing a stronger instrument, one sees them very well; they don’t contain between them any nebulosity: their extension is about 9 minutes of arc. I have compared the middle of this cluster with the star Phi Aurigae, and I have determined its position; its right ascension was 80d 11′ 42″, and its declination 34d 8′ 6″ north.”

M36 Open Cluster. Credit: NOAO/AURA/NSF

It would be observed again by Caroline, William and John Herschel who would be the first to note the double star in M36’s center. Although none of their notes are particularly glowing on this awesome star cluster, Admiral Symth does come to the historic rescue!

“A neat double star in a splendid cluster, on the robe below the Waggoner’s left thigh, and near the centre of the Galaxy stream. A [mag] 8 and B 9, both white; in a rich though open splash of stars from the 8th to the 14th magnitudes, with numerous outliers, like the device of a star whose rays are formed by very small stars. This object was registered by M. [Messier] in 1764; and the double star, as H. [John Herschel] remarks, is admirably placed, for future astronomers to ascertain whether there be internal motion in clusters. A line carried from the central star in Orion’s belt, through Zeta Tauri, and continued about 13deg beyond, will reach the cluster, following Phi Aurigae by about two degrees.”

Locating Messier 36:

Locating Messier 36 is relatively easy once you understand the constellation of Auriga. Looking roughly like a pentagon in shape, start by identifying the brightest of these stars – Capella. Due south of it is the second brightest star which shares its border with Beta Tauri, El Nath. By aiming binoculars at El Nath, go north about 1/3 the distance between the two and enjoy all the stars!

You will note two very conspicuous clusters of stars in this area, and so did Le Gentil in 1749. Binoculars will reveal the pair in the same field, as will telescopes using lowest power. The dimmest of these is the M38, and will appear vaguely cruciform in shape. At roughly 4200 light years away, larger aperture will be needed to resolve the 100 or so fainter members. About 2 1/2 degrees to the southeast (about a finger width) you will see the much brighter M36.

The location of M36 in the Auriga constellation. Credit: IAU and Sky and Telescope Magazine (Roger Sinnott & Rick Fienberg)

More easily resolved in binoculars and small scopes, this “jewel box” galactic cluster is quite young and about 100 light years closer. If you continue roughly on the same trajectory about another 4 degrees southeast you will find open cluster M37. This galactic cluster will appear almost nebula-like to binoculars and very small telescopes – but comes to perfect resolution with larger instruments.

While all three open star clusters make fine choices for moonlit or light polluted skies, remember that high sky light means less faint stars which can be resolved – robbing each cluster of some of its beauty. Messier 36 is intermediate brightness of the trio and you’ll quite enjoy its “X” shape and many pairings of stars!

Has the central double changed with time? Why not observe for yourself and see!

Object Name: Messier 36
Alternative Designations: M36, NGC 1960, Pinwheel Cluster
Object Type: Galactic Open Star Cluster
Constellation: Auriga
Right Ascension: 05 : 36.1 (h:m)
Declination: +34 : 08 (deg:m)
Distance: 4.1 (kly)
Visual Brightness: 6.3 (mag)
Apparent Dimension: 12.0 (arc min)

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier Objects, , M1 – The Crab Nebula, M8 – The Lagoon Nebula, and David Dickison’s articles on the 2013 and 2014 Messier Marathons.

Be to sure to check out our complete Messier Catalog. And for more information, check out the SEDS Messier Database.

Sources:

Posted on by Matt Williams

How Far is Mercury from the Sun?

Transiting
NASA's Hinode X-ray telescope captured Mercury in transit against the Sun's corona in Nov. 2006. Similar views are possible in H-alpha light. Credit: NASA

Mercury is famously known for being a scorching hot world. On the side that is facing towards the Sun, conditions can get pretty molten, reaching temperatures of up to 700 K (427 °C; 800°F) in the equatorial region. The surface is also airless, in part because any atmosphere it could generate would be blown away by solar wind. Hardly surprising, considering it is the closest planet to our Sun.

But just how close is it? On average, it’s slightly more than one-third the distance between Earth and the Sun. However, its orbital eccentricity is also the greatest of any planet in the Solar System. In addition, its orbit is subject to perturbations, ones which were not fully understood until the 20th century. Because of this, Mercury goes through some serious changes during its orbital period.

Perihelion and Aphelion:

Mercury orbits the Sun at an average distance (semi-major axis) of 0.387 AU (57,909,050 km; 35,983,015 mi). However, due to its eccentricity of 0.205 – the highest in the Solar System, with the exception of Pluto (0.248) – its distance from the Sun ranges considerably. When it is at its closest (perihelion), it is 46,001,200 km (28,583,820 mi) from the Sun; and when it is farthest away (aphelion), it is 57,909,050 km (35,983,015 mi) from the Sun.

A timelapse of Mercury transiting across the face of the Sun. Credit: NASA

Orbital Resonance:

At one time, scientists believed that Mercury was tidally-locked, meaning that it kept one side facing towards the Sun at all times. However, it has since been discovered that the planet actually has a slow rotational period of 58.646 days. Compared to its orbital period of 88 days, this means that Mercury has a spin-orbit resonance of 3:2. This means that the planet makes three completes rotations on its axis for every two orbits around the Sun.

Another consequences of its spin-orbit resonance is that there is a significance difference between the time it takes the planet to rotate once on its axis (a sidereal day) and the time it takes for the Sun to reappear in the same place in the sky (a solar day). On Mercury, it takes a 176 days for the Sun to rise, set, and return to the same place in the sky. This means, effectively, that a single day on Mercury lasts as long as two years!

It’s slow rotation also means that temperature variations are extreme. On the Sun-facing side, temperatures can reach as high as 700 K (427 °C; 800°F) in the equatorial region and 380 K (107 °C; 224 °F) near the northern polar region. On the side facing away from the Sun, temperatures reach a low of 100 K (-173 °C; -280 °F) in the equatorial region and 80 K (-193 °C; -316 °F) near the northern polar region.

Diagram of Mercury’s eccentric orbit. Credit: solarviews.com

Perihelion Precession:

In addition to its eccentricity, Mercury’s perihelion is also subject to precession. What this means is, during the course of a century, Mercury’s orbit around the Sun shifts by 42.98 arcseconds (0.0119 degrees). This means that after twelve million orbits, Mercury will have performed a full excess turn around the Sun and returned to where it started.

