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Physics of Stars ---
Power Sources: How do Stars Shine?
- Review of H-R Diagram (T, L, R, M of stars)
- A Beautiful Balance: Gravity vs. Pressure
- Stars are Nuclear Reactors
Terms to Know
4 forces of Nature (gravity, electromagnetism, strong, weak)
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,
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
- 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:
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?
gas heats up
star ignites nuclear fusion in core
provides pressure support (and makes star shine!)
stops gravitational collapse.
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