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    Categories: Deep-Sky Objects

Deep-Sky Objects in Andromeda

Andromeda Galaxy (M31) and two of its satellite galaxies (M32 and M110)

Andromeda is the 19th biggest constellations, with its brightest star, Alpheratz (Alpha Andromedae), a +2.06 magnitude double star 97 light years distant with an orbital period of 96.7 days The best time to view Andromeda is during the month of November when it is highest in the sky, but note that it is not visible for observers south of latitude 40° S. The constellation Andromeda has a number of stars with confirmed planets, and one meteor shower, the Andromedids (also known as the Bielids), although it is better known for the large number of galaxies it contains, the most notable of which are briefly explored in this list.

Andromeda Galaxy (Messier 31,  M31, NGC 224)

At a distance of 2.5 million light years, the Great Andromeda Galaxy is not only our closest large neighbor in the Local Group of galaxies; it is also the most distant object that can be observed without optical aid, largely because it is very bright with an apparent visual magnitude of 3.4. While M31 is estimated to have about the same mass as the Milky Way, it contains about one trillion stars, which is several times the number our galaxy (200 – 400 billion) is thought to contain. Like the Milky Way, the Andromeda Galaxy also has about 30 or so satellite galaxies, including at least 14 dwarf galaxies, among which are M32 and M110.

The remainder of the Andromeda Galaxies’ satellite galaxies are rather faint, and the bulk of them were not even discovered until the 1970’s, when it was found that some satellite galaxies fall outside the borders of the Andromeda constellation. Briefly, some of M31’s satellites that are outside of the Andromeda constellation include the Cassiopeia Dwarf (Andromeda VII) in the constellation Cassiopeia, the Pegasus Dwarf Spheroidal galaxy in the constellation Pegasus, and Andromeda XXII (also known as Pisces VI or Triangulum I), which is located in the constellation Pisces. Note though that while M32 in the constellation Triangulum is often listed as a satellite of M31, there is still some doubt about whether or not is in fact a true satellite of M31.

Messier 110 (NGC 205)

Credit: Johannes Schedler

Located about 2.9 million light years away, M110 is a dwarf spheroidal galaxy that is a satellite of the Andromeda Galaxy. What sets it apart from other galaxies of its type, however, is the fact that evidence of recent star formation have been found, which is atypical, if not downright strange. M110 also does not appear to have a super massive black hole at its core, or if there is one present, no evidence of its existence has been found to date. The galaxy does, however, host 8 large globular star clusters in its outer halo.

While Charles Messier first observed and described M110 in 1773 along with M31 and several other galaxies, he did not include it in his original catalogue. The galaxy was then largely forgotten until 1784/5 when Caroline Herschel rediscovered it, although it was not until 1967 that Kenneth Glyn Jones assigned the designation M110 to the galaxy.

NGC 891

Credit: Bob Franke

Located about 27.3 million light years away, and about four degrees eastward of the star of Almach (Gamma Andromedae), this large spiral galaxy seen edge-on was initially thought to be almost identical to the Milky Way in term of size and structure.

In this view, the large central bulge of the galaxy is just out of view toward the bottom of the frame, but what is interesting about this image is the many filaments of dust and gas that can be seen extending outwards from the plane of the galaxy, seen here as the brownish-red lines/filaments that are silhouetted against the galaxy’s bright disc. The exact origin of these streamers of gas and dust is still somewhat uncertain, but most investigators believe they are the result of dust and gas being blown out of the plane of the galaxy by large numbers of violent supernova events.

While there is no direct evidence that this is indeed the case, it seems a likely explanation since NGC 891 forms part of a small group of galaxies that are gravitationally bound to each other. In this scenario, gravitational interactions between the members of the group seem to be the most likely explanation for the high rate of star births and deaths.

Arp 65 (NGC 90 and NGC 93)

Image: NASA/JPL-Caltech/M.L.Giroux

This pretty pair of interacting galaxies consisting of NGC 90 and NGC 93 is located 333.8 and 259.7 million light years away, respectively. Note the two elongated arms of NGC 90, which is direct evidence of the interaction that is both deforming the galaxy, and causing intense star forming activity in its outer regions, hence the blue color of the arms.

These interacting galaxies, known collectively as Arp 65, are visible with medium to large amateur telescopes. However, they
form part of a large cluster of galaxies (SRGb 063) situated around 240 million light years distant.

Andromeda’s Cluster (Mayall II)

Named after its discoverer, Nicholas Mayall, this great globular cluster orbits the Andromeda Galaxy at a distance of about 130,000 light years from its core. With an apparent visual magnitude of 13.7, it is the most luminous globular star cluster in the entire Local Group of galaxies.

In terms of its size, Mayall II is at least twice as massive as Omega Centauri, the biggest and most massive of the Milky Way’s globular clusters. In fact, Mayall II is so big and massive that most investigators believe it contains an intermediate-mass black hole in its core. Many astronomers also believe that Mayall II is not a true globular cluster, but the remnant of a dwarf galaxy that was destroyed and partly assimilated by the Andromeda galaxy in the distant past.

Blue Snowball Nebula (NGC 7662)

Perhaps better known simply as the Snowball Nebula, this pretty, 1.6-light year diameter planetary nebula has an apparent visual magnitude of 8.6, and can be seen about one degree to the west of the 4th magnitude star, Kappa Andromedae.

Note that while the nebula can be observed even with small telescopes, it only appears as a star-like object surrounded by some nebulosity that may or may not be conspicuous, depending on local seeing conditions. For the best views, it is recommended that the Snowball Nebula be observed with a large telescope under dark skies.

Peter Christoforou :