This is much larger than the perihelion precession of other Solar planets – which range from 8.62 arcseconds (0.0024°) per century for Venus, 3.84 (0.001°) for Earth, and 1.35 (0.00037°) for Mars. Until the early 20th century, this behavior remained a mystery to astronomers, as Newtonian mechanics could not account for it. However, Einstein’s General Theory of Relativity provided an explanation, while the precession provided a test for his theory.

You might say Mercury and the Sun are pretty cozy. They dance pretty close, and the dance is powerful and full of some pretty wide swings!

We have written many interesting articles about the distance of the planets from the Sun here at Universe Today. Here’s How Far Are the Planets from the Sun?, How Far is Venus from the Sun?, How Far is Mars from the Sun?, How Far is the Earth from the Sun?, How Far is the Moon from the Sun?, How Far is Jupiter from the Sun?, How Far is Saturn from the Sun?, How Far is Uranus  from the Sun?, How Far is Neptune from the Sun? and How Far is Pluto from the Sun?

If you’d like more info on Mercury, check out NASA’s Solar System Exploration Guide, and here’s a link to NASA’s MESSENGER Misson Page.

We’ve also recorded an entire episode of Astronomy Cast all about Mercury. Listen here, Episode 49: Mercury.

Sources:

Posted on by Tammy Plotner

The Cepheus Constellation

The Cepheus constellation, located in the northern hemisphere. Credit: Torsten Bronger (2003)/Wikipedia Commons

Welcome back to Constellation Friday! Today, in honor of the late and great Tammy Plotner, we will be dealing with the King of Ethiopia himself, the Cepheus 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 northern constellation of Cepheus, named after the mythological king of Ethiopia. Today, it is one of the 88 modern constellations recognized by the IAU, and is bordered by the constellations of Camelopardalis, Cassiopeia, Cygnus, Draco, Lacerta, and Ursa Minor.

Name and Meaning:

In Greek mythology, Cepheus represents the mythical king of Aethiopia – and husband to the vain queen Cassiopeia. This also makes him the father of the lovely Andromeda, and a member of the entire sky saga which involves jealous gods and mortal boasts. According to this myth, Zeus placed Cepheus in the sky after his tragic death, which resulted from a jealous lovers’ spat.

Cepheus as depicted in Urania’s Mirror, a set of constellation cards published in London c. 1825. Credit: Library of Congress/Sidney Hall

It began when Cepheus’ wife – Cassiopeia – boasted that she was more beautiful than the Nereids (the sea nymphs), which angered the nymphs and Poseidon, god of the sea. Poseidon sent a sea monster, represented by the constellation Cetus, to ravage Cepheus’ land. To avoid catastrophe, Cepheus tried to sacrifice his daughter Andromeda to Cetus; but she was saved by the hero Perseus, who also slew the monster.

The two were to be married, but this created conflict since Andromeda had already been promised to Cepheus brother, Phineus. A fight ensued, and Perseus was forced to brandish the head of Medusa to defeat his enemies, which caused Cepheus and Cassiopeia (who did not look away in time) to turn to stone. Perhaps his part in the whole drama is why his crown only appears to be seen in the fainter stars when he’s upside down?

History of Observation:

As one of the 48 fabled constellations from Greek mythology, Cepheus was included by Ptolemy in his 2nd century tract, The Almagest. In 1922, it was included in the 88 modern constellations recognized by the International Astronomical Union (IAU).

Notable Features:

Bordered by Cygnus, Lacerta and Cassiopeia, it contains only one bright star, but seven major stars and 43 which have Bayer/Flamsteed designations. It’s brightest star, Alpha Cephei, is a white class A star, which is located about 48 light years away. Its traditional name (Alderamin) is derived from the Arabic “al-dira al-yamin“, which means “the right arm”.

This Hubble image shows RS Puppis, a type of variable star known as a Cepheid variable. Credit: NASA/ESA/STScI/AURA/H. Bond/STScI/Penn State University

Next is Beta Cephei, a triple star systems that is approximately 690 light years from Earth. The star’s traditional name, Alfirk, is derived from the Arabic “al-firqah” (“the flock”). The brightest component in this system, Alfirk A, is a blue giant star (B2IIIev), which indicates that it is a variable star. In fact, this star is a prototype for Beta Cephei variables – main sequence stars that show variations in brightness as a result of pulsations of their surfaces.

Then there’s Delta Cephei, which is located approximately 891 light years from the Solar System. This star also serves as a prototype for Cepheid variables, where pulsations on its surface are directly linked to changes in luminosity. The brighter component of the binary is classified as a yellow-white F-class supergiant, while its companion is believed to be a B-class star.

Gamma Cephei is another binary star in Cepheus, which is located approximately 45 light years away. The star’s traditional name is Alrai (Er Rai or Errai), which is derived from the Arabic ar-r?‘?, which means “the shepherd.” Gamma Cephei is an orange subgiant (K1III-IV) that can be seen by the naked eye, and its companion has about 0.409 solar masses and is thought to be an M4 class red dwarf.

Cepheus is also home to many notable Deep Sky Objects. For example, there’s NGC 6946, which is sometimes called the Fireworks Galaxy because of its supernovae rate and high volume of star formation. This  intermediate spiral galaxy is located approximately 22 million light years distant. The galaxy was discovered by William Herschel in September 1798, and nine supernovae have been observed in it over the last century.

The Fireworks Galaxy (NGC 6946). Credit: Simon Driver (University of St. Andrews)

Next up is the Wizard Nebula (NGC 7380), an open star cluster that was discovered by Caroline Herschel in 1787. The cluster is embedded in a nebula that is about 110 light years in size and roughly 7,000 light years from our Solar System. It is also a relatively young open cluster, as its stars are estimated to be less than 500 million years old.

Then there’s the Iris Nebula (NGC 7023), a reflection nebula with an apparent magnitude of 6.8 that is approximately 1,300 light years distant. The object is so-named because it is actually a star cluster embedded inside a nebula. The nebula is lit by the star SAO 19158 and it lies close to two relatively bright stars – T Cephei, which is a Mira type variable, and Beta Cephei.

