It ranks among the most important images ever produced by astronomers. The Hubble eXtreme Deep Field (XDF) showed that an incredibly small patch of sky in the constellation Fornax, thought to have nothing in it, was in fact filled with thousands of distant galaxies, the earliest of which had formed within half a billion years of the Big Bang.
Assembled by combining data collected during a decade by the NASA Hubble Space Telescope, the whole history of galaxy formation and evolution – from the earliest “seedling” galaxies to magnificent spiral galaxies similar to our own and Andromeda – is laid out in that single, spectacular image.
Yet, as the astronomer Jocelyn Bell Burnell has pointed out on several occasions, there is a price to be paid for the clarity of the XDF. Given that the faintest galaxies in the image are one ten-billionth the brightness that the human eye can detect, Hubble had to stare at that small point in the night sky for quite a while; the combined exposure time for the final image is equivalent to some two million seconds – that’s more than three weeks!
“It’s superb stuff,” she says. “That’s what everyone’s been chasing over the last few decades – how deep can we go? Can we see the earliest galaxies? What do they look like? What do the first stars look like? But the main snag about having these very long exposures is that if something changes brightness or something moves, it gets blurred out. You’re not going to see a very rapid change in brightness lasting a second on the final photograph. Equally, if something moves, you lose a lot of the information.”
Or, to put it another way, if you were to take a very long exposure photograph of a set of traffic lights, you’d know the lights were illuminated but you wouldn’t be able to tell the sequence in which they lit up.
Bell Burnell is, famously, the British astronomer who discovered pulsars while completing her PhD at Cambridge University in the late 1960s. The study of transient astronomical events is something that has always interested her, and she’s understandably gratified that the field has become much more mainstream in recent years.
So what are transient astronomical phenomena? “We’re used to seeing the night sky and for it to be pretty constant in its appearance,” says Marek Kukula, Public Astronomer at the Royal Observatory Greenwich. “Of course the sun and the moon change their position against the background stars fairly rapidly on a timescale of days, weeks and months. The planets move against background stars, and you can see that motion on a timescale of days, weeks and months, years. So, to some extent, those are transient phenomena.
“But really, professional astronomers are talking about a class of phenomena which can happen on timescales of seconds, minutes, hours, and days. Sometimes weeks or months, but things that are either a regular event or, more often, one-off events that, when they happen, you need to grab the opportunity to study.”
Some of these have been noted for centuries; for example, stars which were not visible to the naked eye which then become much brighter very suddenly – perhaps in a timescale of hours – and then shone for a few days, weeks, or months before fading away again. We now know these to be exploding stars, supernovae – arguably among the most extreme explosions in the universe.
Flaring can also be caused when a star or planet-sized body passes in front of a much more distant star and, for a brief time, its gravity lenses the light of the background object, causing it to flare and brighten. “We have absolutely no means of predicting when and where they’re going to occur in the sky,” says Kukula, “and when they do occur we know that we’re never going to see that particular object do the same thing again.”
This unpredictability means there are two things you need in place in order to study the dynamic sky. “You need to be scanning a lot of the sky a lot of the time, and then you also need the capability – either with the same telescope or with a complementary telescope – to swing into action and home on these things and study them for that brief period – of perhaps a few hours or days – when its active,” he says.
“This is where the amateur community can really help,” Kakula adds. “There are far more amateur telescopes out there than there are professional ones. And, of course, amateurs live in pretty much every continent – there is somebody looking at the sky pretty much every hour of the day, somewhere around the world, so the chance of picking up something unusual flaring up is pretty good. Amateur astronomers still have definitely something to bring to the table because they’re not computers, and this isn’t data that’s going to be looked at later; this is somebody looking through their telescope now.
“With the internet, there’s this network where people can inform other people, and then not only other amateurs but also professionals can turn their telescopes and start to study. There’s this nice synergy of amateur and professional facilitated by the internet, enabled by the fact that amateur astronomers now have pretty good kit and are really knowledgeable about what to look for.”
But why are astronomers now so interested in these transient phenomena? “From an astronomer’s point of view, transient astronomical events – stellar explosions, gravitational micro-lensing and tidal disruption where one star pulls an object apart, or a black hole or neutron star that has material orbiting it that gets to a certain critical point and disintegrates – tend to be very extreme in terms of the energy and forces involved. The attraction of that for astrophysicists is to be able to observe Nature in extremis – to observe some of the most powerful events in the cosmos, and that tells you something about how the laws of nature, the laws of physics, work.”
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KEEP WATCHING THE SKIES!
1: SUPERNOVAE
“These things are so bright; one supernova can outshine all the billions of stars in its galaxy,” says Marek Kakula. “There are all these great stories about supernova discoveries where people are going: ‘Hang on, that doesn’t look right!’, comparing it to their charts, and going: ‘That wasn’t there before!’”
2: COMETS
Professional astronomers don’t officially classify comets as transient phenomena, but arguably there’s no way anyone can predict (with 100% accuracy) the scale and luminosity of even the most reliably plotted comet on each of its circuits round the Sun. And, of course, a one-off comet always remains a possibility!
3: METEORS & METEORITES
As the explosive Chelyabinsk meteor proved in 2013, meteors can be equally unpredictable! Despite increased observations, we still can’t detect every near-Earth object, so useful information can be gleaned from a meteor’s direction and angle of flight – even colour, which tells us something about the material that’s burning up.
4: CITIZEN SCIENCE
You don’t even need a telescope! Citizen science projects such as Galaxy Zoo – which utilise the huge amounts of visual data from major observation platforms such as Hubble – still rely on our innate pattern recognition abilities to pick up those anomalies that even the best current computer algorithms can miss.
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INTERVIEW
Edo Berger is a Professor of Astronomy at Harvard University.
Why are you interested in transient astronomical phenomena?
My research interests when I started in graduate school were on trying to understand gamma ray bursts which, at the time, very little was known about. The sense of exploring something new in the universe that was very little understood appealed to me from day one. A lot of what we’re trying to do is actually discover new types of objects, and I find that intellectually stimulating.
What difference have technological advances made in recent decades?
People have known about supernova explosions for hundreds of years; they didn’t really understand them, but they knew about them. We now have the ability to build telescopes and faster computers that can process data in real time, and that really allows us to make very rapid progress in understanding these sources.
We’re also in an era of astronomy where we’re not just using visible light telescopes; radio, gamma ray and X-ray telescopes give us a much broader view of the universe that goes beyond the capabilities of our human eyes. For example, if we find a super nova using a visible light telescope, we can then go and observe that object in radio waves, in x rays and in gamma rays and that gives us a much more complete picture of the nature of the explosion, the underlying physics, and what kind of object it was.
Are you excited about the future?
The LIGO gravitational wave detectors will open up a completely new view on finding transient sources and time-domain phenomena. I think that will be a complete revolution in the field; not only will we be able to see these sources through their production of gamma rays, X-rays, light or radio waves, but we’re now going to discover them through their gravitational waves.
First published in BBC Sky at Night Magazine, June 2016.
