1. Star Formation and the Interstellar Medium
OBJECTIVES:
Know the phases of star formation starting with the interstellar cloud.
Understand how the life cycle of both low and high mass stars contribute to the makeup of the universe.
Know some of the particular stages of the life of low and high mass stars.

3. The Interstellar Medium
The interstellar medium of the milky way consists of clouds containing 90% Hydrogen, 9% Helium, and 1% dust grains.
It has an extremely low density of 1 atom per cm3, about 10 billionth the density of gas created by our best vacuums here on earth.

4. Interstellar clouds
Interstellar clouds: dense regions where stars may form.
They are not uniform, some areas are denser and collapse faster.
The collapsing cloud fragments into dense star forming cores called molecular cloud cores.

5. Protostars
Protostars: gravitational energy is being converted into thermal energy, and material falls on the accretion disk.
The protostar is thousands of time more luminous than the star it will form, and hundreds of times larger.

7. From Protostar to Star
A balance between gravity and pressure is always maintained.
As a protostar radiates out thermal energy it shrinks, and the center becomes more dense.
As more material falls on the protostar it contracts, temperature and pressure in the core climb until fusion of hydrogen begins.
A star is cable of nuclear fusion.

8. Star Clusters
A closely knit collection of stars, such as Pleiades (seven sisters).
It would take approximately 30 million years for the collapse of the molecular cloud to form a star similar to sun.

9. Low-Mass Stellar Evolution
Our sun is typical low-mass star with a size of 1 M (M stands for Mass of the sun)
Low mass stars are 3M or smaller.

10. Luminosity of a Star
Luminosity is determined by the amount of fuel that it is burning.
Our sun fuses 4 billion kilograms of hydrogen per second.
In some 5 billion years from now the sun’s life will come to an end.
The more massive a star the shorter its life time, since increasing the mass/gravity increases the rate of nuclear fusion.

11. Subgiant
When a low mass star fuses the majority of its hydrogen, it begins to expand and cool forming a subgiant.

13. Red Giant When the star can cool no further it becomes a red giant.
Helium builds up in the core creating a degenerate helium core.
As the remaining hydrogen fuses, in the H-burning shell, the Helium core grow in density but not in volume.

14. Asymptotic giant branch (AGB)
Helium begins to fuse in the core creating carbon-12 mass amounts of alpha particle radiation.
This carbon core drives up the strength of gravity and pressure in the core.
The radius may now be 100 of times an typical 1 M star.

15. White Dwarf
Within 50,000 years a post-AGB star burns all fuel on its surface leaving a tiny cinder of carbon with a remaining mass of less than 70% of the original star.
White dwarfs have immense gravity, some with the mass of sun and a volume of the earth.

17. Planetary Nebula Planetary nebula may form around low-mass dying stars
Form when the mass ejected by the AGB star piles up in a dense expanding shell.
Planetary nebula are visible for about 50,000 years of so, and can be illuminated by a white dwarf.

18. nova
A possible fate of a white dwarf that is part of a binary star system.
A white dwarf’s immense gravity can pull matter from a companion star forming an accretion disk around the white dwarf.
More material more gravity, more gravity the white dwarf shrinks, temperatures increase and any hydrogen gas explodes in a nova.
This cycle can repeat itself many time.

20. Type 1 supernovae
Through millions of years of mass transfer and countless nova outbursts, the mass of the white dwarf increases.
There comes a point when the pressure can no longer balance gravity and the temperature of 6 X 108 K fuse carbon nuclei up to element 26 Iron (Fe).
At that point the energy of fusion is liberated in an explosion that sends the material across the cosmos destroying the white dwarf.