Energy
Dec 16th, 2005, 3:36 PM
How come,you haven't seen these news?
http://eu.spaceref.com/news/viewpr.html?pid=18165
Astronomers have gotten their deepest glimpse into the heart of our Milky Way Galaxy, peering closer to the supermassive black hole at the Galaxy's core then ever before. Using the National Science Foundation's continent-wide Very Long Baseline Array (VLBA), they found that a radio-wave-emitting object at the Galaxy's center would nearly fit between the Earth and the Sun. This is half the size measured in any previous observation.
We're getting tantalizingly close to being able to see an unmistakable signature that would provide the first concrete proof of a supermassive black hole at a galaxy's center," said Zhi-Qiang Shen, of the Shanghai Astronomical Observatory. A black hole is a concentration of mass so dense that not even light can escape its powerful gravitational pull.
The astronomers used the VLBA to measure the size of an object called Sagittarius A* (pronounced "A-star") that marks the exact center of our Galaxy. Last year, a different team announced that their measurements showed the object would fit inside the complete circle of Earth's orbit around the Sun. Shen and his team, by observing at a higher radio frequency, measured Sagittarius A* as half that size.
A mass equal to four million Suns is known to lie within Sagittarius A*, and the new measurement makes the case for a black hole even more compelling than it was previously. Scientists simply don't know of any long-lasting object other than a black hole that could contain this much mass in such a small area. However, they would like to see even stronger proof of a black hole.
"The extremely strong gravitational pull of a black hole has several effects that would produce a distinctive 'shadow' that we think we could see if we can image details about half as small as those in our latest images," said Fred K.Y. Lo, Director of the National Radio Astronomy Observatory and another member of the research team. "Seeing that shadow would be the final proof that a supermassive black hole is at the center of our Galaxy," Lo added.
Many galaxies are believed to have supermassive black holes at their centers, and many of these are much more massive than the Milky Way's black hole. Also, in many other galaxies, the gravitational energy of the black hole is powering superfast "jets" of subatomic particles at nearly the speed of light. Such jets in other galaxies extend outward for thousands of light-years.
The Milky Way's central black hole is much less active than that of many other galaxies, presumably because it has less nearby material to "eat." Astronomers believe that the radio waves they see coming from Sagittarius A* probably are generated by particle jets much shorter than those of more-active galaxies. By observing the object at higher radio frequencies, scientists have detected parts of the jets ever closer to the black hole. The results announced last year were based on observations at 43 GHz, and the latest observations were made at 86 GHz.
"We believe that if we can double the frequency again, we will see the black-hole shadow produced by effects of Einstein's General Relativity theory," Lo said.
In a few years, when the Atacama Large Millimeter Array (ALMA) comes on line, it may be used in conjunction with other millimeter-wave telescopes to make the higher-frequency observations that will reveal the telltale black-hole shadow.
At a distance of 26,000 light-years, the Milky Way's central black hole is the closest such supermassive object. That makes it the most likely one to finally reveal the concrete evidence for a black hole that astronomers have sought for years.
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A Quest to See a Black Hole's Shadow:
http://www.space.com/scienceastronomy/051011_blackhole_shadow.html
At the core of the Milky Way is a supermassive black hole that sucks in light, rendering it virtually invisible. But astronomers say they will be able to see the black hole's overall shadow within a few years.
"The Holy Grail of black hole astronomy is within our grasp," says Avery Broderick of the Harvard-Smithsonian Center for Astrophysics. "We could see the shadow that the black hole casts on surrounding material, and determine the size and spin of the black hole itself."
Nothing can escape the intense gravitational field of a black hole, not even light. And since they can't emit light, or any other form of matter, there's no visible evidence of their existence. But as matter gets pulled in, it heats up and radiates energy in "hot spots."
Some of this radiation escapes and can be detected.
Astronomers have already detected radiation from hot spots just outside the black hole, and they believe that these will paint a background against which the black hole's profile, or shadow, will stand out.
Since the technology to view the shadow won't be in place for another few years, Broderick and Avi Loeb of the Harvard-Smithsonian Center for Astrophysics have designed a model that anticipates what the shadow will look like.
