The most luminous event in the universe May 19, 2008Posted by Peter Hornby in astronomy.
Something astonishing happened in the sky earlier this year, something unique in human experience. I’d like to talk a little about what this event was, and why it was so remarkable.
Let’s start with some background. Since the late sixties, astronomers have been recording, analyzing – and puzzling over – enigmatic objects called gamma ray bursts. Gamma rays are the most energetic form of electromagnetic radiation, so when we see something shining at gamma ray wavelengths, we know that something extraordinarily powerful is behind it. The gamma ray burst (GRB) is a flash of gamma rays, typically lasting no more than a few seconds, coming from a place in the sky where nothing had previously been recorded. As they fade, GRBs normally show an afterglow at X-ray, ultraviolet or visible wavelengths.
For decades, no visible light counterpart – a star or galaxy – could ever be found, but the suspicion was that, since the distribution of GRBs on the sky showed no relation to the structure of our galaxy, they must be extragalactic, and consequently extraordinarily luminous. This suspicion was confirmed in 1997 when a faint galaxy was found at the location of a GRB.
We’re now in the midst of a revolution in GRB study, dating from the launch of the SWIFT orbiting observatory in late 2004. SWIFT combines a sensitive gamma ray detector with the ability to rapidly slew its other instruments to the location of a burst within seconds. Not only that, there’s now an amazing network which permits co-operating ground-based observatories, both professional and amateur, to be activated, totally automatically, within seconds of SWIFT’s detection of a GRB.
So why I am I so excited? Well, the July “Sky and Telescope” magazine arrived this morning, with a brief analysis of a March 19 GRB detected by SWIFT. What’s totally amazing is that the visible-light afterglow of this event was, for a few seconds, bright enough to be visible to the unaided eye. This struck me as the most astonishing event and I threw a few calculations together to get a sense of just how unbelievable it really was – what’s actually meant when we describe this event as the most luminous single event ever seen, anywhere in the universe.
So, let’s start with some numbers. The March 19 event, called GRB 080319B, occurred in a galaxy 7.5 billion light years away. The galaxy itself can only be seen by the most rigorous imaging techniques. By contrast, the galaxies we can see with our backyard telescopes are only tens of millions light years away – the famous Andromeda galaxy (M31) is just over 2 million light years away and it’s about 60 million light years to the huge Virgo cluster of galaxies, a favourite telescopic target for amateur astronomers in the spring sky.
That’s million, by the way, not billion. At 7.5 billion light years, GRB 080319 B was a long, long way away.
So, how bright was it?
Let’s digress a bit to discuss the way astronomers talk about the brightness of celestial objects. The key thing to realise is that how bright an object appears to us in the sky is no guide to its intrinsic luminosity. Is it a dim candle close by, or a distant searchlight? Astronomers use the apparent magnitude scale to describe how bright an object looks to us. How apparent magnitude is defined doesn’t really matter here. It’s enough to understand that bigger numbers mean fainter objects, on a scale where the faintest star visible to the unaided eye is about magnitude 6, the brightest stars in the sky are around magnitude -1, Venus is around -4, the full moon around -12 and the sun around -26.
Following our analogy, if the candle were ten feet away and the searchlight ten miles, they might have the same apparent magnitude. However, if you move them so that they’re at the same distance from you, it becomes immediately obvious that the searchlight is vastly more powerful than the candle. The absolute magnitude, which astronomers use to describe the intrinsic brightness of an object, is defined in an analogous way. It’s just the apparent magnitude an object would have if you moved it to a standard distance – in this case 32.6 light years.
Let’s look at the Sun as an example. It’s so bright because it’s so close. When you compare it to other stars, though, is it a candle or a searchlight? The answer is – it’s a candle. Move the Sun out to 32.6 light years, and it becomes a dim yellow star of magnitude 4.8, barely visible to the naked eye. By contrast, Rigel, the brilliant white star in the lower right of the famous Orion figure, is a distant celestial searchlight. Even at a distance of 700-800 light years, it appears as one of the brightest stars in our sky, with an apparent magnitude of 0.1. Move it in to 32.6 light years, and it dazzles at magnitude -7.0, ten times brighter even than Venus.
If we go a little crazy, and try to estimate the absolute magnitude of an entire galaxy, we come out with, as you’d expect , some very negative numbers. The absolute magnitude of our own Milky Way – the combined luminosity of its 200 billion or so stars – is estimated at -21.
The estimated absolute magnitude of GRB 080319B was -34.
I still can’t quite believe this. GRB 080319B was a single object – probably a massive star collapsing catastrophically into a black hole – which was, for its few final moments of life, a million times brighter than the entire galaxy it lived in, bright enough to be visible to an unaided human eye halfway across the universe.
Let me conclude with a couple of broad comparisons and a pen picture.
- Place it anywhere at all in our galaxy. GRB 080319B would have appeared as a point source way, way brighter than the full moon. Anywhere within 2000 light years, it would have appeared as bright as the Sun.
- Place it in the Andromeda Galaxy, an association of several hundred billion stars 2 million light years from us, and visible as a faint naked eye object, around magnitude 4. GRB 080319B would have appeared around magnitude -10, still almost as bright as the full moon.
Finally, I want you to imagine that you’re observing the galaxy M58, 60 million light years away, with your backyard telescope. It’s a dark, steady, moonless night, and you’re concentrating on a faint, elliptical haze, trying to detect the spiral structure you know is there. Suddenly, a point of light appears. In a few seconds, it’s so bright that looking at it through your telescope hurts your eyes. You step away from the telescope and look up. The object continues to brighten, until it outshines Venus. A few seconds more and it starts to fade, and within a couple of minutes, it’s lost to your sight.
We’ve only been studying gamma ray bursts seriously for a few years. Maybe, one day, some lucky amateur astronomer will be in the right place, at the right time, to make the observation of a lifetime, to see, with his own eyes, the brightest object anywhere in the universe.
One last note. Sky and Telescope notes that GRB 080319B occurred seven hours after the death of the revered science-fiction writer and visionary Arthur C. Clarke. Some astronomers are already referring to GRB 080319B as the “Clarke Burst”, the universe’s spectacular sendoff to one of our own brightest stars.