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Life and death of Massive Stars

[Article posted on 20-January-2012]
Pankaj S Joshi
©  Pankaj S Joshi
Gravitational collapse of a massive star to black hole and naked singularity.

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):
  1. "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 ).
  2. "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).
  3. "Gravitational collapse and Spacetime Singularities", Pankaj S. Joshi, Cambridge Univ Press, Cambridge, 2008.

FAQ

Click on the question to see the answers and comments

Arun Ragothaman : According to Einstein, heavy objects like stars planets create a small dent in the space -time fabric due to their gravity. He says that it is possible to travel faster than light when one travels across these dents in space-time fabric. My Question is, as singularities are the heaviest points in space-time they will create an enormous dent in the fabric, allowing us to travel many times faster than light if we are able to just stay at the edge of the event horizon without getting sucked in. This becomes even more feasible if naked singularities really exists. Is this theory correct and is it possible ?
  Pankaj S Joshi : Dear Arun, It is true that high matter energy densities create bigger spacetime curvatures in that region. Sometimes in such regions the causality violation occurs, that is faster than light, or what are called `closed timelike curves' do occur. However, that is not Always the case. Namely that in ultra-strong gravity regions also, the causality need not be necessarily violated always. Thus, there are naked singularity spacetimes, resulting from gravitational collapse, where there are no closed timelike curves. In general, I fully agree that the implications of singularities, and in particular those of visible singularities, are quite intriguing... much work is going on to examine such astrophysical and quantum gravity consequences. PSJ


Abraham Kattil : what kind of effect does a supernovae has on the dark matter if that star is in a place where is some very good presence of dark matter.? Is there any effect on dark energy by a black hole perhaps as big as the one in the center of the milky way?
  Pankaj S Joshi : Dear Abraham, The main effect of a supernova on its environment is the generation and propagation of a powerful shock that moves outwards from the vicinity of the star. Thus the dark matter, if present around there, also comes under and experiences this shock effect. As for dark energy, its really a cosmic phenomena, operating at scales even much more larger than the galaxies. This just at the scale of a galaxy it need not have necessarily any significant effect.


Tomin : I want to know whether chandrashekhar limit decides the formation of a blackhole or the internal conditions of a neutron star as in the case of naked singularities.
  Pankaj S Joshi : The Chandrasekhar limit decides the formation of a black hole in the sense that the star must be much more massive than that limit if it is to form a black hole. Similarly, there is a neutron star limit (of about 5-8 solar masses), and the star must be bigger than that to enter a continual collapse state which terminates in a black hole or naked singularity. In the collapse of such massive stars, it is the internal conditions that decide whether it will go to black hole or naked singularity.


Aniket : Do black holes produce thermal radiation, as expected on theoretical grounds? Does this radiation contain information about their inner structure, as suggested by Gauge-gravity duality, or not, as implied by Hawking's original calculation? If not, and black holes can evaporate away, what happens to the information stored in them
  Pankaj S Joshi : This is one of the profound mysteries, or unresolved issue, or contradiction if you say so, associated with black holes. As of now, if quantum mechanics is brought in, we get the radiation as predicted by Hawking. But this in turn creates `information paradox', and probably the riddle is as yet not solved. Another unsolved problem is that of the spacetime singularity sitting at the center of black holes.


Aniket : What is dark matter? How is it generated? Is it related to supersymmetry?
  Pankaj S Joshi : It is the matter that is other than all the usual known matter that we know, by and large. Its exact nature is not as yet known. It is believed mostly it could be some kind of supersymmetric particles such as neutralinos. But further details are not known or confirmed.

  Related questions:
Lone Irfan: It is usually said that when the big bang happened it created an equal amount of positive & negative energies. Is that negative energy the same thing we commonly call the dark energy or is it something else and does this negative energy have corresponding to it a negative matter as suggested by the Einstein equation E=mc2?
Pankaj S Joshi : Its a different form of matter than the one known usually, it does not produce or reflect light. As of now it is not clear what the dark matter particles are really, though they are sure to exist in one form or other. Supersymmetry does offer some possible candidares such as nutralinos.


