Friday, 19 March 2010

The Supernova That Bounces

Here is the transcript for a podcast that I wrote for the 365 Days of Astronomy. If you'd rather listen to it than read it (and I'd recommend that) then feel free to visit the 365 Days website here: The Supernova That Bounces

Supernovae are amongst the most spectacular of all known physical phenomena in the known universe. Pretty much everyone who gets into physics or astronomy in some way, soon takes an interest in these amazing happenings. Essentially, a supernova is a humungous explosion. When I say humungous, I mean really big, in ways it’s hard to wrap our heads around.

Many stars, at the end of their normal life, reach a point where the fuel burning processes within the star can no longer maintain equilibrium with the other forces occurring within the star. As you can imagine, when a giant ball of incandescent gas (i.e. a star) gets to the point where it’s out of balance – something cataclysmic happens.

I’m going to talk about just one of the several types of Supernovae, as we haven’t much time. I’m going to describe for you the before, during and after of a Type II supernova – also called by a much better name: A “Core Bounce Supernova”. To understand why they happen, we need to understand what’s going on in the star before the supernova, and to understand that, we’ll need a little physics – but DON’T PANIC (or indeed switch off)! It’s really quite straight forward.

So let’s start with a star that is at least 8 times the mass of our sun – they’re not hard to find out there, as our sun is quite a modestly sized star really. Now, the reason why stars shine is that they are hot – simple as that. Anything that is a few thousand degrees Kelvin gives off photons that we can see. And the reason why stars are hot is because they are gigantic nuclear reactors. So why do stars burn in this way? When a star is born, it is formed out of a huge, spread out cloud of Hydrogen gas (and maybe a few other elements that are hanging around, but basically Hydrogen). Sometimes these clouds of gas are perturbed by some nearby event (think of wisps of wood smoke rising from a camp fire disturbed by drafts, then scale that thought up billions of times). This disturbance can result in gravity getting a hold, and the giant gas cloud starts to collapse in on itself, shrinking in size until all those Hydrogen atoms start coming into much closer contact with each other. And when that happens, they heat up – the more atoms you have in a certain space, the more those atoms collide with each other and interact. Eventually the pressure and density in the (now spherical) gas cloud reaches a point where the Hydrogen atoms are coming into contact with each other so often, and with so much force, that they actually fuse together – a fusion reaction has started.

Atomic physics is a strange creature indeed if you are not used to it (and when you do get used to it it seems even stranger!). Without going into lots of detail, when two Hydrogen atoms fuse together, they create a Helium atom. But, because one Helium atom has less mass than the two Hydrogen atoms, this missing mass is released as energy. Energy is mass and mass is energy, so said the world’s most famous Swiss Patent Clerk. So every time we fuse two Hydrogen atoms to make one Helium atom, we also liberate energy which is given off as photons.

So, to recap, our young star is burning Hydrogen, giving out large amounts of energy as photons and leaving behind ashes – the Helium atoms. The energy liberated by nuclear fusion heats up the core of the young star and this produces an outward pressure in the core of the star which holds up against the inward force of gravity, so our young star is in a delicate balance, keeping its size and spheroidal shape precisely because of these balancing forces. So, what next? Well, after a long while, when most of the Hydrogen has been burnt into Helium ashes, the star wants to contract because there is less Hydrogen burning going on to keep the pressure up in the core. So the star squeezes down a bit – and guess what, this causes the internal temperature to rise to the point where the Helium atoms, left over from the first stage of burning, can themselves fuse together to give Carbon & Oxygen atoms and, you guessed it, lots of energy output which, yep you guessed it again, serves to hold the star up against it’s desire to collapse in on itself due to gravity, and we’re back in the balancing business. This sort of burning and rebalancing process goes on several more times, with each cycle burning through its fuel faster than the previous cycle, producing ever heavier atoms as the ashes – Neon, Sodium, Magnesium, Aluminium, Silicon, Sulphur, Argon, Calcium, Nickel and Iron. Physicists call this balancing act hydrostatic equilibrium, but don’t be put off by the jargon – it’s just a balancing act between the pressure in the core wanting to bloat the star outwards, and the gravitational attraction of all the stuff in the star which wants to collapse it.

OK, back to our dying star. So, when the star has contracted enough, after a few billion years of burning through the ashes of the previous fusion cycles, the star’s core pressure is immense and the core temperature reaches about 3 billion degrees or so, at which point it can start burning Silicon. This is a very fast process, lasting only about 1 day, and it produces Nickel and Iron. So, if you’re guessing that this process just goes on and on, producing ever heavier elements with each new stage of burning then that would be a good guess, but for one thing: The Iron and Nickel atoms cannot fuse together to produce even heavier elements and liberate energy. Fusion has stopped. The internal furnace of the star has been switched off.

