Sunday, March 12, 2023

3914 - FASTEST STARS - tell us about our galaxy?

 

-   3914  -  FASTEST  STARS  -  tell us about our galaxy?   Fastest stars in the Milky Way are 'runaways' from another galaxy.  The fastest-moving stars in our galaxy are traveling so fast that they can escape the Milky Way.  They are in fact runaways from a much smaller galaxy in orbit around our own.


------------  3914 -    FASTEST  STARS  -  tell us about our galaxy?

-   Astronomers used data from the Sloan Digital Sky Survey and computer simulations to demonstrate that these stellar sprinters originated in the Large Magellanic Cloud (LMC), a dwarf galaxy in orbit around the Milky Way.

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-   These fast-moving stars, known as hypervelocity stars, were able to escape their original home when the explosion of one star in a binary system caused the other to fly off with such speed that it was able to escape the gravity of the LMC and get absorbed into the Milky Way.

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-    Astronomers first thought that the hypervelocity stars, which are large blue stars, may have been expelled from the center of the Milky Way by a supermassive black hole. Other scenarios involving disintegrating dwarf galaxies or chaotic star clusters can also account for the speeds of these stars but all three mechanisms fail to explain why they are only found in a certain part of the sky.

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-   To date, roughly 20 hypervelocity stars have been observed, mostly in the northern hemisphere, although it's possible that there are many more that can only be observed in the southern hemisphere.  The hypervelocity stars are mostly found in the Leo and Sextans constellations.

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-    An alternative explanation to the origin of hypervelocity stars is that they are runaways from a binary system. In binary star systems, the closer the two stars are, the faster they orbit one another. If one star explodes as a supernova, it can break up the binary and the remaining star flies off at the speed it was orbiting.

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-   The escaping star is known as a runaway. Runaway stars originating in the Milky Way are not fast enough to be hypervelocity because blue stars can't orbit close enough without the two stars merging. But a fast-moving galaxy could give rise to these speedy stars.

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-    The LMC is the largest and fastest of the dozens of dwarf galaxies in orbit around the Milky Way. It only has 10% of the mass of the Milky Way, and so the fastest runaways born in this dwarf galaxy can easily escape its gravity.

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-   The LMC flies around the Milky Way at 400 kilometers per second (895,000 miles per hour) and, like a bullet fired from a moving train, the speed of these runaway stars is the velocity they were ejected at plus the velocity of the LMC. This is fast enough for them to be the hypervelocity stars.

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-  This also explains their position in the sky, because the fastest runaways are ejected along the orbit of the LMC towards the constellations of Leo and Sextans.

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-  The researchers used a combination of data from the Sloan Digital Sky Survey and computer simulations to model how hypervelocity stars might escape the LMC and end up in the Milky Way. They simulated the birth and death of stars in the LMC over the past two billion years, and noted down every runaway star.

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-    The orbit of the runaway stars after they were kicked out of the LMC was then followed in a second simulation that included the gravity of the LMC and the Milky Way. These simulations allow the researchers to predict where on the sky we would expect to find runaway stars from the LMC.

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-    They predict that there are 10,000 runaways spread across the sky. Half of the simulated stars which escape the LMC are fast enough to escape the gravity of the Milky Way, making them hypervelocity stars.

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-  If the previously known hypervelocity stars are runaway stars it would also explain their position in the sky.

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-    Massive blue stars end their lives by collapsing to a neutron star or black hole after hundreds of millions of years and runaway stars are no different. Most of the runaway stars in the simulation died 'in flight' after being kicked out of the LMC.

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-    The neutron stars and black holes that are left behind just continue on their way and so, along with the 10,000 runaway stars, the researchers also predict a million runaway neutron stars and black holes flying through the Milky Way.

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-     Hypervelocity stars (HVS) are stars that move so fast they can escape the gravity of the Milky Way. In 2019, astronomers discovered a star “the S5-HVS1”, which covers an astounding 1,755 kilometers per second,  (3,926,000 miles per hour).   Dozens of these stars have since been found. But there are about a thousand of them within our galaxy.

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-   We can assume with fairly great certainty that some of the hypervelocity stars that have now been discovered were ejected following a gravitational encounter with the massive black hole in the center of the Milky Way: Sagittarius A*.

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-    We see a similar effect in the Large Magellanic Cloud, another galaxy that we have reason to believe also contains a black hole." In the right conditions supernovae, exploding stars, could also eject hypervelocity stars.

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-    The stars that turn into supernovae are incredibly rare in our Milky Way and the event is so short-lived that it is difficult to measure.   There are so many stars and so much dust flying around Sagittarius A* that astronomers can't properly see what is going on there.

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-    Some hypervelocity stars are flying in more visible parts of space and can tell us more about where they come from. For example, about the gravity of black holes or the amount of energy a supernova produces.

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-      A thousand kilometers per second is extremely fast. You could fly around the world in under a minute.  Hypervelocity stars  also have a story to tell about processes in the universe about which we know little and still have much to discover.

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                   March 11, 2023       FASTEST  STARS  -  tell us about our galaxy?        3914                                                                                                                         

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--------------------- ---  Sunday, March 12, 2023  ---------------------------

 

 

 

 

         

 

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