1. Lecture 5: Measuring the Milky Way
Astronomy 5: The Formation and Evolution of the Universe
Sandra M. Faber
Spring Quarter 2007
UC Santa Cruz

2. Longer-period Cepheid variables are brighter
Note: another funny plot in which each tickmark is the same FACTOR. 4
This kind of plot is called a log-log plot because each tickmark is one step in the logarithm. 3 2

3. The nearby globular cluster Messier 5

4. Circling around a globular cluster

5. Globular clusters are spherical because their orbits are scrambled

6. The simplest nuclear reaction that makes stars shine

7. Globular clusters in the spheroid of the “Sombrero “ galaxy

8. Milky Way in visible light (0
Milky Way in visible light (0.5 microns): stars are badly obscured by interstellar dust clouds

9. Visible light is 350-700 nm, or 0.35-0.7 microns
Visible light averages around 500 nm, which is 0.5 microns (m).
One micron is one-thousandth of a millimeter.

10. Near-infrared “light” lies at 1-3 microns, between visible light and infrared (i.e., heat radiation)
Near-infrared has slightly longer wavelengths than visible light. Lies between 1-3 microns, part way towards “heat” radiation, which is called “infrared.”
HEAT

11. Milky Way at 1-3 microns: stars seen through dust
Milky Way in visible light (0.5 microns): stars obscured by dust clouds

12. 21 cm radiation is in the short-wavelength radio region
21 cm is a special wavelength that is emitted by clouds of neutral hydrogen gas (H I).
HEAT

13. The Very Large Array of radio telescopes, which observe 21 cm radiation. The VLA can cover up to 27 km and is located in New Mexico.

14. The Very Large Array of radio telescopes, which observe 21 cm radiation. The VLA can cover up to 27 km and is located in New Mexico.

15. 21 cm wavelengths (radio) reveal the hydrogen gas layer in the disk
21 cm wavelengths (radio) reveal the hydrogen gas layer in the disk. This layer fuels star formation.

16. Major structural components of the Milky Way

17. A Milky Way-like external galaxy seen edge on
bulge
disk
NGC 891

18. The “Sombrero “ is similar, but its spheroid is relatively bigger

19. The orbits of spheroid stars in the Milky Way are scrambled like those in a globular cluster
Spheroidal systems have scrambled orbits Disk systems have orderly orbits marching in circles.

20. Map of hydrogen gas made with 21 cm radio telescopes
Gas has density concentrations that look like spiral “arms”

21. Stars form from dense clouds of gas
Giant H II region in Messier 33
Messier 33 galaxy, a nearby member of the Local Group

22. The simplest nuclear reaction that makes stars shine
Blue is clouds of hydrogen gas in Messier 33. H II regions, where stars are forming, are red. Notice how they line up.

23. Three views of the nearby spiral Messier 83
Visible light shows stars of all ages. Blue are massive, youngest, most recently formed. Found only in disk.
21 cm shows hydrogen gas arranged in spiral arms. This is where most stars ar forming.
Near-infrared minimizes blue stars and maximizes cooler, older stars, which populate both disk and bulge.

24. Schematic explanation of long-lived spiral arms
The naturally circular disk orbits are deformed by the gravity of the spiral arms in to ellipses. Successively larger ellipses are rotated slightly with respect to smaller ones. Even though the stars (hardly) change speed as they rotate around the center, their orbits converge where the ellipses nearly touch. This spiral pattern is what is needed to deform the ellipses in the first place, and so the pattern is self-sustaining.