by Laurance Doyle - Astronomer
Did you know that the sunshine that you enjoyed today was made in the center of the Sun over 1 million years ago? It took the light that long to work its way to the solar surface. It then took an additional eight and a half minutes to journey the remaining 93 million miles (149.7 million kilometers) to our planet where it could make the flowers and trees grow, warm the oceans, power our weather patterns and light our way around the globe.
The Sun is equivalent to 80 billion hydrogen bombs exploding every second, but it is just a little star what astronomers call a "dwarf" or "main-sequence" star. (Youll read about what makes a "main-sequence" star in subsequent articles.) You would need to detonate a hydrogen bomb every second for the next 3,200 years to equal the Suns energy output every second. This energy output comes from a process called nuclear fusion, which fuses hydrogen atoms into helium atoms.
Dwarf stars come in many different colors. Our Sun is a yellow dwarf, laying in the middle zone between cooler, redder stars and hotter, bluer stars. Red-hot stars are not as hot as white-hot stars, which, in turn, are not as hot as blue-hot stars. Hot blue-dwarf stars are the most massive and burn out quickly (in a few tens of millions of years) while the cooler red-dwarf stars could last more than a hundred billion years!
Planetary scientists generally define the livable region around the Sun as the distance at which liquid water can exist on a planets surface for long periods of time. This is referred to as the "Circumstellar Habitable Zone" or CHZ. Liquid water is essential for biological systems for many reasons. Among them is the fact that water can hold a lot of heat. (This is the reason its used in your cars radiator.) Water can also dissolve and hold an abundance of different chemicals, providing the medium for all sorts of biogenic reactions. But one of its most important properties is that its common solid state (ice) floats on top of its liquid state (water).
Had it not been for this last feature, Earth would have formed into a solid ice cube. Instead, when the bottom of the ocean got cold enough to produce ice, the ice broke off, floated to the surface, and was melted by the Suns heat.
The Sun is also getting brighter. This is due to the balancing act between gravity, which pulls the Sun inward and nuclear explosions, which push it outward. The result is an almost perfectly balanced star. But because the center is combining hydrogen into helium, more and more helium is building up inside the Suns center. To balance this increasingly heavy center, the Sun explodes more. This is why the Sun is about one-third brighter now then when it was born.
Then why wasnt Earth colder in the past? And will our planet burn up in the remote future because the Sun is still getting brighter? The answer to these questions, as well as an interesting discussion of what makes a Circumstellar Habitable Zone around other stars, will be the topic of next months column.