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 = mc2

(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 GravityGravity

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 Temperature Color Examples

O 30,000 K blue

B 20,000 K bluish Rigel

A 10,000 K white Vega, Sirius

F 8,000 K white Canopus

G 6,000 K yellow Sun, Centauri

K 4,000 K orange Arcturus

M 3,000 K red 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

O B A F G K M Type… 0123456789 0123456789 012345…

1

10

100

1000

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