Introduction to Astrophysics/Sun

The Sun is an ordinary star that was born out of a cloud of matter called the primordial nebula 4½ thousand million years ago (4.5 billion years), along with the eight known planets in our corner of space called the Solar System. The Sun is mainly composed of hydrogen, a much smaller amount of helium together with other chemical elements. It is a globe of intensely hot plasma, 1,391,785,000 km in diameter, more than a million times bigger than our Earth. It shines through an atomic process called nuclear-fusion: four bits of hydrogen join to four bits of helium; the Sun gives off heat and light, and loses solid material into space. This all happens every second and our Sun loses 4 million tonnes of matter during that brief time. The Sun's centre has a temperature of 15 million degrees centigrade, cooling to 6 thousand degrees at its gaseous surface. Our Sun is not motionless in space; in fact, it has two kinds of motion. One is a seemingly straight-line motion in the direction of the constellation Hercules at the rate of about 12 Km per second. But since the Sun is a part of the Milky Way system, and since our Milky Way galaxy rotates slowly around its own centre, the Sun also moves at the rate of 175 Km per second as part of the rotating Milky Way system.

Our Sun is one star in 100,000 million, which goes to make up our galaxy, the Milky Way. Some stars are small and dim while others are super-giants by comparison. Our Milky Way is shaped like two fried eggs back to back and is a noticeable spiral formation of stars. In terms of size, our galaxy is huge, at the speed of light of 300,000 km a second it takes 4.2 years to travel to Earth from our nearest star - Proxima Centauri. On this scale it takes a single pulse of light 100,000 years to travel across the length of our galaxy from end to end.

As the Sun ages, it gradually expands and cools; although; prior to that period, the Sun will "burn" helium or other even heavier elements as the core of the Sun reaches higher and higher temperatures and densities. As a result of the Sun's increased temperature life on Earth will cease. The extreme heat generated will be catastrophic for Earth: the oceans will boil away and life as we know it will end. Long before the Sun reaches the Red Giant phase, the surface of the Earth will literally be molten as the temperature of the Sun increases. It is estimated that the Sun's brilliancy will increase by 10% over the next 1.1 billion years or more, and in about 6.5 billion years, our aging star will have doubled its present luminosity.

Eight billion years from now, when the Sun does reach the Red Giant phase, the Sun's radius will extend beyond the present orbit of Venus, causing the total destruction of Earth.

Our own Sun will continue to shine for generations to come, until its supply of hydrogen fuel is used up, and then its central regions will shrink as different fusion reactions compensate. The Sun will then expand into a huge red-giant star eating up all of the planets from Mercury to Mars (Earth included), and vaporising the atmospheres of the remaining gas giant planets until these too are destroyed. Fortunately by this time our human race will have become technically advanced, so we will have set off on journeys to find safe havens on other planets among the stars - Star Trek style.

The sun’s brilliant surface layer is called the photosphere (which means the ‘sphere-of-light’). Incorrect observation of the Sun causes blindness because of the tremendous amount of visible and invisible light coming off of its surface.

Sunspots are appropriately named. They appear as spots on the disk of the Sun. A sunspot will have a very dark central region known as the umbra. It is often surrounded by a less dark halo known as the penumbra.

The umbra is dark because it is cooler (around 3,500°C) than the surrounding sunspace (around 5,500°C). They appear move across the Sun as the Sun spins on its axis. Because the Sun is gaseous in nature and behaves like a fluid it does not spin as a rigid body. A spot near the equator will take about 25 days to complete one rotation. A spot near a pole, if there were ever one there, will take over a month to make the trip. Collections of sunspot sketches over a period of several years reveal the 11-year cycle of sunspots. Over that period the numbers of spots goes from a maximum to a minimum and back.

There also appears to be a relationship with the size and number of sunspots and Earth’s weather. During the Maunder minimum 1645–1715 there were no observable sunspots and Britain plunged into a mini ice age. However in recent years 2003–2004 there have been large sunspot numbers with some unusually large sunspots. This has coincided with hot summers and is believed to be one of the causes for global warming.

