As you read these words, a former star is blasting its remains into space, spreading the chemical elements forged during its life into the interstellar brew of atoms….
…well, not exactly as you’re reading these words. In fact, the stellar death in question took place 21 million years ago, but so far away that its light is only now reaching Earth.
You can see it yourself. The supernova is located in the galaxy M-101 in Ursa Major, the Big Bear–which most of us identify as the Big Dipper. It can be found above the Dipper, forming a squat triangle with the two end stars of the handle. You might be able to spot the supernova with a good pair of binoculars, or a small telescope.
Better still, you can come to Chabot Space & Science Center and, weather permitting, gaze at it through one of our large telescopes with the assistance of one of our staff astronomers or astro-savvy volunteers. Our observatory deck is normally open every fair-weather Friday and Saturday night from 7:30 to 10:30.
The supernova, named SN2011fe, was first sighted on August 24th in one of M-101’s spiral arms by a team from Oxford University and the Palomar Transient Factory (PFT). And while it’s the 136th supernova spotted this year, SN2011fe is relatively close to us: 21 million light years, as compared to most supernovae, which are much farther away. In addition to being nearby, what is exciting astronomers is that this is a “Type 1A” supernova.
That statement begs the question, are there different kinds of supernovae, and if so, what makes a Type 1A special?
Yes, there are different kinds, and I’d say that they’re all special. But 1As have one quality that makes them useful for things other than spectating a celestial fireworks show. A typical “core collapse” type supernova, of which Type 1A is not, occurs when a really massive star (significantly more massive than our Sun) burns through all of its nuclear fuel and collapses. The sudden collapse produces incredible pressure and temperature at the star’s core, enabling it to fuse the heavier atoms it produced from lighter ones over its lifetime. It goes off like a star-sized nuclear bomb, its outer layers blown away into space and its compressed core collapsing even further into a neutron star or a black hole, depending on its mass.
The amount of light released by this star death scenario depends on the mass of the original star and can vary widely, like a set of light bulbs with a range of wattages.
But a Type 1A supernova is produced by a different star death scenario. Picture a less massive star than one of those incredible hulk supernova producers, a star with about the mass of our own Sun, in fact, give or take. When this star runs out of nuclear fuel, it also collapses, but not violently. The star’s core collapses into an object called a White Dwarf, a compact and hot “cinder” roughly the size of the Earth. At the same time, the star’s outer shell sloughs off into space, expanding gracefully as a planetary nebula.
But there is a limit to the size that a White Dwarf can be. Below 1.38 times the mass of our Sun, a White Dwarf will remain a hot, though gradually cooling ball of material. If the situation arises where a White Dwarf below 1.38 solar masses acquires more mass—say, by pulling in material from a neighboring star, a stellar companion—and exceeds the magic limit, the White Dwarf suddenly collapses and, much as in the case of a “typical” supernova, becomes a stellar fusion bomb.
The trick is that this stellar fusion bomb is powered by the very precise physics of 1.38 solar masses suddenly collapsing from a White Dwarf, the result of which is that all Type 1A supernovae are equivalent in power and brightness. You’ve seen one 1A, you’ve seen them all….
Since all 1As have the same brightness, like a bunch of light bulbs of the same wattage, they’re excellent for measuring distances in the Universe. If we know the actual brightness of a Type 1A supernova, we can compare that to how bright a particular one appears to us, do a little math, and calculate with good precision how far away it, and by extension its home galaxy, is.
Type 1A supernovae are what made possible the recent, and unexpected, discovery that not only is the Universe expanding (as we knew), the rate of expansion is actually accelerating (something scientists did NOT expect).
And with SN 2011fe going off relatively close to us, in M-101, scientists have a chance to refine their understanding of the nature of the Type 1A supernova, increasing the power of the measuring stick….
That’s an eyeful. Come to Chabot and grab a peek this weekend (Sept 9 and 10), before it fades away….