Date: December 15th, 2012

Title: Encore: The Science of Sunshine

Podcaster: Rob Berthiaume

Organization: York Observatory –


This podcast has been aired on May 6th, 2011

Description: Sunshine drives the global water cycle, allows plants to photosynthesize, and creates warm spots by the window where my cat likes to sleep. But just how do these bright, warm, comforting rays get to Earth? This podcast looks at some of the science of sunshine.

Bio: Robert Berthiaume is working towards an MSc in atomic physics at York University in Toronto, Canada. When he can get away from building diode lasers, he rides his motorcycle when the sun is up, and shares the stars with the public at the observatory when it’s not.

Sponsor: This episode of “365 Days of Astronomy” is sponsored by — NO ONE. We still need sponsors for many days in 2011, so please consider sponsoring a day or two. Just click on the “Donate” button on the lower left side of this webpage, or contact us at


Hi there. I’m Robert Berthiaume bringing you today’s edition of the 365 Days of Astronomy Podcast from the York University Observatory in Toronto, Canada. Sunshine drives the global water cycle, allows plants to photosynthesize, and creates warm spots on the carpet by the window where my cat likes to sleep. But just how do these bright, warm, comforting rays get to Earth? This podcast is going to look at some of the science behind sunshine.

A logical place to start would be looking at where, and how, sunshine is produced. Stars are giant, hot, glowing balls of gas, and no one can be blamed for drawing a connection to hot, glowing fires that burn here on Earth. Stars do not burn, however, the way fire burn here on Earth. Burning, known scientifically as combustion, can be explained by the chemistry of molecules. Different types of molecules are broken apart with some heat and joined back together as different, new molecules, giving off even more heat. Think of having a red and yellow lego or k’nex piece clicked together, with a blue and green piece together beside it. Add some heat, and you’ll get a red/green combo, and a yellow/blue combo, along with even more heat released.

Inside the Sun, however, the heat and light that ends up being sunshine is explained by physics. All you need is individual atoms inside the core of the Sun and with the temperature and pressures being high enough. Imagine taking four yellow pieces and slamming them together so hard and so fast that they fuse into a single, bigger brick. In the process, you’ll probably hear a sound as they go from being many into one piece, and the newly formed piece will be hot; the event releases energy. This is exactly what happens in the Sun. Four Hydrogen atoms are squeezed together so hard that they turn into a brand new, heavier atom called Helium – the same stuff inside party balloons.

The fact that sunshine results from fusion, and not combustion, makes for some really cool physics. For one, if the Sun were burning the same way a campfire does, it would burn through all of itself, the fuel, in a time that can be measured in hours. But our star has been giving off sunshine for billions of years, and will continue to do so for billions more.

The amount of energy released in the form of light, or sunshine, is described by the arguably most well-known equation in physics: E=mc^2 . Einstein’s famous equations show that energy is equal to mass, multiplied by a number called c-squared, or the speed of light squared. A single Helium atom weighs a little less than 4 hydrogen atoms. In the Sun’s core, lots and lots of Hydrogen atoms are turning into Helium atoms, and each time, the difference in mass doesn’t just disappear, it changes form to become energy in the form of light. Now the difference is only a few tenths of a percent in mass from the Hydrogen you star with to the Heluim you end up with, but this process happens billions upon billions of times each second. The end result is that the Sun is losing about 4 billion kilograms per second.

The Sun is massive, figuratively and actually. 4 billion kilograms is a super small amount of the total mass of the Sun, but it adds up. The Sun has been fusing for about 5 billion years, and is only halfway done. By the time it runs out of fuel in the core to keep this fusion going, it will be 10 billion years old, and it will have lost about one thousandth of it’s mass.

The thing that keeps this fusion going is gravity. In the lego example, the blocks didn’t just automatically blob together and get hot and loud by themselves. There needs to be an external force, in that case, your big, beefy, lego fusing arms to create the conditions where that will happen. The Sun is set up in this condition due to big, beefy gravity. Every atom in the Sun wants to get closer and closer to all the others, and they are all trying to fall towards the center. This creates the high pressures, and in turn, high temperatures, that are required to start getting Hydrogen to fuse into Helium. They start releasing energy, in the form of sunshine, which on average travels outwards, away from the center of the Sun. Now each little particle of sunshine, each individual piece of light, called a photon, actually exerts a little bit of force on other stuff when it shines on to other atoms.

You heard me right. Light exerts a force when it shines on stuff. Take a strong enough spotlight and shine it on some stage performer, the stage performer will be pushed backwards as if it were windy in the theater…but not before they burst into flames due to the sheer intensity of that light. But I digress…

This is something called radiation pressure. When enough sunshine is being created, the outwards radiation pressure pushes on the atoms outwards and keeps them from falling inwards due to gravity. These two forces set up the conditions for the Sun to stay the size that is is, the temperature that it is, and outputting the amount of Sunshine that it does. But as I just mentioned, the mass of the Sun does decrease over time, and more and more of it is Helium as time goes on, and these changes actually mean that the Sun gets hotter and brighter over time, by tens of percent over its life.

The photons that are created in the core of the Sun travel at the speed of light, which is about 300000 kilometers per second. They go in all directions though, and they keep bumping into atoms, exerting force each time. A photon travelling straight out of the Sun reaches the surface, and starts its journey through space gets there in 3 seconds. But this is very very unlikely. Most photons end up taking about a million years to go from the core to the surface due to all this bumping into other stuff. Some make it out faster, some slower, and in fact, it occurs to me that there is a chance that some photons that were created when the Sun was just a baby, 5 billion years ago, have been bumping around ever since, and have never made it out to space.

Once they make it to space, however, there’s not much more stuff to bump into, and some of that sunshine travels straight to us here on Earth. Again, it takes time for light to travel a certain distance. 300000 kilometers in one second, 10 trillion kilometers in a year, and so on. Astronomers actually use this distance that light travels in a given amount of time as a measure of scale in the universe, the familiar light-year, and similarly light seconds or light minutes. Earth happens to be about 8 light minutes away from the Sun, meaning that if you go outside right now and feel some sunshine on your face, that sunshine left the surface of the Sun about the time you started listening to this podcast.

Thanks for listening, I hope you learned something and had a little fun. Until next time, this is Robert Berthiaume wishing you all clear skies and good times.

End of podcast:

365 Days of Astronomy
The 365 Days of Astronomy Podcast is produced by the New Media Working Group of the International Year of Astronomy 2009. Audio post-production by Preston Gibson. Bandwidth donated by and wizzard media. Web design by Clockwork Active Media Systems. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at or email us at Until tomorrow…goodbye.

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