Discrete Sources
Apparently, by late 1968, no X-rays had been detected from the Andromeda Galaxy. A balloon flight on October 20, 1970, set an upper limit for detectable hard X-rays from M31.
Multiple X-ray sources have since been detected in the Andromeda Galaxy, using observations from the ESA's XMM-Newton orbiting observatory. Robin Barnard et al. hypothesized that these are candidate black holes or neutron stars, which are heating incoming gas to millions of kelvins and emitting X-rays. The spectrum of the neutron stars is the same as the hypothesized black holes, but can be distinguished by their masses.
There are approximately 460 globular clusters associated with the Andromeda Galaxy. The most massive of these clusters, identified as Mayall II, nicknamed Globular One, has a greater luminosity than any other known globular cluster in the local group of galaxies. It contains several million stars, and is about twice as luminous as Omega Centauri, the brightest known globular cluster in the Milky Way. Globular One (or G1) has several stellar populations and a structure too massive for an ordinary globular. As a result, some consider G1 to be the remnant core of a dwarf galaxy that was consumed by M31 in the distant past. The globular with the greatest apparent brightness is G76 which is located in the south-west arm's eastern half. Another massive globular cluster -named 037-B327-, discovered in 2006 as is heavily reddened by the Andromeda Galaxy's interstellar dust, was thought to be more massive than G1 and the largest cluster of the Local Group; however other studies have shown is actually similar in properties to G1.
Unlike the globular clusters of the Milky Way, that show a relatively low age dispersion, Andromeda's globular clusters have a much larger range of ages: from systems as old as the galaxy itself to much younger systems, with ages between a few hundred million years to five billion years
In 2005, astronomers discovered a completely new type of star cluster in M31. The new-found clusters contain hundreds of thousands of stars, a similar number of stars that can be found in globular clusters. What distinguishes them from the globular clusters is that they are much larger–several hundred light-years across–and hundreds of times less dense. The distances between the stars are, therefore, much greater within the newly discovered extended clusters.
Read more about this topic: Andromeda Galaxy
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