A white dwarf is what remains of a star like our Sun after it sheds its outer layers and stops burning fuel though nuclear fusion.
A "dead" star is one that has no more nuclear fusion going in it. When a star dies, it leaves some remnant behind. Depending on the mass of the star, the remnant can be a white dwarf, neutron star or a black hole. White dwarfs are what was once the core of a star.
After the star's outer layer has escaped, the much smaller inner layer collapses into a white dwarf. This star, which is hotter and brighter than the red giant it came from, illuminates and warms the escaped gas, until the gas starts glowing by itself – and we see a planetary nebula.
After a star dies, there is still some residual heat left over. That heat makes the star (white dwarf or neutron star) glow, even though it is not producing any energy. Eventually, the star cools off and does indeed simply become a hunk of ash, which we call a "black dwarf."
But now, a new study published in Astronomy & Astrophysics has found that dying stars can give birth to planets, too. An international team of astronomers led by KU Leuven in Belgium made this discovery while looking at evolved binary stars in the Milky Way.
No. While none exist to date it would be possible for a dead star to have cooled to a safe temperature. However, such objects are inherently supported by degeneracy pressure--they're very dense.
Once there is no fuel left, the star collapses and the outer layers explode as a 'supernova'. What's left over after a supernova explosion is a 'neutron star' – the collapsed core of the star – or, if there's sufficient mass, a black hole.
Stars aren't supposed to come back to life after the death throes of the supernova stage, but astronomers have spotted three that have done just that – surviving the catastrophic explosion that usually marks the end of a star's life, and heading off through the galaxy on a new adventure.
Top. For the most part, the stars you see with the naked eye (that is, without a telescope) are still alive. These stars are usually no more than about 10,000 light years away, so the light we see left them about 10,000 years ago.
One Star's End is a Black Hole's Beginning
Most black holes form from the remnants of a large star that dies in a supernova explosion. (Smaller stars become dense neutron stars, which are not massive enough to trap light.)
Most stars take millions of years to die. When a star like the Sun has burned all of its hydrogen fuel, it expands to become a red giant. This may be millions of kilometres across - big enough to swallow the planets Mercury and Venus.
No. A neutron star has such an intense gravitational field and high temperature that you could not survive a close encounter of any kind.
The most massive stars can burn out and explode in a supernova after only a few million years of fusion. A star with a mass like the Sun, on the other hand, can continue fusing hydrogen for about 10 billion years.
The white dwarf is considered “dead” because atoms inside of it no longer fuse to give the star energy. But it still “shines” because it is so hot. Eventually, it will cool off and fade from view. Our Sun will reach this death about 8 billion years from now.
Using our knowledge of the death rate in the entire Milky Way, the death rate for visible stars works out at about one star every 10,000 years or so. Given that all those stars are closer than 4,000 light-years, it is unlikely – though not impossible – that any of them are already dead.
All of the stars you can see with the unaided eye lie within about 4,000 light-years of us. So, at most, you are seeing stars as they appeared 4,000 years ago.
Stars are born within dust clouds. When there is instability in the depth of these clouds, it creates clumps with enough mass for gas and dust to start collapsing under its own gravitational pull. The elements that make up the cloud's core start to heat up as the cloud breaks up.
While most stars quietly fade away, the supergiants destroy themselves in a huge explosion, called a supernova. The death of massive stars can trigger the birth of other stars. Neutron stars are the fastest-spinnng objects in the universe. They can rotate 500 times in just one second.
As the cloud collapses, the material at the center begins to heat up. Known as a protostar, it is this hot core at the heart of the collapsing cloud that will one day become a star.
What has happened to the stars? Of course they're still there, but we can't see them because of light pollution: the excessive and misdirected anthropogenic and artificial light that has invaded our night skies. Stars have helped shaped human culture for thousands of years.
In the stellar remnants of a supernova, however, there are no longer forces to oppose that gravity, so the star core begins to collapse in on itself. If its mass collapses into an infinitely small point, a black hole is born.
When the helium fuel runs out, the core will expand and cool. The upper layers will expand and eject material that will collect around the dying star to form a planetary nebula. Finally, the core will cool into a white dwarf and then eventually into a black dwarf. This entire process will take a few billion years.
Eventually, the entire contents of the universe will be crushed together into an impossibly tiny space – a singularity, like a reverse Big Bang. Different scientists give different estimates of when this contraction phase might begin. It could be billions of years away yet.