Stellar Spectra Relativity and Astrophysics Lecture 12 Terry Herter Outline What is a star? Stellar Spectra Kirchhoff s Laws Spectral Classification Spectral Types: O B A F G K M L T Stellar Photometry A2290-12 Stellar Spectra 2 A2290-12 1
What is a star? A2290-12 Stellar Spectra 3 What is a star? Stars shine at night (during the day too!). A star is a self-luminous sphere of gas. Stars are held together by gravity. Thermal energy from nuclear reactions, hydrogen fusion powers stars We use a telescope with a spectrograph to measure and record the spectrum of a star. This gives us the brightness of the star at different wavelengths. Almost all stars show a continuum spectrum with absorption lines. Some stars show emission lines. All stars do not have the same spectrum! A2290-12 Stellar Spectra 4 A2290-12 2
Examples of Stellar Spectra Energy Wavelength OR A2290-12 Stellar Spectra 5 Stellar Spectra The spectral (absorption) lines we see in stars are very important. The missing photons give us info on: Chemistry Temperature Density Kirchhoff s laws tell us about the region which gives rise to the spectrum. A2290-12 Stellar Spectra 6 A2290-12 3
Kirchhoff s Laws These are three laws that govern the spectrum we see from objects. They allows us to interpret the spectra we observe. 1 A hot solid, liquid or gas at high pressure has a continuous spectrum. 2 A gas at low pressure and high temperature will produce emission lines. 3 A gas at low pressure in front of a hot continuum causes absorption lines. A2290-12 Stellar Spectra 7 Continuum Spectrum Hot Gas Emission Line Spectrum Cold Gas Absorption Line Spectrum A2290-12 Stellar Spectra 8 A2290-12 4
1 A hot solid, liquid or gas at high pressure has a continuous spectrum. Intensity Wavelength There is energy at all wavelengths. A2290-12 Stellar Spectra 9 2 A gas at low pressure and high temperature will produce emission lines. Intensity Wavelength Energy only at specific wavelengths. A2290-12 Stellar Spectra 10 A2290-12 5
3 A gas at low pressure in front of a hot continuum causes absorption lines. Intenstiy Wavelength Dark lines appear on the continuum. A2290-12 Stellar Spectra 11 Formation of Absorption Lines in a Star Photons Star See this Photosphere: Continuum Source Outer layers are Cooler -- Absorb Photons Like Kirchhoff s 3 rd law A2290-12 Stellar Spectra 12 A2290-12 6
Example Stellar Spectrum H H H H... 4000 5000 6000 7000 (A) The Hydrogen-Balmer spectrum (in the visible) is present for most stars. A2290-12 Stellar Spectra 13 Classification of Stars In the late 19th century astronomers categorized stars according to the strength of the hydrogen absorption lines in the spectrum. They labeled these A, B,... from strongest to weakest. Unfortunately, this was the wrong way to do it! Annie Jump Cannon arranged the spectra of stars in a sequence which corresponds to the temperatures. Classified over 500,000 stars! The spectral sequence is a temperature sequence: O, B, A, F, G, K, M hotter ------------> cooler. A2290-12 Stellar Spectra 14 A2290-12 7
Spectral Sequence = Temperature Sequence Hotter Cooler O B A F G K M Violet Blue Yellow Red Henry Norris Russell (1920): Oh, Be AFine Girl (Guy), Kiss Me Each of these class (O, B, etc.) can be subdivided into tenths, i.e. G0, G1,... G9, K0, K1,... K9 (G0 is hotter than G9) The Sun is a G2 star. A2290-12 Stellar Spectra 15 O5V B5V Spectral Sequence A5V F5V G4V K5V M5V 350 nm 750 nm A2290-12 8
Stellar Spectra Na A2290-12 Stellar Spectra 17 Changes to spectral sequence For the first time in over 100 years the spectral sequence has needed another letter. Very low temperature stars discovered with infrared surveys of the sky. Now have L and T stars! Hotter Cooler O B A F G K M L T Violet Blue Yellow Red A2290-12 Stellar Spectra 18 A2290-12 9
Stellar Photometry & Colors It is not necessary to measure the entire spectrum do determine the spectral peak of a star. We can use color filters to determine the colors of a star. By this we mean how much flux is seen in each color filter. A green filter transmits only green photons. A2290-12 Stellar Spectra 19 The UBV Filter System Transmission 1 U B V U at 3500 A => ultraviolet B at 4300 A => blue V at 5500 A => visible (yellow) 0 3000 5000 7000 A Wavelength A set of color filters which give coarse spectral information. A hot star will have more flux in the U filter than the V filter compared to a cool star. A2290-12 Stellar Spectra 20 A2290-12 10
Colors of a hot star vs. a cool star 1 Hot Star Cool Star Energy 0 U V 3000 5000 7000 A Wavelength flux at U flux at V Hot Star flux at U flux at V Cool Star M O A2290-12 Stellar Spectra 21 Absorption Line Information High Temperature Stars: Lines of Ionized Atoms Intermediate Temperature Stars: Lines of Neutral Atoms Low Temperature Stars: Molecular Lines A2290-12 Stellar Spectra 22 A2290-12 11
Strength of Lines vs. Spectral Class Ionized Helium Neutral Helium Hydrogen Ionized Metals Neutral Metals Molecules Strengths of Lines O B A F G K M Na A2290-12 Stellar Spectra 23 The picture is not yet complete. The spectral type (class) of a star gives us temperature information, but we don t know its luminosity. Must know distance to star And we don t know the mass Look at stars orbiting one another. A2290-12 Stellar Spectra 24 A2290-12 12