Chapter 3: The Sun
The star that dominates our daytime sky and provides the heat and light to support life on Earth is, of course, the Sun. In astronomical terms it is very close, 150 million kilometers (93 million miles), so we can study its properties in great detail. Light from the Sun takes just over eight minutes to reach the Earth. For comparison, light from the next nearest star takes four years to reach us.
The Sun is composed primarily of hydrogen and helium gas. The Sun generates its energy by nuclear reactions at its core. the energy generated at the core makes its way to the surface of the Sun and is radiated away into space. the Sun is very dynamic and the gas is always in motion; the surface resembles a stormy boiling liquid.
The surface of the Sun has a lot of structure and has different layers that constitute its atmosphere. A thin surface layer, called the photosphere, is at a temperature of 6,000 degrees Celsius (11,000 Fahrenheit). Most of the light from the Sun is radiated away from the photosphere. Above the photosphere is the chromosphere, which is hotter and is transparent at visible wavelengths. The outermost layer of the Sun is called the corona. The corona is very rarefied but very hot, with a temperature of more than a million degrees. By observing the Sun in different parts of the spectrum, we can get information about the different layers in its atmosphere to better understand the Sun as a whole.
Figure 6: The Sun - visible image. In visible light, the Sun appears uniformly bright, but touch the Sun at three, eight and nine o'clock on its disc and you discover small dark regions called sunspots. These cooler regions mark the locations of strong magnetic fields. The Sun is a ball of ionized gas, and the motion of the gas generates electric currents and magnetic fields. The number of sunspots tells you how active the sun is. more sunspots mean more magnetic activity, which increases the chance of solar storms.
Figure 7: The Sun- radio image. Electrons caught up in the magnetic fields of the sunspots spiral along the invisible field lines and generate radio waves. The radio image therefore maps out the magnetic fields around the sunspots. these are irregular outlines and the solid areas are radio emission, which correspond with the dark sunspots seen in the previous visible image.
Figure 8: The Sun - ultraviolet image. Ultraviolet light from the Sun originates from the hot chromosphere and corona, the outermost layer of the Sun (sand-like texture). this gas is heated by the strong magnetic fields generated around the sunspots. In ultraviolet light, the Sun appears dark except for the most active areas, the hotspots associated with the magnetic fields. As you touch the ultraviolet image along the edge of the disk at two, four and ten o'clock you can feel the gases following the curved magnetic fields (loops) rising above the disc and into the corona.
Figure 9: The Sun - X-ray image. The X-ray image shows us the gas regions which extend into the hot corona. In X-ray light, the disk of the Sun is dark, but you can study the sand-textured glow of the corona extending high above the Sun's surface. The irregular patterns in the corona are produced by gas heated by the magnetic fields that also create the sunspots. When these magnetic field lines get bunched and twisted they can snap, releasing huge amounts of energy and hot gas into space. This is called a coronal mass ejection. If a coronal mass ejection happens to be aimed at Earth, it can damage the sensitive electronics and detectors on board satellites and space telescopes.
The Sun is a relatively long-lived and stable star. Both these characteristics make it an ideal star for life-bearing planets to orbit. Many stars in the galaxy are much less stable and lead short, dramatic lives. Let's now pay a visit to one of the greatest superstars in our galaxy!