The Sun
The Sun

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The Sun

1.1 The Sun at visible wavelengths

The Sun is seen as a blindingly bright, yellow object in the sky. The part of the Sun that you normally see is called the photosphere (meaning 'sphere of light'); this is best thought of as the 'surface' of the Sun, although it is very different from the surface of a planet such as Earth. Its diameter is about 1.4 million kilometres, making the Sun's volume roughly one million times that of the Earth. The photosphere is not solid. Rather, it is a thin layer of hot gaseous material, about 500 kilometres deep, with an average temperature of about 5500 °C (degrees Celsius).

Detailed studies of the body of the Sun usually require special equipment. However, the natural phenomenon known as a total eclipse of the Sun provides an opportunity to gain further insight into the nature of the Sun (see Figure 1 below). A total eclipse happens when the Moon passes in front of the Sun and blocks out the bright light from the photosphere.

When the Moon just eclipses the bright photosphere, it is often possible to see part of a narrow, pink-coloured ring that encircles the Sun. This is the chromosphere (meaning 'sphere of colour'), the lower or 'inner' part of the Sun's atmosphere. It is actually another layer of gaseous material, a few thousand kilometres thick, that sits on top of the photosphere. The lower parts of the chromosphere are cooler than the photosphere, while the higher parts are considerably hotter, but the chromospheric material is so thin that it emits relatively little light, and is therefore unseen under normal conditions.

Figure 1
Figure 1 A total eclipse of the Sun, revealing the outer part of the Sun's atmosphere, the corona, and the inner part, the chromosphere, which can just be seen as a reddish tinge on the upper right limb.

As a total solar eclipse proceeds, a third part of the Sun is seen - the corona (meaning 'crown'). This is the extremely tenuous (i.e. thin) upper atmosphere of the Sun that extends out to several times the Sun's photospheric radius. The corona seems to be composed of streamers or plumes, but its shape changes from eclipse to eclipse, although it will not usually show any changes during the few minutes of totality that characterise a typical total eclipse. The corona is very hot (temperatures of several million degrees Celsius are not unusual) but it is so thin that its pearly white light is very faint compared with the light from the photosphere.

Answering in-text questions

Throughout this course there are in-text questions marked by a bullet point, which are immediately followed by their answers. To gain maximum benefit from these questions you should think of your own response before clicking to reveal the answer. You will probably find it helpful to write down your answer, in note form at least, before reading the answer in the text.

Question 1

  • The corona may be faint, but it does glow. Why are we not normally aware of the Sun's corona?

Answer

The bright light from the Sun's photosphere is scattered by the Earth's atmosphere. This makes the sky blue and generally rather bright. As a result, we cannot observe the much fainter light from the corona (rather as the light from a dim torch is unnoticeable on a bright sunny day).

Sometimes in eclipses observers also see prominences - great spurts of hot material at the edge of the Sun, extending outwards from the solar surface for many thousands of kilometres. Prominences and the changing shape of the corona indicate that the Sun is an active body, not just a quietly glowing source of light. There is further evidence of this in the images that you will look at shortly. This will introduce you to other features of the visible Sun, including the seething pattern of granules seen all across the photosphere, and the relatively cool sunspots that appear as small dark patches on the photosphere. Individual granules come and go in a few minutes, often to be replaced by other granules. Sunspots are larger and longer-lived, typically surviving for a week or so, and sometimes for many weeks. The longer-lasting sunspots can be photographed repeatedly as they cross the face of the Sun. They can even be used to investigate the rate at which the Sun rotates.

Activity 1 Viewing images

0 hours 15 minutes

Briefly examine the images below. They are accompanied by a paragraph beneath the image, which provides further detail on the image. Read the descriptions carefully, paying particular attention to any reference to sunspots.

Figure 2
Figure 2 A sunspot

Click below to view a larger version of this image.

https://www.open.edu/ openlearn/ ocw/ mod/ resource/ view.php?id=26564 [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)]

A false colour picture of a sunspot, taken with the National Solar Observatory's Vacuum Tower Telescope at the Sacramento Peak Observatory. The central dark area of the spot is called the umbra and the surrounding lighter region is called the penumbra. Solar granulation is clearly visible around the spot. The spot is about 25 000 kilometres in diameter; the smallest features seen in the picture are on a scale of about 100 kilometres. The umbra represents a depression in the solar surface, where the temperature is about 1600 degrees less than in the photosphere; in the penumbra the temperature is about 500 degrees below that of the surrounding photosphere. Well developed sunspots of the kind shown in the picture live for a few weeks, though some (particularly the larger ones) can persist for many weeks, or even a few months.

Figure 3
Figure 3 Movements below a sunspot

Click below to view a larger version of this image.

https://www.open.edu/ openlearn/ ocw/ mod/ resource/ view.php?id=26565

It has long been known that, in the penumbra of a sunspot, there is a generally outward flow of material, away from the centre of the spot. More recent research, based on the surface effects of sound waves travelling through the solar interior. has revealed the movement of material directly below a sunspot. This image shows a complicated flow of material that includes inward motion. "We discovered that the out-flowing material was just a surface feature," said one of the researchers. "If you can look a bit deeper, you find material rushing inward, like a planet-sized whirlpool or hurricane. This inflow pulls the magnetic fields together." The intense magnetic field below a sunspot strangles the normal up-flow of energy fiom the hot solar interior. As a result a sunspot is cooler and therefore darker than its surroundings.

Figure 4
Figure 4 A large sunspot group

Click below to view a larger version of this image.

https://www.open.edu/ openlearn/ ocw/ mod/ resource/ view.php?id=26566

Sunspots often appear in pairs or groups. The members of a pair will typically have opposite magnetic polarity (one having north magnetic polarity, the other south). In more complex groups the magnetic field may also be arranged in a more complicated way. This picture, taken in September 2000 shows a large complex sunspot group covering 2,140 millionths of the visible solar surface, an area about a dozen times larger than the entire surface of the Earth.

Figure 5
Figure 5 Sunspots crossing the sun

Click below to view a larger version of this image.

https://www.open.edu/ openlearn/ ocw/ mod/ resource/ view.php?id=26567

This sequence of images, each individually dated and timed, shows a number of sunspots crossing the face of the Sun. The observed motion is partly the result of the Earth's orbital motion around the Sun, but is mainly a consequence of the Sun's own rotation about its axis. Once the effect of the Earth's motion has been taken into account, such image sequences make it possible to measure the rate at which the Sun rotates. The measurements reveal that the Sun does not rotate as a solid body. Rather the time taken for any pan of the photosphere to make a complete rotation depends on its distance from the equator. At the equator, the Sun takes just under 26 days to complete a rotation. Near the poles, the rotation period is more like 36 days.

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