The Milky Way’s Closest Neighbors Our Local Group

Galactic Classification Just like a street address, the planet earth is part of many different systems. First it is a member of our Solar System, consisting of the Sun (also called Sol) and seven other planets. As we move further out our solar system is part of a Solar Interstellar Neighborhood, which includes the stars Sirius, Alpha Centauri, Vega and Altair. All of these stars, as well as billion others, make up our enormous galaxy called the Milky Way.

Galactic Classification The Milky Way is the second largest galaxy in our Local Group, which consists of approximately 40 galaxies. Further out still, our Local Group is part of the Virgo Super Cluster. Other neighboring superclusters include the Centaurus Supercluster, the Perseus-Picses Supercluster, the Hydra-Centaurus Supercluster, the Coma Supercluster, and a smaller supercluster called Hydra. In between superclusters are huge empty expanses of space simply called “voids”. Local voids include the Canes- Major Void and the Sculptor Void.

Galactic Classification

Our Local Group contains 10 galaxies which are not (usually) classified as dwarfs. In order of distance from the Sun these are: The Milky Way (Spiral) The Large Magellanic Cloud (Irregular, 0.16 mil. Light Years) The Small Magellanic Cloud (Irregular, 0.19 mil. Light Years) NGC 6822, aka Barnard’s Galaxy (Irregular, 1.6 mil. Light Years) M31, aka Andromeda (Spiral, 2.5 mil. Light Years) M32, Andromeda’s Satellite (Elliptical, 2.6 mil. Light Years) M110, Andromeda’s Satellite (Elliptical, 2.7 mil. Light Years) M33, aka Triangulum (Spiral, 2.7 mil. Light Years) Wolf-Lundmark-Melotte (Irregular, 3 mil. Light Years) NGC 3109 (Irregular Spiral, 4.1 mil. Light Years) Source: Pearson text. “The Essential Cosmic Perspective”. Fifth Edition

The Milky Way The term “Milky Way” derives from the Latin Via Lactea, literally “milky road”. This in turn derives from the Greek kyklos galaktikos, or “milky circle” with milk being the root word for the modern term galaxy, so called because it resembles a band of milk splashed across the sky.

The Milky Way It was recently proven that the Milky Way is a barred spiral galaxy. The nature of this bar, located at the center of the galaxy, is hotly debated. It is estimated to be about 13.2 billion years old, nearly as old as the universe. It’s diameter is between 100,000 and 120,000 light years, with the outer halo extending much further. It bulges in the middle, approximately 15,000 light years thick, but the thickness of most of the disk is about 1,000 light years. The Milky Way has a rotational rate between 1 per 15 million and 1 per 50 million years, or between 552 and 630 km per second. It’s luminosity is 15,000 million times that of the Sun. Every star which is visible by the naked eye is part of the Milky Way.

The Large and Small Magellanic Clouds are the closest galaxies to our own. They are both readily visible to the naked eye and can only be viewed from the southern hemisphere. Both are classified as irregular and are gravitationally bound, even currently colliding with Milky Way. Of all the Milky Way’s satellites only the Magellanic Clouds have been known since prehistoric times. In fact, all the Milky Way’s other satellites (dwarfs) have been discovered since the turn of the 20 th century, with the vast majority being discovered only in the last ten years.

Barnard’s Galaxy A barred irregular galaxy located with the constellation Sagittarius, NGC 6822 was discovered by E. E. Barnard in After identifying 11 Cepheid Stars, Edwin Hubble utilized the Cephied Period-Luminosity Relationship to determine NGC 6822’s distance from Earth. Other than the large and Small Magellanic Clouds, this was the first galaxy to have its distance accurately determined (Hubble’s estimate 700,000ly). This realization effectively ended the Shapely-Curtis so-called “Great Debate” of the 1920’s, proving the universe was much larger than 300,000ly. Afterwards all “spiral nebula” were reclassified as distant spiral galaxies.

Andromeda Andromeda, so-called because of its apparent location within the constellation of the same name, is the largest galaxy in the Local Group. Andromeda contains about 1 trillion stars, at least twice that of the Milky Way. However, recent surveys suggest that the Milky Way may contain more mass (dark matter). It has a luminosity of about 21,000 million times that of our Sun First described as a “small cloud” in the Book of Fixed Stars by Persian astronomer Abd al-Raman al-Sufi in 964 AD, it was labeled “Little Cloud” on star charts for many years. Later texts referred to it as the Great Andromeda Nebula. Well into the 20 th century it was believed to be a nebula within our own galaxy, until red (blue)shifting and standard candle measurements proved it to be entirely too far away. Today it is estimated Andromeda is 2.5 million light years from earth.

Andromeda

M32 and M110 Both M32 and M110 are elliptical galaxies gravitationally bound to Andromeda in much the same way the Large and Small Magellanic Clouds are bound to the Milky Way,. M32 collides with the very outer edge of Andromeda’s disk. M32 is the larger and brighter of the two with a luminosity of 380 million times that of the sun. M110 shines with 370 million L sun. In accordance with most elliptical galaxies, M32 shows little evidence of star formation. Recent computer models have hypothesized that before collision with Andromeda, M32 may have been a spiral galaxy. The gravity of Andromeda has stripped away the outer layers (disk) of M32 leaving only the luminous bulge (and supermassive black hole). On the contrary, M110 has spectrographically been shown to contain a large amount of star dust and indicators of new star formation, a rarity in elliptical galaxies as a whole.

In the below picture M32 is the bright spot on the upper left of Andromeda. M110 is the bright patch on the right bottom center.

Triangulum The last large galaxy in the Local Group is an un-barred spiral called Triangulum, also known as M33. M33 likely contains some 40 billion stars, making it substantially less massive than the Milky Way and Andromeda. In fact, it is possible that this galaxy is itself gravitationally bound to Andromeda. Triangulum is one of the most distant objects which is visible with the naked eye, although only under near- perfect conditions. Perhaps the most striking feature of this galaxy is its huge potential for star formation. Spectrographs shows a tremendous amount of CO, with about 10% of the total gas in Triangulum existing in molecular form. This galaxy is sometimes called the Pinwheel Galaxy by amateur astronomers, however, officially this name belongs to the smaller M101.

Triangulum

Wolf-Lundmark-Melotte Discovered by 1909 by Max Wolf and located on the very edge of the Local Group, this irregular galaxy is located within the constellation Cetus. The discovery of its “nature”, or composition is accredited to Knut Lundmark and Philibert Jacques Melotte in 1926, thus the name.

NGC 3109 A small “irregular spiral” this galaxy was discovered by John Herschel in 1835 while he was in South Africa. The view of NGC 3109 from earth is edge-on, but it is suspected to have a disk and halo. It is believed that NGC 3109 and the Antila Dwarf galaxy are tidally bound. Spectrographic analysis of red giants within the galaxy show it to be metallically poor. NGC 3109 likely contains a massive amount of Dark Matter. This galaxy is receding quickly from the Local Group and its inclusion in the neighborhood is disputed.

NGC 3109

Etcetera… In addition to those previously elaborated, our Local Group contains about 30 dwarf galaxy, both elliptical and irregular. In general, spiral galaxies are too massive to ever be classified as dwarfs. Most of the dwarfs in the Local Group have been discovered since the turn of the 21 st century using telescopes that detect infrared, ultraviolet and radio radiation, rather than visible light which is often obscured. It is likely that in coming years dozens more dwarf galaxies will be discovered in our close neighborhood as telescopes become more and more advanced. Since the universe is generally accepted as being isotropic, our Local Group can be viewed as a representative sample of galaxies as a whole, of which there are an immeasurable number.

The Local Group