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Life of the Sun

Artist's impression of the Solar Nebula. Image credit: NASA

The Sun is a perfectly normal example of a star, formed from the solar nebula 4.6 billion years ago.

The Birth of the Sun
The Sun (and all the planets) started their lives in a giant cloud of cold molecular gas and dust. And then about 4.6 billion years ago, something bumped into the cloud, like the gravity from a passing star, or shockwaves from a supernova, causing the cloud to collapse. With the collapse, the mutual gravity from the particles in the cloud pulled together, and formed pockets of denser material in the cloud. These were star forming regions, and one of them was to become the Solar System.

As the cloud collapsed, conservation of momentum for all the particles in the cloud made it start spinning. Most of the material ended up in a ball at the center, but this was surrounded by a flattened disk of material. The ball at the center would eventually form the Sun, while the disk of material would form the planets. The Sun spent about 100,000 years as a collapsing protostar before temperature and pressures at the core ignited fusion at its core. The Sun started as a T Tauri star – a wildly active star that blasted out an intense solar wind. And then, just a few million years later, it settled down into its current form. The life of the Sun had begun.

The Sun captured by NASA's Solar Dynamics Observatory Spacecraft.

The Sun captured by NASA's Solar Dynamics Observatory Spacecraft.

The Main Sequence
The Sun, like most stars in the Universe, is on the main sequence stage of life. Every second, 600 million tons of hydrogen are converted into helium in the Sun’s core, generating 4 x 1027 Watts of energy. For the Sun, this process got going 4.6 billion years ago, and it has been generating energy this way every since. But there isn’t an unlimited amount of hydrogen in the core of the Sun. In fact, it’s only got another 7 billion years worth of fuel left.

As the Sun creates more helium at its core, the Sun burns a little more hydrogen. This causes the output of the Sun to go up. You won’t notice it now, but in about a billion years, the output from the Sun will have increased by 10%.

A more luminous Sun is bad news for Earth.

In 1.1 billion years from now, the Sun will be 10% brighter than it is today. This extra energy will cause a moist greenhouse effect in the beginning, similar to the runaway warming on Venus. But then the Earth’s atmosphere will dry out as the water vapor is lost to space, never to return.

In 3.5 billion years from now, the Sun will be 40% brighter than it is today. It will be so hot that the oceans will boil and that water vapor will be lost to space as well. The ice caps will permanently melt, and snow will be ancient history; life will be unable to survive anywhere on the surface of the Earth. The Earth will resemble dry hot Venus.

The planetary nebula NGC 6751. The future of the Sun. Image credit: Hubble Space Telescope

The planetary nebula NGC 6751. The future of the Sun. Image credit: Hubble Space Telescope

The Death of the Sun
All things must end. That’s true for us, that’s true for the Earth, and that’s true for the Sun. It’s not going to happen tomorrow, but one day in the far future, the Sun will run out of fuel and end its life as a main sequence star and die.

In about 6 billion years, the Sun’s core will run out of hydrogen. When this happens, the inert helium ash built up in the core will become unstable and collapse under its own weight. This will cause the core to heat up and get denser. The Sun will grow in size and enter the red giant phase of its evolution. The expanding Sun will consume the orbits of Mercury and Venus, and probably gobble up the Earth as well. Even if the Earth survives, the intense heat from the red sun will scorch our planet and make it completely impossible for life to survive.

When Will the Sun Burn Out?
Once it has become a red giant, the Sun’s death is just around the corner. It’ll still have enough heat and pressure at its core to begin a second stage of fusion, burning helium this time to form carbon. This phase will last for about 100 million years until this source of fuel is exhausted. Finally, the shell of helium becomes unstable causing the Sun to pulse violently. It will blow off a large fraction of its atmosphere into space over the course of several pulses.

When the Sun has blasted off its outer layers, all that will remain will be central core of carbon. In fact, it will be an Earth sized diamond with the mass of a star. This is a white dwarf, and it will still be hot enough to shine with thermal radiation. But it’s no longer generating solar fusion, and so it will slowly cool down until it becomes the same temperature as the rest of the Universe; just a few degrees above absolute zero. This will take about a trillion years to happen.

The Sun’s death will be complete.

The Crab Nebula. Image credit: Hubble

The Crab Nebula. Image credit: Hubble

Will the Sun Explode?
First, there’s no possible way that the Sun will ever explode. It might seem huge to us, but the Sun is a relatively low mass star compared to some of the enormous high mass stars out there in the Universe. When our Sun runs out of hydrogen fuel, it will expand up as a red giant, puff off its outer layers, and then settle down as a compact white dwarf star; slowly cooling down for trillions of years.

But let’s say that our Sun has about 10 times as much mass. Now we’re talking explosion. When this super massive Sun runs out of hydrogen fuel in its core, it switches over to converting atoms of helium, and then atoms of carbon. It keeps consuming heavier and heavier fuel in concentric layers, like an onion. Each layer takes a shorter period of time, all the way up to nickel, which might take a mere day to burn through.

Then iron starts to build up in the core of the star. And iron doesn’t give off any energy when it undergoes nuclear fusion. Because of this, the star has no more outward pressure in its core stopping it from collapsing inward. When about 1.38 times the mass of the Sun in iron collects at the core, it catastrophically implodes, releasing an enormous amount of energy.

Within 8 minutes, the amount of time it takes for light to travel from the Sun to Earth, an incomprehensible amount of energy would sweep past the Earth, and destroy everything in the Solar System. Supernovae can briefly shine more than an entire galaxy. A new nebula, like the Crab Nebula, would be visible to nearby star systems, expanding outward for thousands of years.

All that would remain of the Sun would be a rapidly spinning neutron star, or maybe even a stellar black hole.

But remember… the Sun has too little mass to ever explode. It won’t explode, so don’t worry.

And the details…

A few papers on star formation:


Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

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