Astronomy Picture of the Day
Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.
2002 August 24 
Posted on 08/23/2002 9:40:28 PM PDT by petuniasevan
Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.
2002 August 24
Explanation: The complex shell of a star seen to explode 300 years ago is helping astronomers to understand how that star exploded. This Chandra Observatory image of supernova remnant Cassiopeia A (Cas A) shows unprecedented detail in three x-ray colors. The relationship between brightness, color, and position of material in the image indicates where in the star this material was just before the explosion. Bright knots on the left, for example, contain little iron, and so are hypothesized to originated from a higher layer than outer red filaments, which are iron rich. The blue region on the right is seen through absorbing dust, and so appears depleted of low-energy x-rays. It takes light ten years to cross the gas shell of the Cas A supernova remnant, which is 10,000 light-years distant. Most of the elements that make people and planets were produced in supernova explosions.
This supernova should have been visible on Earth around 300-350 years ago. So why was it missed?
Too much intervening dust? Misidentification as an ordinary star? Much dimmer than our understanding
of physics would indicate?
Get on the 
list!
We've decided it was too far away. It would have been just a new star for a while, not particularly overwhemingly bright, and maybe not in the night sky at the time.
Casseiopeia is a very northern constellation, and is visible at my latitude (45 north) every (clear) night of the year.
In only 3 years Chandra has sent more valuable data Earthside than all other x-ray platforms before it combined.
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