Now, something really wild is about to happen. Remember from earlier that the only thing holding the star from gravitationally collapsing in on itself was the pressure of the internal furnace at the core? Well, the furnace has just been turned off so the star starts to collapse. Eventually the the temperature in the core reaches about 10 billion Kelvin, at which point neutrinos start streaming out of the star, carrying away energy really quickly. One of the defining characteristics of neutrinos is that they carry energy but cannot be physically stopped by almost anything, so not only has the star’s internal furnace been switched off, but also the star starts radiating away its internal energy. This accelerates the collapse of the star even further, and it contracts from the size of the earth to the size of a city in about one second. Now we have a star that is about 1.5 times more massive than the sun but is only the size of a city. That’s a phenomenal density. It’s so dense, in fact, that it cannot collapse any further because it’s already a tightly packed sphere of neutrons and protons. The collapse stops. Stone dead. Don’t forget the neutrinos, though, they’re still being generated in the core, but now the core is so dense that even neutrinos can’t get through – the temperature therefore rises in the outer part of the core and nuclear fusion kicks off again in a very big way – such a big way, in fact that no amount of gravity can contain it and the outer envelope of the star explodes away at 10,000 km per second. So the core is left behind as a hyper-dense object (a black hole or a neutron star), and the outer layers detonate, giving off huge amounts of energy, much of it in the visible spectrum and thus the supernova shines many thousands of times brighter than the star.

It’s worth a pause at this point to reflect on this star that is a giant factory for producing elements. If you are listening to this podcast on an aeroplane, then many parts of your plane will be made of Magnesium and Aluminium alloys. The actual atoms around you and of you were made in the nuclear furnaces of stars aeons ago. I think that fact alone is amazing. To mangle a famous quote from Dr Johnson - “If you are tired of knowing you came from stardust, you’re tired of life!”.

Saturday, 6 June 2009

The Things That Google Does

Now that Bing's the thing, I have stumbled across some Google capabilities that I didn't know existed. I have been using Google as my search engine of choice for many, many years but I had absolutely no idea it could do things other than search. So, without further ado, here's a list of ways to have fun with Google ...

Its a calculator ...
Type (1+1)/0.5 into the search box, or even 36% of 134, or 2^8.
It Tells You the Weather Forecast ...
Type weather:PO29JY, or weather London.
It Converts Units for You ...
Type 100 parsecs in light years, 1 kg in pounds etc.
It's a Capitalist's Tool ...
Type stock MSFT, or even just a stock code e.g. VOD.
Don't wake up Your Relatives Abroad ...
time adelaide
Speak to Your Relatives Abroad ...
Answer the Ultimate Question ...
Type this into the search box: the answer to life the universe and everything

I'll bet there are many other useful features. Let me know if I've missed something!

Wednesday, 13 May 2009


While you are reading this blog, and in fact while you are doing most things on your computer, its Central Processing Unit (CPU) is doing next to nothing. It's probably using only about 1%-5% of its computational ability. Wouldn't it be great if you could put the CPU's 'idle time' to good use? You won't be surprised to learn that you can - and here's how it works.

Academic research groups around the world frequently need access to very powerful supercomputers to perform very long calculations, or to run short calculations millions of times under slightly different conditions. Supercomputers are relatively few and far between - it's difficult to book time on them, and expensive if you can.

This is where BOINC comes in. From your point of view, it's a program that runs on your computer that allows scientific computation programs to run when your CPU would otherwise be idle. That's about the nub of it: You and about a million others donate your idle CPU time to academic research groups.

There's also a social side to it. You can choose which projects you want to use your idle CPU cycles. There are currently about 50 active projects, ranging from advanced cosmology simulations to computer experiments on protein molecule folding dynamics to give insight into disease such as cancer. All of the active projects have flourishing forums where you can chat to like-minded people (you're all there because you have decided that project is worth supporting). There are also "crunching teams" you can join (e.g. BOINC UK), which themselves have active forums and usually have competitive "crunching drives" on the go - often against other teams in a spirit of, mostly, friendly competition.

Nice idea - how do I get started?

Simply download and install the BOINC software from the link below, then choose your projects.

BOINC - Compute for Science

BOINC always needs new contributors. I've been BOINCing for years - I hope you get into it as well, it's very satisfying being able to provide practical support to projects you believe in.

Some links to further reading ...

BOINC homepage at UC Berkeley
BOINC on Wikipedia

Some of my favourite projects ...


Thursday, 30 April 2009

In a Galaxy Far, Far Away ...

... or "GRB 090423 (And Why it Matters)"

This is a detective story concerning the explosion of a star in a galaxy nearly at the edge of the universe, the tale of how it was discovered, and also a few words on why it is an important discovery.

