Black Dwarf Stars: Corpses of Creation

Around 90 percent of the observed stars in the universe are like our Sun. Called Main Sequence stars by astronomers, our Sun and others like it dominate the heavens.

Published by Tony Darnell on 22nd Sep, 2016

Around 90 percent of the observed stars in the universe are like our Sun. Called Main Sequence stars by astronomers, our Sun and others like it dominate the heavens.

And when they die, many will ultimately leave behind a dark, stellar corpse known as a black dwarf. A stellar remnant that has yet to be found, because the time it takes for them to form is much longer than the age of the universe.

As stars go, our sun is rather ordinary. It began its life as a cloud of gas and dust that eventually condensed by gravity into a protostar - a young stellar body powered by heat generated from collapsing material.

Main sequence stars can range in size from about a tenth of the mass of the sun up to 8 times as massive - and how long they live depends on their size.

The sun will shine for roughly 10 billion years but a star about 20 times larger will only last a fraction of that, say 20 million years or so due to the higher temperatures and faster consumption of its nuclear fuel.

When our Sun, and stars like it, begin to die, they don’t explode into fiery supernovae, instead, as their nuclear fuel runs out the pressure holding them up against gravity lowers, causing them to contract a bit.

This contraction heats up remaining material and helium begins to fuse, creating more heat. This causes the outer shell of the star to expand in size from 100 to 1000 times the diameter of the sun today.

The sun has now become a red giant.

Main Sequence stars spend anywhere from 1000 to one billion years as a red giant until eventually the helium begins to run out.

The star shrinks again, this time blowing the outer layers out into space as a beautiful planetary nebula.

If the star is less than 1.4 times the mass of the sun, something extraordinary happens. The remaining core collapses in on itself so fast that the atoms of the star are packed together so closely that their electrons orbital shells are pressing against one another.

This stellar remnant is known as a white dwarf, a compact sphere of atoms so tightly crushed that the entire mass of the sun would fit in an area the size of the Earth.

This is the fate of our sun in 5 billion years. This white dwarf is very bright, but its not shining due to fusion. We are looking at the remnant heat of the core collapse, no new energy is being added, which means the core is slowly cooling.

What happens next has never been seen because there hasn’t been enough time in the universe to see it, but all white dwarf stars in the universe are cooling. Very slowly.

Astronomers estimate that it will take a white dwarf star tens of hundreds of billions of years to completely cool and become what is known as a black dwarf.

For comparison, if there was a white dwarf star shining at the beginning of the universe, it would have cooled to only 3,000 kelvin 13.7 billion years later, that's just 2,000 degrees below the sun’s present temperature.

This means no one has yet observed or indirectly measured a black dwarf star, the universe is simply not old enough for them to have been formed.

But some 100 trillion years hence, as the universe leaves the stelliferous era of the stars to the degenerate era, 90 percent of the celestial bodies in our cosmos will be these black stellar corpses, hiding in the blackness of eternity.


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