Looking at Old Stars in New Ways: Gravitational Waves Help Us See Inside a Neutron Star

Author:  Jessica Kloss
Institution:  Princeton University
Date:  July 2008

You might have heard the stories: neutron stars are so dense that one teaspoon would weigh about a billion tons on Earth. These stars were once not so different from our Sun, generally about 4 to 8 times more massive. But when these stars run out of fuel to burn, they have a supernova explosion, and eject their outer layers – and in this particular case, the ejected outer layers formed what we now call our beautiful Crab Nebula (see photo). The inner part of the star, with no more fuel, collapses under its own weight to a sphere about 20 kilometers in diameter. The pressure is so great that protons and electrons come together to become neutrons, and a neutron star' is born. And now, for the first time, an international team of scientists has found a way to look inside these mysterious objects, using an instrument intended to pick up gravitational waves.

The strange part is that this groundbreaking discovery was made based on observations of an object that was first noted by Chinese and Arab astronomers nearly a thousand years ago. The Crab Nebula, which contains the object in question, has been observed countless times and is widely considered among the most beautiful objects in the sky. But even though this object is hardly new to us, the technology used to observe it is: the Laser Interferometer Gravitational-Wave Observatory (LIGO) is an instrument designed specially to detect gravitational waves. With LIGO's cutting edge technology observations of the Crab Pulsar' have given us a long-anticipated look into the inside of a neutron star. This discovery also marks the first progress on gravitational wave detection made in a long time.

The star's long history dates all the way back to 1054, when the supernova explosion of the star was spectacular, visible even during the day for more than three weeks. Chinese texts referred to it a "guest star." Today, it's called a pulsar' because it emits rapid pulses of radio signals that we detect on Earth. To understand what causes these "pulses" one must first understand two important properties of the neutron star: its rapid spinning motion, and the energetic jets' it emits at its poles, which happen to be pointed toward the Earth.

To get an idea of why neutron stars spin so quickly, one need only picture an ice skater. The wider the ice skater spreads her arms initially, spinning slowly, the faster she spins when she brings them in close to herself. For stars, the effect is much more dramatic , picture our ice skater spinning slowly, arms open, only now her arms are as wide as the diameter of the Sun (about one and a half million kilometers wide). Imagine the effect if now she were to bring her arms inward and wrap them around herself such that she was 20 kilometers wide. This effect is indeed why neutron stars spin so rapidly.

Highly magnetized neutron stars not only spin fast but emit energy along their poles. However, these emissions can only be detected if the jets' of emission are pointing toward the Earth, where we pick it up in the form of radio waves. For neutron stars for which this is the case, such as the Crab Pulsar, the emissions combined with the fast spinning motion of the star (30 times per second!) means that the jets of emission sweep by the Earth very quickly, looking like quick pulses. These pulses generally come at a very regular rate, changing extremely slowly.

"However, [the Crab Pulsar's] rotation rate is decreasing rapidly relative to most pulsars, indicating that it is radiating energy at a prodigious rate," said Graham Woan of the University of Glasgow, co-leader of the group studying the Crab Pulsar with LIGO. Previously there had been theories suggesting that gravitational waves could be responsible for much of the energy loss. But with the help of LIGO, researchers now say that no more than 4 per cent of the energy loss of the pulsar is due to gravitational wave emissions – a very small amount.

According to the group's co-leader, Michael Landry of the LIGO Hanford Observatory, "The remainder of the loss must be due to other mechanisms, such as a combination of electromagnetic radiation generated by the rapidly rotating magnetic field of the pulsar and the emission of high-velocity particles into the nebula."

LIGO monitored the Crab Pulsar from November 2005 to August 2006, looking to detect gravitational waves. None were found – a significant result, since if the star's shape were even deformed by even a few meters from that of a perfect sphere, they would have been detected. Scientists can now conclude that the Crab Pulsar is very smoothly spherical, which limits the possibilities of what could be going on beneath the surface.

"The physics world has been waiting eagerly for scientific results from LIGO. It is exciting that we now know something concrete about how nearly spherical a neutron star must be, and we have definite limits on the strength of its internal magnetic field," said Joseph Taylor, Nobel Prize-winning radio astronomer and physics professor at Princeton University.

LIGO's observations of the Crab Pulsar provide much-needed data to help determine what's really going on inside neutron stars. For instance, scientists had previously considered that neutron stars could have very strong internal magnetic fields. Other theories predicted that the surface of neutron stars be only semisolid. But now, with LIGO, both of these theories must either be refined or ruled out. So with the help of LIGO, we've moved one step closer to understanding the inner workings of these mysterious, fantastically dense objects that have mystified scientists for decades.

Written by Jess Kloss

Reviewed by Nira Datta

Published by Pooja Ghatalia.