Two new studies have put Einstein's General Theory of Relativity to the test like never before, using observations of galaxy clusters to study the properties of gravity on cosmic scales. These results, made using data from NASA's Chandra X-ray Observatory, show Einstein's theory is still the best game in town. Such studies are crucial for understanding the evolution of the universe, both in the past and the future, and for probing the nature of dark energy, one of the biggest mysteries in science.

This composite image of the Abell 3376 galaxy cluster shows X-ray data from the Chandra X-ray Observatory and the ROSAT telescope in gold, an optical image from the Digitized Sky Survey in red, green and blue, and a radio image from the VLA in blue. The bullet-like appearance of the X-ray data is caused by a merger, as material flows into the galaxy cluster from the right side. The giant radio arcs on the left side of the image may be caused by shock waves generated by this merger.

Chandra observations of galaxy clusters have previously been used to show that dark energy has stifled the growth of these massive structures over the last 5 billion years and to provide independent evidence for the existence of dark energy by offering a different way to measure cosmic distances.

Basic life cycle of a star

                                                               

A Nuclear Furnace

A star is like a gigantic nuclear furnace. The nuclear reactions inside convert hydrogen into helium by means of a process known as fusion. It is this nuclear reaction that gives a star its energy. Fusion takes place when the nuclei of hydrogen atoms with one proton each fuse together to form helium atoms with two protons. A standard hydrogen atom has one proton in its nucleus. There are two isotopes of hydrogen, which also contain one proton, but contain neutrons as well. Deuterium contains one neutron while Tritium contains two. Deep within the star, A deuterium atom combines with a tritium atom. This forms a helium atom and an extra neutron. In the process, an incredible amount of energy is released. When the star's supply of hydrogen is used up, it begins to convert helium into oxygen and carbon. If the star is massive enough, it will continue until it converts carbon and oxygen into neon, sodium, magnesium, sulfur and silicon. Eventually, these elements are transformed into calcium, iron, nickel, chromium, copper and others until iron is formed. When the core becomes primarily iron, the star's nuclear reaction can no longer continue. This is because the temperature required to fuse iron is much too great. The inward pressure of gravity becomes stronger than the outward pressure of the nuclear reaction. The star collapses in on itself. What happens next depends on the star's original mass. 

The Circle of Life

Stars begin their lives as clouds of dust and gas called nebulae. The gravity of a passing star or the shock wave from a nearby supernova may cause the nebula to contract. Matter in the gas cloud will begin to coalesce into a dense region called a protostar. As the protostar continues to condense, it heats up. Eventually, it reaches a critical mass and nuclear fusion begins. This begins the main sequence phase of the star. It will spend most of its life in this stable phase. The life span of a star depends on its size. Very large, massive stars burn their fuel much faster than smaller stars. Their main sequence may last only a few hundred thousand years. Smaller stars will live on for billions of years because they burn their fuel much more slowly. Eventually, the star's fuel will begin to run out. It will expand into what is known as a red giant. Massive stars will become red supergiants. This phase will last until the star exhausts its remaining fuel. At this point, the pressure of the nuclear reaction is not strong enough to equalize the force of gravity and the star will collapse. Most average stars will blow away their outer atmospheres to form a planetary nebula. Their cores will remain behind and burn as a white dwarf until they cool down. What will be left is a dark ball of matter known as a black dwarf. If the star is massive enough, the collapse will trigger a violent explosion known as a supernova. If the remaining mass of the star is about 1.4 times that of our Sun, the core is unable to support itself and it will collapse further to become a neutron star. The matter inside the star will be compressed so tightly that its atoms are compacted into a dense shell of neutrons. If the remaining mass of the star is more than about three times that of the Sun, it will collapse so completely that it will literally disappear from the universe. What is left behind is an intense region of gravity called a black hole.

The nebula that was expelled from the star may continue to expand for millions of years. Eventually, the gravity of a passing star or the shock wave from a nearby supernova may cause it to contract, starting the entire process all over again. This process repeats itself throughout the universe in an endless cycle of birth, death, and rebirth. It is this cycle of stellar evolution that produces all of the heavy elements required for life. Our solar system formed from such a second or third generation nebula, leaving an abundance of heavy elements here on Earth and throughout the Solar System. This means that We are all made of star stuff. Every atom in our bodies was created either in the nuclear furnace of a star or in the cataclysmic explosion of a supernovas.

What is Relativity !!??

•Does every assertion make sense:      Obviously not . Even if we take some words and link them together in strict accordance with the rules of grammar , the result may be complete nonsense . There is no sense whatsoever ,for example , in the assertion that  “water is triangular”.               However , not all nonsense is so obvious . All too often an assertion which appears quite reasonable at first glance turns out to be absolute nonsense under closer scrutiny.                      
Right or left: On what side of the street-right or left-is the house ? You cannot possibly answer this question offhand.                   If you go from the bridge towards the woods , it will be on your left-hand side , and if you go in the opposite direction , it will be on your right hand side . Speaking of the left-or right hand side of a street you must mention the relative direction.                                  
Who is bigger??.....Consider a cow and a cowherd separated by a definite distance . When viewed from near the cowherd , the  cow seems to be smaller in size.               But when viewed from near the cow , the cowherd seems to be smaller in size . As the two cases are viewed from different points-one closer to the cowherd and the other closer to the cow , it is not the true dimensions of an object that are essential for analysis , but the angle from which they are viewed from . And these angular dimensions of the objects are quite obviously relative . It is senseless to speak about angular dimensions of the objects unless the latter are pinpointed in space . For instance , there is no sense in saying that a tower is seen from an angle of  45 degree .But if you say that a tower of 15metres away from you is seen at an angle of 45 degree , that is quite reasonable . It follows , moreover , that the tower is 15metres high.



