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>  News Releases >   2007 >   May

Dartmouth undergrad solves longstanding astronomical puzzle

Dartmouth College Office of Public Affairs • Press Release
Posted 05/29/07
Rebecca Bailey • (603) 646-3661

A green-light image of the Crab Nebula
A green-light image of the Crab Nebula taken with the Subaru 8.2 meter telescope in October 2005. (Image credit: Toru Yamada/NAOJ)
A magnified green-light image of the northern jet
A magnified green-light image of the northern jet taken with the Subaru 8.2 meter telescope in October 2005. (Image credit: Toru Yamada/NAOJ)

A team of astronomers has recalculated the explosion date of the famous Crab Nebula supernova and found excellent agreement between their measurements and the classic date of the 1054 A.D. appearance of a bright "guest star" seen in the constellation of Taurus the Bull.

The research was led by Gwen Rudie, a senior physics major at Dartmouth. She and her adviser, Robert Fesen, along with Toru Yamada from Japan's Subaru Observatory in Hawaii, used photographs taken 17 years apart to study the expansion speed of the Crab Nebula, in results released May 29 in Honolulu at the 210th meeting of the American Astronomical Society.

These astronomers found that the outermost part of the supernova remnant, a very faint "jet" of stellar debris, shows clearly for the first time that the Crab exploded around the middle of the 11th century, in perfect accord with historic records. "For the past 100 years astronomers have puzzled over the discrepancy between the measured age of the Crab Nebula versus the age suggested by numerous historical sightings around the ancient world," Rudie said. "This work verifies some long-standing assumptions about the nature of the Crab-with our result, science and history finally agree."

The Crab Nebula is one of the most studied remains of a stellar explosion and is widely accepted to be due to a supernova seen in the year 1054 A.D. by Chinese, Japanese, Korean, and Arab astronomers, who reported sighting a new bright star in the heavens. The star was so brilliant that it was visible even during the day for nearly three weeks and only faded from view nearly two years later.

While the Crab Nebula's location in the sky agreed very well with the reported position of this bright new star, several studies of the expanding cloud of stellar debris unexpectedly indicated that it was expanding much too fast to be associated with a supernova explosion in 1054.  Instead, these studies pointed later in time, toward an explosion date in the first half of the 12th century.

This dilemma led astronomers to the idea that the remnant's central pulsar—the rapidly spinning and extremely dense neutron star left over from Crab's supernova explosion—emitted such copious amounts of energy that it actually accelerated the expanding cloud of debris, making it move faster with time.

"Previous studies have focused on the main body of the Crab Nebula, which has been accelerated with time," Rudie explained.  "We chose to look at the northern jet because it is farther from the pulsar. That was the key to our new age determination."

The images used in this study were taken 17 years apart: one in October 2005 with the Prime Focus Camera (Suprime-Cam) on the Subaru 8.2-meter telescope on Mauna Kea by astronomers from the National Astronomical Observatory of Japan, Tohoku University, and Kyoto University, and one taken with t he National Science Foundation's Mayall 4-meter telescope at Kitt Peak National Observatory, part of the National Optical Astronomy Observatory, Tucson, Arizona, by Robert Fesen of Dartmouth in November 1988.

The new Subaru image used in the study was released in March 2007 as part of an image-only press release.  The results announced today pertain to a scientific study of the movement of the supernova debris across the plane of the sky.  By measuring this proper motion over the course of 17 years and tracing it backward in time, this group of astronomers was able to confirm the 1054 explosion date.

The Subaru image and a comparison of the two images are available.

Further-processed imaging data of the Crab Nebula from Subaru is available.

Located approximately 7,200 light-years from Earth, the Crab Nebula was named due to its resemblance to a crab's claw in an early sketch made in 1855 by astronomer R.J. Mitchell. The nebula was probably first noticed in 1731 by John Bevis, and it was significant enough to be the first entry in Charles Messier's list of nebulae (compiled to avoid mistaking these nebulae for comets).

The supernova debris cloud has a diameter of about 10 light-years, with material rushing outward at more than 1,500 kilometers per second.  There is a faint-blue neutron star at the center of the Crab Nebula. It rotates rapidly, at a rate of 33 times per second, and emits radio waves, x-rays and gamma rays, in addition to visible light.

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