LBL astrophysicists have announced the discovery of three of the exploding stars known as "supernovas" that are among the most distant from Earth ever detected. Three additional supernova candidates were also found and are now being examined for further identification.
Four of the sightings took place in a single week--the most ever within such a short period of time. All were observed using a new technique developed over a five-year period to make the discovery of supernovas easier.
All the supernovas were consistent with the "Type Ia" classification. They were located in galaxies approximately 3 to 5 billion light years away, which means they exploded roughly around the time the earth was formed.
Because all Type-Ia supernovas give off about the same amount of light, they can be used to measure distances in space. With the identification of perhaps no more than 50 Type-Ia supernovas, scientists believe they can answer the question of whether the universe is infinite and will expand forever, or whether it is finite and will eventually contract.
The first four supernovas were sighted in early January at the 2.5-meter Isaac Newton Telescope in La Palma, Canary Islands, Spain. The other two were sighted in February using the National Optical Astronomy Observatory's Four Meter Telescope at Kitt Peak, Arizona.
Leading the team that made the deep-space supernova discoveries were Saul Perlmutter, Gerson Goldhaber, and Carl Pennypacker, all of whom are affiliated with LBL's Physics Division and with UC Berkeley's Center for Particle Astrophysics and UCB's Space Sciences Laboratory. They also led last year's discovery of a single deep-space Type-Ia supernova. (See Currents, Nov. 13, 1992).
Type-Ia supernovas, which can shine almost as brightly as an entire galaxy for up to a month before beginning to fade, are among the easiest of all the supernovas to spot. However, they are still a rare and random event, occurring only about twice a millennium in any galaxy.
The successful technique used to identify the 1993 supernovas is a refined and more sensitive version of the one used in 1992. An ultrasensitive electronic camera attached to a telescope is used to photograph thousands of deep-space galaxies. These images are then compared to images of the same galaxies taken at the same telescope from as recently as two weeks earlier to as long as a year earlier. Old image light is then subtracted from new image light to reveal the appearance of supernovas their light curves are rising.
"By examining tens of thousands of galaxies in a few days, we can schedule the discovery of a batch of supernovas on demand," Perlmutter says. "For example, the best time to discover faint objects is at the new moon."
For measuring distances, it is critical that a Type-Ia supernova be discovered just before its brightest moments. A type Ia occurs when a white dwarf, an aging star about the size of the earth but with the same mass as the sun, accretes too much matter from a companion star and implodes under the gravitational pressure.
Describing the process, Pennypacker says, "The implosion smashes matter together and creates a giant nuclear conflagration that rips the star to shreds."
The visible light created in a Type Ia supernova's implosion-explosion is consistent enough to be used as a standard of reference. Given this point of reference--dubbed a "standard candle"--scientists can measure the light from a Type-Ia and compare its observed brightness to its known brightness. The resulting ratio yields a precise measurement of its distance from Earth.
"Distance has proved to be one of the most difficult things to measure in the universe," says Goldhaber. "When a bright object is observed in the sky, it is hard to tell whether it is intrinsically bright or simply closer to us than a dimmer object."
Analyzing the spectrum of a Type-Ia supernova and comparing it to the spectrum of its parent galaxy will determine the velocity at which the galaxy is receding from Earth, which in turn reveals the rate at which the universe was expanding at the time the supernova explosion occurred.
Other LBL-UCB members of the team behind the latest discoveries (many of whom were members of the earlier team) were Silvia Gabi, Ariel Goobar (from the University of Stockholm), Alex Kim, Matthew Kim, Reynald Pain (from CNRS of Paris), and Ivan Small.
Hughes Pack, a physics and astronomy teacher at Northfield-Mount Hermon High School in Northfield, Mass., also participated in the observations as part of LBL's nationwide educational program called "Hands-On Universe."