You didn't need to be a solar physicist to be riveted by the "solar storm" that sent a blast of charged particles our way this month. That particular flare-up fizzled, but in the long term, the sun's temper is worthy of our attention.
Our sun changes, and living things adapt or die.
Our planet circled a very different star when life first emerged on Earth some four billion years ago. The sun was dimmer and cooler, but more violent, sending deadly blasts of X-rays as well as particles that would have lit up the skies with spectacular auroras.
The displays would have been visible worldwide, but probably had no spectators, since life needed to stay deep underwater or buried inside minerals to survive until the sun calmed down.
For most of human history no one realized that the sun was fickle, breaking out in spots, flares, and eruptions, and would eventually kill all life on our planet.
"It was a huge part of Western culture that the heavens were forever and unchanging," said University of Michigan astronomer Fred Adams, who has written books on the beginning of the universe and the end.
Galileo was the first to see spots on the sun, which did not ingratiate him with the church. Even Einstein was influenced by the cultural bias toward unchanging heavens, Adams said, altering his theory of general relativity to work in a static universe. Soon after he published his theory, Edwin Hubble showed the universe was in fact expanding.
It wasn't until the mid-20th century that people realized the sun was running on nuclear fusion, and that when its fuel started to run low, the sun would die a violent death, blowing up into an enormous red giant.
For those concerned that the Mayans have forecast the end of the world this year, the astronomers' threat of more solar storms may seem even more ominous.
It's true we're moving into a stormy season that should last into 2013, but this happens every 11 years, said Douglas Duncan, an astronomer at the University of Colorado and director of the Fiske Planetarium. Astronomers still don't know why solar storms come in cycles or why it takes 11 years, he said. Duncan has catalogued similar cycles on other stars, and learned that sunspots and solar storms come in cycles all over the galaxy.
The cycles vary in length depending on a star's age - the cycles lengthening as stars get older.
During the peaks, or solar maxima, the spots on the sun increase, and the sun bursts with flares and storms. The sun always sends us a solar wind of protons and electrons, but during a solar storm, these shoot out in gusts. When the particles reach Earth, they light up molecules in our atmosphere as if it were a giant fluorescent bulb.
The effects on Earth are more dramatic if the gusts are released on a direct path to Earth, as scientists thought happened earlier this month. That would be unlikely to affect human health directly, but it could have disabled satellites, particularly ones that channel GPS signals.
When Duncan was comparing sunspot cycles on different stars, he said he got a call from Carl Sagan wanting to know how solar activity might influence the course of life on Earth. That, Duncan said, would take an expert on our planet's early history.
We humans couldn't have tolerated the ultraviolet radiation and X-rays that pummeled our planet during life's early history. About three billion to four billion years ago, the UV intensity was between 8 and 20 times what we have now, said geochemist Stephen Mojzsis of the Université Claude Bernard in Lyon, France. So for several billion years, life survived protected by water. As the sun cooled down and oxygen began to rise with the advent of blue-green algae, he said, life expanded to fill up the land as soon as it became habitable.
The sun was also cooler and was red rather than yellow, and we may carry an evolutionary fossil of that time in our eyes, he said. On the early Earth, microbes that were just starting to use photosynthesis began manufacturing a pigment called rhodopsin, which is good for absorbing red light. As the sun became yellow, the ability to make rhodopsin persisted, though different organisms used it for other purposes.
We use it in our retinas for night vision.
The sun was also 30 percent dimmer in the distant past, said Mojzsis. If it dimmed that much now, the Earth would freeze solid, but on the early Earth, different configurations of land masses and a different atmospheric chemistry kept the oceans liquid under such a cool sun.
The sun is getting hotter because it's fusing hydrogen into the heavier element helium. That's causing the sun to get denser and the nuclear fusion that powers it to become more efficient.
Scientists estimate that in 500 million to 1.5 billion years, the sun will be hot enough to wipe out all life on Earth. Moving to Mars would only postpone the apocalypse.
Our neighbor, Alpha Centauri, shines in a brighter, more bluish light because it's older and hotter than our sun. If it had any habitable planets, they are now burnt to a crisp, said Mojzsis.
In an additional five billion years, the sun will start to run out of fuel, and before it dies, it will expel its outer layers, becoming a red giant. Astronomers used to assume that the sun would swallow our planet, said Duncan, but more recent calculations show it will expand to just about the size of Earth's orbit. Either way, it will broil us.
As for those pessimists who worry about the Mayan predictions, Duncan said he's looked into the matter and the ancient civilization didn't really predict the world would end this year. Mayans did create an advanced calendar that was so good they extended it many centuries into the future. It just happened to end with 2012.