The hot spot of radiation rotates around the black hole, but researchers don't know if the hole itself is spinning or not, so Broderick and Loeb created two scenarios – one with a motionless black hole, and another with one spinning at the maximum rate.
In each scenario, the hot spot is depicted as a rainbow-colored blob rotating around a solid blue disk representing the black hole's accretion disk, where matter heats up and collects before finally being sucked into the black hole.
"It will be really remarkable when observers can see all the way to the edge of the Milky Way's central black hole – a hole 10 million miles in diameter that's more than 25,000 light-years away," Broderick said.
To view the shadow, astronomers will need a radio telescope as large as the Earth. One is already in the works, more or less. Instead of building one impossibly giant telescope, astronomers will combine readings from a collection of submillimeter telescopes from across the continent.
This technique, known as interferometry, has already been used to study long wavelength emissions from outer space. Astronomers believe that studying short wavelength emissions could yield a high-resolution view of the outer region of the black hole.
The gravity well at the Milky Way's center is the best target for observation by interferometry because it covers the largest area in the sky of any known black hole. And an even higher resolution image could be achieved by combining observations from infrared instruments.
"Submillimeter and infrared observations are complementary," said Lincoln Greenhill, also of the Harvard-Smithsonian Center for Astrophysics. "We need to use both to tackle the problem of getting high-resolution observations. It's the only way to get a complete picture of the galactic center."
But a clear picture of this black hole won't be the only benefit of spotting its shadow. This data will ultimately help astronomers test Einstein's general theory of relativity within the intensely strong gravitational field of a black hole.
"When astronomers achieve it, that first image of the black hole's shadow and inner accretion disk will enter textbooks, and will test our current notions on gravity in the regime where spacetime is strongly curved," Loeb said.
AND WHO SAYS THAT BLACK HOLE CAN'T BE SEEN DIRECTLY,IT WOULD BE IN THE NEXT 10 YEARS WHEN ASTRONOMERS SEE BLACK HOLE'S SHADOW!
http://eu.spaceref.com/news/viewpr.html?pid=18165
Astronomers have gotten their deepest glimpse into the heart of our Milky Way Galaxy, peering closer to the supermassive black hole at the Galaxy's core then ever before. Using the National Science Foundation's continent-wide Very Long Baseline Array (VLBA), they found that a radio-wave-emitting object at the Galaxy's center would nearly fit between the Earth and the Sun. This is half the size measured in any previous observation.
We're getting tantalizingly close to being able to see an unmistakable signature that would provide the first concrete proof of a supermassive black hole at a galaxy's center," said Zhi-Qiang Shen, of the Shanghai Astronomical Observatory. A black hole is a concentration of mass so dense that not even light can escape its powerful gravitational pull.
The astronomers used the VLBA to measure the size of an object called Sagittarius A* (pronounced "A-star") that marks the exact center of our Galaxy. Last year, a different team announced that their measurements showed the object would fit inside the complete circle of Earth's orbit around the Sun. Shen and his team, by observing at a higher radio frequency, measured Sagittarius A* as half that size.
A mass equal to four million Suns is known to lie within Sagittarius A*, and the new measurement makes the case for a black hole even more compelling than it was previously. Scientists simply don't know of any long-lasting object other than a black hole that could contain this much mass in such a small area. However, they would like to see even stronger proof of a black hole.
"The extremely strong gravitational pull of a black hole has several effects that would produce a distinctive 'shadow' that we think we could see if we can image details about half as small as those in our latest images," said Fred K.Y. Lo, Director of the National Radio Astronomy Observatory and another member of the research team. "Seeing that shadow would be the final proof that a supermassive black hole is at the center of our Galaxy," Lo added.
Many galaxies are believed to have supermassive black holes at their centers, and many of these are much more massive than the Milky Way's black hole. Also, in many other galaxies, the gravitational energy of the black hole is powering superfast "jets" of subatomic particles at nearly the speed of light. Such jets in other galaxies extend outward for thousands of light-years.
