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Chapter 11: Chapter 11: Characterizing Stars. How near is the closest star other than the Sun? How near is the closest star other than the Sun? Is the.

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Presentation on theme: "Chapter 11: Chapter 11: Characterizing Stars. How near is the closest star other than the Sun? How near is the closest star other than the Sun? Is the."— Presentation transcript:

1 Chapter 11: Chapter 11: Characterizing Stars

2 How near is the closest star other than the Sun? How near is the closest star other than the Sun? Is the Sun brighter than other stars, or just closer? Is the Sun brighter than other stars, or just closer? What colors are stars? What colors are stars? Are brighter stars hotter? Are brighter stars hotter? What sizes are stars? What sizes are stars? Are most stars isolated from other stars, as the Sun is? Are most stars isolated from other stars, as the Sun is? WHAT DO YOU THINK?

3 Apparent Magnitude Scale – brightness of a star as seen from Earth Several stars in and around the constellation Orion labeled with their names and apparent magnitudes Astronomers give the brightness of objects in the sky by apparent magnitudes. Stars visible to the naked eye have magnitudes between m = –1.44 and about m = +6.

4 The Inverse-Square Law The farther a star is from Earth, the dimmer it looks to us. Doubling the distance makes the star look one-fourth as bright. Tripling the distance decreases the stars brightness by a factor of 9.

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6 Absolute Magnitude – the actual brightness of a star Absolute magnitude tells how bright a star really is, no matter how far from Earth it is. Absolute magnitude tells how bright a star really is, no matter how far from Earth it is. Are the car lights actually dimmer as the car moves away? Are the car lights actually dimmer as the car moves away?

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8 No. Their actual brightness (absolute magnitude) is the same no matter the distance. No. Their actual brightness (absolute magnitude) is the same no matter the distance. But they look dimmer (apparent magnitude) to us when the car is farther away. But they look dimmer (apparent magnitude) to us when the car is farther away.

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10 Temperature and Color (review) Hottest = blue color Medium = orange/yellow color Coolest = red color

11 Oh, Be A Fine Guy/Girl, Kiss Me! Spectral Classes (Color and Temperature)

12 Hertzsprung- Russell (HR) Diagram (Textbook page 305)

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14 Hertzsprung-Russell (HR) Diagram Star brightness is plotted against star spectral types (color / temperature). Star brightness is plotted against star spectral types (color / temperature). Brightness and spectral type are related. Brightness and spectral type are related. Main-sequence stars (fusing hydrogen to helium) fall along the red curve. Main-sequence stars (fusing hydrogen to helium) fall along the red curve. Giants are to the upper right and super-giants are on the top. Giants are to the upper right and super-giants are on the top. White dwarfs are below the main sequence. White dwarfs are below the main sequence.

15 HR Diagram Basics Thanks to Dan Enriquez

16 Star Size Is Also Important Hotter stars are brighter than cooler stars (of the same size). Hotter stars are brighter than cooler stars (of the same size). Bigger stars are brighter than smaller stars (of the same temperature). Bigger stars are brighter than smaller stars (of the same temperature). So the brightest stars are the biggest, hottest ones. So the brightest stars are the biggest, hottest ones. L = R 2 T 4 (L = brightness, R = radius, T = temperature) L = R 2 T 4 (L = brightness, R = radius, T = temperature)

17 Each dot = a main- sequence star. Each dot = a main- sequence star. The dots number is the mass of that star in solar masses (Sun = 1). The dots number is the mass of that star in solar masses (Sun = 1). Mass, brightness, and temperature of main-sequence stars increase from lower right to upper left. Mass, brightness, and temperature of main-sequence stars increase from lower right to upper left. Mass-Temperature-Brightness

18 WHAT DID YOU THINK? How near is the closest star other than the Sun? How near is the closest star other than the Sun? Proxima Centauri is about 40 trillion kilometers (25 trillion miles) away. It takes light about 4 years to reach the Earth from there. Proxima Centauri is about 40 trillion kilometers (25 trillion miles) away. It takes light about 4 years to reach the Earth from there. How luminous is the Sun compared with other stars? How luminous is the Sun compared with other stars? The most luminous stars are about a million times brighter and the least luminous stars are about a hundred thousand times dimmer than the Sun. The most luminous stars are about a million times brighter and the least luminous stars are about a hundred thousand times dimmer than the Sun. What colors are stars? What colors are stars? Stars are found in a wide range of colors, from red through violet, as well as white. Stars are found in a wide range of colors, from red through violet, as well as white.

19 Are brighter stars hotter than dimmer stars? Are brighter stars hotter than dimmer stars? Not necessarily. Many brighter stars, such as red giants, are cooler but larger than hotter, dimmer stars, such as white dwarfs. Not necessarily. Many brighter stars, such as red giants, are cooler but larger than hotter, dimmer stars, such as white dwarfs. What sizes are stars? What sizes are stars? Stars range from more than 1000 times the Suns diameter to less than 1/100 the Suns diameter. Stars range from more than 1000 times the Suns diameter to less than 1/100 the Suns diameter. Are most stars isolated from other stars, as the Sun is? Are most stars isolated from other stars, as the Sun is? No. In the vicinity of the Sun, two-thirds of the stars are found in pairs or larger groups. No. In the vicinity of the Sun, two-thirds of the stars are found in pairs or larger groups. WHAT DID YOU THINK?


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