From Stars to White Dwarfs: the Saga of a Sun-like Star

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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.

White dwarfs are curious objects that many stars morph into as part of their "old age." Most began as stars similar to our own Sun. It seems rather odd that our Sun would somehow turn into a weird, shrinking mini-star, but it will happen billions of years from now. Astronomers have seen these weird little objects all around the galaxy. They even know what will happen to them as they cool: they'll become black dwarfs.

 

The Lives of the Stars

To understand white dwarfs and how they form, it's important to know the life cycles of stars. The general story is pretty simple. These giant seething balls of superheated gases form in clouds of gas and shine by the energy of nuclear fusion. They change throughout 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.

Once stars 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. There are some really exotic objects that really massive stars evolve to become, such as black holes and neutron stars. Others end their lives as a different type of object called 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. Our Sun isn't a massive star, so it, and stars very similar to it, become 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 runs out, 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 any star survives is directly proportional to its mass. The low-mass stars that become helium white dwarf stars would take longer than the age of the universe to get to their final state. They cool very, very slowly. Therefore no one has seen one actually cool completely down, yet and these oddball stars are quite rare. 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 many 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 endpoints 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 in their cores.

Once they run out of their hydrogen fuel, the cores compress and the star expands to become a red giant. It heats up the core until helium fuses to create carbon. When the helium runs out, then the carbon starts to fuse to create heavier elements. The technical term for this process is 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. 

The Deaths of White Dwarfs: Making Black Dwarfs

When a white dwarf stops 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 the last stages of a white dwarf's life as more like the dying embers of a fire. Over time it will cool, and eventually get so cold that will become a cold, dead ember, what some call a "black dwarf".  No known white dwarf has gotten this far yet. That's because it takes billions and billions of years for the process to occur. Since the universe is only about 14 billion years old, even the first white dwarfs haven't had enough time to completely cool down to become black dwarfs. 

Edited by Carolyn Collins Petersen.