Planets Vs. Stars
Stars are made up mostly of hydrogen, stars produce energy through the process of nuclear fusion. Astronomers classify stars according to their physical characteristics. Some stars look reddish. Others yellow or blue. A star's color reveals it surface temperature. The coolest stars have a surface temperature of about 3,200 degrees Celsius appear red. Our yellow sun has a surface temperature of about 5,500 degrees Celsius. The hottest stars, with surface temperature of over 20,000 degrees Celsius, appear bluish. Many stars are actually about the size of the sun. However, some stars are much larger than the sun. Very large stars are called giant stars or super giant stars. White dwarf stars are about the size of Earth. Neutron stars are even smaller, about 20 kilometers in diameter. Planets must be round, orbit the sun, and have cleared out the region of the Solar System along its orbit. The first four planets are small and are mostly made of rock and metal. The last four planets are very large and are mostly made from gas and liquid. Like Earth, each planet has a "day" and a "year". Its day is the time the planet takes to rotate on its axis. The planet's year is the time the planet takes to orbit the sun. A dwarf planet is an object that orbits the sun and has enough gravity to be spherical but has not cleared out the region of the Solar System along its orbit.