Runaway Star

By Phil Plait | August 31, 2005 9:31 pm

First, the Venus-Jupiter update (a continuation of Monday’s, Tuesday’s, and Wednesday’s entries), and then I’ll get to today’s entry.

August 28, 2005August 29, 2005
August 30, 2005August 31, 2005

Tonight: the main event! I’ll have new images up around 9:00 p.m. Pacific time Thursday night.

When you look up at the night sky, the overwhelming sense is one of permanence. Things don’t change. Sure, you might get a meteor, and if you watch some stars very carefully you’ll see them change brightness. But the stars are always there, and they don’t move.

But we see the stars from a terrible distance, the scale of which numbs the mind. The nearest star to the Sun, Proxima Centauri, is an incredible 40 trillion kilometers (24 trillion miles) away. That’s a long way to walk. Even light takes 4 years to get here from there.

That distance crushes our sense of perspective. The stars appear motionless in the sky, but in fact they are rushing around at amazing speeds. The Sun is orbiting the center of the Galaxy at 200 km/sec (120 miles/sec). That’s faster than any man-made macroscopic object has ever moved. No rocket, no hyper-accelerated bullet, nothing we have ever made of any size has gone that fast, yet the Sun makes it look stately. Of course, distances are vast: in 240 million years, it’ll circle the Galaxy just once.

All the stars up there are moving at those crazy speeds. Mostly it’s just gravity at the root of this; just like it’s the Sun’s gravity that causes the Earth to orbit it at 30 km/sec (18 miles/sec). The Galaxy has a lot of mass, and so its gravity is strong. An orbiting star has to book along pretty quickly to keep from falling into the center.

But amazingly enough, some stars make those speeds look positively motionless. Astronomers were shocked some years back to discover a class of stars moving considerably faster– some at speeds of more than 500 km/sec. They’re called (for obvious reasons) runaway stars. The problem is, how do you accelerate an entire star to such ferocious speeds?

The stars themselves were already in a special class: they were neutron stars, the ultradense cores of previously normal stars which had exploded as supernovae (I have a series of webpages describing supernovae and neutron stars on my main website). That was a major clue to their speed. For one thing, the stars may have started life in a binary system, in a tight orbit around another star. The leading theory was that when one of the stars exploded, creating the neutron star, the binary was disrupted, and the neutron star shot off like a rock from a sling. This idea is a good one, but it can only account for runaway stars up to a certain speed.

The problem is that runaways are being found with substantially higher velocities. A paper has just come out describing one such star– the poetically named B1508+55 (here’s the press release for it in layman’s terms). The astronomers involved were able to directly determine the distance to this star using parallax, and found it to be 7700 light years away! That’s amazing all by itself. For aficionados, that’s a parallax of 0.415 milliarcseconds, the smallest I’ve ever heard of. They used a radio interferometer to do it.

With the distance in hand, the astronomers could find the speed of this errant star: 1100 km/sec (about 700 miles/second). Think about that: that’s fast enough to cross the diameter of the Earth in about 12 seconds! Yikes.

What could possibly accelerate a star that masses as much as the Sun (more even) to such an incredible speed? It may have been part of a binary system; that would give it some speed. But there must have been a second effect. There are a few possibilities, but they mostly boil down to an asymmetric kick given to the star when it exploded. The explosion energies are vast, certainly enough to do the trick. If the explosion were off-center somehow, it will give the star a push. Magnetic fields might focus that push further, and the matter ejected in the explosion would act like a rocket. It would be like puncturing a hole in the side of a tank of compressed air. Blam! The material rushing away gives the star a huge kick. Getting it up to 1100 km/sec is still puzzling; the models don’t seem to provide that much momentum. But it’s close. It’s probably a combination of several factors. We’ll know better as more of these objects are found, and their characteristics compared.

Imagine! This object is only 10 kilometers or so across, yet masses as much or more than the Sun. It’s already weird; a cubic centimeter of the star weighs billions of tons, far more than the weight of every person on Earth combined. You’d think something that massive would stay put, but it’s the fastest single solid object known!

And the best part, the absolute bestest thing about all this? We humans figured this out. I love that. We’re clever, us apes. That object is so far away it’s invisible to the eye, even using huge telescopes. But we didn’t let that stop us. It’s moving across the sky at a rate equal to watching a guy walking across the street… from 1300 kilometers away. Yet we measured it. The forces involved are so titanic that they make the nuclear arsenal of our entire planet look pathetic, but we figured ’em out.

There’s hope for us yet. There are smart people out there, and they like to solve puzzles. Curious apes we are, in a Universe full of wonders.


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