A graph of sunspot numbers from 1700 through 1993, showing the 11-year sunspot cycle. Click on image for full size (99K JPEG) Image courtesy NOAA/NGDC.

We tend to think of the Sun as being remarkably constant and reliable. The most obvious feature of the Sun, the amount of light it shines upon our planet, is indeed extremely stable from day to day and even over the course of many years or even human generations. However, most aspects of our neighborhood star do vary, some in dramatic fashion, over time scales as short as minutes and as long as billions of years.

Dark (relatively!) spots appear on the face of the Sun from time to time. The number, size, and locations of these sunspots varies over the years. Observational records of sunspots, going back centuries, show that sunspot counts vary dramatically and regularly in an 11-year cycle. Sunspots are the most obvious indicator of the cyclical nature of solar activity in general. When sunspots are few, the Sun is sedate and solar storms are few. When sunspots are plentiful, the Sun becomes unsettled and much more active. During peaks in solar activity huge explosions on the Sun (called solar flares) generate bursts of radiation and energetic particles that hurtle outward through the Solar System. Active periods bring increased levels of ultraviolet radiation and X-ray emissions, greater prevalence of streamers and prominences above the Sun's surface, and an expansion of the solar atmosphere and heliosphere. The Sun looks differently and displays many different features during active times than during its more quiet periods.

The level of activity on the Sun has consequences throughout our Solar System, including here on Earth. Satellites and astronauts are vulnerable to damage and injury from increased radiation levels at active times. Solar storms generate beautiful auroral light shows high in our atmosphere - the Southern and Northern Lights! Solar activity can also interrupt communications, disrupt electrical power systems, and confuse migratory birds that use Earth's magnetic field to orient themselves. Variations in the Sun probably influence our weather and climate change, though the exact form that those influences take is still being researched.

The Sun also changes over longer timescales. The 11-year sunspot cycle has been interrupted at times, and some scientists believe there are longer-term cyclical variations in solar activity. The Sun has gradually brightened (by roughly 30%) over its six billion year history, dramatically increasing the heat delivered to Earth's climate. Our star was also much more active in its youth, displaying wilder variations in activity level in its youth as compared to modern times.

You can go to bed tonight secure in the notion that the Sun will rise in the morning. Remember, though, that tomorrow's Sun will be a tad different that the one that sets tonight!

A graph of sunspot numbers from 1700 through 1993, showing the 11-year sunspot cycle. Click on image for full size (99K JPEG) Image courtesy NOAA/NGDC.

We tend to think of the Sun as being remarkably constant and reliable. The most obvious feature of the Sun, the amount of light it shines upon our planet, is indeed extremely stable from day to day and even over the course of many years or even human generations. However, most aspects of our neighborhood star do vary, some in dramatic fashion, over time scales as short as minutes and as long as billions of years.

Dark (relatively!) spots appear on the face of the Sun from time to time. The number, size, and locations of these sunspots varies over the years. Observational records of sunspots, going back centuries, show that sunspot counts vary dramatically and regularly in an 11-year cycle. Sunspots are the most obvious indicator of the cyclical nature of solar activity in general. When sunspots are few, the Sun is sedate and solar storms are few. When sunspots are plentiful, the Sun becomes unsettled and much more active. During peaks in solar activity huge explosions on the Sun (called solar flares) generate bursts of radiation and energetic particles that hurtle outward through the Solar System. Active periods bring increased levels of ultraviolet radiation and X-ray emissions, greater prevalence of streamers and prominences above the Sun's surface, and an expansion of the solar atmosphere and heliosphere. The Sun looks differently and displays many different features during active times than during its more quiet periods.

The level of activity on the Sun has consequences throughout our Solar System, including here on Earth. Satellites and astronauts are vulnerable to damage and injury from increased radiation levels at active times. Solar storms generate beautiful auroral light shows high in our atmosphere - the Southern and Northern Lights! Solar activity can also interrupt communications, disrupt electrical power systems, and confuse migratory birds that use Earth's magnetic field to orient themselves. Variations in the Sun probably influence our weather and climate change, though the exact form that those influences take is still being researched.

The Sun also changes over longer timescales. The 11-year sunspot cycle has been interrupted at times, and some scientists believe there are longer-term cyclical variations in solar activity. The Sun has gradually brightened (by roughly 30%) over its six billion year history, dramatically increasing the heat delivered to Earth's climate. Our star was also much more active in its youth, displaying wilder variations in activity level in its youth as compared to modern times.

You can go to bed tonight secure in the notion that the Sun will rise in the morning. Remember, though, that tomorrow's Sun will be a tad different that the one that sets tonight!

A graph of sunspot numbers from 1700 through 1993, showing the 11-year sunspot cycle. Click on image for full size (99K JPEG) Image courtesy NOAA/NGDC.

We tend to think of the Sun as unchanging. In some ways it is! The Sun rises every morning and sets every night. The amount of light it shines on Earth is very stable from day to day and even over the course of many, many years. But, some things do change about the Sun.

The number and placement of sunspots on the face of the Sun changes over time. The number of sunspots tells us roughly how active the Sun will be. Lots of sunspots means the Sun will be very active and stormy. That means solar flares and CMEs. Very few sunspots means the Sun will be calm. How active the Sun is does affect life here on Earth.

The Sun, like all stars, has changed over the course of its very long life. When it was younger, the Sun was more active then it is now. The Sun has also gotten brighter over its lifetime.

You can go to bed tonight knowing the Sun will rise in the morning. Remember, though, that tomorrow's Sun will be a tad different that the one that sets tonight!