The night sky, is the night sky, is the night sky. The constellations you learned as a child are the same constellations that you see today. Ancient people recognized these same constellations. Oh sure, they might not have had the same name for it, but essentially, we see what they saw.
But when you see animations of galaxies, especially as they come together and collide, you see the stars buzzing around like angry bees. We know that the stars can have motions, and yet, we don’t see them moving?
How fast are they moving, and will we ever be able to tell?
Stars, of course, do move. It’s just that the distances are so great that it’s very difficult to tell. But astronomers have been studying their position for thousands of years. Tracking the position and movements of the stars is known as astrometry.
We trace the history of astrometry back to 190 BC, when the ancient Greek astronomer Hipparchus first created a catalog of the 850 brightest stars in the sky and their position. His student Ptolemy followed up with his own observations of the night sky, creating his important document: the Almagest.
Printed rendition of a geocentric cosmological model from Cosmographia, Antwerp, 1539. Credit: Wikipedia Commons/Fastfission
In the Almagest, Ptolemy laid out his theory for an Earth-centric Universe, with the Moon, Sun, planets and stars in concentric crystal spheres that rotated around the planet. He was wrong about the Universe, of course, but his charts and tables were incredibly accurate, measuring the brightness and location of more than 1,000 stars.
A thousand years later, the Arabic astronomer Abd al-Rahman al-Sufi completed an even more detailed measurement of the sky using an astrolabe.
One of the most famous astronomers in history was the Danish Tycho Brahe. He was renowned for his ability to measure the position of stars, and built incredibly precise instruments for the time to do the job. He measured the positions of stars to within 15 to 35 arcseconds of accuracy. Just for comparison, a human hair, held 10 meters away is an arcsecond wide.
Also, I’m required to inform you that Brahe had a fake nose. He lost his in a duel, but had a brass replacement made.
In 1807, Friedrich Bessel was the first astronomer to measure the distance to a nearby star 61 Cygni. He used the technique of parallax, by measuring the angle to the star when the Earth was on one side of the Sun, and then measuring it again 6 months later when the Earth was on the other side.
With parallax technique, astronomers observe object at opposite ends of Earth’s orbit around the Sun to precisely measure its distance. Credit: Alexandra Angelich, NRAO/AUI/NSF.
Over the course of this period, this relatively closer star moves slightly back and forth against the more distant background of the galaxy.
And over the next two centuries, other astronomers further refined this technique, getting better and better at figuring out the distance and motions of stars.
But to really track the positions and motions of stars, we needed to go to space. In 1989, the European Space Agency launched their Hipparcos mission, named after the Greek astronomer we talked about earlier. Its job was to measure the position and motion of the nearby stars in the Milky Way. Over the course of its mission, Hipparcos accurately measured 118,000 stars, and provided rough calculations for another 2 million stars.
That was useful, and astronomers have relied on it ever since, but something better has arrived, and its name is Gaia.
Launched in December 2013, the European Space Agency’s Gaia in is in the process of mapping out a billion stars in the Milky Way. That’s billion, with a B, and accounts for about 1% of the stars in the galaxy. The spacecraft will track the motion of 150 million stars, telling us where everything is going over time. It will be a mind bending accomplishment. Hipparchus would be proud.
With the most precise measurements, taken year after year, the motions of the stars can indeed be calculated. Although they’re not enough to see with the unaided eye, over thousands and tens of thousands of years, the positions of the stars change dramatically in the sky.
The familiar stars in the Big Dipper, for example, look how they do today. But if you go forward or backward in time, the positions of the stars look very different, and eventually completely unrecognizable.
When a star is moving sideways across the sky, astronomers call this “proper motion”. The speed a star moves is typically about 0.1 arc second per year. This is almost imperceptible, but over the course of 2000 years, for example, a typical star would have moved across the sky by about half a degree, or the width of the Moon in the sky.
A 20 year animation showing the proper motion of Barnard’s Star. Credit: Steve Quirk, images in the Public Domain.
The star with the fastest proper motion that we know of is Barnard’s star, zipping through the sky at 10.25 arcseconds a year. In that same 2000 year period, it would have moved 5.5 degrees, or about 11 times the width of your hand. Very fast.
When a star is moving toward or away from us, astronomers call that radial velocity. They measure this by calculating the doppler shift. The light from stars moving towards us is shifted towards the blue side of the spectrum, while stars moving away from us are red-shifted.
Between the proper motion and redshift, you can get a precise calculation for the exact path a star is moving in the sky.
Credit: ESA/ATG medialab
We know, for example, that the dwarf star Hipparcos 85605 is moving rapidly towards us. It’s 16 light-years away right now, but in the next few hundred thousand years, it’s going to get as close as .13 light-years away, or about 8,200 times the distance from the Earth to the Sun. This won’t cause us any direct effect, but the gravitational interaction from the star could kick a bunch of comets out of the Oort cloud and send them down towards the inner Solar System.
The motions of the stars is fairly gentle, jostling through gravitational interactions as they orbit around the center of the Milky Way. But there are other, more catastrophic events that can make stars move much more quickly through space.
When a binary pair of stars gets too close to the supermassive black hole at the center of the Milky Way, one can be consumed by the black hole. The other now has the velocity, without the added mass of its companion. This gives it a high-velocity kick. About once every 100,000 years, a star is kicked right out of the Milky Way from the galactic center.
A rogue star being kicked out of a galaxy. Credit: NASA, ESA, and G. Bacon (STScI)
Another situation can happen where a smaller star is orbiting around a supermassive companion. Over time, the massive star bloats up as supergiant and then detonates as a supernova. Like a stone released from a sling, the smaller star is no longer held in place by gravity, and it hurtles out into space at incredible speeds.
Astronomers have detected these hypervelocity stars moving at 1.1 million kilometers per hour relative to the center of the Milky Way.
All of the methods of stellar motion that I talked about so far are natural. But can you imagine a future civilization that becomes so powerful it could move the stars themselves?
