The Sun, Stars, and Galaxies

The Sun, Stars, and Galaxies — page 7 — THE SUN STARS AND GALAXIES An illustration of the solar system with the Sun at its center Shutterstock com 12

The Sun, Stars, and Galaxies — page 8 — THE SUN TuE CENTER OF THE SOLAR SrsTEM Because the Sun is so close to Earth it seems much larger and brighter than other stars It is the only star whose surface details can be observed from Earth BASIC PROPERTIES Stars vary greatly in size and color They range from giant stars which are much larger than the Sun to dwarf stars which can be much smaller than the Sun In color they range from whitish blue stars with very high sur face temperatures more than 30 000 Kelvin or 53 500° F to relatively cool red stars less than 3 500 K or 5 840° F The Kelvin tem perature scale uses degrees of the same size as Celsius or centigrade degrees but it is numbered from absolute zero 273 15° C The Sun is a yellow dwarf star a kind that is common in the Milky Way galaxy It has a surface temperature of about 5 800 K 10 000° F Its diameter is about 864 950 miles 1 392 000 kilometers which is about 109 times the diameter of Earth Its volume is about 1 300 000 times Earth's volume and its mass or quantity of matter is some 333 000 times as great as Earth s mass More than 90 percent of the Sun's atoms are hydrogen Most of the rest are helium 13

The Sun, Stars, and Galaxies — page 9 — THE SUN STARS AND GALAXIES with much smaller amounts of heavier ele ments such as carbon nitrogen oxygen magnesium silicon and iron By mass the Sun is about 71 percent hydrogen and 28 per cent helium The Sun has no fixed surface It is much too hot for matter to exist there as a solid or liquid Rather the Sun s matter consists of gas and plasma a state in which gases are heated so much that the electrons are stripped away from their atomic nuclei The heated gas is said to be ionized because it consists of a group of ions or electrically charged particles The free electrons carry a neg ative charge and the atomic nuclei carry a positive charge Like the planets the Sun rotates Because the Sun is not solid different parts of it rotate at different rates The A false color Doppler image parts of the surface near the equa shows rotation speeds of the Sun over a 12 month period tor spin the fastest completing one Red areas represent the fast rotation about every 25 Earth days est rotation blue the slowest The parts of the surface near the The cutaway section repre poles take 36 days to complete a sents rotation at the Sun's core SOHO ESA NASA rotation 14

The Sun, Stars, and Galaxies — page 10 — THE SUN THE CENTER OF THE SoLAR SYSTEM STUDYING THE SUN The scientific study of the Sun began with the invention of the telescope in the early 17th century A major breakthrough in solar stud ies came in the 19th century with the use of spectroscopy the analysis of the light emitted by the Sun This approach allowed scientists to determine the composition of the Sun's atmo sphere Scientists in the relatively new field called helioseismology study the Sun s interior by examining waves that travel through it UsING THE SUN’s SPECTRUM When a ray of sunlight which appears white is passed through a prism or a diffraction grating it spreads out into a series of colors called a spectrum Scientists analyze this spectrum to determine what chemicals make up the Sun as well as their abundance loca tion and physical states In 1814 Joseph von Fraunhofer began a thorough study of the Sun's spectrum He found that it was crossed by many dark lines which are now called absorption lines or Fraunhofer lines Meanwhile other scien tists had been studying the light emitted and absorbed by elements in the gaseous state 15

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