Probing the mysterious heart of the Crab Nebula, the tattered remains of a stellar cataclysm witnessed more than 900 years ago, astronomers using NASA's Hubble Space Telescope have found that the Crab is even more dynamic than previously understood, based on a cosmic "movie" assembled from a series of Hubble observations.
The results promise to shed new light on a variety of high energy phenomena in the universe, from nearby neutron stars to remote quasars.
Though changes in most astronomical objects are barely perceptible over a human lifetime, Hubble shows that the interior of the nebula "changes its stripes" every few days, according to Jeff Hester of Arizona State University in Tempe, AZ, who leads the team of astronomers that took the Wide Field and Planetary Camera 2 (WFPC2) images.
"We took the images a few weeks apart because we knew that it might be possible to observe slight differences in the Crab over a short time," said Hester. "But I don't think that any of us were prepared for what we saw."
Though ground-based images of the Crab had shown subtle changes in the nebula over months or years, the Hubble movie shows sharp wisp-like features streaming away from the center of the nebula at half the speed of light.
The powerhouse at the center of the nebula responsible for these changes is a rapidly spinning neutron star – the compact core of the exploded star. Only about six miles (10 kilometers) across, the neutron star would fit inside a small town, "yet its small size belies its significance and the punch that it packs," said Hester.
As the neutron star spins on its axis 30 times a second, its twin searchlight beams sweep past the Earth, causing the neutron star to blink on and off. Because of this flickering, the neutron star is also called a "pulsar." In addition to the pulses, the neutron star's rapid rotation and intense magnetic field act as an immense slingshot, accelerating subatomic particles to close to the speed of light and flinging them off into space.
In a dramatic series of images assembled over several months of observation, Hubble shows what happens as this magnetic pulsar "wind" runs into the body of the Crab Nebula. The glowing, eerie shifting patterns of light in the center of the Crab are created by electrons and positrons (anti-matter electrons) as they spiral around magnetic field lines and radiate away energy. This lights up the interior volume of the nebula, which is more than 10 light-years across.
The Hubble team finds that material doesn't move away from the pulsar in all directions, but instead is concentrated into two polar "jets" and a wind moving out from the star's equator.
The most dynamical feature in the inner part of the Crab is the point where one of the polar jets runs into the surrounding material forming a shock front. The shape and position of this feature shifts about so rapidly that the astronomers describe it as a "dancing sprite," or "a cat on a hot plate." The equatorial wind appears as a series of wisp-like features that steepen, brighten, then fade as they move away from the pulsar to well out into the main body of the nebula.
"Watching the wisps move outward through the nebula is a lot like watching waves crashing on the beach - except that in the Crab the waves are a light-year long and are moving through space at half the speed of light," said Hester. "You don't learn about ocean waves by staring at a snapshot. By their nature waves on the ocean are ever-changing. You learn about ocean waves by sitting on the beach and watching as they roll ashore. This Hubble 'movie' of the Crab is so significant because for the first time we are watching as these 'waves' from the Crab come rolling in."
The Crab Nebula, the result of a supernova explosion witnessed by Chinese astronomers in 1054 AD, also is widely studied because it offers a unique opportunity to study high energy astrophysical phenomena. The physical processes that are at work in the centers of distant active galaxies and quasars are thought to be much like the processes at work in the center of the Crab, only on a vastly larger scale. "The difference is that while astronomers may never truly 'see' into the very heart of an active galaxy, the Crab allows the properties and behavior of high energy winds and jets to be studied up close and personal," Hester said.
"The Hubble results aren't the end of the story," Hester emphasized. "Rather, they are a piece of a larger puzzle. For example, the jets seen streaming away from the pulsar in the Hubble data are of particular interest because they help explain two lobes of X-ray emission seen extending away from the pulsar in images taken with the Einstein and ROSAT X-ray satellites."