In 1987, the Russian astrophysicist Leonid Shkadov presented a technique that could move a star over vast lengths of time. By building a huge mirror and positioning it on one side of a star, the star itself could act like a thruster.
An example of a stellar engine using a mirror and a Dyson Swarm. Credit: Vedexent at English Wikipedia (CC BY-SA 3.0)
Photons from the star would reflect off the mirror, imparting momentum like a solar sail. The mirror itself would be massive enough that its gravity would attract the star, but the light pressure from the star would keep it from falling in. This would create a slow but steady pressure on the other side of the star, accelerating it in whatever direction the civilization wanted.
Over the course of a few billion years, a star could be relocated pretty much anywhere a civilization wanted within its host galaxy.
This would be a true Type III Civilization. A vast empire with such power and capability that they can rearrange the stars in their entire galaxy into a configuration that they find more useful. Maybe they arrange all the stars into a vast sphere, or some kind of geometric object, to minimize transit and communication times. Or maybe it makes more sense to push them all into a clean flat disk.
Amazingly, astronomers have actually gone looking for galaxies like this. In theory, a galaxy under control by a Type III Civilization should be obvious by the wavelength of light they give off. But so far, none have turned up. It’s all normal, natural galaxies as far as we can see in all directions.
For our short lifetimes, it appears as if the sky is frozen. The stars remain in their exact positions forever, but if you could speed up time, you’d see that everything is in motion, all the time, with stars moving back and forth, like airplanes across the sky. You just need to be patient to see it.
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.
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.
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.
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.
View of the night sky in North Carolina, showing the constellations of Orion, Hyades, Canis Major and Canis Minor. Credit: NASA
Welcome back to Constellation Friday! Today, in honor of the late and great Tammy Plotner, we will be dealing with the “little dog” – the Canis Minor 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 was Canis Minor, a small constellation in the northern hemisphere. As a relatively dim collection of stars, it contains only two particularly bright stars and only faint Deep Sky Objects. Today, it is one of the 88 constellations recognized by the International Astronomical Union, and is bordered by the Monoceros, Gemini, Cancer and Hydra constellation.
Name and Meaning:
Like most asterisms named by the Greeks and Romans, the first recorded mention of this constellation goes back to ancient Mesopotamia. Specifically, Canis Minor’s brightest stars – Procyon and Gomeisa – were mentioned in the Three Stars Each tablets (ca. 1100 BCE), where they were referred to as MASH.TAB.BA (or “twins”).
The Winter Hexagon, which contains parts of the Auriga, Canis Major, Canis Minor, Gemini, Monoceros, Orion, Taurus, Lepus and Eridanus constellations. Credit: constellation-guide.com
In the later texts that belong to the MUL.APIN, the constellation was given the name DAR.LUGAL (“the star which stands behind it”) and represented a rooster. According to ancient Greco-Roman mythology, Canis Minor represented the smaller of Orion’s two hunting dogs, though they did not recognize it as its own constellation.
In Greek mythology, Canis Minor is also connected with the Teumessian Fox, a beast turned into stone with its hunter (Laelaps) by Zeus. He then placed them in heaven as Canis Major (Laelaps) and Canis Minor (Teumessian Fox). According to English astronomer and biographer of constellation history Ian Ridpath:
“Canis Minor is usually identified as one of the dogs of Orion. But in a famous legend from Attica (the area around Athens), recounted by the mythographer Hyginus, the constellation represents Maera, dog of Icarius, the man whom the god Dionysus first taught to make wine. When Icarius gave his wine to some shepherds for tasting, they rapidly became drunk. Suspecting that Icarius had poisoned them, they killed him. Maera the dog ran howling to Icarius’s daughter Erigone, caught hold of her dress with his teeth and led her to her father’s body. Both Erigone and the dog took their own lives where Icarius lay.
“Zeus placed their images among the stars as a reminder of the unfortunate affair. To atone for their tragic mistake, the people of Athens instituted a yearly celebration in honour of Icarius and Erigone. In this story, Icarius is identified with the constellation Boötes, Erigone is Virgo and Maera is Canis Minor.”
Canis Minor, as depicted by Johann Bode in his 1801 work Uranographia. Credit: Wikipedia Commons/Alessio Govi
To the ancient Egyptians, this constellation represented Anubis, the jackal god. To the ancient Aztecs, the stars of Canis Minor were incorporated along with stars from Orion and Gemini into as asterism known as “Water”, which was associated with the day. Procyon was also significant in the cultural traditions of the Polynesians, the Maori people of New Zealand, and the Aborigines of Australia.
In Chinese astronomy, the stars corresponding to Canis Minor were part of the The Vermilion Bird of the South. Along with stars from Cancer and Gemini, they formed the asterisms known as the Northern and Southern River, as well as the asterism Shuiwei (“water level”), which represented an official who managed floodwaters or a marker of the water level.
History of Observation:
Canis Minor was one of the original 48 constellations included by Ptolemy in his the Almagest. Though not recognized as its own asterism by the Ancient Greeks, it was added by the Romans as the smaller of Orion’s hunting dogs. Thanks to Ptolemy’s inclusion of it in his 2nd century treatise, it would go on to become part of astrological and astronomical traditions for a thousand years to come.
For medieval Arabic astronomers, Canis Minor continued to be depicted as a dog, and was known as “al-Kalb al-Asghar“. It was included in the Book of Fixed Stars by Abd al-Rahman al-Sufi, who assigned a canine figure to his stellar diagram. Procyon and Gomeisa were also named for their proximity to Sirius; Procyon being named the “Syrian Sirius (“ash-Shi’ra ash-Shamiya“) and Gomeisa the “Sirius with bleary eyes” (“ash-Shira al-Ghamisa“).
The constellation Canis Minor, shown alongside Monoceros and the obsolete constellation Atelier Typographique. Credit: Library of Congress
The constellation was included in Syndey Hall’s Urania’s Mirror (1825) alongside Monoceros and the now obsolete constellation Atelier Typographique. Many alternate names were suggested between the 17th and 19th centuries in an attempt to simplify celestial charts. However, Canis Minor has endured; and in 1922, it became one the 88 modern constellations to be recognized by the IAU.
