A Type Ia supernova, the brightest and most energetic kind of stellar explosion – and the type that astronomers use to measure the accelerating expansion of the universe – has been spotted in a nearby galaxy, making it the closest such event in nearly 40 years.

Astronomers working with the Palomar Transient Factory (PTF), an automated sky survey based at the Mount Palomar observatory in California say their robotic telescope detected the new supernova (arrow) on 23 August in the galaxy M101 located some 21 million light years away.

“We are very excited to get one this close,” says Oxford University astronomer and PTF collaborator Mark Sullivan. “We essentially discovered it the day it blew up.”

The early detection, coupled with the supernova’s relative nearness (in cosmic terms) creates a rare opportunity for researchers trying to refine their understanding of an important class of stellar explosion. In the coming hours and days telescopes on the ground and in space, including the Hubble Space Telescope, and at the Keck Observatory on Mauna Kea, Hawaii, will be swiveling over to gather data from the rapidly brightening object.

“We know it’s the youngest Type Ia ever observed,” says Peter Nugent of Lawrence Berekely National Laboratory in Berkeley, California, who leads the Type Ia search group with PTF. “This thing just shot up out of nowhere.”

Now the race is on to accumulate as much data as possible and as quickly as possible, says Nugent. This will help clarify what goes on in the earliest stages of a Type 1a supernova, when most of the light is emanating from the outermost layers of the exploding star. “At this early epoch, the supernova can change hour to hour,” he says.

A thorough set of observations could catapult this supernova into the ranks of those few textbook cases that are used by astronomers as callibration tools for distance measurements.

Unlike other kinds of supernovae, triggered when massive stars undergo core collapse, Type Ias are produced when white dwarf stars are pushed over a theoretical mass limit. The result is a thermonuclear detonation with a well predicted peak brightness, which makes such events ideal ‘standard candles’ for measuring extragalactic distances. Observations of Type Ia supernovae in remote galaxies led to the surprising revelation in 1998 that the expansion of the universe is speeding up over time due to the presence of dark energy, a poorly understood phenomenon. Subsequent studies have helped confirm dark energy’s fundamental role in shaping the cosmos, but the picture depends on astronomers’ understanding of how Type Ia supernovae shine. Unknown factors in the physics of Type Ias may have a bearing on the way they are used to calculate precise distances.

“One of the biggest unknowns is the effect of metalicity,” says Sullivan. Metallicity is a measure of a star’s accumulation of heavier elements relative to its hydrogen. If the star goes supernova, its metalicity may influence how bright the explosion ultimately appears. Among the observations planned for the new supernova are spectroscopic studies across a broad range of wavelengths, Sullivan says, particularly in the very early stages of the explosion, “when the effects of metalicity are most profound.”

Sullivan predicts that by the time the supernova peaks in early September it will have a visual magnitude of 9-10, placing it within reach of observers using small telescopes or binoculars under clear, dark skies.

M101, better known as ‘the Pinwheel Galaxy’ because of its striking spiral form, is a popular target for astrophotographers. Its location in the northern constellation of Ursa Major (just above the handle of the Big Dipper, or the Plough) virtually guarantees that the supernova will be frequently imaged by amateur astronomers in Europe, the United States and Japan over the next few weeks and followed closely – potentially for years – as its luminosity gradually diminishes.

Although there has not been a Type Ia supernova as near as this since 1972, the modern record holder for all supernovae types remains SN1987a, which exploded in the Large Magellanic Cloud, a satellite system of the Milky Way, in February 1987 and was visible to the naked eye. A supernova has not been seen exploding within our own galaxy since 1604.

Image: Peter Nugent, Palomar Transient Factory