To us here on Earth, sunrise and sunset have been clicking along without a hitch as usual, meaning for the past 4.5 billion years or so. But last year the helio-astronomers got a little antsy when sunspots (cool places on the sun that form when twists of the magnetic field break through the surface) failed to start reappearing as expected after the “minimum” period in their 11-year cycle.
The number of sunspots was expected to bottom out in late 2008 and early 2009, and then start increasing again toward the maximum around 2013. But in 2009 there were 260 days with no sunspots, a startling number to the solar astronomers. There was some uneasy talk of a prolonged absence of sunspots encouraging an impromptu ice age here on Earth, though evidence of a relationship between sunspots and climate is tenuous at best. Then February 2010 was the first month in a long time when there were sunspots every day. Since then, the sun has been acting up as expected.
The sun’s activity goes largely unnoticed by us, unless for example you spotted some spectacular auroras last August — they were triggered by a coronal mass ejection, or huge explosion on the sun’s surface that sent off a plasma of charged particles. The plasma struck Earth’s magnetic field and excited particles there, producing the auroral lights. Large solar explosions can cause electrical failures on Earth, though the August CME didn’t do that. Another fairly large CME occurred in early September.
Solar activity like this, though huge in its own right, occurs in a steady, untroublesome range because our sun is a pretty steady-lived star. It’s a “dwarf,” meaning that in the general run of stars it’s of medium size and brightness, with nuclear fires burning steadily in its center. It’s about 864,000 miles in diameter (Earth’s diameter is about 8,000 miles). It’s about 4.6 billion years old and is expected to pass through several life stages as it burns up its nuclear fuel, eventually blows off its outer layers, turns into a white dwarf star and fades to cool extinction, somewhere around 6 billion years from now. Meanwhile, it is so massive it holds the planets, dwarf planets, asteroids, comets and a lot of other interplanetary riffraff in its gravitational field. Earth-life depends on its steady supply of light and heat.
The sun’s surface is called the photosphere, a sea of gases bubbling up from the nuclear interior at around 10,000 degrees Fahrenheit. Just above the photosphere is the chromosphere, or inner atmosphere, where the temperature rises to about 36,000 degrees. The chromosphere is invisible except in the moment just before and after total eclipses, when it is seen as a bright pink or red flash. The outer atmosphere is the corona, the familiar blazing ring photographed during total eclipses.
The sun drives the space weather in our vicinity, which involves conditions of radiation, magnetic fields and flows of charged particles from the sun called the solar wind, which travels interplanetary space at about 1 million mph. Spacecraft have been designed with sails to catch the solar wind and propel them like sailboats in ocean breezes. Everything electrical here on Earth is subject to disturbances from space weather generated by the sun’s activity. Flares, prominences and ejections of material which amount more or less to stupendous explosions take place pretty frequently there, and seem to be associated to some extent with sunspots.
Meanwhile, life goes on here on Earth. In the Northern Hemisphere we are deep into winter, which is our experience of the Earth’s tilt taking us out of the direct rays that lance from the chromosphere. It takes just this little leaning away from the sun to turn a planet cold enough to be a matter of life and death. Fortunately, the direct rays will return. It is all humming along pretty steadily, as usual. For now and the next few billion years, anyway.
More Amateur Naturalist and other essays on space, tme and nature can be seen at: