When the core hydrogen has been depleted, the star gets larger, cooler, and brighter. It becomes first a subgiant, then a giant. Especially massive stars may become bright giants or supergiants.
Fusion processes in the cores of these stars cease once the helium has been converted to carbon, since the contracting carbon core does not reach a high enough temperature to ignite. Instead, it contracts until it squeezes all of its electrons into the smallest possible space they can occupy. The resulting electron pressure arises due to quantum mechanical effects, and stops gravity from compressing the core further.
A white dwarf is therefore supported by the pressure of electrons rather than energy generation in its core. Once the core has stopped contracting, the white dwarf has a temperature of over , Kelvin and shines through residual heat.
These young white dwarfs typically illuminate the outer layers of the original star ejected during the red giant phase, and create a planetary nebula.
Our red giant Sun will still be eating up helium and cranking out carbon. But when it's finished its helium, it isn't quite hot enough to be able to burn the carbon it created. What now? Since our Sun won't be hot enough to ignite the carbon it its core, it will succumb to gravity again.
When the core of the star contracts, it will cause a release of energy that makes the envelope of the star expand. Now the star has become an even bigger giant than before! Our Sun's radius will become larger than Earth's orbit! The Sun will not be very stable at this point and will lose mass. This continues until the star finally blows its outer layers off. The core of the star, however, remains intact, and becomes a white dwarf. The white dwarf will be surrounded by an expanding shell of gas in an object known as a planetary nebula.
They are called this because early observers thought they looked like the planets Uranus and Neptune. There are some planetary nebulae that can be viewed through a backyard telescope. In about half of them, the central white dwarf can be seen using a moderate sized telescope. Planetary nebulae seem to mark the transition of a medium mass star from red giant to white dwarf.
Stars that are comparable in mass to our Sun will become white dwarfs within 75, years of blowing off their envelopes. Eventually they, like our Sun, will cool down, radiating heat into space and fading into black lumps of carbon.
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