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Multiwavelength Astronomy

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. To do that, we look at a single source, the Crab Nebula, in several different wavelengths to illustrate how its appearance changes from one wavelength to another.

Photograph of the pictograph in Choco Canyon, New Mexico, believed to be a depiction of the supernova of 1054 AD.

A pictograph found in Chaco Canyon, New Mexico, believed to be a depiction of the supernova of 1054 AD. (Credit: Flickr user John Fiveash (jsfiveash))

In July of 1054 A.D., Chinese astronomers and members of the Ancient Pueblo peoples – ancient Native Americans living in present-day Southwest region of the United States – 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. In the 19th century, French comet hunter Charles Messier recorded a fuzzy ball of light in the same location as the 1054 supernova. The fuzzy ball looked like a comet to Messier, but it did not move across the sky. Messier recorded the nebula, called "the Crab" for its supposedly crab-like appearance. This was the first object in his catalog of non-comet objects, and so 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, after the constellation it appears with in the night sky.

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. The collapse released a tremendous amount of energy, blasting the star's surface layers 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 Observations

Radio image of the Crab Nebula

Radio image of the Crab Nebula. (Credit: NRAO/AUI)

This radio image of the Crab Nebula was taken by the Very Large Array and shows what the Crab nebula looks like in radio light. There are two distinctive features that can be seen in this image. The bright, white dot toward the center of the image is the pulsar at the heart of the nebula. This is the core of the star that has collapsed into a neutron star. The neutron star generates pulses at radio frequencies about 60 times a second. In this image, the pulsar's flashes are blurred together, since the image was exposed for much longer than a single 1/60 second pulse. Surrounding the pulsar, the green shows the radio emission from unbound electrons spiraling around inside the nebula.

Infrared Observations

Infrared image of the Crab Nebula

Infrared image of the Crab Nebula. (Credit: NASA/JPL-Caltech/R. Gehrz)

The infrared image of the Crab Nebula, taken by the Spitzer Space Telescope, shows two types of features. The blue-white region is emitted by a cloud of energetic electrons that are trapped in the pulsar's magnetic field. The cloud is driven by the rapidly-rotating pulsar. The red features follow the nebula's filaments that permeate the nebula (explained further in the "Optical Observations" section below).

Optical Observations

Optical image of the Crab Nebula

Optical image of the Crab Nebula. (Credit: NASA/ESA/ASU/J. Hester)

Like the infrared image, the Crab nebula in the visible light shows a web of reddish filaments at the outer edges of the nebula and emission around the bluish core of the nebula.

The blue core of the nebula comes from electrons within the nebula that are 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 Observations

Ultraviolet image of the Crab Nebula

Ultraviolet image of the Crab Nebula. (Credit: NASA)

The Crab Nebula in the ultraviolet (or UV) shows a nebula that is slightly larger than what is seen in X-rays (see below). Because of this, the cooler electrons (responsible for the UV emission) extend out beyond the hot electrons that are responsible for the X-ray emission, which supports the idea that the central pulsar is responsible for energizing the electrons.

X-ray Observations

X-ray image of the Crab Nebula

X-ray image of the Crab Nebula. (Credit: NASA/CXC/SAO/F. Seward et al.)

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 than other wavelengths 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: September 2013


A service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA/GSFC

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