Introductory Astronomy: Black Holes

If the mass of stellar core left after a supernova explosion is greater than 3 solar masses, the stellar remnant will become a black hole. At this point, the force of gravity is strong enough to overcome the degenerate neutron pressure, and nothing stops the object from collapsing. Because nothing balances gravity, the object collapses to a point, called a singularity, where all of the mass is concentrated. It is then known as a Black Hole.

Because black holes are so massive, they exert strong gravitational forces. At distances close to the black hole, even light cannot escape its pull. Based on Newton's gravitational law, there is a quantity, called the escape velocity, that describes the speed a body needs to escape the gravitational pull of an object. Because the gravitational force the body feels decreases as it moves farther from the central object, the escape velocity it needs is smaller the farther away the body moves from the massive object. In other words, the closer you are to a massive object, the faster you must go in order to escape the gravitational pull of that object.

The distance at which the escape velocity equals the speed of light (in other words, the point at which light just barely has enough speed to escape the pull of the black hole) is called the Event Horizon. Any object within this radius is unable to escape the pull of the black hole. Objects outside of the event horizon do have a chance to escape from the black hole. The radius of the event horizon is known as the Schwartzchild radiusand is given by the formula: Rs = 2 G M / c^2 where G is the gravitational constant, M is the mass of the black hole, and c is the speed of light. This formula can also be written as: Rs = 3 M where M is the mass of the object in solar masses. Using this equation, the units of the radius are kilometers.

Because not even light can escape the force of the black hole (once it is inside the Event Horizon), it is hard to detect black holes. Astronomers must try to discover them without directly observing them. One way is to observe the effect that the black hole has on nearby objects. Newton's laws state that mass creates gravitational forces. Since black holes are very massive objects, the force they exert on other objects should be noticeable.

Another way to detect them is through the use of X-ray binaries. An X-ray binary consists of a normal star and a compact object (which is just the end product of a normal star's evolution) orbiting each other. If the outer layers of the normal star are loosely bound to that star, they may feel a stronger gravitational attraction to the compact object and move toward it. As the normal star's material falls toward the compact object (this is known as accretion), it forms a disk surrounding that object. This process heats the material to high temperatures (on the order of a million degrees Kelvin). According to Wien's law, hot material radiates at small wavelengths. Material at a temperature of a million Kelvin is known to radiate in the X-ray regime. Therefore, if X-rays are detected from a binary pair, it is a sign that material is accreting onto a compact object. Scientists can determining the mass of the compact object in the binary and, if it has a mass greater than 3 solar masses, it is thought to be a Black Hole. Examples of Black Hole candidates are Cygnus X-1 and the Red Rectangle nebula.

What would you see if you were close to a Black Hole? Click here to find out what NASA scientists think!

Or, click here to go to a page containing simulations of what it would be like to travel to a Black Hole.