You have never encountered speed until you witness a hypervelocity star escaping a galaxy. In fact, the latest occurrence was the fastest event ever!
US 708 apparently left its galaxy, traveling at over 4 million kilometers an hour (2.7 million mph). The popular belief is that a supernova could be the culprit of this momentum.
Just last year, a multitude of star clusters were expelled by the large galaxy M87 and thought to be related to the black hole. It seems that superfast stars are quite common. The normalcy of these events are the main reason for the collective naming of these heavenly bodies – Hypervelocity Stars (HVS).
Differences in US 708
Yes, we have another super fast star, but this one is different. US 708 is the only known helium star, compact and moving at great speed. This attribute alone makes it unique and interesting to study. There is something else, however, that makes scientists think twice about the origins of US 708.
Recent measurements cast doubt about how this hypervelocity star got hurled across space. The previous theory, well known at that, says that hypervelocity stars are propelled by an interaction with the supermassive black hole and a binary. The close binary is disturbed by the black hole, which resides in the center of the galaxy, and components are sent flying at high speeds. One or more of these components becomes the hypervelocity stars.
US 708 doesn’t quite fit this bill. Although it is possible that US 708 was propelled in the same manner – it happens very often – it is unlikely that this is what happened. This is where the idea of the supernova comes into discussion.
What PROBABLY happened…
Let’s say that a Supernova was the culprit. There are types of supernovae, called 1a. These Supernovas occur in binary systems that are close neighbors, where gases are stripped from one star and collected onto a white dwarf star. In this event, the pieces absorb well. The HVS could be the product of a shockwave from a nearby 1a Supernova!
Reason for this hypothesis
Authors from the European Southern Observatory, including Dr. Stephen Geier, traced the path of US 708 back to its origin. The star, apparently did not come from the center of the galaxy. Calculations concluded that US 708 crossed the galactic disc about 14 million years ago. Scientists think that US 708 was once paired with a white dwarf, larger than our own sun!
The orbit of the stars is a 10 minute period, which is common for helium based stars. Since these partners were so close, the smaller star, the subdwarf, would spin very fast-approximately 115 km/s-thus building the speed for propulsion.
Although most stars propelled from space come from the force of black hole disruption, this one seems different. US 708 has taught us there are still many mysteries of the universe waiting to be solved, and that supernovae are more than they seem.
Image credit: NASA, ESA, and G. Bacon (STScI)
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