February 03, 2008

Andromeda

Today, I'd like to discuss the constellation Andromeda. This constellation is pretty low in the Western sky during evenings in the Winter, but it isn't quite gone yet, and she goes along with the story of Perseus I've talked about recently. Andromeda the princess, daughter of Cepheus and Cassiopeia was chained to a rock on the shore of the sea as a sacrifice to the gods before she was rescued by Perseus. In the sky, she is connected to the flying horse, Pegasus.

Among the stars in Andromeda, there are several of interest. The brightest is Alpheratz, a star that forms one corner of the Great Square of Pegasus. This star, also called Sirrah, was once a part of the Pegasus, but it is now by convention considered to be the sole property of Andromeda. Alpheratz is a hot blue B-type star with a companion only detectable in spectra of the system as the spectral lines shift back and forth about every 100 days. It also has an odd chemical composition (at least in its outer layers) as some elements which are more likely to be pushed outward by radiation pressure are overrepresented compared to their usual cosmic abundance.

Next is Beta Andromedae, Mirach, which has a similar apparent luminosity but is actually quite different. Mirach is a red giant star, with a diameter roughly equal to the orbit of Mercury and a luminosity about 2000 times solar. Mirach has a distant orbiting companion, a red dwarf much fainter than the Sun. Completing the graceful three-star curve of Andromeda, at the tip of one of her feet, is Gamma Andromeda, Almaak, a very nice double star that is among the best in the Northern sky. This is a pair of stars, the brighter one a yellow giant and the fainter a blue dwarf main sequence star, separated by about 10 arcsec. Both of the members of the double are themselves binary systems, but those companions are not so easy to find visually, and the fainter binary system is actually a triple (since one of the widely orbiting stars has a very close companion itself).

This arc of stars can be followed in sequence, and if you continue on the curve, you will run directly into Alpha Persei, or Mirphak. Pretty close to the line connecting Mirach and Almaak, on the Cassiopeia side, we find Upsilon Andromedae, a sun-like star with a distant binary companion and also three detected orbiting planets, all with masses roughly equal to or larger than Jupiter. This system was the first system detected in our surveys with multiple planets, though now many others are known.

Over close in the sky to the Andromeda Galaxy, we find one of the brightest examples of a long period variable in R Andromedae (for which I have no direct link since my primary source is Burnham). This pulsating red giant takes over a year to complete one cycle, during which time its brightness varies by a factor of about 20,000 (!), bringing it almost into a range visible to the naked eye. The archetype for this kind of star is Mira in the constellation Cetus. Like Mira, this star is on the verge of transitioning from an unstable red giant to a planetary nebula with all of its outer layers blown off into the surrounding interstellar medium.

There are several other interesting variable stars here, but nothing of a type we haven't seen before, so I'll move on to the deep sky objects in Andromeda. Any discussion of deep sky objects in Andromeda has to begin with the majestic Andromeda Galaxy, the nearest large galaxy to our own Milky Way. Also known as Messier 31 (M 31), the full extent of this galaxy in deep photographs is nearly 10 times the diameter of the full moon. When astronomers first observed it through telescopes, it was believed to be a spiral nebula within our own galaxy, perhaps even a forming solar system nearby.

The development of spectroscopy revealed that this object didn't have the same kind of spectra as the nebulae in our galaxy we are more familiar with, such as the Great Nebula of Orion. Instead of a spiky emission line spectrum, the spectrum of M 31 was much smoother and more continuous. That's because this is the aggregated light of billions of individual stars, along with reflection from glowing clouds of gas and dust.

Edwin Hubble ultimately made the crucial observations that showed us the true nature of M 31. Working at the 100-inch telescope at Mount Wilson Observatory, Hubble closely monitored the galaxy for many weeks, finding evidence of several variable stars on his deep photographs. Many of these variable stars turned out to be Cepheids, stars for which we can deduce the absolute luminosity by measuring their pulsation period. Knowing the absolute luminosity, we could then find the distance to these stars (and thus the galaxy) using the inverse square law. Instead of being relatively nearby within our own galaxy (which extends about 50,000 to 100,000 light years around us depending on which way you look and what you consider the edge), the stars in M 31 turned out to be 2.5 million light years away. To be fair, Hubble's original estimate was closer to 1 million, but that has since been corrected.