Pranav Upadhyay : What is Symmetry breaking ? how energy released in the process depends on it ?
  Pankaj S Joshi : The term `symmetry breaking' is generally used in the context of the particle physics theories, with special reference to unifying the strong and weak nuclear forces, and the electromagnetic force. Here we are discussing essentially the extreme gravity effects when a massive star collapses on exhausting its internal nuclear fuel. In that context, moving away from symmetry generally means considering spacetime models which are not symmetric but more general. The main question then would be can we avoid spacetime singularities in such a case by going to more general non-symmetric models. The answer to that is no, singularities still form, at least within the classical gravity framework of general theory of relativity.


Aakash Narayan : what do you think in PBH this naked singularity take place , if there is then suggest some density parameter equation .applicable in JBD theory.
  Pankaj S Joshi : I do not quite understand what you meant by PBH, and sorry for my ignorance on the JBD theory. All our considerations are valid in the Einstein theory of gravity, with various physical conditions assumed for the regularity of the initial data from which the collapse commences, and such other conditions. For other details, please see the references mentioned below the article. For other theories of gravity, the conclusions could be still valid, but a detailed check may be necessary..

  Related questions:
Aakash Narayan : kindly please suggest the initial density,charge,angular momentum of PBH with thermodynamics equation applicable to it.
Pankaj S Joshi : Already answered.


Aakash Narayan : what do you think in PBH this naked singularity take place , if there is then suggest some density parameter equation .applicable in JBD theory.
  Pankaj S Joshi : The conclusions are generally applicable to PBH also. There are papers which also worked out for scalar fields in the JBD theory.


Aakash Narayan : SIR please provide relevant equation of the state which present the present phenomena of such massive star , say something about atlas star. aakash narayan lucknow university.
  Pankaj S Joshi : Such a phenomena can happen for various equations of state, including the case p=k \rho. Of course, for dust collapse also such a phenomena can happen.


Pranav Upadhyay : What is a quark star? is there any such star has been observed? in what way it is different from neutron stars?
  Pankaj S Joshi : Just as a neutron star is made of pure neutrons, even at higher densities possibly we may have a stable star made of quark particles only, which has been called a `Quark Star'. Of course, unlike neutron stars, this has not been verified as of today and it is at the moment a theoretical possibility only. Typically it should be even more compact and smaller as compared to the neutron stars, and will be also more denser.


Aakash Narayan : sir previously i hv mail 2 question but have'nt got any answer for how long does a blackk hole hold a charge . what happen iff hypernovae exceed bremstrahlung phase.
  Pankaj S Joshi : Typically, it is believed the black holes can have very little free charge. So the effect of charge should be quite negligible in general. But the effect of spin should be quite important in contrast.

  Related questions:
Aakash Narayan : According to Friedman cosmology.do you think is there any condition in which whole universe can be compress into small ball of infinite density or singularity.
Pankaj S Joshi : The stars are supposed to have very little free charge. So a black hole will have a very little charge, if at all. In the Friedmann models, there is always an initial singularity, called the bigbang.


Ajay Kumar Singh : What is the boundary of black hole? Is it space? Then what is the boundary of space?
  Pankaj S Joshi : The boundary of the black hole is called an `event horizon'. It separates the region of space and time that cannot communicate with any external observer, from the region that can. As no light or particle signal from that region can travel to external observer, that is why this region is called `black hole'.


Ajay Kumar Singh : What is the boundary of black hole? Is it space? Then what is the boundary of space?
  Pankaj S Joshi : The boundary of the balckhole is the region in the spacetime from which no light will escape the faraway observer. This is precisely the event horizon in the spacetime. But event horizon is an abstarct geometric surface, unlike the hard real surface of a neutron star, for example.


Gils Mathew : what is meant by dark energy ?
  Pankaj S Joshi : It is very unlike the visible form of energy such as that in visible matter such as stars and interstellar medium. It is basically an energy field that has positive energy density but negative pressure.


Gils Mathew : what happens for dark energy during expansion of universe
  Pankaj S Joshi : Nothing happens to it directly. It is an energy field that is not directly affected by the expansion or contraction of the universe.


Paras Singh : if there exist black holes that regularly take in matter and energy then there must also be white holes that continuously emit matter and energy, do they exist?
  Pankaj S Joshi : The existence of white holes, the time reversed black hole solutions is not yet confirmed. There also do not seem to be any definite signals that indicate their presence.