Q: How does the Sun compare with other stars in the universe?
The Sun is an average-sized star. Astronomers know this because they are able to determine distances to other stars, and then using that information, determine their sizes based on how bright they seem. Distances are measured by parallax (depth perception). As the Earth moves 6 months around the Sun, you can view a group of stars from two different vantage points separated by nearly 200 million miles. The nearer stars will seem to shift their position slightly compared to the farther ones (similar to apparent shifts in object positions when you alternately close one eye and then the other). It should be noted that photographs of stars do not depict their relative sizes. The ones that appear biggest in the photographs are the brightest ones that have overexposed the film, causing chemicals in the film to spread out from what started as a pinpoint. All stars except the Sun are too far away to see as spheres with even the most powerful telescope.
The Sun is a medium-temperature star. Astronomers know this because they are able to tell the surface temperatures of other stars by looking for the brightest color in the radiated light. Blue-white stars are the hottest, red stars are the coolest, and the Sun is yellow (medium temperature).
Q: What makes stars shine?
All stars radiate the energy released by nuclear fusion. A star is made of so much gas that gravity squeezes the hydrogen together at the core. The tremendous heat and pressure first strips away the electrons, then forces the protons close enough together that they can't repel. At close enough range, the nuclear force takes over and allows the protons to bind together. Nobody has ever seen the interior of the Sun, but there is ample spectroscopic evidence from the outside that fusion is going on deep inside.
Most of the Sun's energy comes from missing mass that has been converted to energy in the fusion process. A deuteron (one proton combined with one neutron) and a proton have masses which can be measured in the laboratory. When they fuse together, they form the nucleus of a helium atom (isotope of atomic weight 3 amu). But the mass of the helium nucleus is slightly smaller than the masses of the deuteron and proton put together. The missing mass is the m in Einstein's Equation E=mc2; E is energy released and c is the speed of light. The speed of light is very large so even a tiny amount of mass can be converted into a large amount of energy. People are trying to develop fusion reactors to allow us to duplicate this process on Earth. Once perfected, fusion reactors will allow us to convert the hydrogen in ordinary water into helium, releasing tremendous amounts of clean energy from a limitless fuel supply.
Q: Which kind of eclipse is which, and how often do they occur?
Eclipses of each kind occur about twice each year, when the Sun, Earth and moon are in a straight line. This doesn't happen every month at new moon and full moon, because the moon's orbit is slightly tilted. But the motion of the moon is regular and predictable, so we know where and when to look for eclipses.
Solar Eclipse: when the moon moves between the Sun and the Earth, and casts its shadow on the Earth. The shadow is tapered, is only a few miles wide at the Earth's surface, and moves quickly because the moon moves quickly, so you have to be lucky enough to live in exactly the right place at the right time to see a solar eclipse. The moon blocks the photosphere, making the chromosphere and corona visible for a short time. The daylight disappears and stars become visible!
Lunar Eclipse: when the moon moves to the side of Earth opposite the Sun, and enters the Earth's shadow, the moon may disappear from view for a short time or may remain visible but reddish in color. The Earth's shadow is large enough to cover the moon, but the red part of sunlight can refract through the Earth's atmosphere to slightly illuminate the moon. Lunar eclipses are visible to more people at once than solar eclipses: you just have to be somewhere on the night side of Earth with clear weather when the moon moves into the shadow.
Q: How did the Sun and Solar System form?
They formed out of a very large interstellar cloud of gas and dust called a nebula. We can see nebulae with telescopes from Earth. These clouds are able to gravitationally collapse (albeit very slowly; we can only see the collapses at various stages as we look around in space). Even though the hydrogen atoms in space are very far apart, the force of gravity has infinite range and will pull the atoms toward each other as long as they start out moving slowly. As the cloud collapses, its initial rotation will cause it to spin into a flattened disk shape (think of how pizza dough is spun by the chef to flatten it out!). The fact that the Sun's rotation and the planets' orbits are all turning in the same direction supports this "Nebula Hypothesis". The strongest gravity is at the center of the nebula, where the star forms. Weaker gravitational collapsing gathers material together for planets. Heavier elements settle far from the heat source.
Q: How will the Sun end its life?
Things happen in space much more slowly than one human lifespan. By looking at many stars, we can see populations with certain characteristics. Some stars are red giants, some are white dwarfs, and some are surrounded by expanding clouds of glowing gas. Astronomers have identified these as stages in the life of a medium star (like the Sun). Evidence suggests that the Sun will deplete its hydrogen supply sufficiently, about 5 billion years from now, to cause a change in its state. The helium core will collapse when it grows large enough, because the heat source surrounds it rather than being at its center. When the helium core of the Sun collapses, the gravitational energy that is released will push away the hydrogen shell. The result will be a very large but cooler star called a Red Giant. The Sun's photosphere will extend beyond the orbit of Mars, and Earth will be incinerated. Eventually the hydrogen shell will be blown away, leaving the helium core behind (white hot and relatively small).
Q: Will other stars evolve differently?
The universe is mostly hydrogen, with lesser amounts of helium. Stars that begin their lives with more hydrogen than our Sun will live fast and die hard. Evidence suggests that the intense heating at the core of a very large star will produce rapid fusion that grows the core to collapsible size much quicker. Not only that, but there might be so much helium accumulated that it will collapse and heat to the temperature needed for a different fusion reaction to commence. Astronomers believe that the elements of the periodic table that are heavier than helium but lighter than iron are all produced in the cores of large stars. These large stars develop onion skin layers of elements formed by fusion, until iron begins to form by fusion. Iron fusion absorbs more energy than it releases, so this "puts out the fire" and causes catastrophic implosion followed by explosion. Astronomers have seen distant stars suddenly flare up to thousands of times their original brightness, then fade away. Such supernova explosions have been recorded in Earth history (stars suddenly so bright they can be seen during the day).
Astronomers believe that the elements on Earth and in the materials that form our bodies were all once inside the core of a giant star. That star exploded long ago, and the Solar System formed later out of the remnants. So we are all "star stuff!"