1. Chapter 20: The Milky Way

2. William Herschel’s map of the Milky Way based on star counts In the early 1800’s William Herschel, the man who discovered the planet Uranus, tried to understand the shape of the Milky Way by counting stars. He counted how many stars there were in each of dozens of different directions and then plotted the count versus direction. The result was a small flat disc with Earth at the center

3. Modern Views of the Milky Way IRAS COBE Compton The problem is we are in the galaxy so it is difficult to discern its shape

4. Edge on view of a spiral galaxy One of the ways we can learn about our Milky Way is to study other galaxies. According to the cosmological principle, we are not in a special place. That means that our Milky Way isn’t any special kind of galaxy. It must look like other galaxies do so if we study them we might be able to learn about ourselves.

5. Looking at other galaxies can give hints at what our galaxy looks like

6. This is our best estimate of what the Milky Way looks like We are probably an SBb barred spiral galaxy

7. What are the Spiral Arms? If the spiral arms are due to the rotation of stars and nebulae around the center of the galaxy, they should “wind up”. We don’t see wound-up galaxies like this so the formation of spiral arms must have a different cause.

8. The Spiral Density Wave Model The arms are like a slow moving truck on the freeway. The gas and dust piles up as it enters the arms, make new stars which then move our of the arm.

9. The part of the Milky way we “see” is about 100,000 Ly across The visible matter in the Milky Way is about 100,000 lightyears across. We are located about 27,000 lightyears from the center. The galaxy appears to have a small central bar, possibly four main arms and a few spurs (arm fragments) although the most recent observations indicate that there may only be two arms, each wrapped around the center more than once. Two arms would be typical of a barred spiral.

10. The Milky Way seems to have two stellar age groups Globular clusters, found in the halo, are the oldest stars. The youngest globular cluster is over 8 billion years old while the oldest is over 12 billion years old. Open clusters and the stars that have drifted out of them are found in the disk and are the young stars. The oldest open clusters are less than 5 billion years old.

11. Astronomers use the “metal” content of stars to find their age

12. The history of star formation in the Milky Way The Milky Way seems to have undergone a number of star forming periods followed by periods of relatively little star formation.

13. Even the disk has different stellar age groups Because of the different episodes of star formation, there are stars of many different ages in the disk of the galaxy.

14. Cosmic rays give us a picture of the galactic magnetic field Cosmic rays are high energy particles, mostly protons, that are accelerated to speeds close to the speed of light in supernova explosions and near black holes.

15. We see the galactic magnetic field is tied to the gas clouds in the disk Cosmic ray studies indicate that the galactic magnetic field is not uniform. It tends to be concentrated where there are giant molecular clouds and spread out and weaker in the region between the spiral arms.

16. The rotation velocity of stars tells us there is a lot of Dark Matter in the Milky Way

17. The Dark Matter, along with globular clusters and old stars, form a halo around the galaxy The dark matter halo extends our at least 150,000 lightyears from the center of the galaxy while the visible part only goes out 50,000 lightyears.

18. The very center of the Milky Way is a very crowded place At the very center of the galaxy, in a space only a few hundred lightyears across, we find a very crowded place.

19. A close-up of the very center of the Milky Way

20. Things move fast around the center While Sagittarius A* isn’t moving, the things around it most definitely are, and fast

21. Sagittarius A* is a 4 million solar mass black hole When we do the Kepler’s 3 rd law calculation we find that Sgr A* is four million times the mass of the Sun. At present it doesn’t seem to be eating much but it does occasionally have a snack.

22. While it may not be eating now, the supermassive black hole at the center of the Milky Way has eaten in the past. Images in x-ray and gamma ray indicate a pair of bubbles and jets above and below the disk of the galaxy.

23. Understanding the formation of the Milky Way helps us understand galactic formation Big galaxies eat smaller galaxies and get even bigger. As they are eaten, they often get shredded into long tidal tails. We see the remnants of several of them around the Milky Way

24. The Milky Way doesn’t live in isolation

25. In the next billion years or so we will collide with the Andromeda Galaxy The merger may take half a billion years. After it is over we will probably be a giant elliptical galaxy: The AndroMilky Galaxy