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Hertzsprung-Russell Diagram The Classification of Stars Why Classify? Use of spectral lines. 1880-1920 Spectral Lines Mercury spectral lines. Spectral Lines • • • • Represent electron “jumps” Specific to each element Used to identify elements present Universal Spectral Lines Most stars have very similar spectral lines indicating the same elements, but the intensity of the lines vary. The various intensities are caused by the energy (temperature) available for the jumps to occur. Classification Based on Spectra Early astronomers (1880-1920) grouped the stars according to the intensity of the hydrogen spectral line. The star with the brightest hydrogen line was classified as a group A star. The stars were grouped according to their intensity from A to P. Modern Atomic Theory The evolution of the modern atomic theory in the 1920’s allowed for an explanation of the spectral lines. The classification system was re-organized to coincide with the temperature required to produce the spectral lines. The H-R Diagram The H-R Diagram is a graph showing the relationship between the surface temperature of a star and the luminosity of the star. Modern Classification Order The original letter classification system was retained for purely historical reasons, but the letters were rearranged. In order of decreasing temperatures, the stellar classification is now: O, B, A, F, G, K, M. Or an easy way to remember them…. Stellar Classification • • • • • • • O oh B be A a F fine G girl K kiss M me New Classification Requires L Class Stars • • • • • • • • Officially Bill Always Felt Guilty Kissing Monica Lewinsky Stellar Classification Each letter classification is further subdivided into ten divisions, 0-9. Our sun is a class G2 star, slightly hotter than a G3, but slightly cooler than a G1. Spectral Class O Surface Temperature (K) 30,000 B 20,000 A 10,000 F 7,000 G 6,000 K 4,000 M 3,000 Prominent Absorption Lines Ionized helium strong; multiply ionized heavy elements; hydrogen faint Neutral helium moderate; singly ionized heavy elements; hydrogen moderate Neutral helium very faint; singly ionized heavy elements; hydrogen strong Singly ionized heavy elements; neutral metals, hydrogen moderate Singly ionized heavy elements, neutral metals; hydrogen relatively faint Singly ionized heavy elements; neutral metals strong; hydrogen faint Neutral atoms strong; molecules moderate; hydrogen very faint Familiar Examples Rigel (B8) Vega (A0), Sirius (A1) Canopus (F0) Sun (G2), Alpha Centauri (G2) Arcturus (K2), Aldebaran (K5) Betelgeuse (M2) Barnard’s Star (M5) Working Independently The Danish astronomer Ejnar Hertzsprung and the American, Henry Norris Russell, devised a method of graphing the stars based on luminosity on the vertical axis and temperature on the horizontal axis. The temperature is read from high to low for historical reasons. A plot of stars within 5 pc of the Sun. Diagonal lines are constant stellar radius. An H—R diagram for the 100 brightest stars in the sky. Such a plot is biased in favor of the most luminous stars. Absolute Magnitude The absolute magnitude is the apparent magnitude we would measure if the star was located at a standard distance of 10 pc from us. Absolute magnitude is equivalent to luminosity. The Main Sequence Most stars seem to fall into a continuous band from the lower right to the upper left. Cool stars tend to be faint, and hot stars tend to be bright. This band is called the main sequence. Stars spend the greatest portion of their lives on the main sequence. The Main Sequence Surface temperatures of main sequence stars range from 3000 K (Class M) to over 30,000 K (Class O) This is a factor of 10. The Main Sequence The stars on the main sequence have a range in luminosity of about 100 million! L is proportional to R^2T^4 The Main Sequence At one end, the stars are big, hot and bright. Due to their color and size they are called blue giants, and the very largest are blue supergiants. At the other end they are small, cool and dim and are known as red dwarfs. The sun is right in the middle. Spectroscopic Parallax Knowledge of star's apparent brightness and distance allows us to determine its luminosity using the inverse-square law. But we can also turn the problem around. Spectroscopic Parallax If we somehow knew a star's luminosity and then measured its apparent brightness, the inverse-square law would give us its distance from Earth. For a star, the trick is to find an independent measure of the luminosity without knowing the distance. The H—R diagram can provide just that. Spectroscopic Parallax A star’s spectrum allows us to identify it’s temperature and luminosity based on location on the main sequence. (It has to be assumed the main sequence is a line for this process to work.) Spectroscopic Parallax If we then measure the amount of energy hitting the earth (energy flux) we can use the inverse square law to determine the distance to the star. Confidence in our measurements. The Principle of Mediocrity Virtually every statement made in this presentation rests squarely on the premise that the laws of physics, as we know them here on Earth, apply everywhere else too, without modification and without exception. Spectroscopic Parallax The process just discussed to find the distance to distant stars is called spectroscopic parallax. The Error of Our Ways The main sequence is not really a line. The sun fits on the main sequence when it has a luminosity of 0.5 to 1.5 the actual luminosity. The reason is due to the age and composition of the star. The Error of Our Ways This correlates to an error in distance of just about 25%. That is a fairly big error, but better than no estimate at all. Luminosity Class But stars can be a giant or a dwarf and have the same luminosity as a star on the main sequence. In this case, the width of the spectral lines is used to determine the luminosity, and gives a quite valid answer. Stellar Luminosity Classes CLASS DESCRIPTION Ia Bright supergiants Ib Supergiants II Bright giants III Giants IV Subgiants V Main-sequence stars/dwarfs Stellar luminosity classes in the H—R diagram Fundamental Properties Mass and composition are fundamental properties of any star. They are set once and for all at the time of a star's birth. They even determine the eventual death process of that star. More than any other factor, mass determines the star’s characteristics. The H-R Diagram Like all good scientific instruments, the H-R Diagram provides astronomers a method for determining the age, distance composition, size, and relative motion of a star just by knowing how to look.