- 3773 - CRAB NEBULA - when light can reach us? If you are interested in astronomy you would recognize the Crab Nebula in images and videos which are easy to find on the web.. This supernova remnant, first seen on Earth in the year 1054, consists of a super-dense neutron start that spins about 30 times an second, making it a pulsar that swings a beam of radiation towards Earth, like a lighthouse.
--------------------- 3773 - CRAB NEBULA - when light can reach us?
- The Fermi Gamma-ray Space Telescope is one of many that look for high-energy radiation sources. Recently astronomers have seen some rather dramatic variations within the nebula, with Fermi and other telescopes noticing X-ray flares a hundred times brighter than seen ever before.
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- Since 2009 Fermi has detected several short-lived gamma-ray flares at energies greater than 100 million electron volts (eV), which is much higher than the flares seen before. Fermi detected a flare that grew about 30 times more energetic than the nebula’s normal gamma-ray output and about five times more powerful than previous outbursts.
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- These superflares are the most intense outbursts astronomers have seen and they are all extremely puzzling events. Astronomers think they are caused by sudden rearrangements of the magnetic field not far from the neutron star, but exactly where that’s happening remains a mystery.
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- When Fermi noted the variances in 2010 it alerted NASA’s Chandra X-ray Observatory, which began routinely monitoring the nebula to identify X-ray emissions associated with the outbursts. When Fermi scientists alerted the astronomers at Chandra about the spike a pre-planned set of observations using the observatory was initiated.
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- Unfortunately, no clear evidence was seen for correlated flares in the Chandra images, so the reason for the sudden extreme variations is still a mystery. Theorists have deduced that the flares must arise within about one-third of a light-year from the neutron star, but efforts to locate them more precisely have been unsuccessful.
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- Scientists believe the flares occur as the intense magnetic field near the pulsar undergoes sudden structural changes. Such changes can accelerate electrons to velocities near the speed of light. As these relativistic electrons interact with the magnetic field, they emit gamma rays.
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- To account for the observed emission the electrons must have energies 100 times greater than can be achieved in any particle accelerator on Earth. This makes them the highest energy electrons associated with any source within our galaxy.
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- The astronomers took an image of the Crab Nebula. The width of this image is estimated to be 5 light years. The elliptical ring near the center is thought to be a circular ring seen at a tilted angle.
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----------------------- ( 1 light year = 5.9 trillion kilometers).
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- The scale of this image is found using a millimeter ruler. When printed, the image is about 70 mm. The scale is then 5 lightyear / 70mm = 0.071 ly/mm. The radius of the ring will be the maximum radius of the elliptical ring, which you can see by drawing a circle on a piece of paper and tilting it so it looks like an ellipse. On the image, the length of the major axis of the ellipse is 10 mm, so the radius of the circle is 5 mm.
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- Using the scale of the image we get 5 mm x 0.071 ly/mm = 0.36 light years.
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- The radius in kilometers is just 0.36 ly * 5.9 trillion kilometers / lightyear = 2.1 trillion km.
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- The high-energy particles that make-up the ring were created near the neutron star at the center of the ring. If they are traveling at a speed of 95% the speed of light, to the nearest day, how many days did it take for the particles to reach the edge of the ring?
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-------------------------- (Speed of light = 300,000 km/s)
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-------------------------- Time = distance/speed, so for s = 0.95x300,000 km/s = 285,000 km/s, and d = 2.1 trillion km, we get T = 2,100,000,000,000 / 285,000 = 7,368,421 seconds.
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-------------------------- Converting to days: 7,368,421 seconds x (1 hour/3600 sec) x (1 day/24 hours) = 85.28 days.
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- To the nearest day, this is 85 days.
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- Suppose the pulsar ejected the particles and was visible to astronomers on Earth as a burst of light from the central neutron star 'dot'. If the astronomers wanted to see the high-energy particles from this ejection reach the ring and change its shape, how long would they have to wait for the ring to change after seeing the burst of light?
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- They would have to wait 85 days after seeing the burst of light because light travels faster than the matter in the particles.
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- Another way to appreciate how much faster light travels, calculate the number of days it would take for the pulse of light to reach the ring, compared to the 85 days taken by the particles. The light pulse would take 2.1 trillion km / 300,000 km/s = 7 million seconds or 81 days. So astronomers would have to wait 81 days to see whether the light pulse affects the ring, and then another 4 days for the particles to arrive.
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- You need math to do astronomy.
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December 5, 2022 CRAB NEBULA - when light can reach us? 3773
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