There is a reason why most of the major observatories in the U.S. are west of the Mississippi River. The clouds and rain we have had in Virginia for the last several weeks have reduced our view of the heavens to about 10,000 feet above us. I know my friends and loved ones in Texas would love to swap climates with us for just a little while, but it has been frustrating for those of us who would like to see a little farther than the clouds.
The major phenomenon we are missing is one of the nearest and brightest supernovae in decades. But before we get going on this particular supernova, let’s provide a little background on the subject.
What is a supernova?
A supernova is a stellar explosion that results in a burst of energy being released, appearing to be a “new star” in the heavens before it fades over weeks or months. Before it does so, it can outshine an entire galaxy of hundreds of billions of stars.
What are the different types of supernovae?
There are basically two. Type II (type two) supernovae result from the deaths of massive stars, ones that are ten, twenty, or fifty times as massive as our sun. They do not go quietly at the end of their lives. There comes a point in their cores when they can no longer sustain themselves against the crushing gravitational force of their huge masses. A complex series of events ensues, not all of which are fully understood, the end result of which is a massive explosion visible across billions of light years of intervening space.
Type Ia (type one a) supernovae are different. These can only occur in binary star systems, ones where two stars orbit each other in close proximity. This allows one star to “steal” material from the other, as its gravity pulls matter from its companion.
The star that eventually becomes a Type Ia supernova is a white dwarf. This is the eventual fate of our sun and of all low-mass stars. Our sun will not become a supernova because it lacks a stellar companion. But a star in a close binary system, instead of quietly fading away, can acquire matter from its partner, steadily increasing its mass. A white dwarf sustains itself against further collapse by something called electron degeneracy. Not a comment on the morals of subatomic particles, this term refers to the ultimate crowding together of atomic matter, pushing atoms so close to each other that the electrons making up their outer regions start pushing back.
But if there is enough matter, and the consequent gravitational force is strong enough, even this is not enough to prevent further collapse. One solar mass is not enough—our sun will simply cool and fade over hundreds of millions of years. But the progenitor of a Type Ia supernova? It is gradually adding more and more mass until it reaches a critical limit at 1.4 solar masses: the Chandrasekhar limit. At this point the star undergoes further collapse, and a runaway nuclear reaction essentially consumes the entire white dwarf star. These events are among the most energetic in the universe.
What’s so special about Type Ia supernovae?
Remember that the explosion occurs when a particular mass limit is reached, so the runaway nuclear reactions consumes the same amount of matter for each Type Ia explosion. The explosions have consistent characteristics, and can be used as “standard candles” useful in determining cosmic distances. If a bright object’s actual luminosity (energy output) is known, then a comparison of its brightness (its appearance) with its luminosity (its actual energy output) can yield its distance. A candle held next to your eye appears much brighter than any star. We recognize that the difference in appearances is the result of a vast difference in distance.
How often do supernovae occur?
In our own Milky Way galaxy, the estimated rate is once every 50 years, although the last one actually observed was in 1604, five years before the invention of the telescope. The cloud of hot expanding gas that is its remnant is shown below.
SN 1604A
As our telescopes became able to see farther and farther into space, we found more and more supernovae in distant galaxies. This is illustrated nicely by the naming system for supernovae. The first supernova of any given year is designated as (for example) SN (for supernova) 2011A, the next is SN 2011B, and so on. Once the alphabet is run through, the next discovery is SN 2011aa, then SN 2011ab, etc. In 1987, a naked-eye supernova was discovered in the Large Magellanic Cloud, a small companion galaxy to our Milky Way. Discovered on February 23, it was the first of the year: SN 1987A. By contrast, the recent supernova, discovered on August 24 from images taken on the nights of August 22 and 23, is designated SN 2011fe—the 161st of the year. A supernova discovered on February 21 of this year is SN 2011ap. The last supernova of 2010 was SN 2010ma. For a list of recent supernovae, go here: http://cbat.eps.harvard.edu/lists/RecentSupernovae.html
What are the particulars of this supernova?
It is in a relatively nearby galaxy (21 million light years away is a close neighbor on the galactic scale) known variously as M101 and the Pinwheel Galaxy. It is easily visible even in modest telescopes, above the “handle” of the Big Dipper, as shown below. It reached its maximum brightness on September 13, and even then it was not visible to the naked eye. It will fade slowly, and should be visible in telescopes for several weeks to come.
Here is a diagram indicating where to find the galaxy and the supernova:
Where to find the supernova
And here is an image of the supernova taken on September 2:
SN 2011fe
The main significance of this event is how quickly after the initial explosion images were captured. This should give us sharper insight into the mechanics of these titanic outbursts, and make them even more reliable as yardsticks for a very big universe.
Leave a Reply