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TOP > Report & Column > The Forefront of Space Science > 2010 > THE BEAUTY OF BLACK HOLES

The Forefront of Space Science

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The English poet Lord Byron stated that truth is stranger than fiction. In the Universe, we find phenomona on mind-boggling scales, vistas of dazzling beauty, and forces more explosive than any imagined by man. But none is stranger than black holes. With a gravitational attraction strong enough to curve space-time infinitely, and an insatiable hunger for swallowing up all light matter that happens to stray too close, it is no wonder that black holes are associated with death and destruction. But discoveries over the past decade have completely changed our view of black holes. We now understand that birth and evolution goes hand in hand with their destructive power. We have also seen beautiful new patterns of light and energy generation in black hole environments; patterns which arise here only because of their enormous powers. It is this new perspective that I would like to introduce to you here briefly.

Figure 1
Figure 1. The author, made insignificant by the Very Large Telescope in Chile.

Ultra-fast black hole brightness changes

Once stars much more massive than our Sun have consumed their supply of Hydrogen and other elements through nuclear fusion, what is left is a dark and massive, compact object; this is a black hole. But in the blackness of space, how can one find something 'black' in the first place? It turns out that the massive and compact nature of black holes naturally creates conditions that make their immediate surroundings visible, if one has the right tools. Firstly, their extreme gravitational forces cause any surrounding matter to be heated to temperatures of millions of degrees before it falls, or 'accretes' onto the hole. Any gas at such high temperatures will blaze brightly in X-ray and ultraviolet light, and even weak black holes in our Galaxy can emit as much X-ray power in 1 second as our Sun does in 50 years! Secondly, their very compact sizes (about the size of Tokyo city) mean that light can traverse their areas very quickly; so any emission from these objects can easily vary on very short times, faster than the 'blink of an eye'. In fact, identifying new black holes in the Universe as bright and variable sources of X-ray emission is now almost common-place -- a real tribute to the advances that have been made in the field of X-ray astronomy over the past few decades.

But black holes are also known to emit light at all other known wavelengths - from radio waves, to visible light up to gamma rays. My recent research has concentrated on trying to understand the origin of fluctuating visible light from black holes. In collaboration with an international team of astronomers, I used a new "ultra-fast" astronomical camera to observe several black holes in our Galaxy, and found fast flaring of visible light. In one particular case, we saw the black hole brightness doubling in times as small as 1/20th of a second (Figure 2). If our Sun were to change in brightness so suddenly, it would surely have catastrophic consequences for life on Earth! Such fast changes can only occur if the emission region is very compact, and close to the black hole itself. This was an unexpected result, because it is generally thought that the temperatures in black hole vicinities are too hot to produce visible light.

Figure 2
Figure 2. Two 'instants' in a long sequence of observations of a Milky Way black hole whose gravity is strong enough to tear away from the surface of a companion star. On the left, the black hole disc and jet system are very bright, and on the left, they are about 3 times fainter. The images are just artistic representations. The actual brightness measurements are at the bottom. The black hole brightness can change dramatically in bursts and spikes as short as 1/20th of a second.

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