An artist’s rendition of a hypervelocity star, being ejected away from a supermassive black hole — S5 Collaboration

Fastest moving Star discovered

Astronomers have just discovered a hypervelocity star that has shattered previous records

Faisal Khan
Nov 23, 2019 · 3 min read

he calmness of the night sky that we see is totally contradictory to the chaos of the Cosmos. And while everything looks stationary to us when we look up at the sky, it is anything but that. Everything is in a constant state of motion in the universe. Take, for example, our own planet Earth which rotates at a speed of 1670 km/h (1037 mph). This is dwarfed by the motion of the sun, constantly moving at a speed of 720,000 km/h (450,000 mph).

Recently, a hypervelocity star, known as S5-HVS1, was discovered by Sergey Koposov from Carnegie Mellon University as part of the Southern Stellar Stream Spectroscopic Survey (S5). Astronomers at the Macquarie University and Australian Astronomical Optics were part of the discovery team.

S5-HVS1 is traveling through space at an astonishing speed of 6 million km/h (3.7 million mph) and is the fastest known star yet. The previous record-holder was a white dwarf called US708, discovered in 2015, which clocked in at 4.3 million km/h.

Hypervelocity stars are normally defined as the ones which are traveling faster than 1.8 million km/h. This speed enables them to escape Milky Way’s gravitational pull to rocket through into intergalactic space. Only a handful of these hypervelocity stars have been discovered so far & the reasons behind their enormous speed are still a subject of some debate.

“We think the black hole ejected the star with a speed of thousands of kilometers per second about five million years ago. This ejection happened at the time when humanity’s ancestors were just learning to walk on two feet.”

~ Sergey Koposov, Lead Author of Study

The most commonly accepted hypothesis behind these superfast stars is Hills Mechanism — where a binary star system strays too close to the supermassive black hole Sagittarius A*. In the ensuing tussles, one of the stars gets ripped apart by the black hole as the other one gets slingshot at extreme speeds into space. This is what is thought to have happened with S5-HVS1 as well.

This theory, however, still leaves some questions unanswered — for example, why these hypervelocity stars are only found in one specific part of the sky & secondly it is improbable that binary stars in our Milky Way galaxy can launch fast enough to achieve hypervelocity. The led the University of Cambridge researchers to explore the idea further that these hypervelocity stars were not local to our galaxy.

For their research, they studied the data from the Sloan Digital Sky Survey & the neighboring dwarf galaxy Large Magellanic Cloud (LMC). The team ran two simulations, one looking at the life cycles of all the stars in the LMC for the past two billion years that became a runaway & the second tracked the effect of gravity of both these galaxies on these runaway stars. Validation of the study would confirm their hypothesis.

Observations from 3.9m Anglo-Australian Telescope (AAT) & European Space Agency’s (ESA) Gaia satellite led astronomers to calculate S5-HVS1's speed & trajectory with accuracy. This new discovery will lead to a better understanding of lesser-known hypervelocity stars.

The S5 collaboration brings astronomers together from the United States, United Kingdom, Australia & Chile. The research was published in Monthly Notices of the Royal Astronomical Society.


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Faisal Khan

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