Monday, December 9, 2013

Nova Centauri 2013: a crucial 24 hours

My long-suffering wife is clearly bemused why I keep getting up at 3.00 am to "go and look at the nova".  So I thought I'd try to explain some of the excitement.

I posted in some depth about the nova here.  So the short version is simply that a small, incredibly dense star -a white dwarf - has staged a gigantic runaway thermonuclear explosion, blasting clouds of hydrogen and other elements into space at incredible speed, while pumping enormous energies at visible wavelengths, X-ray and gamma ray wavelengths.  This started last Tuesday, nearly a week ago.  On Saturday it reached its peak brightness, and it's now fading fast.

This star is quite close to the top pointer to the Southern Cross - so invisible to most (Northern hemisphere) variable star enthusiasts.  The upshot is, I and a handful of Australian, New Zealand, South African and South American observers have been hauling ourselves out of bed to measure and observe the thing. 

A few of us have rudimentary spectroscopes - devices that can spread the white light of the star into its constituent spectrum. It's a simple filter which screws into the front of the camera.  With this one can measure the intensity of each wavelength of light, from deep violet into the red and infra-red.  You can see this at the bottom of the photo below:


The graphs simply plot the intensity of each wavelength of light.

So what does this tell us?  The key to the whole process is to understand that, for some fascinating quantum-physical reasons, particular elements radiate and absorb light at particular wavelengths.  For instance if hydrogen is heated or bombarded with ultraviolet or otherwise energised, it will radiate light at a very specific set of wavelengths - 4861 angstroms, (in the light blue part of the spectrum);  6562 angstroms (in the red zone); and others.  Other elements emit light at different wavelengths.  So, by analysing the spectra of glowing clouds of gas, we can work out their chemical compositon.

The same effect works in reverse.  If a cloud of cool hydrogen is in between us and a bright light source (like an exploding white dwarf star), the gas will absorb light at the same wavelengths.  This also applies to other elements and compounds.  For instance, the big dip in the far right of the spectra above, labelled "Telluric", has nothing to do with the nova: these are the wavelengths absorbed by the water vapour and oxygen in the Earth's atmosphere.

The red line in the graph above shows the nova's spectrum when it was at its brightest, on 6 December.  The red line is the spectrum a day later.  An awful lot has changed; as you'd expect given that the star is in the process of exploding. 

First, and perhaps least interesting - we can see what sort of elements the star is flinging into space.  This particular nova is a "Helium-Nitrogen" nova; slightly less common than the "Iron" novae.  We can tell because we see particular wavelengths glowing that correspond to ions of these elements.  Uninteresting - but it's pretty cool that I can measure the chemical composition of an exploding star 25,000 light years away from the comfort of my own balcony in Canberra. 

Next, we can build a picture of exactly what's happening to the star through comparing successive spectra.  First, let's visualize the explosion.  A cloud of gas has been blasted away from the surface of the star, and this cloud is then being bombarded by high energy waves from the star, casing the gas to glow.






This is a photo the Hubble Space Telescope took of a nova that exploded in 1992.  The ejected gas has expanded to the point where we can actually see it.  In the current nova in  Centaurus, the gas is far too close to the star for us to see it.  But we can observe it with our spectrographs.

As you can see, the gas is moving away from the star, and glowing owing to the intense energies it is being bombarded with,  This causes "emission lines"in the spectra.  You can see in my spectra from 7 December (the blue line in the graph) that the hydrogen line at 6562 A has begun to glow much more than was the case even 24 hours before (the red line in the graph). This accounts for the peaks in the spectra - the energetic gas expanding outwards from the central star.

But remember that elements also absorb energy.  Part of the expanding, glowing sphere of gas is coming straight towards us, and being "backlit" by the intense energy of the star itself.  In this case, the high-intensity light from the star itself is actually absorbed by the expanding gas cloud.

Now here's the best bit.  In the circled part of the red graph, you can see both the emission lines from the expanding cloud of hydrogen (the peak), and the absorption line that comes from the part of the cloud directly between us and the star, attenuating its energy (the trough).   These are at exactly the same wavelength - 6562 Angstrom.  But hang on - if both the emission and the absorption are at the same wavelength, why don't they just cancel each other out?

The answer is is the Doppler effect.  The emission lines come from the gas that is moving out at a right angle to us - so is neither getting closer nor further away.  But the absorption line comes from the gas coming directly towards us, so the emission line appears bluer than it is, because the energy of the velocity at which the gas is moving towards us is added to the energy of the light waves.  It's the same principle that causes the engine tone of a car moving towards us sounds higher, while a car moving away sounds lower.

By measuring the shift between the emission lines and the absorption lines, we can calculate how fast the gas is moving towards us.  The emission line is at 6562, while the absorption line is at about 6505.  6505/6562 = 0.99131.  Thus, the gas is moving towards us at 1-0.99131 = 0.00868 times the speed of light.  The speed of light is 299 792 458 m/s, so the velocity of the expanding gas cloud is 299 792 458 * 0.00868 = 2,604 km/s.  Which is about right for this type of nova; but bloody fast if you think about it.

4 comments:

  1. The red line of Dec 7 is blue, I gather. Very enlightening. Keep getting up to fill us in!

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