What determines whether a star becomes a giant a white dwarf a neutron star or a black hole?

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White dwarfs are what is left when stars like our sun have exhausted all of their fuel. They are dense, dim, stellar corpses — the last observable stage of evolution for low- and medium-mass stars.  

Whilst most massive stars will eventually go supernova, a low or medium mass star with a mass less than about 8 times the mass of the sun will eventually become a white dwarf, according to NASA (opens in new tab). Approximately 97% of the stars in the Milky Way will eventually become white dwarfs, according to researchers (opens in new tab).

Compared to our sun (opens in new tab), a white dwarf has a similar carbon and oxygen mass though it is much smaller in size — similar to Earth (opens in new tab), according to New Mexico State University (opens in new tab) (NMSU). 

White dwarf temperatures can exceed 100,000 Kelvin (opens in new tab) according to NASA (that's about 179,500 degrees Fahrenheit). Despite these sweltering temperatures, white dwarfs have a low luminosity as they're so small in size according to NMSU.

Related: Red dwarfs: The most common and longest-lived stars (opens in new tab)

Main-sequence stars (opens in new tab), including the sun, form from clouds of dust and gas drawn together by gravity. How the stars evolve through their lifetime depends on their mass. The most massive stars, with eight times the mass of the sun or more, will never become white dwarfs. Instead, at the end of their lives, white dwarfs will explode in a violent supernova (opens in new tab), leaving behind a neutron star (opens in new tab) or black hole (opens in new tab).

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According to NASA, a teaspoon of white dwarf matter would weigh 5.5 tons on Earth — about the same as an elephant!  

Smaller stars, however, will take a slightly more sedate path. Low- to medium-mass stars, such as the sun (opens in new tab), will eventually swell up into red giants. After that, the stars shed their outer layers into a ring known as a planetary nebula (opens in new tab) (early observers thought the nebulas resembled planets such as Neptune (opens in new tab) and Uranus (opens in new tab) ). The core that is left behind will be a white dwarf, a husk of a star in which no hydrogen fusion occurs.

Smaller stars, such as red dwarfs, don't make it to the red giant state. They simply burn through all of their hydrogen, ending the process as a dim white dwarf. However, red dwarfs take trillions of years to consume their fuel, far longer than the 13.8-billion-year-old age of the universe, so no red dwarfs have yet become white dwarfs.

White dwarf characteristics

When a star runs out of fuel, it no longer experiences an outward push from the process of fusion and it collapses inward on itself. White dwarfs contain approximately the mass of the sun but have roughly the radius of Earth, according to Cosmos (opens in new tab), the astronomy encyclopedia from Swinburne University in Australia. This makes them among the densest objects in space, beaten out only by neutron stars and black holes. According to NASA, the gravity on the surface of a white dwarf is 350,000 times that of gravity on Earth. That means a 150-pound (68-kilogram) person on Earth would weigh 50 million pounds (22.7 million kg) on the surface of a white dwarf.

White dwarfs reach this incredible density because they are collapsed so tightly that their electrons are smashed together, forming what is called "degenerate matter." The former stars will keep collapsing until the electrons themselves provide enough of an outward-pressing force to halt the crunch. The more mass, the greater the pull inward, so a more massive white dwarf has a smaller radius than its less massive counterpart. Those conditions mean that, after shedding much of its mass during the red giant phase, no white dwarf can exceed 1.4 times the mass of the sun (opens in new tab).

When a star swells up to become a red giant, it engulfs its closest planets. But some can still survive. NASA’s Spitzer spacecraft (opens in new tab) revealed that at least 1 to 3 percent of white dwarf stars have contaminated atmospheres that suggest rocky material has fallen into them.

"In the quest for Earth-like planets, we have now identified numerous systems which are excellent candidates to harbor them," Jay Farihi, a white dwarf researcher at the University of Leicester in England, told Space.com (opens in new tab). "Where they persist as white dwarfs, any terrestrial planets will not be habitable, but may have been sites where life developed during a previous epoch."

In one exciting case, researchers have observed the rocky material as it falls into the white dwarf.

"It's exciting and unexpected that we can see this kind of dramatic change on human timescales," Boris Gänsicke, an astronomer at the University of Warwick in England, told Space.com (opens in new tab).

The fate of a white dwarf

Many white dwarfs fade away into relative obscurity, eventually radiating away all of their energy and becoming so-called black dwarfs (opens in new tab), but those that share a system with companion stars may suffer a different fate. 

If the white dwarf is part of a binary system, it may be able to pull material from its companion onto its surface. Increasing the white dwarf's mass can have some interesting results.

One possibility is that the added mass could cause it to collapse into a much denser neutron star.

A far more explosive result is the Type 1a supernova (opens in new tab). As the white dwarf pulls material from a companion star, the temperature increases, eventually triggering a runaway reaction that detonates in a violent supernova that destroys the white dwarf. This process is known as a "single-degenerate model" of a Type 1a supernova. 

Related: Know Your Novas: Star Explosions Explained (Infographic) (opens in new tab) 

In 2012, researchers were able to closely observe the complex shells of gas surrounding one Type 1a supernova in fine detail.

"We really saw, for the first time, detailed evidence of the progenitor for a Type 1a supernova," Benjamin Dilday, the study's lead author and an astronomer at Las Cumbres Observatory Global Telescope Network in California told SPACE.com (opens in new tab).

If the companion is another white dwarf instead of an active star, the two stellar corpses merge together (opens in new tab) to kick off the fireworks. This process is known as a "double-degenerate model" of a Type 1a supernova.

At other times, the white dwarf may pull just enough material from its companion to briefly ignite in a nova, a far smaller explosion. Because the white dwarf remains intact, it can repeat the process several times when it reaches that critical point, breathing life back into the dying star over and over again.

"These are the brightest and most frequent stellar eruptions in the galaxy, and they're often visible to the naked eye," Przemek Mróz, an astronomer at Poland’s Warsaw University, told Space.com in a previous article.

Additional resources

You can learn more about white dwarfs with ESA (opens in new tab) and explore different types of stars with NASA (opens in new tab). Discover the evolution of binary star systems with this free educational material from Lumen Learning (opens in new tab). Explore the physics of the universe with white dwarfs in this informative material from The University of Texas at Austin (opens in new tab).

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