Notable Features:
Canis Minor contains two primary stars and 14 Bayer/Flamsteed designated stars. It’s brightest star, Procyon (Alpha Canis Minoris), is also the seventh brightest star in the sky. With an apparent visual magnitude of 0.34, Procyon is not extraordinarily bright in itself. But it’s proximity to the Sun – 11.41 light years from Earth – ensures that it appears bright in the night sky.
The star’s name is derived from the Greek word which means “before the dog”, a reference to the fact that it appears to rise before Sirius (the “Dog Star”) when observed from northern latitudes. Procyon is a binary star system, composed of a white main sequence star (Procyon A) and Procyon B, a DA-type faint white dwarf as the companion.
Procyon is part of the Winter Triangle asterism, along with Sirius in Canis Major and Betelgeuse in the constellation Orion. It is also part of the Winter Hexagon, along with the stars Capella in Auriga, Aldebaran in Taurus, Castor and Pollux in Gemini, Rigel in Orion and Sirius in Canis Major.
The stars of the Winter Triangle and the Winter Hexagon. Credit: constellation-guide.com
Next up is Gomeisa, the second brightest star in Canis Minor. This hot, B8-type main sequence star is classified as a Gamma Cassiopeiae variable, which means that it rotates rapidly and exhibits irregular variations in luminosity because of the outflow of matter. Gomeisa is approximately 170 light years from Earth and the name is derived from the Arabic “al-ghumaisa” (“the bleary-eyed woman”).
Canis Minor also has a number of Deep Sky Objects located within it, but all are very faint and difficult to observe. The brightest is the spiral galaxy NGC 2485 (apparent magnitude of 12.4), which is located 3.5 degrees northeast of Procyon. There is one meteor shower associated with this constellation, which are the Canis-Minorids.
Finding Canis Minor:
Though it is relatively faint, Canis Minor and its stars can be viewed using binoculars. Start with the brightest, Procyon – aka. Alpha Canis Minoris (Alpha CMi). If you’re unsure of which bright star is, you’ll find it in the center of the diamond shape grouping in the southwest area. Known to the ancients as Procyon – “The Little Dog Star” – it’s the seventh brightest star in the night sky and the 13th nearest to our solar system.
For over 100 years, astronomers have known this brilliant star had a companion. Being 15,000 times fainter than the parent star, Procyon B is an example of a white dwarf whose diameter is only about twice that of Earth. But its density exceeds two tons per cubic inch! (Or, a third of a metric ton per cubic centimeter). While only very large telescopes can resolve this second closest of the white dwarf stars, even the moonlight can’t dim its beauty.
The Winter Triangle. Credit: constellation-guide.com/Stellarium software
Now hop over to Beta CMi. Known by the very strange name of Gomeisa (“bleary-eyed woman”), it refers to the weeping sister left behind when Sirius and Canopus ran to the south to save their lives. Located about 170 light years away from our Solar System, Beta is a blue-white class B main sequence dwarf star with around 3 times the mass of our Sun and a stellar luminosity over 250 times that of Sol.
Gomeisa is a fast rotator, spinning at its equator with a speed of at least 250 kilometers per second (125 times our Sun’s rotation speed) giving the star a rotation period of about a day. Sunspots would appear to move very quickly there! According to Jim Kaler, Professor Emeritus of Astronomy at the University of Illinois:
“Since we may be looking more at the star’s pole than at its equator, it may be spinning much faster, and indeed is rotating so quickly that it is surrounded by a disk of matter that emits radiation, rendering Gomeisa a “B-emission” star rather like Gamma Cassiopeiae and Alcyone. Like these two, Gomeisa is distinguished by having the size of its disk directly measured, the disk’s diameter almost four times larger than the star. Like quite a number of hot stars (including Adhara, Nunki, and many others), Gomeisa is also surrounded by a thin cloud of dusty interstellar gas that it helps to heat.”
Now hop over to Gamma Canis Minoris, an orange K-type giant with an apparent magnitude of +4.33. It is a spectroscopic binary, has an unresolved companion which has an orbital period of 389 days, and is approximately 398 light years from Earth. And next is Epsilon Canis Minoris, a yellow G-type bright giant (apparent magnitude of +4.99) which is approximately 990 light years from Earth.
The location of Canis Minor in the northern hemisphere. Credit: IAU/Sky&Telescope magazine
For smaller telescopes, the double star Struve 1149 is a lovely sight, consisting of a yellow primary star and a faintly blue companion. For larger telescopes and GoTo telescopes, try NGC 2485 (RA 07 56.7 Dec +07 29), a magnitude 13 spiral galaxy that has a small, round glow, sharp edges and a very bright, stellar nucleus. If you want one that’s even more challenging, try NGC 2508 (RA 08 02 0 Dec +08 34).
Canis Minor lies in the second quadrant of the northern hemisphere (NQ2) and can be seen at latitudes between +90° and -75°. The neighboring constellations are Cancer, Gemini, Hydra, and Monoceros, and it is best visible during the month of March.
The northern constellation of Bootes, one of the 88 modern constellations recognized by the IAU. Credit: smokymtnastro.org
Welcome back to Constellation Friday! Today, in honor of our dear friend and contributor, Tammy Plotner, we examine the Bootes constellation. Enjoy!
In the 2nd century CE, Greek-Egyptian astronomer Claudius Ptolemaeus (aka. Ptolemy) compiled a list of the then-known 48 constellations. Until the development of modern astronomy, his treatise (known as the Almagest) would serve as the authoritative source of astronomy. This list has since come to be expanded to include the 88 constellation that are recognized by the International Astronomical Union (IAU) today.
