Our nearest star, of course, is the sun. And no one can dispute its enormous power. Even though it is some 150 million kilometres away (149 597 870.691 kilometres, to be precise) it can still burn your skin. Good job that only one-billionth of the total energy produced by the sun reaches Earth.
All this power comes from a process we call “fusion”, the combining of elements to make heavier elements. In the case of any star the process is at the very heart of the creation of all matter.
A star is just like an engine. The fuel is hydrogen and the exhaust is luminosity in the form of enormous amounts of heat, electromagnetic radiation (including X-rays, gamma rays and radio waves, infrared and ultraviolet light and, of course visible light), cosmic rays (mainly hydrogen protons and helium alpha particles), and neutrinos. The waste product, if you like, is the helium left behind.
The numbers are terrifying. The sun converts 700 million tonnes of hydrogen into 695 million tonnes of helium per second. In this process the sun loses about 4.4 million tonnes of mass per second, ejected as energy (heat and light). This is equivalent to about 383 billion billion megawatts (383 x 1018) of power in terms of the sun’s luminosity, which more precisely defined by the International Astronomical Union as 3.839 x 1026 Watts. In one second it releases more energy than has ever been used in the entire period of man’s civilisation. This loss of mass affects the sun’s gravitational pull—and the reason why Earth is drifting away from the sun at about 1.5 centimetres per year. Indeed, every planet drifts away from its parent star. In the case of the solar system this is no different.
The sun makes up 99.8% of the mass of the entire solar system. The gas giants account for over 90% of the rest. In an imaginary sphere encompassing Neptune’s orbit the volume occupied by the sun, all the planets and their moons, would take up little more than one trillionth of the enclosed space. Outer space really is relatively empty!
The Solar System
The solar system is approximately 26 000 light years from the centre of our galaxy—the Milky Way. As the planets orbit the sun, so the sun orbits the galactic centre of the Milky Way, and it does so at a speed of about 230 kilometres a second, taking its planets and asteroids along with it.
For each orbit the sun travels a distance of 150 000 light years, each taking between 225 million and 250 million years each to complete. Since its birth the sun has completed about 20 orbits. This is the so-called cosmic year (also sometimes referred to as the galactic year). Astronomers have been unable to determine more accurate figures. But what they do know is that the sun does not orbit on a level plane. Instead it oscillates, up and down as if on a roller coaster. This means the solar system passes through the plane of the Milky Way about once every 32 million years. Intriguingly this correlates with mass extinctions on Earth in the past, about once every 25 million to 32 million years.
Though the earth orbits the sun, for the purposes of astronomy we assume our planet is the centre of the universe, and therefore stationery and it is the sun that is moving. It takes four minutes longer for the Sun to return to the same spot in the sky each day – a solar day. Its apparent path across the sky is called the ecliptic. And this crosses the celestial equator at two nodes called the spring (or vernal) equinox and the autumnal equinox: nodes at which, as the name suggests, both day and night are of equal length.
The solar system is like a disc comprising the eight planets orbiting the sun – aside from Pluto, which maybe an escaped moon of Neptune, and of course the asteroids and other matter. From Earth then, it follows that wherever you search for any of the planets you will find them along and close to the path of the ecliptic. During the daytime you can trace the path made by the sun across our sky and it is along this path that at night you will find the planets.
Interestingly the constellations of the zodiac are also arranged along the ecliptic. And this brings us on to the question as to why the tropics are so named: the Tropic of Cancer and the Tropic of Capricorn. Their latitudes correspond to the northern most and southern most declinations of the sun respectively. In the Northern Hemisphere the summer solstice occurs around June 21st each year when the sun is directly overhead at the Tropic of Cancer (at 23°27′ north of the equator). By the same token the winter solstice occurs around December 21st each year when the sun is directly overhead the Tropic of Capricorn in the southern hemisphere (at approximately 23°27′ south of the terrestrial Equator).
Today, however, the sun at solstice appears in neither of these constellations. It did 24 000 years ago! Currently at the northern summer solstice the sun actually appears in the constellation Gemini. Similarly, at the winter solstice (Northern Hemisphere) the sun lies within the boundaries of the constellation Sagittarius.
Because of the gradual change in the direction of the Earth’s axis of rotation, the sun will not reappear in the constellations of Cancer and Capricorn for another 24 000 years.
The sun revolves anti-clockwise, as do most of the planets. Its revolutionary period is a bit odd because it rotates once every 25 days at the equator, but once every 36 days at the polar regions. This is because the surface of the sun is a liquid and rotation rates slow down towards the poles.
Back to Top
By Nigel Benetton, science fiction author of Red Moon Burning and The Wild Sands of Rotar.
Last updated: Thursday, 25th February 2021