In addition to heat and light, the Sun also generates solar wind, a stream of ionized particles that radiates outward through the solar system at high speeds. One of the effects of solar wind is that it forces the tails of comets to point away from the Sun.

The solar wind also interacts with the Earth's magnetic field, causing the aurora and other phenomena. Solar flares — eruptions — of hydrogen gas on the surface of the Sun can also cause disturbances in the Earth's magnetic field.

The chromosphere is above the photosphere. Solar energy passes through this region on its way out from the centre of the Sun.

Faculae and flares arise in the chromosphere. Faculae are bright luminous hydrogen clouds, which form above regions where sunspots are about to form. Flares are bright filaments of hot gas emerging from sunspot regions.

Sunspots are dark depressions on the photosphere with a typical temperature of 4,000°C. In 1998, scientists observed for the first time solar flares producing seismic waves in the Sun's interior that resemble those created by earthquakes. They observed a flare-generated solar quake equivalent to an 11.3 magnitude earthquake. It contained about 40,000 times the energy released in the great 1906 San Francisco earthquake.

The corona is the outer part of the Sun's atmosphere. It is in this region that prominences appear. Prominences are immense clouds of glowing gas that erupt from the upper chromosphere. The outer region of the corona stretches far into space and consists of particles travelling slowly away from the Sun. The corona can only be seen during total solar eclipses.

Eclipses occur when the Sun, Earth and Moon line up. They are rare because the Moon usually passes above or below the imaginary line connecting Earth and the Sun. In a solar eclipse the Moon passes directly in front of the Sun. This can only happen when the phase of the Moon is "new." That occurs because for Earth-based observers, the far side of the Moon is illuminated while the side facing Earth is in darkness. The Moon like any sphere, casts a shadow. A solar eclipse occurs when that shadow sweeps across Earth. The black cone is called the umbra. An observer anywhere inside that region is completely in shade. None of the Sun is visible from there.

Surrounding the umbra is the penumbra. An observer there will see some, but not all, of the Sun. Outside of these regions all of the Sun is visible. Note that the tip of the umbra barely touches Earth. At the current time the position of the Moon relative to the Sun is such that the Moon, which is 1/400 as large as the Sun, is 400 times closer! This means that the two objects appear to be the same size in the sky. Only observers at the tip of the umbral cone will see a total solar eclipse. A large number of observers across the globe will see a partial solar eclipse if they are in the penumbra. An annular eclipse is a special partial solar eclipse. Because the Moon's orbit around Earth is an ellipse, not a circle, the Moon's distance from Earth varies. When the Moon is far from Earth it appears slightly smaller in the sky. (Earth's orbit around the Sun is also an ellipse, and during January, Earth is at its closest point to the Sun. The Sun's size is slightly larger than during the rest of the year.) With a "small" Moon and a "large" Sun the Moon will not completely block out the Sun. The umbra does not touch Earth. An observer would have to be above the surface of Earth to see a total eclipse. For individuals in just the right location, the Sun appears as a ring (annulus) around the silhouetted Moon.

In a lunar eclipse the Moon moves into Earth's shadow. They can only occur when the moon is "full." Observers on the night side of Earth see the Moon take on a reddish hue as it moves into Earth's umbra. If the entire disk of the Moon moves into the umbra it is a total lunar eclipse. If only a portion does, then it is a partial lunar eclipse.

Penumbral lunar eclipses are very difficult to detect because the Moon dims only slightly while moving through that region. Lunar eclipses are more common than solar eclipses. Total eclipses of the Sun and Moon are partial before and after totality.

Moving away from the sun at the centre of our solar system we arrive at the closest planet, Mercury, while there are eight known planets altogether including Earth. A planet is a sizeable body that orbits around a star. A planet also has no light of its own and therefore is only visible by reflecting starlight, in our case sunlight off its surface.