What is our galaxy's halo made of, and how was it formed?
What is the halo around the Milky Way made of? And why is it there? How did it get there?
The halo is a (nearly) spherical region surrounding the galaxy (like the diffuse light around the heads of saints, not like the yellow rings around angels' heads in cartoons), made up mostly of dark matter. Dark matter is called dark because it does not radiate brightly enough to be observed. Because its radiation is not observed, the dark matter is studied through its gravitational interaction with light passing by, an effect known as "gravitational lensing". This suggets the dark matter is made up of "MACHO's" (MAssive Compact Halo Objects) and "WIMP's" (don't blame me, I didn't name them--Weakly Interacting Massive Particles). MACHO's are familiar matter, like dead stars and eggbeaters, and possibly some particles that have not been detected yet. WIMP's are purely theoretical particles and are like neutrinos in that they rarely interact, but unlike neutrinos they can be extremely heavy. The makeup of galactic haloes is important to cosmology, since much of the mass of the universe is in galactic haloes, and how the universe evolves depends strongly on how much of each sort of dark matter there is. It is the subject of enthusiastic study.
Not all of the matter is dark. The halo is home to some ancient star clusters, known as globular clusters. They are known to be old because their stars contain low levels of heavy elements, which weren't present in the young universe and were built up over time in supernovae. The clusters move about the galactic nucleus in (nearly) random orbits.
So how did it get there? Well, it seems to be a remnant from the formation of the galaxy. Some of the stars are fossils from that time, and the lack of free gas means no new star formation. Other stars were taken from small galaxies that were disrupted by our galaxy, an idea suggested by some stripes of stars in the halo, like galactic roadkill.
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