This month Earth returns to the point in its orbit which marks the beginning of spring. This change of season brings a renewal of life as the cold of winter slowly gives way to the coming warmth. But not all change in nature is so gradual and reassuring. Change can sometimes be sudden and cataclysmic, as in the cases of earthquakes or storms. We have an example of a cosmic catastrophe in the northern sky this month.
This change was first noticed on Jan. 21 when a teaching fellow at London College Observatory noticed approaching clouds and decided to cancel the scheduled astronomy lesson and allow the students a few minutes to enjoy the view through the telescope before the clouds obscured the sky. They pointed the telescope at a favorite target of amateur astronomers, the galaxy M82, which lies between the bowl of the Big Dipper and the North Star. Galaxies like M82 normally appear in small telescopes as a fuzzy blur in which the light of the galaxy's hundred billion stars blends together. The instructor and students were surprised to see an unexpected bright point of light in M82 standing out against the galaxy's usual wispy glow. After observations with different filters to confirm what they saw, they immediately notified the astronomical community of the discovery. Soon astronomers around the world confirmed that this abrupt change in M82 was a supernova - an exploding star suddenly shining brighter than the combined light of the other hundred billion stars of the galaxy. The images show the galaxy as it has appeared for as long as astronomers have observed it and the suddenly new view with the bright supernova.
Astronomers were excited because supernovae are rare. The last one in our own Milky Way galaxy visible from Earth was seen in 1604 - five years before Galileo turned his first telescope toward the sky. So we have had to content ourselves with observing supernovae in other, distant galaxies. The closest occurred in 1987 in our companion galaxy known as the Large Magellanic Cloud. That supernova was one of the kind that astronomers creatively call "Type II," but we are excited about the new supernova not only because it is in a relatively close galaxy (M82 lies about 12 million light years from Earth) but also because it is an example of a "Type Ia" supernova which can serve as a yardstick for measuring galactic distances.
Type Ia supernovae occur on white dwarf stars. These are the corpses of stars about the size of our Sun that have exhausted their nuclear fuel and blown off their outer layers, leaving the compressed remnant of their core to cool in the galactic night.
The compression they go through in formation crams a star's mass, about a million times that of Earth, into an object about the size of Earth. These objects are so dense that a spoonful of one would weigh tons. Unlike ordinary stars, a white dwarf has no nuclear reactions to generate heat, and a lonely white dwarf will gradually radiate away the heat of its formation and cool to a darkened ember. This will be the fate of the Sun. But single stars like the Sun are a minority; most stars are members of multiple star systems, orbiting companion stars. If one of these stars goes through its death to become a white dwarf before its companion, the stage is set for the possibility of a Type Ia supernova.
When a white dwarf's companion begins its own death throes, it will expand into a red giant. The Sun will do this in about five billion years, expanding to about the size of Earth's current orbit. The red giant star, Betelgeuse, marking Orion's right shoulder, would fill Jupiter's orbit if it replaced the Sun. When such a star is a companion of a white dwarf, some of its hydrogen-rich outer layers are pulled away by the gravitational force of the white dwarf. As the smaller star greedily gobbles up the material from its companion, it accumulates hydrogen-rich gas until its intense gravity forces the nuclei of hydrogen atoms together in nuclear fusion in an explosion that destroys the white dwarf as fuel that would normally sustain a star for millions of years is consumed in a matter of days.
How many planets, if any, might have been destroyed by this cosmic cataclysm that we've just witnessed in M82? Previous estimates have shown that the radiation from a blast such as this should be able to cause mass extinctions on any Earth-like world within about 33 light years of the blast, mainly by destroying that world's ozone layer, leaving it vulnerable to fatal levels of radiation from its own star. So, the question becomes, "How many worlds were within 33 light years of the supernova?"
We can't see planets at the distance of M82, but we can estimate an average number of planets based upon recent detections of planets in our own galaxy. Current results from NASA's Kepler spacecraft suggest that our galaxy may contain 100 billion planets. Only a fraction of them are of a size and temperature similar to that of the Earth. Based upon the fraction of systems in which we find such planets, we can estimate that there are probably about 40 billion Earth-like planets in our galaxy. Given the size of our galaxy, we would expect to find one Earth-like planet in every spherical volume with a radius of about five light years. Almost 305 light-year volumes can fit into the 33 light-year diameter supernova destruction sphere.
If Earth-like planets are as common in M82 as they are here, that means more than 200 Earth-like worlds were probably destroyed by the light shining in our cold northern sky tonight.
We have no way of knowing if any of these worlds were homes to any kind of life. But given what we know about how life develops, it's likely that some of them may have harbored life. This should remind us that nothing lasts forever -?all worlds die. But, as on Earth, death is not really the end. Supernova explosions like this create atoms of heavy elements like iron which are spewed into space where eventually, over billions of years, they will coalesce under the influence of their own gravity to form the next generation of stars and planets. Just as the coming spring brings forth new life from the dead of winter, new life may arise from the death and destruction of Nature's greatest cataclysm, a supernova.
With our roll-off-roof facility up and running, astronomers of the Adirondack Public Observatory are eager to dazzle you with telescopic views of the Moon and cosmos every other Friday night (next on February 14). Go to the APO web site at apobservatory.org and click on "events" for more information and directions to our site above Little Wolf Pond in Tupper Lake. Listen for Aileen ODonoghue on North Country Public Radio about once a month during "The Eight O'Clock Hour" or email Aileen with any questions at firstname.lastname@example.org.