Ten things you should know about the Milky Way Galaxy

So you’ve lived here all your life — in So you’ve lived here all your life — in fact, everyone has — but what do you fact, everyone has — but what do you

really know about really know about

the Milky Way galaxy? the Milky Way galaxy?

1. It’s a barred spiral.1. It’s a barred spiral.

The Milky Way is a The Milky Way is a spiral galaxy: arms spiral galaxy: arms sweeping out from a sweeping out from a central hub or bulge of central hub or bulge of glowing stars. glowing stars.

1. It’s a barred spiral.1. It’s a barred spiral.

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

1. It’s a barred spiral.1. It’s a barred spiral.

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

2. There’s a supermassive black 2. There’s a supermassive black hole at its heart.hole at its heart.

We know it’s there due to We know it’s there due to the effect of its gravity: the effect of its gravity: stars very near the center stars very near the center of the galaxy orbit at of the galaxy orbit at phenomenal speeds of phenomenal speeds of thousands of kilometers thousands of kilometers per second!per second!

2. There’s a supermassive black 2. There’s a supermassive black hole at its heart.hole at its heart.

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

2. There’s a supermassive black 2. There’s a supermassive black hole at its heart.hole at its heart.

… … and we’re in no danger and we’re in no danger of plunging into it since it’s of plunging into it since it’s 250,000,000,000,000,000 250,000,000,000,000,000 kilometers away.kilometers away.

3. It’s a cannibal.3. It’s a cannibal.

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

3. It’s a cannibal.3. It’s a cannibal. The Milky Way is currently eating several other The Milky Way is currently eating several other

galaxies. galaxies. These galaxies have been ripped into long, curving These galaxies have been ripped into long, curving

arcs of stars that orbit the center of the Milky Way. arcs of stars that orbit the center of the Milky Way. Eventually they’ll merge completely to create a slightly Eventually they’ll merge completely to create a slightly larger galaxy. larger galaxy.

4. 4. We live in a nice We live in a nice neighborhood…neighborhood…

The Milky Way is not alone in space. We’re part of The Milky Way is not alone in space. We’re part of a small group of nearby galaxies called the Local a small group of nearby galaxies called the Local Group. Group.

4. 4. We live in a nice We live in a nice neighborhood…neighborhood…

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

4. 4. We live in a nice We live in a nice neighborhood…neighborhood…

There are approximately three dozen galaxies in There are approximately three dozen galaxies in the Local Group, with most being smaller dwarf the Local Group, with most being smaller dwarf galaxies that are faint and difficult to detect. galaxies that are faint and difficult to detect.

5. 5. … and we’re in the suburbs.… and we’re in the suburbs.

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

6. 6. You can only see 0.000003% You can only see 0.000003% percent of it.percent of it.

On a clear night, we can On a clear night, we can see thousands of stars. see thousands of stars. But the Milky Way has But the Milky Way has two hundred billiontwo hundred billion stars stars so you’re only seeing a so you’re only seeing a fraction of the number fraction of the number of stars in the galaxy. of stars in the galaxy.

7. 7. 90% of it is invisible.90% of it is invisible.

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

7. 7. 90% of it is invisible.90% of it is invisible.

Whatever it is, it has Whatever it is, it has mass, but doesn’t glow. mass, but doesn’t glow. So we call it So we call it Dark Dark MatterMatter. We just don’t . We just don’t know what it know what it isis yet yet. .

8. 8. Spiral arms are an illusion.Spiral arms are an illusion.

The number of stars The number of stars inin the spiral arms of our the spiral arms of our galaxy isn’t very galaxy isn’t very different from the different from the number of stars number of stars betweenbetween the arms! The the arms! The arms are like cosmic arms are like cosmic traffic jams traffic jams – regions regions where the local density where the local density is enhanced. is enhanced.

8. 8. Spiral arms are an illusion.Spiral arms are an illusion.

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

9) 9) It’s seriously warped.It’s seriously warped.

The Milky Way is a flat The Milky Way is a flat disk roughly 100,000 disk roughly 100,000 light years across and a light years across and a few thousand light years few thousand light years thick (depending on thick (depending on how you measure it). how you measure it).

9) 9) It’s seriously warped.It’s seriously warped.

It has the same proportion as a stack of four It has the same proportion as a stack of four DVDs..DVDs..

Have you ever left a DVD out in the Sun? It Have you ever left a DVD out in the Sun? It can warp as it heats up, getting twisted. The can warp as it heats up, getting twisted. The Milky Way has a similar warp!Milky Way has a similar warp!

10. 10. We’re going to get to know the We’re going to get to know the Andromeda galaxy a Andromeda galaxy a lotlot better. better.

Andromeda is visible to Andromeda is visible to the naked eye on a the naked eye on a clear, moonless night. clear, moonless night. It’s faint, but big; it’s four It’s faint, but big; it’s four or more degrees or more degrees across, eight times the across, eight times the apparent size of the apparent size of the Moon on the sky.Moon on the sky.

10. 10. We’re going to get to know the We’re going to get to know the Andromeda galaxy a Andromeda galaxy a lotlot better. better.

In two billion years we In two billion years we will have a will have a muchmuch better better view. Eventually the view. Eventually the two galaxies will merge two galaxies will merge and become…. and become….

Milkomeda? Milkomeda?