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Welcome to the World of Multiwavelength Astronomy!

Until the 20th century, astronomers learned virtually all they knew about sources in the sky from only the tiny fraction of electromagnetic radiation that is visible to the eye. However, as astronomers have discovered how to collect radiation outside this part of the spectrum, they have been able to learn much more about the universe. Many objects reveal different aspects of their composition and behavior at different wavelengths. Other objects are completely invisible at one wavelength, yet are clearly visible at another.

This section explains a little about what is revealed by observing at each wavelength, and includes an example image of the Crab Nebula to illustrate how one source varies its appearance from one wavelength to another.

The Crab Nebula

In July of 1054 A.D., Chinese astronomers and members of the Anasazi tribe — native Americans living in present-day Arizona — recorded the appearance of a new star. Although it was visible for only a few months, it was bright enough to be seen even during the day.

At the same location in the sky, near the constellation Taurus, the 19th Century French comet hunter Charles Messier recorded a fuzzy ball of light that looked similar to a comet, but did not move across the sky. Messier recorded the nebula, called the Crab for its supposedly crab-like appearance, as the first object in his catalog. For this reason, it is sometimes called M1. It was also one of the first sources of X-rays identified in the early 1960s, when the first X-ray astronomy observations were made, and at that time it acquired another name, Tau X-1.

Scientists now know that the Crab Nebula is the remains of a star that suffered a supernova explosion. The core of the star collapsed and formed a neutron star, which released a tremendous amount of energy, sufficient to blast the surface layers of the star into space. The expelled gases have formed the nebula, which is still expanding. When the central star collapsed, its magnetic fields and rotation collapsed with it, so the neutron star is now a rapidly rotating object with an intense magnetic field near its surface. The strobe effect of the rotating star generates pulses observed at radio, optical, and X-ray wavelengths. Thus we see flashes from the neutron star each time one of the magnetic poles is pointed toward Earth. Such a neutron star is called a pulsar.

Radio Astronomy

Crab nebula at radio frequencies

What the Crab Nebula looks like in the radio can be seen in this image made at the National Radio Astronomy Observatory (© 1992 ( This image shows two distinctive physical features. First, the colored regions (in this false-colored image, blue is less intense, green is a little more intense, yellow more intense still, and red the most intense) represent the radio emission that comes from unbound electrons spiraling around inside the nebula. Second, the pulsar at the heart of the Crab Nebula generates pulses at radio frequencies roughly 60 times a second. In this image, the pulsar's flashes are blurred together (since the image was "exposed" for much longer than 1/60 s) and it appears as the bright white spot near the middle of the nebula.

Optical Astronomy

Crab nebula in visible light

The Crab Nebula in the visible spectrum (photograph courtesy of the Anglo-Australian Observatory) shows two distinct features: a reddish web of filaments at the outer edges of the nebula and a bluish core.

The blue core of the nebula is from electrons within the nebula being deflected and accelerated by the magnetic field of the central neutron star. The radiation appears blue because this process emits more light in the shorter (bluer) wavelength portion of the visible spectrum than in the longer (redder) wavelength portion.

The filaments surrounding the edges of the nebula are what is left of the original outer layers of the star. The red color comes from emission of hydrogen. Blown off the star by the supernova, the filaments are still expanding outward into space, away from the central star. Scientists can measure this expansion by comparing pictures taken several years apart and tracing the motion of these filaments. Extrapolating backward in time shows that the filaments first started expanding away from the center around 1040-1070 A.D. This agrees well with the 1054 A.D. supernova explosion.

Ultraviolet Astronomy

Crab Nebula in the far ultraviolet

The Crab Nebula in the ultraviolet (or UV) shows a nebula that is slightly larger than what is seen in X-rays (photograph from the Ultraviolet Imaging Telescope). This reveals that cooler electrons (responsible for the UV emission) extend out beyond the hot electrons near the central pulsar. This supports the theory that the central pulsar is responsible for energizing the electrons.

X-ray Astronomy

Crab Nebula in X-rays

The Crab Nebula in X-rays reveals a condensed core near the central neutron star. The central star is seen to pulse in X-rays, just like it does at radio and optical wavelengths.

The Crab Nebula appears smaller and more condensed in X-rays because the electrons that are primarily responsible for the X-ray emission exist only near the central pulsar. Scientists believe the strong magnetic field near the surface of the neutron star "heats up" the electrons in it. These "hot" electrons are responsible for the X-ray emission.

Last Modified: November 2010

Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.

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All material on this site has been created and updated between 1997-2012.

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