1st draft ScienceNow! presentation. Black holes are Pink. How could anyone possibly think that a black hole is any colour other than black? Actually, professional astronomers have long been happy with the idea that black holes could be a brilliant blue-white colour. Our new breakthrough is to show that they can be pink too! Black holes are the densest things in the universe: they are what you get when you compress something weighing far more than the Earth down to smaller than the head of a pin. As a result, their gravity is so intense that not even light can escape: that is why they are called "black". They are thought to form when massive stars come to the end of their lives. If this was the whole story, nobody would ever study black holes: they would be tiny black dots against the blackness of space: quite invisible. Luckily for astronomers, and very strangely, some black holes shine incredibly brightly. These black holes are called quasars (which means "quasi-stellar objects" - ie. things that look like stars but aren't). Even with the Hubble Space Telescope, quasars look just like brilliantly glowing dots: no details can be seen. They are so bright that if you replaced the Sun with a quasar, it would vaporise the Earth in less than a second. So: how can a black hole be so very un-black? We don't really know: but most theories suggest that it isn't the black hole itself that is so bright, but the gas swirling down its throat. Imagine that there was a black hole at the back of this room. If I were to drop this peanut, what would happen? It would shoot up towards the back of the room, and by the time is passed the head of those of you in the last row, it would be travelling at virtually the speed of light, before being swallowed by the black hole. Now what would happen if one of you were so foolish as to put something in the way of this relativistic super-peanut? The explosion when the peanut hit would be equivalent to a fair sized atom bomb going off: it would devastate Melbourne. So, if you drop things into a black hole (such as passing stars, planets and gas clouds that blundered a little bit too close), as they fall in, the gravity will rip them to shreds, and as the shreds swirl around the throat of the black hole, the debris will be smashed together with incredible violence, producing (so the theory goes) the brilliant light we see. The laws of physics tell us that when you do incredibly violent things to matter (like feed it into a black hole) it becomes enormously hot: 30,000 degrees or more, and shines with an intense blue-white light. You can see this colour from arc welding, or from the hottest, most violent stars in the nights sky, like Vega or sirius: they look distinctly blue. Astronomers therefore expected that quasars should look blue-white in colour. And for decades, people thought they did. (slides of blue-white quasars) That's where we came in. Back in 1993, my collaborators and I decided to try and find some more quasars, using an unusual technique: instead of using visible-light telescopes (slide of a visible light telescope) we used data from the Parkes Radio Telescope (slide of Parkes Telescope). Sure enough, we found lots of quasars: 323 at the latest count. But many of these quasars were very strange: they were extremely pink in colour (slides showing pink quasars, and comparing them with the normal blue quasars). What were these things? Could they really be pink black holes? At first we thought they might be strange stars or galaxies. But using the Anglo-Australian Telescope (slide) up near Coonabarabran in the western plains of NSW, we were able to show that these were indeed quasars: ie. black holes. But this was bizarre: how could the searingly hot gas swirling down the throat of a black hole be any colour other than blue? We soon came up with a theory: perhaps these black holes really are blue: they just look pink because we are seeing them through a cloud of interstellar dust. Space is full of these clouds of interstellar dust (slides). If something hides behind one of these clouds, you may not see it at all. But if you do, it will appear a dim yellowish colour (slide), as the dust blocks blue light but lets red and green light through. You see them same with smoke from forest fires: when you look through the smoke plume, things look a dim reddish colour. If this theory were true, the pink quasars should look a very particular shade of yellow: slide. We could calculate this shade quite precisely. So, over the last couple of years, I've spent many long nights on these two telescopes (slides), the ANU's 2.3m and 1m telescopes (also at Siding Spring Observatory, near Coonabarabran), carefully measuring the colours of hundreds of these red quasars. And what did I find? Slides: A few of the quasars were just the colours you'd expect of truely blue quasars hidden behind clouds of interstellar dust. But most, however, were a quite different shade of purplish pink (slides). Dust cannot explain their strange colour: these black holes really must be pink. That's where we are today. We've pretty conclusive evidence that some black holes are pink: extremely pink. But we've really very little idea why. We are guessing that these pink quasars shine due to light swirling down into the black holes, just like normal blue quasars. But why does it come out so pink? If you force us to speculate, we think that the violent swirling motion of the gas around the black hole is acting as a natural particle accelerator, generating an intense beam of sub-atomic particles. Somehow, this beam, as it blasts out into space, is producing the strong pink glow: we're not at all sure how.