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Speedy star gets expelled from Milky Way

  • 23 July 2010
  • Written by 
  • Published in Space

U.S. astronomers using the Hubble Space Telescope have conjectured that a very rare hypervelocity star (a really fast one) was flung out of our Milky Way galaxy by its supermassive black hole over one hundred million years ago.


The hypervelocity star, called HE 0437-5439, is one of the fastest stars ever detected by humans.

It travels at a speed of approximately 1.6 million miles (2.6 million kilometers) per hour'”or about 723 kilometers per second'”which is about three times the speed at which our own Sun travels around the galaxy.

HE 0437-5439, discovered in 2005, is considered by astronomers to be a massive, unbound hypervelocity main sequence B-type star--and a super-hot, massive, rejuvenated (formed from two lighter stars) blue star, or what is called a blue straggler.

It is a young star, being only about 30 million years old. The star's mass is almost nine times greater than our Sun's mass.

The star is located 200,000 light years away in the direction of the Dorado constellation, just northwest of the Large Magellanic Cloud (LMC) and further away in distance than the LMC.

According to the NASA media brief 'NASA'S Hubble Shows Hyperfast Star Was Booted From Milky Way,' here is what astronomers think happened about one hundred million years ago, based on data provided by the Hubble Space Telescope:

'The star may have been created in a cosmic misstep. A hundred million years ago, a triple-star system was traveling through the bustling center of our Milky Way galaxy when it wandered too close to the galaxy's giant black hole. The black hole captured one of the stars and hurled the other two out of the Milky Way. The two outbound stars merged to form a super-hot blue star traveling at incredible speeds.'

NASA states that the star is 'one of the fastest ever detected.'

Page two continues with more on hypervelocity stars and how valuable Hubble is in detecting them.




So far, 16 hypervelocity stars have been discovered since 2005. This one is the first hypervelocity star to be linked directly to being born out of the center of the galaxy.

Most of these stars have been thrown out of the galaxy from the massive black hole residing at its center.

U.S. astronomer Warren R. Brown, of the Harvard-Smithsonian Center for Astrophysics (Cambridge, Massachusetts), stated, "Using Hubble, we can for the first time trace back to where the star came from by measuring the star's direction of motion on the sky.'

Brown, one of the members of the discovery team for HE 0437-5439, added, 'Our measurements point directly to the Milky Way center."

He continued with, "These exiled stars are rare in the Milky Way's population of 100 billion stars. For every 100 million stars in the galaxy, there lurks one hypervelocity star."

The discovery team consists of Dr. Brown, along with Jay Anderson, Mario Livio, and Howard E. Bond (all three from the Space Telescope Science Institute, Baltimore, Maryland), Oleg Y. Gnedin (from the Department of Astronomy, University of Michigan, Ann Arbor), and Margaret J. Geller and Scott J. Kenyon (both from the Smithsonian Astrophyscial Observatory).

Page three continues.



They published the paper 'A Galactic Origin for HE 0437-5439, The Hypervelocity Star Near the Large Magellanic Cloud' within the Astrophysical Journal Letters.

They state in the abstract to their paper: 'We use Hubble Space Telescope imaging to measure the absolute proper motion of the hypervelocity star (HVS) HE 0437-5439, a short-lived B star located in the direction of the Large Magellanic Cloud (LMC).'

And, 'The velocity vector points directly away from the center of the Milky Way....'

In conclusion: 'The flight time of the HVS from the Milky Way exceeds its main-sequence lifetime, thus its stellar nature requires it to be a blue straggler. The large space velocity rules out a Galactic-disk ejection. Combining the HVS's observed trajectory, stellar nature, and required initial velocity, we conclude that HE 0437-5439 was most likely a compact binary ejected by the Milky Way's central black hole.'

The discovery of HE 0437-5439 will help astronomers learn more about how galaxies form and how these very fast traveling stars originated and evolved over time.

However, right now, astronomers are still puzzled over this star.

Page four concludes with two possible scenarios on this puzzle: the age of the star.



NASA states, 'The star's age is another mystery. Based on the speed and position of HE 0437-5439, the star would have to be 100 million years old to have journeyed from the Milky Way's core. Yet its mass -- nine times that of our sun-- and blue color mean that it should have burned out after only 20 million years -- far shorter than the transit time it took to get to its current location.'

Astronomers are hypothesizing two scenarios: (1) 'The star either dipped into the Fountain of Youth by becoming a blue straggler' or (2) '[I]t was flung out of the Large Magellanic Cloud, a neighboring galaxy.'

Please read the rest of the NASA article, mentioned earlier, for more details on how the U.S. astronomy team used the Hubble Space Telescope to make this exciting discovery.





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