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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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"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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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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