STARGAZING: Holes in the Sun

Astronomy books often describe the Sun as a "ball of hot gas".

Astronomy books often describe the Sun as a “ball of hot gas”.

This falls far short of the truth. A big ball of hot gas would be a fuzzy blob, densest in the centre and gradually fading away, getting fuzzier and fuzzier as we get farther out. Thanks to the extensive media coverage of the Sun over the last few years, usually associated with reports of “solar storms,” we all know the Sun does not look like that at all. It appears to have a surface, with loops and arches of hot gas, and darker areas — sunspots. That is because the fuzzy ball of hot gas idea misses out one critically important ingredient.

In medieval times it was widely believed that the objects in the heavens are perfect and unblemished — perfect spheres moving in perfectly circular orbits. The invention of the telescope in the 17th Century rather upset the apple cart. Galileo saw the Moon is covered with mountains and craters, and is very similar to the Earth, and then he and Scheiner saw spots on the Sun. The two scientists monitored sunspot for some time, seeing them come and go, and watching them move across the solar disc, carried by the Sun’s rotation. It was also clear that some sunspots are far larger than the Earth. However, that still left the big question as to what sunspots actually are.

In the late 18th and early 19th Century, William Herschel was probably the World’s leading astronomer. He and his sister Caroline discovered the planet Uranus, catalogued nebulae (cosmic gas and dust clouds), and made many other contributions to astronomy. One of these was to count sunspots and use them as an indicator of the Sun’s activity. They compared changes in solar activity with climate, wheat yields and other phenomena on Earth that are likely to be affected by the Sun. They laid the foundations for the solar activity monitoring programmes carried out in Canada and around the world today.

However, their idea for what sunspots might be was rather odd. They suggested the Sun is a big ball of rock, a sort of superplanet, surrounded by a layer of flaming clouds. Sunspots they suggested to be holes in these clouds, revealing the colder land beneath; the Sun might even be inhabited. What they did not take into account is if you take a big ball of rock and surround it with a ball of flame, that rock will receive heat from the flame.

The Second Law of Thermodynamics says that heat won’t flow from a cold object to a hotter one. So completely enclosed in a fiery envelope, the superplanet would eventually end up at the same temperature as the surrounding flame. The Herschels were wrong here, but they were right on many other aspects of astronomy they researched. Careful studies of sunspots have revealed what they are and also given us the missing ingredient that makes the Sun something much more interesting than a ball of hot gas — magnetic fields. There are magnetic fields in the Sun, which give us the loops, arches and what looks like a surface. When concentrated, they affect the transfer of heat from deeper inside, leading to cooler and darker patches on the surface – sunspots. These are still very hot, 3,000 C compared with their 6,000 C surroundings. Sunspots are huge, bowl-like depressions in the Sun’s surface, so the Herschels were at least on the right track with that.

You might be tempted to look at sunspots for yourself. However, unlike other astronomical objects, the Sun is a hazardous thing to observe. Staring at it with unaided eyes is very dangerous. Pointing binoculars or a telescope at it can be deadly. Unless you know exactly how to make such observations you will run a very serious risk of permanent eye damage or even blindness. Without expert guidance, … DON’T DO IT.

Jupiter rises in the late evening and is high in the southern sky before sunrise. Mars is fainter, lower and left of Jupiter. Venus and Saturn lie close together low in the southeast before sunrise. The Moon will be New on Jan. 9.

Ken Tapping is an astronomer with the National Research Council’s Dominion Radio Astrophysical Observatory, Penticton.


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