Components of the Milky Way's Halo
The main disc of our galaxy rotates at a speed of 500,000 mph and it is surrounded by the inner halo rotating at 50,000 mph in the same direction. The outer halo rotates in the opposite direction at about 100,000 mph. These regions mainly consist of (a) Population II stars and globular clusters, (b) ionized gas and (c) dark matter. More specifically:
a) The inner part of the halo is free of dust and mostly consists of individual Population II stars and globular clusters. Population II stars are older and with lower metallicity compared to the younger Population I stars that belong to the main galactic disc. Globular clusters, on the other hand, are spherical concentrations of very old stars found only in the halo of our galaxy. Since they contain no gas, it is not possible to create new stars within the globular clusters.
b) Another component of the Milky Way's halo is the hot ionized gas that originates from supernova remnants being expelled through galactic chimneys. These are pipes or vents of hot gas that transport this matter away from the galactic disc. After the gas is cooled, it is pulled back into the disc by gravitational forces.
c) The outer part occupies a larger region and mostly consists of dark matter. The presence of dark matter can only be detected through its gravitational effect on the motion of stars. For more information on its mysterious nature, click here.
Image: This is Palomar 1, a young globular cluster only 6.3 to 8 billion years old. Several galaxies can be seen at the background. The picture was created by images taken by Hubble’s Advanced Camera for Surveys.