The Crab Nebula (Messier 1), located in the constellation of Taurus, is a supernova remnant (SNR), the result of a cataclysmic supernova explosion in the year 1054. This explosive death of a star was so bright that it could be seen in the daytime sky for 23 days, and was documented by astronomers throughout the Far East.
Visible: DSS (left) and Visible: TIE (right)
Let us begin by comparing two visible-light photos (above), taken with different telescopes. Note that the DSS image is a longer exposure than the TIE photo. How can you tell? Because the diffuse (fuzzy) emission from the nebula is overexposed in the DSS picture, and because the surrounding field shows a greater density of stars. In general, a longer photographic exposure will reach out to deeper space and to fainter objects; in other words, it will achieve greater sensitivity. Apart from this difference, the optical images appear similar -- as they should!
Visible: Color Credit: FORS Team, 8.2-meter VLT, ESO
This visible-light color photo illustrates the complex composition of the Crab Nebula. The gaseous filaments are the result of the cataclysmic explosion that blasted the star's outer shell into space. At the center of the nebula, a rapidly spinning neutron star is spinning at the rate of about 30 times a second! This pulsar is too faint to be seen in these images.
Near-Infrared: 2MASS (left) and Visible: TIE (right)
The near-infrared image also appears similar to the visible-light photographs. This is partly because near-infrared wavelengths are only slightly longer than the (red) visible-light region of the electromagnetic spectrum. Try selecting any star pattern near the edge of the nebulosity in the near-IR image. [There will not be a perfect correlation between relative stellar brightnesses, because some stars appear brighter in visible than in near-IR, and vice versa.] Now locate that same pattern of stars in the visible-light (TIE) image. You should find that the pattern appears farther away from the diffuse emission, suggesting that the 2MASS near-IR image reaches a slightly greater sensitivity than the TIE optical image. How can you confirm this?
Near-Infrared: 2MASS (left), Mid-Infrared: Spitzer (middle) and Far-Infrared: IRAS (right)
Now examine the mid-infrared (middle) and far-infrared images (right) of the Crab Nebula, and compare them with the near-infrared picture (left). Like most images of astronomical phenomena, the mid- and far-IR images are false-colored.
The Spitzer image is a composite of images from Spitzer's Infrared Array Camera (IRAC) and Multiband Imaging Photometer (MIPS) at 3.6 (blue), 8.0 (green), 24 (red) microns. This view of the supernova remnant obtained by the Spitzer Space Telescope shows the mid-infrared view of this complex object. The blue region traces the cloud of energetic electrons trapped within the star's magnetic field, emitting so-called "synchrotron" radiation. The yellow-red features follow the well-known filamentary structures that permeate this nebula. Though they are known to contain hot gasses, their exact nature is still a mystery that astronomers are examining. The energetic cloud of electrons are driven by a rapidly rotating neutron star, or pulsar, at its core.
In the far-infrared image, the brightest regions are depicted in red, intermediate brightness in green, and the faintest emission is in blue. We do not see much detail at the far-infrared wavelengths! You see a central concentration of infrared emission, but not much else. Why does the far-infrared emission look so different from the near and mid-infrared? There are two reasons. First, these types of infrared radiation are produced by different physical processes. Near-infrared light is similar to very long-wavelength red light, and is often generated by the same processes that produces visible light from stars. Mid- and far-infrared radiation, on the other hand, is true thermal emission (or heat). Its presence is often a signature of cosmic dust, and results from the dust particles absorbing ultraviolet and visible light, and re-radiating at lower energies (longer wavelengths) in the infrared. Second, the spatial resolution (ability to discern detail) of the IRAS far-infrared detectors was about 10 times worse than the optical and near-infrared data shown above. Hence, far less detail is seen in the IRAS maps, and individual picture elements (pixels) can be seen clearly.
Radio: NRAO (left) and Mid-Infrared: Spitzer (right)
Now turn your attention to the radio image (above left). Supernova remnants typically emit large amounts of non-thermal synchrotron radio emission, and the Crab Nebula is no exception. This type of radiation is a result of fast-moving, high-energy electrons spiraling about magnetic field lines. The general shape and orientation of the radio emission from the nebula resembles the previous images. In general, the distribution of radio emission often resembles that of mid- and far-IR light.
The X-ray image shows high-energy emission from fast-moving particles in the central region of the Crab Nebula and reveals a brilliant ring around the nebula's heart. The image shows tilted rings or waves of high-energy particles that appear to have been flung outward over the distance of a light year from the central star, and high-energy jets of particles blasting away from the neutron star in a direction perpendicular to the spiral.
Ultraviolet: ASTRO-1 UIT (left) Visible: TIE (center) and Near-Infrared: 2MASS (right)
Finally, the ultraviolet image (above left) resembles the shape and configuration of the optical and near-infrared images. Note that the notch to the east of center (at the 9 o'clock position) matches the shape and size of the near-infrared image. The point sources scattered about the field of view are foreground and background stars in our Milky Way. The most likely candidates for UV emission are the youngest, hottest and most massive stars (denoted as types O and B).