WHAT DO YOU THINK? • How near is the closest star other than the Sun? • Is the Sun brighter than other stars, or just closer? • What colors are stars? • Are brighter stars hotter? • What sizes are stars? • Are most stars isolated from other stars, as the Sun is?
Apparent Magnitude Scale – brightness of a star as seen from Earth 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. Several stars in and around the constellation Orion labeled with their names and apparent magnitudes
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 star’s brightness by a factor of 9.
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. • Are the car lights actually dimmer as the car moves away?
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.
Temperature and Color (review) Hottest = blue color Medium = orange/yellow color Coolest = red color
Spectral Classes (Color and Temperature) “Oh, Be AFine Guy/Girl, Kiss Me!”
Hertzsprung-Russell (HR) Diagram • Star brightness is plotted against star spectral types (color / temperature). • Brightness and spectral type are related. • 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. • White dwarfs are below the main sequence.
HR Diagram Basics Thanks to Dan Enriquez
Star Size Is Also Important • Hotter stars are brighter than cooler stars (of the same size). • Bigger stars are brighter than smaller stars (of the same temperature). • So the brightest stars are the biggest, hottest ones. • L = R2T4 (L = brightness, R = radius, T = temperature)
Mass-Temperature-Brightness • Each dot = a main-sequence star. • The dot’s 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.
WHAT DID YOU THINK? • 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. • 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. • What colors are stars? • Stars are found in a wide range of colors, from red through violet, as well as white.
WHAT DID YOU THINK? • 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. • What sizes are stars? • Stars range from more than 1000 times the Sun’s diameter to less than 1/100 the Sun’s diameter. • 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.