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“All men have the stars,” he answered, “but they are not the same things for different people. For some, who are travelers, the stars are guides. For others they are no more than little lights in the sky. For others, who are scholars, they are problems. For my businessman they were wealth. But all these stars are silent. You – you alone – will have the stars as no one else has them.” Antoine de Saint-Exupery (1900 – 1944) from The Little Prince
WHAT DO YOU THINK? • How near to us is the closest star other than the Sun? • What colors are stars, and why do they have these colors? • How luminous is the Sun compared with other stars? • Are brighter stars hotter than dimmer stars? • Compared to the Sun, what sizes are other stars? • Are most stars similar to the Sun, one star with planets, or in multiple-star groups?
A Snapshot of the Heavens • How can we learn about the lives of stars, which last tens of millions to hundreds of billions of years? • we will never observe a particular star evolve from birth to death • so how can we study stellar evolution?
How can we study the “Life Cycles” of stars? • Key: All stars were NOT born at the same time. • stars we see today are at different stages in their lives • we observe only a brief moment in any one star’s life • by studying large numbers of stars, we get a “snapshot” of one moment in the history of the stellar community • we can draw conclusions just like we would with human census data…we do stellar demographics!
A Snapshot of the Heavens • What two basic physical properties do astronomers use to classify stars? • What does that classification tell us?
Classificationof Stars • Stars were originally classified based on: • their brightness • their location in the sky • This classification is still reflected in names of the brightest stars…those we can see with our eyes:
Classification of Stars Order of brightness within a constellation Latin Genitive of the constellation Orionis Geminorum
Classification of Stars • The old classification scheme told us little about a star’s true (physical) nature. • a star could be very bright because is was very close to us; not because it was truly bright • two stars in the same constellation might not be close to each other; one could be much farther away
Classification of Stars • In 20th Century, astronomers developed a more appropriate classification system based on: • a star’s luminosity • a star’s surfacetemperature • These properties turn out to depend on: • a star’s mass and • its stage in life • measuring these=> reconstruct stellar life cycles
WHAT DO YOU THINK? • How near to us is the closest star other than the Sun? • What colors are stars, and why do they have these colors? • How luminous is the Sun compared with other stars? • Are brighter stars hotter than dimmer stars? • Compared to the Sun, what sizes are other stars? • Are most stars similar to the Sun, one star with planets, or in multiple-star groups?
WHAT DO YOU THINK? • How near are stars?DISTANCE • What colors are stars?TEMPERATURE • How luminous are stars?LUMINOSITY • Are brighter stars hotter?TEMP vs. LUMIN. • What sizes are stars?SIZE • Single or Multiple?ORBITS MASS
Step 1: Distance! • How near are stars?DISTANCE • What colors are stars? TEMPERATURE • How luminous are stars? LUMINOSITY • Are brighter stars hotter? TEMP vs. LUMIN. • What sizes are stars? SIZE • Single or Multiple? ORBITS MASS
PARALLAX • Determines distance based on Earth’s Orbit around Sun. • SMALL angular shift! • Even closest stars (4.3 light years) show shift less than 1/4000th of a degree!
PARALLAX • Examplehttp://www.solstation.com/stars/61cygni2.htm • Animationhttp://ircamera.as.arizona.edu/NatSci102/NatSci102/lectures/otherstars.htm • Current telescopes can measure angles as small as 1/400,000th of a degree (400+ light years)
Step 2: TEMPERATURES! • How near are stars?DISTANCE • What colors are stars?TEMPERATURE • How luminous are stars? LUMINOSITY • Are brighter stars hotter? TEMP vs. LUMIN. • What sizes are stars? SIZE • Single or Multiple? ORBITS MASS
COLORSof stars lead toSurface Temperatures!
