Wider and possibly brighter than our own Milky Way, the Andromeda Galaxy was once thought to be the dominant member of the Local Group of galaxies. Although it is Milky Way's nearest large galactic neighbor, this large spiral galaxy (type Sb with two arms) lies around 2.52 million light-years from the Solar System. It can be found in Constellation Andromeda, the Chained Maiden. It is located northwest of Mu and Beta Andromedae (Mirach); west of Nu Andromedae; northeast of Theta and Sigma Andromedae; north of Pi, Delta, and Epsilon Andromedae; and south of Theta and Omega Cassiopeiae. Andromeda can be seen by Human eyes from Earth without a telescope as a "little cloud" Andromeda has a bright disk that is now believed to span as much as 228,000 ly in width. In 2005, astronomers announced that Andromeda's disk actally extends far further out, so that the disk spans at least 260,000 light-years The outer disk emits nearly 10 percent of the galaxy's total light and may be comprised of metal-poor stars stripped from smaller galaxies that strayed too close. ---------- Andromeda is the nearest major galaxy to our own Milky Way Galaxy. Our Galaxy is thought to look much like Andromeda. Together these two galaxies dominate the Local Group of galaxies. The diffuse light from Andromeda is caused by the hundreds of billions of stars that compose it. The several distinct stars that surround Andromeda's image are actually stars in our Galaxy that are well in front of the background object. Andromeda is frequently referred to as M31 since it is the 31st object on Messier's list of diffuse sky objects. M31 is so distant it takes about two million years for light to reach us from there. Much about M31 remains unknown, including why the center contains two nuclei. ---------- Also known as M31, Andromeda is located only about 2.5 million light-years from Earth, making it our nearest galactic neighbor. Like our own Milky Way, Andromeda is a classic spiral galaxy, which typically consists of three main parts: a flattened disk, a bright central bulge of densely packed stars and an extended spherical halo where stars are more sparsely distributed. Using the Mayall Telescope at Kitt Peak and the DEIMOS spectrograph on the the Keck II Telescope in Hawaii, the researchers found previously unseen red giant stars out to a distance of at least 500,000 light years from Andromeda's center. The researchers picked out Andromeda's faint halo stars using a technique developed by Karoline Gilbert, a UCSC graduate student, which distinguishes the halo stars from the more numerous foreground stars in our Milky Way. A dim foreground star and a bright star located much farther away-whose light can be diminished by interstellar gas-can be hard to tell apart because they appear to have similar luminosities as stars in our own galaxy. The researchers liken the effect to distinguishing between the light of a firefly 10 feet away and that of a powerful beacon in the distance. According to current galactic formation theories, the halo is the first part of a galaxy that forms. Stars in the halo are predicted to be metal poor because they formed during a time when the universe had much less heavy metal content than it does now. Heavy metals are created as stars evolve and then spewed out into interstellar space when ancient stars either explode as supernovas or shed their outer layers to become white dwarfs. "The first stars are expected to be chemically deficient, and as these other components such as the disk of the galaxy form later, it is contaminated by the products of those first stars, so those stars are more metal rich," Kalirai said. However, instead of being metal-poor, previous studies have found that Andromeda's halo stars were actually 10 times richer in metals than halo stars in our galaxy. This finding puzzled astronomers because both Andromeda and the Milky Way should have similar formation histories. The new findings could solve this discrepancy because the red giant stars are anemic, as is expected from galaxy formation theories and what is known about the Milky Way. ---------- M31 is a large spiral galaxy, very similar in appearance to, and slightly larger than, our own Galaxy, and our closest normal-galaxy companion (the very close Magellanic clouds are classified as irregular galaxies). In fact, from a distant vantage point, Andromeda and the Galaxy would appear as a pair, a binary or double galaxy system, if it were not for the rather smaller, though still significant, spiral galaxy M33. As our nearest neighbor, Andromeda is extremely large on the sky. This picture extends for over two and a quarter degrees, or more than four times the width of the full moon, and still does not include the full extent of M31. M31 is visible to the naked eye, although we can only see the bright inner bulge, and it has therefore been known since at least the year 964AD, when Persian astronomer Al-Sufi described it as a `little cloud'. We can see that the western (right) side of M31 is closer to us, by the fact that the dark dust lanes belonging to the inner spiral arms show up in silhouette against the nucleus on that side only. At the very center of the Andromeda Galaxy is a brilliant point of light, which is a very tightly packed star cluster, but this is not visible in this saturated image. The entire galaxy is rotating in space, with the lower portions approaching while the upper parts recede. The rotation is not completely smooth, showing `bumps' where the spiral arms occur, which are probably due to the spiral density wave that maintains the arms. By applying gravitational theory to this rotation, we can `weigh' M31, and when we do it seems that there may be ten times as much material as we can see in the visible portions of the galaxy, distributed in a huge dark halo. Photographic plates are hardly used in astronomy any more, due to their very low efficiency (a few percent at best) compared to electronic detectors such as charge-coupled devices (CCDs) which can collect more than 50% of the light falling on them. Plates are also less accurate for measuring light intensities, but they can still be of benefit for imaging large areas at one time.