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.

Wednesday, December 4, 2013

A third pointer to the Southern Cross

For Australians and New Zealanders, the Southern Cross is an icon of identity.  The stars of the cross adorn our flags, and the constellation, together with its two bright pointers, has hung always visible in our southern skies. It has been a constant and unchanging beacon to indigenous peoples, to European explorers, and to all modern Australasians.

Until this week.

For a few nights only, there is a third pointer to the Southern Cross.  A new quite bright star has appeared very close to Beta Centauri, the topmost of the two pointers, effectively creating a third pointer to the Southern Cross.    This star is what is known as a “nova” (from the Latin for “new”).  In fact, an existing star – a very faint and hitherto undistinguished star invisible in any but powerful professional telescopes – has, over a period of mere hours, brightened spectacularly.  Last night, it was 25,000 times brighter than it was on Monday, and it’s still getting brighter.

The nova was discovered in the early hours of Tuesday morning by John Seach, an amateur astronomer from Chatsworth Island in NSW.  John regularly scans the skies for novae and supernovae, using a nothing more elaborate than a DSLR camera with a wide-angle lens.  He already has several discoveries to his name.  On Tuesday morning, it was still only barely visible to the naked eye, but 24 hours later near dawn on Wednesday it had become as visible as in the image above.

The study of novae is extremely important to astronomers.  A lot of the physics that underpins the formation, evolution and behaviour of star happens at such colossal temperatures and gravitational forces that they cannot possibly be duplicated in a laboratory.  Therefore, a lot of stellar physics is theoretical.  However, when a nova erupts, astronomers get a chance to watch the physics at work inside stars in real time.  Therefore, they closely monitor the emissions from stars in nova – visible light at all wavelengths, X-rays, infra-red – to build up a picture of what is occurring within.

In fact, stars that “go nova” are part of a binary system – two stars in close orbit around each other.  One of the pair is a normal star, much like our sun; or perhaps a bit larger and redder.  The other is what is known as a “white dwarf” star.  White dwarfs are extraordinarily small and dense – as much mass as our sun, packed into the size of a planet.  At these densities, matter becomes “electron degenerate” – the electrons become stripped from their nuclei, which float together in an ultra-dense soup.  One teaspoon of degenerate matter has a mass of several tonnes.  These two stars whirl around each other in an orbit that takes mere hours.

In nova systems, the gravitational force of the white dwarf continually pulls matter (mostly hydrogen) away from the larger star.  This matter gradually spirals down onto the surface of the white dwarf, where it accretes, and is compressed and heated.  Pouring hydrogen onto the surface of a white dwarf star is a bit like pouring petrol on a barbecue - eventually a runaway nuclear fusion reaction starts in the accreted hydrogen, which explodes cataclysmically and blows the hydrogen and other accumulated gases out into space at thousands of kilometres per second.  By examining the spectra of novae in outburst, we can determine the chemical composition of the gases hurtling into space; the temperatures of the reactions occurring; and much else of scientific interest.  By measuring the Doppler shift of the spectrum, we can even determine how fast the gas is being blasted into space.  So in a real sense, novae are astronomers’’ practical laboratories for observing stellar physics in action.

If it behaves as a typical nova, Nova Centauri 2013 might continue to brighten for another day or (if we are lucky) several days, before beginning a similarly rapid fade back into obscurity over the following weeks and months.  At this time of year the Southern Cross and its pointers rise a decent distance above the horizon in the hours before dawn.  So if you want to grab a look at the third pointer to the Southern Cross you’ll have to get up early over the next few days.