- 3516 - SUPERNOVA - EXPLOSIONS. NEW FINDINGS MIGHT EXPLAIN TYPE IA SUPERNOVAS THAT HAPPEN WITHIN A BILLION YEARS OF A WHITE DWARF'S FORMATION, AS THEIR URANIUM HAS NOT YET ALL RADIOACTIVELY DECAYED. WHEN IT COMES TO OLDER WHITE DWARFS, TYPE IA SUPERNOVAS MIGHT HAPPEN THROUGH MERGERS OF TWO WHITE DWARFS.
--------------------- 3516 - SUPERNOVA - explosions.
- A “NOVA” is a “white dwarf” star that pulls matter off of a companion “red giant star” until a powerful nuclear fusion explosion occurs on the dwarf’s surface. The star is not destroyed and additional explosions can occur. It is a phenomenon called a “recurrent nova“.
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- A SUPERNOVA is much more brilliant than a nova, a supernova can shine brighter than an entire galaxy for a brief time.
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- A “Type I Supernova“ is a white dwarf star pulls matter from a companion star until the dwarf’s dead core re-ignites in a thermonuclear explosion that destroys the star. This is similar to a nova but the explosion is much more powerful. A Type I supernova has no hydrogen in its spectrum.
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- A Type II Supernova is a star several times more massive than the sun that runs out of nuclear fuel and collapses under its own gravity until it explodes. A Type II supernova has hydrogen in its spectrum.
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- A “Superluminous Supernova” or “Hypernova” has a burst 5 to 50 times more energetic than a supernova. A hypernova may or may not be associated with a powerful burst of gamma radiation.
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- A burned-out stellar core has produced a shockwave that pushed particles to their theoretical speed limit. Astronomers used a gamma-ray observatory in Namibia called the “High Energy Stereoscopic System” (HESS) to show how an eruption creates a shock wave that accelerates material around it super-fast speeds.
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- This research focused on “RS Ophiuchi“, a nova that erupts every 15 to 20 years, most recently, in 2021. RS Ophiuchi's system includes one normal star and one white dwarf, the cold dense core that remains after a star explodes.
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- The white dwarf pulls matter off the star, and when the stellar corpse has swallowed enough material, it produces the eruption scientists call a “nova“. As the nova erupts, the resulting shock wave collides through the surrounding area, pulling particles along with it and creating an accelerator that turns out to be incredibly powerful.
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- When the nova exploded in August 2021, the HESS telescopes allowed astronomers to observe a galactic explosion in very-high-energy gamma rays for the first time.
Particles at RS Ophiuchi reached rates hundreds of times faster than scientists have ever observed at other novas. The acceleration was so powerful that the particles reached the maximum speed predicted in theoretical models.
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- The observation that the theoretical limit for particle acceleration can actually be reached in genuine cosmic shock waves has enormous implications for astrophysics. It suggests that the acceleration process could be just as efficient in their much more extreme relatives, supernovas.
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- This has implications not only for novas and supernovas, but perhaps also for better understanding the origin story of cosmic rays. Cosmic rays are energetic explosions that appear to come from every direction in space, making their source difficult to trace.
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- White dwarfs are the dim, fading, Earth-size cores of dead stars that are left behind after average-size stars have exhausted their fuel and shed their outer layers. Our sun will one day become a white dwarf, as will more than 90% of the stars in our galaxy.
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- White dwarfs can die in nuclear explosions known as type Ia supernovas. Much remains unknown about what triggers these explosions, but prior work suggested they may happen when a white dwarf acquires extra fuel from a binary companion, perhaps due to a collision. In contrast, type II supernovas occur when a single star dies and collapses in on itself.
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- As a white dwarf cools, uranium and other heavy radioactive elements known as “actinides crystallize” within its core. Occasionally the atoms of these elements spontaneously undergo nuclear fission, splitting into smaller fragments. These instances of radioactive decay can release energy and subatomic particles, such as neutrons, which can break up nearby atoms.
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- If the amount of “actinides” within a white dwarf's core exceeds a critical mass, it can set off an explosive, runaway nuclear fission chain reaction. This outburst can then trigger nuclear fusion, with atom nuclei fusing to generate huge amounts of energy. In a similar fashion, a hydrogen bomb uses a nuclear fission chain reaction to detonate a nuclear fusion explosion.
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- Computer simulations found that a critical mass of uranium can indeed crystallize from the mixture of elements usually found in a cooling white dwarf. If this uranium explodes due to a nuclear fission chain reaction, the resulting heat and pressure in the white dwarf's core could be high enough to trigger fusion of lighter elements, such as carbon and oxygen, resulting in a supernova.
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- These findings might explain type Ia supernovas that happen within a billion years of a white dwarf's formation, as their uranium has not yet all radioactively decayed. When it comes to older white dwarfs, type Ia supernovas might happen through mergers of two white dwarfs.
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March 24, 2022 SUPERNOVA - explosions. 3516
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