Monsters, Dwarfs, and Everything in Between

I went to the third Lowell Series Lecture last night, Monsters, Dwarfs, and Everything in Between. Last week I loved the one on discovering extra-solar planets. This week Dr. Sallie Baliunas of the Harvard Center of Astrophysics talked about different types of stars the largest and smallest and why they are what they are. It's only recently that I started to really understand the relationship between astronomy and the nature of the atom and I'm finding it fascinating. This talk was under an hour and covered a lot of ground so it had to be at a very high level. If you knew this stuff I could see getting bored, but if it was new to you, it's pretty amazing.

She began by reading a bit from an astronomy book of 1912 about what stars are. It said that they were glowing balls of gas and some stars are different colors which probably means different temperatures, that's it. Nothing about why they glow or how. She went back to ancient Greece where the notion that matter is made of atoms came from and Aristotle's four basic elements: fire, air, water, earth. 2200 years later, we basically had the same thing, stars are fire.

The she walked through discoveries at the beginning of the 20th Century about the nature of atoms which could explain what stars are. Max Planck in 1900 found energy is in discrete parts (as opposed to continuous waves) he named quanta. Albert Einstein in 1905 theorized that light is made of photons that act some times like waves, sometimes like particles, and sometimes neither. Ernest Rutherford in 1911 discovered that most of the mass of an atom is in a positively charged core with negatively charged particles around it and a lot of empty space which led to Neils Bohr to publish in 1913 his model of atoms where electrons orbit the nucleus in various energy states and that in changing them emits photons. So in just 13 years we have a model of atoms and how they work and how much engergy is involved in keeping the parts together (positively charged protons should repell each other and negatively charged electrons should be attracted to protons). So what does this have to do with stars?

Stars like the Sun are made of hydrogen and there is a lot of it. Gravity holds all this mass together and the force is strong and builds pressure in the core of the Sun such that the hydrogen undergoes nuclear fusion and creates Helium releasing a lot of engergy which we see as sunlight and feel as heat (it also pushes out from the center, counteracting gravities effort to collapse the star). So what about other stars? Well to get fusion you need 20 million degrees so that's a minimum mass of about 8% of the sun (or 75 times the size of Jupitor). With more mass you get more pressure (more gravity acting) so the fusion goes faster and you run out of fuel and the star dies sooner. At about 130 times the size of the sun the fusion overcomes gravity and the star breaks apart. So those are limits on the smallest and largest stars. When stars exhaust their hydrogen they have helium left. Our sun will ignite the Helium and fuse it into carbon, but it's not large enough to ignite this carbon (you need 8 times the mass of the sun for that). More massive stars can keep this up until they get to iron, which can't fuse into larger elements. Smaller stars like our sun turn into nebulas when they exhaust their fuel, larger ones with iron left will supernova which will turn the iron into the larger elements. Since we don't see that many supernova, it explains why heavy elements like gold and uranium are so rare.

In fact, according to the Big Bang theory, at the beginning there was just hydrogen and helium and all other elements were formed by fusion in stars afterwards. It's why understanding quantum mechanics and how atoms work explains the stars and heavens and everything we are and find on earth. The carbon that's in humans, was formed by previous generations of stars that died and were reformed into new stars and planets. We have theories and formulas that support this explanation and we have conducted experiments whose results match those expected by the theories. Or it could have been that God created everything in six days or any number of other creation myths, I'm enjoying learning about the Big Bang one :).