Wednesday, 9 April 2014

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By: faiza khan On: 10:00
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  • How the Crab Nebula’s Pulsing Heart Stayed Hidden for Centuries

    Ancient astronomers chronicled the shifting heavens, diligently charting the movements of our star-studded canopy. The moon’s face morphed nightly, our planetary neighbors came and went, and occasionally a brilliant, icy vagrant would sweep by.

    But the stars? Those stayed pretty much in the same place, relative to one another. So when new, starry points of light briefly appeared and then faded away, Earth’s sky gazers noticed.

    Nearly 1,000 years ago, one of these new stars began shining brightly in the northern sky. It was July 4, 1054, and the people of Earth – from North America to China – turned their attention skyward. Glimmering near the star Zeta Tauri, the new star was much more than a distant, pale point of light: For almost a month, it even shone during the day. Chinese astronomers, who politely referred to the newcomer as a “guest star,” kept detailed records of the stellar visitor. Those records show that the star stuck around for more than two years before slowly fading from the sky like a guest saying good night for the evening.

    Color-inverted reproduction of William Parsons' 1844 drawing of the Crab. (Wikimedia)
    Color-inverted reproduction of William Parsons’ 1844 drawing of the Crab Nebula. (Wikimedia)
    Seven centuries years later, French astronomer Charles Messier was peering through his telescope in Paris, looking for comets. One night in 1758, Messier saw a strange, fuzzy object in the constellation Taurus. He briefly thought it might be the comet Edmund Halley had predicted would return that year. But the object wasn’t moving; it was fixed in the sky, near where Chinese stargazers had marked the appearance of their guest star almost exactly 700 years earlier. That fuzzy dot, which Messier came to realize was a gassy nebula, became known as M1 – it was the first entry in his new catalogue of astronomical objects.

    By the mid-1800s, the nebula had another name: The Crab Nebula, a result of Irish astronomer William Parsons sketching the object and thinking it looked vaguely crab-like.

    It wasn’t until the early 20th century that a series of observations finally revealed what the Chinese guest star was: In 1054, a massive star had exploded and died. At 6,500 light-years away, the supernova, as these stellar explosions are called, was so close by that its light pierced the heavens and arrived on Earth with no difficulty at all. The explosion produced a bright, expanding shell of gas — the nebula Messier, Parsons, and others had seen. When astronomers in the 1920s measured how fast the nebula was growing, they realized they were looking at an object that began ballooning outward nearly 900 years earlier.

    By 1942, there no doubt the nebula was linked to the observations from 1054. But the story isn’t quite over yet.

    For most of its lifetime in Earth’s skies, the Crab Nebula has only been observed in optical wavelengths – that small sliver of the electromagnetic spectrum that humans have evolved to perceive as colors. In addition to visible light, there are also such things as X-rays, gamma rays, infrared, ultraviolet and radio waves. They’re all part of the same spectrum, but vary in wavelength and energy. It’s only been in the last 100 years that astronomers have finished working out how to view the skies through all these different lenses.

    “These are not just different ways of seeing the same thing,” says Neil DeGrasse Tyson in this week’s episode of Cosmos: A Spacetime Odyssey. “These other kinds of light reveal different objects and phenomena in the cosmos.”


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