http://www.bbc.com/earth/story/20150907-the-fastest-stars-in-the-universe

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:

1. Earth’s rotational speed: 1,670 kph (1,040 mph)
   (varies according to latitude)
   (zero at poles; the above speed is at the equator: 40,075km (24,901 miles) in 23hr 56m 4s)
2. Earth’s escape speed: 40,270 kph / 11.2 kps (25,020 mph / 7.0 mps)
   (the speed to be able to escape from the earth’s gravity)
   (so, about 7 times faster than a rifle bullet)
3. Earth’s speed around the Sun: 107,000 kph / 30 kps (66,000 mph / 18.5 mps)
   (970 million km (600 million miles) in 1 year)
4. Sun’s escape speed: 151,560 kph / 42.1 kps (94,174 mph / 26.2 mps)
   (escape velocities vary according to the starting point)
   (these speeds are those at the earth/moon orbit)
   (the value from the sun’s surface is 617.5 kps)
5. Sun’s speed around the Milky Way: 720,000 kph / 200 kps (450,000 mph / 124 mps)
   (240 million years for the Sun to make a complete circuit)
6. Milky Way’s escape speed: (approx) 1.8 million kph / 500 kps (1.1 million mph / 310 mps)
   (these speeds are those at the solar system orbit)
7. HyperFast Stars: 1,000 kps (621 mps)
8. Hyper-HyperFast Stars: 10,000 kps (6,214 mps)
9. Galactic-Centre Stars: 12,000 kps (7,456 mps)
   (the speed of the fastest stars that circle around the galactic black-hole)
10. Black Hole escape speed: 299,792 kps (186,282 mps)
    (these speeds are those at the event horizon)
    (this is the speed of light, which is why a ‘Black Hole’ is black)

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:-

1. In 1988, astrophysicist Jack Hills (Los Alamos National Laboratory, USA) described a hypothetical encounter between a supermassive black hole and a binary star system. The latter consists of two stars closely orbiting each other, and there are lots of those in every galaxy.
   
   Jack realised that it would be possible for the pair to get too close. In that situation, one of the binary stars could be pulled into a tight orbit (or swallowed), whilst it’s partner could be flung out at extraordinary speed. He dubbed the latter “hypervelocity stars”.
2. In 2005, astronomer Warren Brown (Harvard-Smithsonian Center for Astrophysics, USA) was searching for a certain type of bright, blue star in the Milky Way. By tracking their motions, and thus the galaxy’s gravitational influence on them, he was trying to measure the mass of the galaxy. What he found instead was a really fast star. Too fast. It was leaving the galaxy at 853 kps - more than 3 million kph (the galaxy escape velocity is at [6] above).
   
   Warren had found the first HyperFast Star. Eventually he came across Jack Hills’ paper, and has now found a couple of dozen of them. Jack’s mechanism suggests that there should be some ultra-fast stars circulating around the central black-hole ([9] above), which is exactly what has been found.
   
   He reckons that there must be ~1,000 HyperFast Stars. It’s just that the others will be very much dimmer, and hard to measure.

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 ([8] 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.

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Alex Kemp