The constellation Boötes (pronounced Bu-Oh-Tays) is one of these constellations, and was also among those listed in the Almagest. It is frequently called the “Watcher of the Bear”, guarding over the northern constellations of both Ursa Major and Ursa Minor (the Greater and Lesser Bears). It is bordered by Canes Venatici, Coma Berenices, Corona Borealis, Draco, Hercules, Serpens Caput, Virgo and Ursa Major.
Name and Meaning:
According to myth, Boötes is credited for inventing the plough, which prompted the goddess Ceres – a goddess of agriculture, grain crops, fertility and motherly love – to place him in the heavens. There are also versions where Bootes represents a form of Atlas, holding up the weight of the world as it turns on its axis (yet another of Hercules’ labors).
Most commonly, Boötes is taken to represent Arcas, the son of Zeus and Callisto. In this source, Arcas was brought up by Callisto father, the Arcadian king Lycaon. One day, Lycaon decided to test Zeus by serving him his own son for a meal. Zeus saw through Lycaon’s intentions and transformed the king into a wolf, killed his sons, and brought Arcas back to life.
Boötes as depicted in Urania’s Mirror, a set of constellation cards published in London c.1825. Credit: Wikipedia Commons/Sidney
Having heard of her husband’s infidelity, Zeus’ wife Hera transformed Callisto into a bear. For years, she roamed the woods until she met her son, who was now grown up. Arcas didn’t recognize his mother and began to chase her. To avoid a tragic end, Zeus intervened by placing them both in the sky, where Callisto became Ursa Major (aka. The Big Dipper, or “Great Bear”) and Arcas became Boötes.
In another story, Boötes is taken to represent Icarius, a grape grower who was given the secret of wine-making by Dionysus. Icarius used this to create a wonderful wine that he shared with all his neighbors. After overindulging, they woke up the next day with terrible hangovers and believed Icarius had tried to poison them. They killed him in his sleep, and a saddened Dionysus placed his friend among the stars.
Notable Features:
Bootes contains the third brightest star in the night sky – Arcturus (aka. alpha Boötis) – whose Greek name “Arktos” also means “bear”, and is associated with all things northern (including the aurora). Arcturus is quite important, being a type K1.5 IIIpe red giant star. The letters “pe” stand for “peculiar emission,” which indicates the spectrum of the star is unusual and full of emission lines. This is not uncommon in red giants, but Arcturus is particularly strong.
The location of the Bootes contellation. Credit: IAU/Sky and Telescope
Arcturus is about 110 times more luminous than our nearest star, but the total power output is about 180 times that of the Sun (when infrared radiation is considered). Arcturus is also notable for its high proper motion, larger than any first magnitude star in the stellar neighborhood other than Alpha Centauri. It is now almost at its closest and is moving rapidly (122 km/s) relative to the Solar System.
Arcturus is also thought to be an old disk star, and appears to be moving with a group of 52 others of its type. Its mass is hard to determine exactly, but it may have the same mass as Sol, or perhaps 1.5 times as much. Arcturus may also be older than the Sun, and much like what the Sun will be in its Red Giant Phase.
Arcturus achieved fame when its light was used to open the 1933 Chicago World’s Fair. The star was chosen because it was thought that light from the star had started its journey at about the same time of the previous Chicago World’s Fair (1893). Technically the star is 36.7 light years away, so the light would have started its journey in 1896. Arcturus’ light was still focused onto a cell that powered the switch for the lights that eventually shined so bright that Arcturus was no longer visible.
Arcturus, along with its neighboring stars, also form the curious “Colonial Viper” formation, a triangular asterism invented by dedicated SkyWatcher, Ed Murray. It is so-named because it resembles a Colonial Viper being launched from a tube on the TV series Battlestar Galactica. The “Launch Tube” is formed by the intersection of Arcturus, Alphekka (Alpha Corona Borealis) and Gamma Bootis, while Izar (Epsilon Bootes) is the Viper.
A Colonial Viper leaving the Launch Tube aboard the Battlestar Galactica. Credit: battlestararies-bsr26.net
Other notable stars include Nekkar (Beta Boötis), a yellow G-type giant that is 219 light years from Earth. It is a flare star, which is a type of variable star that shows dramatic increases in luminosity for a few minutes. The name Nekkar derives from the Arabic word for “cattle driver”. Then there’s Seginus (Gamma Boötis), a Delta-Scuti type variable star that is approximately 85 light years from Earth. It shows variations in its brightness due to both radial and non-radial pulsations on its surface.
Izar (Epislon Boötis) is a binary star located approximately 300 light years away which consists of a bright orange giant and a smaller and fainter main sequence star. Epsilon Boötis is also sometimes knows as Pulcherrima, which means “the lovieliest” in Latin. The name Izar comes from the Arabic word for “veil.” The star’s other traditional names are Mirak (“the loins” in Arabic) and Mizar.
Muphrid (Eta Boötis) is a spectroscopic binary star that is 37 light years from Earth and close to Arcturus in the sky. The star’s traditional name is Muphrid, derived from the Arabic phrase for “the single one of the lancer.” It belongs to the spectral class G0 IV and has a significant excess of elements heavier than hydrogen.
Boötes is also home to many Deep Sky Objects. This includes the Boötes void (aka. the Great Void, the Supervoid). This sphere-shaped region of the sky is almost 250 million light years in diameter and contains 60 galaxies. The void was originally discovered by Robert P. Kirshner – a Harvard College Professor of Astronomy – in 1981, as part of a survey of galactic redshifts.
The very loose globular cluster NGC 5466 located in the Boots consetllation, Credit: NASA, ESA/Wikisky
Then there is the Boötes Dwarf Galaxy (Boötes I), a dwarf spheroidal galaxy located approximately 197,000 light years from Earth that measures about 720 light years across. It was only discovered in 2006, owing to the fact that it is one of the faintest galaxies known (with an absolute magnitude of -5.8 and apparent magnitude of 13.1). Boötes I orbits the Milky Way and is believed to be tidally disrupted by its gravity, as evidenced by its shape.
