And then … just to make the idea of 'Standard Candles' even less firm:
High-energy surprises found in 'constant' Crab Nebula (w/ Video)
Quote:
The combined data from several NASA satellites has astonished astronomers by revealing unexpected changes in X-ray emission from the Crab Nebula, once thought to be the steadiest high-energy source in the sky.
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"For 40 years, most astronomers regarded the Crab as a standard candle," said Colleen Wilson-Hodge, an astrophysicist at NASA's Marshall Space Flight Center
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For decades, astronomers have regarded the Crab's X-ray emissions as so stable that they've used it to calibrate space-borne instruments. They also customarily describe the emissions of other high-energy sources in "millicrabs," a unit derived from the nebula's output.
"The Crab Nebula is a cornerstone of high-energy astrophysics," said team member Mike Cherry at Louisiana State University
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The results confirm a real intensity decline of about 7 percent at energies between 15,000 to 50,000 eV over two years. They also show that the Crab has brightened and faded by as much as 3.5 percent a year since 1999.
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Careful use going forward in the concept of 'Standard Candles', eh ?
Have to remember this one when wrangling over distance measurement issues.
Cheers