How Do Astronomers Measure the How Do Astronomers Measure the Brightness of a Star?Brightness of a Star?

Stars vary greatly in brightnessStars vary greatly in brightnessEarly peoples observed bright stars and Early peoples observed bright stars and

grouped them into constellations (88)grouped them into constellations (88)Ancient Greeks established classification Ancient Greeks established classification

system based on star brightnesssystem based on star brightnessApparent magnitude-Apparent magnitude- brightness of a star as brightness of a star as

viewed from Earthviewed from EarthA difference of 1 magnitude corresponds to a A difference of 1 magnitude corresponds to a

factor of 2.5 in brightnessfactor of 2.5 in brightnessThe smaller (more negative) the #, the The smaller (more negative) the #, the

brighter the starbrighter the star

Examples of Apparent MagnitudeExamples of Apparent Magnitude

*REMEMBER- lower magnitude = brighter stars!!

Problem: Apparent Magnitudes only Problem: Apparent Magnitudes only measure measure relativerelative brightness! brightness!

1 light-year

10 light-years

Apparent magnitudes only tell us how bright stars appear to be, NOT how bright they actually are. Look at the above example:

-There are 2 stars that both shine with the exact same amount of light, BUT one of them is 10x further than the other

-The further one will send us 102 = 100x less light (Brightness varies inversely with the square of the distance.)

-100x less light means the further star will be 5 magnitudes dimmer than the closer star

Solution: Absolute MagnitudeSolution: Absolute Magnitude

Absolute magnitude- the brightness a star Absolute magnitude- the brightness a star would have if it were placed at 10 parsec would have if it were placed at 10 parsec (or 32 light-years) from Earth; (or 32 light-years) from Earth;

Shows the star’s inherent (or true) Shows the star’s inherent (or true) brightness brightness Star Apparent Mag.Star Apparent Mag. Absolute Mag. Absolute Mag.

Sun Sun -26.74-26.74 4.83 4.83

SiriusSirius -1.44-1.44 1.45 1.45

ArcturusArcturus -0.05-0.05 -0.31 -0.31

VegaVega 0.030.03 0.58 0.58

AntaresAntares 1.001.00 -4.7 -4.7

Hertzsprung-Russell DiagramHertzsprung-Russell Diagram

H-R diagram H-R diagram demonstrates the demonstrates the relationship between relationship between mass, luminosity and mass, luminosity and temperaturetemperature

An H-R diagram plots An H-R diagram plots the absolute mag on the the absolute mag on the vertical axis and temp vertical axis and temp (or spectral type) on the (or spectral type) on the horizontal axishorizontal axis Sometimes the y Sometimes the y

axis is luminosity axis is luminosity

H-R DiagramH-R Diagram

The The mainmain sequencesequence, which , which runs diagonally runs diagonally from the upper from the upper left corner (hot, left corner (hot, very luminous very luminous blueblue giantsgiants) to ) to the lower right the lower right corner (cool, faint corner (cool, faint redred dwarfsdwarfs), ), represents about represents about 90% of stars90% of stars

Remaining 10% of stars include:Remaining 10% of stars include: red giantsred giants and and supsupergergiantsiants- large, cool, - large, cool,

luminous stars plotted at the upper right luminous stars plotted at the upper right cornercorner

white dwarfs-white dwarfs- small, dim, hot stars in the small, dim, hot stars in the lower left cornerlower left corner

Spectral TypeSpectral Type Since around 1900, astronomers have Since around 1900, astronomers have

categorized the absorption spectra they received categorized the absorption spectra they received from stars using lettersfrom stars using letters

Stars are classified into 7 main stellar spectra Stars are classified into 7 main stellar spectra from from ““OO”” (hottest, bluest) to (hottest, bluest) to ““MM”” (coolest, (coolest, reddest)reddest)The color of a star tells is its temperature!!The color of a star tells is its temperature!!

Our sun is a G5Our sun is a G5