Milky Way

Scorpius and Summer Milky Way, photo by Brad Snowder.
If our solar system was the size of a coffee cup, the Milky Way Galaxy would be the size of the North American Continent.

Since ancient times people have speculated about the nature of the hazy band of light that stretches around the entire sky. It is widest and brightest in the summer sky, especially in Sagittarius. There is a long twisty dark lane through Cygnus known as the Great Rift. In autumn the path winds north past Cassiopeia and Perseus, in winter past Orion, and in Spring it reaches down to the Southern Cross.

Our galaxy appears to be in the shape of a big pancake with a bulge in the middle. Our solar system is embedded inside the pancake about half way between the edge and the middle. When we try to look out along the edges we see the combined light of billions of stars. Most of those stars are too far away to pick out individually but together they add up to a milky haze.

Our galaxy is about 100,000 light years across. The central bulge is about 16,000 light years thick. The thinner region where our solar system resides is about 3000 light years thick. Our solar system orbits around the core once every 200 million years. The total number of stars in the Milky Way is probably several hundred billion.

The core of our galaxy lies in the direction of Sagittarius. We have detected that stars in that region are circling the center at very high speed. The simplest explanation for why those stars can travel so fast without flying completely out of the galaxy is that there is a supermassive black hole in the core. The mass of the black hole is estimated at 3 to 4 million times the mass of the Sun.

The disk and central bulge are only the obvious parts of the galaxy; the parts that glow in the dark. There is also a part that we can't see with our eyes but can be detected by other means, directly and indirectly. We can directly measure light outside the range of human eyes, such as infrared and ultraviolet. We can also deduce where mass exists by its gravitational effect on other objects. We have concluded that the visible disk of the galaxy is surrounded by a huge sphere of material we call the halo.

One of the very visible populations in the halo is the globular clusters. These are the oldest objects made out of stars in the Universe. The globulars formed long before the birth of galaxies. When the galaxies came along, the globulars were caught by the gravity and have been orbiting around them ever since. There are about 200 globular clusters orbiting in the halo of the Milky Way.

Harlow Shapley The globular clusters were one of the keys in the research that revealed the size and shape of the Milky Way galaxy. Another key is variable stars, which reveal their intrinsic brightness by the rate at which they vary. The astronomer who used the keys was Harlow Shapley. From his measurements of the distance to variable stars in globular clusters, Shapley could see that the clusters were not evenly distributed around our solar system, thus inferring that our Sun is not in the center of the galaxy. The globulars are distributed and orbiting around a central point 30,000 light years in the direction of Sagittarius, a point we now accept as the galactic core. At that time, (early 20th century), most astronomers still thought that the Milky Way was the whole of the Universe.

Map of the arms of the Milky Way Galaxy
Map of the arms of the Milky Way. Image by Richard Powell