One of the greatest debates of American astronomy occurred in 1920. It centered on the true nature of the spiral nebulae – pinwheel shaped hazy patches of light visible in the night sky. Harlow Shapley believed they were located relatively nearby inside our own Milky Way. In contrast, Heber Curtis insisted the spiral nebulae are actually ‘island universes’ far outside the Milky Way and are comparable in size and nature to our own galaxy.
Edwin Hubble (namesake of the Hubble Space Telescope) settled the Great Debate by using Cepheid variable stars to measure the distance to the Great Andromeda Nebula – the biggest and brightest the spiral nebulae. Using the 100″ Hooker Telescope, Hubble showed that Andromeda was actually a galaxy like the Milky Way and was located at a distance of about 1.5 million light-years.
Modern measurements of the Andromeda Galaxy place it at a distance of approximately 2.5 million light-years from Earth. Located in the constellation Andromeda, the Andromeda Galaxy can be seen with the naked eye from a dark moonless sky as a hazy oval patch about twice the diameter of the Full Moon. During December evenings, Andromeda passes overhead around 8 pm from mid-northern latitudes. A pair of 7×50 binoculars provides a good view of the galaxy.
The 18th century French comet hunter Charles Messier included the Andromeda Galaxy as entry M31 in his famous catalog of deep sky objects. In August of 1764, Messier gave the following description:
“The beautiful nebula of the belt of Andromeda, shaped like a spindle; M. Messier has investigated it with different instruments, and he didn’t recognise a star: it resembles two cones or pyramides of light, opposed at their bases, the axes of which are in direction NW-SE; the two points of light or the apices are about 40 arc-minutes apart; the common base of the pyramids is about 15 (arc-minutes).”
The Andromeda Galaxy is the closet spiral galaxy to the Milky Way and it resembles how our own galaxy would appear from a great distance. Recent research indicates that M31 was formed from the collision and merger of two smaller galaxies about 10 billion years ago. Some 2 to 4 billion years ago, M31 experienced a close encounter with the Triangulum Galaxy (M33). The encounter triggered high levels of star formation in M31, and disturbed M33’s outer disk.
M31 is accompanied by two satellite elliptical galaxies known as as M32 (NGC 221) and M110 (NGC 205). Both are easily seen and photographed with amateur telescopes. Research with larger telescopes has identified a total of 14 dwarf satellite galaxies gravitationally associated with M31.
The Andromeda Galaxy and its satellites, along with the Milky Way Galaxy and the Triangulum Galaxy (M33), are all part of the Local Group of about 54 nearby galaxies. The gravitational center of the group is located somewhere between the Milky Way and M31.
The Andromeda Galaxy is approaching the Milky Way at about 2 billion miles per year and is expected to collide with us in about 4.5 billion years. Although the outcome is uncertain, it is likely that the galaxies will merge to form a giant elliptical galaxy. This is a common event among interacting galaxies. In any case, the collision will take place near the end of the Sun’s expected lifetime.
What an amazing view it will be when the Andromeda Galaxy dominates the night sky a few hundred million years before the collision! (see Andromeda Collision for a preview)