Messier 31: The Great Andromeda Galaxy...

M31: The Great Andromeda Galaxy...

Object: Messier 31 (The Great Andromeda Galaxy, M31, NGC 224)

Type: Spiral galaxy (classification SA(s)b)
Constellation: Andromeda
Distance: 2.54 million light years

Equipment: Atik 460/EFW 2, Samyang 135mm lens@ F2, Vixen GPDX mount, guiding with Lodestar X2/PHD
Date: November 4th. 2021
Subframes: 12 x 300s and 12 x 20s for Luminance, 200s for RGB each, flats, no darks (hot pixel removal in Astroart).

This object has always been a bit of a nemesis for me, and this image still shows my struggle with it. It is noisy and the star shapes are appalling, but it will have to do for now until I have another go next year. In many ways, my earlier monochrome image of it is better.  Nevertheless, it does show the spiral structure and the two satellite galaxies M32 and M110 (the two white ovals at 7 o’clock and 1 o’clock respectively, relative to the centre of the main galaxy).

Messier 31 (M31), better known as the Andromeda Galaxy, is a large spiral galaxy located in the constellation Andromeda and is the nearest major galaxy to our own.

The Andromeda Galaxy is one of the most distant deep sky objects visible to the naked eye. It is relatively easy to find high in the autumn Northern hemisphere sky as it is one of the brightest Messier objects.

Stellarium map showing location of M31...

The earliest record of M31 comes from the Persian astronomer Abd al-Rahman al-Sufi, who mentioned the object as being in the constellation of Andromeda (the Chained Maiden) in his Book of Fixed Stars in 964, describing the galaxy as a “small cloud.”

The first documented telescopic observation of the galaxy was provided by the German astronomer Simon Marius on December 15, 1612. He described the object as resembling “the flame of a candle as seen through transparent horn”.

Charles Messier credited Marius for the discovery of M31, unaware of the Persian astronomer’s earlier observations.

In 1887, Welsh engineer and amateur astronomer Isaac Roberts took the first photographs of the Andromeda Galaxy from Sussex, England. His long-exposure images revealed the galaxy’s spiral structure for the first time.

Binoculars and small telescopes reveal only the galaxy’s bright core, but larger instruments show its full size, which is six times larger than the apparent diameter of the full Moon. The Andromeda Galaxy has a total of 14 satellite galaxies, of which Messier 32 and Messier 110 are the largest and easiest to observe.

Messier 31 is inclined at about 77 degrees relative to Earth. As a result of gravitational interaction with the nearby galaxies, it has a notable S-shaped warp rather than a flat disk.

Messier 31 is the largest and most massive member of the Local Group of galaxies, which also includes our Milky Way, the Triangulum Galaxy (M33) and more than 40 smaller galaxies. The Andromeda Galaxy contains a trillion stars, more than twice as many as the Milky Way, which is home to 200 to 400 billion stars. In 2005, M31 was discovered to have a large extended stellar disk, spanning more than 220,000 light years in diameter. The galaxy was previously thought to have a diameter between 70,000 and 120,000 light years.

The Andromeda Galaxy was long believed to be a nebula in our own galaxy and was known as the Great Andromeda Nebula. It wasn’t until 1917 that this belief started to be questioned. American astronomer Heber Curtis saw a nova within the galaxy and, after going over the photographic record, found 11 more novae in the region. He noticed that the novae within M31 were about 10 magnitudes fainter than those observed elsewhere in the sky and came up with a new distance estimate for the object: 500,000 light years.

Curtis became a proponent of a new theory which introduced the idea that the objects known as spiral nebulae were in fact independent galaxies. The theory was known as the “island universes” hypothesis. The term “island universes” came from German philosopher Immanuel Kant, who also believed that spiral nebulae were not part of our galaxy.

In 1920, Curtis debated the nature of spiral nebulae and the size of the universe with Harlow Shapley in what is known as the Great Debate or the Shapley-Curtis Debate. The debate took place on April 26 at the Smithsonian Museum of Natural History. Shapley argued that spiral nebulae were part of the Milky Way and that the universe was composed of only one large galaxy, while Curtis contended that spiral nebulae were separate galaxies and that the Milky Way was just one of many galaxies.

The true nature of M31 was not proven until 1923, when Edwin Hubble established the intergalactic distance between Andromeda and the Milky Way. Using the 100-inch Hooker Telescope at Mount Wilson Observatory in Los Angeles, Hubble identified Cepheid variable stars on astronomical images of M31.

Hubble’s original estimate placed M31 at an approximate distance of 750,000 light years from Earth, which finally proved that the object resided outside of our galaxy.  Further observations by German astronomer Walter Baade identified that the stars within the galaxy fell into two population types: Type I and Type II, with each type having a distinct kind of Cepheid variable and which led to distance estimates for M31 doubling.

Messier 31 is approaching the Milky Way at about 110 km/s. It is one of the few blue-shifted galaxies (moving toward us) from our point of view. The two galaxies are roughly equal in mass and will collide in about 3.75 billion years. The collision will most likely result in a merger of the two large galaxies into a giant elliptical galaxy, and possibly even a large disk galaxy.