60 million years ago a supergiant star exploded in the galaxy NGC 4790 in Virgo. 22 million years later another overgrown and underfed supergiant star ended its life in the galaxy M95. After all that time traveling through space, the light from each explosion arrived within two days of each other in the skies over planet Earth. What a joy to see them both in their final glory.
Supernova 2012 aw has been getting most of the attention lately, because it was discovered in a bright galaxy not far from the planet Mars in Leo. Most supernovae are caught on the rise to maximum light. This one was no exception. 2012 aw was first spotted at 15th magnitude (dim!) by Paolo Fagotti and Alessandro Dimai of the Italian Supernovae Search Project, and independently by Jure Skvarc (Crni Vrh Observatory, Slovenia) on March 16. A day later it rose to 13th magnitude which put it within range of many amateur telescopes. Today March 21, the supernova still hovers at around 13th magnitude, though it’s uncertain if it will brighten further or plateau.
Studying its light with a spectrograph, an instrument that drags a fine-toothed comb though a star’s light to determine its chemical makeup, speed of rotation and the like, astronomers discovered that M95’s supernova is a Type IIP. Type II tells us this was a supergiant star that used up all the available nuclear fuel in its core. With nothing left to burn, the star’s internal “furnace” shut down, gravity took hold and the whole works collapsed in upon itself at speeds up to 45,000 miles per second.
When the outer layers reached the core, they crushed it into a dense ball of subatomic particles and sent a powerful shock wave back towards the surface that helped tear the star apart, creating a supernova. New radioactive forms of elements like nickel and cobalt were created by the tremendous pressure of the explosion; their decay into stable forms releases energy that contributes to the supernova’s light. The “P” by the way stands for plateau. Type IIP supernovae level off in brightness more slowly, plateauing for a time before fading away.
A supernova is really the only way beginning and amateur astronomers can see a star in another galaxy. Galaxies beyond the Milky Way system – which includes our nearby satellite galaxies the Magellanic Clouds – are too far away for most telescopes to resolve into stars. All those billions of stars in the thousands of galaxies visible in amateur telescopes look like pale white fuzz that remind me of cocoons. Not so when a supernova blows. We finally get to see across the light years at an individual star waving farewell.
To get an idea of how bright the supernovae in M95 and NGC 4790 are, take a look at Arcturus, the bright star in the northeastern sky found below the arc of the Big Dipper’s Handle. If you placed either SN 2012 aw or SN 2012 au at Arcturus’ distance of 34 light years, each would shine at better than magnitude -15 or a full magnitude brighter than the full moon. Image all that light squeezed into a point in the sky. Think how bright that would be and the shadows it would cast at night. What power!
I was asked today whether you can see M95’s supernova in a small, 4-inch telescope, and the answer is yes – if you observe from a dark sky in good seeing (steady, non-turbulent air) using higher magnifications (150x). The limit for a 4-inch is 13.2 magnitude. A 6-inch scope is better and it should be easy in an 8-inch. By good fortune, the star is far from the galaxy’s center where it might otherwise be camouflaged by a haze of spiral arms.
But what about SN 2012 au? Let’s not pass it by. After all, it’s even brighter than the one blazing in M95. This Type II supernova is nearly dead center in NGC 4790 in Virgo and currently shines at 12.7 magnitude or nearly a half-magnitude brighter than 2012 aw – well within the 4-inch limit. The reason it might be overlooked is that 4790 is a smaller, fainter galaxy and not as easy to find. Try using this finder chart for NGC 4790 from this earlier blog. Once you spot the galaxy, you can’t miss the supernova.