Astronomy 100


Lectures Table of Contents Astro 100

The Sun -- Our Star

The Sun's Atmosphere

Terms to Know

solar wind

The Sun: Our Star

The Sun is an ordinary star; there are many millions just like it in our Milky Way Galaxy alone. It shines the same way other stars of its type do, and it has similar color, size, brightness, and composition.

The Sun provides virtually all of the energy necessary for life on Earth. All of our food depends on it. Weather depends on it. Without its heat and light, the temperature on the Earth's surface would be close to absolute zero. Even coal and oil, extracted from deep within the Earth, were once plant forms that depended on the Sun for their energy.

What can we observe about the Sun just from looking at its surface?

  • The Sun's surface, with a temperature of 6000 K, is an almost perfect black body. The wavelength of peak emission (from Wien's Law) is peak = 2.9 / T = 5 x 10-4 mm = 5000 Angstroms, close to the peak of the response curve of the human eye.
  • The bright part of the Sun you normally see is called the photosphere .
  • During total solar eclipses, when the photosphere is blocked out by the Moon, the bright red chromosphere can be seen just outside of the photosphere, with the ghostly white corona extending out far from the photosphere. Although the corona is much fainter than the photosphere, it is much, much hotter: about 1 million degrees K! (How does this show that the corona is NOT a perfect black body?)
  • The photosphere is often marked with sunspots that come and go over periods of days or weeks. Their motion across the face of the Sun indicates that the Sun rotates once every 25 days at the Sun's equator, but it takes longer -- closer to 30 days -- at high latitudes, close to the solar north and south poles. This differential rotation shows that the Sun's surface is fluid, not solid; it's a BIG BALL OF GAS!
  • Other details of the photosphere change daily as well: the surface is broken up into tiny granules that form and disappear like the surface of boiling water. But the total rate of energy radiated by the Sun has remained constant to within 5% or so over the entire course of human history.
  • During an eclipse, or viewed through a special filter, the chromosphere often shows prominences and flares , which are huge mountains of glowing solar material arching up from the Sun's surface. The "feet" of prominences and flares are usually associated with sunspots, which often are seen in pairs.
  • The level of sunspot activity obeys an 11-year cycle from maximum activity to minimum and back to maximum. During times of peak activity, there are more prominences as well, and the corona is larger and more symmetrical. Electrical interference in the Earth's atmosphere is also higher, with increased aurorae (northern and southern lights) and disruption of satellite communication; this is due to the solar wind, a river of particles (protons, electrons,...) that constantly stream away from the Sun. In 1998 we are heading towards maximum, though we can't say precisely when the peak will be.
  • Absorption lines in the spectrum of the Sun indicate that it is composed of many elements found commonly on Earth, including hydrogen, carbon, oxygen, nitrogen, sodium, iron, and many others. It also includes the element helium (from "helios," Greek for Sun), which was discovered on the Sun before it was identified on Earth.
  • The mass of the Sun (measured by observing the planets orbiting it) is 2 x 1033 grams, and its radius is 7 x 1010cm (easy to determine once you know its distance and angular size).

    Solar Activity


    Terms to Know

    11-year solar cycle

    What Makes the Sun's Appearance Change?

    Why do sunspots, granules, flares, prominences, and the 11-year solar cycle occur?

    Three reasons:

    1. The Sun rotates (see above);
    2. The Sun has a strong magnetic field that (probably) gets wound up by the rotation; and
    3. There is a tremendous source of energy inside the Sun's interior, and that energy diffuses from the center outwards until it reaches the surface, and then radiates out into the solar system. What could that source of energy be?

    The Sun's magnetic field is just like a bar magnet, with a north end and a south end. But it completely flips upside down every 11 years! So really the Sun's activity cycle is 22 years long -- because that's how long it takes to go from north up to south up and back to north up again.

    Sunspots are also probably magnetic storms related to the internal magnetic field and the energy flowing out from the Sun's center. They are slightly cooler than the surrounding surface, which makes them look darker.

    Lectures Table of Contents Astro 100

Houjun Mo Astronomy 100