Astronomy 100


Lectures Table of Contents Astro 100

Physics of Stars ---
Power Sources: How do Stars Shine?


  1. Review of H-R Diagram (T, L, R, M of stars)
  2. A Beautiful Balance: Gravity vs. Pressure
  3. Stars are Nuclear Reactors

Terms to Know

4 forces of Nature (gravity, electromagnetism, strong, weak)
binding energy
mass-luminosity relation
mass-lifetime relation

 1. Stars are Nuclear Reactors

Nuclear fusion is the process of combining (or fusing) light nuclei (such as hydrogen nuclei, which are just single protons) into heavier nuclei (such as helium). It is the energy source of stars! Fusion is the opposite of fission, the process used in nuclear energy generators here on Earth; in fission, heavy elements are broken apart to form light elements.

The stars, including the Sun -- as well as most of the Universe -- are made up of around 75% H and 25% He.

The end product of fusion (e.g., a helium atom) is a tiny bit lighter than the ingredients that go into the process ( why? ). This miniscule mass is converted into pure energy according to Einstein's famous equation, E=mc2. The energy escapes in the form of photons (light) and neutrinos, which are like light but which don't interact strongly with matter (and they also probably have a tiny bit of mass, which photons don't).

The amount of energy released per reaction is very small: 4.3 x 10-5 ergs (1 erg = 1 mosquito hitting your forehead). But the huge number of fusion reactions taking place in a typical star every second makes the total luminosity very large.

The two common processes for converting H to He are the "pp chain" (proton-proton) and the "CNO cycle" (carbon, nitrogen, oxygen).

Note that fusion is totally different from chemical reactions such as combustion (fire), which are merely breaking electromagnetic bonds between molecules or between atoms in molecules. Fusion is much more powerful and requires much hotter temperatures -- millions of degrees K.

In fusion, the strong force of physics comes into play. Like charges -- such as two protons -- repel each other via the EM force, but if you can get them close enough together, the strong force will "latch on" like Velcro and bind them to each other.

How do you get them that close together? Heat them up so they fly around very fast and crash into each other! That's what the gravity of stars does, and that's why stars need to be hot to work.

2. Review of H-R Diagram (T, L, R, M of stars)

  • The Hertzsprung-Russel Diagram is a plot of temperature T (or color) vs. luminosity L (or absolute visual magnitude MV -- don't confuse with mass, also denoted M!) of stars.
  • But it also contains information on radius R and mass M, as well as the lifetimes and relative numbers of stars! All stars, including the Sun, can be plotted somewhere on the H-R Diagram.
  • The masses of stars are measured from binary systems (more than half the stars you can see with your naked eye are double stars!) -- it's like weighing our Sun using the Earth's orbit.
  • Most stars, including the Sun, appear on the Main Sequence in the H-R Diagram. On the Main Sequence, stars appear to obey a Mass-Luminosity relation:
    L M3.5
    The more massive a Main Sequence star is, the hotter, bluer, and more luminous!
  • The spectral types of stars are related to their temperature (or color).

3. A Beautiful Balance: Gravity vs. Pressure

Stars are balanced! Two forces are exactly equal and opposite in most stars: Gravity pulls in (like the rubber of a balloon) while pressure pushes out (like the air inside the balloon).

The force of gravity at the center of a star is immense, billions of times greater than at the bottom of the deepest ocean trench on Earth. How can a star possibly support that weight?

Gravity compresses gas gas heats up star ignites nuclear fusion in core releases energy provides pressure support (and makes star shine!) stops gravitational collapse.

Lectures Table of Contents Astro 100

Houjun Mo Astronomy 100