Post by glactus on Jun 21, 2011 2:57:59 GMT
There are many popular myths concerning black holes, many of them perpetuated by Hollywood. Television and movies have portrayed them as time-traveling tunnels to another dimension, cosmic vacuum cleaners sucking up everything in sight, and so on. It can be said that black holes are really just the evolutionary end point of massive stars. But somehow, this simple explanation makes them no less mysterious, and no easier to understand.
A Black Hole illustration
So What is a Black Hole?
Black holes are the evolutionary endpoints of stars at least 10 to 15 times as massive as the Sun. If a star that massive or larger undergoes a supernova explosion, it may leave behind a fairly massive burned-out stellar remnant. With no outward forces to oppose gravitational forces, the remnant will collapse in on itself. The star eventually collapses to the point of zero volume and infinite density, creating what is known as a "singularity." Around the singularity is a region where the force of gravity is so strong that not even light can escape. Thus, no information can reach us from this region. It is therefore called a black hole, and its surface is called the "event horizon."
A wandering Black Hole
But contrary to popular myth, a black hole is not a cosmic vacuum cleaner. If our Sun was suddenly replaced with a black hole of the same mass, Earth's orbit around the Sun would be unchanged. Of course, Earth's temperature would change, and there would be no solar wind or solar magnetic storms affecting us. To be "sucked" into a black hole, one has to cross inside the Schwarzschild radius. At this radius, the escape speed is equal to the speed of light, and once light passes through, even it cannot escape.
Black hole Cygnus x1
Cygnus X-1 (Cyg X-1) is the longest known of the black hole candidates. It is a highly variable and irregular source, with X-ray emission that flickers in hundredths of a second. An object cannot flicker faster than the time required for light to travel across the object. In a hundredth of a second, light travels 3,000 kilometers. This is one fourth of Earth's diameter. So the region emitting the X-rays around Cyg X-1 is rather small. Its companion star, HDE 226868 is a B0 supergiant with a surface temperature of about 31,000 K. Spectroscopic observations show that the spectral lines of HDE 226868 oscillate with a period of 5.6 days. From the mass-luminosity relation, the mass of this supergiant is calculated as 30 times the mass of the Sun. Cyg X-1 must have a mass of about 7 solar masses,
If the Sun was replaced with a black hole that had the same mass as the Sun, the Schwarzschild radius would be 3 km (compared to the Sun's radius of nearly 700,000 km). Hence the Earth would have to get very close to get sucked into a black hole at the center of our Solar System.
If we can't see them, how do we know they are there?
Since stellar black holes are small (only a few to a few tens of kilometers in diameter), and light that would allow us to see them cannot escape, a black hole floating alone in space would be hard, if not impossible, to see in the visual spectrum.
However, if a black hole passes through a cloud of interstellar matter, or is close to another "normal" star, the black hole can accrete matter into itself. As the matter falls or is pulled towards the black hole, it gains kinetic energy, heats up and is squeezed by tidal forces. The heating ionizes the atoms, and when the atoms reach a few million Kelvin, they emit X-rays The X-rays are sent off into space before the matter crosses the Schwarzschild radius and crashes into the singularity. Thus we can see this X-ray emission.
Looking at a Black Hole
Credits: These are NASA/JPL images
Text by NASA
Telescope in avatar: Meade 16" LX 200
Astronomer in avatar: Glactus
A Black Hole illustration
So What is a Black Hole?
Black holes are the evolutionary endpoints of stars at least 10 to 15 times as massive as the Sun. If a star that massive or larger undergoes a supernova explosion, it may leave behind a fairly massive burned-out stellar remnant. With no outward forces to oppose gravitational forces, the remnant will collapse in on itself. The star eventually collapses to the point of zero volume and infinite density, creating what is known as a "singularity." Around the singularity is a region where the force of gravity is so strong that not even light can escape. Thus, no information can reach us from this region. It is therefore called a black hole, and its surface is called the "event horizon."
A wandering Black Hole
But contrary to popular myth, a black hole is not a cosmic vacuum cleaner. If our Sun was suddenly replaced with a black hole of the same mass, Earth's orbit around the Sun would be unchanged. Of course, Earth's temperature would change, and there would be no solar wind or solar magnetic storms affecting us. To be "sucked" into a black hole, one has to cross inside the Schwarzschild radius. At this radius, the escape speed is equal to the speed of light, and once light passes through, even it cannot escape.
Black hole Cygnus x1
Cygnus X-1 (Cyg X-1) is the longest known of the black hole candidates. It is a highly variable and irregular source, with X-ray emission that flickers in hundredths of a second. An object cannot flicker faster than the time required for light to travel across the object. In a hundredth of a second, light travels 3,000 kilometers. This is one fourth of Earth's diameter. So the region emitting the X-rays around Cyg X-1 is rather small. Its companion star, HDE 226868 is a B0 supergiant with a surface temperature of about 31,000 K. Spectroscopic observations show that the spectral lines of HDE 226868 oscillate with a period of 5.6 days. From the mass-luminosity relation, the mass of this supergiant is calculated as 30 times the mass of the Sun. Cyg X-1 must have a mass of about 7 solar masses,
If the Sun was replaced with a black hole that had the same mass as the Sun, the Schwarzschild radius would be 3 km (compared to the Sun's radius of nearly 700,000 km). Hence the Earth would have to get very close to get sucked into a black hole at the center of our Solar System.
If we can't see them, how do we know they are there?
Since stellar black holes are small (only a few to a few tens of kilometers in diameter), and light that would allow us to see them cannot escape, a black hole floating alone in space would be hard, if not impossible, to see in the visual spectrum.
However, if a black hole passes through a cloud of interstellar matter, or is close to another "normal" star, the black hole can accrete matter into itself. As the matter falls or is pulled towards the black hole, it gains kinetic energy, heats up and is squeezed by tidal forces. The heating ionizes the atoms, and when the atoms reach a few million Kelvin, they emit X-rays The X-rays are sent off into space before the matter crosses the Schwarzschild radius and crashes into the singularity. Thus we can see this X-ray emission.
Looking at a Black Hole
Credits: These are NASA/JPL images
Text by NASA
Telescope in avatar: Meade 16" LX 200
Astronomer in avatar: Glactus