She Blinded Me With… A Nova?

Be careful whose advice you buy, but, be patient with those who supply it. Advice is a form of nostalgia, dispensing it is a way of fishing the past from the disposal, wiping it off, painting over the ugly parts and recycling it for more than it’s worth.

But trust me on the sunscreen. –Baz Luhrmann

Our Sun gives us practically all of the light and energy our planet receives, and it does it, at its core, by fusing light elements into heavier ones.

And even though this nuclear fus releases a tremendous amount of very high energy photons (known as gamma rays), the photons that reach us are mostly visible light, much lower in energy.

Why? Because passing through the entirety of the Sun, from the core all the way to the surface, takes tens of thousands of years and trillions upon trillions of collisions, knocking the energy down in the process.

But we get plenty of gamma rays on Earth; they just don’t usually originate from the Sun. In the physical world, there are plenty of ways to make them, including radioactive decay, smacking some antimatter into normal matter, and by accelerating charged particles fast enough in strong enough magnetic fields. In space, gamma rays are made near pulsars, black holes, and in the tremendously violent deaths of massive stars: supernovae!

But what about the supernova’s non-super little brother, the nova? Unlike a supernova, which destroys the entire parent star, a regular nova is much more restrained. When a star like our Sun fuses all of the light elements that it can, what’s left over is a solid core of that nuclear ash: things like carbon, nitrogen, and oxygen in the case of our Sun. Without sufficient mass, temperature, and pressure to fuse these elements further (into things like Silicon and Iron), the leftover matter from a used-up star contracts down to around the size of the Earth, giving off a small amount of light.

The leftover, dim collection of ultra-dense atoms is known as a White Dwarf star, and is the eventual fate of our Sun.

But despite their tiny size, they’re often just as massive as a full-blown Sun-like star, which makes them excellent at gathering up interstellar gas and dust. This is why white dwarf stars have thick atmospheres.

Over time, these atmospheres get thicker and thicker, and the pressure at the surface gets higher and higher. For many white dwarf stars — the ones that can collect enough matter — something spectacular happens. If you get the pressure of your (mostly hydrogen gas) atmosphere to be high enough, you can ignite nuclear fusion again! And this tremendous release of energy is what a nova is!

Novae aren’t all that violent, when you look at the numbers. Despite being thousands of times brighter than a star like the Sun, that brightness usually falls off quickly after a month (or a few). The white dwarf star only loses about one-thousandth of 1% of its mass. And after a few thousand years, enough matter will fall back onto the star to make it go nova again.

But one thing we’ve never seen before from a nova? The highest-energy form of light there is: gamma rays. Until, that is, right now.

Image credit: Fermi Space Telescope.

The gamma-ray telescope, Fermi, was observing, when Nova Cygni 2010 went off in March. The “before” picture, on March 10th, is to the left. There are gamma rays visible here, but look at the after picture on the right, 19 days later. There’s a brand new source of gamma rays, corresponding perfectly with the location of that nova!

This is the first time ever that gamma rays have been associated with a nova, a surprise for both observers and theorists! So how do these novae, with their relatively mild energy releases (for astrophysics, at any rate) manage to make gamma rays?

They must be making magnetic fields much stronger than anyone anticipated. To me, it’s absolutely an amazing bit of science, because it’s a shining example of one new observation forcing everybody to rethink how they “know” these things work.

So can you make gamma rays with a nova? Hell yes! Just don’t forget your sunscreen!