Stars
Star field taken with Hubble Space Telescope
Hertzsprung–Russell diagram
Pleiades
Star
• Massive, luminous ball of plasma
• Held together by gravity
• Shines due to thermonuclear fusion of
hydrogen in its core
Betelgeuse
Spectra of stars
Allow astronomers to determine a star’s:
• Composition
• Luminosity
• Velocity
• Mass
Spectrum of the Sun
3 types of spectra
1. Continuous- produced by a glowing body
2. Absorption line (dark line)- produced when a
cooler gas lies between observer and glowing
body – this is the type used to ID stars
3. Emission line (bright line)- emission from a
glowing gas, used to study nebulae
Star classification
• Stars are all made of the same material
• Spectral differences are due to temperature
Coolest stars are red, hottest stars are blue
Classification system
• O B A
• O hottest
• M coolest
F
G
K
M
H R (Hertzsprung Russell) Diagrams
Stars plotted according to:
1. Luminosity (absolute magnitude) – brightest
stars at the top
2. Temperature (spectral class) – hotter stars to
the left
H R Diagram (cont.)
1. Main sequence
– Band from hot (blue) to cool (red)
– 90% of stars
– e.g. the Sun
2. Giants
– Luminous and cool
– 10-100x size of the Sun
– e.g. Aldebaran in Taurus
3. Super Giants
– 1000x larger than the Sun
–
e.g. Betelguese in Orion
Antares in Scorpius
4. White Dwarfs
– Very hot, but low luminosity due to small size
– e.g. Sirius B
– NO fusion in core, glow due to contraction?
Gravitational forces
Binary star
• http://www.astronomy.ohiostate.edu/~pogge/TeachRes/Movies162/#spbin
• http://www.calvin.edu/~lmolnar/anim/algol.html
Albireo (the Cal star)
Stellar Evolution
Fusion
Carbon-carbon fusion
supernova
http://www.pbs.org/wgbh/nova/origins/progra
m-3114.html
http://www.youtube.com/watch?v=8CtjhWhw2I8
Stellar Evolution
• Stars do not live forever
• Eventually nuclear fuel runs out and star dies
Stages of Stellar Evolution
1. Pre-main sequence
– Stars form in a dense, cool cloud of dust and gas
– Gravitational attraction causes it to start to glow
– NO fusion yet
 Protostar
• Once fusion starts the star enters the main
sequence