Astro News
- Recent Updates of Astro News
- Active Mercury(07/09)
- Hubble Status Report: Directly Observes a Planet Orbiting Another Star(01/09)
- A Non-trivial Answer to a Trivial Astronomical Question-The Origin Of Absolute Magnitude(07/08)
- Assault by a Black Hole(04/08)
- New Lakes Discovered on Titan(01/08)
- ˇ§Deviant Behaviourˇ¨ in the Solar System(10/07)
- Cosmic Ripples - Cosmic Microwave Background - CMB(07/07)
- Interplanetary Superhighway(04/07)
- Is Pluto a Planet?(01/07)
- Breathing Moonrocks(10/06)
- My Thoughts on the Theory of Relativity, Quantum Mechanics, Superstring Theory and Dark Matter(07/06)
- Space-time Vortex(04/06)
- Radio Astronomy(01/06)
- Neutrino Astronomy(10/05)
- The Active Earth(07/05)
- What is Dark Energy?(04/05)
- The Mysterious Black Holes(01/05)
- Intermediate-Mass Black Holes And Quasisoft X-Ray Sources(10/04)
- Time Travel: From a Scientific Approach(07/04)
- What is Astrobiology?(04/04)
- Black Hole: From Fantasy To Reality (II)(01/04)
- Black Hole: From Fantasy To Reality (I)(10/03)
- From The Oldest Light In The Universe To The Fate Of The Universe(7/03)
- The Cosmic HERO(4/03)
- Quaoar - the Tenth Member of the Solar System?(1/03)
- The First Chinese Telescope in Space(10/02)
- Diamonds and Other Stardust(7/02)
- Supermassive Black Hole in Andromeda Galaxy(4/02)
- Detection of Solar Neutrinos(1/02)
- Simultaneous Multiple Wavwlength Observation(10/01)
- Celestial Distance(7/01)
- Solar-Terrestrial Relations(7/00)
- Fundamental Particles in Astronomy(4/00)
- The Solar Maximum in 2000(1/00)
- Hubble Constant(10/99)
- New Findings on Cosmology(7/99)
- Strange Stars(4/99)
- How Strong Stellar Magnetic Field Can Be?(1/99)



Important notices






Black holes are some of the most mysterious objects of the Universe. A mere mention of the name invokes a sense of enormous cosmic power at work. It is not surprising that they have been frequently portrayed in science fiction movies, maybe as the time-tunnel to travel to another universe, or as the dangerous region where everything near and far can get sucked into - sort of like a cosmic vacuum cleaner*. In this article I would like to discuss the basic theories about black holes, and some exciting new developments in the field.

The theory of black holes starts from the theory of gravity. The gravitational force is one of which we are probably most familiar with in our daily life; it is the force that keeps our feet firmly planted on the ground and the force which keeps the Earth in orbit around the Sun. Einstein theory of relativity told us that mass and energy determine the geometry of spacetime and any curvature of this spacetime shows itself as the gravitational force. 

Black holes are objects with such huge gravitational field that not even light, traveling at the highest speed allowed in the Universe, can escape. In 1971 John Wheeler named such objects black holes because even light cannot escape from them. If an object with the mass of the Sun were a black hole, its radius would only be 3 km (or 0.0004% of the solar radius). 

So how can we detect the existence of black holes? We know for sure that a black hole itself is invisible because no light can escape from it. However, they can be detected indirectly through their influence on the gas and dust from the stellar environment nearby. 

A black hole can accrete matter into itself as the matter falls or is pulled towards it. The accreting material then orbits around the black hole, forming an accretion disk. The tidal force of the orbiting material converts its own kinetic energy into heat, thus heating it up to a temperature as high as a few million degrees. We can deduce the existence of a black hole by measuring the strength and variation of the X-ray emitted by this hot gas. 

Astronomers have long thought that black holes come under two categories. The first is stellar-mass black holes, having a mass of a few solar masses as the name indicates. They are the end products of stellar evolution for very massive stars. We know that a star with mass lower than 8 solar masses, such as the Sun, will end its life as a cold, compact object called white dwarf. Higher mass stars will die in spectacular stellar explosions called supernovae, leaving behind even more compact objects - neutron stars (for stars of mass lower than 25 solar masses) and black holes (for stars of mass higher than 25 solar masses). 

The second category is the supermassive black holes found at the center of galaxies, each millions to billions of solar masses in size. Many of them were discovered in the distant Universe when it was at a young age. At that time, there were significantly more interactions between stars and galaxies, particular near the core of galaxies. We can deduce the existence of black holes by studying the dynamics of such Active Galactic Nuclei.

Despite the progress we made in the studies of black holes, numerous questions remain. Is there a black hole at the center of all galaxies, including our own? Are there micro-black holes formed in packages of extremely high density soon after the big bang explosion? Does a third class of black hole, intermediate mass black hole, exist? Recent theoretical calculations with super-computers using results from the Chandra X-Ray Observatory seem to answer to last question in the positive. However, in that case, is there any chance we can derive more direct observational evidence? With an array of new astronomical satellites soon to be launched in the next few years, we sure would be able to learn more about these mysterious objects.

* In fact, black holes do not behave like such cosmic vacuum cleaners. They only suck in everything that crosses the border (event-horizon) of the black holes. If our Sun were to suddenly collapse into a black hole of the same mass in a split second, the Earth's orbit would not change a bit! (Of course, we will feel a large drop in temperature though because we won't be receiving any energy from it!)

Close-up image of the galactic center M82 taken with the Chandra X-Ray Observatory. Theoretical modeling suggests that the star cluster MGG-11 is the first intermediate mass black hole candidate of 1000 times the mass of the Sun. (Courtesy: NASA/CXRO)

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Image of the jet streaming from the galactic center of the galaxy M87 taken with the Hubble Space Telescope. The mass of the supermassive black hole at the center of the galaxy is estimated to be 2 billion times the mass of the Sun. (Courtesy: NASA/HST)

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Image of Cygnus X-1, the first stellar black hole candidate. The central bright star HD 226868 is the binary companion of the black hole, currently undergoing mass loss to Cygnus X-1. (Courtesy: NASA)


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