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?

As the Earth orbits theSun, a nearby star appears to shift its position against the background of distant stars.

DistanceDistance ParallaxParallax

The closer the star is to us,the greater the parallax angle p.

Telescopes on earth can measure stars up to 100pc and space based telescopes can measure stars up to 150pc

Distance (in parsecs) = 1 / parallax angle in arcsecondsDistance (in parsecs) = 1 / parallax angle in arcseconds

The nearest star, Proxima Centauri, has a parallax angle The nearest star, Proxima Centauri, has a parallax angle of .77 arcsec. What is its distance?of .77 arcsec. What is its distance?

Apparent MagnitudeApparent Magnitude

Brightness of a star Brightness of a star without regard to without regard to distancedistance

The more negative The more negative the brighter it isthe brighter it is

Stars visible to the naked eye have magnitudes between m = –1.44 and about m = +6.

Absolute MagnitudeAbsolute Magnitude

Brightness of a star at Brightness of a star at a distance of 10pca distance of 10pc

Can be calculated Can be calculated using the star’s using the star’s distance and distance and apparent magnitude apparent magnitude using the inverse using the inverse square lawsquare law

The Inverse-Square LawThe Inverse-Square Law

The same amount of radiation from a light source must illuminate an ever-increasing area as the distance from the light source increases. The decrease in brightness follows the inverse-square

law. This means, for example, that tripling the distance decreases the brightness by a factor of 9.

Temperature and ColorTemperature and Color

The intensity of light emitted by three hypothetical stars is plotted against wavelength. The range of visible wavelengths is indicated. Where the peak of a star’s intensity curve lies relative to the visible light band determines the apparent color of its visible light.

Principal Types of Stellar SpectraPrincipal Types of Stellar Spectra

Classifying the Spectra of StarsClassifying the Spectra of Stars

Williamina Fleming (standing) Annie Jump Cannon

“Oh, Be A Fine Guy/Girl, Kiss Me!”

Spectral ClassesSpectral Classes

Hertzsprung-Russell DiagramHertzsprung-Russell Diagram Luminosities (absolute Luminosities (absolute

magnitude) of stars are magnitude) of stars are plotted against their spectral plotted against their spectral types.types.

Main-sequence stars fall Main-sequence stars fall along the red curve.along the red curve.

Giants are to the right and Giants are to the right and supergiants are on the top.supergiants are on the top.

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

Types of Stars and Their SizesTypes of Stars and Their Sizes

Stellar luminosities are Stellar luminosities are graphed against the graphed against the surface temperatures.surface temperatures.

Dashed diagonal lines Dashed diagonal lines indicate stellar radii.indicate stellar radii.

For stars of the same For stars of the same radius, hotter stars are radius, hotter stars are more luminous than more luminous than cooler stars.cooler stars.

Luminosity ClassesLuminosity Classes Luminosity classes permit Luminosity classes permit

finer distinctions between finer distinctions between giants and supergiants.giants and supergiants.

Ia and Ib encompass the Ia and Ib encompass the supergiants.supergiants.

II, III, and IV indicate giants II, III, and IV indicate giants of different brightness.of different brightness.

V is the main-sequence V is the main-sequence stars.stars.

White dwarfs do not have a White dwarfs do not have a luminosity class.luminosity class.

Stellar MassStellar Mass

Found by its Found by its gravitational effects gravitational effects on other objectson other objects

Keplers 3’rd Law Keplers 3’rd Law (a(a33/p/p22))

Most stars are binary Most stars are binary starsstars

Ursae Majoris

The Mass-Luminosity RelationThe Mass-Luminosity Relation

Luminosities and Luminosities and masses are plotted masses are plotted using logarithmic using logarithmic scales.scales.

The more massive The more massive a star, the more a star, the more luminous the star.luminous the star.

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

The number next to The number next to each dot is the mass of each dot is the mass of that star in solar that star in solar masses.masses.

Mass, luminosity, and Mass, luminosity, and surface temperature of surface temperature of main-sequence stars main-sequence stars increase from lower increase from lower right to upper leftright to upper left

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?