FINAL FATE OF A MASSIVE STAR Modern science has introduced the world to plenty of strange ideas, but surely one of the strangest is the fate of a massive star that has reached the end of its life. Having exhausted the fuel that sustained it for millions of years, the star is no longer able to hold itself up under its own weight, and it starts collapsing catastrophically. Modest stars like the sun also collapse but they stabilize again at a smaller size, whereas if a star is massive enough, its gravity overwhelms all the forces that might halt the collapse. From a size of Fig. 1 Black Hole (Artist’s conception) millions of kilometers across, the star crumples to a pinprick smaller than the dot on an “i.” What is the final fate of such massive collapsing stars? This is one of the most exciting questions in astrophysics and modern cosmology today. An amazing inter-play of the key forces of nature takes place here, including the gravity and quantum mechanics. This phenomenon may hold the key secrets to man’s search for a unified understanding of all forces of nature. It also may have most exciting connections and implications for astronomy and high energy astrophysics. This is an outstanding unresolved issue that excites physicists and the lay person alike. The story really began some eight decades ago when Subrahmanyan Fig. 2 An artist’s conception of Chandrasekhar probed the question of final fate of stars such as the Sun. He naked singularity showed that such a star, on exhausting its internal nuclear fuel, would stabilize as a `white dwarf', which is about a thousand kilometers in size. The British masters were in disbelief, refused to accept his results, saying how a star can become so small. Ultimately Chandrasekhar left Cambridge to settle in the USA. After many years, the prediction of white dwarfs was verified. It also then became known that stars three to five times the Sun give rise to what are called Neutron stars, just about ten kilometers in size, after causing a supernova explosion. But when the star has a mass more than these limits, the force of gravity is supreme and shrinks the star in a continual gravitational collapse. No stable configuration is then possible, and the star which lived for millions of years would then catastrophically collapse within matter of seconds. The Einstein’s theory of relativity then predicts that the outcome is a spacetime singularity. This is an arena where an infinitely dense and extreme physical state is reached. Such a state is ordinarily not encountered in any of our usual experiences of the physical world. As per our current understanding of physics, it was one such singularity, called the `big bang’ that created our expanding universe as we see it today. Such singularities will be again produced when massive stars die and collapse in the cosmos. The singularity is the most amazing place at the boundary of universe, if there is one. It is a region of arbitrarily huge densities, billions of times over the Sun's density. A tremendous creation and destruction of particles will take place in its vicinity. One could imagine it as the `cosmic dance' of basic forces of nature, which may come together in a unified manner. This is because the energies and all physical quantities reach their extreme values in the vicinity of such a singularity. The quantum gravity effects should dominate in such a regime. Thus, the collapsing star may hold secrets vital for man's search for a unified understanding of all forces of nature. The question then arises, whether such singularities are visible to faraway observers, or they are always hidden in the universe. When they are visible, they are called `Quantum stars' or a `naked singularity'. The visibility or otherwise of such a super-ultra-dense fireball the star has turned into, is one of the most exciting questions in astrophysics and modern cosmology today. Because, in the case of their being Fig 3: The fate of a star depends on its mass visible, the unification of the fundamental key forces of nature which takes place in their vicinity, becomes observable and testable. As the star collapses, an `event horizon' of gravity can possibly develop as the shrinkage of the star progresses. The horizon is essentially a kind of one way membrane that allows entry, but no exits are permitted. In such a case, if the star enters the horizon before it collapsed to singularity, the result is a `Black Hole' (Fig. 1) that hides the final singularity. It is the permanent graveyard for the collapsing star. While gravitation theory implies that singularities must form, we have no proof today that the horizon must necessarily form during collapse. Therefore, an assumption is made that an event horizon always does form, hiding all the singularities of collapse. This is called the `Cosmic Censorship' conjecture, which is the foundation of the exiting theory of black holes and their astrophysical applications. But what if the horizon did not form before the singularity, as the star collapsed? We will then be able to visualize and observe the super ultra-dense regions that form due to collapse of the massive star. Then the quantum gravity play taking place near such visible ultra-dense regions or the naked singularity would become observable. (Fig. 2) In recent years, a series of collapse models have been developed where the horizon failed to form in the collapse of a massive star. This is a most exciting scenario. Because, the singularity now becomes visible to external observers in the universe, who can then actually see the extreme physics taking place in the vicinity of such ultimate ultra-dense regions that formed in the collapse of the star. It turns out that the collapse of the massive star will give rise to either a black hole or a naked singularity (Fig. 3), depending on the internal conditions within the star, such as its densities and pressure profiles, and velocities of the Fig 4: An exploding star collapsing shells (for further details, see e.g. Joshi P. S., `Naked singularities', Scientific American, Feb 2009. See also the talk by SciAm Editor in Chief, at the link http://www.scientificamerican.com/podcast/episode.cfm?id=the-naked-singularity-meets-social-09-02- 04). When a naked singularity happens, small inhomogeneities in matter densities, very close to the singularity, could then spread out and magnify enormously to create extremely energetic shock waves. This, in turn, may have connections to extra-ordinary high energy astrophysical phenomena (figure 4), such as the cosmic Gamma rays bursts, which are totally non-understood today. Also, clues to constructing quantum gravity--a unified theory of forces, may emerge through observing such ultra-high density regions. Shall we be able to see this `Cosmic Dance' drama of collapsing stars in the theater of skies? Or will the `black hole' curtain always hide and close it forever, even before the cosmic play could barely begin? Only the future observations of massive collapsing stars in the universe would tell. Prof. Pankaj S. Joshi is a Senior Professor in the Department of Astronomy & Astrophysics at TIFR. He can be contacted on 022-22782486, email: [email protected] .