Sky Lights

How to read a picture, in well under 1,000 words

By Bob Berman|Thursday, February 01, 2001



Photo by VLT/ESO

Photo by Raghvendra Sahai, John Trauger (JPL), and the WFPC2 Science Team/NASA

Photo by Solar System Visualization Project/Magellan Science Team/JPL

Photo by SOHO/EIT Consortium
With new high-tech telescopes hard at work on Mauna Kea in Hawaii and Cerro Paranal in Chile and with NASA continuing its barrage of planetary probes, space photos are becoming as common as postcards. Staggeringly dramatic, these images suffer only one fault: Most people have no idea what they're looking at. Fortunately, all it takes is a little insider knowledge to break the code.

GALAXIES These groupings of billions of stars generally have oval or spiral shapes. They betray their distance by how detailed they appear. If the galaxy shows intricate structure, as does NGC 1232 (image 1, at right), it lies within 65 million light-years of Earth, in the Local Group or the Virgo Cluster. Throngs of small, featureless galaxies that all fit within a single frame usually lie at least a quarter of a billion light-years away. Spirals have yellowish or reddish centers and bluish arms that may be blotched with pink nebulae. Any additional colors are fake.

NEBULAE When you see a cloud in space, you're seeing a nebula. All well-observed nebulae lie relatively nearby within our own galaxy. Yellow, orange, aqua, or violet probably means the image was colorized. Scientists working with the Hubble Space Telescope seem to favor baby blue or turquoise, as they did with the famous image of the Eagle nebula. But the stirring red of Hubble's Hourglass nebula (image 2), featured on a recent Pearl Jam album and on Star Trek: Voyager, is real.

PLANETS Views of the same planet can look remarkably different from telescopes and from spacecraft. Any decent shot of Mercury was snapped by the Mariner 10 spacecraft in 1974 or 1975, case closed. Ground-based telescopes show Venus as a bright blob of white, featureless clouds. Venus' distinctive D-shaped bands show up only in infrared and ultraviolet images taken by the Pioneer and Mariner probes in the 1970s. Garish, orange-tinged views of the Venusian landscape (image 3) are radar maps made in the early 1990s by the Magellan spacecraft. Mars photos that reveal craters or riverbeds were acquired by spacecraft, while vistas of the rusty Martian surface came from two Viking landers in 1976 or from Mars Pathfinder in 1997. Mars, Jupiter, Saturn, Uranus, and Neptune always appear as full or nearly full globes from Earth. A crescent shape indicates a spacecraft took the picture after it had passed the planet. Well-defined detail of Jupiter's Red Spot means the photo was taken from space, and vivid colors prove the image was enhanced. Likewise, a photo showing more than three of Saturn's rings came from a spaceborne camera, and any hue other than yellow-white is not real. All crisp images of aquamarine Uranus and blue Neptune came from a single probe, Voyager 2, during the 1980s.

THE SUN Our star appears most dynamic in ultraviolet or X rays, which can only be detected above Earth's atmosphere. The Japanese Yohkoh spacecraft produces stunning X-ray shots in which the sun usually appears deep orange with eruptions of yellow-white filaments. Ultraviolet pictures from the international SOHO probe survey the sun's disk or outer rim (image 4), while NASA's TRACE has an ultraviolet camera that zeroes in on specific regions, often rendered in bright yellow or pink and blue. All of these colors are made up to depict radiation that is not visible to the human eye.

SMALL STUFF If it looks like an Idaho potato, it's probably an asteroid. Any shot of an asteroid or a moon other than our own will be short on detail unless it came from a spacecraft. But bright comets like Comet West, with their long, fuzzy tails, are spectacular even when photographed by backyard amateurs. The art of astrophotography still includes some images that speak for themselves— no enhancement needed.









For more space pix on the Internet, see NASA's Planetary Photojournal, photojournal.jpl.nasa.gov, and the Hubble Space Telescope's Public Pictures gallery, oposite.stsci.edu/pubinfo/pictures.html.

For a look at photos of space from Earth, try the Very Large Telescope site (www.eso.org/outreach/ info-events/ ut1fl/ astroimages.html) or the collection of David Malin (www.aao.gov.au/images/index.html).
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