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

White dwarfs are curious objects. Theyre small and not very massive (hence the dwarf part of their names) and they radiate mainly white light. Astronomers also refer to them as degenerate dwarfs because theyre really the remnants of stellar cores that contain very dense, degenerate matter. Many stars morph into white dwarfs as part of their old age. Most of them 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: theyll become black dwarfs.   An artist impression of a white dwarf star in orbit with pulsar PSR J2222-0137. It may be the coolest and dimmest white dwarf ever identified. (Larger version available at: https://public.nrao.edu/images/non-gallery/2014/c-blue/06-23/ColdRemnant.jpg). B. Saxton (NRAO/AUI/NSF) The Lives of the Stars To understand white dwarfs and how they form, its 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. The Sun will someday evolve into a white dwarf. NASA/SDO 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 its on the main sequence—will explode as a supernova and create a neutron star or black hole. Our Sun isnt 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. Massive stars die in supernova explosions like this one. The leftovers of this star will not form a white dwarf, but instead have created a rotating neutron star called a pulsar. Hubble Space Telescopes view of the Crab Nebula supernova remnant. NASA/ESA/STScI Low-mass stars (those with about half the Suns 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 stars hydrogen fuel runs out, its core cannot resist the weight of layers above it, and it all collapses inward. Whats 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. Thats not to say they dont 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. Our star will swell to become a red giant on its way to becoming a white dwarf surrounded by a planetary nebula. B. Jacobs/Wikimedia Commons 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 stars outer layers are gently blown away and form a planetary nebula. Whats left behind is the carbon-oxygen core, the heart of the white dwarf. Its very likely that our Sun will start this process in a few billion years.   Theres 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. The Deaths of White Dwarfs: Making Black Dwarfs When a white dwarf stops generating energy via nuclear fusion, technically its no longer a star. Its a stellar remnant. Its still hot, but not from the activity in its core. Think of the last stages of a white dwarfs 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. Thats 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 havent had enough time to completely cool down to become black dwarfs.   Key Takeaways All stars age and eventually evolve out of existence.Very massive stars explode as supernovae and leave behind neutron stars and black holes.Stars like the Sun will evolve to become white dwarfs.A white dwarf is the remnant of a stellar core that has lost all its outer layers.No white dwarfs have cooled completely in the history of the universe. Sources NASA, NASA, imagine.gsfc.nasa.gov/science/objects/dwarfs1.html.Stellar Evolution, www.aavso.org/stellar-evolution.â€Å"White Dwarf | COSMOS.†Ã‚  Centre for Astrophysics and Supercomputing, astronomy.swin.edu.au/cosmos/W/white dwarf. Edited by Carolyn Collins Petersen.