- 2842 - STARS - at the extremes of astronomy? The stars in the night sky can become boring. Also the same except for the fact that the Earth is going around in circles to our star. But the same pattern seems to repeat itself every year. Along comes astronomy and we see these stars in a whole new perspective.
--------------------------- 2842 - STARS - at the extremes of astronomy?
- The Sun is a pretty boring star. Still burning through the hydrogen in its core, our middle-aged Sun is comfortable at its current, relatively petite size. And though it will stay this way for about 5 billion years more, our star will eventually run low on hydrogen and switch to fusing helium deep within.
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- This will inflate the Sun into a “red giant” over the span of just a couple of hundred million years. After engulfing the innermost planets, possibly including Earth, the Sun will continually shed its outer layers, eventually leaving behind a smoldering “white dwarf “ surrounded by a beautiful “planetary nebula” of glowing gas.
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- The largest known star in the universe is “UY Scuti“. One day, the Sun will become a red giant. But if it had started its life with a dozen or so times its current mass, it could have eventually evolved into a “red super giant“. UY Scuti has already shed a lot of mass.
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- The biggest of these stars, called hypergiants, can swell to more than 1,000 times the size of the Sun. UY Scuti, located near the center of the Milky Way in the constellation Scutum, is around 1,700 times the Sun’s width.
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- UY Scuti’s brightness changes over a period of about 740 days, leading astronomers to reclassify it as a “variable star“. Intrinsic variables like UY Scuti experience physical changes within, such as pulsations. In the case of UY Scuti, it varies in brightness because it’s constantly yo-yoing in terms of size making exact measurements a challenge.
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- Like any “red super giant“, including “Betelgeuse“, UY Scuti is destined to end its life with a bang. After exhausting the helium fuel in its core, it will ferociously forge increasingly heavy elements. And as long as UY Scuti doesn’t expel too much mass over the course of its remaining life, it will eventually start producing iron.
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- Making iron is a death sentence for stars. Unlike when it combines lighter elements, when a star forces two iron nuclei together, it doesn’t release any energy; it instead takes energy away from the environment.
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- This causes a runaway collapse where the star no longer generates enough outward pressure to keep it from imploding under its own gravity. The end result? A powerful core-collapse (type II) supernova that will finally make UY Scuti visible to the naked eye from Earth.
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- Another massive and luminous star, RMC 136a1, have extremely powerful stellar winds, which are streams of charged particles flowing from the star’s surface. They also emit intense ultraviolet radiation that would be strong enough to sterilize the surface of Earth.
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- Just because a star is a certain size doesn’t mean it has a certain mass. That’s absolutely the case with the most massive known star in the universe, RMC 136a1, which packs a lot of heft into a surprisingly slim frame. Although thought to be more than 300 times the mass of our Sun, RMC 136a1 is only about 30 times as wide as our home star.
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- Located in the Milky Way’s largest satellite galaxy, the Large Magellanic Cloud, RMC 136a1 is just one of many blazing stars that’s ionizing the gas within NGC 2070. This huge open star cluster lies in the heart of the Tarantula Nebula, which is the brightest star-forming region in our galactic neighborhood.
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- Hubble Space Telescope observations allow astronomers know that RMC 136a1 is just one of more than 200 bright, massive stars in the immediate area, all found within a cluster called RMC 136. RMC 136a1 is the brightest of these beacons. In addition to holding the title for the most massive star, RMC 136a1 is the most luminous star.
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- Although the exact age of this stellar heavyweight is still uncertain, according to a 2016 study, RMC 136a1 could be as young as a few hundred thousand to a million years old, so it’s thought to still be burning hydrogen in its core.-
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- Because RMC 136a1 is a rare “Wolf-Rayet star“, it’s incredibly hot, chock-full of heavy elements, and sports extremely powerful stellar winds that are blowing off its outer layers.
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- These stellar winds are so powerful, reaching a velocity of around 5.8 million mph that by the end of its life, the star is expected to expel enough gas to end up weighing just over 50 solar masses. For comparison, Supernova 1987A, also located in the Large Magellanic Cloud, was only about 20 solar masses.
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- Astronomers found tiny star EBLM J0555-57Ab only when it passed in front of its larger binary companion, which blocked some of the bigger star’s light. Detecting such a transit is also the way researchers find many exoplanets.
