Mission: Galaxy Evolution Explorer (GALEX)
Spitzer Space Telescope
Spacecraft: GALEX Orbiter

Instrument: Multiband Imaging Photometer (MIPS)
Ultraviolet/Visible Camera

The Andromeda Galaxy is part of the Local Group which the Milky Way Galaxy also belongs to. It is found in the constellation Andromeda and can be seen with the naked eye. It is the largest galaxy in the Local Group. It has 700 billion solar masses. 1

Discovery and Details

The largest neighbor to the Milky Way Galaxy is Messier 31, the famous Andromeda Galaxy. First discovered by Persian astronomer Abd-al-Rahman Al-Sulfi in about 905 AD, it was first recorded in his Book of Fixed Stars in 963 AD. He referred to it as the “little cloud”. Many other astronomers after Al-Sufi claimed to have discovered Andromeda, however, it wasn't until August 3, 1764, when Charles Messier cataloged it, that its name because Messier 31. This was because it was Charles Messier's 31st discovery.2

It was first believed to be the “Great Andromeda Nebula”, which would make it one of the nearest nebular to the Milky Way. However, William Huggins, a well known spectroscoper, noted, in 1864, the difference between the spectra of Andromeda and the spectra of a gaseous nebula. The spectra of Andromeda was of a star-like, continuous spectra which is the spectra of specific nebulae known as galaxies.3

In 1887, Isaac Roberts captured the first photograph of Andromeda, which shows its spiral structure and basic features.

The Once and Future Stars of Andromeda
The Once and Future Stars of Andromeda

It was calculated that Andromeda is moving at 300 kilometers per second towards the sun. Edwin Hubble calculated that Andromeda is 2.5 million light years away from earth and later astronomers calculated that it is approximately 220,000 light years across which is twice the size of the Milky Way. This makes it the closest spiral galaxy to the Milky Way. In about 2.5 billion years, it is said that Andromeda and the Milky Way will collide to form one huge elliptical galaxy. 4

The Hubble Space Telescope revealed that Andromeda has a double nucleus. This could either mean it has two bright nuclei because it ate another galaxy or that parts of its core are obscured by dark material, such as dust. Supernova 1885 was the first and only supernova recorded to take place in the Andromeda galaxy. This was also the first supernova to be discovered outside of the Milky Way galaxy. Ernst Hartwig at Dorpat Observatory in Estonia, discovered this supernova on August 20, 1885. Although it was found by many independent observers, it was only Hartwig who knew of its significance and made sure it was recorded.5

As observed in this video (right), the Andromeda galaxy has lost its once spiral shape. In taking a look at the components of a galaxy, the parts and what they contain can be known. The arms in the diagram (below) of a spiral galaxy show what Andromeda would have looked like before the collision with its neighbor M32.


Globular Clusters: gravitationally bound concentrations of approximately ten thousand to one million stars, spread over a volume of several tens to about 200 light years in diameter.6

Bulge: a round structure consisting of old stars. gas and dust. Sometimes the outer parts of the bulge are difficult to distinguish from the halo because it reaches out such a large area.7

Nucleus: the core of the galaxy that contains some hot young stars but mainly old stars that are found in the halo. It is said that many galaxies contain a massive black hole at their core.8
Disk: a flattened region that surrounds the bulge in a spiral galaxy. It contains mostly young stars, gas and dust, which are concentrated in spiral arms. Some old stars are also present.9

Halo: contains individual old stars and clusters of old stars ("globular clusters"). The halo also contains "dark matter," which is material that we cannot see but whose gravitational force can be measured.10


What is fascinating about the Andromeda galaxy is that it can be seen with the naked eye. There are steps to take by following the constellations to locate our giant neighbor.

Step 1: Use binoculars for your first attempt at finding the Andromeda Galaxy.11

external image SkyChart1201_0100hrsZ_755.gif

Step 2: Locate three constellations to get your bearings: Pegasus, Cassiopeia and Andromeda.12

external image Andromeda_Pegasus_Labelled_647.GIF

Step 3: Look for distinguishing features. Pegasus looks like a giant rectangle and Cassiopeia looks like a gieant "M" or "W". Andromeda lies between them.13

external image Andromeda_Pegasus_Sir_Ruch_453.GIF

Step 4: Draw a line from the star Sirrah on the edge of Pegasus and Andromeda to the star Ruchbah in Cassiopeia.14

external image Andromeda_Pegasus_Sir_Ruch_Mirrach_mu_921.GIF

Step 5: Draw a line from Mirach through mu Andromedae and on through the first line.15

external image Andromeda_Pegasus_to_M31_360.GIF

Step 6:Scan the area just to the southeast of where the lines meet, along the second line, with binoculars. You'll notice an oval of faint light. This is the Andromeda Galaxy.16

external image In-Distant-Galaxies%2C-New-Clues-to-Century-Old-Molecule-Mystery.jpg

Step 7: Use a telescope for a more details examination.17


7, 9-10.