This week, three unexpected astronomical discoveries. The first relates to white dwarfs. A star like our sun is argued to eventually run out of hydrogen, at which point its core collapses somewhat and it starts to burn helium, which it converts to carbon and oxygen, and gives off a lot more energy. This is a much more energetic process than burning hydrogen to helium, so although the core contracts, the star itself expands and becomes a red giant. When it runs out of that, it has two choices. If it is big enough, the core contracts further and it burns carbon and oxygen, rather rapidly, and we get a supernova. If it does not have enough mass, it tends to shed its outer matter and the rest collapses to a white dwarf, which glows mainly due to residual heat. It is extremely dense, and if it had the mass of the sun, it would have a volume roughly that of Earth.
Because it does not run fusion reactions, it cannot generate heat, so it will gradually cool, getting dimmer and dimmer, until eventually it becomes a black dwarf. It gets old and it dies. Or at least that was the theory up until very recently. Notice anything wrong with what I have written above?
The key is “runs out”. The problem is that all these fusion reactions occur in the core, but what is going on outside. It takes light formed in the core about 100,000 years to get to the surface. Strictly speaking, that is calculated because nobody has gone to the core of a star to measure it, but the point is made. It takes that long because it keeps running into atoms on the way out, getting absorbed and re-emitted. But if light runs into that many obstacles getting out, why do you think all the hydrogen would work its way to the core? Hydrogen is light, and it would prefer to stay right where it is. So even when a star goes supernova, there is still hydrogen in it. Similarly, when a red giant sheds outer matter and collapses, it does not necessarily shed all its hydrogen.
The relevance? The Hubble space telescope has made another discovery, namely that it has found white dwarfs burning hydrogen on their surfaces. A slightly different version of “forever young”. They need not run out at all because interstellar space, and even intergalactic space, still has vast masses of hydrogen that, while thinly dispersed, can still be gravitationally acquired. The surface of the dwarf, having such mass and so little size, will have an intense gravity to make up for the lack of exterior pressure. It would be interesting to know if they could determine the mechanism of the fusion. I would suspect it mainly involves the CNO cycle. What happens here is that protons (hydrogen nuclei) in sequence enter a nucleus that starts out as ordinary carbon 12 to make the element with one additional proton, which then decays to produce a gamma photon, and sometimes a positron and a neutrino until it gets to nitrogen 15 (having been in oxygen 15) after which if it absorbs a proton it spits out helium 4 and returns to carbon 12. The gamma spectrum (if it is there) should give us a clue.
The second is the discovery of a new Atira asteroid, which orbits the sun every 115 days and has a semi-major axis of 0.46 A.U. The only known object in the solar system with a smaller semimajor axis is Mercury, which orbits the sun in 89 days. Another peculiarity of its orbit is that it can only be seen when it is away from the line of the sun, and as it happens, these times are very difficult to see it from the Northern Hemisphere. It would be interesting to know its composition. Standard theory has it that all the asteroids we see have been dislodged from the asteroid belt, because the planets would have cleaned out any such bodies that were there from the time of the accretion disk. And, of course, we can show that many asteroids were so dislodged, but many does not mean all. The question then is, how reliable is that proposed cleanout? I suspect, not very. The idea is that numerous collisions would give the asteroids an eccentricity that would lead them to eventually collide with a planet, so the fact they are there means they have to be resupplied, and the asteroid belt is the only source. However, I see no reason why some could not have avoided this fate. In my ebook “Planetary Formation and Biogenesis” I argue that the two possibilities would have clear compositional differences, hence my interest. Of course, getting compositional information is easier said than done.
The third “discovery” is awkward. Two posts ago I wrote how the question of the nature of dark energy might not be a question because it may not exist. Well, no sooner had I posted, than someone came up with a claim for a second type of dark energy. The problem is, if the standard model is correct, the Universe should be expanding 5 – 10% faster than it appears to be doing. (Now, some would say that indicates the standard model is not quite right, but that is apparently not an option when we can add in a new type of “dark energy”.) This only applied for the first 300 million years or so, and if true, the Universe has suddenly got younger. While it is usually thought to be 13.8 billion years old, this model has it at 12.4 billion years old. So while the model has “invented” a new dark energy, it has also lost 1.4 billion years in age. I tend to be suspicious of this, especially when even the proposers are not confident of their findings. I shall try to keep you posted.