Discovered by Sir William Herschel on October 18, 1794, Herschel made the correct assumption of, “A star of 7th magnitude. Affected with nebulosity which more than fills the field. It seems to extend to at least a degree all around: (fainter) stars such as 9th or 10th magnitude, of which there are many, are perfectly free from this appearance.”

So where did the confusion come in? It happened in 1931 when Per Collinder decided to list the stars around it as a star cluster Collinder 429. Then along came Mr. van den Berg, and the little nebula became known as van den Berg 139. Then the whole group became known as Caldwell 4! So what’s right and what isn’t?

The Wizard Nebula (NGC 738). Credit: NASA/JPL-Caltech/WISE Team

According to Brent Archinal, “I was surprised to find NGC 7023 listed in my catalog as a star cluster. I assumed immediately the Caldwell Catalog was in error, but further checking showed I was wrong! The Caldwell Catalog may be the only modern catalog to get the type correctly!”

Finding Cepheus:

Cepheus is a circumpolar constellation of the northern hemisphere and is easily seen at visible at latitudes between +90° and -10° and best seen during culmination during the month of November. For the unaided eye observer, start first with Cepheus’ brightest star – Alpha. It’s name is Alderamin and it’s going through stellar evolution – moving off the main sequence into a subgiant, and on its way to becoming a red giant as its hydrogen supply depletes.

What’s very cool is Alderamin is located near the precessional path traced across the celestial sphere by the Earth’s north pole. That means that periodically this star comes within 3° of being a pole star! Keeping that in mind, head off for Gamma Cephei. Guess what? Due to the precession of the equinoxes, Errai will become our northern pole star around 3000 AD and will make its closest approach around 4000 AD. (Don’t wait up, though… It will be late).

However, you can stay up late enough with a telescope or binoculars to have a closer look at Errai, because its an orange subgiant binary star that’s also about to go off the main sequence and its accompanied by a red dwarf star. What’s so special about that? Well, maybe because a planet has been discovered floating around there, too!

The location of the northern Cepheus constellation. Credit: IAU/Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

Now let’s have some fun with a Cepheid variable star that changes enough in about 5 days to make watching it fun! You’ll find Delta on the map as the figure 8 symbol and in the sky you’ll find it 891 light-years away. Delta Cephei is binary star system and the prototype of the Cepheid variable stars – the closest of its type to the Sun.

This star pulses every 5.36634 days, causing its stellar magnitude to vary from 3.6 to 4.3. But that’s not all! Its spectral type varies, too – going from F5 to G3. Try watching it over a period of several nights. Its rise to brightness is much faster than its decline! With a telescope, you will be able to see a companion star separated from Delta Cephei by 41 arc seconds.

Are you ready to examine two red supergiant stars? If you live in a dark sky area, you can see these unaided, but they are much nicer in binoculars. The first is Mu Cephei – aka. Herschel’s Garnet Star. In his 1783 notes, Sir William Herschel wrote: “a very fine deep garnet colour, such as the periodical star omicron ceti” and the name stuck when Giuseppe Piazzi included the description in his catalog.

Now compare it to VV Cephi, right smack in the middle of the map. VV is absolutely a supergiant star, and it is of the largest stars known. In fact, VV Cephei is believed to be the third largest star in the entire Milky Way Galaxy! VV Cephei is 275,000-575,000 times more luminous than the Sun and is approximately 1,600–1,900 times the Sun’s diameter.

Artist’s impression of VV Cep A, created using Celestia, with Mu Cephei (Garnet Star) in the background. Credit: Wikipedia Commons/Rackshea

If placed in our solar system, the binary system would extend past the orbit of Jupiter and approach that of Saturn. Some 3,000 light years away from Earth, matter continuously flows off this bad boy and into its blue companion. Stellar wind flows off the system at a velocity of approximately 25 kilometers per second. And some body’s Roche lobe gets filled!

For some rich field telescope and binocular fun from a dark sky site, try your luck with IC1396. This 3 degree field of nebulosity can even be seen unaided at times! Inside you’ll find an open star cluster (hence the designation) and photographically the whole area is criss-crossed with dark nebulae.

For a telescope challenge, see if you can locate both Spiral galaxy NGC 6946 – aka. the Fireworks Nebula – and galactic cluster NGC 6939 about 2 degrees southwest of Eta Cepheus. About 40 arc minutes northwest of NGC 6946 – is about 8th magnitude, well compressed and contains about 80 stars.

More? Then try NGC 7023 – The Iris Nebula. This faint nebula can be achieved in dark skies with a 114-150mm telescope, but larger aperture will help reveal more subtle details since it has a lower surface brightness. Take the time at lower power to reveal the dark dust “lacuna” around it reported so many years ago, and to enjoy the true beauty of this Caldwell gem.

The Iris Nebula (NGC 7023). Credit: Hewholooks

Still more? Then head off with your telescope for IC1470 – but take your CCD camera. IC1470 is a compact H II region excited by a single O7 star associated with an extensive molecular cloud in the Perseus arm!

Yes, Cepheus has plenty of viewing opportunities for the amateur astronomer. And for thousands of years, it has proven to be a source of fascination for scholars and astronomers.

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

The Orbit of Venus. How Long is a Year on Venus?

Venus captured by Magellan.

Venus and Earth have many similarities. Both are terrestrial planets, meaning that they are composed predominately of metal and silicate rock, which is differentiated between a metal core and a silicate mantle and crust. Both also orbit the Sun within its habitable zone (aka. “Goldilocks Zone“). Hence why Venus and Earth are often called “sister planets”.

However, Venus is also starkly different from Earth in a number of ways. It’s atmosphere, which is composed primarily of carbon dioxide and small amounts of nitrogen, is 92 times as dense as Earth’s. It is also the hottest planet in the Solar System, with temperatures hot enough to melt lead! And on top of all that, a year on Venus is much different than a year on Earth.

Orbital Period:

Venus orbits the Sun at an average distance of about 0.72 AU (108,000,000 km/67,000,000 mi) with almost no eccentricity. In fact, with its farthest orbit (aphelion) of 0.728 AU (108,939,000 km) and closest orbit (perihelion) of 0.718 AU (107,477,000 km), it has the most circular orbit of any planet in the Solar System.

