Distance from Sun to Earth:
1 AU = 1.5 x 108 km = 8.3 light-minutes
The Sun is a normal G2 star, one of more than 100 billion stars in our galaxy. The Sun is by far the largest object in the solar system. It contains more than 99.8% of the total mass of the Solar System (Jupiter contains most of the rest). It is often said that the Sun is an "ordinary" star. That's true in the sense that there are many others similar to it. But there are many more smaller stars than larger ones; the Sun is in the top 10% by mass. The median size of stars in our galaxy is probably less than half the mass of the Sun.
The outer layers of the Sun exhibit differential rotation: at the equator the surface rotates once every 25.4 days; near the poles it's as much as 36 days. This odd
behaviour is due to the fact that the Sun is not a solid body like the Earth. Similar effects are seen in the gas planets. The differential rotation extends considerably down into the interior of the Sun but the core of the Sun rotates as a solid body.
The surface of the Sun, called the photosphere, is at a temperature of about 5800 K. Sunspots are "cool" regions, only 3800 K (they look dark only by comparison with the surrounding regions). Sunspots can be very large, as much as 50,000 km in diameter. Sunspots are caused by complicated and not very well understood interactions with the Sun's magnetic field.
Recent data from the spacecraft Ulysses show that during the minimum of the solar cycle the solar wind emanating from the polar regions flows at nearly double the rate, 750
kilometres per second, that it does at lower latitudes. The composition of the solar wind also appears to differ in the polar regions. During the solar maximum, however, the solar wind moves at an intermediate speed.
The solar wind has large effects on the tails of comets and even has measurable effects on the trajectories of spacecraft. Spectacular loops and prominences are often visible on the Sun's limb (left).
Heat leaks out from the Sun's hot interior to its relatively cool photosphere by two mechanisms. Most of the interior of the Sun is stable, so the heat energy is carried out through the matter by photons (mainly X-rays). Moving at the speed of light, a photon could travel from the centre of the Sun to the photosphere in only about 2 seconds if there was nothing to stop it. But actually, a photon can travel only a few centimetres through the Sun's interior before it will be deflected or absorbed by an electron or atom. Thus, instead of travelling straight out from the Sun's interior, a photon will rattle around for thousands of years before it eventually finds its way out. This process is called radiative diffusion. Thus, the Sun's envelope serves as a very effective insulating blanket that lets the intense heat of the Sun's core leak out only very slowly. That's a good thing, because without this insulation, the radiation from the Sun's core would melt the Earth's surface in a very short time.
In the outer 30% of the Sun's radius, the envelope is literally boiling. Hot gases at the bottom become buoyant and rise to the top, causing an overturning motion called convection. The heat is carried from the bottom to the top of the convective layer by the motion of the rising hot gas. The top of the convective layer is the photosphere, where we can see the overturning motion of the convective cells. The gas and radiation beneath the Sun's surface are so hot that electrons are being knocked free from the hydrogen and helium atoms constantly, by atomic collisions and by photons. As a result, at any given time about one percent of the hydrogen atoms are separated into free electrons and positively charged ions (atoms lacking electrons), and so the gas becomes a very good conductor of electricity. We call such an ionized gas plasma. The physical behaviour of plasmas is a very interesting and complicated subject. One very important property of plasmas is that they can cause magnetic fields to increase when they flow. That certainly happens inside the Sun. There are two obvious kinds of motions that can increase the Sun's magnetic field. The first is the differential rotation seen at the Sun's surface: the gas at the equator rotates faster than that the gas at higher latitudes.
When the magnetic fields within the Sun's interior become strong enough, they become buoyant and tend to rise toward the photosphere. They eventually break through the photosphere, forming loops of magnetism with relatively cool sunspots at their footprints. Some sunspots are larger than the Earth. Galileo was the first to observe sunspots with a small telescope in 1610 (he must have put very dark glass in front of the telescope; otherwise he would have been blinded). Since then, people have been keeping records of the number of sunspots at any given time. The number varies with cyclically with time, reaching a solar maximum every 11 years and a solar minimum of almost no sunspots between maxima.
Violent activity occurs above the Sun's photosphere. It is caused both by solar convection and by instabilities in the solar magnetic field. The convection, which is a relatively steady rolling motion below the photosphere, turns into a violent splashing motion above the photosphere, just as relatively smooth waves in the open sea become violent breakers when they reach the shore. We can see the splashing motion above the Sun's photosphere as spicules. Moreover, the magnetic field often becomes unstable above the Sun's photosphere and erupts outward, causing solar flares, which can be seen with radio telescopes as well as with optical telescopes. The violent release of magnetic energy in sunspots and flares heats the very tenuous gas above the photosphere to temperatures of millions of degrees, creating the corona, which extends far beyond the optical disk of the Sun. We can see the relatively faint optical radiation from the corona during solar eclipses, when the Moon blocks the much greater optical light from the Sun's photosphere. A few solar radii above the photosphere, the Sun's gravity is no longer strong enough to hold in the hot gas of the corona, and the corona turns into the solar wind, which flows outward through the solar system. Moving at velocities of 400 - 500 km/s, the solar wind takes about four days to travel from the Sun to the Earth.
Disturbances in the corona resulting from solar flares propagate out through the solar wind as coronal mass ejections, which may reach the Earth a couple of days after the flare. Although the fraction of solar power in the corona and wind is relatively small (about 10-5) compared to the power radiated by the photosphere, these disturbances can have noticeable effects on Earth. In fact, a few days after the coronal mass ejection that you just saw in the movie above, the cloud of high energy particles reached the Earth's orbit and destroyed the electronics on at least one telecommunications satellite that cost hundreds of millions. Such events also cause high-energy particles to enter the Earth's atmosphere in rings surrounding the North and South Poles. (They are channelled into these rings by the Earth's magnetic field.) When these particles hit oxygen and nitrogen molecules in the Earth's atmosphere, people at northern latitudes can see the spectacular optical displays called the northern lights, or aurora borealis (or near the South Pole, the aurora australis).
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