1. Spectroscopy – the study of the colors of light (the spectrum) given off by luminous objects. Stars have absorption lines at different wavelengths where the energy is precisely correct to excite the electrons to a new level.

9. Different elements show different absorption lines, so the composition can be determined by the spectrum of the light produced.

13. However, differences in the absorption spectra of different stars is not due to differences in composition, but due to differences in temperature.

14. Stars over 25,000K show intense lines of singly-ionized helium and multiply-ionized heavier elements (O, N, Si). There are no Hydrogen lines because the hydrogen is mostly ionized, so no lines due to excited electrons.

15. Only blue stars are hot enough to completely ionize hydrogen.

16. Stars about 10,000K show mostly H-lines. These lines are produced by electrons moving between the 2nd and 3rd orbitals eliminating those wavelengths.

17. Stars about 10,000K show no He, O, or N lines because these electrons are too tightly held in their orbitals. Calcium and Titanium lines are common because they easily lose their electrons.

18. Stars about 6000K, like our Sun, have few strong lines of ionized elements; the elements are too cool to ionize. They have few H-lines.

19. Stars about 3000K, red stars, have weak H-lines, show weak lines for neutral heavy atoms. No lines are seen from ionized elements.

20. Spectral Classifications Early researchers designed a scheme of classification based on the spectra of stars.

21. At this time atomic theory was lacking so all the lines were not understood. (Most importantly, the absence of H-lines was not understood.) It was believed that the abundance of hydrogen varied from lots to none.

22. Stars were classified by Hydrogen-line intensity. They used a system of letters from A through P, thinking A had more hydrogen than P due to the strength of the H-line.

23. The abundance of Hydrogen is actually similar for all stars, the different intensities of H-lines from one star to another is due to differences in temperature causing different levels of ionization.

24. Stars are more meaningfully classified by surface temperature. So, the A to P classes were realigned by temperature. The result is the spectral classes: OBAFGKM (the other letters have been dropped from usage)

25. In this system, O is the hottest type of star, M is the coolest. A mnemonic device for remembering these classes is: Oh, Be A Fine Guy, Kiss Me!

26. Each letter is subdivided into subclassifications, 0 through 9. The lower the number, the hotter the star.

27. Using this system our Sun is a G2 (cooler than a G1, hotter than a G3). The star Vega is a A0. Barnard’s Star is a M5. Betelgeuse is a M2.

28. Prominent Spectral SurfaceAbsorption ClassTemp Lines Example O 30000K Strong Ionized He Multiply-ionized Heavies Faint Hydrogen lines B 20000K Neutral He Moderate Singly-ionized Heavies Rigel (B8) Hydrogen lines moderate A 10000K Faint Neutral He Singly-ionized Heavies Vega (A0) Hydrogen lines strong Sirius (A1) F 8000K Singly-ionized Heavies Neutral metals Canopus (F0) Hydrogen lines moderate

29. Prominent Spectral SurfaceAbsorption ClassTemp Lines Example G 6000K Singly-ionized Heavies Neutral metals Sun (G2) Hydrogen lines moderate Alpha Centauri (G2) K 4000K Singly-ionized Heavies Neutral Metals Strong Arcturus (K2) Hydrogen Faint Aldeberan (K5) M 3000K Neutral Atoms strong Molecules moderate Betelgeuse Hydrogen-very faint (M2) Barnard’s Star (M5)

31. To construct a Hertzsprung-Russell diagram astronomers must first find the star’s surface temperature. This can be done using a Plank curve or the spectrum.

32. Second, the luminosity must be found. Finding the luminosity is either: easy, if the distance to the star and the apparent brightness are known they can be used to find the luminosity from the inverse square law; or it is impossible, if the distance isn’t known.

33. The Main Sequence - The stars are not evenly distributed on a Hertzsprung-Russell diagram. Most of the stars range from high temperature and high luminosity to low temperature and low luminosity. (In other words, cool stars are faint and hot stars are bright, duh.)

34. Hot stars tend to be larger, cooler stars tend to be smaller.

36. Large, hot, bright stars are found to the upper left. These are blue giants and blue supergiants. Small, cool, faint stars are to the lower right. These are the red dwarfs. Our Sun is in the middle of the range.

37. HR diagrams are biased in favor of blue giants (so much easier to see) and against red dwarfs (hard to see). Actually red dwarfs make up approximately 80% of all the stars in the galaxy.

38. Most stars lie on the main sequence, but there are some notable exceptions: the red giants and the white dwarfs.

41. While red giants are rare, they are very visible. The distribution of types of known stars: 90% of all stars are on the main sequence, 9% of all stars are white dwarfs, 1% of all stars are red giants.

42. The main sequence can be used to find the distance to a star using the apparent brightness and the temperature (color). This use of a Hertzsprung-Russell diagram to find the distance of very distant stars is called spectroscopic parallax.

45. Luminosity is comparable to absolute brightness; this is compared to the apparent brightness to find the distance by the inverse square rule.

46. Spectroscopic parallax depends on the Principle of Mediocrity; we assume that distant stars are similar to nearby stars.

47. Spectroscopic parallax is simple to use if the star is on the main sequence. Fortunately, 90% of all stars lie on the main sequence. But, what if the star is not on the main sequence?

48. The width of the spectral line seen in the spectra of stars is determined by the density of the gas producing the light. The densities of these gases is less for a red giant and more for a white dwarf.

49. This lets astronomers using spectroscopic parallax to distinguish between red giants and red dwarfs and between white dwarfs and white giants. Therefore the distance to the other 10% of stars not on the main sequence can be found.

51. Another way to distinguish types of stars is by luminosity class: I a - bright supergiants I b - supergiants II - bright giants III - giants IV - subgiants V - main sequence dwarfs VI - sub dwarfs