The Case for a Creator - Lee Strobel [94]
“But,” I said, “the sun has flares too.”
“That’s right. And the intensity of flares on red dwarfs is about the same as on our sun. The difference is that red dwarfs as a whole emit much less total light, so they’re much less luminous. That means in comparison to the luminosity of the star, the output of the flare is high.”
“Whoa!” I said, putting up my hand in protest. “You’ve lost me.”
Gonzalez regrouped. “Okay, let me get to the bottom line: for this kind of star, flares cause the star’s total luminosity to vary. In fact, astronomers call them ‘flare stars,’ and they watch as they get much brighter for a while and then dimmer again. We don’t pay too much attention to the solar flares of our sun, because the sun is so luminous that the flares are like a little blip. You barely notice them.”
“And remember we’re ninety-three million miles from the sun,” Richards said. “With a red dwarf, your planet would have to be much closer to the star.”
“Right,” said Gonzalez. “The luminosity increase would cause temperature spikes on the surface of an orbiting planet. But just as bad would be the increased particle radiation that would result from the flares. On Earth, we get a very mild effect called the aurora borealis. This is where there’s a flare on the sun, the particles eventually reach the Earth, they’re funneled down the magnetic field to the north and south poles, and we see the aurora borealis as these beautiful lights in the northern hemisphere.
“However, particle radiation has the effect of quickly stripping away the atmosphere, increasing the surface radiation levels, but most importantly, destroying the ozone layer, which we need to protect from radiation. All of this would be deadly for any life on a planet near a red dwarf.
“And then red dwarfs have one more problem: they don’t produce much ultraviolet light, which you need early on to build up oxygen in the atmosphere. Scientists believe that the oxygen in the Earth’s atmosphere was built up at first by the ultraviolet radiation that broke up water into oxygen and hydrogen. The oxygen was allowed to build up in the atmosphere, while the hydrogen escaped into space, because it’s lighter. But you get very little blue light from a red dwarf, so this phenomenon wouldn’t occur as rapidly and you wouldn’t get the build up of the oxygen you need to sustain life.
“Fortunately, our sun is not only the right mass, but it also emits the right colors—a balance of red and blue. As a matter of fact, if we were orbiting a more massive star, called an F dwarf, there would be much more blue radiation that would build up the oxygen and ozone layer even faster. But any momentary interruption of the ozone layer would subject the planet to an immediate flood of highly intense ultraviolet radiation, which would be disastrous to life.
“Also, the more massive stars don’t live as long—that’s the major problem. Stars that are even just a little more massive than the sun live only a few billion years. Our sun is expected to last a total of about ten billion years on its main sequence, burning hydrogen steadily, whereas stars just a few tens of percent more massive have considerably less lifetime on the main sequence. And while on the main sequence, they change luminosity much faster. Everything on their lifecycle happens faster.”
“Anything else that makes our sun unusual?” I asked.
“Yes, the sun is metal-rich; in other words, it has a higher abundance of heavy elements compared to other stars of its age in this region of the galaxy. As it turns out, the sun’s metallicity may be near the golden mean for building Earth-size habitable terrestrial planets.
“And the sun is highly stable, more so than most comparable stars. Its light output only varies by one-tenth of one percent over a full sunspot cycle, which is about eleven years. This