Edward Sion, astronomer and astrophysicist at Villanova University, discusses the end of the world if one day the sun swallows the earth.
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How the Sun Will One Day Swallow the Earth I think the world would end, leaving out all these other catastrophes and leaving aside all the possibilities that they could completely destroy Earth, which is not clear. I think that the real end of Earth will take place when the sun, which is now a very average star on what we call the main sequence stage of evolution, when the sun runs out of hydrogen in its core. It’s building up helium right now. When this happens, the sun will drastically change its structure. When it uses up hydrogen it no longer has thermonuclear fusion energy to provide its luminosity. In other words, it's shining because of its thermonuclear fusion of hydrogen to helium. When that hydrogen fuel source runs out, the sun will drastically change its structure and evolve into what we call a red giant star. A red giant star is one like, for example, you may be familiar with Antares, which is the brightest star in the constellation Scorpius. It’s a very red star. It’s a red super giant star. Another example is Betelgeuse in the constellation Orion, a famous wintertime constellation here in the northern hemisphere. These stars, these red supergiants, for example Antares, if you placed it where the sun is located our orbit would be inside of that star. So I think the ultimate end of the earth is going to be when the sun expands to a red giant. Its outer layers swell up to beyond 93 billion miles. Earth will then start to experience viscous drag as it orbits the red giant sun. This viscous drag will cause the orbit of earth to decay much like an artificial satellite we launch around earth will eventually burn up in the atmosphere. Earth will eventually be incinerated inside the sun. Mercury, Venus, and Earth, and possibly Mars will undergo what we call death spirals. Their orbits will decay as they loose orbital angle due to the viscous drag they will decay and spiral into the sun where they encounter very high temperatures and essentially Earth will vaporize. Now this won’t happen for at least, well the sun has approximately 5.4 billion more years as a main sequence star. Then a few hundred million years beyond that. So, I would say, in roughly six billion years from now, the inter planets should be engulfed by the giant sun. But by then, presumably advanced life here on earth will have colonized other worlds and so we don’t have to worry about it. But I think that’s perhaps the best answer to your question is that eventually it’s got to happen. The sun will become a red giant star and the inner planets will then decay and burn up inside the sun, vaporize. And then Jupiter will start to accumulate mass and if Jupiter accumulates more hydrogen-rich gas, than its present mass. Its present mass is 1/1000 of the Sun’s mass. If Jupiter creates about, I’d say roughly reaches 10 times its present mass and it could do this by sweeping up solar gas as it orbits the sun, or the future giant sun. It could undergo a thermonuclear ignition and become a star and so then you’d have the sun and Jupiter as a binary system. But this is, of course, something that is billions of years into the future.