## Life and death of Massive Stars

*Gravitational collapse of a massive star to black hole and naked singularity.*

- 20-Jan-2012

Modern science has introduced us to many strange ideas on the universe, but one of the strangest is the ultimate fate of a massive star that reached the end of its life-cycle. 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 the force of self-gravity, and it starts collapsing and shrinking catastrophically. Modest stars like the Sun also collapse but they stabilize later at a smaller size. However, if a star is massive enough, with tens of times the mass of the Sun, its gravity overwhelms all the forces that might halt the collapse. From a size of millions of kilometers across, the star crumples to a pinprick size, 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 secrets to man's search for a unified understanding of all forces of nature. It also may have 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 Chandrasekhar probed the question of final fate of stars such as the Sun. He 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. Eminent scientists of the time, in particular Arthur Eddington, refused to accept this result, saying how a star can become so small. Finally Chandrasekhar left Cambridge to settle in the USA. After many years, the prediction of white dwarfs was verified. Then it also 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 indeed supreme and it overtakes to shrink 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 outcome of such a collapse, as predicted by Einstein's theory of relativity, is a space-time singularity: an infinitely dense and extreme physical state of matter. Such a state is ordinarily not encountered in any of our usual experiences of the physical world.

As the star shrinks, an "event horizon" of gravity can possibly develop as the collapse progresses. The horizon is essentially a one way membrane that allows entry, but no exits are permitted. If the star enters the horizon before it collapsed to singularity, the result is a "Black Hole" that hides the final singularity. It is the permanent graveyard for the collapsing star.

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. This is the amazing place at the boundary of universe, if there is one, a region of arbitrarily huge densities which are billions of times the Sun's density.

An enormous creation and destruction of particles could take place in its vicinity, and one could imagine this as the "cosmic inter-play" of basic forces of nature which come together here 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 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 or the super-ultra-dense regions that arise in nature are visible to faraway observers, or they would be always hidden in the universe in a black hole. When they are visible, they are called a "Naked Singularity", or a "Quantum Star". 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 being visible, the unification of the fundamental forces which takes place in their vicinity becomes observable and testable.

While gravitation theory implies that singularities must form, we have no proof that the horizon must necessarily form during collapse. Therefore, an assumption was 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 existing theory of black holes and their modern astrophysical applications. But if the horizon did not form before the singularity, we will then be able to observe the super ultra-dense regions that form due to collapse of the massive star and the quantum gravity effects near such visible ultra-dense region or the naked singularity would become observable.

In recent years, a series of collapse models have been developed where the horizon failed to form in the collapse of a massive star. The mathematical models of collapsing stars and numerical simulations show that such horizons do not always form as the star collapsed. This is an 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. It turns out that the collapse of the massive star will give rise to either a black hole or a naked singularity, depending on the internal conditions within the star, such as its densities and pressure profiles, and velocities of the collapsing shells.

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, such as the cosmic Gamma rays bursts, which we do not understand 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.

References (popular to more technical):

- "Naked Singularities", Pankaj S. Joshi, "Scientific American", Feb 2009, Vol 300, No 2. (The talk by SciAm Editor in Chief, on the same topic is available at http://www.scientificamerican.com/podcast/episode.cfm?id=the-naked-singularity-meets-social-09-02-04 ).
- "Recent developments in gravitational collapse and spacetime singularities", Pankaj S. Joshi and Daniele Malafarina, Invited Review article in Int. J. Mod. Phys. D: No 14,Vol 20 (2011).
- "Gravitational collapse and Spacetime Singularities", Pankaj S. Joshi, Cambridge Univ Press, Cambridge, 2008.