White Dwarfs: Compact Corpses of Stars

White Dwarfs: Compact Corpses of Stars

White Dwarfs: Compact Corpses of Stars







		The stars in the sky may seem eternal and immutable, but eventually most of them will become white dwarfs, the last observable stage of evolution for low and medium mass stars. These dark stellar corpses dot the galaxy, remnants of stars that once burned brightly.



		Training



	Main sequence stars, including the sun, is formed from clouds of dust and gas joined by gravity. How stars evolve throughout their lives 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, they will explode into a violent supernova, leaving behind a neutron star or dungeon.

		The smaller stars, however, will take a slightly quieter path. Stars from low to medium mass, like the sun, it will eventually swell in red giants. After that, the stars shed their outer layers in a ring known as planetary nebula (The first observers thought that the nebulae looked like planets like Neptune and Uranus). The nucleus that will remain behind will be a white dwarf, a shell of a star in which the fusion of hydrogen does not take place.

		Smaller stars, such as red dwarfs, do not reach the status of red giant. They simply burn all their hydrogen, ending the process as a thin white dwarf. However, red dwarfs take billions of years to consume their fuel, much more than the age of 13.8 billion years of the universe, so no red dwarf has become a white dwarf.



		Features

		When a star runs out of fuel, it no longer experiences an external push from the fusion process and collapses inward on itself. White dwarfs contain approximately the mass of the sun but have approximately the radius of the Earth, according to Cosmos, the astronomy encyclopedia of the Swinburne University in Australia. This makes them among the most dense objects in space, vanquished only by neutron stars and black holes. According to POTThe gravity on the surface of a white dwarf is 350,000 times greater than gravity on Earth. That means that a person of 150 pounds (68 kilograms) 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 collapse so strongly that their electrons break apart, forming what is called "degenerate matter". The previous stars will continue to collapse until the electrons themselves provide enough force to stop the crack. The greater the mass, the greater the pull inward, so that a more massive white dwarf has a smaller radius than its less massive counterpart. These conditions mean that, after losing much of their mass during the red giant phase, no white dwarf can overcome 1.4 times the mass of the sun.

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

		"In the search for Earth-like planets, we have now identified numerous systems that are excellent candidates to house them," Jay Farihi, a white dwarf researcher at the University of Leicester in England, he told Space.com. "Where they persist as white dwarfs, the terrestrial planets will not be habitable, but they may have been places where life developed during an earlier era."

		In an exciting case, researchers have observed the rocky material when it falls on the white dwarf.

		"It is exciting and unexpected that we can see this kind of dramatic change in human time scales," Boris Gänsicke, an astronomer at the University of Warwick in England, he told Space.com.





						


Two white dwarfs are heading towards a collision in this artist's illustration. New research suggests that the preponderance of positrons from the Milky Way could come from a specialized supernova type of low-mass white dwarf collisions, an explosion difficult to detect, but rich in an isotope that generates this type of antimatter.

            						    Credit: NASA / Tod Strohmayer (GSFC) / Dana Berry (Chandra X-ray Observatory) 




		One last kick

		Many white dwarfs fade in relative darkness, eventually radiate all their energy and become black dwarfs, but those who 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 mate material onto its surface. Increasing the mass of the white dwarf can have some interesting results.

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

		A much more explosive result is the Supernova type 1a. As the white dwarf extracts material from a companion star, the temperature increases, which eventually causes an uncontrolled reaction that detonates in a violent supernova that destroys the white dwarf. This process is known as a "single degeneration model" of a Type 1a supernova.

The[[Know your novas: Explosions of stars explained (infographic)]

		In 2012, researchers were able to observe closely the complex gas deposits surrounding a Type 1a supernova in great detail.

		"We really saw, for the first time, detailed evidence of the progenitor of a Type 1a supernova," Benjamin Dilday, lead author of the study and astronomer of the World Telescope Network at the Las Cumbres Observatory in California. he told SPACE.com.

		If the partner is another white dwarf instead of an active star, the two stellar corpses merge to start the fireworks. This process is known as a "double degeneration model" of a Type 1a supernova.

		At other times, the white dwarf can extract enough material from her partner to light briefly on a nova, a much smaller explosion. Because the white dwarf remains intact, you can repeat the process several times when it reaches that critical point, returning life to the dying star over and over again.

		"These are the brightest and most frequent stellar eruptions in the galaxy, and they are often visible to the naked eye," said Przemek Mróz, an astronomer at the Warsaw University of Poland. he told Space.com.



	This article was updated on October 11, 2018 by Space.com Associate Editor, Sarah Lewin. 

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