Mass Extinctions: Other Astrophysical Mechanisms The Universe is a dangerous place, and life can survive and thrive only in places that are relatively well protected from harmful external influences such as radiation. Any such place will be able to protect life for only a limited time, before some disaster strikes, as has happened many times in Earth's history. For example, a relatively stable star such as our own Sun (a "Main Sequence" star) produces a slowly-varying heat output for only a few thousands of millions of years before it starts running out of nuclear fuel. When this happens, such a star expands dramatically. It becomes a "Red Giant star, whose surface may engulf a planet orbiting it. Supernovae At the ends of their lives, stars that are significantly heavier than our Sun explode as supernovae. These produce for short periods as much light as an entire galaxy, as well as intense radiation. In a galaxy like our Milky Way, there is about one supernova explosion every 100-500 years. Famous supernovae include the one that exploded in the constellation Taurus (the Bull) in the year 1054, and "Tycho's Supernova" in Cassiopeia in 1572. Both of these were fortunately very far away. A supernova explosion within about one hundred light-years from the Earth would challenge severely the survival of life on our planet, either by direct radiation damage or by destroying the ozone layer that shields us from the Sun's ultraviolet radiation. Such nearby supernovae might have occurred several times during the history of the Earth, but there is no clear evidence yet that they might have caused any of the identified mass extinctions. Gamma-ray bursts About once a day throughout the observable Universe, we detect an intense burst of gamma radiation from some distant explosion. About once every hundred million years, it is thought that the Earth might be hit by a pulse of radiation from such a gamma-ray burster exploding within our own galaxy. Such an event might well be as disastrous for us as a nearby supernova. Other dangers from space As if these possible disasters were not enough, astrophysicists have raised other sinister possibilities, such as the passage of our solar system through a dust cloud that might obscure the Sun, passage through the galactic plane that disturbs the Oort cloud of comets and other debris surrounding the Solar System, or a collision with a cloud of dark matter that might cause genetic damage.

The well-known Crab Nebula obtained with the VLT. It is a remnant of a supernova explosion. The rapid fading of the optical counterpart of Gamma Ray Burst GRB 000131 (slightly left of the center), by means of exposures with the VLT. It is no longer visible on the last photo.