Black Holes: Objects of Attraction
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Ever since Princeton physicist John Wheeler coined the term nearly 50 years ago, black holes have evoked a sense of mystery and wonder for astronomers and space enthusiasts. But unlike comets, stars and other beautiful objects in the night sky, black holes can't actually be seen – they trap light, after all. From this infinitely dark void, myths and misconceptions have taken flight, spurred by the sci-fi depictions of black holes as cold cosmic villains with a bottomless appetite for nearby planets, stars and mighty spacecrafts whizzing through the galaxy at warp speed.
"Watching sci-fi movies is fun in part because as a scientist, I like to try to find out what’s wrong with it. On the other hand, you know, some of what we used to think was sci-fi became reality," said Alex Filippenko, a UC Berkeley professor of astronomy who has been studying and hunting black holes for 30 years. He also teaches one of the most popular classes at UC Berkeley, an introduction to astronomy course which attracts thousands of students a year, many of whom are non-science majors.
Professor Filippenko helped debunk for me some of the fanciful myths about black holes, including the ability to travel through time by surfing a 'wormhole', a theoretical portal which connects two black holes and leads to another universe. He told me, "People sometimes think that by jumping into a black hole you can travel backward in time. But that’s not really possible because after you jump into the black hole, you’ll get crushed or vaporized."
Even though black holes don't behave like the celestial monsters with insatiable appetites they're sometimes caricatured to be, there is still plenty of wonder and unanswered questions about them to satisfy astronomers for years to come. For example, scientists have distinguished between two major classes of black holes – stellar black holes (also known as stellar-mass black holes) and supermassive black holes. The former are roughly six to 30 times the mass of the sun and the latter are a whopping million to billions of times the mass of the sun. So are there intermediate-mass black holes, on the order of hundreds to thousands of times the mass of the sun?
Quite possibly, yes. In August, a team from Keio University in Japan announced their discovery of a region of space 30,000 light years away which they suspect might contain young, intermediate-mass black holes. One of these black hole candidates is nearby Sagittarius A*, a supermassive black hole which lurks at the center of our Milky Way galaxy.
But as I mentioned in my story on black holes, simply accumulating the observational evidence to infer the existence of a black hole is a daunting and time-consuming process. It took two separate teams of astronomers years to estimate the mass of Sagittarius A* (four million times the mass of the sun) from meticulous calculations of its gravitational effects on orbiting stars.
NuSTAR, a NASA telescope about the size of a large fridge which launched in June, can much more nimbly and swiftly detect the presence of black holes by viewing, with its sophisticated optics, the glow of high-energy x-rays emitted from the vicinity of black holes currently hidden behind thickets of cosmic dust and gas. Caltech's Fiona Harrison, the Principal Investigator, explained how the NuSTAR telescope will beam its observational results to the the team of astronomers, physicists and engineers who've toiled diligently to ensure the success of this low-cost, high-impact mission.
"Four times a day, NuSTAR will send its data down to a ground station, in Malindi, Kenya…and the data from these surveys comes down to scientists around the world. We take this data and we turn it into images; we look in these images for pinpoints of X-ray light. Each one will be a hallmark of a black hole. NuSTAR will see hundreds and hundreds of black holes," she said.
Harrison and her colleague, William Craig, spent ten years working on NuSTAR, in part because the technology didn't exist at the time to focus high-energy x-rays – the kind used in dental and medical offices – emitted from cosmic sources billions of light years away.
To do this, Craig and his team took 4,000 pieces of glass twice the thickness of a human hair and stacked them in a nested array which greatly increased the surface area for reflecting the high-energy x-rays onto NuSTAR's digital detector 33 feet away from its lens. But to even get the pieces of glass to reflect the x-rays was a feat of ingenuity, requiring 17 months of research and the use of nanotechnology to coat the surface of the glass with hundreds of layers of nanoparticles.
"Looking for black holes has always been kind of like looking for a needle in a haystack," Craig said. "But looking with these X-rays just like a medical X-ray can look through your skin and see the bone, it’s like looking through the haystack to see the needle. It’s like the haystack’s not there," he added.
In late July, the NuSTAR team announced that the telescope had successfully taken its first images of the black hole Cygnus X-1 and had participated in joint observations with the Keck and the Chandra telescopes of Sagittarius A*. In addition, NuSTAR has also made observations of the "optically brightest quasar in the sky", located more than two billion light years away.
More observations and excitement are sure to follow on the heels of NuSTAR's observations, from quasars to stars orbiting powerful black holes and the cosmic ashes of supernova explosions which can seed the universe with these space oddities that have perplexed and intrigued the sharpest minds in astronomy.
"NuStar is gonna find all kinds of black holes in the centers of galaxies that might be otherwise hidden from our perspective by a bunch of other gas…This is gonna revolutionize the study of black holes in our universe," said Alex Filippenko.
If you'd like to learn more about black holes and test your knowledge about these space oddities, be sure to check out this neat interactive feature produced by former QUEST content intern C.K. Hickey.