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 = mc 2 (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 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 17 th 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 TemperatureColorExamples O30,000 Kblue B20,000 KbluishRigel A10,000 KwhiteVega, Sirius F8,000 KwhiteCanopus G6,000 KyellowSun,  Centauri K4,000 KorangeArcturus M3,000 KredBetelgeuse 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 O B A F G K M Type … … L Aldebaran Sun (G2V) 160

The Hertzprung- Russell 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