Earth and Venus’ orbit compared. Credit: Sky and Telescope

The planet’s orbital period is 224.65 days, which means that a year on Venus is 61.5% as long as a year on Earth. Unlike most other planets in the Solar System, which rotate on their axes in an counter-clockwise direction, Venus rotates clockwise (called “retrograde” rotation). It also rotates very slowly, taking 243 Earth days to complete a single rotation.

Sidereal vs. Solar Day:

While a year on Venus lasts the equivalent of 224.65 Earth days, it only lasts the equivalent 1.92 days on Venus. This is due to the fact that Venus rotates quick slowly and in the opposite direction of its orbit. Because of this, a Solar Day – the time it takes for the Sun to rise, set, and return to the same place in the sky – takes 116.75 Earth days.

This means, in effect, that a single day on Venus lasts over half a year. In other words, in the space of just over a single Venusian year, the Sun will appear to have circled the heavens twice. In addition, to someone standing on the planet’s surface, the Sun would appear to rise in the west and set in the east.

Variations:

Because of its dense atmosphere and its highly circular rotation, Venus experiences very little in the way of temperature variations during the course of a year. Similarly, its axial tilt of 2.64° (compared to Earth’s 23.44°) is the second-lowest in the Solar System, behind Mercury’s extremely low tilt of 0.03.

This means that there is virtually no variation in Venus’ surface temperature between day and night, or the equator and the poles. All year long, the mean surface temperature of Venus is a scorching 735 K (462 °C/863.6 °F), with the only variations occurring as a result of elevation.

Yes, Venus is a truly hellish place. And unfortunately, that’s a year-round phenomena! The days are extremely hot, the nights extremely hot, and a day lasts over half as long as a year. So if you’re planning on vacationing somewhere, might we recommend somewhere a little less sunny and balmy?

We’ve written several articles about years on other planets here at Universe Today. Here’s How Long is a Year on the Other Planets?, Which Planet has the Longest Day?, How Long is a Year on Mercury?, How Long is a Year on Earth?, How Long is a Year on Mars?, How Long is a Year on Jupiter?, How Long is a Year on Saturn?, How Long is a Year on Uranus?, How Long is a Year on Neptune?, How Long is a Year on Pluto?

If you’d like more info on Venus, check out Hubblesite’s News Releases about Venus, and here’s a link to NASA’s Solar System Exploration Guide on Venus.

We’ve also recorded an episode of Astronomy Cast all about Venus. Listen here, Episode 50: Venus.

Sources:

Posted on by Tammy Plotner

Messier 35 – the NGC 2168 Open Star Cluster

The open star cluster Messier 35, with NGC 2158 and IC 2157 shown nearby. Credit: Wikisky

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the open star cluster known as Messier 35. Enjoy!

During the 18th century, famed French astronomer Charles Messier noted the presence of several “nebulous objects” in the night sky. Having originally mistaken them for comets, he began compiling a list of them so that others would not make the same mistake he did. In time, this list (known as the Messier Catalog) would come to include 100 of the most fabulous objects in the night sky.

One of these objects is known as Messier 35, a large open star cluster located in the northern constellation Gemini. M35 is the only Messier Object located in Gemini, and lies near the border with the adjacent constellations of  Taurus, Auriga and Orion. It consists several hundred stars that are scattered over an area that is about the same size as a Full Moon.

What You Are Looking At:

Messier 35 is 2,800 light years away from Earth and is relatively young as star clusters go, having formed only about 100 million years ago. The cluster occupies a region of space that is roughly 24 light years in diameter, and an area of 28 arc minutes on the sky – which is roughly equal to the size of the full Moon.

Image of Messier 35 obtained by the Two Micron All Sky Survey (2MASS). Credit: NASA/2MASS

M35 has a central mass that spans 11.4 light years (3.75 parsecs), with an estimated mass of 1600 to 3200 solar masses. While most of the molecule cloud from which it formed has been blown away, some of the material resides in the immediate vicinity of its stars. This can be seen in the way that light from its particularly bright blue stars is scattered to create a diffuse glow.

These are the hottest main sequence stars in the cluster, which correspond to a spectral classification of B3. M35 also contains more evolved stars, including several orange and yellow giants, which have longer lifespans than the more-massive blue stars (only a few tense of millions of years).

As a result, these stars will likely die out in the near future while the smaller stars continue to evolve, drastically affecting the cluster’s luminosity and appearance. In short, it will become redder and dimmer over time.

History of Observation:

This wonderful star cluster was discovered by Philippe Loys de Chéseaux 1745-46 and recovered again by John Bevis before 1750. However, we know and love it best as Messier Object 35, when it was penned into being by Charles Messier. As he wrote of the cluster upon observing it for the first time:

“In the night of August 30 to 31, 1764, I have observed a cluster of very small stars, near the left foot of Castor, little distant from the stars Mu and Eta of that constellation [Gemini]. When examining this star cluster with an ordinary refractor of 3 feet, it seemed to contain nebulosity; but having examined it with a good Gregorian telescope which magnified 104 times, I have noticed that it is nothing but a cluster of small stars, among which there are some which are of more light; its extension may be 20 minutes of arc. I have compared the middle of this cluster with the star Eta of Castor; its right ascension has been concluded at 88d 40′ 9″, and its declination at 24d 33′ 30″ north.”

Close-up of the Messier 35 open star cluster, showing its blue stars. Credit: Wikisky

How long would it be before the companion cluster was observed as well? My guess is Sir William Herschel’s time. Although Herschel would not publish his notes on Messier objects, they do state while observing M35 that “There is no central condensation to denote a globular form.”

And what of Admiral Smyth? He observed the cluster in September of 1836, though he appeared to have missed its companion cluster. As he recorded of M35 at the time:

“A cluster, near Castor’s right foot, in the Galaxy, discovered and registered by Messier in 1764. It presents a gorgeous field of stars from the 9th to the 16th magnitudes, but with the center of mass less rich than the rest. From the small stars being inclined to form curves of three, four, and often with a large [bright] one at the root of the curve, it somewhat reminds one of the bursting of a sky-rocket.”

A nice description, but if you see the companion cluster, you’ll know it!

Locating Messier 35:

Locating M35 in binoculars is fairly easy once you recognize the constellation of Gemini. You’ll find it just a little more than the average field of view north of Eta – the center most of the three “foot” stars on the northernmost twin. In the finderscope of a telescope, begin with Eta and starhop north until you spot a faint fuzzy in the finderscope.

