If you have a stain

The sunspots were probably discovered by Chinese astronomers as early as the first millennium AD. Sunspots are not dark, but only places where the temperature is several hundred degrees lower than that of the hot environment 

Written by: Dr. Netzah Farbiash, Young Galileo

How many of you are probably familiar with the trivia question "How many stars are there in the solar system?" And the correct answer: in the solar system there is only one star, and it is the sun itself (the others are planets). Because the sun is the only star in the solar system, it is also the closest star to us, to the earth. Thanks to its relative proximity, it is possible to make observations on it that are not possible on more distant stars, and besides, thanks to the fact that the Sun is a fairly ordinary star in the universe, it is possible to learn from the study of it on other, more distant stars as well.

First of all, our desire to study phenomena related to the sun stems from its effect on the earth and the space between it and the sun, such as climate changes, magnetic storms and the sunspot cycle.

What is the "language of the sun"?

Before we explain about sunspots, let's mention a few facts: the sun is a ball of gas (and plasma), therefore, unlike a solid body (a planet for example), the definitions of the "rim of the sun" and its atmosphere depend on the wavelength at which it is viewed. One can think of the "rim of the sun" as the layer of the photosphere, which can be seen in light in the visible range.

Think of the orange ball we see at sunset - its boundaries define what we will call here the "rim of the sun", although the gases that make up the sun extend far beyond that. The thickness of the photosphere is about one hundred kilometers - very thin compared to the inner part of the sun (the radius of the sun is 695,990 kilometers), and the prevailing temperature in it is about 6,000 degrees (at such values ​​it is no longer particularly important if you use Kelvin or Celsius units).

Are the spots really dark?

When you look at the sun (with the appropriate means only! It is dangerous to look at the sun with the naked eye or using "homemade" filters!), you can sometimes notice dark spots on its surface. The dark color of the spots is due to the fact that the temperature prevailing in them is several hundred degrees lower than that of their surroundings. In fact, as you can see in the photographs, the spots look completely dark. However, this is an optical illusion: the spot appears dark due to the large contrast between the intensity of the light emitted from the sunspot and the area surrounding it, but in reality a sunspot is brighter than the moon.

Although the dimensions of the spots in relation to the sun are not large, their diameter may reach about thirty thousand kilometers (for comparison, the diameter of the entire earth is about 12,756 kilometers); This fact made it possible to identify them even without sophisticated means of observation.

Who discovered the spots?

Sunspots were probably discovered by Chinese astronomers as early as the first millennium AD, but only after the invention of the telescope in 1609 did Galileo Galilei recognize that sunspots are part of the sun itself. Since the invention of the telescope, sunspots have been observed and studied more thoroughly.

Sunspots survive on the surface of the sun for several days. The largest among them may even survive a few weeks. Most often, the spots are seen in sets of a pair of spots or more. In 1843, the German astronomer Heinrich Schwabe found in his observations that the total number and average strength of sunspots change in a cycle of about 11 years. This phenomenon has since been observed repeatedly and has been nicknamed the "sunspot cycle" or the "butterfly diagram". There are various hypotheses regarding the cause of the sunspot cycle, but the cause is not entirely clear. However, the number of sunspots and the intensity of the sun's activity have effects on Earth.

The American astronomer George Ellery discovered in 1908 that there is a connection between sunspots and magnetic fields. In fact, even in Galileo's early drawings, from the beginning of the 17th century, it is possible to notice that the sunspots have a structure in which a dark part in the center of the spot, called the umbra, and a lighter part surrounding it, called the penumbra. Later photographs, taken with advanced telescopes, show that the penumbra is a structure that appears to be made of fibers that point in different directions. The presence of this structure and also the direction of the fibers suggest the existence of magnetic field lines, in accordance with Ellery's discovery.

Today it is clear to scientists that the strength and direction of magnetic fields in the sun are responsible for the existence of sunspots, and not only them. Other important phenomena that occur in the sun, such as the heating of the corona, the acceleration of the solar wind and solar storms, are also related to the magnetic fields.

The writer is Vice President of Science at Caruso Science Park

The article was published in the Galileo Young Monthly for curious children. For a gift digital sheet Click