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 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.
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