Pickering’s “Harem” of women computers at Harvard College Annie Cannon
Spectral Type Classification System O BAFGKM (L) Oh Be A Fine Girl/Guy, Kiss Me! 50,000 K 3,000 K Temperature
Starlight Follows 2 Key Radiation Laws Peak of radiation is at shorter wavelengths for hotter stars MUCH more intensity at every wavelength for hotter stars
Step 3: BRIGHTNESS! • How near are stars?DISTANCE • What colors are stars?TEMPERATURE • How luminous are stars?LUMINOSITY • Are brighter stars hotter? TEMP vs. LUMIN. • What sizes are stars? SIZE • Single or Multiple? ORBITS MASS
How LUMINOUS are stars? • Bright because they are nearby…or… • Bright because they are really, truly bright?
Rigel is bright because it is REALLY bright! Sirius is bright because it is CLOSE!
Apparent Magnitudes Ancient method for measuring stellar brightness from Greek astronomer Hipparchus (c. 190 – 120 B.C.) Magnitude Scale This scale runs backwards: The bigger the number, the fainter the star Brightest stars are #1, next brightest are #2, etc.
Inverse Square Law for Light (how distance relates to brightnes)
Absolute Magnitudes • Takes into account distance of star! • Treats all stars as if they were same distance away! • True measure of star’s real brightness.
Relating Measurable Quantities • Measure distances of nearby stars • Deduce how bright they really are • Determine surface temperatures from spectraHOW do temperatures and brightness relate to one another?
Step 4: HR Diagram! • How near are stars?DISTANCE • What colors are stars?TEMPERATURE • How luminous are stars?LUMINOSITY • Are brighter stars hotter?TEMP vs. LUMIN. • What sizes are stars?SIZE • Single or Multiple? ORBITS MASS
Sun is much, much dimmer than most “bright” stars Our Star! The Sun Sun is brighter than most “nearby” stars
BRIGHT HOT COOL FAINT
Hypotheses to Explore • Stars are born like children, cool and small, and then heat up and grow brighter over time. • Stars are like candles, hot and bright, and then cool off and get dimmer over time. • Stars are born with different temps and brightnesses, and change little over 90% of their lives.
Hypotheses to Explore • IF…. Stars are born like children, cool and small, and then heat up and grow brighter over time. • THEN… new clusters of stars should all be hot O stars, and old clusters only M stars.
Hypotheses to Explore • IF…Stars are like candles, hot and bright, and then cool off and get dimmer over time. • THEN… new clusters should show only M stars, and older ones should show O stars. • So… DO the EXPERIMENT!
Hypotheses to Explore • IF…. Stars are born like children, cool and small, and then heat up and grow brighter over time. • IF…Stars are like candles, hot and bright, and then cool off and get dimmer over time. “New” clusters like the Pleiades show all kinds of stars!
Step 5: Size! • How near are stars?DISTANCE • What colors are stars?TEMPERATURE • How luminous are stars?LUMINOSITY • Are brighter stars hotter?TEMP vs. LUMIN. • What sizes are stars?SIZE • Single or Multiple? ORBITS MASS
Diameter (Size) of Stars • Calculated from known values: • Luminosity • Temperature • Laws of Physics • Stefan-Boltzmann Law: Luminosity ~ Surface Area x T4 • PingPong balls, volleyballs, and Stars…
HUGE Bigger tiny! Smaller
Determining Size with SPECTRA! • Larger stars can be less dense at edges • Less dense gas changes absorption lines
Luminosity Classes of Stars • Based on Spectral Line Shapes & Density • Tied to physical SIZE
Step 6: Mass! • How near are stars?DISTANCE • What colors are stars?TEMPERATURE • How luminous are stars?LUMINOSITY • Are brighter stars hotter?TEMP vs. LUMIN. • What sizes are stars?SIZE • Single or Multiple? ORBITS MASS
Masses from BINARY STARS • Doppler Shift of Spectra Lines tells us orbital velocities • Velocities get us Orbital Sizes • Orbits get us Mass
