The Incomprehensible Power of a Supernova

The Incomprehensible Power of a Supernova
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In a brief flash, the supernova of a single star can burn brighter than the billions of suns of an entire galaxy:

Supernova 1994D: NASA/ESA

That supernova at bottom left is not sitting in front of the galaxy NGC 4526. It's in the outer edge of that galaxy, 55 million light years away.

Last summer, astronomers found the most powerful supernova they had ever seen, an event called ASSASN-15lh. Their report published in the journal Science last week contained a measurement of the total power of this explosion: (2.2+/-0.2) x 1045 Ergs per second. That's an esoteric number phrased in unfamiliar units. What's the real meaning of this much power?

Astronomers look at a stellar object and measure its luminosity: the amount of energy it releases per second. (Further, this is a measure called bolometric luminosity: the total power radiated out across all frequencies of electromagnetic waves.) This sort of measurement is very familiar to us, as we use several scales that measure energy per unit time. A watt, a horsepower, and calorie burned per hour are human measures of power.

The numbers we're discussing are so big that we have to use exponential notation -- unless you want to read numbers like 220000000000000000000000000000000000000 watts.

A quick review on exponential notation: 102 = 100; 104 = 10,000; 3.5 x 104 = 35,000.

Let's start by getting rid of ergs. An erg is a ten-millionth of a joule. ASSASN-15lh radiated 2.2 x 1038 joules of energy per second, which happens to be exactly the definition of a watt. It's like the universe turned on a couple of 1038 W light bulbs. 1038 W is one hundred billion, billion, billion, billion watts. Now we've got a different problem: energy scales that are hard to imagine. What can we compare this to?

Converting a chunk of uranium smaller than a pea directly to energy via E = mc2 produced the nuclear blast that leveled Hiroshima. The energy of ASSASN-15lh is comparable to converting the entire moon to pure energy every 30 seconds. The biggest thermonuclear blast ever created was a billion trillion times less energy than one second of this supernova.

Our sun produces 3.8 x 1026 watts of power. So, this supernova was about 580 billion times brighter than our sun. The explosion radiated, every second, as much power as the sun has produced total over the past 18 millennia.

The Milky Way galaxy in which we live burns with roughly 8 x 1036 watts. For its few dying days, the supernova is nearly 30 times more luminous than our entire galaxy.

All of our smaller human endeavors are still more incomparable.

For example, 746 watts is equal to a single horsepower. A redlining Ferrari engine might produce 600 HP, or about 450,000 watts. Our supernova is like 1032 Ferrari engines at full throttle.

A massive power plant produces about 109 watts. The total electrical energy generated by all power plants on earth is about 7.9 x 1019 Joules per year. In a single nanosecond, ASSASN-15lh expelled more energy than those power plants could produce, operating at full capacity, for 2.8 billion years.

As intelligent as we are, our brains and our experiences don't prepare us to comprehend numbers and scales so big. Cosmic events like supernovas boggle our minds. Human works are dwarfed by the magnitudes of interstellar space like a single-celled organism by the vastness of the ocean in which it floats. Astronomy reminds us of how intelligent we are, but also how very tiny.

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