The Milky Way Galaxy Structure & Evolution Ohio University - Lancaster Campus slide 2 of 35 Spring 2009 PSC 100

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Milky Way’s Structure Nuclear Bulge in center –12,000 LY wide x 10,000 LY thick –Made of old (mostly yellow & red) stars –Contains much of the visible mass in the galaxy. Ohio University - Lancaster Campus slide 5 of 35 Spring 2009 PSC 100

The Galactic Center The center of the galaxy is a very busy place. It’s obscured by dust and hard to see, but we do know there’s a 4 million solar mass black hole inside! This intense X-ray and radio source is called Sagittarius A.

Center of the galaxy There are so many old, red stars there that “night” on a planet near the galactic center would be twice as bright as twilight is to us. A fun place to live, except for all that pesky deadly X-ray radiation.

Thin Disk surrounds nuclear bulge –120,000 LY wide, but <10,000 LY thick –Most new star formation goes on here, so stars are much younger (Population I type stars – all colors) –Much free-floating gas & dust. –BUT…contains < 50% of the visible mass of the galaxy. Ohio University - Lancaster Campus slide 9 of 35 Spring 2009 PSC 100

A halo of stars and globular clusters surrounds the entire galaxy. –Contains almost exclusively VERY old stars (population II stars). –These stars are low in heavy elements, meaning that they’re as old as the universe (before supernovas formed heavier elements.) –Contains little visible mass, but about ½ total mass of galaxy. Ohio University - Lancaster Campus slide 11 of 35 Spring 2009 PSC 100

Total Mass of the galaxy is equivalent to 1 trillion stars, but the galaxy actually contains only about 200 billion stars. Loose gas & dust only account for about another 200 billion stars’ worth. Where and what is the missing 600 billion stars’ worth of mass? Ohio University - Lancaster Campus slide 13 of 35 Spring 2009 PSC 100

Where is our sun in the MW? We have a difficult problem in trying to understand where we live in the galaxy. Imagine yourself being plopped down on a street corner in an unfamiliar city, and being told to map the entire city. Ohio University - Lancaster Campus slide 14 of 35 Spring 2009 PSC 100

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Here’s our galactic street corner. Ohio University - Lancaster Campus slide 16 of 35 Spring 2009 PSC 100

Where do we live in the galaxy? You’d only be able to clearly map the area right around you. You might be able to guess at some other streets if you could see light poles, trees, or rooftops. The other side of town would remain unknown. But if you could go 1000’ up in a helicopter, it would be much easier.

From up above, we can see the layout.

It’s hard for us to know the layout of our own galaxy, because we live in the plane of the galaxy, not above it. We have to find out indirectly… –by looking at other similar galaxies… –by looking at globular clusters… –by mapping the positions of bright sources (nebulae, bright stars.) Ohio University - Lancaster Campus slide 20 of 35 Spring 2009 PSC 100

The Andromeda Galaxy is similar to ours. Ohio University - Lancaster Campus slide 21 of 35 Spring 2009 PSC 100

We believe that our solar system is about 2/3 out from the center toward the edge of the galaxy. Here’s why: Our galaxy is surrounded by a halo of evenly-distributed globular star clusters. Ohio University - Lancaster Campus slide 22 of 35 Spring 2009 PSC 100

A globular cluster from a nearby planet.

If we lived near the center of our galaxy, we’d see globular star clusters evenly distributed everywhere in the sky…

If we live nearer the edge of the galaxy, globular star clusters won’t be evenly distributed – this is what we actually see.

How fast are we moving? By comparing how our position changes relative to the average speed of the galaxy’s globular clusters, we know that the sun moves at a speed of 220 km/second around the galaxy’s center. The sun takes 1000 years to move 1 light year, or it takes 280 million years to orbit the nucleus of the galaxy once. The galaxy has only made about 50 complete rotations since the universe began!

Here’s a problem: by looking at the visible mass in the galaxy, and the way that the galaxy rotates, we can predict how fast the sun should move through space, revolving around the center of the galaxy. It moves 220 km/sec, but it should only move at 160 km/sec. Ohio University - Lancaster Campus slide 27 of 35 Spring 2009 PSC 100

So what does this mean? The galaxy rotates so fast, that it should fly apart. (There’s not enough visible mass to hold it together.) The way that the galaxy rotates tells us where the galaxy’s mass is located.

it rotated like a rigid object, (a spinning DVD), where most of the mass is located in the outer half of the disk, then the outer edges would move faster than the center. If it rotated like a rigid object, (a spinning DVD), where most of the mass is located in the outer half of the disk, then the outer edges would move faster than the center.

If the galaxy was like the solar system, where most of the mass is in the center, the edges would rotate much slower than the center. Eventually, the galaxy’s arms would wind up, like a watch spring twisted too tight.

However, the galaxy rotates at just about the same speed everywhere. This means that the mass is evenly distributed inside and outside the sun’s orbit.

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Dark Matter Since more than half of the visible (bright) mass is inside the sun’s orbit, this means is that there is a vast amount of dark matter in our galaxy, about 600 billion sun’s worth. We can’t see this dark matter. We don’t even know what it is, but most of it is found in the galaxy’s halo.

There are >30 candidates for the identity of the dark matter. black holesdustneutrinos tiny red dwarf stars types of elementary particles that we don’t yet know anything about dark energy – a strange repulsive “anti-gravity”