What Are White Dwarfs? - Stellar Remnant Characteristics

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  • 0:01 Geology & Space
  • 0:47 What Are White Dwarfs?
  • 1:28 How White Dwarfs Form
  • 5:06 Characteristics of…
  • 6:17 Black Dwarfs
  • 7:25 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

White dwarfs are incredible little stellar remnants. Which begs the question, are they actually stars or not? This lesson will tell you if and why and how white dwarfs come to be. That, and you'll learns some incredible facts about them as well.

Geology & Space

When I was a wee little lad, I wanted to be a geologist. I loved collecting all sorts of cool rocks and crystals. I even got one of those chemistry sets that lets you grow crystals at home. Little did I know then, digging in the dirt, that some of the coolest gems and metals and whatnot were actually in the opposite direction, up in space.

But it wasn't only other planets and meteorites that potentially contained wondrous untapped riches and even new kinds of elements - stars were also fair game. Kind of odd, no? I thought they were made of gas, not crystals.

You'll soon learn why that may be true when we discuss the characteristics of some pretty cool celestial objects called white dwarfs.

What Are White Dwarfs?

White dwarfs are dying remnants of stars initially of lower to intermediate mass (less than 8 solar masses). Medium-sized stars like our sun will one day eject their gas into space and contract into white dwarfs, which are about the size of planet Earth yet about half the mass of the sun because they are so incredibly dense.

White dwarfs are the second-most abundant type of star in our universe, second only to red dwarfs. About 95% of the stars in the Milky Way will become white dwarfs and thus the most likely outcome of stellar evolution of most stars is a white dwarf.

How White Dwarfs Form

The reason our star isn't a white dwarf right now is because it can resist the compressive forces of gravity trying to mush it into a little ball by generating enough energy through nuclear fusion: nuclear fusion stemming from the burning of hydrogen fuel. Such energy production helps the sun push outwards, against gravity, to maintain its shape.

You can relate this concept to a hot air balloon symbolizing a star. The forces of gravity, among others, will want to contract or deflate the balloon. But, if you burn a fuel, you heat up gas (or air) in the balloon, which then expands outwards, inflates the balloon and therefore resists the gravitational contraction of the balloon.

With just the right amount of fuel being used, you can keep the balloon inflated just enough so it doesn't burst or fly away but doesn't deflate either. This is how our sun stays the same shape, by maintaining this type of equilibrium of inward gravitational contraction vs. outward expansive gas pressure generated by burning hydrogen fuel by way of thermonuclear reactions.

But one day, our sun will run out of fuel like any balloons, planes, trains, and automobiles would without refueling. The silver lining is, like automobiles can refuel, stars roughly the size of our sun can also refuel by using helium fuel: helium that was actually generated as a byproduct from the burning of hydrogen before most of it ran out.

As the helium burns, it produces a carbon-oxygen interior as a byproduct. Once the helium fuel runs out, that's it. There are no more pit stops for our sun, no more refueling, and the race is over.

This is because our sun isn't massive enough to compress its new carbon interior to ignite it for fuel. Here is what I mean by this. Let's take you and an elephant. The elephant is much more massive than you are. If you stand atop a table you will not be able to compress or crush it much, if at all. But that elephant is so massive that it will crush it and compress it without much effort at all. The more massive you are, the more massive a star is, the bigger the capability of compressing something. And you need to have a lot of compressive strength to ignite carbon for use as a tertiary fuel.

Therefore, once it runs out of energy, a star like our sun will lose its gases and begin to collapse inwards because it can no longer resist gravity. Thus, the star will begin converting gravitational energy into thermal energy as it contracts.

But once again, this sort of process doesn't get the carbon-oxygen core hot enough to ignite. As a result, the star contracts more and more to a point where the star will eventually resist further contraction not through nuclear fusion, not through thermal energy created due to gravity, but by the pressure created by degenerate electrons, electrons so tightly packed they cannot compress any further. This is why another name for a white dwarf, a degenerate dwarf, is sometimes used.

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