And there’s also NGC 5466, a globular cluster approximately 51,800 light years from Earth and 52,800 light years from the Galactic center. The cluster was first discovered by the German-born British astronomer William Herschel in 1784. It is believed that this cluster is the source of a star stream called the 45 Degree Tidal Stream, which was discovered in 2006.
History of Observation:
The earliest recorded mentions of the stars associated with Boötes come from ancient Babylonia, where it was listed as SHU.PA. These stars were apparently depicted as the god Enlil, who was the leader of the Babylonian pantheon and special patron of farmers. It is likely that this is the source of mythological representations of Bootes as “the ploughman” in Greco-Roman astronomy.
The name Boötes was first used by Homer in The Odyssey as a celestial reference point for navigation. The name literally means “ox-driver” or “herdsman”, and the ancient Greeks saw the asterism now called the “Big Dipper” or “Plough” as a cart with oxen. His dogs, Chara and Asterion, were represented by the constellation of Canes Venatici (the Hunting Dogs) who drove the oxen on and kept the wheels of the sky turning.
The Big Dipper, the asterism that neighbors the Bootes constellation. Credit: Jerry Lodriguss
In traditional Chinese astronomy, many of the stars in Boötes were associated with different Chinese constellations. Arcturus was one of the most prominent, variously designated as the celestial king’s throne (Tian Wang) or the Blue Dragon’s horn (Daijiao). Arcturus was also very important in Chinese celestial mythology because it is the brightest star in the northern sky, and marked the beginning of the lunar calendar.
Flanking Daijiao were the constellations of Yousheti on the right and Zuosheti on the left, which represented the companions that orchestrated the seasons. Dixi, the Emperor’s ceremonial banquet mat, was north of Arcturus. Another northern constellation was Qigong, the Seven Dukes, which was mostly across the Boötes-Hercules border.
The other Chinese constellations made up of the stars of Boötes existed in the modern constellation’s north. These are all representations of weapons – Tianqiang, the spear; Genghe, variously representing a lance or shield; Xuange, the halberd; and Zhaoyao, either the sword or the spear.
Finding Bootes:
Bootes can be found south of Ursa Major, just off the handle of the Big Dipper. Because the Big Dipper is easy for most observers to find, the handle is used to point to other important stars. Bootes’ brightest star, Arcturus, is also part of a mnemonic device used to orient people, which goes: “Arc to Arcturus, speed on to Spica.” This means you follow the curve in the Dipper’s handle away from Ursa Major until you run into Arcturus. The other star – Spica – is part of the neighboring Virgo constellation.
Arcturus, the brightest star in the Boötes constellation. Credit: astropixels.com
For those using binoculars, check out Tau Bootis, a yellow-white dwarf approximately 51 light-years from Earth. It is a binary star system, with the secondary star being a red dwarf. In 1999, an extrasolar planet was confirmed to be orbiting the primary star by a team of astronomers led by Geoff Marcy and R. Paul Butler. Maybe you’d like to look at long term variable star R Boötis? It ranges from 6.2 to 13.1 every 223.4 days.
For those using telescopes, there are plenty of excellent binary star systems to be seen. Pi Boötis is located approximately 317 light years from our solar system and the primary component, P¹ Boötis, is a blue-white B-type main sequence dwarf with an apparent magnitude of +4.49. It’s companion, P² Boötis, is a white A-type main sequence dwarf with an apparent magnitude of +5.88.
Now try looking at Xi Boötis, a binary star system which lies 21.8 light years away. The primary star, Xi Boötis A, is a BY Draconis variable, yellow G-type main sequence dwarf with an apparent magnitude that varies from +4.52 to +4.67. with a period just over 10 days long. Small velocity changes in the orbit of the companion star, Xi Boötis B – an orange K-type main sequence dwarf – indicate the presence of a small companion with less than nine times the mass of Jupiter.
The AB binary can be resolved even through smaller telescopes. The primary star (A) has been identified as a candidate for possessing a Kuiper-like belt, based on infrared observations. The estimated minimum mass of this dust disk is 2.4 times the mass of the Earth’s Moon.
The location of Mu Bootis (Alkalurops) in the Bootes constellation. Credit: universeguide.com
Then there’s the triple system, Mu Boötis. The primary component, Mu¹ Boötis, is a yellow-white F-type sub giant with an apparent magnitude of +4.31. Separated from the primary by 108 arc seconds is the binary star Mu² Boötis, which has a combined spectral type of G1V and a combined brightness of +6.51 magnitudes. The components of Mu² Boötis have apparent magnitudes of +7.2 and +7.8 and are separated by 2.2 arc seconds.
They complete one orbit about their common center of mass every 260 years. How about colorful yellow and blue Kappa Boötis? Kappa2 Boötis is classified as a Delta Scuti type variable star and its brightness varies from magnitude +4.50 to +4.58 with a period of 1.83 hours. The companion star, Kappa¹ Boötis, has magnitude +6.58 and spectral class F1V.
For deep sky observers with large telescopes, try checking out the globular cluster NGC 5466, which is about a fist’s width north of Arcturus. This class XII, 9th magnitude globular was discovered in 1784 by Sir William Herschel and presents an nice challenge for experienced stargazers and amateur astronomers.
Or try compact spiral galaxy NGC 5248. It’s about a fist width south of Arcturus and about a finger width southwest. It’s part of the Virgo cluster of galaxies and could be as far as 50 million light years away. It’s another great grand design spiral which shows spiral galaxy structure when viewed in long exposure photographs. You can mark it on your list as Caldwell 45.
The NGC 5248 spiral galaxy, as imaged with a 32-inch telescope. Credit and Copyright: Adam Block/Mount Lemmon SkyCenter/University of Arizona
But if you’d just like to have some fun, then why not try picking out the aforementioned “Colonial Viper and Launch Tube” asterism. If you’re a longstanding Battlestar Galactica fan, then you’ll recognize this ultra-cool spaceship as it sits in its triangular shaped launch tube. To find it, just draw a line between Arcturus, Alphekka (Alpha Corona Borealis) and Gamma Bootis which make up the “Launch Tube”, while Izar (Epsilon Bootes) is the Viper.
