Wednesday, 11 December 2013

Supernovae: The Possibly True Story of the Christmas Star

It should come as no shock to anyone to say that science often makes trouble for religion. Over the eons, people have been jailed, tortured, and killed for questioning the lessons in The Good Book. Occasionally though, science can fit nicely into a biblical story; and in the spirit of the holiday season we thought we could explore a fun topic that blends facts and folklore.

Anyone who has ever heard the nativity story describing the birth of Jesus (or has seen National Lampoon’s Christmas Vacation) knows about the Christmas Star. On the night that the protagonist of the New Testament was born a star appeared in the eastern sky over Bethlehem that guided the three magi (read wise-men) to the first ever Christmas, complete with presents. But how can a star just appear?

We generally don’t think of the night sky as something that changes. The stars move too slowly for us to perceive in real time and the patterns they make are so fixed that we have named them and written stories about the characters they depict. It’s all a lie though. The night sky is dynamic, and nothing can change things up quicker than the events known as supernovae.

Supernovae are the biggest things in the universe that we know about. They occur when a star many times bigger than our own sun (8-15 times bigger is a generally accepted estimate) dies. As a big star burns up the last of its fuel, nuclear fusion produces heavy elements in its core. It’s an interesting fact that everything in the universe that isn’t made of hydrogen or helium (mountains, trees, your desk, your lunch, your body, etc.) owes its existence to a star that exploded once upon a time.

When heavy elements accumulate in its core a star begins to collapse in on itself. Once things reach a critical density, known as the Chandrasekhar limit, the bomb goes off. Most of the material gets flung off into space and what remains is called a neutron star. Neutron stars themselves are amazing. They are tiny by star standards (20km across on average), but they are the densest things we know of, other than black holes. One teaspoon of neutron star would weigh 400 million tons, or about as much as the mass of all humans.

When a big star dies, the explosion is bigger than anything you have the ability to imagine. Supernovae can destroy worlds. They release more energy in an instant than the sun will produce in its entire lifetime. Supernovae outshine entire galaxies in the night sky and one in 1054 was so bright that it was visible in the middle of the day for over a month. Fortunately for us there are no stars big enough and close enough to threaten us with their inevitable boom, but there are plenty of chances to enjoy the light show from a safe distance.

A supernova happens about once every 50 years in a galaxy the size of our Milky Way (about once per second in the universe at large). They usually appear only as a new point of light among the countless stars in the sky, but occasionally they stand out.

The religious website “Answers in Genesis” lists a supernova as one possible explanation for the Christmas Star, and they aren’t the only ones. It has long been thought that the star that allegedly hung in the sky over Bethlehem was in fact an explosion of literally biblical proportions.

That star eventually inspired the ones that adorn Chirstmas trees the world over. So this December, as you take in the festive air and admire a finely decorated tree, take some time to appreciate that the focal point might just symbolize the most badass event in all of creation.

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