Last Wednesday my friend Sharon and I got together to write. We usually write for 20 minutes and then read what we’ve written. Sharon had a page of a local paper to show me. It talked about a star which would be going nova soon. So I wrote about what I had learned as an Astronomy major in college. After I got home, I checked the web and discovered that T Coronae Borialis goes nova every 80 years. The word “nova” means “new” in Latin, and centuries ago, an exploding star would suddenly appear in the sky and observers would think it was a new star because they didn’t know it was an old one exploding.
Here’s what I wrote:
We were looking at the newspaper. They’ve been watching a star that will go nova soon. They said it will be as bright as the North Star — which isn’t hugely bright but should be noticeable. It will be near the constellation Corona Borealis, Northern Crown, which is easy to find from the Big Dipper. What I would like to know is how they know it’s getting ready to explode. Something about how it’s siphoning off hydrogen from a nearby star. I suppose the mass of hydrogen will condense and eventually create a thermonuclear explosion, which is generally how stars start. A cloud of hydrogen gas condenses, and the density and temperature rise as it condenses. Eventually the temperature reaches one million degrees and sets off an explosion which is a source of electromagnetic radiation. The radiation is absorbed and re-emitted until it emerges at the surface of what is now a star, as electromagnetic radiation of a whole spectrum of wavelengths, some of which are visible to human eyes. … and there was light. And it was evening and it was morning, the first day. Of course, that’s only if you’ve got a planet to observe from. If I were describing our sun, it would be a long time before the planet cooled enough to have hard rock floating on the surface. On our planet, continents of hard rock float on a core of molten rock.
Once upon a time, I had an astrophysics textbook where a chapter began “In the beginning there was hydrogen.” My mind went on to “and the hydrogen was without form, and void, and darkness was upon the face of the deep…” and I went on to write something that I called Genesis II.
If what we’re describing is an ordinary star becoming a nova, conditions are somewhat different. The star is also burning and putting out some light. If the star was small to begin with, like our sun, it will one day use up its fuel and die. If it’s a bigger star, when enough hydrogen has burned into helium, which has been gathering at the center of the star, the helium will get denser and hotter until it does what the hydrogen did before it, explodes, and the whole star expands into a red giant with two sources of energy: hydrogen burning into helium, and helium burning into lithium. Depending on the initial mass of the star, it will go on burning, with successive layers burning into beryllium, boron, carbon… or at some time it will blow up. In fact, it’s very necessary for it to blow up, so the original hydrogen becomes seeded with more heavy elements. A second generation star, like our sun, has a different history and can even develop planets. Eventually planets will produce life.
Note that there would be no life on Planet Earth if some earlier star had not produced carbon, oxygen, nitrogen, and then blown up and scattered them into the primordial hydrogen, so that the second generation stars would condense out of clouds containing all these other elements.
The story of the star in Corona Borialis is that it is a white dwarf paired with a red giant. The white dwarf is very heavy and sucks hydrogen gas off the red giant. Eventually, after 80 years or so, the amount of hydrogen gas becomes enough to explode away from the dwarf, creating the bright nova spectacle. It disappears after a few days, and the cycle starts again.