Spectrum and Temperature

• What distinguishes one star from another? Mainly color and brightness (temperature and luminosity, size is related to those quantities), and to some extent chemical composition; For example, Betelgeuse is red, Rigel bluish (Zubeneschamali greenish).
• Temperature from spectrum: Look at the wavelength of the brightest part (or absorption lines); example of the Sun.
• Temperature from color index: Compare the intensity of B and V light, and find the ratio (B intensity / V intensity), which is related to T (not affected by velocity).
• Temperature range: T usually ranges from 3,000 K to 50,000 K [but colder stars are being found, and classified as L and T; others seem to be as hot as 75,000 K!].
• Spectral type/class: Based on the intensity of some spectral lines, classes are O-B-A-F-G-K-M (each from 0 to 9), in order of decreasing temperature.
• Examples: the Sun 6000 K, G2; [Betelgeuse 3000 K, M2; Arcturus 5000 K, K2; Procyon 7000 K, F5; Sirius 10000 K, A1; Rigel B8; others 30,000 K.]
• New spectral classes: Brown dwarfs are in colder classes, L from 1,300 to 2,000 K, T from 750 to 1,300 K, and Y when even colder.

The Hertzsprung-Russell Diagram

• The idea: Are a star's T and L related? Hotter stars should also be brighter (and more massive); Plot to find out.
• Sorting stars by luminosity: L ranges from about 1/10,000 to 10,000 times the Sun's luminosity.
• The Main Sequence: Where most stars lie; Includes blue supergiants, giants, Sun-like stars, red dwarfs.
• Other kinds of stars: Above the MS, red supergiants and giants, below the MS, white dwarfs; How do we know that their position on the diagram is related to size?
• Examples of H-R diagrams: A diagram of nearby stars shows that most of them are dim, cool stars; Can also plot all stars in a cluster.
• Luminosity Classes: Stars can be classified as Ia & Ib (supergiants), II, III (giants), IV (subgiants), V (main sequence), corresponding to their density.
• Classes: Sun, G2V; [Betelgeuse, M2Ia; Rigel, B8Ia; Sirius, A1V; Alpha Centauri A&B G2V & K1V; Proxima is a red dwarf].
• Masses: Can be measured directly only for stars with a companion; The mass has now been measured for enough stars that we know what it is at each point of the H-R diagram; Values range from 0.08 to more than 100 solar masses (150 for LBV 1806-20).

  Applications: Distances to Stars

• Finding distances to stars: Locate the star on the H-R diagram using only its spectrum, read off L, and compare the apparent and absolute magnitudes to get the distance ("spectroscopic parallax").
• How good is the method? Works if we can see the spectrum well, out to 1000s of pc [with about 10% uncertainty in distances].
• Main sequence fitting: The method is applied to a whole cluster, using apparent luminosity instead of absolute luminosity; Can get the distance from the pattern, and uncertainties are reduced.