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Peering into the heart of a Crab

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Peering at the heart of a Crab

New Chandra image released today

September 28, 1999: "Modern astrophysics," an astrophysical wag once said, "has two areas of study: The Crab Nebula and everything else."

Right: The Crab Nebula as seen by the Chandra X-ray Observatory. The image links to a 533x533-pixel, 54K JPG. Click here for a 3,000x2,984-pixel, 1.3MB JPG. Credit: NASA and Chandra Science Center

It's a bit of hyperbole that illustrates a point: The Crab Nebula seems to have most of what's in the celestial bestiary. It is one of the most spectacular nebulas in the sky. It's a supernova remnant. It has a pulsar that emits in radio, visible, ultraviolet, and X-ray wavelengths. It even has a well-established pedigree since it was sighted by royal Chinese astronomers when light from the supernova arrived here in 1054.

"The Crab Nebula and the star at the center of it are the Rosetta Stone of modern astrophysics," said Dr. Martin Weisskopf, Project Scientist for the Chandra X-ray Observatory. The Rosetta Stone is a block of black granite (discovered in 1799) inscribed in Greek, Demotic, and Egyptian hieroglyphs. From this, archaeologists were able to start decoding the texts of ancient Egypt.

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Provenance for a supernova

Like an antiques dealer, astrophysicists often are faced with the challenge of estimating the age of an artifact such as a supernova remnant. Calculations can yield reasonably good estimates, but because most art happened long before modern instruments, the estimates have margins of error.

Like the antiques business, the most valuable artifacts are the ones with a provenance, a record that removes all doubt about its origins and history. The Crab Nebula has a provenance, starting with records kept by royal court astronomers in China and Native Americans.

Right: The many colors of the Crab Nebula as recorded over the years by various observatories on Earth and in orbit. Links to 244x1369-pixel, 147KB JPG. Credit: NASA/Marshall Space Flight Center.

The Crab appeared in July or August A.D. 1054, according to Chinese records, probably on July 5, according to Native American cave drawings White Mesa and Navajo Canyon. Appearing in the sky above the southern horn of the constellation Taurus was a star the Chinese described as six times brighter than Venus, about as brilliant as the full Moon - and visible during the day for almost a month, and at night for a year.

Small wonder. At its peak it blazed with the light of about 400 million suns. That was enough energy to have destroyed all living things on any planet within 50 light years. Fortunately for us, the Crab is more than 7,000 light years away, so the pulse Earth received was about 1/20,000th what it would have been for a closer world.

Then it faded from view and memory until 1731 when 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, spotted it as he started his list of objects that are not comets, his real quarry. The nebula became M1 in his famous "Catalogue of Nebulae and Star Clusters," 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 (he was only off by a century, a remarkably accurate estimate). Historians later linked the Crab with the "guest star" of 1054.

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 power source? 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. Scientists soon concluded that the pulsar was a neutron star because theory suggested that these stars existed at the centers of supernova remnants.

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. That's bright enough to draw the constant attention of astrophysicists from across the planet and the spectrum.

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In like manner, the Crab Nebula has features serve as clues to the inner workings of a range of astrophysical phenomena. In the last couple of weeks, Chandra and its remarkable X-ray telescope targeted the Crab Nebula to collect more clues with the High Resolution Camera. Those images are to be released today in a Space Science Update at NASA headquarters.

"Right now [before Chandra] we're looking at the glow of activity near the center of the nebula as you might see the glow of city lights from a distance," Weisskopf said in a 1998 interview. "Examining the pulsar in the center using Chandra will be like using a telescope to focus on a single street light in the middle of the city."

Right: I'll take Manhattan, plus the rest of Earth if a neutron star was near our planet. This artist's concept shows the relative scale of a neutron star to New York City. While no one knows if a neutron star is dark gray, the sunlight glaring off it probably is real since the star's intense gravity would make it the smoothest object in the universe. Links to 600x533-pixel 362KB jpg, or click here for a 4041x3593-pixel, 6.8 MB JPG. Credit: NASA/Marshall Space Flight Center.

As it happens, that single light has " a brilliant ring around a cosmic powerhouse at the heart of the Crab Nebula," the NASA press announcement promises.

Aside from being the most observed of all pulsars, the Crab Pulsar is also believed to be the youngest of more than 700 known to astronomers.

"Since it is the youngest, it's also the hottest," explained Weisskopf, "and X-rays offer the best way to observe it at these temperatures." Neutron stars cool as they age and the temperature offers evidence of the physical activity occurring inside the star.