So Hubble revealed that the Andromeda Galaxy is an "island universe" well outside the boundaries of our own galaxy. He followed up this groundbreaking work by observing Cepheids in other galaxies. Using those to determine the distances to many galaxies, he then used the velocities of these galaxies (using Doppler shift), found by other Astronomers such as Vesto Slipher, and developed (what would become known as) Hubble's Law, a relationship between galaxy distance and radial velocity that shows us the Universe is expanding.

Like our own galaxy, Andromeda has many regions of ongoing star formation. The largest and brightest star cloud in Andromeda is near the southern tip and is known as NGC 206. Just a few hundred stars are easily picked out in deep photographs, but all of them are over 10,000 times the luminosity of our Sun, with some comparable to Rigel or Deneb in our own galaxy. With so many massive stars, there are bound to be supernovae in this galaxy, at a rate roughly equal to that of our own galaxy (we expect a supernova on average every 100 years or so). The first extragalactic supernova ever detected was S Andromedae in 1885, though at the time, Astronomers didn't realize what they were looking at.

Like our galaxy, Andromeda has many smaller galaxies in orbit nearby, the most prominent of which is M 32, easily visible in any photograph of M 31 as the brightest of the smaller companion objects, separated from the nucleus of the Andromeda Galaxy by about one full moon diameter. This galaxy recently collided with Andromeda along Andromeda's polar axis and passed through, creating chaos in both bodies. Some simulations show that Andromeda is transitioning from a classic grand-design spiral into a more ring-shaped galaxy as a result, and M 32 will be slowly torn apart through repeated interactions with Andromeda over the next few billion years or so.

Moving on to the other worthy objects in Andromeda, I'll start with the area near her feet on the border with the constellation Perseus. Probably the easiest galaxy to see here aside from the little M 31 group is the beautiful edge-on spiral NGC 891. With a total luminosity somewhat less than our Milky Way or M 31 and a distance 10 times further away than M 31, it is no surprise that this is a challenge for small telescopes. To really see the pretty pictures like this one requires a large telescope and lots of integration time.

About twice as far away is a faint face-on spiral known as IC 239. I found a very nice photo of the region here which also includes the elliptical galaxy NGC 1023 in Perseus. IC 239 has a low surface brightness and is tough to observe due to relatively bright stars nearby, and it is on the fringes of our local group of galaxies. Also on the border with Perseus is the S-shaped open cluster known as NGC 956 (also Collinder 27), for which I found a very nice picture here.

Moving toward the horizon, away from Perseus and toward Pegasus, you can find the open cluster NGC 752 about five degrees due south of Almaak, on the border between Andromeda and the tiny Triangulum constellation. This cluster is about 50% larger than the full moon and contains just under 100 noticeable stars. At about 1300 light years away, it is just bright enough to picked up with a good set of binoculars. What makes it interesting to Astronomers is that it seems to be a cross between a galactic open cluster and a globular cluster (more common in the halo), probably because of its advanced age (which I see cited in various sources as between 1 and 2 billion years) compared to other galactic clusters (which are usually much less than a billion years old).

Moving west from here toward Mirach, we run into a small galaxy cluster known as Hickson 10. We're talking a very deep field here, 100 times more distant than the Andromeda Galaxy. This is a group of four spiral galaxies, two of which are clearly warped due to their mutual gravitational interaction. Right next to Mirach is a little elliptical galaxy (NGC 404) known as Mirach's ghost. In a small telescope, it almost looks like a reflection of the bright star Mirach off of the internal optics of the lenses of the telescope, but it is a real galaxy first seen by Herschel in 1784. Like IC 239, this little guy is on the fringes of the local group, about 10 million light years away.

For the last two objects I want to mention, we look to the far Western end of the constellation, near the border it shares with Cassiopeia and Lacerta, a region of the sky about 10 degrees north of Alpheratz. First is a small open cluster of only a few dozen stars, NGC 7686 (or Collinder 456), for which I found a nice picture here. Also present is NGC 7662, also known as the Blue Snowball Nebula, a fairly easy planetary nebula to observe with a small telescope. Burnham notes that with a 10-inch telescope, the annular nature of the nebula is easy to see, though the central star is tough to pick out. The real puzzle with these nebulae is trying to reconstruct what was going on before to give them their unusual and varied shapes, and this is the main research interest of a faculty member where I got my PhD.

Posted by Observer at February 3, 2008 02:02 PM
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