The northern constellation Auriga, showing the brightest stars of Capella, Menkalinan, and proximate Deep Sky Objects. Credit: stargazerslounge.com
Welcome back to Constellation Friday! Today, in honor of our dear friend and contributor, Tammy Plotner, we examine the Auriga constellation. Enjoy!
In the 2nd century CE, Greek-Egyptian astronomer Claudius Ptolemaeus (aka. Ptolemy) compiled a list of the then-known 48 constellations. His treatise, known as the Almagest, would serve as the authoritative source of astronomy for over a thousand years to come. Since the development of modern telescopes and astronomy, this list has come to be expanded to include the 88 constellation that are recognized by the International Astronomical Union (IAU) today.
One of these is the constellation of Auriga, a beautiful pentagon-shaped collection of stars that is situated just north of the celestial equator. Along with five other constellations that have stars in the Winter Hexagon asterism, Auriga is most prominent during winter evenings in the Northern Hemisphere. Auriga also belongs to the Perseus family of constellations, together with Andromeda, Cassiopeia, Cepheus, Cetus, Lacerta, Pegasus, Perseus, and Triangulum.
The Aries constellation and nearby Deep Sky Objects. Credit: thinglink.com
Welcome back to constellation Friday! Today, in honor of our dear friend and contributor, Tammy Plotner, we examine the Aries constellation. Enjoy!
In the 2nd century CE, Greek-Egyptian astronomer Claudius Ptolemaeus (aka. Ptolemy) compiled a list of the then-known 48 constellations. His treatise, known as the Almagest, would serve as the authoritative source of astronomy for over a thousand years to come. Since the development of modern telescopes and astronomy, this list has come to be expanded to include the 88 constellation that are recognized by the International Astronomical Union (IAU) today.
Of these constellations, Aries – named in honor of the Ram from classical Greek mythology – is featured rather prominently. This faint constellation has deep roots, and is believed to date all the way back to the astrological systems of the ancient Babylonians. Positioned on the ecliptic plane, it is bordered by constellations of Perseus, Triangulum, Pisces, Cetus and Taurus, and is also the traditional home of the vernal equinox.
Aquarius the "Water Bearer" is a large but faint constellation in the Southern sky. Credit: Stellarium
Welcome back to Constellation Friday! Today, we will be dealing with one of the best-known constellations, that “watery” asterism and section of the sky known as Aquarius. Cue the soundtrack from Hair!
In the 2nd century CE, Greek-Egyptian astronomer Claudius Ptolemaeus (aka. Ptolemy) compiled a list of all the-then known constellations. This work (known as the Almagest) would remain the definitive guide to astronomy and astrology for over a thousand years. Among the 48 constellations listed in this book was Aquarius, a constellation of the zodiac that stretches from the celestial equator to the southern hemisphere.
An illustration of the Ptolemaic geocentric system by Portuguese cosmographer and cartographer Bartolomeu Velho, 1568 (Bibliothèque Nationale, Paris)
During the many thousand years that human beings have been looking up at the stars, our concept of what the Universe looks like has changed dramatically. At one time, the magi and sages of the world believed that the Universe consisted of a flat Earth (or a square one, a zigarrut, etc.) surrounded by the Sun, the Moon, and the stars. Over time, ancient astronomers became aware that some stars did not move like the rest, and began to understand that these too were planets.
In time, we also began to understand that the Earth was indeed round, and came up with rationalized explanations for the behavior of other celestial bodies. And by classical antiquity, scientists had formulated ideas on how the motion of the planets occurred, and how all the heavenly orbs fit together. This gave rise to the Geocentric model of the universe, a now-defunct model that explained how the Sun, Moon, and firmament circled around our planet.
Andreas Cellarius's illustration of the Copernican system, from the Harmonia Macrocosmica (1708). Credit: Public Domain
The Scientific Revolution, which took place in the 16th and 17th centuries, was a time of unprecedented learning and discovery. During this period, the foundations of modern science were laid, thanks to breakthroughs in the fields of physics, mathematics, chemistry, biology, and astronomy. And when it comes to astronomy, the most influential scholar was definitely Nicolaus Copernicus, the man credited with the creation of the Heliocentric model of the Universe.
Based on ongoing observations of the motions of the planets, as well as previous theories from classical antiquity and the Islamic World, Copernicus’ proposed a model of the Universe where the Earth, the planets and the stars all revolved around the Sun. In so doing, he resolved the mathematical problems and inconsistencies arising out of the classic geocentric model and laid the foundations for modern astronomy.
While Copernicus was not the first to propose a model of the Solar System in which the Earth and planets revolved around the Sun, his model of a heliocentric universe was both novel and timely. For one, it came at a time when European astronomers were struggling to resolve the mathematical and observational problems that arose out of the then-accepted Ptolemaic model of the Universe, a geocentric model proposed in the 2nd century CE.
In addition, Copernicus’ model was the first astronomical system that offered a complete and detailed account of how the Universe worked. Not only did his model resolves issues arising out of the Ptolemaic system, it offered a simplified view of the universe that did away with complicated mathematical devices that were needed for the geocentric model to work. And with time, the model gained influential proponents who contributed to it becoming the accepted convention of astronomy.
An illustration of the Ptolemaic geocentric system by Portuguese cosmographer and cartographer Bartolomeu Velho, 1568. Credit: Bibliothèque Nationale, Paris
The Ptolemaic (Geocentric) Model:
The geocentric model, in which planet Earth is the center of the Universe and is circled by the Sun and all the planets, had been the accepted cosmological model since ancient times. By late antiquity, this model had come to be formalized by ancient Greek and Roman astronomers, such as Aristotle (384 – 322 BCE) – who’s theories on physics became the basis for the motion of the planets – and Ptolemy (ca. 100 – ca.?170 CE), who proposed the mathematical solutions.
