Galaxy of Andromeda

The Persian astronomer Abd ar-Rahman as-Sufi (Al Sufi) was the first to describe the Andromeda galaxy in 964 according to hand-written parchment.

Abd al-Rahman al-Sufi’s Book of the Fixed Stars, dating from around AD 964, is one of the most important medieval Arabic treatises on astronomy. This major work contains detailed star charts and an extensive star catalogue, which lists star co-ordinates and magnitude estimates.

1,000 years ago in his book he wrote: “The two lines (of stars) begin from the nebula which is close to the 14th star [in the constellation of Andromeda] …”. Here he is describing the constellation of Andromeda, and for the first time in recorded history he mentions the location of a nebula which is now identified as M31, the spiral galaxy in Andromeda.

Simon Marius was the first observer with telescope of the Andromeda Nebula in December 1612, and gave the first description of this object based on telescopic observation. He described it as resembling the light of a candle flame seen through translucent horn.

In 1781 Charles Messier of France listed it as No. 31 in his catalogue of nebulous objects, and to this day the Andromeda nebula is also commonly known as Messier 31, or M3l.

Modern observations of M31 date from 1914, when V. M. Slipher, using the 24-inch refracting telescope at the Lowell Observatory in Flagstaff, Ariz., determined that the solar system and the center of M31 are approaching each other at a speed of 300 kilometers per second.

In 2012 researchers concluded that Milky Way and Andromeda are going to collide in around four billion years. Andromeda is approaching Milky way at about 110 km/s.
Galaxy of Andromeda

The Birth of the Galaxy

Because globular clusters contain the oldest stars associated with the Galaxy the halo marks the fossil remains of its birth.

Within it, globulars orbit the Galaxy on extremely elongated elliptical paths. Most of the time, the globulars move slowly through the halo at the outer extremes of their orbits; only briefly do they whip in and around the nucleus.

These stars exhibit the motions of the cloud from which they were formed. So the Galaxy must have been born form a gas cloud that was initially huge- at least 300,000 ly in radius.

Imagine a tremendous, ragged cloud of gas roughly twice as big as the Galaxy’s halo today. Its density is low. This proto Galaxy cloud probably is turbulent, swirling around with random churning currents.

Slowly at first, the cloud’s self-gravity pulls it together, with it central regions getting denser faster than its outer parts.

Throughout the cloud, turbulent eddies of different sizes form, break up, and die away. Eventually, the eddies become dense enough to contain sufficient mass to hold themselves together. These might be hundreds of light years in size – incipient globular clusters.

Each blob then splits up to form individual stars – all born at about the same time.

Meanwhile, the gas contracts more and fall slowly into a disk. Why a disk? Because the original cloud had a little spin, and the conservation of angular momentum requires that it spin faster around its rotational axis as it contracts.

The kinetic energy energy of the cloud slowly decreases, as gas clouds collide and heat is radiated away.

The disk rapidly flattens. As the disk forms, its density increases and more stars form. Each burst of starbirth leaves behind representative stars at different distances from the present disk.

Finally, the remaining gas and dust settle into the narrow layer as we see today. Somehow density waves appear and drive the formation of spiral arms.

During this time, massive stars were manufacturing heavy elements and flinging them back into the cloud by supernova explosions.

So as stars were born in succession, each later type had more heavy elements. That enrichment continues today in the disk of the Galaxy.
The Birth of the Galaxy