Astronomers from Baltimore's Space Telescope Science Institute and the University of Texas recently made what could be a giant leap in our understanding of the cosmos by detecting black holes that are neither "supermassive" nor as small as a single decayed star.
A quick black hole history lesson might prove helpful in putting this latest development into perspective. The idea of black holes sprung up in the early20th century. According to Astronomy Prof. Steven Balbus, Robert Oppenheimer first proposed the existence of black holes in the 1930s, but his ideas met skepticism in the scientific community -- even Albert Einstein vehemently disputed his theory.
In the 1960s, Stephen Hawking and Roger Penrose mathematically proved that black holes have to exist, and this result has been corroborated by data gathered from astronomical observation. This data does not necessarily provide direct visual information because black holes are invisible, but scientists still can detect them by studying the orbits of stars close to black holes.
It also is possible to detect the X-ray radiation that is given off when material sucked into the hole is crushed at the event horizon.
"As many as half of all the X-ray sources in the universe could be black holes," Astronomy Prof. Robert O'Connell said.
We now know how scientists discovered black holes, but why do they exist in the first place?
"Black holes are formed when giant stars collapse to give a region in space where there is enough high-density mass to form a pocket in the space-time continuum called an event horizon," O'Connell said.
While the mass of the black hole remains the same as the mass of the original star, the matter now is in such a highly compressed state that the force of gravity at the event horizon is greatly increased.
In fact, in order for an object at the event horizon to escape from the black hole's gravitational pull, its velocity would have to equal the speed of light. But the gravitational field only is so strong at the event horizon.
"If our sun were to form a black hole, nothing would change at the Earth except the loss of sunlight," Balbus said.
There were only two known categories of black holes prior to the recent discovery of mid-size black holes -- "small" and "supermassive." The smallest black holes form from ordinary stars that are at least 10 times as massive as our sun. When these stars are unable to support themselves against their own gravity, they collapse until all their matter is compressed into a much smaller volume.
"Supermassive" black holes exist at the centers of galaxies and are between a million and a billion times as massive as the sun. The thought of such gigantic objects might be scary, but according to Balbus, "bigger black holes are actually safer around the event horizon."
He also said that "supermassive" black holes lack the vicious tidal forces of smaller ones because the event horizon is farther away from the hole.
The recent discovery of mid-size black holes put a brand new spin on the science of stars.
"The discovery of mid-size black holes in these star clusters is very surprising because they have been probed for years," O'Connell said.
Scientists also discovered that the ratio of the size of these mid-size black holes to the size of the star systems in which they were found was similar for "small" and "supermassive" black holes.
"This means that the formation of galaxies is related to the presence of black holes," O'Connell said.
This might ultimately help explain what happened after the Big Bang, and how the universe became filled with clusters, constellations and galaxies.
So what's next on the minds of black hole scientists? Two interesting questions remain unanswered. First, do black holes lead to other space-times? And second, exactly where exactly is the mass of these black holes? These and other problems are currently being worked on in a branch of physics called string theory. Perhaps one day our knowledge of black holes will help us pass through short cuts in the fabric of space-time.
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