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






When we talk about black hole, you may think of concepts like "wormhole" or "time tunnel" which give you a heavy flavour of sci-fi. In science fictions or movies, spaceship may enter a black hole, travel through spacetime via "wormhole" and finally emerge from somewhere in the Universe. However, most of the present astronomers will not take the concepts like spacetime travel seriously for reasons that they can neither find any mechanism leading to the formation of "wormhole", nor can they prove the existence of "wormhole" by way of astronomical observations. Moreover, recent studies on the topics demonstrate that even if "wormhole" does exist, it will be very unstable. An extremely small amount of matter passing through it will suffice to make it collapse. Up to now, the concept of spacetime travel by way of black hole can only be regarded as subjects of sci-fi and not a serious science.

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Other than some improvable guesses, interests in black hole studies in recent years are on the rise. What new discoveries have pushed astronomers into such relentless researches in everything connected with the black hole? How does the study of black hole help to the understanding of the evolution of the Universe?

Back in the 18th century, scientists like Laplace has already pointed out that highly compact objects might prevent their nearby light from escaping. Soon after Einstein published his General Theory of Relativity in the beginning of 20th century, Karl Schwarzschild found a mathematical solution of the theory to describe the spacetime structure of such object with spherical symmetry. That was the prelude to the study of black holes. Later on, Oppenheimer and others through calculation proved that supermassive stars under gravitational force could really collapse to form black holes. By the 70's, astronomers started to carry out systematic observations to look for evidence of black holes in binary system. The flush of observational evidence from the Hubble Space Telescope launched at the end of the 20th century further convinces us that black holes really exist. To the surprise of astronomers, black holes come in various sizes and origins, and are far more complicated than we can think of. For example, the sizes of black holes can vary immensely from a few to a few billions of solar masses! What is more important, the existence of these different kinds of black holes and the respective astronomical phenomena associated with them always brings far-reaching revelation to our understanding of the evolution of stars, galaxies and at last the whole Universe.

In the early days, man turned their eyes to the binary systems to look for black holes. From the spectral analysis of the orbit, if the invisible companion of a star in a binary system is 3.5 times heavier than that of the Sun, this dark celestial object is most probably a black hole. We take 3.5 solar masses as the benchmark for judging whether something is a black hole because we know we know that theoretically the mass of other compact objects (like neutron star) cannot exceed that maximum threshold. Otherwise those bodies will collapse under their own gravitational force into black holes. However, it is never an easy task to determine the mass of companion stars in binary systems just by way of spectral analysis. Miscalculations did always happen, as it is difficult to accurately measure factors like the luminosity of the visible stars in the pairs and the tilting of the orbits. Accretion disks formed when compact objects suck in matters of companion stars may also betray the existence of black holes. For neutron stars and black holes, accretion disk will emit high-energy X-rays when matters are spiraling in the compact objects, since immense gravitational force can cause substantial heat up of the matters. Searching for X-ray sources in the sky becomes the most important ways to locate neutron stars and black holes among binary system. 

In recent years, with the help of X-ray satellites, astronomers make remarkable progress in the search of the Holy Grail. The main difference between neutron stars and black holes lies in the fact that neutron star has a solid surface whereas the black hole does not. Studies show that, in a binary system, huge energy is released when matter of companion star fall onto the surface of neutron star. On the contrary, when matter falls into black hole, it together with the energy generated will disappear behind the event horizon. For that reason, X rays emitted by neutron star binaries are stronger and their spectra exhibit special characteristics. For the total energy generated during the accretion process, the part confiscated by a black hole could be 100 times higher than the radiation that can narrowly escape from the formidable gravitational force. Astronomers are almost sure that the dark companions of many X-ray binaries are black holes and not neutron stars. V404 Cygni is one of the most well-known examples.

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For a long time, black hole's event horizon and its association bizarre behaviours are only mathematical game. But observation by the Hubble Space Telescope in recent years provided convincing evidence for the existence of the event horizon. Researchers analysed a huge amount of ultra-violet radiation data coming from a compact object called Cygnus XR-1. They found two events showing the shortening of pulsating cycle and decaying of radiations intensity. The signatures matched theories of what scientists would predict to see. When matter is falling so close to the event horizon, it will be circling the black hole with increasing speed and its light will rapidly dim as it is stretched by gravity to ever-longer wavelengths. However, it is impossible for astronomers to see the even horizon directly due to the current technical limitation. Therefore what scientists discovered so far is only an indirect evidence of the gravitational redshift or similar phenomenon caused by black holes. But those results undoubtedly become an important bridge for linking black hole theory with actual observation.

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