Investigating the Crab Nebula

(Adapted from and courtesy of the University of Washington)

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Summary

The student will review supernova phenomenon, predict the characteristics of the Crab Nebula, and relate observations of the Crab to the predictions.

Introduction

No celestial phenomenon is as powerful as a supernova explosion, and few objects in the sky have been studied as much as the Crab Nebula and Pulsar. When observing the Crab, we view tremendous forces and energy at work, things that could never be recreated here on Earth. The story of the Crab starts in the year A.D.1054, when the Chinese astronomers spotted a brilliant new star that blazed brighter than all other stars and even the planet Jupiter. We know today that this "guest star" was the explosion of a star much more massive than our Sun.

The Crab Nebula consists of a pulsar surrounded by a bright diffuse nebula. A pulsar is a rapidly rotating neutron star that happens to beam radiation in our direction as its magnetic axis is offset from its rotation axis. The nebula is about six light years across. It is expanding at roughly 5 million kilometers per hour. If the nebula were located at our Sun's position, it would take a little over a day for the expanding gas to reach us!

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Part 1: Background and Predictions

Tale of a Supernova

Read this part of the lab carefully. It summarizes how a massive star spends the last years of its life, and the processes involved in the supernova explosion. Try to picture the events described as you read. When you are finished with your reading, take a blank, white sheet of paper and make two sketches:

1. how you think the explosion will look immediately after it happens and
2. how it will look 1,000 years into the future. If you are so inclined, you may use more than pencil and paper: colored pencils, crayons, charcoal, paint, whatever.

Don't go onto the further parts of the lab until you have completed the first part. Your pictures will serve as your prediction as to what to look for when hunting down a supernova.

For additional background on supernovae, it is strongly recommended that you check out the site that covers numerous violent acts in the Universe, including Type I and Type II supernovae.

Physical Processes in the Crab

After you have made your sketches. Go on to read more about the Crab nebula. Do some preliminary study of the images included in this section.

The Crab Nebula at Four Wavelengths

Both the nebula and the pulsar are bright sources of radiation at all wavelengths. The radiation is produced mainly by high-energy particles accelerated by the energy of the neutron star. These energetic particles spiral around magnetic field lines in the nebula and give off radiation; this process is known as synchrotron process.

Comparing the X-ray, optical, infrared, and radio images of the Crab shows that the nebula appears most compact in X-rays and largest in the radio. This can be understood by following the history of energetic electrons produced by the neutron star. Review these images of the Crab at four wavelengths. The images have been resized to place them roughly at the same scale.

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Part II : Investigating the Images

1. Take another look at this optical image of the crab. Make a rough sketch of the image on a blank sheet of paper and label the two regions: the oval-shaped distribution of gas and the chaotic network of filaments.

   Answer these questions:

   1. Which region gives rise to a continuous spectrum?
   2. Which region gives rise to an emission spectrum?

2. Now zoom in once again to the core of the nebula: the region around the pulsar. Notice in the bottom right-hand image, the one identifying the various parts of the region, the scale for 10,000 AU. Based on this scale, it appears that the white-line "wave" moved about 6,000 AU's between February 1 and April 16, 1996.

   1. What is the approximate speed for this "wave"? (How many kilometers per second?) You need the following data to calculate the answer to this question:
      • 1 AU = 150,000,000 km
      • 1 day = 60 * 60 * 24 = 86,400 seconds
   2. At what fraction of the speed of light, 300,000 km/s, was this wave moving?

3. Return to the images of the Crab seen in four wavelengths, and answer these questions:

   1. Which type of radiation is revealing events with the highest energy?
   2. Which type of radiation represents events having the lowest energy?

4. Write a short paragraph describing the Crab Nebula at each of these wavelengths: X-ray, optical, infrared, and radio. Be sure to mention any fine substructure that you notice.

5. Write another brief paragraph comparing your "predictions" to the actual images.

   1. How close were you?
   2. Which image (which energy or wavelength) does your original sketch most resemble?

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For more information, be sure to visit the following sites:

• HST images of the Crab
• The Crab at four wavelengths

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© 1999 University of Washington
Revised: 3 February, 2000
Revised by R. Knop : 2006 Oct 19