Chapter 11:Chapter 11:CharacterizingStars

How near is the closest star other than the How near is the closest star other than the Sun?Sun?

Is the Sun brighter than other stars, or just Is the Sun brighter than other stars, or just closer?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, Are most stars isolated from other stars,

as the Sun is?as the Sun is?

WHAT DO YOU THINK?WHAT DO YOU THINK?

Apparent Magnitude Scale – brightness Apparent Magnitude Scale – brightness of a star as seen from Earthof a star as seen from Earth

Several stars in and around the constellation Orion labeled with their names and apparent magnitudes

Astronomers give thebrightness of objects in the sky by apparent magnitudes. Stars visible to the naked eye have magnitudes between m = –1.44 and about m = +6.

The Inverse-Square LawThe 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 Absolute Magnitude – the actual brightness of a starbrightness of a star

Absolute magnitude tells how bright a star Absolute magnitude tells how bright a star really is, no matter how far from Earth it is.really is, no matter how far from Earth it is.

Are the car lights actually dimmer as the Are the car lights actually dimmer as the car moves away?car moves away?

No. Their actual brightness (absolute No. Their actual brightness (absolute magnitude) is the same no matter the magnitude) is the same no matter the distance.distance.

But they But they looklook dimmer (apparent dimmer (apparent magnitude) to us when the car is farther magnitude) to us when the car is farther away.away.

Temperature and Color (review)Temperature and Color (review)

Hottest = blue colorMedium = orange/yellow colorCoolest = red color

“Oh, Be A Fine Guy/Girl, Kiss Me!”

Spectral Classes (Color and Temperature)Spectral Classes (Color and Temperature)

Hertzsprung-Hertzsprung-Russell (HR) Russell (HR)

DiagramDiagram(Textbook(Textbookpage 305)page 305)

Hertzsprung-Russell (HR) Hertzsprung-Russell (HR) DiagramDiagram

Star brightness is plotted against star spectral Star brightness is plotted against star spectral types (color / temperature).types (color / temperature).

Brightness and spectral type are related.Brightness and spectral type are related. Main-sequence stars (fusing hydrogen to Main-sequence stars (fusing hydrogen to

helium) fall along the red curve.helium) fall along the red curve. Giants are to the upper right and super-giants Giants are to the upper right and super-giants

are on the top.are on the top. White dwarfs are below the main sequence.White dwarfs are below the main sequence.

HR Diagram BasicsHR Diagram Basics

Thanks to Dan Enriquez

Star Size Is Also ImportantStar Size Is Also Important

Hotter stars are brighter than cooler stars Hotter stars are brighter than cooler stars (of the same size).(of the same size).

Bigger stars are brighter than smaller stars Bigger stars are brighter than smaller stars (of the same temperature).(of the same temperature).

So the brightest stars are the biggest, So the brightest stars are the biggest, hottest ones.hottest ones.

L = RL = R22TT44 (L = brightness, R = radius, (L = brightness, R = radius,T = temperature)T = temperature)

Each dot = a main-Each dot = a main-sequence star.sequence star.

The dot’s number is The dot’s number is the mass of that the mass of that star in solar masses star in solar masses (Sun = 1).(Sun = 1).

Mass, brightness, Mass, brightness, and temperature of and temperature of main-sequence main-sequence stars increase from stars increase from lower right to upper lower right to upper left.left.

Mass-Temperature-BrightnessMass-Temperature-Brightness

WHAT DID YOU THINK?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 Proxima Centauri is about 40 trillion kilometers (25 trillion

miles) away. It takes light about 4 years to reach the miles) away. It takes light about 4 years to reach the Earth from there.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 The most luminous stars are about a million times

brighter and the least luminous stars are about a brighter and the least luminous stars are about a hundred thousand times dimmer than the Sun.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 Stars are found in a wide range of colors, from red

through violet, as well as white.through violet, as well as white.

Are brighter stars hotter than dimmer stars?Are brighter stars hotter than dimmer stars? Not necessarily. Many brighter stars, such as red giants, Not necessarily. Many brighter stars, such as red giants,

are cooler but larger than hotter, dimmer stars, such as are cooler but larger than hotter, dimmer stars, such as white dwarfs.white dwarfs.

What sizes are stars? What sizes are stars? Stars range from more than 1000 times the Sun’s Stars range from more than 1000 times the Sun’s

diameter to less than 1/100 the Sun’s diameter.diameter to less than 1/100 the Sun’s 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 No. In the vicinity of the Sun, two-thirds of the stars are

found in pairs or larger groups.found in pairs or larger groups.

WHAT DID YOU THINK?WHAT DID YOU THINK?