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.

Saturday, November 16, 2013

Staying power

Recently Alan Kerlin posted Peter Treyde's photo of M41 on the CAS blog which instantly brought back memories of when I first started out with Astronomy. It was 1986, I was 19, and we all had Halley fever ... I had my first job, and my first scope soon followed: a Vixen 5" f/5 Newtonian on a very simple GEM mount. I used to run into M41 while looking at Sirius - the unexpected ones are always better I even tried some astrophotography with my dad's old SLR (no 'D') - somewhere I have some pink smudges that were my proud attempts at M42 and Eta Carina.

I used the scope obsessively for a couple of years, then it gradually drifted into uncollimated disrepair. It came with me for ten years in the UK, and saw a lot of light pollution and not much else. But basically for the last 25 years has essentially been a spider breeding station and less of a light bucket and more of a rain bucket. It has been variously under the house, in the roof cavity, and outside on the verandah for the past 15 years. I just couldn't bring myself to throw it away. I always vowed that one day I'd return to astronomy.

So, last year I finally got the opportunity to get set up with some functioning equipment - an SCT 10" on a decent mount, with a guiding setup and a Canon 60Da. At the same time, out of nostalgia, and while waiting for my new scope to arrive (it actually arrived the day Patrick Moore died, 9/12/2012 - that's another story) I decided to clean up my old Vixen. I pulled it out, cleaned up the structure, replaced all the rusted screws, cannibalized the old 0.95" fittings to build something that would accept my new 1.25" eyepieces and camera adapter, and sent the mirrors off for recoating.

Earlier this year, while learning the ropes with the new state-of-the-art kit, I suddenly on a whim threw the old 5" Vixen on to my new mount and attached the camera. This image of the Horsehead was the result - 20 x 4 min subs. And you know, it's not half bad, especially for a dear old scope that cost $300 in 1986.

So now the old scope comes with me to Mt Stromlo public nights and is operated by my 15yo son while I run the 10" SCT. He had at least a hundred fascinated kids looking at lunar craters in September.

Not a bad way to have spent my first ever paycheque, all things considered.

Sorry for the rambling story. It just all came flooding back thinking about M41.

Monday, September 30, 2013

EV Ceti

I spent Saturday night watching an eclipse.

The faint (11th magnitude) star EV Ceti is in fact two stars, very close together, that orbit each other in less than a day. We can't see this directly - they are far too close. But we can measure the consequences. Mostly, the light we see comes from both stars in the binary pair.  But every ten hours or so one passes in front of the other and the total light drops.

On Saturday I took one 50s image each minute for four hours, and captured and measured the brightness of the star.  And of course duly submitted the observations to the venerable American Association of Variable Star Observers.

Sunday, August 11, 2013

Music and Walking Pace

 Marching to a different tune: researchers unlock the motivational power of music

  This is the text of my article for The Conversation published there on 12 July 2013.

 “Music has charms to soothe a savage breast, to soften rocks, or bend a knotted oak”, wrote William Congreve in 1697.  We take this for granted, and therefore often tend to overlook how genuinely mysterious it is that simple patterns of sound vibrations can have profound effects on our minds and bodies. This power of music to on the one hand soothe, but equally to energise, has long fascinated musicians and philosophers. Most recently, psychologists and neurophysiologists have turned their attention to music, and have sought to measure, and explain in empirical terms, how music can have so much influence our moods and levels of energy.

A Belgian study published today has shed some further light on how this might work. Marc Leman and colleagues at the Institute for Psychoacoustics and Electronic Music and Ghent University analysed the effects of listening to different pieces of music on the walking speed of 18 adults.  Researchers have long known that people will synchronise their steps with the tempo of music – after all, this is why we have marching bands.  So for this study, the researchers chose 52 pieces of instrumental music with contrasting moods and styles, but exactly the same musical tempo – 130 beats per minute.

Sure enough, almost all the participants stepped in time with the music.  What was more interesting was that certain pieces of music caused the participants to walk more energetically – to take larger strides, and cover a larger total distance – while other pieces caused the opposite effect.

For the record, the piece of music that created the most vigorous walking in the study was:

While the gentlest response was to:

After the walking test, the participants were then asked to rate the pieces they listened to in terms of opposed pairs of adjectives: was the piece good or bad? Stuttering or flowing?  Tender or aggressive?  Soft or loud?  Unsurprisingly, the participants walked with more of a spring in their step to music rated as stuttering, loud, or aggressive, while gentler, softer, flowing or more complex music had a relaxing effect.

This effect appeared to be independent of musical genre: the list of the most arousing music included classical, techno, world music and house, while the top ten most relaxing pieces ranged from Baroque solo viol music to contemporary Korean dance tracks.  The effect also seemed to be independent of the participants own musical preferences – the music had the observed affect whether or not the participants liked that particular style or genre.  Leman and colleagues speculate that this musical effect on the vigour of physical response might happen at an autonomous or subliminal level.  This suggests several possible practical applications of the research, for instance in sports performance or physical rehabilitation.

What makes this study relatively unusual is that the researchers then analysed these objective cognitive results in terms of a sophisticated music theoretical model.  They were attempted to discover exactly what the musical features were that were associated with the arousing or relaxing effects.  Nearly 200 sonic features of each piece were analysed – the loudness of various parts music, the sharpness of the attack, the structure of the beats, the distribution of pitches and so on – and this musical analysis was then correlated with the results of the walking experiment.  Surprisingly, only a handful of features were shown to cause arousal and relaxation, and these all had to do with the regular structure of the rhythm, which musicians call “metre”.  Put simplistically, music with a march-like rhythm (“binary metre”) causes more arousal, while music with a waltz-like rhythm (“ternary metre”) causes greater relaxation.

