30Kpc (100,000 light years)
- Bulge 3 Kpc
- Disk 1.5 Kpc thick
- No. of Stars 100 billion
- Bulge 3 Kpc
The center of the Milky Way is in Sagittarius, and is obscured from view by dust.
The Sun is XXX from center, in the Orion Arm.
The Solar System orbits around the Galaxy in 200 million years.
During the Northern Hemisphere summer, the Milky Way we are viewing across the sky is in the direction of inwards towards the center of the Galaxy, and during the winter we are viewing 'outwards'. Because there is more 'spiral arm' material 'inwards' than 'outwards', the summer Milky Way is brighter. On the other hand, a cold sky is drier than the summer sky, resulting in less scatterring of light allowing us to see the Milky Way with more contrast.
Galaxies are arranged in clusters. The Milky Way belongs to a cluster called the Local Group. This Local Group is about 3 million light years across. Other objects in the Local Group include, for example
- M31 - Andromeda Galaxy The largest member of the Local Group. 20% more massive and 40% larger than the Milky Way. It is tilted at 12.5° from our line of sight
- M32 , elliptical, companion of M31
- M33 in Triangulum, spiral galaxy
- M110, elliptical, companion of M31
Large Magellanic Cloud satellite galaxy of the Milky Way. Spreads across the constellations Dorado and Mensa. It lies about 165-179 thousand light years away. It was the location of supernova
1987A. Although classed as an 'irregular', there are definite signs of a spiral structure.
- Small Magellanic Cloud in Tucana. 200,000 light years distant. Spans 4 degrees in the sky.
- Sculptor faint dwarf galaxy
- Fornax System faint dwarf galaxy. Gets its name because it is in the Fornax constellation. It is 420,000 light years away and is 7000 light years in diameter.
- Leo I very close to regulus
The Local Group is a member of the Virgo Supercluster, which contains several hundred clusters.
The Virgo cluster, one of the other members of the Virgo supercluster (I know the naming is a bit confusing), is considerably larger than the Local Group - it contains massive elliptical galaxies in its heart which are about the size of the Local Group in themselves. It also contains M87. A total of about 2500 galaxies have been identified in the cluster.