The Milky Way Galaxy

2. The Milky Way Galaxy Structure & Evolution

5. 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. • Contains galaxy’s nucleus at center: a black hole with mass of 4,000,000 suns. • Nucleus is called the “Shapley Center”

6. Milky Way’s Structure 2 • Thin Disk surrounds nuclear bulge • 120,000 LY wide, x <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. • Contains less than ½ of the visible mass of the galaxy.

7. Milky Way’s Structure 3 • 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.

10. Milky Way’s Structure 4 • 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. • The galaxy is about the same age as the universe: 12-15 billion years old.

11. 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.

13. Here’s our galactic street corner.

14. 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 up in a helicopter, 1000’ up, it would be much easier.

15. From 1000’ up, you can see the layout.

17. Where do we live in the galaxy? • 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 (the streetlights).

18. The Andromeda Galaxy is similar to ours.

19. A globular cluster from a nearbyplanet.

20. Where do we live in the galaxy? • We know that our solar system is about ½ way from the center to the edge of the galaxy. • Our galaxy is surrounded by a halo of evenly-distributed globular star clusters.

21. If we lived in the center of our galaxy, we’d see globular star clusters evenlydistributed everywhere in the sky…

22. If we don’t live at the center of the galaxy,globular star clusters won’t be evenlydistributed – this is what we actually see.

23. 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!

24. An interesting puzzle !! • 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.

26. So what does this mean? • The way that the galaxy rotates tells us where the galaxy’s mass is concentrated.

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

28. If the galaxy waslike the solarsystem, wheremost of the massis in the center,the edges wouldrotate much slowerthan the center. Eventually, the galaxy’s arms would wind up, like a watch spring twisted too tight.

29. However, thegalaxy rotates atjust about thesame speedeverywhere.This means thatthe mass is evenlydistributed insideand outside thesun’s orbit. Even this means that thegalaxy should “wind up”, just more slowly.

31. 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.

32. Some ideas about dark matter • What is the dark matter? There are over 30 candidates: • black holes dust neutrinos • tiny red dwarf stars • types of elementary particles that we don’t yet know anything about • dark energy – a strange repulsive “anti-gravity”

33. Let’s go back a step • Let’s go back to mapping the galaxy for just a moment. How do we know where the galaxy’s arms are located? • We look for bright signposts: • Bright O & B stars • Gas & dust clouds where new stars are forming. • Cepheid variable stars

34. What are Cepheid Variable stars? • Cepheid variables are red supergiant stars near the ends of their lives. • They slowly flicker or pulsate, like fluorescent lights ready to burn out. • We’ve discovered that the longer a Cepheid variable star takes to pulsate, the brighter it is. This gives us a way to measure distance to it.

35. Here’s the light curve of a Cepheid variable.

36. This is theperiod-luminosityrelationship.

37. This star (-Cephii) has an absolutemagnitude, M, of -3.2 and an apparentmagnitude, m, of +4.0. If we use thedistance formula….we can calculateits distance: Distance = 10(4-(-3.2)+5) / 5 = 102.44 = 275 parsecs or 897 LY.Do this for enough Cepheid variable stars & you’ll outline an arm of the galaxy!!

38. What’s the center of the galaxy like? • 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.

40. 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? Yeah, except for all that pesky deadly X-ray radiation.