- 3552 - NOVAE - exploding stars? Astronomers have discovered a new way for stars to explode. They describe small, fast stellar explosions, named “micro novae“. A micronova can flare up in minutes and fade in just a few hours.
--------------------- 3552 - NOVAE - exploding stars?
- Micronovae happen when a white dwarf (a small, dense remnant of a Sun-like star that burned up all its fuel) pulls in material from a nearby star. If white dwarf’s magnetic field is strong enough, it channels that incoming material to a small area around its magnetic poles, where it triggers a relatively small, contained thermonuclear reaction. The resulting explosion is about a millionth as powerful as a typical nova.
-
- The discovery was made by astronomers combing through data from NASA’s “Transiting Exoplanet Survey Satellite” (TESS), looking for images of white dwarfs. They wanted to learn more about how these super-dense stars capture, or accrete, material from their partners in binary star systems (where two stars orbit a shared center of gravity). But a series of bright, fast bursts of light from three white dwarfs caught their attention.
-
- “TV Columbae“, a white dwarf, suddenly flared twice as bright as normal within about half an hour. Within another 12 hours, the sudden brightness had faded again. Three days later, it happened again and repeated at least one more time.
-
- Another white dwarf star, “El Ursae Majoris“, did something very similar. So did a recently-discovered object called “ASASSN-19bh“; an x-ray instrument on the European Southern Observatory (ESO)’s evocatively-named “Very Large Telescope” (VLT) confirmed that it, too, was a white dwarf. Clearly, some white dwarfs were up to something very strange.
-
- Based on how quickly the stars brightened, how slowly they faded, and the amount of energy being released in each burst, giant thermonuclear explosions are happening in the stars’ upper layers.
-
- These reactions are triggered by gas pulled in from the companion piling onto the area around the white dwarf’s magnetic poles, compressing atoms together under enough pressure to trigger nuclear fusion, but, only in the area around the poles. That’s different from a full-fledged nova, in which accreted gas triggers thermonuclear reactions that engulf the entire white dwarf.
-
- Dubbed a “micronova,” one of these stellar explosions releases about a millionth of the energy of a typical nova. Each burned up an average of 20 million trillion kilograms of stellar gas in a matter of minutes or hours (for comparison, Earth’s mass is about 6 trillion trillion kilograms).
-
- When a white dwarf and its companion star orbit each other too closely, the white dwarf’s gravity can actually pull gas away from the other star. Astronomers call this “accretion“, and when that accreted gas lands on the surface of the already super-dense white dwarf, the heat and pressure can squish hydrogen atoms until they start fusing together into helium atoms.
-
- One thermonuclear reaction sets off a series of others, so the whole surface of the star is engulfed in what’s called a thermonuclear runaway. The whole star burns brighter for several days or weeks, and then, once all the extra nuclear fuel is used up in those runaway reactions, it settles back to its usual luminosity as if nothing ever happened.
-
- Because the bursts of optical and UV light were so brief, compared to the several days or weeks of a proper nova, they suspect that micronovae involve a much smaller amount of material and a much smaller area.
-
- Some white dwarfs, like “TV Columbae” and “ASASSN-19bh“, have especially strong magnetic fields, which funnel all the incoming gas straight to the star’s magnetic poles and hold it there. So when that extra mass triggers a thermonuclear runaway, it’s confined to the poles instead of swallowing up the whole star’s surface.
-
- The white dwarf TV Columbae flared up three times, each about three days apart. El Ursae Marjoris flared up twice, with a day in between. As gas flows in from the companion star, stars like TV Columbae and El Ursae Majoris may be burning up just part of that flowing stream of material in each short burst.
-
- ASASSN-19bh, on the other hand, seems to gobble up its whole accretion column at once, in a single big burst with about twice the energy of TV Columbae’s whole cluster of micronovae combined.
-
- This explains the strange flashes of light astronomers have noticed from some white dwarf stars for the last 40 years. Micronovae may also shed some light on the strange goings-on of neutron stars (the small, incredibly dense ex-cores of collapsed supergiant stars).
-
- Astronomers watching neutron stars with X-ray telescopes have noticed short, bright bursts of X-ray radiation that behave a lot like the short, bright bursts of optical and UV light from white dwarfs, only faster, and at much shorter wavelengths.
-
- To catch a micronova in action astronomers will have to search a huge swath of the sky, then react quickly to get a closer look with the right instruments when they see a white dwarf star suddenly brighten. Micronovae are fast, transient, events.
-
April 22, 2022 NOVAE - exploding stars? 3552
----------------------------------------------------------------------------------------
----- Comments appreciated and Pass it on to whomever is interested. ---
--- Some reviews are at: -------------- http://jdetrick.blogspot.com -----
-- email feedback, corrections, request for copies or Index of all reviews
--- to: ------ jamesdetrick@comcast.net ------ “Jim Detrick” -----------
--------------------- --- Friday, April 22, 2022 ---------------------------
No comments:
Post a Comment