The Crab Nebula is arguably the most spectacular event in the cosmos. A colour photograph taken in modern times shows its highly disturbed state with filaments curving across a cloudlike structure and evidence of fast-moving particles. When French astronomer Charles Messier began his catalogue of cosmic objects in 1771, not surprisingly, he began the list with the Crab Nebula.

Astronomers today know that what we are seeing here is the remnant of a star that was seen to explode on July 4, 1054. For the exact date and meticulous records we must thank the Chinese and Japanese astronomers of the time. In China, those were the days of the Sung dynasty and one of the prevailing beliefs was that if the emperor strayed from his prescribed righteous path, a sign from the heavens would warn him. Not to miss out on the timely warning the ruler had stargazers maintaining detailed records of the sky.
These records state that on that day a “guest” star appeared in the sky and stayed there for a few nights. Initially, it was so bright as to be visible even in day time. The Chinese observers were not aware of the existence of the star before July 4 and could not see it after it eventually faded out. For its brief presence in the sky the Chinese dubbed it a “guest” star. Today we know that as the star exploded its brightness shot up to a very high level, so as to be comparable with that of the whole galaxy of a hundred billion stars.
Astronomical studies of stars show that as stars several times as massive as the Sun evolve, they become unstable and explode. Radiation and fast-moving particles known as cosmic rays emerge from the deep interior of the shattered star which is now left with a dense inner core only. Such a star is called a supernova. The Crab picture shows the debris left behind by the supernova explosion. The distance of the object from us is around 6,000 light years. A light year is around 10 thousand billion kilometres. This is just as well! For if the Crab were not so far away, the high energy ejected from the explosion would have torn apart the protective layers of the Earth’s atmosphere leaving us vulnerable to harmful radiation existing beyond the supernova. One can say that such a danger arises from a supernova if it explodes within a distance of 30 light years from us.
Another cosmic threat to us on the Earth comes from within our own solar system. It is probably the same catastrophe that destroyed all those Jurassic animals long ago. The solar system is not limited to eight planets. There are other smaller objects like dwarf planets (to whose class Pluto was relegated in 2006), asteroids, meteors and meteorites, and comets, all circling the Sun under its force of gravity. There is a small but finite chance that one of these may collide with the Earth, and if that happens, it would produce a big catastrophe. As I mentioned in my sci-fi story The Comet, a comet seen to be on a collision course with the Earth is diverted by exploding a small but potent nuclear device near the incoming object. But a large collision would be hard to prevent. The dinosaurs may well have been destroyed by an impacting meteor or a comet, an event that is estimated to have taken place some 150 million years ago.
A meteor can be devastating too if it strikes the Earth. The meteor crater in Arizona and our own Lonar Crater Lake in the Buldhana district of Maharashtra show the aftermaths of the impact. The Lonar impact may have generated as much energy as 500 bombs of the kind that destroyed Hiroshima. But forewarned is forearmed, and to be able to take a preventive step like the one described above one must know at least a year beforehand that the Earth is going to be hit. To this end, under the name Project Spacewatch, astronomers are meticulously charting the trajectories of all the objects in the solar system which are likely candidates for such impacts.
A threat to our existence might come from a relatively sedate object, our Sun. The Sun has been steadily shining for nearly five billion years. Until recently many scientists thought that the Sun’s output of radiation only varied by a fraction of a percent over many years. However, recent measurements by satellites have suggested that the “solar constant”, that is, the Sun’s energy output at the Earth, may not be all that constant. Monitoring measurements during the early 1980s suggests a decrease of 0.1 per cent in the solar constant over just an 18-month time period. If this trend were to extend over several decades, it could influence global climate. For example, computer simulations predict that a change in solar output of only one per cent per century would alter the Earth’s average temperature by between 0.5 and 1.0° Celsius.
Why should the Sun be so fickle? As a physical object whose constitution and behaviour are governed by the laws of physics, calculations show that minor internal fluctuations can result in changes in external properties, like the Sun’s luminosity. Astronomers are getting more adept with time, at monitoring the Sun non-stop and at making accurate models of its internal constitution.
These are some examples of how cosmic threats are being perceived today. The more we examine them, the more we come to appreciate how fortunate we are in having such a benign location for our living. And the best we can do is to keep it that way and not further threaten it by our actions!

Jayant V. Narlikar, a renowned astrophysicist, is professor emeritus at Inter-University Centre for Astronomy and
Astrophysics, Pune University Campus