No one has ever seen a black hole with their own eyes but everyone has heard about black holes. New technology for making telescopes has helped scientists learn more details about black holes but the principles of physics give us our basic understanding.
When a massive star dies the type of black hole formed is a stellar black hole. The star caves in on itself when it dies forming a black hole. (Figure 1) When a star with a huge mass dies, part of the start explodes into space – that is a supernova. The area of a black hole is small because the dead star’s matter is compressed into a small space. (Figure 2) The human eye cannot see a black hole because it does not reflect light. It absorbs all light.
Types based on rotation
There are two types of black holes based on rotation; they are a called Schwarzchild and Kerr. (Freundenrich 2006) The Schwarzchild type of black hole does not rotate. They are not very common. The Kerr type of black hole is the type of black hole that rotates. It rotates because it formed when a rotating star dies. A star dies by collapsing on itself which makes the core of the black hole. The black hole continues to rotate based upon the Conservation of Angular Momentum. The collapsed core of the dead star is called the ‘singularity.’ The place where the black hole opens is called the ‘event horizon.’
The third part of the Kerr black hole is called the ‘ergosphere.’ An ergosphere is the space around the opening of the black hole (the event horizon) that is distorted. It is egg shaped because the distortion is caused by the rotation which pulls or drags space while it rotates. The boundary between the ergosphere (the distorted space) and the normal space (space that is not distorted) is called the ‘static limit.’
Type of rotating black holes
The three types of Kerr black holes that have been identified are (a) primordial, (b) stellar and (c) supermassive. (Smith 2008) Black holes are categorized based on their mass and their size. Primordial black holes are the smallest in size, perhaps only one atom but the mass is as large as Mount Everest or any other large mountain. The mid-sized black hole can have a ten mile diameter. The mass might be as large as approximately twenty times the mass of our sun. There may be “dozens of stellar mass black holes within the Milky Way galaxy” (Smith 2008). The way to visualize a supermassive black hole is to image a ball the size of our solar system that has the mass of one million of our suns. (Smith 2008)
Measuring and Detecting
Measurements can be taken of a black hole even though we cannot visually ‘see’ the black hole. Measuring can be done by using mass, electric charge or angular momentum (based on rotation). The rotation of the black hole can be determined by the way other objects like stars move around the black hole. The rotation radius or orbital speed of the star or some ‘disk of material’ around the black hole can be measured. (Freundenrich 2006)
If objects are near enough the black hole and large enough the “difference in angular direction between one boundary and another is directly proportional to its size” (Cambridge Relativity n.d.). Because the relationship is proportional the calculations can be reliably made:
Size = Angular Separation*Distance
The astronomer Johannes Kepler developed a way to mathematically calculate the relationship between objects in space. Kepler’s Modified Third Law of Planetary Motion is
T2 = R3
where T equals the time for one orbital period and R equals the average orbital radius we can calculate For example in our planetary system, the earth can be used to calculate the average distance from the sun of the other planets. For a black hole an object close to the black hole is used.
The black hole emits X-rays which explains the use of X-ray technology to study them. The extreme intensity demonstrated by a black hole “pulls in dust particles from a surrounding cloud of dust or a nearby star.” (Smith 2003) The gravity pull causes the particles to accelerate (speed up) which in turn causes the particles to emit X-rays. The X-rays are not coming from the black hole because that would be impossible. But the X-rays emitted from the particles being pulled into the black hole can be seen by astronomers. The matter of the dead star falls into itself and only takes up a very small amount of space.
The bending of light waves around a black hole alerts astronomers to the location of a black hole. The Physics describing the phenomena was explained by Albert Einstein. He called it a “gravity lens” (Smith 2003). In 1919 Einstein proved “that light follows in the path of the bent time and space which is warped due to the gravitational force of a massive object” (Smith 2003).
Figure 1. Relationship of the size of a singularity to an event
(Version of a diagram from www.eclipse.net)
Figure 2. Death of Star three times the mass of our sun creates a Black Hole
(Version of a diagram from www.eclipse.net)
Freudenrich, Ph.D., Craig. "How Black Holes Work" 26 November 2006. HowStuffWorks.com.
Miller, Chris. Black Holes and Neutron Stars. Eclipse.net, 27 April 1996. Web. 17 January 2013.
Miller, Chris. How do we detect them? Eclipse.net, 17 September 2003. Web. 10 January 2013.
Smith R., Heather. What is a Black Hole? National Aeronautics and Space Administration, (NASA), 8 September 2008. Web. 10 January 2013.