If you've ever sat by a bonfire on a chilly evening, you would have noticed that without constant stoking, the fire would soon die out; but the great ball of fire, our sun, has been burning non-stop for over 4.5 billion years without any signs of dying out. It will eventually burn out, but no need to hurry up on that intergalactic shift. It has enough fuel left to last for another 5.5 billion years. Although it burns up around 620 million tons of fuel a second, till date, it has only used up about half of its hydrogen fuel.
A day in the life of the sun.
The sun is mainly composed of hydrogen; the rest is helium (along with small quantities of other common elements like oxygen, carbon, nitrogen, silicon, magnesium, neon, iron, sulfur, aluminum, calcium, sodium, nickel, and argon). Hydrogen is converted by nuclear fusion into helium, which is also combustible (the reaction is similar to the one that happens in an H-bomb). In this way, it both burns and creates fuel. It is a very efficient furnace, and over 92 percent of the hydrogen burnt is converted into helium, while the remaining is lost as energy in the form of heat, light, and x-rays.
Our vast Earth is only able to capture a miniscule percentage of this energy, the rest of which is lost to space. Even then, each square meter of the Earth receives 1.4 kilowatts of solar power each second.
Sun of a huge gun!
The sun, which is a yellow dwarf star, is the closest star to Earth. It is around 150 million kilometers or 8 light minutes away, thus, light originating from it would take around 8 minutes to reach the Earth's surface. Although the sun is composed of light gases, it weighs about 330,000 times the Earth's mass. It is about 1.3 million times the Earth's volume, and around 1.4 million kilometers in diameter (around 100 times the Earth's diameter).
At its core, the temperature is about 15,000,000 K (27 million degrees Fahrenheit), but by the time the heat reaches the surface, it cools down to 5,800 K (10,400 degrees Fahrenheit).
And God created light!
By studying other stars, which are at various stages in their life cycles, it has been determined that the sun started off as a large collapsing cloud of cold molecular hydrogen gas and dust. Slowly, over a period of millions of years, the cloud transformed into a rotating disk with a huge central bulge, which further collapsed due to its large gravitational force, and ended up with a temperature of several million degrees. At this temperature, the hydrogen nuclei began to fuse into helium, and thus, the present form of the sun came into existence. At about this time, a powerful solar wind blew away the remaining gas clouds and dust, which till then had obscured its visibility.
The beginning of the end!
In about a billion years, the sun will be around 10 percent brighter than what it is now. This will cause the Earth's atmosphere to dry out; wiping out most of the land-based species. 2.5 billion years later, it would have become 40 percent brighter than today. This would cause all the oceans to evaporate, leading to total annihilation of most life forms on Earth. Two billion years later, i.e., 5.5 billion years from now, its core would run out of hydrogen, causing it to shrink, while its atmosphere will expand to about 100 times the current size; Mercury and Venus will be engulfed, and the Earth will turn into a massive ball of fire, although it will survive this holocaust.
The sun will then become a white dwarf, a star with a white-hot core. It would produce no energy and would be highly unstable. Slowly, it will change its color from white, to yellow, then red, and finally become a black dwarf, turning invisible to the naked eye. Our solar system will be altered drastically. Two of the planets will have vanished, and the remaining will be frozen in complete darkness for all eternity. Although the Earth will survive, it is likely to have a glassy surface once all the molten rock has cooled down.
A new beginning?
It is highly probable that humans, assuming that they survive till then, would have already inhabited other planets and even galaxies, but it is improbable that the Earth will be able to rejuvenate itself and become fertile enough to host life yet again.