Long available here, below is a description of the 2008 December 3rd episode of this radio show with focused on black holes. On this program, I discussed the following:
- Black Holes: Black Holes are objects believed to be so dense that light can not escape if it gets to close (i.e., past the "event horizon"). Additionally, its gravity is so strong that material that makes up a black hole can not arrange itself into a structure that can withstand its own gravity, so this material is thought to collapse into a point. But if one can't see too close to a black hole, how does one know? Well, it the above is correct, that black holes should not have a surface, unlike a neutron star. If a neutron star or a black hole is close enough to a normal star, its gravity is so strong that it will rip material off the surface of the normal star and cause it to fall on itself. This process is called "accretion", and as discussed previously on this radio show by Dr. Tod Strohmayer, this has process have been observed for many neutron stars and black hole candidates. In the case of a neutron star, this material will pile up on the surface of the neutron star, and this pile will got hotter and denser until it is so hot and dense that the hydrogen in this material fuses into helium, releasing a burst of heat and light which is observable by X-ray telescopes such as the Rossi X-ray Timing Explorer. One would not expect such bursts from material falling onto a black hole - which is more massive than a neutron star - since their is no surface for the material to collect. These X-ray bursts have been detected from neutron stars (for example this link), but not from any black hole candidates. Therefore, even there seems to be evidence that black holes indeed do not have surfaces. Additionally, black holes appears to have a mass either a few times that of the Sun (called "stellar-mass black holes"), or millions to billions times that of the Sun - like the black hole believed to reside in the center of our galaxy, the Milky Way (called "super-massive black holes"). Why there seems to be a lack of black holes with a mass between these two extremes (for example, a thousand solar masses) isn't known. One possibility is that such black holes exist, but reside in the middle of globular clusters where they would be hard to detect. A recent survey of globular cluster RZ2109 did not find such a black hole, suggesting that if this was correct they are extremely rare (link). Studies of the super-massive black holes suggest that their may be an upper-limit to how massive they can be, around 10 billion times that of the Sun (link). Since black holes do not have any structure, they are often thought to be some of the simplest objects in the universe (really!) - according to Einstein's theory of General Relativity, in order to completely describe the properties of a black hole you need to know its mass, its spin, and its electric charge (and real black holes in the universe and expected to have zero electrical charge). If so, stellar-mass black holes and super-massive black holes should accrete matter the same way - as observed for black holes in the nearby galaxy M81 (link). As mentioned before, it a normal star passes too close to a black hole, the black hole's gravity will cause material from the star to fall towards, and eventually fall inside, the black hole. As this material falls towards the black hole, it gets hotter - so hot that it produces a lot of X-rays. By looking at the periodic flickering of this light, it is possible to estimate the mass of the black hole. This was recently done for one black hole in the Milky Way, at they estimate a mass of just 3.8 times that of the Solar Mass, the lowest mass black hole known (link). Periodic flickering has also been observed from the super-massive black holes in the centers of other galaxies, and can be used to estimate their mass as well. This was done for galaxy RE J1034+396, which has a mass a million times that of the Sun (link). What causes the light from gas falling into a black hole to flicker is not known, but a recent study of the visible light flickering and X-ray light flickering from a Galactic black hole suggests that its magnetic field plays an important role (link). As mentioned before, there is a super-massive black hole in the center of our galaxy called Sgr A*, and material falling into this black hole also produces regular flares of light (link). In fact, there is evidence that, around 300 years ago, it produced a flash of light so bright that today we are seeing some of the light reflecting off molecular clouds near the Galactic center (link). A major goal of astronomers today is to make an image of light coming from the event horizon around a black hole. The best chance to do this is using radio telescopes to observe Sgr A*, and radio astronomers have gotten down to only 3 times the expected size of the event horizon (link 1, link 2). It is possible to use to orbits of objects around black holes to test general relativity (GR), just as one does this with neutron stars. This was recently done for the two black holes believed to be orbiting each other in center of galaxy OJ 287, and the time between the closest approach of these two black holes agreed with what GR predicts. The material that flows into a black hole is believed to form a disk (called an "accretion disk") around the black hole before it passes the event horizon. Quasars are believed to be galaxies where the optical light of this disk shines much, much brighter than all the stars in the rest of the galaxy. If so, the spectrum of the quasar - how bright the quasar is at different colors - should resemble that of a disk. This is hard to measure in great detail since the light at some colors is absorbed by material between the Earth and the galaxy, but a recent study suggests that it does (link). Additionally, for some unknown reason, the presence of gas flowing into the black hole is often associated with the presence of gas flowing AWAY from the black hole with very high velocities (>60 million mph). Astronomers at UCSC and University of Florida recently observed this outflow turn on around a quasar (link). Sometimes this very collimated outflow (called a "jet) is pointed directly at the Earth, in which case the galaxy is called a "blazar." Recent radio observation of a blazar suggest that these jets are powered by the magnetic field of the black hole (link). Quasars come in very different varieties, with one class recently observed to produce many more X-ray than previously thought to be possible (link). The presence of very, young stars around super-massive black holes like Sgr A* strongly suggest that stars can form in the accretion disk around them (article, link 1). Optical observations of these accretion disk have also detected weird optical filaments around them, suggesting this gas is confined by a weak magnetic field possibly generated by the black hole (link). While all galaxies are currently thought to have a super-massive black hole in their center, very few galaxies are quasars. Why some galaxies are quasars are most are not is unknown, but it is though that quasars are the result of two galaxies recently merging together.
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