Hypervelocity Stars (updated)

© Ruth Bazinet/Harvard-Smithsonian
Center for Astrophysics

Hypervelocity stars are stars that are moving, on average, around 7x than the average for, well, your average star. OK, in other words the average velocity for most stars is around 223,694 mph, which seems plenty fast but your average hypervelocity star moves at 1,615,068 mph (1.6 million mph)!

What causes these stars to become dragsters? A binary star system tangling with a black hole. If the conditions are right, one of the binary will be captured by the black hole, while the other star gets flung away at great velocity.

Not many of these hypervelocity stars have been found so far. One recent discovery, HE 0437-5439, has not been ejected from the Milky Way, but from the Large Magellanic Cloud (LMC), a small neighbor galaxy. This star is strong indirect proof of a black hole somewhere in the LMC.

What would that mean for any life on a habitable planet around such a star? They would see constellations change about 7 times faster than we do and they would experience a slight relativistic time dilation (1.5 minutes per year).

But most importantly, they would either pass by other stars several times more often than our own Sun, or be flung out into intergalactic space - far from any other star. The latter would be lonely civilizations indeed, unless they could somehow develop faster than light speeds, as it may take a few billion years for sentient life to arise on such a planet; after a few billion years they would be thousands of light years out into intergalactic space by the time they developed a technological civilization. They would be even further out if they had the unfortunate luck of being ejected from the galaxy, or the LMC, in the opposite direction of the galaxy's trajectory (the Local Group of galaxies, of which the Milky Way and the LMC are members of, is moving at 1.34 million miles per hour in the direction of the constellation Hydra) ¹.

There is an additional effect of being flung out into intergalactic space: being so far from other stars would mean the chances of being effected by a nearby supernova would be nearly zero. As mentioned in a previous post, for the Earth, a supernova 30 light years or closer would be quite devastating for life - for other planets, the distance could be greater, depending upon how thick their protective atmospheres are (to show you how protective our atmosphere is, for astronauts outside the Earth's atmosphere, a supernova 3,000 light years away could be deadly). Some scientist conjecture that maybe a supernova was involved in past extinction events on Earth.

In addition, the solar system would not be affected by galactic disturbances (compression waves, for instance). Thus, the system might be more "boring" than our own, and thus allow sentient life to form more rapidly. HE 0437-5439 is a young star, only 35 million years old, so if there are any planets around it (and it is a big if), and if one of those planets is habitable and in the habitable zone, most likely hasn't arisen yet. But if it does, and it evolves into a sentient race, it will be a very isolated race.

Would they feel themselves blessed by being alone in the universe? Will it appear, to them, that the entire universe revolves around them - even the galaxies? Though if their parent galaxy is racing away from them, what would the make of it? Would they instead feel abandoned? Any thing they create would be lost when their star dies - there would be no one else to ever come along to explore their world. Would this affect how they lived? And if so, how?

Of course, if faster than light travel is somehow possible, by some "trick" (like worm holes), maybe they would be more pressured to discover it than would other, galactic, civilizations; and solve their isolation that way.

Another example of counsel given by Hamlet: "There are more things in heaven and earth, Horatio, Than are dreamt of in your philosophy" (Hamlet Act 1, scene 5, 159–167). Speculating on on possible extrasolar (alien) biological, psychological, societal, technological, and theological realities stretches the dreams of any of our philosophies here on Earth!

Notes:

1. A light year is the distance light travels in mph: 186,000 miles/second * 60 seconds/minute * 60 minutes/hour = 669,600,000 mph.

The distance light travels in a year: 186,000 miles/second * 60 seconds/minute * 60 minutes/hour * 24 hours/day * 365 days/year = 5,865,696,000,000 miles/year.

A hypervelocity star traveling at 1,615,000 mph would be traveling at 0.04145 light years per year: 1,615,000 mph/669,600,000 mph = 0.0024119 or 0.24119 % the speed of light.

Thus, the hypervelocity star would cover 0.24119% of 5,865,696,000,000 miles/year; it would take the star 414.611 years to cover a light year.

For a hypervelocity star leaving a galaxy, in 4 billion years it could be (depending upon relative velocity of it with its parent galaxy) 9,647,597.386 light years away from its parent galaxy! If sentient life takes as long to develop on a planet around such a star as it did on Earth (4.5 billion years), it would be 10,853,547.06 light years away.


References:

Kraan-Korteweg, Renée C. & Ofer Lahav. "Galaxies Behind The Milky Way." Scientific America. October 1998.

Przybilla, N. et al. "LMC origin of the hyper-velocity star HE 0437-5439. Beyond the supermassive black hole paradigm." Astrophysical Journal Letters. Submitted on 29 Jan 2008. 17 Feb 2008. <http://arxiv.org/abs/0801.4456>.

"What is a Light Year?" How Stuff Works. 17 February 2008. < http://www.howstuffworks.com/question94.htm>.