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- If a star is exceptionally massive, it gobbles up its fuel, causing it to live fast and die hard. However, if a star is small and light, it has a slow metabolism, allowing it to live an extremely long life. But just how small can a star be? EBLM J0555-57Ab is right at the limit.
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- At just 85 times the mass of Jupiter and a sliver wider than Saturn, EBLM J0555-57Ab skirts the lower boundary of what it takes to be a star. Had the star formed with only a slightly lower mass, the fusion reaction of hydrogen in its core could not be sustained, and the star would instead have transformed into a brown dwarf.
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- Brown dwarfs are not-quite-planets, not-quite-stars , whose cores can only fuse a heavy form of hydrogen called deuterium, as well as possibly lithium. EBLM J0555-57Ab may be tiny, but there are other stars out there that compare with its mass. For instance, the star TRAPPIST-1, which hosts at least seven rocky planets, tips the scales at just a little more massive than EBLM J0555-57Ab.
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- Because stars with less than 25 percent the Sun’s mass are the most common type of stars and excellent candidates for hosting Earth-sized planets, learning more about the lives of the smallest stars may help researchers uncover potentially habitable, Earth-like planets around them.
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- The faster a star burns through its fuel, the shorter its life. And this is surely the case for Wolf-Rayet stars. These stars not only burn incredibly hot and bright, but their stellar winds also blast much of their potential fuel into space. The hottest known star, WR 102, is one such Wolf-Rayet, sporting a surface temperature more than 35 times hotter than the Sun.
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- Wolf-Rayet stars come in a variety of flavors. The most massive star, RMC 136a1, has a spectral type of WN, meaning it’s rich in ionized nitrogen as a result of rapidly converting hydrogen to helium in its fiery core via the C-N-O cycle, carbon - nitrogen - oxygen cycle.
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- The hottest star, WR 102, is an especially rare WO-type Wolf-Rayet, which is a late-stage star that has a surface heavily enriched with ionized oxygen. Astronomers only know of about 10 WO-type Wolf-Rayet stars in the entire universe.
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- Even for a Wolf-Rayet star, WR 102 has intense stellar winds. Currently, they are blowing about a Sun’s worth of mass from the star’s surface every 100,000 years. That means WR 102 is losing several hundred million times more mass each year than the Sun. Although that may not seem like much for a massive star, at this rate, WR 102 would be completely gone in less than 2 million years.
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- Astronomers are interested in WR 102 not just because of its exceptionally hellish surface temperature and rapid mass loss, but also because the star is a prime candidate to go supernova in the relatively near future.
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- WR 102 is a post-core helium burning star and has a remaining lifetime of less than 2,000 years. Astronomers think S5-HVS1 achieved such a breakneck speed following its ejection from a binary system that passed too close to the Milky Way’s central black hole, as seen in this artist’s concept.
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- The Sun zips through space at 490,000 mph relative to the Milky Way. That’s fast, but the fastest known star (that’s not a white dwarf) belong to a speed demon known as S5-HVS1. This middle-aged, hypervelocity star is fleeing our galaxy at more than 3.9 million mph (6.3 million km/h). That is 0.6 percent the speed of light.
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- Astronomers first found the star streaking through the southern constellation Grus in 2019. After tracing its orbit back in time, they quickly realized it is coming from the center of the Milky Way, near our roughly 4-million-solar-mass supermassive blackhole, Sagittarius A*.
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- We think the blackhole ejected the star with a speed of thousands of kilometers per second about 5 million years ago. This ejection happened at the time when humanity’s ancestors were just learning to walk on two feet.
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- Though single now, researchers suspect S5-HVS1 wasn’t always alone. The evidence suggests the star was ejected thanks to a process called the “Hills mechanism“, which was outlined some three decades ago by astronomer Jack Hills.
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- The idea is that S5-HVS1 was once part of a binary system that tangled with Sagittarius A*. When the stellar pair ventured too close, the blackhole captured the companion star, releasing S5-HSV1 from its binary dance and flinging it through space.
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- That may not be an ideal life for a star, but at least it didn’t suffer the fate of its companion. That poor star disappeared into the blackhole.
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- September 27, 2020 2842
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