The "Ring Nebula" in the constellation of Lyra is perhaps the most famous example of a planetary nebula -- gaseous shells produced by mass ejection from low-mass stars near the end of their lives.
Unlike many of the other galleries in this Messier Museum, the images shown here have small fields of view. That is, the photographs are the result of "zooming in" on the relatively small nebula, and you could fit ten of these images within the diameter of the full moon. The actual distance across the nebula is about 1 light-year, or nearly 10 trillion kilometers.
Visible: HST (left), Visible: TIE (center) and Visible: DSS (right)
Begin your examination with the colored image (left) taken with the Hubble Space Telescope (HST). By clicking on the image at the top of the page, you should be able to see a faint central star. It is the outer layers of gas have that been ejected from this star that produces the spectacular ring. This ring is essentially a geometric result of our vantage point. The expulsion of the star's outer layers produces a spherical shell of ionized gas. We have an oblique view of the outer shell, and our line-of-sight passes through more of the luminous gas, producing a bright outer ring.
The color image was assembled from three black-and-white photos taken through different color filters. Blue reveals the emission from very hot helium, which is located primarily close to the hot central star. Green represents ionized oxygen, which is located farther from the star. Red shows ionized nitrogen, which is radiated from the coolest gas, located farthest from the star. The rich palette of colors seen in most planetary nebulae (plural of nebula) are due to a mixture of strong spectral emission lines. Careful study of these spectral lines permits astronomers to determine the chemical composition of nebulae. You should also be able to see filaments of dust in the outer layers of the nebula.
The short-exposure TIE image (center) was taken with a far smaller telescope than HST, and one that is ground-based and therefore looking through the Earth's atmosphere. The spatial resolution is hence poorer than the HST image. Like the nebula itself, the surrounding stars are within our Milky Way Galaxy. The long-exposure DSS image (right) is clearly over-exposed in the central regions. It reveals filamentary structure in the outer shells of gas, not otherwise seen in short-exposure photographs. How else can you determine that the DSS image is a longer exposure, and therefore more sensitive, than the TIE photo? Answer here.
Near-Infrared: 2MASS (left) and Mid-Infrared: Spitzer (right)
The near-infrared image (above left) shows the same general characteristics as the visible-light pictures above. On the other hand, the mid-infrared photograph of M57 is dramatically different. The Spitzer image is a composite of 3.6 (blue), 4.5 (green), 5.8 (orange), and 8.0 (red) microns. It details additional structure not seen in the other images. The telescope's infrared array camera detected this material expelled from the withering star. Previous images of the Ring Nebula taken by visible-light telescopes usually showed just the inner glowing loop of gas around the star. The outer regions are especially prominent in this new image because Spitzer sees the infrared light from hydrogen molecules. The molecules emit infrared light because they have absorbed ultraviolet radiation from the star or have been heated by the wind from the star.
The obvious "pixelation" of the radio image immediately suggests that it has lower spatial resolution than even the IRAS far-infrared photograph. By counting the equivalent of 12 pixels across the image, one concludes that the effective angular resolution ("seeing") is no better than (3 arcmin/12) = (180 arcsec/12) = 15 arcseconds. Ground-based visible-light telescopes often achieve 1 arcsec seeing, limited primarily by fluctuations within the atmosphere. In truth, the VLA telescope used to make the radio image is capable of far better resolution. However, the NVSS survey was intended to cover the entire sky visible from New Mexico. Astronomers often degrade the resolution of their telescope intentionally (that is, "zoom out" in a photographic sense) in order to cover wide fields of view and hasten large-area surveys of the sky.