The Milky Way's central black hole is much less active than that of many other galaxies, presumably because it has less nearby material to "eat." Astronomers believe that the radio waves they see coming from Sagittarius A* probably are generated by particle jets much shorter than those of more-active galaxies. By observing the object at higher radio frequencies, scientists have detected parts of the jets ever closer to the black hole. The results announced last year were based on observations at 43 GHz, and the latest observations were made at 86 GHz.
"We believe that if we can double the frequency again, we will see the black-hole shadow produced by effects of Einstein's General Relativity theory," Lo said.
In a few years, when the Atacama Large Millimeter Array (ALMA) comes on line, it may be used in conjunction with other millimeter-wave telescopes to make the higher-frequency observations that will reveal the telltale black-hole shadow.
At a distance of 26,000 light-years, the Milky Way's central black hole is the closest such supermassive object. That makes it the most likely one to finally reveal the concrete evidence for a black hole that astronomers have sought for years.
----------------------------------------------------------------------------
A Quest to See a Black Hole's Shadow:
http://www.space.com/scienceastronomy/051011_blackhole_shadow.html
At the core of the Milky Way is a supermassive black hole that sucks in light, rendering it virtually invisible. But astronomers say they will be able to see the black hole's overall shadow within a few years.
"The Holy Grail of black hole astronomy is within our grasp," says Avery Broderick of the Harvard-Smithsonian Center for Astrophysics. "We could see the shadow that the black hole casts on surrounding material, and determine the size and spin of the black hole itself."
Nothing can escape the intense gravitational field of a black hole, not even light. And since they can't emit light, or any other form of matter, there's no visible evidence of their existence. But as matter gets pulled in, it heats up and radiates energy in "hot spots."
Some of this radiation escapes and can be detected.
Astronomers have already detected radiation from hot spots just outside the black hole, and they believe that these will paint a background against which the black hole's profile, or shadow, will stand out.
Since the technology to view the shadow won't be in place for another few years, Broderick and Avi Loeb of the Harvard-Smithsonian Center for Astrophysics have designed a model that anticipates what the shadow will look like.
The hot spot of radiation rotates around the black hole, but researchers don't know if the hole itself is spinning or not, so Broderick and Loeb created two scenarios – one with a motionless black hole, and another with one spinning at the maximum rate.
In each scenario, the hot spot is depicted as a rainbow-colored blob rotating around a solid blue disk representing the black hole's accretion disk, where matter heats up and collects before finally being sucked into the black hole.
"It will be really remarkable when observers can see all the way to the edge of the Milky Way's central black hole – a hole 10 million miles in diameter that's more than 25,000 light-years away," Broderick said.
To view the shadow, astronomers will need a radio telescope as large as the Earth. One is already in the works, more or less. Instead of building one impossibly giant telescope, astronomers will combine readings from a collection of submillimeter telescopes from across the continent.
This technique, known as interferometry, has already been used to study long wavelength emissions from outer space. Astronomers believe that studying short wavelength emissions could yield a high-resolution view of the outer region of the black hole.
The gravity well at the Milky Way's center is the best target for observation by interferometry because it covers the largest area in the sky of any known black hole. And an even higher resolution image could be achieved by combining observations from infrared instruments.
"Submillimeter and infrared observations are complementary," said Lincoln Greenhill, also of the Harvard-Smithsonian Center for Astrophysics. "We need to use both to tackle the problem of getting high-resolution observations. It's the only way to get a complete picture of the galactic center."
But a clear picture of this black hole won't be the only benefit of spotting its shadow. This data will ultimately help astronomers test Einstein's general theory of relativity within the intensely strong gravitational field of a black hole.
"When astronomers achieve it, that first image of the black hole's shadow and inner accretion disk will enter textbooks, and will test our current notions on gravity in the regime where spacetime is strongly curved," Loeb said.
AND WHO SAYS THAT BLACK HOLE CAN'T BE SEEN DIRECTLY,IT WOULD BE IN THE NEXT 10 YEARS WHEN ASTRONOMERS SEE BLACK HOLE'S SHADOW!