The geocentric model essentially came down to two common observations. First of all, to ancient astronomers, the stars, the Sun, and the planets appeared to revolve around the Earth on daily basis. Second, from the perspective of the Earth-bound observer, the Earth did not appear to move, making it a fixed point in space.
The belief that the Earth was spherical, which became an accepted fact by the 3rd century BCE, was incorporated into this system. As such, by the time of Aristotle, the geocentric model of the universe became one where the Earth, Sun and all the planets were spheres, and where the Sun, planets and stars all moved in perfect circular motions.
However, it was not until Egyptian-Greek astronomer Claudius Ptolemaeus (aka. Ptolemy) released his treatise Almagest in the 2nd century BCE that the details became standardized. Drawing on centuries of astronomical traditions, ranging from Babylonian to modern times, Ptolemy argued that the Earth was in the center of the universe and the stars were all at a modest distance from the center of the universe.
The planet Mars, undergoing “retrograde motion” – a phenomena where it appears to be moving backwards in the sky – in late 2009 and early 2010. Credit: NASA
Each planet in this system is also moved by a system of two spheres – a deferent and an epicycle. The deferent is a circle whose center point is removed from the Earth, which was used to account for the differences in the lengths of the seasons. The epicycle is embedded in the deferent sphere, acting as a sort of “wheel within a wheel”. The purpose of he epicycle was to account for retrograde motion, where planets in the sky appear to be slowing down, moving backwards, and then moving forward again.
Unfortunately, these explanations did not account for all the observed behaviors of the planets. Most noticeably, the size of a planet’s retrograde loop (especially Mars) were sometimes smaller, and larger, than expected. To alleviate the problem, Ptolemy developed the equant – a geometrical tool located near the center of a planet’s orbit that causes it to move at a uniform angular speed.
To an observer standing at this point, a planet’s epicycle would always appear to move at uniform speed, whereas it would appear to be moving at non-uniform speed from all other locations.While this system remained the accepted cosmological model within the Roman, Medieval European and Islamic worlds for over a thousand years, it was unwieldy by modern standards.
However, it did manage to predict planetary motions with a fair degree of accuracy, and was used to prepare astrological and astronomical charts for the next 1500 years. By the 16th century, this model was gradually superseded by the heliocentric model of the universe, as espoused by Copernicus, and then Galileo and Kepler.
Picture of George Trebizond’s Latin translation of Almagest. Credit: Public Domain
The Copernican (Heliocentric) Model:
In the 16th century, Nicolaus Copernicus began devising his version of the heliocentric model. Like others before him, Copernicus built on the work of Greek astronomer Atistarchus, as well as paying homage to the Maragha school and several notable philosophers from the Islamic world (see below). By the early 16th century, Copernicus summarized his ideas in a short treatise titled Commentariolus (“Little Commentary”).
By 1514, Copernicus began circulating copies amongst his friends, many of whom were fellow astronomers and scholars. This forty-page manuscript described his ideas about the heliocentric hypothesis, which was based on seven general principles. These principles stated that:
Celestial bodies do not all revolve around a single point
The center of Earth is the center of the lunar sphere—the orbit of the moon around Earth
All the spheres rotate around the Sun, which is near the center of the Universe
The distance between Earth and the Sun is an insignificant fraction of the distance from Earth and Sun to the stars, so parallax is not observed in the stars
The stars are immovable – their apparent daily motion is caused by the daily rotation of Earth
Earth is moved in a sphere around the Sun, causing the apparent annual migration of the Sun. Earth has more than one motion
Earth’s orbital motion around the Sun causes the seeming reverse in direction of the motions of the planets
Thereafter he continued gathering data for a more detailed work, and by 1532, he had come close to completing the manuscript of his magnum opus – De revolutionibus orbium coelestium(On the Revolutions of the Heavenly Spheres). In it, he advanced his seven major arguments, but in more detailed form and with detailed computations to back them up.
A comparison of the geocentric and heliocentric models of the universe. Credit: history.ucsb.edu
By placing the orbits of Mercury and Venus between the Earth and the Sun, Copernicus was able to account for changes in their appearances. In short, when they are on the far side of the Sun, relative to Earth, they appear smaller but full. When they are on the same side of the Sun as the Earth, they appear larger and “horned” (crescent-shaped).
It also explained the retrograde motion of planets like Mars and Jupiter by showing that Earth astronomers do not have a fixed frame of reference but a moving one. This further explained how Mars and Jupiter could appear significantly larger at certain times than at others. In essence, they are significantly closer to Earth when at opposition than when they are at conjunction.
However, due to fears that the publication of his theories would lead to condemnation from the church (as well as, perhaps, worries that his theory presented some scientific flaws) he withheld his research until a year before he died. It was only in 1542, when he was near death, that he sent his treatise to Nuremberg to be published.
Historical Antecedents:
As already noted, Copernicus was not the first to advocate a heliocentric view of the Universe, and his model was based on the work of several previous astronomers. The first recorded examples of this are traced to classical antiquity, when Aristarchus of Samos (ca. 310 – 230 BCE) published writings that contained references which were cited by his contemporaries (such as Archimedes).
Aristarchus’s 3rd century BC calculations on the relative sizes of, from left, the Sun, Earth and Moon. Credit: Wikipedia Commons
In his treatise The Sand Reckoner, Archimedes described another work by Aristarchus in which he advanced an alternative hypothesis of the heliocentric model. As he explained:
Now you are aware that ‘universe’ is the name given by most astronomers to the sphere whose center is the center of the earth and whose radius is equal to the straight line between the center of the sun and the center of the earth. This is the common account… as you have heard from astronomers. But Aristarchus of Samos brought out a book consisting of some hypotheses, in which the premises lead to the result that the universe is many times greater than that now so called. His hypotheses are that the fixed stars and the sun remain unmoved, that the earth revolves about the sun in the circumference of a circle, the sun lying in the middle of the orbit, and that the sphere of the fixed stars, situated about the same center as the sun, is so great that the circle in which he supposes the earth to revolve bears such a proportion to the distance of the fixed stars as the center of the sphere bears to its surface.
