The fastest stars in the universe

Stars exist, normally, inside galaxies: huge stellar accumulations containing many thousand millions of members. The Sun itself belongs to one of such systems, normally called the Galaxy. For this reason, from the Solar System we contemplate a night-sky full of stars: we are seeing the closest companions of the Sun inside the Galaxy. But several astrophysical mechanisms are capable of ejecting stars from inside a galaxy directly into the intergalactic space. Those stars, the so-called hyper-velocity stars, reach speeds of hundreds of kilometres per second. Recently, the first hyper-velocity star from an external galaxy has been detected in a research leaded at the Ruhr-University (Bochum, Germany), and Calar Alto telescopes and instruments have played a key rule in this discovery.

Some 30 hyper-velocity stars in process of being expelled from our Galaxy are known up to date. But finding this kind of elusive object around external galaxies poses an observational challenge. Individual normal stars cannot be detected at the distances of most external galaxies. No doubt, many hyper-velocity stars exist around all the galaxies we can observe on the sky. How could we find at least some of them? Supernova explosions are the key. Statistically, a few of the stars expelled from any galaxy as hyper-velocity stars have to be massive enough to end their lives in the powerful explosions called gravitational (or core-collapse) supernovae. Catching some of these exploding fast stars was the aim of the research performed by scientists P.C. Zinn, P. Grunden and D.J. Bomans, of the Astronomical Institute of Ruhr-University at Bochum (Germany).

Sloan Digital Sky Survey image of galaxy UGC   5434. A hyper-velocity star from this galaxy exploded  as supernova SN 2006bx.

These researchers identified a set of extragalactic gravitational supernovae that, apparently, had happened outside any galaxy close to them, in the middle of the emptiness of intergalactic space. To confirm that, in fact, these supernova explosions were not related to any unseen, vary faint (and thus undetected) parent galaxy, these astronomers performed very deep observations around the explosions, looking for the slightest hint of background light. This was done in 2010 with the camera CAFOS attached to the Zeiss 2.2 m reflector of Calar Alto Observatory. This was complemented with data coming from the database of the space observatory GALEX, and other sources.

The final results, recently published in journal Astronomy & Astrophysics, led to the conclusion that they found one very good case of hyper-velocity star that exploded as a gravitational supernova while it was escaping from its parent galaxy. The galaxy, known as UGC 5434, is located at 250 million light-years from us (78.3 megaparsecs) and the related supernova was labelled SN 2006bx. In the moment of the explosion, the star was receding from the galaxy at the overwhelming speed of 850 kilometres per second, and this is only a lower bound!

The place where SN 2006bx exploded beside galaxy UGC5434  shows no hint of other candidate to parent galaxy in the   observations performed at Calar Alto Observatory.   SDSS/CAHA.

In words of the researchers, ”the progenitor of SN 2006bx is an excellent example of a hyper-velocity star that has most likely been accelerated via the classical scenario involving a close passage of the central supermassive black hole of the parent galaxy. That also applies to most of the hyper-velocity stars in our Galaxy”.

The night sky seen from a planet orbiting an intergalactic star would be extremely different to what we seen from Earth. The parent galaxy would appear as a huge cloud of diffuse light, maybe with a characteristic spiral shape, hanging in the middle of a black sky in which not even one more star could be seen.

The harvest of this quest for the fastest stars in the universe may increase in a near future, because there are more promising supernova explosions, and the authors of this work announce that ”a second observation run has been already allocated time”. So, more news on this line of research are to arise from Calar Alto Observatory and Ruhr-University.

Further information:

RUBIN, das Wissenschaftsmagazin der Ruhr-Universität Bochum (in German)

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