In addition to Hester and Paul Scowen of Arizona State University, other members of the team responsible for this work include Ravi Sankrit of Arizona State University, Curt Michel of Rice University, Jay Gallagher of the University of Wisconsin at Madison, James Graham of the University of California at Berkeley, and Alan Watson of New Mexico State University.
BACKGROUND INFORMATION: A HISTORY OF THE CRAB NEBULA
Hundreds of years before Americans began celebrating Independence Day by peppering the sky with fireworks, a more powerful celestial explosion brightened a summer sky.
It was the spectacular explosion of a supernova, the violent death of a star that may have been as much as 10 times more massive than our Sun. In July or August of 1054, Chinese astronomers saw and recorded the star's demise. Appearing in the sky above the southern horn of the constellation Taurus was a star the Chinese described as six times brighter than Venus and about as brilliant as the full Moon. The remains of this star were later christened the Crab Nebula, a cloudy, glowing mass of gas and dust about 7,000 light-years away from Earth.
This "guest star," as the Chinese called it, was so bright that people saw it in the sky during the day for almost a month. During that time, the star was blazing with the light of about 400 million suns. The star remained visible in the evening sky for more than a year. In two separate accounts, Chinese astronomers described the star as having pointed rays in all four directions and a reddish-white color.
If the blast had occurred 50 light-years from Earth, astronomers believe that all living things could have been destroyed by radiation. In the nine centuries since, astronomers have witnessed only two comparable cataclysms in our Galaxy: the supernova explosions of 1572 and 1604.
By Chinese accounts, the supernova was a tremendous celestial display. But the Europeans may not have agreed, because astronomers have not found any European records of the event.
The American Indians in northern Arizona, however, may have been so inspired by the event that they drew pictures of it. Two pictographs have been found, one in a cave at White Mesa and the other on a wall of Navajo Canyon. Both show a crescent moon with a large star nearby. Scientists have calculated that on the morning of July 5, 1054, the Moon was located just 2 degrees north of the Crab Nebula's current position.
The supernova was forgotten for more than 600 years until the invention of telescopes, which revealed fainter celestial details than the human eye can detect. In 1731, English physicist and amateur astronomer John Bevis observed the strings of gas and dust that form the nebula. While hunting for comets in 1758, Charles Messier spotted the nebula, noting that it had no apparent motion. The nebula became the first entry in his famous "Catalogue of Nebulae and Star Clusters," first published in 1774. Lord Rosse named the nebula the "Crab" in 1844 because its tentacle-like structure resembled the legs of the crustacean.
In the decades following Lord Rosse's work, astronomers continued to study the Crab because of their fascination for the strange object. In 1939, astronomer John Duncan concluded that the nebula was expanding and probably originated from a point source about 766 years earlier.
Astronomer Walter Baade probed deeper into the nebula, observing in 1942 that a prominent star near the nebula's center might be related to its origin. Six years later, scientists discovered that the Crab was emitting among the strongest radio waves of any celestial object. Baade noticed in 1954 that the Crab possessed powerful magnetic fields, and in 1963, a high-altitude rocket detected X-ray energy from the nebula.
Radio waves. X-rays. Strong magnetic fields. Scientists knew that the Crab Nebula was a powerful source of radiation, but what was its origin? They discovered it in 1968: an object in the nebula's center – Baade's prominent star – that emitted bursts of radio waves 30 times per second. Called the Crab Pulsar, it is among the first pulsars discovered, and is the fastest and most energetic pulsar formed from a supernova explosion.
But scientists wondered why the pulsar was spinning so fast. They concluded that the pulsar was a neutron star because theory suggested that these stars existed at the centers of supernova remnants. Neutron stars also are the only stars that can rotate rapidly without breaking apart. An extremely dense, compact object, a neutron star forms from the matter of a collapsed star. The Crab Pulsar acts as a celestial power station, generating enough energy to keep the entire nebula radiating over almost the whole electromagnetic spectrum. Because of the pulsar's power, the nebula shines brighter than 75,000 suns.
