The black hole enigma

By Arif Ali Abbasi

Never has an issue grasped the attention of so many astronomers, as well as astronomy enthusiasts around the world as has that of black holes. First predicted in 1915 by Albert Einstein (1879-1955), it has continued to grab the attention and imagination of the succeeding generations of curious enthusiasts. This is our last discussion on this mesmerising topic, unless of course the need to bring it up is forced on us again. Enough has also been said about what they really are; what remains to be related is whether there are just one, or more kinds of black holes? This aspect of the topic occupies our attention today. The lifecycle of massive stars have higher levels of gravitational force. What is a massive star after all? Let us say that it would be three times as massive as our sun. In the case of massive stars, the temperature in their core (the very centre) can rise to one billion degrees and more (one followed by nine zeros), and elements as massive as iron may be produced before the core collapses. Although a significant portion of the star’s mass will be thrown off in the accompanying supernova explosion, the momentum created in the core cannot be stopped by neutron pressure, and its inevitable fate is a black hole. Based upon Einstein’s work of 1915, Karl Schwarzschild (a German scientist, 1873-1916) predicted in 1916 that when a star collapses to a certain size, depending only upon its mass, the radiation (any radiation) from the star can no longer escape owing to the enormous gravitational forces that will ensue. For example, if a star of three solar masses were to collapse into a black hole, this trapping of radiation will occur after the radius has been reduced to a mere nine kilometres. The collapse will not stop at this incredible stage since it is thought to continue to a point approaching infinite density and infinite tidal forces. This condition is called a singularity. It is hard to imagine the effect of increasing surface gravity that accompanies the acute contraction of a body into a black hole. Imagine that a beam of light is somehow produced. However, it cannot leave the surface of a star that was. It becomes permanently captive and just cannot leave the body. In fact, it is captured the moment the beam is lit. The beam of light has nowhere to go! Such is the audacity of a black hole! But that is not all; the body will continue to shrink to a singularity (as explained above), i.e. absolute limit until it is no longer possible for it to shrink any further. It has a zero kilometre radius, or no radius at all. That is what we mean by singularity. By now you have a fairly good idea of how a star comes into being, and then, billions of years later, how it meets its end or death. But what a stormy death! However, this speculation is not complete without some material available to be sucked into the black hole. One very natural suggestion that made its appearance was that it is a binary star system in which one member is a black hole and the other is a red giant forced to surrender matter to the black hole. Instead of flowing, or being sucked into the black hole, the super-heated gasses tend to form an orbiting disc around the ultra dense object. Gas particles orbit the black hole at very high velocities and become very turbulent just above it, raising the temperature of the gas to many tens of millions of degrees, before being sucked in. Again, all of this must happen when the star is of a mass of about three solar masses. The gasses are eventually sucked in, as stated earlier, and because of the ever increasing speed of their ‘entry’, gain in temperature — million upon millions of degrees. Any less solar mass, as is commonplace, it is another matter. There can be a black hole where its planets will stay in their orbits. From our discussion so far, it is difficult to imagine a black hole giving up or surrendering some of its energy to the outside universe. However, a rotating black hole will do so. Just as a revolving ice skater pulls in her arms, she spins faster and faster, conserving angular momentum. Likewise, a rotating star will spin faster as it collapses. And gives up very little of the energy it is trying to conserve, until it reaches a stage when no energy is lost. Only accreted inward. Remember, from the outset it is accepted that nothing can escape the surface (read interior) of a black hole. After all, if light cannot escape, and nothing can travel faster than light, then nothing can escape it. Also, black holes do not exist in isolation; they have to have a companion, and it is a binary. The companion surrenders matter to the black hole, in the process increasing its mass; besides, the falling matter continues to gain in temperature by leaps and bounds, as you already know, until the star begins to emit X- rays. All black holes differ from one another in small ways. There may be a rotating, or a non-rotating one. And, horror of horrors, there are black holes which are billions, nay, tens of billions of times bigger than our sun! These reside in the nucleus of a galaxy. The centre of galaxies are known to harbour them. Yet, curiously, the nucleus of all galaxies are shown to be bright! If black holes reside in the nucleus of galaxies, and they are the blackest of black, why do they appear so brightly lit? Yet another stigma, or enigma of science!

Comet making closest approach ever of Earth

CAPE CANAVERAL (Florida, US): A recently discovered comet is closer than it’s ever been to Earth, and stargazers in the Northern Hemisphere finally get to see it. Called Pan-STARRS, the comet passed within 160 million kilometres of Earth on Tuesday (March 5), its closest approach in its first-ever cruise through the inner Solar System. The ice ball will get even nearer the sun this weekend — just 45 million kilometres from the sun and within the orbit of Mercury. The comet has been visible for weeks from the Southern Hemisphere. Now the top half of the world gets a glimpse as well. California astronomer Tony Phillips said the comet’s proximity to the moon made it easier for novice sky watchers to find it. He warned onlookers to avoid pointing them at the setting sun. “Wait until the sun is fully below the horizon to scan for the comet in the darkening twilight,” Phillips advised in an email sent from his home and observatory in the Sierra Nevada Mountains. Pan-STARRS’ name is actually an acronym for the Hawaiian telescope used to spot it two years ago: the Panoramic Survey Telescope and Rapid Response System. The volcano-top telescope is on constant prowl for dangerous asteroids and comets that might be headed our way. Thought to be billions of years old, the comet originated in the distant Oort cloud — a cloud of icy bodies well beyond the orbits of Neptune and Pluto — and somehow got propelled toward the inner Solar System. It’s never passed by Earth before, Phillips said. A much brighter comet show, meanwhile, is on the way. Comet ISON may come close to outshining the moon in November. It was discovered last September by Russian astronomers and got its acronym name from the International Scientific Optical Network. Neither Pan-STARRS nor ISON pose a threat to Earth, according to scientists. — AP