Supernovae

I went (via Zoom) to a great lecture last night. Serafina Nance spoke to the The Calgary Centre of the Royal Astronomical Society of Canada on Tracing the Lives, Deaths, and Explosions of Massive Stars.

Supernovae are cosmic events of gigantic power. Their explosions can shine as bright as a galaxy, a pinprick of extraordinarily bright light in the night sky. What is less well-understood, however, is which stars reach the point of explosion and how they evolve to their deaths. Interestingly, their explosions provide astronomers with key tools to uncover fundamental aspects of our Universe. While we know that the Universe is expanding at an accelerated rate due to dark energy, the rate of the expansion of the Universe is not well-constrained. Supernovae provide us with independent ways to measure this expansion and work to resolve one of the most pivotal questions in astronomy: How fast is the Universe really expanding?

There are several types of Supernovae, but this talk was focused on Types Ia and II. Type Ia are binary stars, with at least one partner being a white dwarf. If matter is ejected from the other partner onto the white dwarf, the mass of the white dwarf will increase. If this keeps up long enough the mass of the white dwarf will eventually approach the Chandrasekhar limit (about 1.4 Solar masses), beyond which a white dwarf cannot exist. A runaway nuclear fusion reaction will result causing the white dwarf to explode as a supernova. Since all such supernovae start as objects of the same mass, the supernovae will then be of about the same absolute brightness. Thus a Type Ia supenova is a “standard candles” and can be used to estimate the distance R to the galaxy it is in. We can determine the velocity v at which the galaxy is moving away from us by its redshift. Hubble’s Law, aka the Hubble–Lemaître law, v = H0R, can then be used to determine H0, the Hubble constant, which expresses the rate at which the universe is expanding.

So far, so good, but there is another way of determining H0, from the cosmic microwave background. For a while, given the uncertainty in the measurements involved, they seem to agree. However, in recent years both techniques have improved in precision, to the point that their error bars do not overlap and a Dispute over a Single Number Became a Cosmological Crisis.

Ms. Nance, soon to be Dr. Nance, studies Type II Supernovae. These are massive stars that explode by a different mechanism. Eventually the core of the star runs out of Hydrogen to fuse into Helium, so it starts fusing Helium into Carbon and Oxygen. Then it runs out of Carbon …… until it is left with Iron. This is a dead end. You cannot fuse Iron nuclei into something heavier and get energy out. In fact, you have to add energy. So without anything to fuse there is no source of heat energy to support the core and the outer regions of the star collapse inward. The core then heats up again and produces a supernova explosion.

Type II Supernovae can come in any sufficiently massive size, and so their explosions will be of differing brightness. So they cannot directly be standard candles for the universe. However, Ms. Nance hopes that further study will reveal some properties which will allow them to be used to measure cosmological distances, providing an alternative measurement of H0. I am very curious about what that might be.

Going back to Type Ia supernovae, in the Q & A after the talk somebody mentioned Sirius, which is a binary system with a white dwarf. It is only 8.6 light years away, so if Sirius B became a supernova all life on Earth would be killed and the planet would become uninhabitable. Fortunately, that is very unlikely. Sirius B is only about 1.02 solar masses (I checked), well below the Chandrasekhar limit. It is far enough away from Sirius A that there is no noticeable accretion of mass. Even when Sirius A enters its red giant phase, about half a billion years in the future, Sirius B probably will not capture enough mass to explode.

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