what if the sun were half as big?
A smaller star might last longer, but Earth would be a very different planet
Earth sits in an orbit around the Sun called the habitable zone. This ‘Goldilocks’ region of stellar space is not too hot and not too cold, just right for liquid water to flow freely in Earth’s lakes, rivers and oceans. The position of this zone depends on the size of our star, and if the Sun suddenly halved in mass everything would change. The Sun is a G-dwarf star, also known as a yellow dwarf. It fuses hydrogen atoms together to make helium, generating a surface temperature of between 5,300 and 6,000 Kelvin (around 5,000 to 5,700 degrees Celsius). A star half its size would be an M-dwarf star, also known as a red dwarf, with a much lower temperature of between 2,500 and 4,000 Kelvin (2,200 to 3,700 degrees Celsius). To keep our water liquid around this kind of star we’d need to orbit much closer, and that would cause some problems. Huddling close to a red dwarf star would generate strong tidal forces. This would slow Earth’s spin, lengthening our days. It’s possible that we’d decelerate so much that we’d become locked in one orientation, with one side of the planet always in daylight and the other in perpetual darkness. If this happened our water might freeze or boil away, leaving Earth barren and lifeless.