Astronomers classify Milky way as large spiral galaxy with several spiral arms coiling around a central bulge. Stars in the central bulge are close together while those on the arms are farther apart. Spiral arms also contain clouds of gas and inter stellar dust. The disk is about 100,000 light years in diameter and iis surrounded by a large cloud of hydrogen gas. Surrounding the cloud in turn is a spherical halo that contains many globular clusters of stars mainly lying above or below the disk. The mass is estimated to be about 2000 billion times that of Sun.
The halo stars which are concentrated in compact spherical clusters known as globular clusters. Halo stars are very old; they have very large velocities out of the plane of the galaxy and were formed about 13 billion years ago and are relatively poor in elements heavier than hydrogen and helium.
The bulge stars are known as Population 2. They form the majority and have mass and age comparable to those of the stars in the halo. It is these stars that give the bulge an orange appearance.
With in the disk there are spiral arms which are a concentration of stars and interstellar gas which spiral out from the end of the budge. These spiral arms are characterised by the presence of ionised hydrogen and by the formation of young stars.
Open Clusters and
They differ primarily in age and in the number of member stars.
All stars in the galaxy move in orbit around its center. The Sun takes about 200 million years to complete an orbit.