  The relative appears absolute: If we shift our point of view of observation slightly , the angular dimensions  are often used in astronomy . Stellar maps are supplied with angular distances between the stars , i.e., the angles at which the distance between the stars is seen from the Earth. Regardless of our movements on the Earth , and regardless of  our point of observation , we shall always see the stars at one and the same distance from each other . This is due to the tremendous , inconceivably great distances that separate us from the stars .compared to them our movement on earth from point to point is so insignificant that we may easily disregard it . Therefore , in this case angular distances may be accepted as absolute distances .
  If we shift our point of view of observation slightly , the angular dimensions  are often used in astronomy . Stellar maps are supplied with angular distances between the stars , i.e., the angles at which the distance between the stars is seen from the Earth. Regardless of our movements on the Earth , and regardless of  our point of observation , we shall always see the stars at one and the same distance from each other . This is due to the tremendous , inconceivably great distances that separate us from the stars .compared to them our movement on earth from point to point is so insignificant that we may easily disregard it . Therefore , in this case angular distances may be accepted as absolute distances . The absolute turns out to be relative:We often say “up” and “down” . Are these notions absolute or relative ?                        At different times people gave different answers to this question . When people did not know that our Earth was round and imagined it to be as flat as a pancake , the vertical direction was regarded as an absolute concept . It was assumed that the vertical direction was one and the same at all  points of the Earth’s surface and that it was quite normal to speak of absolute “up” and the absolute ”down”.                      When it was discovered that Earth was round , the notion “vertical” collapsed.                    Indeed , the Earth being round , the direction of a vertical line depends essentially on the position of the point on the Earth’s surface through which that line passes.                 At different points of the globe the vertical direction will be different.               Since the notions of “up” and “ down” thus lost sense , unless the exact point on earth is specified , the absolute became relative.

“Common sense” protests:     All this appears obvious to us today and we do not doubt it in the least . Nevertheless , we know from history that it has not been easy for man to realize the relativity of “up” and ”down” . People are inclined to ascribe absolute sense to concepts if their relativity is not evident from everyday experience.         Let us recall the absurd objection to the fact that the Earth was round , which came down to us from the middle ages: how can people walk upside-down ….?!         If we did not recognize the relativity of the vertical direction and took it to be absolute in Moscow , for example , then , naturally , people in New Zealand would be walking upside-down . But bear in mind that for New Zealanders muscovites , too , are walking upside-down . There is no contradiction in that at all , since the vertical direction is not really an absolute concept , but a relative one.           We begin to feel the true meaning of the relativity of vertical directions only when we consider two points sufficiently far apart on the Earth’s surface-Moscow and New Zealand , for example. But when we take two points close to each other , then we are justified with the absolute concept.
The Tragedy of light: •  Light does not propagate instantaneously        We have convinced ourselves of the principle of the relativity of motion and of the existence of a countless number of “inertial frames” . However , there exists a kind of motion which , at first glance contradicts the principle we have established above . It is the propagation of light.
Principle of relativity of   motion seems to be shaken!!:: The colossal but not infinite velocity of light in vacuum brings us into conflict with the principle of relativity of motion.        Imagine a train hurtling along a tremendous speed of 240,000km/sec . We are riding in the head carriage , and an electric bulb is switched on in the tail carriage . Let us see what results we would get if we measured the time necessary for light to travel from one end of the train to the other.
                        It would seem that this would differ from the one we would obtain if the train were at rest . Indeed , relative to a train moving at 240,000km/sec  the light should travel at a speed of only
300,000 – 240,000=60,000km/sec . It is as if the light has to catch up with the head carriage . If we place the bulb at the head of the train and measure the time necessary for the light to reach the tail carriage , it would seem that its velocity in the direction opposite to the movement of the train should be 240,000 + 300,000 = 540,000km/sec . The light and the tail carriage move towards each other.
                          Thus , it appears that in a moving train light should propagate at different velocities in different directions , while in a train which is at standstill the velocity of light is the same in both directions...


Boarding a train….
Imagine a train 5,400,000km long moving with a uniform velocity of 240,000km/sec along a straight line.§               Suppose a lamp is switched on at some instant of time somewhere in the middle of the train . And suppose the automatic doors in the front and rear of the train carriages open the moment the light of the bulb reaches them . What will the people on board the train and those standing on the station platform see??§               The people in the middle of the train will see the following: as light travels relative to the train at the same velocity in all directions300,000km/sec , it will reach the rear and front carriages simultaneously , 9 seconds later (2,700,000:300,000) and both doors will open at the same time.          Relative to the station platform the light also travels at the speed of 300,000km/sec , but the rear carriage moves to meet the light beam . Therefore , the beam of light will reach rear carriage after  ,2,700,000/(300,000+240,000)i.e., 5 seconds . The beam . The beam must catch up with the front carriage and , therefore , will reach it 45 seconds later , 2,700,000/ (300,000-240,000). It will seem to the people on the platform that the doors open at different times-the rear door first and the front door 45-5=40 seconds later.       Thus , two absolutely identical events-opening of the front and rear doors of the train-will happen at the same time for the people onboard the train and with a 40-second interval for those on the platform. .....