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Chapter 15 Section 1 Stars Bellringer List ways that stars differ from one another. How is the sun like other stars? How is it different? Write your answers in your Science Journal. < Back Next > Preview Main Chapter 15 Section 1 Stars What You Will Learn • Stars differ in size, temperature, composition, brightness, and color. • Distances between stars are very large and are measured in light-years. < Back Next > Preview Main Chapter 15 Section 1 Stars Color of Stars • The color of a star indicates the star’s temperature. • Red stars are the coolest, and blue stars are the hottest. • If two stars differ in color, you can conclude that they differ in temperature too. < Back Next > Preview Main Chapter 15 Section 1 Stars Composition of Stars • Astronomers use an instrument called a spectroscope to separate a star’s light into a spectrum (plural, spectra). • A spectrum is the band of colors produced when white light passes through a prism. < Back Next > Preview Main Chapter 15 Section 1 Stars Composition of Stars, continued • A continuous spectrum shows all of the colors, while an absorption spectrum shows which wavelengths of light are absorbed. • The spectrum of a star is an absorption spectrum because the atmosphere of the star absorbs certain portions of the light produced by the star. < Back Next > Preview Main Chapter 15 Section 1 Stars Composition of Stars, continued • When a chemical element emits light, only some colors in the spectrum show up. The colors that appear are called emission lines. • Every element has a unique set of emission lines that act like a fingerprint for that element. < Back Next > Preview Main Chapter 15 Section 1 Stars Composition of Stars, continued • The pattern of lines in a star’s absorption spectrum is unique to that star. • A star’s absorption spectrum can be used to determine the elements in that star’s atmosphere and the stage the star occupies in its life cycle. • Stars are made of mostly hydrogen and helium gases. < Back Next > Preview Main Chapter 15 Stars, Galaxies, and the Universe Composition of Stars, continued < Back Next > Preview Main Chapter 15 Section 1 Stars Classifying Stars • Stars are now classified by how hot they are. • Temperature differences between stars result in color differences that can be seen. For example, class O stars are blue—the hottest stars. < Back Next > Preview Main Chapter 15 Section 1 Stars Classifying Stars, continued • Magnitude is used to compare the brightness of one object with the brightness of another object. • To express the brightness of stars, astronomers use a system of magnitudes. < Back Next > Preview Main Chapter 15 Section 1 Stars Classifying Stars, continued • Positive magnitude numbers represent dim stars. Negative magnitude numbers represent bright stars. • The brightest star in the night sky, Sirius, has a magnitude of -1.4. < Back Next > Preview Main Chapter 15 Stars, Galaxies, and the Universe Classifying Stars < Back Next > Preview Main Chapter 15 Section 1 Stars How Bright Is That Star? • The apparent magnitude is the brightness of a star as seen from Earth. The absolute magnitude is the brightness that a star would have at a distance of 32.6 light-years from Earth. • If all stars were the same distance away, their absolute magnitudes would be the same as their apparent magnitudes. < Back Next > Preview Main Chapter 15 Stars, Galaxies, and the Universe Absolute and Apparent Magnitude < Back Next > Preview Main Chapter 15 Section 1 Stars Distance to the Stars • Because stars are so far away, astronomers use a unit called a light-year to measure the distance from Earth to the stars. • A light-year is the distance that light travels in one year, about 9.46 trillion kilometers. < Back Next > Preview Main Chapter 15 Section 1 Stars Distance to the Stars, continued • Parallax is an apparent shift in the position of an object when viewed from different locations. • Astronomers use parallax and trigonometry to find the actual distance to stars that are close to Earth. < Back Next > Preview Main Chapter 15 Section 1 Stars Motions of Stars • All of the stars in the sky appear to make one complete circle around Polaris every 24 h. • This apparent motion of the stars is due to Earth’s rotation and its revolution around the sun. • Each star is actually moving in space. But because stars are so distant, their actual motion is hard to see. < Back Next > Preview Main Chapter 15 Stars, Galaxies, and the Universe < Back Next > Preview Main