Protostar- in a nutshell

The current wisdom about the formation of a star goes something like this. first, you need a lot of matter containing various elements, mostly hydrogen, along with a lot of dust that might be the result of earlier happenings in space. 

Large clouds of such matter are visible in many parts of our Galaxy and in other galaxies in the universe. these clouds roam around, sometimes with turbulent motion, rotating and interacting with other clouds and stars. 
All particles in the cloud interact with each other due to force of gravity.
Fluctuations might lead to situation where the concentration of mass is Greater at some point as compared to the neighbourhood. 
This would lead to more and more particles being drawn towards that point of concentration. the big one continuous to become bigger. As the particles fall in together, the progress accelerates.

While this is happening, we have also to remember that the cloud was originally rotating, howsoever slowly . 
the particles that come from the direction of the axis of rotation can fall straight in towards the centre while those along the equator have to content with the centrifugal force. 
Therefore, the concentration of matter does not lead to a ball but to a disc - like shape. All this might take millions of years .In later stages , the particles falling in begin to collide with each other rather frequently and their energy becomes the chaotic energy of heat. 
Does the gravitational energy is converted to heat ;concentrated near the centre of attraction . 

This heat can be sent through infrared emissions. if the cloud is large enough, the process can proceed towards star formation of the clouds .

Before it gets there you can call the child star only a protostar. the true star phase of its life begins when the interior temperature rises to several million degrees or more . 
At those temperature the kinetic energy of protons and light chemical elements are high enough for thermonuclear reactions to occur ; the energies of particles can overcome the electrostatic repulsion of nuclei. fusion of four protons to form a Helium nucleus and a couple of positrons and some neutrinos can produce more energy per mass unit than any other fuel period ,this is the energy that ultimately enables the protostar to shine, and continue to shine till the fuel inside is exhausted .
 Depending on the mass of the stars that might take a few million years to many billions of years . the heavier ones Shine brighter and die earlier . our sun will continue to give us light and heat for several billion years.

Incidentally ,let us not forget the matter around the edges of the accreting disc . it is believed that planets are coagulated in those regions.

DARK ENERGY-THE DARK SECRET

We are passing through a unique period in the history of science. Based on observations and experiments in our laboratories and on internal consistency of the of the theories we had built, we were convinced that our universe is structured and controlled only by four types of forces . These were gravitation, electromagnetism, the weak force, and the strong force. These forces seemed to account for everything we saw or measured , here on earth or amongst stars and galaxies . There was no measurement or observation that was considered unexplainable. The basic principles incorporating these theories were not violated. The world of physics, chemistry, biology, geology, and astronomy threw up nothing that required the existence of an other type of force or energy. With e had also worked out theories that helped us to unify at least theree of these four interactions . The only residual agenda in this connection was the unification of gravity with all the other forces . Some ingenious experiments looked for a possible fifth force . These could only put upper limits of the order of a millionth the strength of gravity. Then came some surprising observations in astronomy and from cosmological theories. Some radical changes in the Constitution and the history of universe has been indicated , even though we are far from understanding the science behind them. The proposal for existence of dark energy was one of the new hypothesis .
This was entirely new . In addition, we did have mounting evidence that a dominant fraction of some matter in the universe is not visible to us. It has gravity like all other matter but it is dark, in the sense that it does not emit any detectable radiation. The nature of dark matter is still under discussion even though it's existence is observationaly strongly indicated.
I find a present situation rather uncomfortable. Till recently, we did our physics through observation and theory, connected with things happenings on earth. We had tremendous success in understanding the external universe using the very same physics. Now the distant universe is beginning to suggest objects and laws that are difficult, if not impossible to test here on earth! I , for one, am beginning to have some doubts about the validity of the evolutionary or bigbang model of the creation of the universe, in spit of the fact in many respect it seems so simple and compelling . In this regard, let me describe a piece of evidence brought forth to suggest that the bulk of the energy in the universe resides in a repulsive field called Dark energy.One of the methods used for determining the rate of expansion of the universe is to explore the relation between the red shift of astronomical objects and their distance . It is clear that we could get an approximate idea of the distance of an object if it were assumed to be a standard "candle" and it's brightness was measured. The rate at which it is receding from us would be known from the red shift of its spectral lines. A class of supernovae has been used to provide standard candles in this regard. It has been recently discovered that some of these supernovae at large red shifts appear to be too dim to be at the distance indicated by their red shifts if we use the standard value of the Hubble constant. This has been interpreted as indicating that universe has started to expand much faster than initially believed. A large repulsive force embedded in the fabric of the universe is believed to be responsible . The energy associated with this force is the so called Dark Energy!!.I cannot help being a little sceptical .Do we have enough confidence that the few supernovae we have used are truly standard light sources? Are we sure that the dimness of the sources is not due to absorption of their light by intervening dust clouds? Perhaps these doubts would be removed In time. However one is still left with a feeling that parameters are being chosen somewhat arbitrarily, to make the universe completely consistent with the dominant model of big bang  cosmology . We might end up getting a good parametric fit without a true physical foundation . That would be , at best , some tentative progress, not a proper understanding.

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