The location of Messier 35 in the norther n Gemini constellation. Credit: IAU/Sky & Telescope magazine/Roger Sinnott & Rick Fienberg

Because Messier 35 is large, you’ll need low magnification to appreciate the size of this cluster in a telecope. It stands up well to moonlight and light polluted skies – as well as less than perfect sky conditions, but you will need around a 10″ or larger telescope to really begin to notice its companion cluster, NGC 2158. In smaller telescopes with good conditions, it will appear as a faint nebulous patch.

And as always, here are the quick facts on M35 to get you started!

Object Name: Messier 35
Alternative Designations: M35, NGC 2168
Object Type: Galactic Open Star Cluster
Constellation: Gemini
Right Ascension: 06 : 08.9 (h:m)
Declination: +24 : 20 (deg:m)
Distance: 2.8 (kly)
Visual Brightness: 5.3 (mag)
Apparent Dimension: 28.0 (arc min)

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier Objects, , M1 – The Crab Nebula, M8 – The Lagoon Nebula, and David Dickison’s articles on the 2013 and 2014 Messier Marathons.

Be to sure to check out our complete Messier Catalog. And for more information, check out the SEDS Messier Database.

Sources:

Posted on by Matt Williams

What is the Weather like on Saturn?

The Saturn hexagon as seen by Voyager 1 in 1980 (NASA)

Welcome back to our planetary weather series! Next up, we take a look at the ringed-beauty, Saturn!

Saturn is famous for many things. Aside from its ring system, which are the most visible and beautiful of any gas giant, it is also known for its extensive system of moons (the second largest in the Solar System behind Jupiter). And then there its banded appearance and gold color, which are the result of its peculiar composition and persistent weather patterns.

Much like Jupiter, Saturn’s weather systems are known for being particularly extreme, giving rise to features that can be seen from great distances. It’s high winds periodically create massive oval-shaped storms, jet streams, hurricanes, and hexagonal wave patterns that are visible in both the northern and southern polar regions.

Saturn’s Atmosphere:

The outer atmosphere of Saturn contains 96.3% molecular hydrogen and 3.25% helium by volume. The gas giant is also known to contain heavier elements, though the proportions of these relative to hydrogen and helium is not known. It is assumed that they would match the primordial abundance from the formation of the Solar System.

The huge storm churning through the atmosphere in Saturn’s northern hemisphere overtakes itself as it encircles the planet in this true-color view from NASA’s Cassini spacecraft. Credit: NASA/JPL-Caltech/SSI

Trace amounts of ammonia, acetylene, ethane, propane, phosphine and methane have been also detected in Saturn’s atmosphere. The upper clouds are composed of ammonia crystals, while the lower level clouds appear to consist of either ammonium hydrosulfide (NH4SH) or water. Ultraviolet radiation from the Sun causes methane photolysis in the upper atmosphere, leading to a series of hydrocarbon chemical reactions with the resulting products being carried downward by eddies and diffusion.

Saturn’s atmosphere exhibits a banded pattern similar to Jupiter’s, but Saturn’s bands are much fainter and wider near the equator. As with Jupiter’s cloud layers, they are divided into the upper and lower layers, which vary in composition based on depth and pressure. In the upper cloud layers, with temperatures in range of 100–160 K and pressures between 0.5–2 bar, the clouds consist of ammonia ice.

The presence of hydrogen gas results in clouds of deep red. However, these are obscured by clouds of ammonia, which are closer to the outer edge of the atmosphere and cover the entire planet. The exposure of this ammonia to the Sun’s ultraviolet radiation causes it to appear white. Combined with its deeper red clouds, this results in the planet having a pale gold color.

Water ice clouds begin at a level where the pressure is about 2.5 bar and extend down to 9.5 bar, where temperatures range from 185–270 K. Intermixed in this layer is a band of ammonium hydrosulfide ice, lying in the pressure range 3–6 bar with temperatures of 290–235 K. Finally, the lower layers, where pressures are between 10–20 bar and temperatures are 270–330 K, contains a region of water droplets with ammonia in an aqueous solution.

Great White Spot:

On occasion, Saturn’s atmosphere exhibits long-lived ovals, similar to what is commonly observed on Jupiter. Whereas Jupiter has the Great Red Spot, Saturn periodically has what’s known as the Great White Spot (aka. Great White Oval). This unique but short-lived phenomenon occurs once every Saturnian year, roughly every 30 Earth years, around the time of the northern hemisphere’s summer solstice.

These spots can be several thousands of kilometers wide, and have been observed in 1876, 1903, 1933, 1960, and 1990. Since 2010, a large band of white clouds called the Northern Electrostatic Disturbance have been observed enveloping Saturn, which was spotted by the Cassini space probe. If the periodic nature of these storms is maintained, another one will occur in about 2020.

Meteorological Phenomena:

The winds on Saturn are the second fastest among the Solar System’s planets, after Neptune’s. This is due in part to Saturn’s high rotational velocity – which is 9.87 km/s (6.13 mi/s), which works out to 35,500 km/h (22,058.7 mi/h). At this rate, it only takes the planet 10 hours 33 minutes to rotate once on its axis. However, due to it being a gas giant, there is a difference between the rotation of its atmosphere and its core.

Data obtained by the Voyager 1 and 2 missions indicated peak easterly winds of 500 m/s (1800 km/h). Saturn’s northern and southern poles have also shown evidence of stormy weather. At the north pole, this takes the form of a hexagonal wave pattern, whereas the south shows evidence of a massive jet stream.

Saturn makes a beautifully striped ornament in this natural-color image, showing its north polar hexagon and central vortex. Credit: NASA/JPL-Caltech/Space Science Institute

The persisting hexagonal wave pattern around the north pole was first noted in the Voyager images. The sides of the hexagon are each about 13,800 km (8,600 mi) long (which is longer than the diameter of the Earth) and the structure rotates with a period of 10h 39m 24s, which is assumed to be equal to the period of rotation of Saturn’s interior.

The south pole vortex, meanwhile, was first observed using the Hubble Space Telescope. These images indicated the presence of a jet stream, but not a hexagonal standing wave. These storms are estimated to be generating winds of 550 km/h, are comparable in size to Earth, and believed to have been going on for billions of years.

