Team discovers 11 new supernovas --
One is most distant ever observed

December 15, 1995

By Lynn Yarris, [email protected]

An international scientific team led by researchers at Berkeley Lab have announced the discovery of 11 of the exploding stars known as supernovas, including one that is the most distant star ever observed. Ten of the supernovas were discovered within a 48-hour period (Nov. 19-20), an unprecedented achievement in the history of astronomy, and a validation of a technique that was designed to make deep space supernova discoveries possible.

The new supernovas were found in galaxies ranging from four to seven billion light years away. (A light year is the distance light can travel in a single year--approximately 6 trillion miles.) This means that the farthest of these explosions took place billions of years before our own solar system was formed; the light is only now reaching Earth.

All of the new supernovas (including one discovered on Oct. 29) were sighted on the telescope at the Cerro Tololo Inter-American Observatory (CTIO) in La Serena, Chile. This optical telescope, with a reflecting mirror that measures four meters across, is one of the largest in the southern hemisphere. The discoveries were made as part of the "Supernova Cosmology Project," which is headed by astrophysicist Saul Perlmutter of the Physics Division, and jointly sponsored by Berkeley Lab and the Center for Particle Astrophysics, located at UC Berkeley. The project is also affiliated with the Lab's new Institute for Nuclear and Particle Astrophysics.

It is known that the universe is still in a state of expansion as a result of the Big Bang that started it. The goal of the Supernova Cosmology Project is to discover enough "Type Ia" supernovas to measure the universe's "deceleration"--the rate at which the expansion is slowing down. If the deceleration is sufficiently large, the universe will eventually stop expanding and reverse course, a scenario sometimes referred to as the "Big Crunch." If the deceleration is sufficiently small, the expansion of the universe will continue forever.

Type Ia supernovas are the nuclear conflagrations that are believed to occur when a white dwarf, an aging star about the size of the earth but with about the same mass as the sun, accretes too much matter from a companion star and implodes under the gravitational pressure.

"Type Ia supernovas can serve as a measurement of distance because they can be calibrated as a standard candle with a known brightness," says Perlmutter. "By measuring the light from a distant Type Ia supernova and comparing its observed brightness to its known brightness from "close-up" measurements, we can precisely calculate the supernova's distance from Earth."

Scientists can then analyze the spectrum of the supernova and compare it to the spectrum of its parent galaxy to determine the velocity at which the galaxy is receding from earth. This reveals how fast the universe was expanding at the time the supernova explosion occurred.

"To the extent that distant measurements of expansion rate differ from nearby measurements (the Hubble Constant), we can thus determine the rate of deceleration," Perlmutter says.

Perlmutter and his colleagues on the Supernova Cosmology Project believe that about 50 type Ia supernovas should be enough to determine the universe's rate of deceleration. With the addition of their latest discoveries--10 of which are believed to be Type Ia-- they now have a total of 18.

The technique used to discover the 11 new supernovas of 1995 is the same as that used by the supernova cosmology project team to discover seven supernovas in 1994 and one in 1993. An ultrasensitive electronic camera attached to a telescope is used to photograph thousands of deep-space galaxies at the time of a new moon. A second set of images of the same galaxies is taken at the same telescope at the beginning of the next new moon. Using a computer, the two sets of images are compared and light from the older image is subtracted from new image light to reveal the appearance of supernovas.

"A Type Ia supernova can shine almost as brightly as an entire galaxy for about a month before it becomes too faint for even the largest telescopes to observe," says Gerson Goldhaber, a physicist in the Physics Division and one of the key members of the project. "For the purpose of measuring distances, it is critical that a Type Ia supernova be discovered just before its brightest moments. By recording our images at the start of a new moon (the best time to discover faint objects), we can follow our supernovas while their light curves are rising to determine their peak output of light."

Light measurements of the new supernovas were collected at both the CTIO and at the WIYN telescope on Kitt Peak. To collect data for spectral analyses, the Supernova Cosmology Project team is now working with UCB astronomers Alex Filippenko and Aaron Barth at the Keck Ten Meter Telescope in Hawaii. The Keck, which is the world's largest optical telescope, was designed at Berkeley Lab specifically for the observation of faint distant objects such as these supernovas.

Perlmutter and his colleagues have put in a request for images from the Hubble Space Telescope that will enable them to determine the nature of the galaxies that hosted their supernovas (i.e., spiral, elliptical, etc.).

In addition to Perlmutter and Goldhaber, other Berkeley Lab and Center for Particle Astrophysics members of the supernova discovery team include Susana Duestua, Ariel Goobar (now at Stockholm), Donald Groom, Isobel Hook, Alex Kim, Matthew Kim, Julia Lee, Jason Melbourne, Reynald Pain (now in Paris), Carl Pennypacker, and Ivan Small.