previously at a temperature below 10 million degrees) gets heated by the gravitational collapse and H burning resumes briefly. The flood of energy produced by the hydrogen burning pushes the outer layers of the Sun, forcing them to expand to 10-100 times their diameter. Thus, the Sun moves away from the main sequence and becomes a Red GIANT Star.

 

The Sun will darken to a bright orange and, although the Sun is dying, it will glow thousand times brighter. The star ejects large amounts of material and after a temporary drop in size there is a final flame, in which the Sun gets larger, brighter and redder than ever. Inside, its structure is unstable and it pulsates slowly in and out with a period of about several months. The Sun's surface eventually extends as far as the surface of the Earth.



Its outer layers keep blowing off into space until the once powerful Sun has lost almost half of its original mass. The last layer around the innermost core is ejected as a glowing shell to create a planetary nebula. 



The core of the Sun, which has shrunk to about the size of the Earth, is finally exposed. The Sun moves once more around the H-R diagram This kind of star is called a White Dwarf Star. In this white-dwarf phase the "Sun" will cool down and fade over a very long period.



This is an H-R diagram which summarizes the life-track of stars.

The Death of  a Massive Star: 

Stars up to approximately 8 times the mass of the sun follow a life track similar to that of the Sun.  Larger stars, however, are vastly different. We can follow a 12 solar mass star as an example. Starting off blue white it  expands, cools and starts to turn yellow.  For a while it becomes unstable pulsing in and out for about a month. As it  shrinks and expands in turn its brightness changes in a regular way. It has become a Cepheid  variable star.

 Eventually the  massive stars becomes a  Red SUPERGIANT one thousand times larger than the Sun,  shedding some of its material along the way. In the star's core a series of different nuclear process keep it shinning. Low-mass stars, such as the Sun, can burn hydrogen and sometimes helium, but the temperature in the core is too low to support nuclear reactions involving nuclei with larger charges. Massive stars, on the other hand, can attain temperatures so high that not only helium but even heavier elements such as carbon can fuse. Eventually, Iron (Fe) is produced in the core and an iron core thousand kilometers across is formed. Iron is the most stable of nuclei so heavier elements can be produced in stars but now at an expense in energy.

At that critical moment, the core collapses rapidly (under a tenth of a second) until it is less than a kilometer across and some of the highest densities ever observed in the Universe are produced. Part of the imploding material produces a shock wave which blows the star apart and produces one of the most cataclysmic events in the universe: A Supernova explosion.

 Just a few years ago (1987) astronomers and high-energy physicists observed a Type II  Supernova in the Large Magellanic Cloud; a small companion  galaxy about 160,000 light years from us.

 The supernova, the explosion of a 15 solar mass blue supergiant star, was named SN1987A.Never before in the history of humanity had such an event been studied and recorded so precisely. About 20 hours before the supernova was detected with optical telescopes, the Earth was bathed in a 13-second burst of neutrinos from the exploding star. These neutrinos were recorded by underground experiments in Japan and the United States. This was the first time in human history that mankind recorded neutrinos from a dying star. In reality, of course, the star exploded about 160,000 years ago. Since then, the neutrino pulse has been expanding into space like a huge inflating bubble. The Earth passed through the bubble in 1987,  at an epoch in our history in which we were technologically able to witness and record the event. If the bubble had passed us just a decade earlier, the whole event would have gone unnoticed - as the means to record the event were not yet in place.

Before and AftER: The brightest star in the left picture is the first supernova to be visible to the unaided eye for almost 400 years. It occured in a region rich in young, blue stars and it was one of these which destroyed itself. When this picture was taken, about 2 weeks after the supernova was discovered, at the end of February, 1987, the expanding shell of material had already changed from blue to orange-red as it cooled. The location of the supernova in the Large Magellanic Cloud (LMC) means that it can only be seen from the southern hemisphere. This was very convenient in the early  stages of the development of the supernova, since the LMC is always above the horizon of the AAT.

Type I Supernovae:

Also caused by the collapse of a star but in this case the star is a white dwarf (typically 0.6 solar masses). The mass of the white dwarf, however, may increase if it has a companion star. If the amount of hydrogen accreted onto a white dwarf causes the star's mass to exceed 1.4 solar masses then the white dwarf will collapse and eventually explode as a Type I supernova, leaving no remnant behind.

Type II Supernovae:
As we will see in the next chapter, leave behind as a remnant a Neutron Star or a Black Hole. In 1928, while on his way to England from India, a graduate student by the name of Chandrasekhar calculated that a star heavier than about 1.4 solar masses cannot remain stable;  it will collapse. If the star's mass is less than 1.4 solar masses, however, it can remain as a stable white dwarf. The 1.4 solar mass limit is known as the Chandrasekhar Limit.

Nova Star:
We know that binary-star systems are very common. So it is possible to have a white dwarf star paired with a companion star. Hydrogen from the companion star can be deposited onto the white dwarf. This is called accretion. The accreted hydrogen forms an accretion disk in orbit about the white dwarf. As the hydrogen swirls down onto the white dwarf it heats up eventually reaching a temperature above 10 million degrees K-  a temperature hot enough for hydrogen to fuse into helium. This triggers an explosion called a nova; a Nova is a thermonuclear explosion on the surface of a white dwarf. Yet, this explosion at the surface, hardly disturbs the white dwarf and its companion star. Thus, sometimes this process can repeat itself. In this case the binary is called a recurrent nova.

The European Space Agency's ESA Faint Object Camera utilizing the corrective optics provided by NASA's COSTAR (Corrective Optics Space Telescope Axial Replacement), has given astronomers their best look yet at a rapidly ballooning bubble of gas blasted off a star. The shell surrounds Nova Cygni 1992. Nova Cygni is 10,430 light years away and is located in the summer constellation Cygnus the Swan. Nova Cygni erupted on February 19, 1992. A nova is a thermonuclear explosion that occurs on the surface of a white dwarf star in a double star system.