So when you next see a batsman stride to the crease or a boxer enter the ring to the blare of aggressive, pounding motivational music, it’s more than just theatre.  There’s a genuine physiological effect at work.  And it might be that the Blues’ best chance in next week’s State of Origin decider will be to make sure the Queensland team runs out to the sound of the Blue Danube Waltz. 

Thursday, March 14, 2013

Four curiosities

Here are four slightly random images that I'm posting simply because they are slightly quirky, and I am rather fond of them.  The first is the Eskimo Nebula, an example of a rather particular type of nebula called a "planetary nebula".  The name is a misnomer: these objects have nothing to do with planets.  They are the remains of stars much like our sun, that die not with a bang but a whimper.  Instead of the fate of larger stars, which end as novae or supernovae, massive explosions and/or collapse into neutron stars or black holes, smaller stars like the sun simply puff their mass out into space as a series of cosmological death-rattles.  These puffs form a sphere of gas that expands outwards from the star over thousands of years.  They appear as small coloured discs - easily mistaken for planets.  Hence the name.  This particular nebula, the "Eskimo", NGC2392, is a rich blue colour not unlike Neptune.  It's the first example of its type I have photographed.

The next photo is f the Tarantula Nebula.  This is not strikingly exceptional as an emission and diffusion nebula - the (slightly colour enhanced) red elements are glowing hydrogen, while the blue/white sections are gas reflecting the bright light of internal stars.  The one really unusual aspect of this object is that it is outside our own galaxy, in the Large Magellanic Cloud.  It's actually a massive object, and if it were in our own galaxy - say, at the same distance as the Orion Nebula - it would take up half the sky and glow as bright as the moon.

The next is an image of a real neighbour.  This is Jupiter with her four brightest moons.  Actually this is a composite of two images - no single exposure can capture the detail of the bright planet as well as the faint moons, so I had to do a bit of Photoshop jiggery-pokery.

Finally, here is a fuzzy incomplete image of galaxy NGC1365 in Fornax.  For various reasons (clouds, trees, my own inexperience) I've only been able to collect 30 mins of total exposure time where it really needs about four times that much. The reason I'm posting it now is because last November a star in this galaxy (56 million light years away) went supernova.  It was quite bright last year, but is beginning to fade.  The star is still visible - it's the blue dot marked with the arrow.  The galaxy is starting to get too low in the Western sky to shoot properly - I'll image it again next year but by then the supernova will be gone.

A matter of perspective ...

This is my favourite astrophoto yet. I took it last week- 16 exposures for a total f 90 minutes. It is Centaurus A, a large, quite close galaxy that has its unusual shape owing to the fact that it is currently in the process of swallowing a smaller spiral galaxy.  At its heart is a supermassive black hole with a mass 100 million times that of the sun.  That sort of size puts things in perspective.

The other issue of perspective, and the reason I really like the photograph, is that you can easily see the foreground stars - the ones in our own galaxy.  The sense of distance is quite acute. 

I say it's a close galaxy, but I'm speaking cosmologically. It's around 13 million light years away, meaning the photons which hit the sensor of my DSLR left Centaurus A and began their long journey to earth at pretty much the same time as this chap, a Sivapithecus, came down out of the trees and on to the African grasslands, and began the complex process of trial and error that led to the tool-use and then ultimately to the invention of the DSLR ... 

The Jewel Box

As a child in the 1970s, I loved astronomy and space. It wasn't until I was 20, in 1986, that I was able to get a telescope. If you were alive then, and at all interested in what might rain down from above, ;-) you will remember that 1986 was all about Halley Comet fever. So in April 1986 I finally got a telescope - a modest 5" Newtonian - and with my reference materials (a newspaper article on Halley, a planisphere, and a Patrick Moore book on astronomy that I'd had for years) pointed my new treasure eagerly at where Halley was supposed to be.

Amazing! The eyepiece was filled with a glorious shperical burst of light. I couldn't believe what I was seeing:

 I looked harder at the image - interesting that the diffuse cometary halo semed to resolve into individual points of light ... And no sign of a tail, but then I had heard that this visit of Halley was disappointing. What I was seeing was anything but disappointing. But the longer I looked, the less it looked like what a comet was supposed to look like.

Well, eventually I realised that I was a few degrees off, and what I was looking at was Omega Centauri. Wonderful it was! I was looking for this comet, but was the sky really full of unexpected treasures? I started to slew the scope around (in those days, "slew" was something we did by hand) and came across something that literally took my breath away. There was a little pocket of gems in the sky that Aladdin had left behind. A sparkle of red, blue and green (I'm SURE I remmeber green!) stars in a tiny little treasure-chest just below the Southern Cross:

It was the Jewel Box, and it hooked my into the delights of the sky in a way that has lasted a lifetime.

So, nearly 30 years later, when I finally fulfilled a lifetime's ambition and acquired a new telescope and a camera, and after three months of technical bedding in during which I finally got autoguiding working, there was only one object I could possibly photograph. This little box of jewels.

Meade 10" SCT, F/10, with a Canon EOS 60Da. Next time I'll go for it with my focal recucer to F/6.7, which will enhance the sense of this little treasure of gems secretly buried in the deep south of the sky.

10x2' exposures, ISO 800.

Thanks for reading this far in the nostalgic musings of a lifetime star addict :-)