1. < BackNext >PreviewMain Section 1 Stars Chapter 15 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.

2. < BackNext >PreviewMain Section 1 Stars Chapter 15 Stars differ in size, temperature, composition, brightness, and color. Distances between stars are very large and are measured in light-years. What You Will Learn

3. < BackNext >PreviewMain Section 1 Stars Chapter 15 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.

4. < BackNext >PreviewMain Section 1 Stars Chapter 15 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.

5. < BackNext >PreviewMain Section 1 Stars Chapter 15 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.

6. < BackNext >PreviewMain Section 1 Stars Chapter 15 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.

7. < BackNext >PreviewMain Section 1 Stars Chapter 15 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.

8. < BackNext >PreviewMain Stars, Galaxies, and the Universe Chapter 15 Composition of Stars, continued

9. < BackNext >PreviewMain Section 1 Stars Chapter 15 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.

10. < BackNext >PreviewMain Section 1 Stars Chapter 15 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.

11. < BackNext >PreviewMain Section 1 Stars Chapter 15 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.

12. < BackNext >PreviewMain Stars, Galaxies, and the Universe Chapter 15 Classifying Stars

13. < BackNext >PreviewMain Section 1 Stars Chapter 15 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.

14. < BackNext >PreviewMain Stars, Galaxies, and the Universe Chapter 15 Absolute and Apparent Magnitude

15. < BackNext >PreviewMain Section 1 Stars Chapter 15 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.

16. < BackNext >PreviewMain Section 1 Stars Chapter 15 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.

17. < BackNext >PreviewMain Section 1 Stars Chapter 15 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.

18. < BackNext >PreviewMain Stars, Galaxies, and the Universe Chapter 15