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Sun & stars

The Sun (Latin: Sol) is the star at the center of the Solar System. The Earth and other matter (including other planets, asteroids, meteoroids, comets and dust) orbit the Sun, which by itself accounts for about 99.8% of the solar system's mass. Energy from the Sun?in the form of sunlight?supports almost all life on Earth via photosynthesis, and drives the Earth's climate and weather.

The Sun is composed of hydrogen (about 74% of its mass, or 92% of its volume), helium (about 25% of mass, 7% of volume), and trace quantities of other elements. The Sun has a spectral class of G2V. G2 implies that it has a surface temperature of approximately 5,500 K (or approximately 9,600 degrees Fahrenheit / 5,315 Celsius), giving it a white color which, because of atmospheric scattering, appears yellow as seen from the surface of the Earth. This is a subtractive effect, as the preferential scattering of blue photons (causing the sky color) removes enough blue light to leave a residual reddishness that is perceived as yellow. (If low enough in the sky, the Sun appears orange or red, due to this scattering.)

Its spectrum contains lines of ionized and neutral metals as well as very weak hydrogen lines. The V (Roman five) suffix indicates that the Sun, like most stars, is a main sequence star. This means that it generates its energy by nuclear fusion of hydrogen nuclei into helium and is in a state of hydrostatic balance, neither contracting nor expanding over time. There are more than 100 million G2 class stars in our galaxy. Because of logarithmic size distribution, the Sun is actually brighter than 85% of the stars in the galaxy, most of which are red dwarfs.

The Sun orbits the center of the Milky Way galaxy at a distance of approximately 26,000 light-years from the galactic center, completing one revolution in about 225?250 million years. The orbital speed is 217 km/s, equivalent to one light-year every 1,400 years, and one AU every 8 days.

The Sun (Latin: Sol) is the star at the center of the Solar System. The Earth and other matter (including other planets, asteroids, meteoroids, comets and dust) orbit the Sun

The Sun is composed of hydrogen (about 74% of its mass, or 92% of its volume), helium (about 25% of mass, 7% of volume), and trace quantities of other elements. The Sun has a spectral class of G2V. G2 implies that it has a surface temperature of approximately 5,500 K (or approximately 9,600 degrees Fahrenheit / 5,315 Celsius), giving it a white color which, because of atmospheric scattering, appears yellow as seen from the surface of the Earth. This is a subtractive effect, as the preferential scattering of blue photons (causing the sky color) removes enough blue light to leave a residual reddishness that is perceived as yellow. (If low enough in the sky, the Sun appears orange or red, due to this scattering.)

, which by itself accounts for about 99.8% of the solar system's mass. Energy from the Sun?in the form of sunlight?supports almost all life on Earth via photosynthesis, and drives the Earth's climate and weather.

Its spectrum contains lines of ionized and neutral metals as well as very weak hydrogen lines. The V (Roman five) suffix indicates that the Sun, like most stars, is a main sequence star. This means that it generates its energy by nuclear fusion of hydrogen nuclei into helium and is in a state of hydrostatic balance, neither contracting nor expanding over time. There are more than 100 million G2 class stars in our galaxy. Because of logarithmic size distribution, the Sun is actually brighter than 85% of the stars in the galaxy, most of which are red dwarfs.

Planets--our neighbours

Mercury is only visible at twilight (in the west) and sunrise (in the east) because its orbit is so close to the sun. Just how close is that, though? The sun is "only" 36 million miles away. Mercury's temperatures change radically during the day: 800° to -200°F between day and night. Although small, Mercury is the densest planet after Earth.

Venus, second-closest planet to the sun, has the highest planetary temperature: 900°F/475°C. It also has the longest rotational period of any of the planets, the equivalent of 243 Earth days. You would need more than an umbrella if you got caught in a storm on Venus: it rains sulfuric acid there! Venus is the third brightest object in the sky, followed by Jupiter.

Mars has a day length just slightly longer than Earth's, but its average surface temperature is a chilly -80°F. Do this experiment to demonstrate one of theories about why Mars has such a red surface. Put a layer of sand in the bottom of a ceramic baking dish. Cut some steel wool into 2 cm (1 inch) pieces and mix them with the sand, then cover the mixture with water. (You could also sprinkle iron filings on top of the sand instead of using steel wool.) You might need to add more water each day as some evaporates. Every day, check on the experiment and record how the surface is changing. How long did it take before the sand turned red like the surface of Mars? Scientists believe Mars' color occurs because of iron oxide (rust) in the soil.

Jupiter, named after the king of the Roman gods, has a diameter more than 11 times the size of Earth's. One day on Jupiter is only 10 hours, though, since the planet rotates very quickly. One of Jupiter's moons, Io, has the most violent volcanic eruptions known in our solar system. You can actually see Io and three of Jupiter's other moons (Europa, Ganymede, and Callisto) with just binoculars. Galileo was the first to see these moons, using a telescope in the 1600s.

Saturn is one of the "gas planets," along with Jupiter, Uranus, and Neptune. Like Jupiter, Saturn probably has a rocky core and outer envelope composed of liquid metallic hydrogen. However, Saturn is notable as the planet with the lowest density: it's less dense than water. Although all of the gas planets have rings around them, Saturn's are the brightest and most famous. In the mid-1800s, Scottish physicist James Clerk Maxwell correctly hypothesized that Saturn's rings must be made up of solid particles (such as ice) or else the rings could not maintain their stability.

Uranus and Neptune are mostly rock and ice, with around 15% hydrogen. Uranus's satellites (moons or huge rocks orbiting it) are named after characters from Shakespeare's plays and from an Alexander Pope poem. Neptune has a really, really long sidereal period or year: 165 Earth years would equal just one year on Neptune! Since Neptune wasn't discovered until 1846, the planet has yet to make a full orbit around the sun while we've been aware of it.

Pluto has the coldest temperatures: -400°F/-250°C. In fact, it's so cold there that it "snows" methane gas crystals. There's been much debate about whether Pluto is really a planet, but it seems unlikely that it will stop being classed as one anytime soon. Due to its rather wacky orbit, Pluto sometimes nips in closer to the sun and leaves Neptune behind as the farthest planet.

Miscelleneous space

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