Visible Galaxies

Two of the many galaxies visible to us in the northern sky are the Andromeda galaxy and the Triangulum galaxy. The Magellanic Clouds, satellite galaxies of the Milky Way, are visible only from the Southern Hemisphere.

Andromeda Galaxy

The Andromeda galaxy (Messier 31, NGC 224) is the closest large galaxy to us. The Andromeda system also contains a number of companions, including two bright dwarf ellipsoid galaxies. Andromeda is visible with the naked eye. It was first described by Al-Sufi in his Book of Fixed Stars from the first century A.D., and has been observed by astronomers ever since.

Andromeda's radial velocity was measured at the Lowell Observatory in 1912. Moving at about 300 kilometers per second, it was the fastest-traveling object ever recorded at that time. Since radial velocity is correlated with distance, this discovery gave an indication of Andromeda's distance from Earth.

In 1923, Edwin Hubble continued the research into Andromeda, eventually determining it to be a separate galaxy instead of a nebula. Hubble had a lifelong interest in space and all things far away. After becoming a lawyer in the early 1900s, Hubble switched professions to become a full-time astronomer. Using Hooker Telescope at the Mount Palomar Observatory, Hubble saw stars all around Andromeda, proving definitively that it was a galaxy.

The Andromeda galaxy is one of the most-observed galaxies today, partly because it's so easy to observe. It has a number of companion galaxies, or neighboring galaxies, that are sometimes assimilated. M32 and M110 are two of the most prominent and the most visible, even with regular binoculars. M32 is a bright, small elliptical galaxy that can be readily observed when viewing Andromeda. It is about 3 billion solar masses in size, quite small compared to Andromeda itself. M110 is Andromeda's other bright companion, and is a small elliptical galaxy. A picture of Andromeda and two of its companion galaxies is visible in Chapter 3.

Triangulum Galaxy

The Triangulum galaxy, initially discovered by Italian astronomer Giovanni Hodierna (1597–1660), is also visible in the northern sky. Denoted in the Messier system as M33, Triangulum was one of the first-known spiral galaxies. M33 is about 3 million light-years away, and often the most distant object you can observe with the naked eye on a very clear night. Its area is about four times that of a full Moon, so it definitely takes up a large part of the night sky. M33 has at least five identified, bright globular clusters, although it may be surrounded by as many as twenty.

Satellite Galaxies of the Milky Way

The Magellanic Clouds, visible from the Southern Hemisphere, are some of the closest galaxies to us, and actually consist of two galaxies—the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC). The LMC was first discovered, as the name suggests, by Magellan in the early sixteenth century. The SMC was discovered at the same time but is slightly farther away from us than the LMC. The SMC orbits the Milky Way at a distance of about 210,000 light-years, where the LMC's orbit is about 179,000 light-years.

The closest external galaxy to the Milky Way is the Sagittarius Dwarf Elliptical galaxy, just discovered in 1994 by Ibata, Irwin, and Gilmore. It is located only 80,000 light-years away, making it by far the closest galaxy. Part of the Local Group, Sagittarius Dwarf and its companions are associated with the Sagittarius constellation, and there are also neighboring globular clusters such as M54.

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