How will our Sun Die?

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Nebula-sun
The Helix nebula, a planetary nebula similar to what the Sun will produce in 8 billion years

Sun

The Sun is the star at the core of the Solar System. It is a nearly perfect sphere of hot plasma, with an inner convective motion that generates a magnetic field via a dynamo process. It is by far the most significant source of energy for life on Earth. Its diameter is about 1.39 million kilometers (864,000 miles), or 109 times that of Earth, and its volume is about 330,000 times that of Earth. It values for about 99.86% of the total mass of the Solar System. Roughly three-quarters of the Sun’s mass consists of hydrogen (~73%); the rest is mostly helium (~25%), with much smaller numbers of heavier elements, including oxygen, carbon, neon, iron and other metals.

The tremendous effect of the Sun on Earth has been known since prehistoric times, and the Sun has been seen by some cultures as a deity. The synodic rotation of Earth and its orbit around the Sun are the basis of solar calendars, one of which is the official calendar in use today.

The Sun is currently 4.603 billion years old.

How will our Sun Die?

In around five to seven billion years from now, the Sun’s life will come to an end. Our star will plump up, becoming something called a “Red Giant” star.

Once the Sun shifts from burning hydrogen within its core to burning hydrogen in a shell around its core, the core will start to contract and the outer envelope will expand. The total radiance will firmly increase over the following billion years until it reaches 2,730 times the Sun’s current luminosity at the age of 12.167 billion years. Most of Earth’s atmosphere will be lost to space and its exterior will consist of a lava ocean with floating continents of metals and metal oxides as well as icebergs of impliable materials, with its surface temperature reaching more than 2,400 K (2,130 °C; 3,860 °F). The Sun will undergo more rapid mass loss, with about 33% of its total mass shed with the solar wind. The loss of mass will mean that the orbits of the planets will grow. The orbital distance of the Earth will stretch to at most 150% of its current value.

The most rapid part of the Sun’s enlargement into a red giant will occur during the final stages when the Sun will be about 12 billion years old. It is likely to grow to swallow both Mercury and Venus, reaching a maximum radius of 1.2 AU (180,000,000 km). The Earth will communicate tidally with the Sun’s outer atmosphere, which would labor to reduce Earth’s orbital radius. Drag from the chromosphere of the Sun would also reduce the Earth’s orbit. These outcomes will act to counterbalance the influence of mass loss by the Sun, and the Earth will be engulfed by the Sun.

The pull from the solar atmosphere may induce the orbit of the Moon to decay. Once the realm of the Moon closes to a distance of 18,470 km (11,480 mi), it will cross the Earth’s Roche limit. This means that tidal communication with the Earth would break apart the Moon, turning it into a ring system. Most of the orbiting ring will then begin to decay, and the debris will impact the Earth. Hence, even if the Earth is not swallowed up by the Sun, the planet may be left moonless. The ablation and evaporation caused by its fall on a decaying trajectory towards the Sun may remove Earth’s mantle, leaving just its core, which will finally be destroyed after at most 200 years. 

Ouch!

After blending helium in its core to carbon, the Sun will start to collapse again, evolving into a dense white dwarf star after expelling its outer atmosphere as a planetary nebula. The foretold final mass is 54.1% of the present value, most likely consisting fundamentally of carbon and oxygen.

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