When you look up into the night sky on a clear night in a place that has little light pollution, you can see thousands of stars spanning the sky, all of which lie in our galaxy, the Milky Way. The Milky Way seems really, really big when seen from the comparatively tiny Earth. What follows is a list of the attributes of our galactic neighborhood, the dimensions of our corner of space.
We’ll start with the mass of the Milky Way. It’s so massive that we have to give its mass in units of something rather large itself: the Sun. When you take into account all of the stars, gas, dust and the copious amounts of dark matter that surround our galaxy in a halo, it has about 3 trillion times the mass of the Sun, according to the most recent estimate as of this writing. Previous estimates put the number at over 1 trillion solar masses. Over 90% of that mass can be attributed to dark matter, matter that cannot be detected except for its gravitational pull.
Of course, the Milky Way isn’t all dark matter – there’s lots of gas, dust, and stars that populate the galactic disk. The number of stars in the Milky Way is estimated to be about 200-400 billion, though you can only see about 5,000-8,000 of those stars with the naked eye, and only about 2,500 of them at any one time from the Earth. For one of the most highly detailed images of our galaxy in all of its stars and splendor from the Spitzer Space Telescope, go here.
The Milky Way is a huge disk, roughly 100,000 to 120,000 light years across. Its thickness is 1,000 light years throughout most of the disk, but there is a spheroidal bulge at the center of the galaxy that is 12,000 light years in diameter. These proportions are similar to a small stack of DVDs with a rubber ball glued into the middle. For a great representation of the proportions of the Milky Way in these terms, check out the video Galaxies by the Bad Astronomer, Phil Plait.