The Physics of the Universe - Difficult Topics Made Understandable

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Main Topics: Black Holes and Wormholes


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A slightly different kind of supernova explosion occurs when even larger, hotter stars (blue giants and blue supergiants) reach the end of their short, dramatic lives. These stars are hot enough to burn not just hydrogen and helium as fuel, but also carbon, oxygen and silicon. Eventually, the fusion in these stars forms the element iron (which is the most stable of all nuclei, and will not easily fuse into heavier elements), which effectively ends the nuclear fusion process within the star. Lacking fuel for fusion, the temperature of the star decreases and the rate of collapse due to gravity increases, until it collapse completely on itself, blowing out material in a massive supernova explosion.

If the mass of the compressed remnant of the star exceeds about 3 - 4 solar masses, then even the degeneracy pressure of neutrons is insufficient to halt the collapse and, instead of forming a neutron star, the core collapses completely into a gravitational singularity, a single point containing all the mass of the entire original star. The gravity in such a phenomenon is so strong that it overwhelms all other forces, to the extent that even light can not escape from it, hence the name black hole. Thus, the gravity of a body just a few times denser than a neutron star would result in its inevitable further collapse into a black hole.

Simulated black hole in front of the Milky Way - click for larger version
(Click for a larger version)
Simulated black hole in front of the Milky Way
(Source: Space Time Travel:
- Credit: Ute Kraus)

Although singularity at the the centre of a black hole is infinitely dense, the black hole itself is not necessarily huge, as is sometimes assumed. A black hole with the mass of our Sun, for example, would have a radius of just three kilometres (roughly two hundred million times smaller than the Sun), while one with the mass of the Earth would fit in the palm of your hand! Having said that, black holes can grow to great size over time as they assimilate more and more matter and even other black holes, and some do become extremely massive.

Contrary to popular belief, a black hole does not just "suck up" everything around it in an uncontrolled orgy of destruction: it actually exerts no more gravitational pull on the objects around it than the original star from which it was formed, and any objects orbiting the original star (and which survived the supernova blast) would now orbit a black hole instead (an object would need to approach quite close to a black hole before being sucked in). The very largest blue stars may skip even the supernova stage, so that even their outer shells become incorporated into the singularity.

By definition, we cannot observe black holes directly, but they can be detected by the gravitational effect they exert on other bodies or on light rays. This is especially easy to spot in the case of binary star systems where an ordinary star is orbiting around a black hole. In the early 1990s, Reinhard Genzel pioneered this work, using the then new technique of adaptive optics to plot and track the motions of stars near the centre of our own Milky Way galaxy, to show that they must be orbiting a very massive, but invisible, object. From the immense speed with which the stars closest to the centre of the galaxy are orbiting - millions of kilometeres per hour - we know that there is a "supermassive black hole" (known as Sagittarius A) at the centre of the Milky Way, with a mass of around 2 - 4 million times that of our Sun. In addition, in the Milky Way galaxy alone, there are many millions of black holes of at least ten solar masses each.

Supermassive black holes lurk in the centres of most galaxies, forming the hubs around which the galaxies rotate. In fact, from observations of the intense radiation of gases swirling around them at close to the speed of light, we can infer that there are much larger supermassive black holes in the centres of other galaxies, some of them weighing as much as several billion suns. The black hole at the centre of a galaxy known as M87 has a mass estimated at around 20 billion solar masses, and may be as large as our entire Solar System.

It seems likely that the early universe, in which very large, short-lived stars were the norm, was scattered with many, many black holes, which gradually merged together over time, creating larger and larger black holes. Observations have shown that is not uncommon for two black holes to swirl around each other in a kind of cosmic dance as their gravitational fields interact. The ripples in space-time caused by two black holes orbiting around each other - typically in a three-leaved clover shape or more complex multi-pass configuration, rather than the simple orbit of an electron within an atom, and ever-smaller and faster as the two objects inevitably approach each other - can be recorded visually and even audibly.

Long and short gamma ray bursts - click for larger version
(Click for a larger version)
Long and short gamma ray bursts
(Source: Internet Encyclopedia of Science:
- Credit: Ute Kraus)

In the case of the largest events, moments after the creation of a black hole, the heat and the hugely amplified magnetic field of the collapsing star combine to focus a pair of tight beams or jets of radiation, perpendicular to the spinning plane of the accretion disk. These beams focus vast amounts of particles and energy (of the order of a billion billion times the energy output of our Sun) away from the black hole at close to the speed of light. The shock waves of this massively enegetic beam cause gamma rays to be emitted in a phenomenon known as a "gamma ray burst" or "hypernova" event (so named because its energy and brightness dwarfs even that of a supernova, by a factor of upto a hundred million times). Gamma ray bursts are by far the brightest electromagnetic events occurring in the universe, and can last from mere milliseconds to nearly an hour - a typical burst lasts a few seconds - usually followed by a longer-lived “afterglow” emitting at longer wavelengths (x-ray, ultraviolet, visible, infrared and radio waves). It is likely that collisions between neutron stars, or between a neutron star and a black hole, can also cause gamma ray bursts.

Interestingly, it appears to be easier for stars with fewer heavy elements to turn hypernova and generate gamma ray bursts. That, and the fact that larger, more short-lived stars were more common earlier in the life of the universe, mean that the phenomenon of gamma ray bursts is actually rarer today than it was. Having said that, NASA's Swift Probe, launched in 2004 with a mission specifically to locate gamma ray bursts throughout the universe, is recording at least one such event each day, so these are not rare incidents. (It should be remembered that any supernovas or gamma ray bursts we observe today in galaxies, say, 9 billion light years away, actually occurred 9 billion years ago.)

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