1. Composition of Stars
Classify stars by their color, size, and brightness. Other properties of stars are chemical composition and mass.
Color and Temperature – a stars color indicates the temperature of its’ surface.
The hottest stars appear blue
The cooler stars appear red

2. Each star has its own spectrum.
Most stars have a chemical makeup that is similar to the sun, with hydrogen and helium together making up to 96 to 99.9 % of a star’s mass.

3. How Stars Form
A nebula is a large cloud of gas and dust spread out over a large volume of space.
Some nebulas are glowing clouds lit from within by bright stars.
A star is formed when a contacting cloud of gas and dust becomes so dense and hot that nuclear fusion begins.

4. Nucleosynthesis = formation of elements Except for H, He
(created in Big Bang),
all other elements created
by fusion processes in
Stars.
FUSION CAUSES
NUCLEOSYNTHESIS!!
Relative abundance

5. Stellar Nucleosynthesis Some H destroyed; all elements with
Z > 2 produced
Various processes,
depend on
(1) star mass
(determines T)
(2) age (determines
starting composition)
Z = no. protons, determines element

6. Life Cycles of Stars
Adult Stars – A star’s mass determines the star’s place on the main sequence and how long it will stay.
The amount of gas and dust available when a star forms determines the mass of each young star.
The larger the star the more energy produce.
Since blue stars burn brightly, they use up their fuel quickly and are short lived

7. The Death of a Star
The dwindling supply of fuel in a star’s core leads to the star’s death as a white dwarf, neutron star, or black hole.

8. Expanding Universe
The observed red shift in the spectra of galaxies shows that the universe is expanding.
Astronomers theorize that the universe came into being in an event called the big bang.
Dark matter can only be detected by observing its gravitational effects on visible matter.

9. Stellar lifecycles: from birth to death
low mass
star (< 5 Msun)
high mass
star (> 5 Msun)

10. Stellar lifecycles: low mass stars
Stellar nucleosynthesis
2. Main Seq.
3. Red Giant
low mass
star
1 & 5.
molecular
cloud
4. Planetary nebula
4. White dwarf
Nucleosynthesis possible
if white dwarf in binary system
(during nova or supernova)

11. Stellar lifecycles: high mass stars
Stellar nucleosynthesis
2. Main Seq.
(luminous)
3. Red Giant/
Supergiant
1 & 6.
molecular
cloud
high mass
Star
5. Neutron star
4. Supernova
5. Black hole

12. Track stellar evolution on H-R diagram of T vs luminosity
Luminosity: energy / time

13. Molecular clouds: Where it begins & ends

14. Molecular clouds Mainly molecular H2, also dust, T ~ 10s of K
cold, dense areas in
interstellar medium (ISM)
Horsehead Nebula
Mainly molecular H2,
also dust, T ~ 10s of K

15. Famous Eagle
Nebula image.
Cool dark clouds
are close to hot
stars that are
causing them to
evaporate.

16. End for high mass star comes as it tries to fuse core Fe into heavier elements– and
finds this absorbs energy
STAR COLLAPSES & EXPLODES AS SUPERNOVA

17. Supernova remnants

18. Crab Nebula
supernova
remnant.
A spinning
neutron star
(pulsar) occurs
in the central
region.