At 07:55:19 UT on 23rd April 2009, the Swift satellite detected a faint Gamma Ray Burst (GRB) using its Burst Alert Telescope (BAT), determining its direction in the sky to within about 3 arcminutes (a rough estimate by modern astronomical standards, but accurate enough for other telescopes and instruments to do follow-up work). The BAT has a large field of view (2 steradians, approximately the same as a single human eye), and is designed to quickly detect gamma ray bursts over a large area. Within seconds, the Swift satellite slewed around to point directly at the burst, and its on-board X-Ray Telescope (XRT) began observing the field at 07:56:31.8 UT, 72.5 seconds after the BAT trigger. The field of view of the XRT is 23.6 arcminutes square. It can record follow-up data as well as localising the source. The XRT found a faint, and fading, source of X-Rays coming from the same direction as the gamma-ray burst. The plot thickens. Swift also carries a 30cm mirror telescope capable of observing in UV and optical wavelengths (UVOT), which points in exactly the same direction as the XRT. It found, on this occasion, absolutely nothing. The plot thickens further - there's a gamma-ray and X-ray source out there but it's not giving off much (if any) ultraviolet or optical light.

The excellent design of Swift gives us a hierarchical way to observe these bursts: Fast detection and rough position calculation (BAT), followed by X-Ray photometry, spectroscopy, and precise localisation of the source (XRT), and UV/visible photometry using UVOT. Meanwhile, through a carefully choreographed and automated communications system, the operators of ground-based telescopes are alerted to the burst. In the case of GRB 090423, a mere 108 seconds after the burst was first detected by the BAT, the 2m Faulkes Telescope South began observing at visible wavelengths in the direction of the burst. Nothing was found. The plot thickens yet again - any optical source would have to be fainter than magnitude 20. About a minute later, the mighty Palomar 60-inch telescope got much the same result - nothing out there unless it is fainter than about magnitude 21.

About 15 minutes later, the 3.8m UK Infrared Telescope (UKIRT), perched atop Mauna Kea on Hawaii, at a lung-busting altitude of over 4000m, was commanded to stare at the source using its huge infrared eye. Bingo! Source found - faint but clearly detectable and not previously seen. The object seen was a point source (so not a big diffuse cloud of glowing gas and dust) shining faintly upon us at a magnitude of about 18 in the mid infrared range (2.2 micron wavelength).

The plot is now getting positively gloopy: Some point source has started spewing out gamma-rays, x-rays and infrared light - but no visible light that we can detect. We now have some very excited astronomers.

The universe is expanding in all directions, which is to say that the distance between galaxies is getting bigger all the time. The further away an object is from us, the faster it is receeding away from us. A side effect of all this extragalactic fleeing is that the light from far away objects is Doppler-shifted towards the red end of the spectrum (the wavelength of the light is stretched out, becoming longer). In fact, if an object is far enough away from us, then it will be receeding away from us very quickly indeed, and any visible light it shines towards us can be red-shifted right out of the optical range and into the infrared range of wavelengths. Spectroscopic measurements taken by ESO's Very Large Telescope (VLT), now give us a redshift for our mysterious point source of 8.2. This is the furthest redshift yet measured, and it tells us that the object is about 13 billion light years away.

Whilst I'm sure you'll agree that this cosmic detective story has so far been gripping enough, in and of itself, we're actually only just getting to the really interesting part. Because light travels at a finite speed, any light we observe from very far away left its source a long time ago - we are looking at old photons that have travelled a long distance. In the case of our mystery object, we are, today, observing light that was produced 13 billion years ago - only 600 million years after the big bang. Whilst 600 million years seems a long time to us humans, it is actually in the infancy of the universe. If the universe were a human, GRB 090423 erupted forth when the universe was a 4 year old toddler. According to Derek Fox at Pennsylvania State University:
"The burst most likely arose from the explosion of a massive star. We're seeing the demise of a star -- and probably the birth of a black hole, in one of the universe's earliest stellar generations."
That is why GRB 090423 matters - it is tantalising evidence that we may be on the right track with our theories of how stars are born, evolve, and sometimes die a violent gamma-ray bursting death. Even in the very early universe.

Some links to further reading ...

The Swift Gamma Ray Burst Explorer Mission
"Gamma-ray burst." Wikipedia, The Free Encyclopedia
The Electromagnetic Spectrum
GCN Circulars
United Kingdom Infra-Red Telescope (UKIRT)
New Gamma-Ray Burst Smashes Cosmic Distance Record (NASA)
The European Southern Observatory's Very Large Telescope array (VLT)
Faulkes Telescope Project

Yogi Stereogram

Here's an image I processed of the famous "Yogi" rock on Mars. The original images were taken by the Mars Pathfinder lander. Go cross-eyed whilst looking at the centre of the image to get the 3d effect.
This is my first ever stereogram and it's made using exactly the same image for both sides of the "stereo input pair", so it isn't the best stereo image I have seen. I think I'll have some more fun with this later and learn how to make good ones.
Thanks to these guys for the free software. And also to NASA et al for spending truly vast quantities of cash in order to take pictures of far-far away rocks (and other science stuff, of course).

And now for something completely different ...

... A test blog. Not all that different, then.