MadSci Network: Astronomy |
It is true that as the Sun evolves, it will cool and eventually evolve into a white dwarf star, which is a very hot but very small remnant star that will continue to shine from residual heat. However, there are important evolutionary steps between the Sun today and its final state as a white dwarf. The Sun is currently a main sequence star. The main sequence is the group of stars that are at stable configurations (gravitational forces pulling inward are balanced by radiation and gas pressure pushing outward) due to hydrogen “burning” (the creation of helium atoms from hydrogen atoms through fusion). A star generally spends ~90% of its lifetime on the main sequence. The evolution time and style of a star depends greatly on the star’s initial mass. Here we’ll consider the evolution of a star like the Sun. Once the hydrogen in a star’s core is consumed, there is not enough energy produced to maintain enough pressure to balance gravity. The core then contracts and increases in temperature, so that remaining hydrogen can continue “burning” in layers outside of the core. The outer layers of the star adjust to the core contraction by expanding. The star has now entered the “red giant” phase. As a result of the star’s larger surface area, its luminosity (energy radiated per second) increases. When this expansion happens to the Sun, its radius will probably engulf the orbits of Mercury, Venus, and the Earth. However, the Sun will then be losing mass due to increased solar wind activity. The lower mass of the Sun will decrease the gravitational force it exerts on the planets, and their orbits will expand outwards. Thus, the Sun probably will not engulf the Earth, though Earth will be much closer to the surface of the Sun than it is today, raising its temperature. In summary, though the Sun will cool as it dies, it will first increase in luminosity and in radius. Venus will probably be engulfed by the red giant Sun. Thus, temperatures on Venus will never become suitable for life.
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