In 2006, the Cassini space probe observed a hurricane-like storm that had a clearly defined eye. Such storms had not been observed on any planet other than Earth – even on Jupiter. This storm appeared to be caused by heat that was generated in the depths of the warm interior of Saturn, which then escaped to the upper atmosphere and escaped the planet.

Saturn has also been noted for its “string of pearls” feature, which was captured by Cassini’s visual and infrared mapping spectrometer in 2006. This feature, which appeared in it’s northern latitudes (and has not been seen on any other gas giant) is a series of cloud clearings spaced at regular intervals that show how Saturn’s atmosphere is lit by its own internal, thermal glow.

So how is the weather on Saturn? Pretty violent and stormy! And not surprising given the planet’s mass, composition, powerful gravity, and rapid rotation. Makes you feel happy we live on Earth, where the Earth is (comparatively speaking) pretty calm and boring!

We have written many interesting articles about planetary weather here at Universe Today. Here’s What’s the Weather Like on Mercury?, What’s the Weather Like on Venus?, What’s the Weather Like on Mars?, What’s the Weather Like on Jupiter?, What is the Weather Like on Uranus? and What is the Weather Like on Neptune?

For more information, check out NASA’s Solar System Exploration – Saturn, and the facts about Saturn from Space Facts.

Astronomy Cast has recorded some interesting episode on the subject. Here’s Episode 59: Saturn, and Episode 61: Saturn’s Moons.

Sources:

Posted on by Matt Williams

The Orbit of Mercury. How Long is a Year on Mercury?

A global view of Mercury, as seen by MESSENGER. Credit: NASA

Mercury is a planet of extremes. As the closest planet to our Sun, it experiences extremely high surface temperatures. But since it has virtually no atmosphere to speak of, and rotates very slowly on its axis, it gravitates between extremes of hot and cold. It also means that it’s Sun-facing side experiences prolonged periods of day while its dark side experiences extremely long periods of night.

It’s proximity to the Sun also means that it orbits the planet quite rapidly. To break it down, Mercury takes roughly 88 Earth days to complete a single orbit around the Sun. Between this rapid orbital period and its slow rotational period, a single year on Mercury is actually shorter than a single day!

Orbital Period:

Mercury orbits the Sun at a distance of 57,909,050 km (35,983,015 mi), which works out to o.387 AU – or slightly more than one-third the distance between the Sun and the Earth. It’s orbit is also highly eccentric, ranging from a distance of 46 million km/28.58 million mi at its closest (perihelion) to 70 million km/43.49 million mi at its most distant (aphelion).

Illustration of the orbit of Kepler-432b (inner, red) in comparison to the orbit of Mercury around the Sun (outer, orange). Credit: Dr. Sabine Reffert.

Like all the planets, Mercury moves fastest when it is at its closest point to the Sun, and slowest when it is at its farthest. However, it’s proximity to the Sun means that its average orbital velocity is a speedy 47.362 kilometers a second or 29.429 miles per second –  approximately 170,500 km/h; 105,945 mph.

At this rate, it takes Mercury 87.969 days, or the equivalent of 0.24 Earth years, to complete a single orbit of the Sun. Thus, it can be said that a year on Mercury lasts almost as long as 3 months here on Earth.

Sidereal and Solar Day:

Astronomers used to think that Mercury was tidally locked to the Sun, where its rotational period matched its orbital period. This would mean that the same side it always pointed towards the Sun, thus ensuring that one side was perennially sunny (and extremely hot) while the other experienced constant night (and freezing cold).

However, improved observations and studies of the planet have led scientists to conclude that in fact, the planet has a slow rotational period of 58.646 days. Compared to its orbital period of 88 days, this means that Mercury has a spin-orbit resonance of 3:2, which means that the planet makes three completes rotations on its axis for every two orbits it makes around the Sun.

Another consequences of its spin-orbit resonance is that there is a significance difference between the time it takes the planet to rotate once on its axis (a sidereal day) and the time it takes for the Sun to reappear in the same place in the sky (a solar day). On Mercury, it takes a 176 days for the Sun to rise, set, and return to the same place in the sky. This means, effectively, that a single day on Mercury lasts as long as two years!

Yes, Mercury is a pretty extreme place. Not only do temperatures on its surface range from molten hot to freezing cold, but a single day lasts as long as six months here on Earth. Add to that the fact that it has virtually no atmosphere, and is exposed to extreme amounts of radiation, and you can begin to understand why life cannot exist there.

At least… not yet!

We have written many interesting articles about Mercury here at Universe Today. Here’s How Long is a Day on Mercury?, How Long is a Year on the Other Planets?, Which Planet has the Longest Day?, How Long is a Year on Venus?, How Long is a Year on Earth?, How Long is a Year on Mars?, How Long is a Year on Jupiter?, How Long is a Year on Saturn?, How Long is a Year on Uranus?, How Long is a Year on Neptune?, and How Long is a Year on Pluto?

If you’d like more info on Mercury, check out NASA’s Solar System Exploration Guide, and here’s a link to NASA’s MESSENGER Misson Page.

We’ve also recorded an entire episode of Astronomy Cast all about Mercury. Listen here, Episode 49: Mercury.

Sources:

Posted on by Matt Williams

What is the Surface Area of the Earth?

Earth, seen from space, above the Pacific Ocean. Credit: NASA

Whoever coined the phrase “it’s a small world” obviously never tried to travel around it! In truth, the planet’s dimensions are quite impressive, and determining just how big it is took many thousands of years. From astronomers determining that Earth was in fact round (and not a flat disc, cube or ziggurat), to the first successful attempts at circumnavigation, our estimates have changed over time.

And in the era of modern astronomy, improvements in instrumentation, methodology, and the ability to see Earth from space have certainly helped. According to modern estimates, the surface area of the Earth is approximately 510 million square km (5.1 x 108 km2) or 196,900,000 square miles. Determining this was not only a matter of ascertaining Earth’s dimensions, but also its proper shape.

Shape of the Earth:

For starters, and contrary to what scientists have believed since classical antiquity, Earth is not a perfect sphere. Since the 17th and 18th centuries – thanks to improvements made in the field of astronomy and geodesy (a branch of mathematics dealing with the measurement of the Earth) – scientists have understood that the Earth is actually a flattened sphere.

