White Dwarfs: Really OLD Stars

You can see a white dwarf at the heart of the Ring Nebula. This is a Hubble Space Telescope image. The Ring Nebula consists of a white dwarf at the center of an expanding shell of gases expelled by the star. It is possible our star could end up like this. NASA/ESA/STScI.

Stars go through life cycles, just as humans, plants, and animals do. The fact that they are giant spheres of seething hot gas means that they change over their lifetimes, going through different and very interesting stages. They spend most of their lives converting hydrogen to helium and producing heat and light. Astronomers chart these stars in a graph called the main sequence, which shows what phase they are in their evolution.

However, stars cannot live forever. Once they get to be a certain age, they transition to new phases of existence. Ultimately, they die in some fashion and leave behind fascinating pieces of evidence about themselves. We've all heard about the really exotic objects that stars become, such as black holes and neutron stars. Those happen to very massive stars. Others end their lives as a different type of object. That would be a white dwarf.

Creating a White Dwarf

How does a star become a white dwarf? Its evolutionary path depends on its mass. A high-mass star — one with eight or more times the mass of the Sun during the time it's on the main sequence — will explode as a supernova and create a neutron star or black hole. Stars not quite so massive can end as white dwarfs, and that includes the Sun, stars lower mass than the Sun, and others that are somewhere between the mass of the Sun and that of the supergiants.

Low-mass stars (those with about half the Sun's mass) are so light that their core temperatures never get hot enough to fuse helium into carbon and oxygen (the next step after hydrogen fusion). Once a low-mass star's hydrogen fuel is depleted, its core cannot resist the weight of layers above it, and it all collapses inward.

What's left of the star will then compress into a helium white dwarf — an object made mainly of helium-4 nuclei

How long such a star survives is directly proportional to its mass. With such low mass, these helium white dwarf stars would take longer than the age of the universe to get to their final state. Therefore we wouldn't expect to see any of them, yet. That's not to say they don't exist. There are some candidates, but they typically appear in binary systems, suggesting that some kind of mass loss is responsible for their creation, or at least for speeding up the process.

The Sun will Become a White Dwarf

We do see other white dwarfs out there that began their lives as stars more like the Sun. These white dwarfs, also known as degenerate dwarfs, are the end points of stars with main sequence masses between 0.5 and 8 solar masses. Like our Sun, these stars spend most of their lives fusing hydrogen into helium.

Once they run out of their hydrogen fuel, their cores compress and the star expands to become a red giant. It heats up the core until fusion into carbon and oxygen is possible. (The star undergoes what's called the triple-alpha process: two helium nuclei fuse to form beryllium, followed by the fusion of an additional helium creating carbon.)

Once all the helium in the core has been fused, the core will compress again. However, the core temperature will not get hot enough to fuse carbon or oxygen. Instead, it "stiffens", and the star enters a second red giant phase. Eventually, the star's outer layers are gently blown away and form a planetary nebula. What's left behind is the carbon-oxygen core, the heart of the white dwarf. It's very likely that our Sun will start this process in a few billion years. 

Death of White Dwarfs - Black Dwarfs

Because the white dwarf is no longer generating energy via nuclear fusion, technically it's no longer a star. It's a stellar remnant. It's still hot, but not from the activity in its core. Think of it more like the dying embers of a fire. Over time it will cool, and eventually get so cold that will become a black dwarf.

Edited by Carolyn Collins Petersen.