Exit Strategy: Neutron Star Kicked Out of Milky Way
It went that-a-way: Over the past 2.5 million years, Pulsar B1508+55 has moved across about a third of the night sky as seen from Earth. Image
Credit: Bill Saxton, NRAO/AUI/NSF

Thanks to the kick it received from a supernova, the fastest known neutron star is speeding out of the Milky Way.

Most stars in the Milky Way lie in a fairly concentrated plane, varying only by plus or minus five degrees.

Two-and-half million years ago, a massive star in the constellation Cygnus--a collection of giant stars within the galactic plane--went supernova. As the star collapsed, the resulting huge explosion gave a powerful "kick" to the neutron star that formed deep inside the supernova.

Now, at 52 degrees latitude and about 7,700 light-years from Earth, the neutron star B1508+55 is well out of the galactic plane. And although it's the fastest neutron star ever observed--traveling at 670 miles-per-second (1,100 km/s) it could make the trip from New York to Los Angeles in under 4 seconds--it will still be sometime before it leaves the galaxy.

"It's on its way out," Shami Chatterjee of the National Radio Astronomy Observatory and the Harvard-Smithsonian Center for Astrophysics told SPACE.com. "In maybe another million or so years it will leave what we nominally think of as our galaxy."

This discovery could settle the argument over how fast an imbalanced supernova explosion can send a neutron star flying.

"We've thought for sometime that supernova explosions can give a kick to the resulting neutron star, but the latest computer models of this process have not produced speeds anywhere near what we see in this object," said Chatterjee.

Three-dimensional computer models, run for the first time this past year, predict ejecting neutron star speeds of only 122 miles-per-second (200 km/s). In computer simulations, material from the outer layers of the collapsing massive star crashes into the neutron star as it's on its way out.

But these simulations factor in variables based on estimations. Speeds predicted by these simulations aren't reliable, says Chatterjee. His measurement, based on pure geometry, is much more accurate.

"It's a straight forward measurement of velocity," Chatterjee said. "Once we know how far away the star is, and we know how much physical space it's covered in two years, we can translate that into kilometers per second."

To determine B1508+55's speed, astronomers first had to figure out how far it is from Earth. They accomplished this using the National Science Foundation's Very Long Baseline Array (VLBA) radio telescope and a trick from high school geometry--parallax.

If you don't remember parallax, it's a way of determining how far away an intermediate object is using two viewing points, a background of known distance, and the angles between all these objects. Here's an example:

First, stand five feet from a wall. Now, hold up one finger arm's length from your face. As you focus on the wall, each eye sees the finger slightly differently. Using these differences, the known distance from eye to eye and the distance and angle from eye to wall, you can figure out how far away your finger is.

"Here, the wall that it's compared against is the background galaxies. Instead of two eyes, we used positions of the Earth six months apart, on either side of the sun," Chatterjee said. "Since we know these other distances very accurately, and we know the angle, we can get a very accurate measurement. It's straight geometry."

Once they determined the distance between Earth and the neutron star, they used the VLBA to track how far it moved across the sky in a given time. Fortunately the VLBA specializes in making observations on the tiniest scales.

"The motion we measured with the VLBA was about equal to watching a home run ball in Boston's Fenway Park from a seat on the Moon," Chatterjee said.

With this information in hand, Chatterjee determined that this object is traveling at 1,100 km/s (670 miles/s), the fastest speed yet recorded for a neutron star.

Since these measurements are based on geometry, they're hard to argue with, Chatterjee says. The computer simulations, he adds, may need to be corrected to account for this observation.

Once Chatterjee calculated how fast it was going, he estimated the neutron star's age at about 2.5 million years old and began tracking it backwards to determine its origin.

"We have a very accurate distance and a very accurate velocity vector for the star. If you trace it back, it comes back straight to the galactic plane to a cluster of very massive stars," Chatterjee said. "It shot right out of the galactic plane."

Although Chatterjee believes the neutron star was thrown out of the Cygnus constellation, he says there is another way it may have gotten this much speed--binary disruption. Massive stars often exist in pairs, spinning furiously and held together by a band of gravity. When one of the stars goes supernova, this shock disrupts the gravity tie and sends both stars flying in opposite directions.

"It doesn't look like you can get 1,100 km/s through this process," Chatterjee said. "Probably only 600 km/s (366 miles/s) from binary disruption. At 1,100 km/s, most people will say there must be a supernova kick involved."

This observation was part of a long-term project to use the VLBA to measure the distances and speeds of numerous pulsars. The VLBA is a system of 10 radio-telescope antennas, each with a dish 25 meters (82 feet) in diameter. The VLBA spans over 5,000 miles - from Mauna Kea on the Big Island of Hawaii to St. Croix in the U.S. Virgin Islands - providing astronomers with the sharpest vision of any telescope on Earth or in space.