Hubble Heritage: How One Project Changed the Look of Space Images

Before 1998, most Hubble pictures reached the public as illustrations for a news story. A research team announced a result, and an image made from its data went out with the press release, framed and colored to support that finding. Whatever happened to sit in the data archive without a story attached stayed there. The Hubble Heritage Project, set up that year by astronomers working at the Space Telescope Science Institute in Baltimore, turned that relationship around: the picture itself became the reason for the release.

The project started a few years after the telescope's launch in 1990 and after the first servicing mission of 1993 had corrected its flawed optics. By then the archive held a large stock of sharp exposures. The Heritage group set out to show the visual qualities of those data while keeping their scientific content intact, and over almost two decades it built the gallery of Hubble pictures that is now reproduced on posters, book covers and postage stamps. A full index of the objects in the series is on the Hubble Heritage archive page.

A picture as the starting point

The difference sounds small but it changed the result. A press image tied to a research paper usually showed the part of an object that the paper was about, in whatever filters the scientists had needed. A Heritage image began with a different question: which object in the archive, rendered with care, would look most striking, and what would a viewer learn from looking at it?

That question often exposed gaps. A favorite object might have been observed in only one filter, or the telescope's field might have covered only a corner of it. For those cases the project could use a small amount of Hubble observing time of its own. Sometimes the new exposures were added to existing archival data so a better picture could be made. Sometimes the team observed a target nobody had proposed for research. In both cases the condition was the same: the new data had to be scientifically useful as well as photogenic, and they went into the public archive for any astronomer to analyze.

Four images, four kinds of subject

The first Heritage release, published on October 21, 1998, was chosen to show the range of what Hubble observes, from a planet in the solar system to a galaxy millions of light-years away.

The Saturn picture at the top of this page was processed to keep the planet's subtle natural colors, the pale yellows, browns and grays of its cloud bands. Saturn's upper clouds are ammonia ice, which has no color of its own. The tints come from a haze above them, formed when sunlight's ultraviolet light breaks down methane gas. The moon Enceladus casts a small shadow onto the disk just above the rings. The rings, mostly chunks of water ice, look like a few broad bands but consist of tens of thousands of narrow ringlets. Two gaps show clearly: the wide Cassini division near the middle and the narrow Encke Gap close to the outer edge.

The second picture looked through a rare clear patch toward the center of the Milky Way. Dust hides most of that direction, but the Sagittarius Star Cloud is a narrow, relatively dust-free window, and Hubble recorded a dense field of stars there, some among the oldest in the galaxy. The colors of the stars were the point. Blue stars are hot and young and burn through their fuel quickly. Red ones are either small, cool stars with long lives or red giants that have swollen near the end of theirs.

The third was the Bubble Nebula, NGC 7635, an expanding shell of gas about seven light-years across blown by the winds of a single massive star, 10 to 20 times the mass of the Sun. The picture was assembled from data taken with Hubble's first camera, the Wide Field/Planetary Camera, which astronauts replaced with WFPC2 in 1993. Even exposures from before the optics were fixed could yield a release-quality image. The nebula was photographed again for Hubble's 26th year in orbit, covered in the Bubble Nebula article.

The fourth showed the spiral galaxy NGC 7742 face-on. Its large yellow core is powered by a central black hole, which places it among the active galaxies known as Seyfert 2 galaxies. A lumpy ring of star formation circles the core about 3,000 light-years out, with faint, tightly wound spiral arms beyond.

What the project changed

Three habits that are now standard in astronomy imaging trace back to the Heritage approach.

The first was treating color as a decision that has to be explained. The team mapped filters to colors by wavelength order where it could and published the assignments alongside each picture, so anyone could see which hue stood for which filter. The reasoning behind those choices is covered in Are the Colors Real?.

The second was the idea that old data deserve a second look. A large share of the Heritage gallery came from exposures already sitting in the archive, made years earlier for other purposes. Re-processing archival data for public release is now routine,, and ESA/Hubble's weekly Picture of the Week series is built on the same principle.

The third was collaboration between scientists and image specialists on equal terms. Decisions about framing, contrast and color were made by the group, with astronomers making sure nothing appeared in the picture that was not in the data, and processors making sure the data were shown as clearly as possible.

Questions readers ask

Who started the Hubble Heritage Project?

A group of astronomers and image specialists at the Space Telescope Science Institute in Baltimore founded it in 1998, with Keith Noll as principal investigator. The institute runs Hubble's science operations for NASA, which gave the team direct access to the data archive and, for selected targets, a small share of the telescope's observing time.

What were the first Hubble Heritage images?

The first release, on October 21, 1998, contained four pictures: Saturn in natural color, a dense star field in the Sagittarius Star Cloud toward the center of the Milky Way, the Bubble Nebula NGC 7635, and the face-on spiral galaxy NGC 7742. Together they covered a planet, stars, a nebula and a galaxy.

Did the Hubble Heritage Project take its own observations?

Yes, in limited numbers. Most images came from archival exposures, but the project had a small allocation of observing time. It used it either to add missing filters or coverage to archival data, or to image new targets. Its first new observation was the Ring Nebula in October 1998.