New Supernova

Discovery of the Week
The Hot New Supernova



Stellar scientists announced last week that they've spotted the youngest supernova remnant ever seen in the Milky Way. Called "G1.9+0.3," the remnant's estimated age is just 140 years--meaning that's when the radio waves started reaching Earth. (The actual supernova happened 26,000 years ago.)

How young is that, in supernova remnant terms? Until last week, the youngest remnant ever seen in our galaxy was a 330-year-old named "Cassiopeia A." So, why hadn't scientists spotted "G1.9+0.3" before, if its radio waves first reached us in the 1860s? Because to see it, they had to peer through a thick cloak of interstellar dust.

While astronomers pore over their super new data, we're here to help you understand what supernovas really are. It's all about the hot life and violent death of a big star.



A Star Is Born

Like humans, stars are born through contractions--though the contractions here are not of muscle, but of massive clouds of gas and dust in interstellar space. Every now and then, such a cloud accumulates enough matter for gravitational forces to pull it together even more. A protostar is born, and gets hot. When the temperature near its center hits 18 million degrees Fahrenheit (10 million degrees Celsius), nuclear reactions kick in.

Newborn stars are made mostly of hydrogen. At their cores, they "burn" hydrogen and generate helium. Of course, they don't use matches or flames. The burning at a young star's heart is a nuclear fusion reaction, in which four hydrogen atoms fuse to produce a single helium atom. The mass of that helium atom is less than the combined mass of the four hydrogen atoms, and the leftover mass is released as energy.

The release of that energy drives the temperature inside the star way up--in some cases to hundreds of millions (even billions) of degrees Fahrenheit. Pressure inside the star increases enough to counteract the gravitational forces still trying to contract it. At the same time, heat pours from the star's core toward its cooler surface, and from there radiates into space. Presto: a relatively stable star is burning bright.

Twinkle, Twinkle, Supergiant Star

Stars survive a long time by human measures, but eventually they all run out of gas, literally. And those that live larger burn out quicker. A relatively small star--like our sun--might burn for 10 billion years, and then linger for eons as a cosmic cinder called a "white dwarf." A star 10 times as massive might live just 10 million years, and then go out with a bang.

When a star's core runs out of hydrogen to burn, it begins to contract again. The core's temperature increases until the helium made earlier ignites. Now a helium fusion reaction produces carbon and oxygen in the star's core, while hydrogen fusion fires up in a thin shell around it. The star generates far more energy than before, and puffs up accordingly. If the star started out modest, it grows into a red giant. If it started out big, it becomes a supergiant.

Smaller stars' nuclear careers generally end with the burning of core helium. But big stars start burning the carbon and oxygen fused in the helium reaction, too. They go on to produce elements like neon, magnesium, silicon, and sulfur. Then they burn the silicon to produce iron. Such stars wind up layered like onions--with a central core of iron, surrounded by layers of burning silicon, magnesium, neon, oxygen, carbon, helium, and hydrogen.

Out with a Bang

After taking several million years to grow up, a supergiant builds its iron core in about a day. At its peak, the iron core is around two-thirds the size of the Earth but contains more mass than the sun. It's also caught in an enormous gravitational crunch. The star's core no longer generates energy to counteract the forces of contraction--to fuse iron requires energy input rather than leading to energy release--so it can't hold out for long.

When it goes, it goes fast. In less than a second, the core collapses from a 5,000-mile-wide sphere (8,000 km) into a 12-mile-wide one (20 km). The sudden crash releases a huge amount of energy--100 times the energy our sun will produce in its entire 10-billion-year life. Tiny particles called neutrinos carry most of that energy off into space. The rest races out through the star's layers in a supercharged shockwave.

The resulting explosion blasts the star's gaseous shell into space at speeds exceeding 10 million miles per hour (16 million km/h). For a few weeks, this "supernova" burns brighter than a billion suns. And for millennia to come, the former star's gaseous shell plows into the interstellar medium. Meanwhile, the star's collapsed iron core carries on as a neutron star--or, in some cases, becomes a black hole. In the cosmos, it seems, big stars burn out and fade away.

--Steve Sampson