Using two different satellites that study X-rays, teams of astronomers have developed a new method for finding out what happens when matter is packed together as tightly as it can possibly be.
Image: An artist depicts a disk of hot gas whipping around a neutron star. The gas in the inner part of the disk whirls around the neutron star at about 40 percent the speed of light, so fast that it experiences effects predicted by Einstein's theories of relativity. Superheated iron atoms in this region emit X-rays at a characteristic wavelength, but the spectral feature is highly distorted by the relativistic effects. + High resolution Credit: NASA/Dana Berry.
One team used a satellite called XMM-Newton, which is operated by the European Space Agency. The other team used the Suzaku satellite, which was built mostly by Japan’s space agency with some help from NASA.
The two teams studied neutron stars. These are among the strangest objects in our Universe. Imagine taking all the gas in our sun (a sphere that could fit a million Earths) and cramming it into a ball about the size of New York City (20 miles across). That’s a neutron star. Matter in neutron stars is crammed so tightly that an ice-cream cone filled with neutron-star material would outweigh Mount Everest — the highest mountain in the world!
Neutron stars are the hardest and most dense objects in the Universe. Their gravity is so powerful that adding just a little bit of matter can cause a neutron star to collapse inward to form a black hole. But what is the structure and behavior of matter when it is scrunched so tightly into neutron stars? This is one of the biggest mysteries in astronomy.
Image: Many neutron stars are accompanied by a companion star, as portrayed in this illustration. The powerful gravity of the neutron star siphons off gas from the companion, which then settles into a slowly in-spiraling disk around the neutron star. + High resolution Credit: NASA.
Because it’s impossible to create these conditions in a lab, astronomers need to find out exactly how much material a neutron star contains, and how big it is. But neutron stars are tiny objects (city sized) and very far away. The closest known neutron star is so distant that it takes light — the fastest thing in the Universe — several hundred years to reach Earth. Trying to measure the exact size of a neutron star is like an astronomer on Earth trying to measure the exact size of a rock on Pluto.
The teams used the two X-ray satellites to take a spectrum of several different neutron stars. Astronomers Sudip Bhattacharyya and Tod Strohmayer at NASA’s Goddard Space Flight Center in Greenbelt, Md., aimed the XMM-Newton satellite at a neutron star named Serpens X-1. They found that superhot iron atoms are whirling around the neutron star at speeds more than a million times faster than a race car. The iron is so hot that it’s in the form of a gas rather than a solid.
Image: XMM-Newton observed this spectral line from superheated iron atoms orbiting the neutron star Serpens X-1 at the inner edge of an accretion disk. Normally, the line would be a symmetrical peak, but it exhibits the classic features of distortion due to relativistic effects. The extremely fast motion of the iron-rich gas causes the line to spread out. The entire line has been shifted to longer wavelengths (left, red) because of the neutron star's powerful gravity. The line is brighter toward shorter wavelengths (right, blue) because Einstein's special theory of relativity predicts that a high-speed source beamed toward Earth will appear brighter than the same source moving away from Earth. Credit: Credit: Sudip Bhattacharyya and Tod Strohmayer..
Normally, iron would show up in a spectrum as a big, sharp spike sticking up above a horizontal line. But the iron is moving so fast, and the neutron star’s gravity is so extreme, that strange effects predicted by Albert Einstein take place. They make the iron spike look much broader and skewed, with the peak shifted to the right side.
Another team was led by Edward Cackett and Jon Miller of the University of Michigan, Ann Arbor, and includes Bhattacharyya and Strohmayer. This group used the Suzaku satellite to look at Serpens X-1 and two other neutron stars. They found the exact same effects around all three neutron stars.
The distorted iron spectral lines confirm that Einstein’s theories are working around neutron stars. But they also provide important clues about the size and weight of the neutron star. The speed of the iron gas tells astronomers the strength of the neutron star’s gravity, which also tells them how much material it contains. And since the gas is orbiting the neutron star just beyond its surface, astronomers can calculate the maximum possible size of the neutron star. In the case of the three neutron stars studied by Suzaku, they are between 18 and 20{1/2} miles across.
These kinds of measurements are very difficult to make, so several techniques are needed to solve this great mystery. "This new method gives us a new instrument in our toolkit," says Bhattacharyya. "This gives us confidence that we will figure out what is going on in neutron stars."
Topic: Probing Scrunched Stars (Read 1833 times)