This gave rise to the notion that there should be an observable parallax with the “fixed stars” (i.e an observed movement of the stars relative to each other as the Earth moved around the Sun). According to Archimedes, Aristarchus claimed that the stars were much farther away than commonly believed, and this was the reason for no discernible parallax.
The only other philosopher from antiquity who’s writings on heliocentrism have survived is Seleucis of Seleucia (ca. 190 – 150 BCE). A Hellenistic astronomer who lived in the Near-Eastern Seleucid empire, Seleucus was a proponent of the heliocentric system of Aristarchus, and is said to have proved the heliocentric theory.
According to contemporary sources, Seleucus may have done this by determining the constants of the geocentric model and applying them to a heliocentric theory, as well as computing planetary positions (possibly using trigonometric methods). Alternatively, his explanation may have involved the phenomenon of tides, which he supposedly theorized to be related to the influence of the Moon and the revolution of the Earth around the Earth-Moon ‘center of mass’.
In the 5th century CE, Roman philosopher Martianus Capella of Carthage expressed an opinion that the planets Venus and Mercury revolved around the Sun, as a way of explaining the discrepancies in their appearances. Capella’s model was discussed in the Early Middle Ages by various anonymous 9th-century commentators, and Copernicus mentions him as an influence on his own work.
During the Late Middle Ages, Bishop Nicole Oresme (ca. 1320-1325 to 1382 CE) discussed the possibility that the Earth rotated on its axis. In his 1440 treatise De Docta Ignorantia (On LearnedIgnorance) Cardinal Nicholas of Cusa (1401 – 1464 CE) asked whether there was any reason to assert that the Sun (or any other point) was the center of the universe.
Indian astronomers and cosmologists also hinted at the possibility of a heliocentric universe during late antiquity and the Middle Ages. In 499 CE, Indian astronomer Aaryabhata published his magnum opus Aryabhatiya, in which he proposed a model where the Earth was spinning on its axis and the periods of the planets were given with respect to the Sun. He also accurately calculated the periods of the planets, times of the solar and lunar eclipses, and the motion of the Moon.
Ibn al-Shatir’s model for the appearances of Mercury, showing the multiplication of epicycles using the Tusi couple, thus eliminating the Ptolemaic eccentrics and equant. Credit: Wikipedia Commons
In the 15th century, Nilakantha Somayaji published the Aryabhatiyabhasya, which was a commentary on Aryabhata’s Aryabhatiya. In it, hedeveloped a computational system for a partially heliocentric planetary model, in which the planets orbit the Sun, which in turn orbits the Earth. In the Tantrasangraha (1500), he revised the mathematics of his planetary system further and incorporated the Earth’s rotation on its axis.
Also, the heliocentric model of the universe had proponents in the medieval Islamic world, many of whom would go on to inspire Copernicus. Prior to the 10th century, the Ptolemaic model of the universe was the accepted standard to astronomers in the West and Central Asia. However, in time, manuscripts began to appear that questioned several of its precepts.
For instance, the 10th-century Iranian astronomer Abu Sa’id al-Sijzi contradicted the Ptolemaic model by asserting that the Earth revolved on its axis, thus explaining the apparent diurnal cycle and the rotation of the stars relative to Earth. In the early 11th century, Egyptian-Arab astronomer Alhazen wrote a critique entitled Doubts on Ptolemy (ca. 1028) in which he criticized many aspects of his model.
Entrance to the observatory of Ulug’Beg in Samarkand (Uzbekistan). Credit: Wikipedia Commons/Sigismund von Dobschütz
Around the same time, Iranian philosopher Abu Rayhan Biruni 973 – 1048) discussed the possibility of Earth rotating about its own axis and around the Sun – though he considered this a philosophical issue and not a mathematical one. At the Maragha and the Ulugh Beg (aka. Samarkand) Observatory, the Earth’s rotation was discussed by several generations of astronomers between the 13th and 15th centuries, and many of the arguments and evidence put forward resembled those used by Copernicus.
Impact of the Heliocentric Model:
Despite his fears about his arguments producing scorn and controversy, the publication of Copernicu’s theories resulted in only mild condemnation from religious authorities. Over time, many religious scholars tried to argue against his model. But within a few generation’s time, Copernicus’ theory became more widespread and accepted, and gained many influential defenders in the meantime.
These included Galileo Galilei (1564-1642), who’s investigations of the heavens using the telescope allowed him to resolve what were seen as flaws in the heliocentric model, as well as discovering aspects about the heavens that supported heliocentrism. For example, Galileo discovered moons orbiting Jupiter, Sunspots, and the imperfections on the Moon’s surface – all of which helped to undermine the notion that the planets were perfect orbs, rather than planets similar to Earth. While Galileo’s advocacy of Copernicus’ theories resulted in his house arrest, others soon followed.
German mathematician and astronomer Johannes Kepler (1571-1630) also helped to refine the heliocentric model with his introduction of elliptical orbits. Prior to this, the heliocentric model still made use of circular orbits, which did not explain why planets orbited the Sun at different speeds at different times. By showing how the planet’s sped up while at certain points in their orbits, and slowed down in others, Kepler resolved this.
In addition, Copernicus’ theory about the Earth being capable of motion would go on to inspire a rethinking of the entire field of physics. Whereas previous ideas of motion depended on an outside force to instigate and maintain it (i.e. wind pushing a sail) Copernicus’ theories helped to inspire the concepts of gravity and inertia. These ideas would be articulated by Sir Isaac Newton, who’s Principia formed the basis of modern physics and astronomy.
Although its progress was slow, the heliocentric model eventually replaced the geocentric model. In the end, the impact of its introduction was nothing short of a revolutionary. Henceforth, humanity’s understanding of the universe and our place in it would be forever changed.
