Perhaps the best question to start this little email with is:-
What exactly is a ‘HyperFast Star’, clever-clogs?
A “HyperFast Star” is a star that is travelling so fast that it can leave the galaxy that it is in & travel across the vastness of the universe, possibly to another galaxy.
Here are some speeds to begin to get your head around:
Understanding HyperFast Stars begins with an extraordinary fact. A fact that, when I first saw it, I went “you got to be kidding”. However, the cross-confirmations are inescapable, and it is now part of ordinary Astronomical lore:
In the same way that at the heart of (almost) every raindrop is a particle of dust, so at the heart of every galaxy is a supermassive black-hole.
Next, the fact of HyperFast Stars begins with 2 things:-
Launched in 2013, ESA’s Gaia spacecraft is measuring the velocities and positions of about a billion of the galaxy’s stars. When it’s done, astronomers expect it will identify yet more hypervelocity stars. And that will help them better understand the galaxy.
But there is one more wrinkle!
In 2005 a team of astronomers discovered US-708, a rare type of star called a hot subdwarf. This year (2015) a team led by Stephan Geier of the European Southern Observatory in Germany found that US-708 is travelling at 1200 kps, and is thus the fastest HyperFast Star ever ( above).
US-708 has not come from the galactic centre like the other fast-stars. It looks likely that US-708 has gained it’s speed as the result of a thermonuclear explosion. This star is a fast rotator. The most likely explanation that matches all this evidence is that it is the remnant of a binary pair that began to spiral in towards each other. It’s partner destroyed itself in a process called a ‘supernova’ (an instantaneous destruction) (caused here by an accumulation of helium from US-708), and that suddenly left US-708 alone, and travelling unbound at truly extraordinary speed.
There is one, final conjecture (none have yet been identified) for an even faster star...
Galaxies tend to attract one another. That can cause the supermassive black-holes at their centre to dance around each other, or even merge. The poor old stars that are in each merging-galaxy are just collateral damage amidst the carnage. Some get swallowed up, some are disturbed but stick around whilst others get ejected. All of these latter will be HyperFast Stars. Some will be as fast as US-708. It’s calculated that 1% (which is about a trillion stars) will reach 30,000 kps (10% of the speed of light) or even 100,000 kps (one third light-speed). A few thousand of that trillion could end up in our neighbourhood, but the trouble is identifying them (there are a hundred million ‘normal’ stars for each ultra-fast star). It’s a needle in a haystack. Maybe Gaia will find them.
--------- Alex Kemp