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Cat on a hot Crab

In 1996, astronomers using NASA's Hubble Space Telescope found that the Crab is even more dynamic than previously understood after they assembled a cosmic "movie" from a series of Hubble observations showing that the interior of the nebula "changes its stripes" every few days.

"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 Dr. Jeff Hester of Arizona State University in Tempe, AZ. "But I don't think that any of us were prepared for what we saw."

Left: A schematic depicts the jets, halo, and other unusual features discovers by the Hubble Space Telescope when it peered inside the Crab in 1996. Links to 580x711-pixel, 35KB JPG. Credit: Space Telescope Science Institute.

The Hubble team found 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 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, brightenand 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."

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"Neutron stars are a unique laboratory for probing various physical phenomena," Weisskopf continued. "Of interest here is the thermal evolution of the stars." The physical activity in the star's superfluid interior, under a 2-kilometer-thick crystalline neutron crust, is impossible to recreate in any laboratory on Earth, so scientists have been working up theories based on observations of the Crab and other neutron stars. Different theories predict different temperature ranges for such stars.

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How to cook a crab

Stars the mass of our sun eventually die quietly, becoming white dwarfs shining for eons with leftover heat. If a star has about 10 times the Sun's mass, the extra internal pressure burns nuclear ash into heavier elements until it fuses silicon into iron and nickel. (No. 1 in the sequence below) Further fusion absorbs rather than yields energy, so the furnace flips off, the star collapses (2) and then rebounds in a massive blast that spews hot gas - including virtually every element in the periodic table - into space (3).

Such a supernova created the Crab Nebula.

Links to 1280x1024, 427KB JPG. Credit: NASA/Marshall Space Flight Center.

Besides the interstellar debris, supernova explosions often leave behind a cinder, a dense, collapsed core created by the compression of electrons and protons. Called a neutron star, the object is about 20 km (12 mi) wide, has a mass greater than our Sun, and a density equivalent to cramming a World War II battleship into the head of a pin.

The star's gravitational field is about 300,000 times stronger than the Earth's. Its rotation also increases dramatically during the collapse, like a skater spinning faster as she retracts her arms. The neutron star in the Crab Nebula rotates 30 times per second or 3.4 million miles per hour at its equator.

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 as the pot point towards then away from Earth. 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.

Their rapid rotation makes them powerful electric generators, capable of accelerating charged particles to energies of millions of volts, lighting up the nebula around it. (The irregular structure comes from the accelerating gas slamming into slower, colder gas in interstellar space). This drags on the pulsar, so that it spins slower over time. It will take about 10,000 years for the pulsar to slow to half its current rotation speed. The Crab's pulses will weaken, 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.

The ACIS image is not the only view of the Crab that will be taken by Chandra. As a guest investigator, Weisskopf has time allocated to observe the Crab Nebula in more detail using Chandra's High Resolution Camera (HRC) which provides X-ray images that approach the rich detail of the Hubble Space Telescope's Wide-Field Camera. The HRC actually is two cameras in one, an imager to make pictures of X-ray sources and a spectrometer to take pictures of their "colors."

Left: The geometry of the Crab extends far beyond the oversize billiard ball sitting atop Manhattan at the lead of this story. It includes an intense magnetic field whose rotation (with the star) controls and drives the activities of plasma for billions of miles around the Crab. Links to either a 640x512 56KB JPG or larger 1280x1024-pixel, 403KB JPG. Credit: NASA/Marshall Space Flight Center.

One of the important features of the HRC is its speed. Its time resolution is 0.000016 second, the equivalent of taking 62,500 pictures a second, letting Weisskopf capture images of the Crab when it is "on" or "off."

Complicating the task is the fact that the star is a pulsar, meaning that the X-ray readings must be synchronized with Crab. Some HRC readings will have to be made when the pulse is off - actually, when the source is not pointed at Earth - and others when it is on - source pointed at Earth.

In addition to providing information on the Crab Pulsar and its neutron star, the HRC will provide pictures of other discrete structures within the nebula. High-resolution spectroscopy of interstellar material and high-resolution spectroscopy of the nebula itself are also part of the mission plan.

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For more information, please contact:
Dr. John M. Horack , Director of Science Communications
Authors: Tom Kelleher and Dave Dooling, with materials from the Space Telescope Science Institute.
Curator: Linda Porter
NASA Official: M. Frank Rose