The New York Academy of Sciences used to hold evening lectures that were open to the public. The building was on the Upper East Side, and once in a while I’d attend a lecture with my ice skates in tow. After the talk I’d walk across the street and go skating at Wollman Rink.
One night the speaker was a guy who reviewed requests for “looking” time on the Hubble Space Telescope. He brought some slides taken by the Hubble that hadn’t been generally released. One of these slides showed two galaxies in collision with each other. But the guy explained that they were mostly passing through one another, not colliding. The average spacing between stars is about four light years, and the average distance between galaxies is one million light years. So his explanation seemed reasonable, since galaxies are mostly empty space (which isn’t really empty).
In this particular slide were about a dozen blue-white specs. These were the result of two stars crashing into one another at great speed (for scale: the sun travels at about half a million miles an hour). Sometimes, such a crash results in a new star, which this slide captured. It was a beautiful image, spectacular, and several things struck me immediately: that two galaxies might pass through one another harmlessly; the time scale that must be involved; and that such star crashes could be recorded on film. All of which got me thinking about the motions of single stars, and the formation of stars in general.
Hydrogen is the least complex atom. One proton in the nucleus, and one electron in a probability cloud around it. It’s by far the most common constituent of the universe. It’s the stuff from which stars are created. Empty space is not “empty” at all; it’s permeated with hydrogen, interstellar dust, and traces of other material. This dust comes from exploding and disintegrating stars. Brian May, formerly of Queen, has a PhD in astrophysics; his specialty was interstellar dust. Interstellar dust is the reason the night sky isn’t bright white at night. If you compress and heat the hydrogen and dust sufficiently, it will start to collapse under the force of gravity. Eventually, it’ll ignite nuclear burning, which creates helium (the next most abundant component in a star) and releases energy. This is fusion. Every second, the sun fuses 600 million tons of hydrogen into 596 million tons of helium. The missing four million tons are converted into energy, in accordance with Einstein’s formula. Mass and energy are equivalent, he said, they’re the same thing. For an indication of how equivalent: the bomb used on Hiroshima converted less than one ounce of Uranium-235 into the destructive force of 14,000 tons of TNT. That was a fission bomb, splitting the atomic nucleus and releasing the energy holding it together.
Fusion’s the opposite fission. Instead of cracking a nucleus apart, stars fuse nuclei together, and that also releases energy. Four hydrogen nuclei are forced together to form a helium nucleus. This is the first reaction in a star’s energy production. Our star operates on that principle, and will continue to do so for billions of years. The sun has used up about half its hydrogen supply so far, and has about five billion year’s worth left.
Fusion only happens under extreme temperature and pressure. Even before the Los Alamos test of the plutonium fission bomb in 1945, a few people at the lab understood that the conditions necessary for fusion might be met with the aid of a fission bomb. Pack a bunch of deuterium and tritium (hydrogen isotopes) in a container with a fission bomb, set the timer, and walk away. A massive blast of x-rays, huge internal temperature, and extreme compression of the isotopes. Edward Teller understood this, and so did Stan Ulam and Enrico Fermi. So did some in the USSR, who were very quick to get a deliverable H-bomb; physics doesn’t hold secrets for long. But the critical thinking, in my opinion, was done by Hans Bethe, for explaining the energy production in stars in 1938. He described two possible mechanisms for fusion: the proton-proton cycle for smaller stars like the sun, and the C-N-O cycle bigger ones. Both pathways have the same outcome: releasing binding energy. This work won Bethe the 1967 Nobel Prize in physics. He continued to produce valuable work into his 90s.
The development of the hydrogen bomb proved Bethe correct; it’s little more than a miniature star. Nuclear burning continues through other stages, such as carbon, oxygen, neon, and silicon. Eventually we come to iron, and fusion stops; there’s no more energy to be gained. Depending on the size of the star (the sun is very average) it might become a supernova, or a red giant, or eject its outer layers as a planetary nebula and become a white dwarf, in which there’s no nuclear burning. All the elements as we know them come from stars. Helium was first detected in 1868 by spectrographic analysis of the sun; it was named after Helios, the Sun god, and was not discovered on Earth until 1895.
The energy leaving a star like ours consists of gamma rays, visible white light, x-rays, radio waves, and ultraviolet light. All these are electromagnetic radiations consisting of photons of various energies. A photon from the sun’s core will take eight minutes to come to earth, but might noodle around on a “random walk” within the sun for tens of thousands of years.
Look at the constellation Orion, and notice the three stars making up the belt. Hanging is a sword containing three stars. The middle one seems fuzzy because it’s a nebula, a place where stars are born. It’s number 42 in the Messier catalog, M42, is sometimes visible with the naked eye, and seen easily with binoculars. Maybe I’m just lucky, but it amazes me that the entire known universe is made from less than 100 building blocks, and that those building blocks all start with hydrogen.