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Stellar break-up likely behind ‘runaway’ star’s fast pace, researcher says

Posted April 6, 2018

During a recent survey of supermassive stars, an international team of astronomers discovered a star that is in quite a hurry. As they report in a paper accepted to The Astronomical Journal and a story by the Lowell Observatory, the team tracked one yellow supergiant star cruising along at about 300,000 miles per hour, a velocity that would get you from the Earth to the Moon in about 48 minutes.

Kathryn Neugent, a University of Washington doctoral student in astronomy, is lead author on the paper — which stems from work she began as a researcher at the Lowell Observatory. Neugent sat down with the UW News Office to answer some questions about this star and its journey.

An infrared image of the Small Magellanic Cloud taken by VISTA, a survey telescope in Chile operated by the European Southern Observatory. Image credit: European Southern Observatory/VISTA VMC.

How did you and your colleagues discover this yellow supergiant traveling at 300,000 miles per hour?

I have been at Lowell Observatory as a part-time researcher for the past 9 years, and we were in the process of searching for yellow supergiant stars in the Small Magellanic Cloud, one of the Milky Way’s satellite galaxies, when we found this star.

What is a yellow supergiant star?

A yellow supergiant is a brief phase in the life of certain “massive stars” — any star generally greater than about 10 solar masses. Yellow supergiants only exist for a relatively short period of time as a star goes from being very hot to very cool or very cool to very hot again. They’re also yellow in color, much like our sun. Polaris, the north star, is a yellow supergiant.

How brief is the yellow supergiant phase?

Massive stars spend only about 10,000 to 100,000 years as a yellow supergiant. Our sun, by comparison, will spend about 10 billion years as a main sequence star. The yellow supergiant phase is a very unstable phase, so stars don’t spend too much time in it. They are more stable either on the blue side, which are hotter stars, or red side, which are cooler stars.

How did you and your colleagues discover that this particular yellow supergiant is moving so fast?

We collected data on yellow supergiants at the Cerro Tololo International Observatory in Chile. Specifically, the way you find yellow supergiants is by observing the spectrum of light coming from the star and looking to see how the spectral lines shift to the right and left. This is called Doppler shifting. This shift gives you the radial velocity, or how fast the star is traveling. Stars in the Small Magellanic Cloud are going to be traveling at very specific radial velocities. One of the stars, this runaway yellow supergiant, was traveling at a much larger radial velocity than we expected and thus we decided to investigate it further.

How old is this runaway star?

We estimate that it is about 30 million years old. Compare this with the age of the sun which is about 5 billion years.

Have fast-moving stars been observed before?

Yes, they have. Up to 50 percent of main-sequence massive stars are thought to be runaway stars, those with radial velocities greater than 40 kilometers per second. What makes this discovery so exciting is that this star is moving much faster — about 150 kilometers per second — and this is a star that has left the main sequence and is slowly dying, making it what astronomers call an “evolved” star. This is only the second runaway evolved star found outside of our own galaxy, and the first runaway yellow supergiant described.

Why is this yellow supergiant moving so quickly?

Something needed to have happened to this star to make it move so fast. There are a few different ways a runaway star can be created but most of them create low-velocity runaways (more like 50 km/s). We believe that this star once had a companion star — and that companion star went supernovae, propelling this star away at an extremely high velocity. It needs to have interacted with something to be propelled through space at such a high velocity. Supernovae explosions in binary systems have been known to create high velocity runaways like the one we’ve found.

As this star hurtles forward, will it disrupt other stars or planets it passes near?

Doubtful. I’m not sure what the probability is of it running into another star, but it is incredibly small. Space is big!

Source: University of Washington

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