1. Ten things you should know about the Milky Way Galaxy Ten things you should know about the Milky Way Galaxy

2. So you’ve lived here all your life — in fact, everyone has — but what do you really know about the Milky Way galaxy?

3. 1. It’s a barred spiral. The Milky Way is a spiral galaxy: arms sweeping out from a central hub or bulge of glowing stars.

4. 1. It’s a barred spiral. … but a lot of spirals have an unusual feature: a rectangular block of stars at the center, instead of a sphere, and the arms radiate away from the ends of the block. Astronomers call this block a bar and the Milky Way’s bar is 27,000 light years end- to-end.

5. 1. It’s a barred spiral. This image of the Milky Way is not a photograph; it’s a drawing– it’s not currently possible to get outside the galaxy to take a photo, like this one, looking back.

6. 2. There’s a supermassive black hole at its heart. We know it’s there due to the effect of its gravity: stars very near the center of the galaxy orbit at phenomenal speeds of thousands of kilometers per second!

7. 2. There’s a supermassive black hole at its heart. It’s possible to determine that the mass needed to accelerate the stars to those speeds must tip the cosmic scales at four million times the mass of the Sun! Yet in the images, nothing can be seen. So what can be as massive as 4,000,000 Suns and yet not emit any light? Right. A black hole. Right. A black hole.

8. 2. There’s a supermassive black hole at its heart. … and we’re in no danger of plunging into it since it’s 250,000,000,000,000,000 kilometers away.

9. 3. It’s a cannibal. Galaxies are big, and have lots of mass. If another, smaller galaxy passes too close by, the bigger galaxy can rip it to shreds and ingest its stars and gas.

10. 3. It’s a cannibal. The Milky Way is currently eating several other galaxies. The Milky Way is currently eating several other galaxies. These galaxies have been ripped into long, curving arcs of stars that orbit the center of the Milky Way. Eventually they’ll merge completely to create a slightly larger galaxy. These galaxies have been ripped into long, curving arcs of stars that orbit the center of the Milky Way. Eventually they’ll merge completely to create a slightly larger galaxy.

11. 4. We live in a nice neighborhood… The Milky Way is not alone in space. We’re part of a small group of nearby galaxies called the Local Group.

12. 4. We live in a nice neighborhood… The Milky Way is the heaviest guy on the block, and our nearest neighbour, the Andromeda Galaxy, is slightly less massive but spread wider. The Triangulum Galaxy is also a spiral, but not as big, and there are other assorted galaxies dotted here and there in the Group

13. 4. We live in a nice neighborhood… There are approximately three dozen galaxies in the Local Group, with most being smaller dwarf galaxies that are faint and difficult to detect.

14. 5. … and we’re in the suburbs. The Local Group is small and cozy. That’s because if you take the long view, we live in the suburbs. The big city in this picture is the Virgo Cluster, a huge collection of about 2000 galaxies, many of which are as large or larger than the Milky Way. The Local Group is small and cozy. That’s because if you take the long view, we live in the suburbs. The big city in this picture is the Virgo Cluster, a huge collection of about 2000 galaxies, many of which are as large or larger than the Milky Way.Virgo ClusterVirgo Cluster

15. 6. You can only see 0.000003% percent of it. On a clear night, we can see thousands of stars. But the Milky Way has two hundred billion stars so you’re only seeing a fraction of the number of stars in the galaxy.

16. 7. 90% of it is invisible. When you look at the motions of the stars in our galaxy, you can determine how much mass is in the galaxy. You can also count up the number of stars in the galaxy and figure out how much mass they have. The problem is the two numbers don’t match: stars (and other visible things like gas and dust) make up only 10% of the mass of the galaxy. Where’s the other 90%?

17. 7. 90% of it is invisible. Whatever it is, it has mass, but doesn’t glow. So we call it Dark Matter. We just don’t know what it is yet.

18. 8. Spiral arms are an illusion. The number of stars in the spiral arms of our galaxy isn’t very different from the number of stars between the arms! The arms are like cosmic traffic jams – regions where the local density is enhanced. –

19. 8. Spiral arms are an illusion. Like a traffic jam on a highway, cars enter and leave the jam, but the jam itself stays. The arms have stars entering and leaving, but the arms themselves persist which is why they don’t wind up like twine on a spindle.

20. 9) It’s seriously warped. The Milky Way is a flat disk roughly 100,000 light years across and a few thousand light years thick (depending on how you measure it).

21. 9) It’s seriously warped. It has the same proportion as a stack of four DVDs.. Have you ever left a DVD out in the Sun? It can warp as it heats up, getting twisted. The Milky Way has a similar warp!

22. 10. We’re going to get to know the Andromeda galaxy a lot better. Andromeda is visible to the naked eye on a clear, moonless night. It’s faint, but big; it’s four or more degrees across, eight times the apparent size of the Moon on the sky.

23. 10. We’re going to get to know the Andromeda galaxy a lot better. In two billion years we will have a much better view. Eventually the two galaxies will merge and become…. Milkomeda?