What’s in a Nova?

There’s a new star in the sky, or rather one that’s on the order of ten thousand times brighter than it was a few days ago.

There’s a new star in the sky, or rather one that’s on the order of ten thousand times brighter than it was a few days ago. A “new” star like this one is known as a nova. National Geographic, among other websites, has an article with details about the nova in the constellation Delphinius, and is offering a live view of the Nova Del 2013 starting Thursday evening as I write this. It’s bright enough to be seen with binoculars, and perhaps by a sharp-eyed person at a dark location, and will take some time to fade.

But what is a nova? And how is it different from a supernova? Or a hypernova? Or a dwarf nova? I have to confess that before I studied astronomy formally, I found this a confusing subject and could have used a handy guide. Here’s one for you.

Let’s start with novas. Novas are stellar explosions of a kind, except an entire star doesn’t explode, just a layer of gas on the star’s surface. The star itself is not a star like our own sun, but a kind of dead star called a white dwarf, that is just the fuel-exhausted core of a star that was once like our own sun but lost its outer envelope. Furthermore, novas do not occur in isolated systems, but only in binary systems where another star is available to dump hydrogen onto the surface of the white dwarf. Over time, this new hydrogen fuel is pulled by the immense surface gravity denser and denser, hotter and hotter, until the conditions trigger nuclear fusion. When that happens, the entire surface effectively explodes. Here’s what a system like this might look like prior to the nova event:


Now, this can happen repeatedly in the same system over time, giving us so-called recurrent nova that can and are predicted.

What about the more popular supernovas? Well, they are much more luminous than mere novas and can rival a galaxy in their luminosity, come in two classes (and a variety of subclasses, too). One of the classes, cleverly labeled “type 1,” is thought to arise in systems very similar to novas. That is, a white dwarf in a close binary with a normal star like our own sun. The difference is that the gas accreted from the sunlike-star doesn’t lead to a nova explosion, but leads instead to an increase in mass and eventually a collapse due to gravity. When this happens you get a crapload (a technical term equivalent to about 1.4 solar masses) of nuclear reactions as everything is fused into neutrons. The entire white dwarf star blows up in an epic event.

Something similar happens in the type II supernovas, which are very massive stars in which the core collapses under its own weight after an accelerated evolution and without need of outside assistance. It’s interesting, at least to me, that it’s a collapse that triggers an explosion (driven by a flood of neutrinos flying out of the core and into the surrounding envelope of gas).

A hypernova is a kind of extreme supernova associated with gamma ray bursts. These events are thought to happen in the most massive stars with the most massive cores collapsing into black holes. Just as a spinning figure skater pulling in his arms accelerates his rotation rate, collapsing cores spin up and can form jets. These jets emerging along the spin axis according to some complicated physics move at relativistic speeds and beam radiation preferentially along the jet direct. When those jets point in our direction, we see something that looks more powerful than a mere supernova.

There’s another class of dwarf novas that are also seen. These are the babies of the nova world, and again, ironically enough, are associated with close binaries consisting of a white dwarf and a sunlike star. In this case, the brightening isn’t an explosion at all, but an instability in the accretion disk feeding the white dwarf. The disk itself brightens for a period of time as it heats up, only later falling back into a quiescent phase. A dwarf nova system (SS Cygni) was the setting of my first novel Star Dragon.

There’s a whole zoo of strange stellar variability and ways for things to explode in space.

While there’s a lot to learn about all the subtle details of these events, let me confess that even as a professional astronomer the appeal to me is mostly this: explosions are fun to watch!

Enjoy the nova.

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