Stars

1. Pgs. 98 - 104 Stars

2. Color of Stars • Most stars look like faint dots of light in the night sky but are actually huge balls of burning gas. • Believe it or not gases will burn at different colors. • The color helps determine the temperature. • Betelgeuse is a red star and Rigel is a blue star.

4. Composition of Stars • If you look at white light through a glass prism, you see a rainbow of colors called a spectrum. • The spectrum contains all the colors we see. • A hot solid object like the glowing wire of a light bulb gives off a continuous spectrum. • Astronomers use a tool called a spectrograph to spread starlight out into its colors. • Since stars are not solid objects they do not give off a continuous spectrum.

5. Hot Dense Gas • Stars are made of gases that are so dense that they act like a hot solid. • The “surface” of a star, the part we see, gives off a continuous spectrum. • The light we see has to pass through the star’s atmosphere and Earth’s atmosphere as well. • The star produces a spectrum with lines in it.

6. Making an ID • Restaurants use neon signs to attract customers. • The neon sign glows when there is an electric current going through it. • Using a spectrograph, we would see emission lines, that are made when certain wavelengths are given off by hot gases. • Each gas has its own set of emission lines that give the gas an identity through a spectrograph.

7. Spectrograph of Gases

8. Trapping the Light • The spectrum produced by a star is not continuous, nor is it bright lines. • The star’s atmosphere is cooler than the star itself and the gases absorb some of the star’s light. • This removes some colors of light from the continuous spectrum of a hot star.

9. Cosmic Detective Work • If light from a hot solid passes through a cooler gas it produces absorption spectrum. • This is a continuous spectrum with dark lines where less light gets through. • The pattern of lines shows some of the elements that are in the stars atmosphere. • Since there are several elements making up a star, it can be a puzzle to figure out its parts.

10. Classifying Stars – Differences in Temperature • Stars are now classified by how hot they are. • We can see temperature differences as colors. • The hottest stars are blue and designated as letter ‘O’. • The order of stars are classified as OBAFGKM.

12. Differences in Brightness • Ancient astronomers classified stars by their brightness using only their eyes. • The brightest stars were first magnitude stars and the faintest were sixth magnitude stars. • Once telescopes were developed, we were able to see more stars than ever before. • Scientists added to the old system using positive numbers for dim stars and negative numbers for bright stars. • Examples: a telescope can see 29th magnitude star and Sirius (the brightest in the night sky) is a -1.4 magnitude star.

13. Apparent Magnitude • How bright a light looks, or appears, is called apparent magnitude. • If you look at a row of street lights, the closest to you appears brightest. • As they get farther away from you, their brightness will decrease by the square of their distance. • A light 10m away will appear 4 times as bright as one 20m away.

14. Absolute Magnitude • The actual brightness of a star is absolute magnitude. • Scientists use the star’s apparent magnitude and its distance from Earth to calculate absolute magnitude. • If all stars were the same distance away, then a star’s absolute and apparent magnitude would be the same. • The sun has an absolute magnitude of 4.8 and an apparent magnitude of -26.8.

16. Distance to the Stars • Astronomers use light-years to describe distances between stars. • A light-year is the distance light travels in one year (9.5 trillion kilometers/ year). • The North Star is 431 light-years. • More distant stars appear to move as the Earth rotates. • This shift is called a parallax.

17. Motions of Stars • The Earth rotates on its axis and different parts of its surface face the sun just as the Earth revolves around the sun. • At different parts of the year, we see different stars in the night sky. • At night, different parts of the Earth will face different parts of the universe.

18. Apparent Motion • If you look at the night sky long enough, the stars appear to move across the sky. • The stars will actually appear to move around the North Star, Polaris. • This motion is caused by the Earth’s rotation on its axis. • The North Star aligns with the Earth’s poles and rotational direction. • The other stars will make a circle around the North Star every 24 hours, called circumpolar stars

19. Actual Motion • Each star in outer space is actually moving. • Since stars are so distant, their real motion is hard to observe. • Over thousands of years, the stars’ patterns in the sky will change over time.