 | WHITE DWARFS |
| NOVAE |
| SUPERNOVAE |
This is the fate of stars like the Sun, that is, low to medium mass stars. As the hydrogen in a star's core is used up the star expands
into a red giant and gradually the outer layers of the star are blown off by the pressure
of the star's radiation.
Planetary Nebula
The outer layers become a planetary nebula-which is nothing more than a shell of expanding gas. (See Hubble's Planetary Nebula Gallery. Also, see animation at this Hubble site.)At the center is left a core star that gradually cools to a white dwarf.
Properties
Radius about 104 km, that is, about the size of the Earth.Mass about one Solar mass.
Density about 106 g per cubic centimeter. One teaspoon of stellar material weighs as much as an automobile!
Accretion Disk
Matter (mostly hydrogen) from the companion star can be stripped from it and deposited onto the white dwarf. This process forms an accretion disk about the white dwarf.The hydrogen flows onto the white dwarf through a narrow channel about the Lagrangian point. The Lagrangian point is the point between two objects where the gravitational pulls are equal.
The hydrogen heats up as it approaches the surface of the white dwarf. Indeed, the temperature can eventually exceed about 107 K, that is, a temperature high enough for hydrogen to fuse into helium. If that temperature is reached an explosive nuclear chain reaction can occur, called a nova.
A nova is a thermonuclear explosion on the surface of a white dwarf; basically, it is a gigantic star-sized hydrogen bomb.
Sometimes this process can occur repeatedly. Then the binary system is called a recurrent nova.SUPERNOVAE
Type I
In 1928, while on his way to England, from India, the Indian graduate student Subrahmanyan Chandrasekhar calculated thata 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 called the Chandrasekhar Limit.
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 star will collapse and explode leaving no remnant. This sort of explosion is called a Type I supernova. (Such supernovae have recently become extremely important as a tool for mapping the universe on very large scales.)
Type II
This is the end-point in the evolution of a massive star like Betelgeuse. The nuclear reaction that creates iron causes the core to cool and the pressure in the core to decrease.At some critical point the pressure in the core is insufficient to withstand the star's gravitational compression, and the iron core collapses. The outer layers of the star fall inward onto the core causing it to heat up rapidly to extremely high temperatures.
This triggers a fantastically powerful thermonuclear explosion called a Type II supernova. The catastrophic destruction of the iron core releases an intense neutrino plasma that races outwards at the speed of light.
The neutrino flux is intense: it carries away about 99% of the energy of the explosion. We noted earlier that neutrinos interact so weakly with matter that they can sail out of the Sun with little resistance. But in supernova explosions, their density is so enormous that the neutrino plasma actually pushes the outer layers of the core and the surrounding matter outwards, in the process reheating the expanding layers and causing them to glow billions of times brighter than normal.
As the outer layers expand the core continues to be crushed. What is left behind is a very strange object: either a neutron star or black hole.
This was the explosion of a 15 solar mass blue super giant star.
About 20 hours before the supernova was detected with optical
telescopes the Earth was hit by the neutrino shock-wave from the exploding
star. The thickness of the shock-wave was such that the flux of
neutrinos lasted 13-seconds. This burst of neutrinos was recorded by underground experiments
in Japan and the United States. It was the first time that neutrinos
from a dying star had been recorded.
In reality, of course, the star exploded about 160,000
years ago. Since then, the neutrino shock-wave has been expanding at the speed
of light 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
the event. If the bubble had passed us a decade earlier it would have gone
unnoticed, for the means to record the event were not yet in place.