Runaway star

Can you get out of here? That’s the question pondered for centuries: How to leave Earth permanently. A cannonball fired upward has always returned. With a greater explosive charge and a higher speed, the shell went farther. It wasn’t hard to figure out what speed an object would need to fall around Earth’s curve and leave our planet permanently: It was 6.9 miles per second. This is Earth’s escape velocity.

Sounds like a simple concept, and it is. For every celestial object, its mass determines how strongly it glues to itself any nearby planets or moons or other stars. For the Sun, the escape velocity is a whopping 384 miles per second, if you start out just above its surface. But by the time an object has managed to flee from the Sun to arrive here at Earth’s distance, the escape speed has fallen off to just 26 miles per second. That’s still faster than our best rockets, which can go only ten miles per second. Thus, it’s easier to escape our own world than to fight our way outward and wrench free from the Sun’s more insistent grasp.

Our Milky Way galaxy, composed of 400 billion suns, has its own escape velocity, and it’s nothing trivial. For anything to leave, it would have to travel over 180 miles per second. Not surprisingly, no star in our galaxy had ever been found to possess such a speed. Put another way, the immense, beautiful azure spirals and golden nucleus of our Milky Way constitute an integrated unit where all components are attached forever.

Forever, that is, until 2005: That’s when astronomers found the first runaway. As early as 1988, astronomer J. G. Hills of Los Alamos had discovered a mathematically possible way for a star to escape the galaxy. If it were a member of a binary system, a double star, and if it passed a precise distance from the supermassive black hole in our galaxy’s center, that could set in motion an intriguing high-stakes pinball game. The black hole could, in theory, yank at one member of the binary system in such a way that the other member gets flung off at incredible speed: fast enough to break away permanently from the entire galaxy.

It could happen. But has it? In 2003, astronomers starting searching for just such objects – and found one two years later. It was discovered using the enormous 6.5-meter telescope at the MMT Observatory in Arizona. It’s a dazzling blue star with the kind of high metal content expected of stars born in the galaxy’s core – except that this star is nowhere near there. Instead, it’s whizzing through the constellation Hydra in the Milky Way’s suburbs at 415 miles per second: more than twice the speed necessary to leave the galaxy behind permanently.

This faint star, about 100 times fainter than Pluto, is named SDSS J090745.0+024507. It is currently located 160,000 light-years from the galaxy’s center, after having made a beeline from there ever since its birth 80 million years ago. This is the fastest-moving star in the galaxy.

Already, astronomers are calling such objects “outcast stars” or “slingshot stars” or “runaway stars” – but the official name is hypervelocity stars or, given the obsessive urge to abbreviate, HVS. In just another 80 million years – a mere coffee break in star-time – SDSS J090745.0+024507 will be traveling alone like “the man without a country” – a passportless intergalactic wanderer. Any life on planets forming around it will gaze up into a starless sky, lit only by the fuzzy outline of its long-ago parent galaxy.

Could there be others? Now that Hills’s 1988 prediction is validated, astronomers are looking, but it’s not an easy quest. Statistically, up to 1,000 stars could have had a similar close encounter with the 4-million solar mass black hole at our galaxy’s center and not have been pulled in, but instead flung violently away. But how to find such faint stars among the 400 billion that populate the Milky Way?

For now, the status of SDSS J090745.0+024507 seems safe, as the “one that got away.” Hurtling at over 1,500,000 miles per hour – more than twice as fast as any other.



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