Stellar 'dots' pinpoint galactic distances.

Stellar 'dots' pinpoint galactic distances

Just as the sun-dappled images in a Seurat painting dissolve into tiny dots of color when viewers near the canvas, close scrutiny of electronically generated galactic images reveals the "dots" of starlight making up those images. Astronomers using state-of-the-art telescope technology to examine and then compare neighboring collections of dots, or pixels, within an image report that their point-illistic approach offers a highly accurate method for measuring a galaxy's distance from Earth.

The technique, they say, promises to generate new data on the universe's rate of expansion and other cosmic enigmas.

To improve upon standard methods of measuring galactic distance, John L. Tonry and his colleagues at the Massachusetts Institute of Technology sought to obtain more precise values for the average brightness of stars within a small patch of sky. In their galactic images, each pixel collects light from such a patch, and each patch contributes to the overall brightness. By calculating the average surface brightness of a star in a patch, and comparing these values with estimates of the star's actual brightness -- which includes far more light than ever falls on the CCD detector -- Tonry's team determined how far away from Earth a star and its galaxy lie.

To analyze galactic light as discrete points, the astronomers relied on a highly sensitive, computer-chip-like detector known as a charge-coupled device (CCD), which contains a grid of light-sensing elements. The researchers examined galaxies in the Virgo cluster, using a CCD attached to the 4-meter telescope at the National Optical astronomy Observatories near Tucson, Ariz. They knew that the total amount of light recorded within each pixel represented the contribution from about 1,000 Virgo stars -- information sufficient for a rough calculation of the average brightness of an individual star. But comparing the varying brightnesses recorded at adjacent pixels on the same CCD image yielded a more accurate average value, Tonry says.

This comparison enabled the group to determine the distances of several Virgo galaxies with an error margin of only 3 percent, or several times the precision of other methods, they report in the November ASTRONOMICAL JOURNAL.

The new technique for assessing distance may help clarify or confirm a variety of cosmological phenomena, says Alan M. Dressler of the Carnegie Institution of Washington in Pasadena, Calif. Since the 1920s, when Edwin P. Hubble discovered that a galaxy's speed increases in proportion to its distance from another galaxy, scientists have equated velocity measurements with distance, Dressler notes.

The comparative approach -- which doesn't require measurements of galactic velocity or mass -- may shed new light on the assumed uniformity of the "Hubble flow" of galaxies, as well as indicate the magnitude of deviations from the flow, he says. Some researchers attribute such deviations to a localized concentration of mass, dubbed the Great Attractor (SN: 1/27/90, p.60).

Tonry told SCIENCE NEWS that he and Dressler have begun collecting data on galaxies in the Fornax and Centaurus clusters. Centaurus, he adds, may contain part of the Great Attractor.