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A Very Ordinary Star - Part II

At the beginning of this month, we compared the Sun to other stars using the Hertzsprung Russell Diagram. We saw there was a relationship between luminosity of a main sequence star and any one of the following: spectral color, mass, temperature, or lifetime. Lets turn to the Sun itself.

The Sun is about 860,000 miles across, weighs about 333,000 times the Earth, and is composed of about 98% hydrogen and helium by weight. We think of hydrogen and helium as light gasses and indeed they are, but they are squeezed so much by the Sun's gravity that the compressed fluid weighs about 40% more than the same volume of water.

The Sun surface is called the photosphere [light globe]. Just above the photosphere is a region called the chromosphere [color globe]. At higher altitudes the chromosphere thins until it becomes the corona. We see spectacular photos of the corona during eclipses. Above the corona is the region of the solar winds. What lies below the photosphere is a matter of conjecture, but scientists are firmly convinced that there is a central core. This core is very dense with a temperature of 15,000,000oK [Kelvin]. These conditions support the nuclear fusion of hydrogen into helium. When hydrogen fuses to form helium, very short wavelength electromagnetic radiation (gamma rays) and neutrinos are generated. [We will have more to say about neutrinos a bit later.] Gamma rays encounter countless atoms as they move out of the core, heating the atoms in the process. The light we see is emitted by these very hot (6100oK) atoms.

The Sun's surface is far from a smooth featureless globe. In many respects it looks like oatmeal bubbling on a stove. The individual flakes are called granulations. Dotting the photosphere are sunspots [raisins?] that come and go in an 11 year period. Sunspots are regions where the surface is about 1500oK cooler. An individual sunspot will last from a few days to a few months. During the Maunder Minimum (fifteen century), sunspots disappeared for many decades. At this same time the Earth had extremely severe weather called the little ice age. Besides the dark sunspots are other regions which are unusually bright called plages. When a plage is particularly bright it is sometimes called a faculae [little torch]. All across the surface of the photosphere, geysers of rising gas spurt up, called spicules.

On occasion, the Sun produces solar storms throwing out massive eruptions. Sometimes these eruptions take the form of huge arcs (hundreds of thousands of miles across) called prominences. Other times they take the form of flares. Motion pictures of prominences are spectacular. Huge domes (hundred of times the size of Earth) erupt and rain back into the Sun. While flares are not as spectacular looking as prominences, are actually more significant. Flares produce very short wavelength radiation in huge amounts (ultraviolet and X-rays). We are protected reasonably well by the ozone layer, but astronauts face a life threatening situation if they are aloft during a flare. To survive, they must ride out flares in protected portions of space craft called storm cellars. Flares and prominences can interact with the Earth's atmosphere. The aurora borealis is often spectacular during these solar storms. Unfortunately, these solar storms can disrupt long distance communications for days on end. Sometimes the Earth is exactly in the path of matter ejected by one of these solar storms. In this case, we can even lose parts of our long distance electric power grid due to voltage surges.

Although we have been looking at the Sun since antiquity, we are still far from understanding it. Today there is a major debate about the "missing solar neutrinos". Neutrinos are subatomic particles that pass through any type of matter almost as easily as a vacuum. They are exceedingly hard to catch. However, sometimes neutrinos react with certain elements to form other elements. Scientists placed huge vats of pure heavy water and dry cleaning fluid deep underground as traps for the neutrinos. They were confident they knew how many neutrinos were being created based on the Sun's power output. The measured rate was only a third of expectations. This error is so great that it requires a revision of our understanding of neutrino physics.

Three of the four outer gas giants, Jupiter, Uranus and Neptune have passed the Sun into the morning sky. Neptune rises shortly before 1 AM, Uranus just before 2 AM and Jupiter rises near 4 AM. Saturn and the Sun are very close together for the next few weeks.

Venus remains bright in the south western sky. Mars rides high throughout the night. If you want to glimpse Mercury, you may see it in the East just before sun rise. We have been lucky this year that at least two planets are up each night. If you remember the last two years, we had a Fall with many planets, but Spring had almost no planets out.

Leslie Coleman
Leslie Coleman
Entry Date:
Apr 1, 1999
Published Under:
Leslie Coleman's Columns
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