This is what is known as an “oblate spheroid”, which is a sphere that is wider at its horizontal axis than it is at its vertical axis. According to the 2004 Working Group of the International Earth Rotation and Reference Systems Service (IERS), Earth experiences a flattening of 0.0033528 at the poles. This flattening is due to Earth’s rotational velocity – a rapid 1,674.4 km/h (1,040.4 mph) – which causes the planet to bulge at the equator.

Because of this, the diameter of the Earth at the equator is about 43 kilometers (27 mi) larger than the pole-to-pole diameter. The latest measurements indicate that the Earth has an equatorial diameter of 12,756 km (7926 mi), and a polar diameter of 12713.6 km (7899.86 mi). This is true for other planets in the Solar System that have rapid rotations (like Jupiter and Saturn), and even stars like the rapidly-spinning Altair.

Calculation:

Given its particular shape, calculating the Earth’s surface area requires a specific equation. Whereas determining the surface area of a sphere is a simple matter of multiplying pi by four, and these by the square of its radius (4 x 3.14159… x r²), to calculate the surface area of an oblate spheroid – where the distance from the center to a pole (c) is less than its semi-axis (a) – the following equation has to come into play:

{\displaystyle S_{\rm {oblate}}=2\pi a^{2}\left(1+{\frac {1-e^{2}}{e}}\tanh ^{-1}e\right)\quad {\mbox{where}}\quad e^{2}=1-{\frac {c^{2}}{a^{2}}}.}

Whereas S equals the surface area, c represents the distance from the center to a pole, and a represents the semi-axis, e represents the eccentricity.  Naturally, Earth’s surface area can also be subdivided between water and land segments (aka. oceans or continental crust).

The assignment of semi-axes on a spheroid. It is oblate if ca (right). Credit: Wikipedia Commons/Ag2gaeh

And since 70% of the Earth’s surface is covered by water, that works out to 361 million km² (139.4 million mi²). Earth’s continents, on the other hand, cover the remaining 149 million km² (57.5 or million mi²). This is a phenomena unique to Earth (at least in our Solar System) since no other Solar planet has liquid water covering a significant amount of its surface.

Other Solar Planets:

Compared to the other planets of the Solar System, Earth ranks somewhere in the middle. Of the terrestrial planets (i.e. Mercury, Venus, Earth and Mars) it is the largest. However, when compared to the gas giants (Jupiter, Saturn, Uranus and Neptune) it comes in dead last! Let’s see just how Earth stacks up against these other worlds…

Mercury is the smallest planet in our Solar System (ever since the 2006 IAU decision that designated Pluto as “dwarf planet”). It has a surface area of 7.48 x 107 km2, which is only about 15% of Earth’s surface area. Venus is similar in size to Earth, hence why it has earned the title of “Earth’s Sister Planet”. Consistently, Venus has a surface area of 4.6 x 108 km2, which is roughly 90% of Earth’s.

Mars is also a small planet, the second smallest in our Solar System. This is evident in Mars’ diminutive surface area of 1.45 x 108 km2, which is roughly 28% that of Earth’s. Moving to the outer Solar System, it is quickly made apparent that all of the gas giants have the four planets of the inner Solar System beat (at least in a size contest)!

An illustration showing the 8 planets of the Solar System to scale. Credit: NASA

Jupiter is the largest planet in our Solar System, with a surface area of 6.14 x 1010 km2 – which is about 122 times greater than the surface area of Earth! Saturn is the second largest planet in our Solar System and has a surface area of 4.27 x 1010 km2 – which is roughly 83.7 times that of Earth.

As for the “ice giants”, Uranus has a surface area of 8.1156 x 109 km2 (15.91 times that of Earth) while Neptune has a slightly smaller surface area of  7.618 x 109 km2, which is close to 15 times that of Earth.

All told, Earth is relatively spacious place, as terrestrial bodies go. But the amount of surface that we humans can actually live on is rather limited. Once you subtract all the space that’s occupied by water, you begin to see that the world may be a little on the smallish side after all.

We have written many interest articles about the Earth for Universe Today. Here’s What is the Diameter of Earth?, How Strong is the Force of Gravity on Earth?, How Much Does the Earth Way?, How Fast Does the Earth Rotate?, 1o Interesting Facts About Planet Earth, What is the Earth’s Average Temperature?, and Why Does the Earth Rotate?

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

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 Fraser Cain

Can We Launch Nuclear Waste Into the Sun?

Can We Launch Nuclear Waste Into the Sun?
Can We Launch Nuclear Waste Into the Sun?

When I look at the Sun, I don’t see a warm life-giving orb, nourishing all living creatures here on Earth. No, I see that fiery ball as a cosmic garbage compactor. A place I can dump all my household garbage, to make room for new impulse purchases.

I mean, the Sun is right there, not doing anything right? It’s hotter than any garbage incinerator, and it’s the gravitational well at the heart of the Solar System. Get me a rocket, let’s blast that waste into oblivion.

Okay, I suspect it’s going to get expensive, so let’s just start with the worst garbage on Earth: nuclear waste. You know, the byproduct of nuclear reactors that generate electricity for many parts of the world. This stuff is highly toxic and it’s going to be around for hundreds of thousands of years.

It’s also pretty dense, maybe it does make sense to get this stuff off Earth and into the Sun? Let’s run the numbers.

Nuclear waste, or radioactive waste, of course, is anything leftover material that still has radioactivity. For the most part, we get this as the leftover material from nuclear power reactors, but it’s also generated by hospitals, and nuclear weapons manufacturing. We’ve got leftover nuclear waste from uranium mining, radium processing, and various civil and military research projects.

Inside this geometric is a demolished uranium mill and its radioactive tailings. Credit: U.S. Department of Energy

For example, when you mine uranium from the ground, you get leftover radium and radioactive rock, soil, and even the water. When you power a nuclear reactor, the spent fuel rods are still highly radioactive and dangerous. In the United States alone, there are hundreds of different sites which are heavily contaminated, over thousands of acres.

According to the World Nuclear Association, OPEC nations generate 300 million tonnes of toxic waste every year. We’re talking about poisonous chemicals, medical waste, coal dust. Really anything that you don’t want anywhere near you, or inside you.

