According to a team of astronomers reporting in 2010, M31 was formed out of the collision of two smaller galaxies between 5 and 9 billion years ago.
A paper published in 2012 has outlined M31’s basic history since its birth. According to it, Andromeda was born roughly 10 billion years ago from the merger of many smaller protogalaxies, leading to a galaxy smaller than the one we see today.
The most important event in M31’s past history was the merger mentioned above that took place 8 billion years ago. This violent collision formed most of its (metal-rich) galactic halo and extended disk and during that epoch Andromeda’s star formation would have been very high, to the point of becoming a luminous infrared galaxy for roughly 100 million years.
M31 and the Triangulum Galaxy (M33) had a very close passage 2–4 billion years ago. This event produced high levels of star formation across the Andromeda Galaxy’s disk – even some globular clusters – and disturbed M33’s outer disk.
While there has been activity during the last 2 billion years, this has been much lower than during the past. During this epoch, star formation throughout M31’s disk decreased to the point of nearly shutting down, then increased again relatively recently. There have been interactions with satellite galaxies like M32, M110, or others that have already been absorbed by M31. These interactions have formed structures like Andromeda’s Giant Stellar Stream. A merger roughly 100 million years ago is believed to be responsible for a counter-rotating disk of gas found in the center of M31 as well as the presence there of a relatively young (100 million years old) stellar population.
Imaging telescopes or lenses: Orion ED80T-CF
Imaging cameras: Nikon D7100
Mounts: Skywatcher AZ-EQ6 GT
Guiding telescopes or lenses: Orion 50mm mini guidescope
Guiding cameras: Orion Star Shoot autoguider (SSAG)
Focal reducers: TeleVue 0.8x Photo Reducer/Flattener TRF-2008
Software: Adobe Lightroom 5, StarTools64, Photoshop CS6, PHD Guiding, Luc Coiffier DeepSkyStacker
Dates: Nov. 8, 2013, Nov. 9, 2013
Integration: 1.8 hours
AstroPhotography of the day by SPONLI 26 May 2014