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Stars and HR Diagrams Nuclear fusion reaction – – – In essence, 4 hydrogen nuclei combine (fuse) to form a helium nucleus, plus some byproducts (actually, a total of 6 nuclei are involved) Mass of products is less than the original mass The missing mass is emitted in the form of energy, according to Einstein’s famous formulas: E= 2 mc (the speed of light is very large, so there is a lot of energy in even a tiny mass) Hydrogen fuses to Helium Start: 4 protons End: Helium + neutrinos + energy Hydrogen fuses to Helium The Standard Solar Model (SSM) • Sun is a gas ball of hydrogen & helium • Density and temperature increase towards center • Very hot & dense core produces all the energy by hydrogen nuclear fusion • Energy is released in the form of EM radiation and particles (neutrinos) • Energy transport well understood in physics Standard Solar Model Hydrostatic Equilibrium • Two forces compete: gravity (inward) and energy pressure due to heat generated (outward) • Stars neither shrink nor expand, they are in hydrostatic equilibrium, i.e. the forces are equally strong Gravity Heat Gravity More Mass means more Energy • More mass means more gravitational pressure • More pressure means higher density, temperature • Higher density, temp. means faster reactions & more reactions per time • This means more energy is produced How do we know what happens in the Sun? • We can’t “look” into the Sun • But: come up with theory that explains all the features of the Sun and predicts new things • Do more experiments to test predictions • This lends plausibility to theory Details • • • • • • • Radiation Zone and Convection Zone Chromosphere Photosphere Corona Sunspots Solar Cycle Flares & Prominences Understanding Stars • “Understanding” in the scientific sense means coming up with a model that describes how they “work”: – Collecting data (Identify the stars) – Analyzing data (Classify the stars) – Building a theory (Explain the classes and their differences) – Making predictions – Testing predictions by more observations Identifying Stars - Star Names • Some have names that go back to ancient times (e.g. Castor and Pollux, Greek mythology) • Some were named by Arab astronomers (e.g. Aldebaran, Algol, etc.) • Since the 17th century we use a scheme that lists stars by constellation – in order of their apparent brightness – labeled alphabetically in Greek alphabet – Alpha Centauri is the brightest star in constellation Centaurus • Some dim stars have names according to their place in a catalogue (e.g. Ross 154) Classification by Star Properties • What properties can we measure? – – – – – – – distance velocity temperature size luminosity chemical composition mass Classification of the Stars: Temperature Class O B A F G K M Temperature 30,000 K 20,000 K 10,000 K 8,000 K 6,000 K 4,000 K 3,000 K Color blue bluish white white yellow orange red Examples Rigel Vega, Sirius Canopus Sun, Centauri Arcturus Betelgeuse Mnemotechnique: Oh, Be A Fine Girl/Guy, Kiss Me The Key Tool to understanding Stars: the Hertzsprung-Russell diagram • Hertzsprung-Russell diagram is luminosity vs. spectral type (or temperature) • To obtain a HR diagram: – get the luminosity. This is your y-coordinate. – Then take the spectral type as your x-coordinate, e.g. K5 for Aldebaran. First letter is the spectral type: K (one of OBAFGKM), the arab number (5) is like a second digit to the spectral type, so K0 is very close to G, K9 is very close to M. Constructing a HR-Diagram • Example: Aldebaran, spectral type K5III, luminosity = 160 times that of the Sun L 1000 160 100 Aldebaran 10 1 Sun (G2V) O B A F G K M Type … 0123456789 0123456789 012345… The HertzprungRussell Diagram • A plot of absolute luminosity (vertical scale) against spectral type or temperature (horizontal scale) • Most stars (90%) lie in a band known as the Main Sequence Hertzsprung-Russell diagrams … of the closest stars …of the brightest stars