WHAT ARE WHITE DWARFS??
Saturday, December 9th, 2023
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. Approximately 97% of the stars in the Milky Way will eventually become white dwarfs, according to researchers.
Compared to our sun, a white dwarf has a similar carbon and oxygen mass though it is much smaller in size — similar to Earth, according to New Mexico State University (NMSU).
White dwarf temperatures can exceed 100,000 Kelvin 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.
Main-sequence stars, 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, leaving behind a neutron star or black hole.
Smaller stars, however, will take a slightly more sedate path. Low- to medium-mass stars, such as the sun, will eventually swell up into red giants. After that, the stars shed their outer layers into a ring known as a planetary nebula (early observers thought the nebulas resembled planets such as Neptune and Uranus ). 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.
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, 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.
When a star swells up to become a red giant, it engulfs 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 rocky material has fallen into them.
Originally published on www.space.com
(Wednesday, December 13th, 2023)
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Driven by powerful stellar winds, the expanding gas and dust frame the hot, luminous star Wolf-Rayet 124 in this sharp infrared view. The 6-spike star pattern is characteristic, taken with the 18 hexagonal mirrors of the James Webb Space Telescope. About 15,000 light-years away toward the pointed northern constellation Sagittarius, WR 124 has over 30 times the mass of the Sun. Produced in a brief and rarely spotted phase of massive star evolution in the Milky Way, this star's turbulent nebula is nearly 6 light-years across. WR 124's impending stellar death will be a supernova explosion and any dusty interstellar debris that survives the supernova will influence the formation of future generations of stars.
Image Credit: NASA
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