As it turns out our nearest star has been quite useful for our existence. It pumps out a consistent amount of energy, it doesn’t flare up that often and hasn’t yet started to swallow up other planets in the Solar System. All of this has helped our own planet have a certain amount of stability over the last 4.5 billion years to allow for life to flourish, with a few hiccups along the way.
Our star, or the Sun as we call it, is huge compared to everything else in the Solar System, all up it has 99.86% of the entire Solar System’s mass. It’s just under 1.4 million km in diameter or about 109 times bigger than the Earth. It’s mass is around 333,000 times that of Earth but it’s density is only about 1/4 on average. It’s mass creates an enormous amount of gravity, so if you could stand on the surface of the Sun you would feel nearly 28 times the pull of gravity than you feel on Earth. That means for someone like me with my weight, I would feel like I weighed nearly 2,400 kg! Of course the sun is also extremely hot at about 5,770 K, and it’s this heat which has been quite helpful for us on Earth! This temperature and the make up of our own atmosphere and our distance from the Sun allows water to be a liquid, it puts us in the so called ‘Goldilocks Zone’, not to hot, not to cold for life to exist.
As far as stars go the Sun is a pretty mediocre type of star. Its classification is G2V which is a grading system based on spectral lines, temperature and luminosity. About 7.6% of stars around us are G type stars, the V indicates that the star is in a stage of life called main sequence, this means that it is stable and slowly churning through its hydrogen fuel, basically it’s at equilibrium between the crushing gravity of its huge mass versus the thermal pressure from fusing hydrogen into helium in the core. This equilibrium will remain until the Sun runs out of hydrogen, this is good news for us because that will take around 5 billion years from now. Below is a picture I took of the sun about 5 years ago so given that the sun consumes around 600 million tons of hydrogen per second then that means the sun has burned 18,921,600,000,000,000 tons of hydrogen each year since I took that photo.
The Sun has been burning away like this for over 4.5 billion years and it is estimated the core is probably about 60% helium. It’s important to remember that the Sun didn’t start out as a ball of pure hydrogen. It formed from a gas cloud that was itself a mix of older stars that had blown up and shed their outer layers as well as the left over dust from around 9 billion years of galactic evolution since the formation of the universe. That proto-star that would eventually ignite into our Sun, was 71.1% hydrogen, already 27.4% helium and the balance is made up of heavier elements. So basically our star is made up of some second material already consumed by more ancient stars that have long gone.
The 600 million tons of hydrogen consumed every second doesn’t just disappear, most of it is converted into helium, but about 4,260,000 tons every second is converted into energy. This energy eventually makes its way our of the core and outer layers of the sun and on its way to us. It takes neutrinos about 2.3 seconds to reach the surface of the sun from the nuclear furnace in the core, this is because nothing interacts with them so they blast out in a straight line at the speed of light. Not so for photons who have a less direct path that can take 10,000 to 170,000 years to reach the surface. From there it’s a short 8 minute hop to Earth.
Eventually the hydrogen in the core will run out and the star will exit the main sequence stage and start expanding. It goes through a complex process of burning hydrogen in the shell before the helium starts burning, all the time expanding and swallowing up the inner planets, including the Earth. Eventually there’s not enough gravity in the sun to hold on to its expanding atmosphere so it starts to lose its outer layers into space like in this picture of the Eskimo Nebula, which was a star similar to our Sun before it’s outer layers headed off into space creating a beautiful planetary nebula.
So next time you’re outside on sunny day (or any day really) then think about those neutrinos passing through you unnoticed that got created only about 8 minutes previously, then think of those photons hitting you that may have taken up to 170,000 years to get out of the Sun. Then have a think about all of the hydrogen getting converted helium in the core – and be thankful there’s a enough of it to last another 5 billion years.