BACKGROUND INFORMATION: HOW PULSARS ARE FORMED The Life of a Star
Throughout their existence, stars fight a dramatic battle against the forces of gravity. Gravity tries to collapse the star by pulling its outer layers towards its center. But the star fights back by releasing nuclear energy, which is fueled by a rich supply of hydrogen. Eventually, usually after billions of years, stars deplete their fuel supply and must give up the fight. Some aging stars die quietly; others suffer violent deaths. The method depends on a star's mass. Stars about the same size as our Sun become white dwarfs, which shine for a very long time from leftover heat. Stars that have about 10 times the Sun's mass blow apart and often form neutron stars. Scientists believe that the Crab Nebula came from such a star.
A Star's Collapse
Once a large star exhausts its fuel supply, gravity takes over and the star is collapsed without opposition. Usually a star will find other sources of fuel like helium, carbon, oxygen and nitrogen, but these offer a short reprieve. Eventually the densities at the center of the star get so high that the star cannot collapse much more at all. Instead all the pressure from the collapse is "stored," ready for release. Finally the conditions become so extreme at the center of the star that all the "stored" pressure from the years of collapse are released in a single brilliant burst: a nova or a supernova, depending on the mass of the star. This explosion throws off the outer layers of the star and compresses its core even more. It was a supernova that created the Crab Nebula. During the explosion, the star gives off more energy than a galaxy of 100 billion stars. The outer layers being ejected create an expanding shell of dust and gas that become a supernova remnant.
The Birth of Neutron Stars
Besides the interstellar debris, supernova explosions often leave behind a cinder, the star's dense, collapsed neutron core, which was created by the compression of electrons and protons. Called a neutron star, the object is about 10 miles wide, has a mass greater than our Sun, and a density of about a billion tons per teaspoonful. Because of its small size and high density, the neutron star possesses a gravitational field 300,000 times stronger than the Earth's. Its rotation also increases dramatically during the collapse. Most celestial objects rotate, but neutron stars rotate very rapidly. The neutron star in the Crab Nebula rotates 30 times per second or 3.4 million miles per hour. It is the only kind of star that can rotate rapidly without breaking apart.
The Formation of Pulsars
Some neutron stars – such as the Crab – emit radio waves, light, and other forms of radiation that appear to pulse on and off like a lighthouse beacon. Called pulsars, they don't really turn radio waves on and off – it just appears that way to observers on Earth because the star is spinning. Astronomers pick up the radio waves only when the pulsar's beam sweeps across the Earth.
Pulsars possess a powerful magnetic field that traps and accelerates charged particles, and shoots them through space as radio waves. Their rapid rotation makes them powerful electric generators, capable of accelerating charged particles to energies of millions of volts. The Crab, the youngest and most energetic pulsar, produces enough energy to power the nebula and make it expand. The real difference between a neutron star and a pulsar is that a pulsar has a magnetic field that is misaligned with the rotation axis –being tilted at an angle of about 30 degrees to the rotation poles.
A pulsar's energy output lights up and expands the nebula around it. This action robs energy from the pulsar's rotation, so that it spins slower over time. This "spin-down" rate is a tiny percentage per year, so that it will take about 10,000 years for the pulsar to slow to half its current rotation speed. As time progresses, the Crab's pulses will become less intense, and its X-ray emissions eventually will end. The nebula itself will disappear after only a few thousands years. Eventually only the radio pulsar, beaming every few seconds, will remain.
First discovered in 1967, scientists jokingly dubbed pulsars LGM for "Little Green Men," because the radio signals were so regular it seemed to be a sign of intelligent life. Scientists can predict the arrival times of pulses a year ahead with an accuracy of better than a millisecond. They have cataloged more than 300 of them. But only two, the Crab and Vela, emit detectable visible pulses. The Crab emits radiation throughout the entire spectrum, including gamma and X-rays.
