What is super Nova ?????

 What is a nova? A supernova?

A nova – the name means “new” in Latin – is a star that suddenly becomes enormously bright. Novae were so named because they appeared where no star had been seen before, but that was simply because they had been too faint to be visible to the naked eye. And when they did become visible, it was because they had undergone a violent nuclear explosion.Most novae are the result of an explosion in a binary star system  in whichone member of the pair has already exhausted its hydrogen to become a white dwarf  and the other is a normal “main sequence” star that has exhausted the nuclear fuel in its interior, its outer layers have expanded, and it has become a red giant.
As the atmosphere of the aging giant expands, the material is captured by its dwarf
companion. Such a transfer of matter onto a stellar surface is called accretion.
The explosion occurs as the material from the red giant is deposited on the surface of
the white dwarf. Compressed under the white dwarf’s gravity and heated to the point
of triggering nuclear fusion, the accreted material releases a vast amount of energy as
Artist’s view of a nova event. The white dwarf on the left is accumulating matter from its companion, a red giant. The accreted matter explodes in a nuclear fusion reaction when it reaches the
surface of the white dwarf. it explodes, blowing the gases away from the white dwarf at incredible speeds,up to thousands of km/s,and causing a sudden brightening of the binary pair by a factor of 50 000 to 100 000. Although the brightening is dramatic,only a small amount of the total mass of the
system is ejected – about 1/10 000th of the mass of the Sun. The process is sometimes repeated peri-
odically: the binary star RS Ophiuchi, for example


 has exploded approx-
imately every 20 years
over the past century. Its
last explosion occurred in20 Stars
The supernova SN 1987A appeared in
February 1987 in the Large Magellanic
Cloud, a companion galaxy to our own.
The event actually occurred 160 000 years
ago, but the light has taken that long to
reach us. The glowing ring is produced by
the shock wave from the supernova as it
encounters gas left behind by previous
events. The image was obtained with the
Hubble Space Telescope in 1994.
If the white dwarf accretes a very large amount of material from its companion, it
undergoes “runaway” nuclear fusion, an event that completely destroys the star in an explosion even more gigantic than that produced in a mere nova: this is a supernova. Such an explosion in which a star is completely destroyed can also occur at the end
of the life of a single massive star. For a star of between one and eight solar masses, life ends with it ejecting most of its material in the form of a planetary nebula . leaving only the core. Very massive stars, those over eight solar masses, annihilate even
their cores in their death throes. Here is how it happens.
In a star of modest mass, when the hydrogen fuel in its core has been exhausted,energy continues to be produced by the fusion of helium into carbon and oxygen, butthe reactions stop there. In truly massive stars, the pressure and temperature becomeso intense that the fusion continues, at first with oxygen being merged to form silicon,and then, in continued fusion events, with the production of elements up to iron.
Fusion of hydrogen Non-burning hydrogen  Iron ,Fusion of helium ,Fusion of carbon ,Fusion of neon
Fusion of oxygen ,Fusion of silicon ,Diagram of layers of fusion in the core of an older, very massive star.The extremely dense nucleus,approximately the size of Earth, is tiny compared to the size of the
star.
Fusion processes must stop once the stellar core has become iron because the merging
of elements heavier than iron actually consumes energy rather than releases it
At this point, since energy is no longer being produced in the core, the internal
pressure drops and gravity forces cause the outer layers to collapse. The massive star
suddenly implodes, compressing the core to the point where its protons and neutrons
are squeezed into close contact with each other. The density of such a core is enormous:
a teaspoon of this degenerate matter would weigh 400 million tons. The core responds
with an explosion of incredible violence, sending a titanic shock wave throughout the
star. The explosion can be so stupendous and the star’s collapse so complete that the
result can be the creation of a black hole.
The explosion blows off a significant amount of hot material which expands rapidly
outward at 5000–20 000 km/s, producing the dramatic brightening of a supernova. The
brightening can be five billion times the brightness of the Sun.
The matter ejected during the explosion is so hot that many nuclear reactions are
triggered and a series of heavy chemical elements are produced. In fact, supernovae are
the primary source of heavy elements in the Universe, including plutonium, uranium,
and other exotic elements. This material, rapidly ejected into the surrounding space,
eventually drifts into contact with clouds of gas and dust in interstellar space and can
eventually be incorporated into new stars and planetary systems like our own.
The most spectacular supernova in our galaxy in historical times occurred in 1054.
Noted by Chinese and Korean observers, the bright new star lasted several weeks and
was visible even during the day. The next supernova in our galaxy could very well be
Eta Carinae, but nobody can predict when this might happen. It could be in the next
few years – or in a million years. The mass of this star is 100 times that of the Sun, and
it has already begun to manifest large variations in brightness.
Of the several types of supernova that are recognized, depending upon the exact
mechanism that produces the explosion, the Type Ia is of particular interest. The time it
The Crab Nebula, the remains of the supernova reported by the Chinese in 1054 AD. Its diameter is enormous,11 LY, and it is expanding at 1500 km/s. In the center is a pulsar
, a neutron star that rotates at 30 times per second and emits strong
gamma radiation. Stars Eta Carinae, a massive star in the sky of the southern hemisphere. The two lobes, which are the size of our solar system, are composed of gas ejected in an explosion
that occurred 150 years ago,takes for supernovae to brighten rapidly and then dim has a profile called a light curve.
.As it happens, the maximum intrinsic luminosity of Type Ia supernovae is remarkably
uniform, with some subtle adjustments depending upon how steeply the light curve
declines. This allows astronomers to use them as “standard candles” to measure distances.
If such a supernova is discovered in a distant galaxy, its intrinsic luminosity can then
be determined which, when compared to its observed brightness, yields the distance to
that galaxy. This is the same method as that used with Cepheid variables  but
supernovae are much brighter, and can reach deeper into the Universe.