December 24, 2007

Cassiopeia

Cassiopeia, the Ethiopian Queen, figures prominently in winter evenings in the Northwestern region of the sky, being the mother of Andromeda and wife of Cepheus. She got on the Greek gods' bad list (namely Poseidon's) because she claimed she was more beautiful than all the sea nymphs (the Nereids), so she is strapped to her throne as she circles the celestial pole, half the time upside down. Poseidon also sent the sea monster Cetus to ravage the coast of their country, and the king and queen responded by chaining their daughter Andromeda to a rock as a sacrifice, only to be rescued by Perseus. To the early Arab Astronomers, this was a large hand stained with henna, with each of the bright stars representing one of the five fingertips.

Shedir (Alpha Cassiopeia, sometimes Schedar) ) is the star in the W closest to Andromeda, and it is actually the 2nd brightest star in the constellation. It is an orange giant somewhat similar in properties to Aldebaran in Taurus, though it is about four times further away than Aldebaran at 230 light years. The name comes from the Arabic word for "breast", and it represents that on most star charts. Caph (Beta Cassiopeia) is the star at the end of the W closest to Cepheus and furthest from Perseus. It is fairly nearby, only about 30 light years away, and its name indicates "hand". It is a spectroscopic binary with a period of about 27 days.

Gamma Cassiopeiae is the central star of the W and one of the most peculiar variable stars in the sky. It is a B-type subgiant that spins very rapidly (every 8 hours, which means the equatorial matter is moving 0.1% the speed of light). Occasionally, instabilities inside cause it to puff up, and it loses a disk of matter thrown off from its equatorial region. This causes the brightness to fluctuate by a factor of 4 or more. It is a spectroscopic binary with a sun-like companion orbiting in a nearly perfect circle with a period of 204 days. It has an incredibly strong magnetic field interacting with this disk of hot gas, and that also causes the spectrum to vary in a crazy way. Ruchbah and Segin make up the last two stars of the W.

About halfway between Alpha and Gamma is Eta Cas, a well-known binary system with a central star almost identical to the Sun. So looking at this, you can see what the Sun would look like at a distance of about 20 light years. Following a line from Beta to Alpha and about that same distance further, you run into Mu Cassiopeiae (also known as Marfak), a sun-like star in terms of mass, but it is metal poor and only about half as bright as the Sun (metals tend to act as a blanket around the core of a star, heating it up and making it more luminous). This is an example of a halo star. While most of the stars in this area of the sky move more or less in the same direction very slowly (relatively speaking) through the disk, Mu moves just about perpendicular to that and very fast in a relative sense. It is a halo star plunging through the disk of the Milky Way right now. That's one reason it is metal poor, because it formed out in the halo long ago (instead of the disk where many generations of stars are constantly enriching the gas and dust that forms the star).

If you go from Gamma to Beta, moving in the general direction of Pegasus from the center of the W, then half again as far, you will run into Rho Cassiopeiae, a yellow hypergiant, one of seven such stars that have been discovered in the Milky Way. If I had to pick one star in the entire galaxy that is the most likely candidate to go supernova next, it would be this one. It is 500,000 times brighter than our Sun and located 10,000 light years away. The "habitable zone" around this star is 450 AU away! If placed in the center of our solar system, it would easily engulf Mars. These types of stars undergoing massive variations in size and color and are very likely close to the end of their lifetimes and about to erupt as Type II supernovae, perhaps on a timescale of tens or hundreds of years.

Moving on to the deep sky objects in Cassiopeia: Alpha, Beta, Gamma and Tycho's Nova form a pretty good square on the sky, only there is nothing visible to see at the nova location now. In 1572, one of only eight supernova explosions in recorded history visible to the naked eye occurred here, and it was closely monitored (though not discovered) by Danish astronomer Tycho Brahe. Now there is a huge expanding supernova remnant here about 7500 light years away, not much further than the Crab. One reason it is much fainter than the Crab and expanding faster is the difference in the initial explosion.

It is thought that Tycho's supernova was a white dwarf star that reached a critical mass and temperature, and nuclear fusion began explosively in its core, blowing the whole star apart and leaving no remnant behind (so no pulsar to pump energy into the surrounding gas). The companion of the white dwarf probably had most of its outer layers blown off, and it was flung away from the site. In 2004, astronomers found a sun-like star moving away from the scene of the crime very rapidly and about the right distance for its speed, and they think that was the original companion.

Another famous object not visible for most amateur astronomers is Cas A, the single brightest source of radio energy in the sky for us outside of our solar system. It is a supernova remnant about 10,000 light years away that first went off 9700 years ago, based on the expansion rate and size. There is no record of it historically (it should've been seen in 1670 or so), probably because there is extensive dust along the line of sight that obscured it. Only with radio telescopes or deep images taken by Hubble have we gotten much detail.

There are lots of interesting nebulae in this constellation since the Milky Way passes right through it. Looking in the direction of Cassiopeia, we are looking almost directly down along our arm (the Orion arm) of the Galaxy. The best is the Heart and Soul Nebula, a region of ongoing star formation about 6000 light years away. The red light comes from Hydrogen emission. Within the Soul Nebula is the Mountains of Creation. This is probably what the Pleiades cluster looked like while it was forming. Bright blue stars are just starting to emerge from their cocoons of dust, blowing away the surrounding gas and dust and causing it to glow.

Closer to the W (near Shedir) is the Pacman Nebula, named for its distinctive shape. There are two Messier objects in Cassiopeia. The first is M52, an open cluster next to the Bubble Nebula, yet another glowing star-forming cloud. M52 is comparable in age to the Pleiades, but it is much fainter, over 10 times further away. Also present near Ruchbah is star cluster M103, one of the faintest of the Messier Objects, a cluster with only a handful of bright stars moving together through space. One of the most massive stars has evolved off the main sequence and turned into a bright red giant.

NGC 147 and NGC 185 are little elliptical galaxies close to the border of the constellation Andromeda that are likely companions of the Andromeda galaxy. They are at the same distance as Andromeda and separated from it by about 250k light years, or 2.5 times the diameter of our galaxy. Another nice cluster present is the Owl Cluster, also known as NGC 457. It is similar to either member of the double cluster (h and chi) in Perseus. Another older clusteris NGC 7789, perhaps a billion years old so there are no really bright stars here that stand out. It is one of the oldest clusters in the disk. Clusters don't get much older because they get pulled apart by the gravity of all the surrounding material.

Posted by Observer at December 24, 2007 09:21 AM
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