The Milky Way

The Milky Way is quite large so you would expect it to weigh lot and you’re right, it does. The Milky Way’s mass is 200 to 600 billion times that of the sun. This number came about by counting all the stars and assuming they all weigh about the same as our sun. The mass also depends on what you define as the edge of the Milky Way to be. This is nit the only way to determine the mass of it. It is possible to measure the mass by measuring how fast stars are rotating around the disk. The heavier the Milky Way is the more affect gravity will have on the orbit of the stars. Using this way to measure it, the results show the mass would be 1 to 2 trillion times that of our sun. A more recent estimate by measuring the velocity of 2, 400 stars shows that the Milky Way and its halo’s mass is 1 trillion solar masses. We can’t see all of the mass because most of it is made up of dark matter. This dark matter makes up 80 to 90 percent of the Milky Way’s mass, which means we can only see a small 10 to 20 percent of our own galaxy.  
The Milky Way Galaxy is a spiral galaxy. It looks a lot like a pinwheel and it is always spinning. Gas and dust rotates at approximately 270 kilometres per second (168 miles per second). At this rate our Solar System will take 225 million years to do one orbit. The last time our Solar System completed its orbit dinosaurs were just starting to appear on the earth.

Some galaxies are blob shaped and some are irregular shapes but our galaxy is spiral shaped. Because of the spiral our galaxy falls into a class called barred spirals. The Milky Way has four main spiral arms. These are the Norma and Cygnus arm, Sagittarius, Scutum-Crux and Perseus. Our sun is in a minor arm or spur. This spur is called the Orion Spur. The galactic disk itself measures a huge 100, 000 light years across and the bar at the centre of the spiral is estimated at be approximately 27, 000 light years long. The Milky Way is a spiral because of its rotation. The stars don’t...