Star Points for February 2010 by Curtis Roelle Some Stars Supernova In February 1987 the brightest supernova since the invention of the telescope, several centuries earlier, erupted in the southern sky. The supernova appeared in the Large Magellanic Cloud (LMC), a far-off dwarf galaxy drifting near our own Milky Way galaxy, in which we live, at a distance of 160,000 light-years from us. The supernova was designated SN1987A – the first supernova observed that year. The discovery excited professional and amateur astronomers. I called a travel agent and flew to Jamaica, wanting to see the first supernova visible to the unaided eye since Johannes Kepler's supernova in 1604. The 1604 supernova remnant's distance is estimated to be around 20,000 light- years away, meaning the star actually blew up 200 centuries before it was seen. Then it took the light from the explosion 20,000 years to reach Earth. But at SN1987A's greater distance, its light took 160,000 years to reach Earth. So, although SN1987A was viewed nearly four centuries after the 1604 supernova, SN1987A actually occurred about 140,000 thousand years prior to it! To understand why certain stars explode requires some knowledge of what is going on inside the star during its life. As Gettysburg College astronomer Laurence Marschall once wrote, "Stars live their lives on the brink of disaster." Stars are believed to form out of coalescing clouds of cold gas. As more gas gathers to form a cloud in space, its gravity increases, enabling it to attract more gas. As its mass increases, its gravity grows stronger. The interior begins to warm under the crushing load. The gas continues to compress down until the temperature at the new star's core is millions of degrees, hot enough to initiate the process of nuclear fusion. In this process, hydrogen is converted into helium with a byproduct of energy. The pressure from this energy stalls the star's contraction, causing it to become stable, neither contracting or expanding. For stars like the sun, this stable period is assumed to last for billions of years even though it consumes more than 700 million tons of hydrogen per second. However, stars that go supernova are not like the sun. They are many times more massive and also hotter. A star with 20 solar masses has 10,000 times the sun's brilliance. Yet, like cosmic gas guzzlers they consume their fuel at a much faster rate and run out in just a few million years. When the end finally comes, and they have exhausted their fuel supply, gravity crushes them mercilessly to smithereens. Like the tortoise racing against the hare, in the end gravity wins. In our galaxy with an estimated 200 billion stars, one or two supernovae are predicted to occur each century. Just because we went nearly four centuries without seeing any doesn't mean they have not occurred in more distant, obscured, regions of the Milky Way. Perhaps if astronomers had radio telescopes and other modern instruments they could have detected them. Indeed, some supernova remnants have been discovered this way, including one believed to have occurred around the year 1680 in the constellation of Cassiopeia that nobody saw at the time. There are thousands of cataloged galaxies, and millions more that aren't. So several times a year astronomers see supernova explosions in some of these. Of course they are only visible in telescopes, but many are bright enough to be easily seen in amateur instruments. As far as our own sun goes, it is too small to become a supernova. It should continue shining much as it does now for another 4.5 billion years before turning into a red giant. By that time our star may become quite hazardous to our health, due to its increasingly searing heat.