What Are Stars?

• History: In the Middle Ages, stars were sometimes thought to be "holes" in the surface of the celestial sphere, that let light from behind shine through. Around 1600, Galileo saw stars in the Milky Way and Thomas Digges suggested that stars are distributed in 3D space, and Giordano Bruno that they are like distant suns. It was only in the 1800s that astronomers were finally able to see the first stellar parallax and spectra.

• What we now know: A star is a body that (at least for some time in its life) generates light and heat by nuclear reactions that fuse hydrogen into helium, under conditions of enormous temperature and density. We can use our knowledge of the Sun to help us understand other stars, and the fact that we see different stars at different stages in their lives to understand their evolution.

How Far Are They?

• Parallax: Remember its importance for the issue of whether the Earth moves; Using the Earth's orbit as baseline, we can see parallax for stars out to a few hundred ly, with angles always less than 1"!
• New distance unit: 1 parsec = 3.3 ly [= 206,000 AU = 33 trillion km], the distance at which the parallax of an object would be 1".
• Nearest stars: The closest is Proxima Centauri, at 1.3 pc = 4.3 ly; then Barnard's star, at 1.8 pc = 6.0 ly; There are about 300 within 30 ly (mostly dim, and new ones were reported even in 2006, about half of which in binary or multiple systems), and 250,000 within 250 ly.
• Motion: Stars move, and their proper motion can be seen directly, but it is extremely slow (arcsec/year); The first who noticed this was Edmund Halley, by comparing his star charts to ancient Greek ones. Radial motion (10s of km/s) can be found from the Doppler shift.

  Luminosity and Brightness

• Apparent brightness and magnitude: The power received per unit area, or apparent magnitude m (introduced by Hipparchus in the 2nd cy BC, it ranged from 1 to 6 for stars visible with the naked eye).
• Luminosity and absolute magnitude: L is the total power emitted; The absolute magnitude M is the apparent magnitude they would have if they were at 10 pc from us.
• Examples: The Brightest one is Sirius [alpha Canis Majoris, m = –1.5, M = 1.4], the second brightest Canopus [alpha Carinae, m = –0.72, M = –3.1]; [the Sun has m = –27 but M = 4.8!].

• What distinguishes one star from another? Mainly temperature and luminosity (size is related to those quantities); And to some extent, chemical composition.
• Temperature from spectrum: Look at the wavelength of the brightest part (or the absorption lines); example of the Sun. (One can also get it from the color index, which is not affected by the star's velocity.)
• Temperature range: It 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 labelled by letters A, B, C, D, E, F, etc, were introduced. It then turned out that those classes corresponded to temperatures, and that the correct order was O-B-A-F-G-K-M (each from 0 to 9), in order of decreasing T.
• Examples: The Sun, at 6000 K, is a G2 star; [Betelgeuse 3000 K, M2; Arcturus 5000 K, K2; Procyon 7000 K, F5; Sirius 10000 K, A1; Rigel B8; others 30,000 K].
• The HR diagram: A plot of luminosity vs temperature; L ranges from about 1/10,000 to 100,000 times the Sun's luminosity, T from about 3000 to 50,000 K. A clear pattern emerges.
• Types of stars: The Main Sequence, where most stars lie; It includes blue supergiants, giants, Sun-like stars, red dwarfs. Other Liminosity classes include supergiants (Ia & Ib), giants (II, III), IV, Main sequence (V), below the MS, white dwarfs

• New method to find distances to stars: Locate the star on the HR diagram using only its spectrum, read off L, and compare the apparent and absolute magnitudes to get the distance ("spectroscopic parallax"). This works if we can see the spectrum well, in practice out to 1000s of pc.