Sun
You can watch the summary of this post as video here.
Overview
The Sun, the star of our planetary system, is the most massive object in our solar system. It holds almost 99% of the mass of the entire solar system. The combined mass of everything else in the solar system accounts for only 1 percent!
It is a simple fact that life on earth cannot exist without the Sun. It supplies the energy needed to sustain life on our planet. That explains why many cultures in the world revere and worship the Sun.
Interesting Facts
- Our home galaxy, Milky Way, has more than 100 billion stars. Our Sun is just one of them, located in a place which is 28,000 light-years away from the center of the Milky Way. Take a look at the picture of the Milky Way below. The red arrow shows where our Sun is located.
- The Sun is made of:
- hydrogen (92.1%)
- helium (7.8%)
- other elements (0.1%).
It is a super-hot ball of hydrogen and helium gases.
- The Sun’s gravity not only holds the planets in their orbits, but also keeps every object around it.
- Its radius is 695,500 kilometers. Compare this with the radius of our Earth which is 6378 kilometers. We can fill the Sun with 1.3 million Earths!
What Powers the Sun
What powers the Sun is the thermonuclear-process known as nuclear fusion – hydrogen nuclei in the core, under enormous pressure and heat due to gravity, fuse together to produce helium nuclei. This nuclear fusion in the Sun’s core produces massive amounts of energy. Any idea how much energy? Keep reading.
Let us quantify the energy generated by the nuclear fusion process. In its core, the Sun burns 600 million tons of hydrogen every second, producing 596 million tons of helium. What about the missing 4 million tons? These four tons of mass are transformed into pure energy. Recall Einstein’s famous mass-energy equivalence principle equation, E=mc². This equation comes into play here. Using this equation, when we compute the energy released by the missing mass, we get an astronomical number: 360 septillion watts (360 followed by 24 zeros!). Just to put a context to this enormity, this is equivalent to 4 million years of electricity usage by the entire world. What powers the Sun is this incredible amount of energy released, every second, from the nuclear fusion process in the core.
Despite burning at this rate, the Sun has enough hydrogen fuel in stock to last another 4 to 5 billion years.
The central core of the Sun – where the thermonuclear process of nuclear fusion has been going on for the last 4.6 billion years – is the hottest part of the Sun with temperature of about 27 million degrees Fahrenheit.
Evolution Of Our Sun
Main Sequence Stage
Our Sun formed 4.6 billion years ago from a disk of rotating solar nebula (cloud). When the solar nebula collapsed to its center due to gravity, the center of the nebula became so compressed and hot that nuclear fusion started, and our star began to shine. An average-sized star, like our Sun, stays in this state for about 10 billion years. This stage is known as the ‘Main Sequence’. Our Sun has been in the ‘Main Sequence’ stage for 4.6 billion years so far, giving us another 4 to 5 billion years to go until the next stage.
While in the ‘Main Sequence’ stage, the nuclear fusion process in the core keeps the Sun stable. The immense amount of energy released in the core by the fusion process creates outward radiation pressure, which supports the huge mass of the Sun and prevents it from collapsing due to gravity. Take a look at the illustration below.
Though this balancing act is delicate, it will continue to be stable until there is no more hydrogen in the core to burn. When this happens, the next stage in the evolution of our star begins.
Red Giant Stage
When the Sun runs out of hydrogen, 4 to 5 billion years from now, hydrogen burning stops and there is no outward radiation pressure anymore. That tips the balance in favor of gravity. The Sun starts collapsing towards the core. This collapsing pressure causes the remaining hydrogen in the shell, wrapped around the core, to fuse. This shell fusion makes the outer layers of the Sun expand to become a red giant star. During this stage, Sun becomes so large that the planet Mercury will be swallowed.
The Sun will be in the Red Giant phase for about 400 million years.
White Dwarf Stage
During the red giant phase, the helium core (the leftover of hydrogen burning) gets compressed and becomes hotter. As a result, the helium starts burning, producing carbon and oxygen. What happens when the helium core is fully burned? Will the carbon core start burning? No, it will not be able to continue the nuclear synthesis any further because the pressure in the core will not be high enough to fuse carbon atoms. The Sun’s mass is not big enough to create the amount of pressure needed in the core to ignite carbon burning!
At this stage, in the absence of any outward pressure, gravity will take over. The force of gravity will collapse the Sun into a dense object known as a white dwarf, while the outer layers are blown away to become a planetary nebula that will engulf the region beyond the orbit of our beautiful Earth.
Remaining inner-orbit planets – Venus, Earth, and Mars – will all be swallowed at this time.
The white dwarf, thus formed with most of the mass of the Sun, will be compressed to the size of our Earth with extremely high density. A teaspoon of white dwarf material would weigh about 15 tons!
Though there is no burning taking place in the core now, the white dwarf is still extremely hot and will be radiating light and heat for billions of years, but of course, in extremely reduced luminosity.
The white dwarf stage may last for 20-100 billion years.
Black Dwarf Stage
After several billion years of cooling, the white dwarf will stop radiating light and will eventually become a black dwarf. We are yet to detect a black dwarf in the universe as the universe is not yet old enough to create one.
You may wonder, what will happen to the black dwarf ultimately? Though we are not sure, we can make a guess. It should remain as a black dwarf for eternity or until some other object, like a black hole, swallows it when our Milky Way and the Andromeda galaxies collide.
That is the life story of our Sun.
Now let us see how other stars in the universe could evolve.
Evolution of Stars
Chandra Limit
After reading the story of our Sun above, you may ask: is this the way every star evolves? The answer is no. A star’s evolution depends on its mass.
To understand the mass dependency, we need to introduce a concept in astrophysics called, ‘Chandra limit’. The astrophysicist and Nobel laureate, S. Chandrasekhar, in 1930s, made outstanding discoveries on the evolution of stars, especially on white dwarfs. He discovered a limiting mass (of a star relic) that determines the path of its evolution during the end stage. You can read more on Chandrasekar’s work here.
Before Chandrasekhar’s discovery, astronomers believed that every star would eventually become a white dwarf. This theory was corrected when Chandrasekhar discovered that:
- Only a star remnant with mass less than 1.44 times the solar mass (the mass of our Sun) will end up as a white dwarf.
- A star remnant with mass greater than 1.44 times the solar mass, becomes a neutron star or a black hole after supernova explosion. The diagram below explains this idea.
For instance, while our Sun is on the left-side path of the diagram above, the well know star in the Orion constellation, Betelgeuse is on the right-side route. Betelgeuse’s mass is almost 20 times the solar mass. It is one of the largest stars known to us. Currently, it is in ‘red super giant’ stage and almost at the end of its nuclear fusion cycles. It can go supernova anytime in the next 100,000 years. When it goes supernova, it will be so bright that it will be visible even during the daytime, though it is 600 lightyears away from us.
If we are lucky, we may be able to see it go supernova in our lifetime.