What are White Dwarfs? White Dwarfs are ancient stars that are no longer burning hydrogen, and are beginning to collapse on themselves, causing them to become very dense.  They are considered smaller stars on the spectrum but, even so, they can reach sizes of up to eight times what our own sun measures.  Actually, our own sun will one day become a white dwarf.  When a star burns up most of it’s hydrogen, the process known as fusion slows, and gravity causes the star to eventually collapse on itself. At this point, the collapsed star is known as a white dwarf.

When a star becomes a white dwarf, it reduces to mere percentages of what it was in size, but weights about the same. That being said, white dwarfs are very dense.  Just an gram of their matter would weigh over 6 tons – that’s about the weight of a large SUV.  To put this in perspective, a white dwarf that is the size of the earth would have the mass of our sun.  That is one heavy object!

The Life-cycle of a Star.

When a star is burning up the last of it’s hydrogen, it’s outer layers begin to expand and cool, white the center remains hot.  When this happens, it becomes a red giant.  Red giants are very large and contain gases that expand outward into space.

Eventually, the outer layers of the red giant expand and create a shell of gas called a planetary nebula. The core of the planetary nebula is extremely hot – about 180,000 degrees Fahrenheit.  The core is also very dense, but the outer layers consist of an expanding shell of gas.

When the gas dissipates, the resulting hot, dense white center is left.  At this stage, it is known as a white dwarf.

Over time the matter cools and the white dwarf no longer emits any energy.  At this stage it becomes known as a black dwarf.  Scientists have yet to discover a black dwarf in the universe, so their existence up to this point is entirely theoretical.

Scientists  have discovered a total of eight white dwarfs in the explored universe, the closest being  Sirius B, which is about 8.6 light years away.  The first of the white dwarfs were discovered by scientists in 1910.  The term white dwarf was first used by Willem Luyten in 1922.

Sources

National Geographic – Article on White Dwarfs

Wikipedia – White Dwarfs