Scientists from N.C. State’s physics department have made discoveries about a famous supernova remnant that may help unlock secrets about the very nature of the universe.

A supernova is a dying star, a star that is at least eight times the mass of the sun.

“Stars have a wide range of masses — from that of a small fraction of the sun to maybe about 50-60 times its size,” Stephen Reynolds, professor of physics, said.

The larger the star, the shorter the life span, and a star with a mass of twice that of the sun will be eight times as bright. Thus it burns fuel faster, even though it has a lot more of it.

“[Stars] first turn hydrogen into helium and then helium into carbon, which is then converted to heavier elements in what are known as ‘fuel cycles,'” Reynolds said. “The temperature will continue to rise until it reaches a stage where fuel cycles are exhausted in one day. This continues until the entire core of the star is turned into iron.”

The weight of more than eight solar masses of iron pressing down on the core causes it to disintegrate and fall inward, crushing the entire mass into high densities. This causes the outer layers to suddenly bounce outward in an explosion. This is known as a “core-collapse supernova.” This incredibly dense core is called a “neutron star,” and if it has an active magnetic field it becomes known as a “pulsar” as it will periodically emit pulses.

“These supernovae are also classified as type-2 supernovae,” Brian Williams, a third year doctoral student working with Reynolds on related topics, said.

Less massive stars — stars with less than eight times the mass of our sun — go out with a whimper and less of a bang.

“Stars constantly blow off material in a phenomenon known as stellar winds,” Reynolds said. “Stars lose a significant amount of their material in these stellar winds, filling the space around them with this expunged material.”

Nature seems to ensure that these kinds of stars will always end up with sizes less than the solar mass — light enough so that the pressure can be supported without much energy. This type of a core is called a “white dwarf.” It is dense, but not as much as a neutron star. It is as if the whole mass of the sun was squeezed into a size similar to that of the Earth.

“These will typically cool forever and are fairly boring,” Reynolds said.

The situation gets interesting in a binary system, a system where two stars are revolving around each other, and one of them becomes a white dwarf.

According to Reynolds, if one of the stars in a binary system becomes a white dwarf then it can draw material, such as hydrogen, off the other star in the system.

“This will cause explosions in a periodic fashion — once every few weeks, months, years or sometimes even centuries,” he said.

The scale of the explosions is also awe-inspiring.

“The amount of energy released is of the order of 1044 joules — the sun won’t release as much energy in its entire lifetime,” Reynolds said.

Why study supernovae?

“Well to begin with, it is just plain fun to study things that blow up,” Reynolds said.

Also, studying supernovae can help scientists better understand our world, since all elements heavier than iron (such as gold, silver, platinum, etc.), are produced in supernova explosions.

“The silver in the quarter in your pocket or the gold in your watch are all produced in supernova explosions. So if you want to know about the past of our universe, we need to study supernovae,” Reynolds said.

There are other reasons why scientists are interested in white dwarves. Reynolds said if a white dwarf is pushed beyond 1.4 times the mass of the sun it will blow up. This number is called the “Chandrashekhar limit.” When the star reaches this size, even a single atom being added on will cause it to blow up. Thus, all white dwarves are the same size and hence serve as cosmological “beacons.” Hence, these white dwarves are used to measure the scale of the universe.

Reynolds states these ideas led to the surprising discovery a decade or so ago that the expansion of the universe is speeding up instead of slowing down. These discoveries led to the development of theories about “dark energy.”

Reynolds’ recent work involves studies related to supernova “remnants” — the remains of a supernova explosion. Apparently, the gas and dust that remains from these explosions emanate X-rays, radio waves and light, which can be seen for tens of thousands of years to come. So, even though supernovae are rare in our galaxy, remnants aren’t.

What Reynolds found

Reynolds’ team could not make sense out of two supernova explosions in a nearby galaxy. They seemed like type 1-A supernovae that ran into some dense material of their own making. But type 1-A supernovae are not supposed to behave in this manner. They speculated that these were a new type of 1-A supernovae which were more massive than usual and had a shorter life span.

Problems with studying these supernovae related to distance — they exist over 150,000 light years away. So they looked around for something in our own galaxy, and found one around 5000 light years away, known as “Kepler’s supernova remnant.”

“In 1604, people around the world saw a new object in the sky — it was the brightest object in the sky for over six months,” Williams said. “This was the last known supernova in our galaxy.”

Johannes Kepler observed this event and made detailed observations before telescopes were invented, and the supernova became known as Kepler’s supernova.

Kepler’s supernova is also important because it is the most famous and the latest “historical supernova,” according to Williams. A historical supernova is one that has been observed in documented human history, and there are supposed to be about half a dozen of these.

“The Chinese kept excellent records and detailed observations about astronomical events and objects. They documented a few supernovae as well,” Williams said.

All other historical supernovae had been classified as either type 1-A or type 2, but Kepler’s supernova kept eluding classification.

Reynolds led the team that classified the supernova.

They observed it using the Chandra orbiting X-ray observatory.

“We were granted an enormous amount of observation time — one week,” Reynolds said.

The scientists collected over 30 million X-ray images over that period of time — enough to last for years of analysis.

Initial analysis of the data received from the x-ray telescope proved that the Kepler’s supernova remnant is actually of type 1-A — in fact a new kind of type 1-A. It is a 1-A supernova that exploded into the leftover mass that the original star (known as “progenitor”) shed before it became a supernova.

Reynolds likened the amount of data collected and the kind of analysis to be done to “being in a candy store,” because there are so many investigations to be taken up. They have also submitted a proposal to the National Science Foundation to perform theoretical calculations on the physics of the star before it exploded.

Supernova at State

Reynolds said that NCSU physics department is the most qualified in the world as far as supernova remnant studies are concerned. While Harvard, Berkeley and other institutions have teams of scientists studying supernovas, no other institution studies their remnants in so much detail.

“N.C. State does not have an astronomy department,” Williams said. “All the astronomy and astrophysics related research happens in the physics department.”

Williams has also spent a lot of time studying Kepler’s supernova remnant in the infra-red spectrum. NASA’s Spitzer telescope is used for this purpose.

“While x-rays give us information about the gases in the vicinity of the supernova, infra-red spectra provide information about dust particles in the area,” Williams said.

Shock waves from the supernova heat up the surrounding gas and dust particles to millions of degrees.

The study of astronomy and supernovae is gratifying in that it provides a sense of perspective while looking out at 13 billion years and at the distances involved, Reynolds said.