The Death of a Massive Star Holds Key to Early Universe

Imagine if paleontologists found the fossilized remains of a long-hypothesized, but never discovered dinosaur deep in the Amazon rainforest. Well, physicists participating in the international Supernova Factory, a Department of Energy sponsored project at Lawrence Berkeley National Laboratory, have found the remains of a massive ‘fossil’ star similar to the kind that first populated the Universe.

Using a robotic telescope located at Palomar Observatory, scientists found the extraordinarily bright, extraordinarily long-lasting supernova named SN 2007bi.

When a massive star dies, it explodes producing a supernova. Not all supernovae are equal. As a result, supernovae are classified according to its energy spectra as Type I or Type II. Type I supernovae are further refined as Type Ia, Type Ib, and Type Ic based on the presence or absence of elements in the energy spectrum.

This particular supernova was found in a nearby dwarf galaxy that is only a few hundred million years old. That is pretty young when you compare it to our home galaxy the Milky Way, which is more than 13 billion years old.

“This is important,” begins astrophysicist Peter Nugent, “because young galaxies are small and contain very few metals.” For clarification, a metal in this context is any element found beyond helium, which is the second element on the periodic table.

SN 2007bi didn’t look like any standard Type 1 supernova. It was ten times brighter than the typical Type Ia supernova. “It was an odd ball so we started to study it” said Nugent. Scientists observed its fiery collapse for 555 days.

By studying the light pattern from the supernova, scientists can learn about the precursor star. “We found something that was incredibly bright. Forty to 50 times brighter than a typical Type 1c supernova” continued Nugent. “We immediately initiated a large follow up program to monitor the supernova’s light curve over the next year. We also studied historical images to see if we cruised by this part of the sky in times past.”

In this case, the data collection paid off. Their analysis indicated that the precursor star could have been a giant, weighing 100 to 200 times more than our Sun. These massive stars are believed to be similar to the first stars of the early Universe and would have been fueled by very few elements — often no more than hydrogen and helium.

Enlarge Photo

Photo credit: Peter Nugent

A false-color image of the supernova. The top image is the region of the Universe prior to the supernova and the bottom image shows the supernova as a bright spot in the center of the sky.

The explosion of the star told the scientists something interesting indeed.

"SN 2007bi was the explosion of an exceedingly massive star,” says Alex Filippenko, a professor in the Astronomy Department at UC Berkeley. “But instead of turning into a black hole like many other heavyweight stars, its core went through a nuclear runaway that blew it to shreds.” All of the energy in this nuclear reaction came from the radioactive decay of nickel.

Where did this nickel come from? Think of a star as nature’s perfect kiln. It begins with two simple elements — hydrogen and helium. These elements are fused together to produce heavier elements, like oxygen and carbon.

“In this case,” said Nugent, “we believe that SN 2007bi became so hot that it started a positive temperature feedback loop as it was burning the oxygen.” As the star became hotter and hotter, it triggered a thermonuclear reaction that burned to produce nickel and led to this unusual supernova.

How much nickel would this take to power the light seen in SN 2007bi? According to Nugent, it would require the nickel equivalent of five of our Suns to produce a supernova as bright as SN 2007bi and an additional 50 to 70 Sun equivalents of other matter (carbon, oxygen, etc.) to trap this light and release it slowly over the 555 days of the event.

Get out your calculators ladies and gentlemen – that is the equivalent of 75 of our Suns in this supernova moving at a whopping 33 million miles per hour!

The only mechanism that has been proposed that could produce such a massive star with that much nickel is called a paired instability supernova. “This type of behavior was predicted several decades ago by theorists, but never convincingly observed until now” said Filippenko.

What's next?

With the advent of new technology, the scientific team expects to soon find more ultrabright, ultramassive supernovae, revealing the role of these fiery explosions in creating the Universe as we know it today.

“This is one of those extreme events that let you go back and say ‘how will this change my understanding of how the universe evolved’” said Nugent. It is an excellent example where luck and science meet in perfect harmony to predict an event hypothesized more than 40 years ago.

The eye-opening implications of this project may offer new opportunities for scientists who study ‘low luminosity anonymous’ galaxies. According to Filippenko, “In the future, we might end up detecting the very first generation of stars, early in the history of the Universe, through explosions such as that of SN 2007bi – long before we have the capability of directly seeing the pre-explosion stars."

This project received partial funding through the Department of Energy (DOE) Office of High Energy Physics. DOE invests in science and solving critical issues impacting people’s daily lives and the nation’s future. To learn more about DOE, visit www.energy.gov.

Additional funding was provided by the National Aeronautics and Space Administration (NASA) and the Israeli Science Foundation.

This article was written by Stacy W. Kish