Just to give you a sense of scale, that’s a cube of toxic poisons nearly a kilometer to a side, assuming the stuff is a little more dense than water.

Out of this, only 97,000 tonnes of nuclear waste is generated across the planet every year. This is radioactive wastes of all types. That’s only .03% of all the toxic waste.

But for the purpose of our calculations, I’m going to zero in on the most toxic, most radioactive material we’re dealing with: the high-level waste produced by nuclear reactors. Now we’re merely talking about 12,000 tonnes per year, or 12% of the nuclear waste showing up on our planet every year.

Now, let’s look at launch costs. Most rocket companies are going to charge you $10,000 to $20,000 per kilogram to blast a payload into Low Earth Orbit. The best deal on the market right now is SpaceX at around $4,000 USD per kilogram. And if they get the Falcon Heavy flying this year, it could bring the price down to around $2,500 per kilogram.

If all we wanted to do was blast all this waste into Low Earth Orbit, the calculations are pretty simple. 12,000 tonnes is 12 million kilograms. Multiply that by $2,500 per kilogram, and you get 30 billion dollars. You’re looking at 240 Falcon Heavy launches per year. Almost a launch every single day carrying a payload of high-level nuclear waste. Out of sight, out of mind.

SpaceX Falcon Heavy rocket poised for launch from the Kennedy Space Center in Florida in this updated artists concept. Credit: SpaceX

That’s a lot of money, but in theory, the world could afford it if they wanted to stop having wars, or something. If they wanted to blast off all the nuclear waste, it would be more like 250 billion. Again. An incomprehensible amount of money, but still within the realm of possibility, assuming that SpaceX gets the Falcon Heavy launching, lofting payloads of nuclear waste 50 tonnes at a time.

But this is Low Earth Orbit, and we don’t want to go there. Anything in LEO still experiences friction from the Earth’s atmosphere, and eventually it’s going to return back to Earth. Imagine regular meteor showers of highly radioactive plutonium. That would be bad.

It would be more safer to launch this stuff into Geostationary Orbit, where the television satellites are broadcasting from. Material in this orbit can be expected to hang around for a long long time.

You’re looking at twice the price to blast off to GEO, so go ahead and double your costs to put that stuff safely out into space. 60 billion dollars for high-level waste. 500 billion for all the nuclear waste.

I’m sure SpaceX will give you a volume discount. And there might be smarter orbits where the waste has totally decayed into something safer by the time it re-enters the Earth’s atmosphere. What I’m saying is, there might be some cost savings.

Let’s say we’ve run all these numbers, and the cost is still worth it. But here’s the problem, rockets fail on a regular basis. They explode on the launch pad, or on their way to orbit. One bad explosion could spray highly toxic plutonium across a huge swath of the planet.

SpaceX Falcon 9 rocket moments after catastrophic explosion destroys the rocket and Amos-6 Israeli satellite payload at launch pad 40 at Cape Canaveral Air Force Station, FL, on Sept. 1, 2016. Credit: USLaunchReport

For one rocket, there’s a pretty low risk. Rockets are about 95% reliable, which means that 1 in 20 is going to fail somehow. If you’re only launching 240 rockets, you’re looking at 12 failure, some of which will be detonations on the launch pad, or explosions at a high altitude. At that rate, we’re guaranteed that it’ll always be cloudy with a chance of plutonium rain somewhere on Earth.

If having thousands of tonnes of nuclear waste hanging over your head makes you nervous, then you’re going to want to hear about more, permanent options. Let’s crash that stuff into the Sun.

It turns out, blasting it into the Sun is much much more expensive. Here’s why: You’d think that just blasting your waste into space means that it would just fall into the Sun, but your waste is still orbiting the Sun at the Earth’s velocity – 30 m/s sideways.

In order to actually get it to drop into the Sun, you need to cancel out the orbital velocity. In other words, you need to give your rocket about 31.7 m/s in velocity, to account for the atmosphere drag of Earth, and then cancel out the orbital velocity.

NASA’s New Horizons spacecraft needed 16.1 m/s to reach Pluto, so you’re talking about double the velocity.

To be fair, New Horizons and other spacecraft use gravitational slingshots to steal velocity from Jupiter and other planets, so it’s possible you could perform some complicated trajectory sweeping past the various planets to get the change in velocity you need. I haven’t done the math, but let’s just assume, there could be savings.

If you don’t cancel out that motion, your nuclear waste is going to just orbit the Sun forever, like an asteroid of garbage.

There’s another path you could take. Instead of trying to drop down into the Sun, you fly outwards until you’ve almost escaped the pull of the Sun. Where the angular momentum, that sideways motion, is almost zero. Cancel that out with a little thrust, and then let the Sun’s gravity pull your waste back down to its doom.

It’ll take hundreds or even thousands of years, but there would be cost savings. Then you only need to gain about 16.5 m/s in velocity.

The Falcon Heavy, once operational, will be the most powerful rocket in the world. Credit: SpaceX

Rockets need to carry more of their payload as fuel if they’re going to gain higher velocities. A Falcon Heavy can carry more than 54 tonnes to Low Earth Orbit, but only 2.9 tonnes to Pluto.

In other words, using the most efficient trajectory, you’d still need about 20 times more rockets to blast your fuel into the Sun. In other words, multiple your costs by a factor of 20.

$1.2 trillion to launch the high-level waste into the Sun on a trajectory that takes a long long time.

The bottom line is that blasting our nuclear waste off into space, into the Sun, is just too expensive – by several orders of magnitude. Not to mention incredibly dangerous for the inevitable rocket failures that will compound the problem.

No, we need to learn how to recycle nuclear waste, to make it less toxic. We need to be willing to spend the resources to properly clean up contaminated sites, and we need to careful consider the long term consequences of how we generate our energy. Not just with nuclear power, but with any polluting form of energy generation.

But you know what idea I like even better? I agree with Jeff Bezos when he says that we’re eventually going to want to move all heavy industry and manufacturing off Earth and out into space.

We could take our manufacturing off-planet to reduce environmental risks. NASA/Denise Watt

Instead of cleaning the waste out of our environment, let’s mine it, refine it and manufacture it out in space in the first place. Then we can send the products back to Earth, and skip most of the pollution.

Now I’ve done the numbers, what do you think? Still worth it to launch nuclear waste into space? Let me know your thoughts in the comments.