Understanding the StarsUnderstanding the Stars• How can we learn about the lives of How can we learn about the lives of

stars, which last millions to billions of stars, which last millions to billions of years?years?

• Consider the story of the Consider the story of the EphemeraEphemera

• Image from: Image from: http://www.startunzflutes.com/graphicshttp://www.startunzflutes.com/graphics

Classification of StarsClassification of Stars• Stars were originally classified based on:Stars were originally classified based on:

• their brightnesstheir brightness• their location in the skytheir location in the sky

• This classification is still reflected in the names of This classification is still reflected in the names of the brightest stars…those we can see with our the brightest stars…those we can see with our eyes:eyes:

Orionis Geminorum

Order of brightness within a constellation

Latin Genitive of the constellation

Summer TriangleSummer Triangle•Which of these three Stars in the Summer Triangle is the brightest (to the naked eye)

•Which is the most Luminous?

•Which of these three are the closest?

Dark Sky image of Summer Dark Sky image of Summer Triangle Triangle

Don’t always trust your Don’t always trust your eyes!eyes!

•Note that Deneb is a bit larger than the other two•Its also a bit farther away—2250 light years! •Vega is 22 light years away—its in the neighborhood•Note that Altair (about 30 lyrs) is probably flattened..its rotating very fast! •Apparent brightness depends on size, temp, and distance!

Luminosity of StarsLuminosity of Stars

• Apparent brightness (I)Apparent brightness (I) refers to the amount of a refers to the amount of a star’s light which reaches us star’s light which reaches us per unit areaper unit area..• its apparent brightness its apparent brightness

decreases as the (distance)decreases as the (distance)22

• Apprent brightness is also Apprent brightness is also known as the known as the IntensityIntensity

I = L / 4d2

Luminosity – the total amount of power radiated by a star into space.

• The apparent brightness of a star depends on two things:• How much light is it emitting: luminosity (L) [watts]• How far away is it: distance (d) [meters]

Apparent BrightnessApparent Brightness

The Inverse Square Law for Light What Determines Apparent Brightness?

Measuring Distances to StarsMeasuring Distances to Starsparallax – apparent wobble of a star due to the Earth’s orbiting of the Sun. Check out annimation at: http://instruct1.cit.cornell.edu/courses/astro101/java/parallax/parallax.html

Measuring Distances to Measuring Distances to StarsStars

p = parallax angled = 1 AU/ pGives distance in Parsecs.

convert p into arcsec

d = 206,265 AU/ p

Measuring Distances to StarsMeasuring Distances to Stars

d = 1 / pIf p is in arcsec and d is in parsecs

A star with a parallax of 1 arcsec is 1 parsec distant

let’s define1 parsec 206,265 A.U. = 3.26 light years

The Brightness of StarsThe Brightness of Stars

Astronomers still use an ancient method for measuring stellar brightness which was proposed by the Greek astronomer Hipparchus (c. 190 – 120 B.C.)

This scale runs backwards:The bigger the number, the fainter the starBrightest stars are #1, next brightest are #2, etc.

The Modern Magnitude The Modern Magnitude SystemSystem

apparent magnitude

• brightness of a star as it appears from Earth

= -2.5 log (app bright)

• each step in magnitude is 2.5 times in brightness

absolute magnitude• the apparent magnitude a star would have if it were 10 pc away

What good is this?What good is this? If you know apparent brightness, If you know apparent brightness,

you can find magnitude. you can find magnitude. If you know magnitude, you can use If you know magnitude, you can use

another relationship to find distanceanother relationship to find distance M – m = 5 – 5log(d)M – m = 5 – 5log(d) M= Absolute magitudeM= Absolute magitude M = m when distance is ten parsecs. M = m when distance is ten parsecs.

An example of how this An example of how this works!works!

Deneb has an apparent visual magnitude of Deneb has an apparent visual magnitude of 1.26 (see chart of brightest stars at end of your text)1.26 (see chart of brightest stars at end of your text)

Deneb has an Absolute visual magnitude of -8.73Deneb has an Absolute visual magnitude of -8.73

(this is about the same brightness as the quarter moon---but (this is about the same brightness as the quarter moon---but at 32.6 light years away!)at 32.6 light years away!)

Using the weird equation, the distance to deneb can be Using the weird equation, the distance to deneb can be calculated: 2500 light years (M – m = 5 – 5log(d))calculated: 2500 light years (M – m = 5 – 5log(d))

One last obvious question: How did we ever know the One last obvious question: How did we ever know the Absolute visual magnitude to Deneb without knowing its Absolute visual magnitude to Deneb without knowing its distance in the first place? distance in the first place?

16.3 Classifying Stars 16.3 Classifying Stars • Hypothysis: Hypothysis: • the Luminoisty and Abolsute Magnitude of Stars can be known if we the Luminoisty and Abolsute Magnitude of Stars can be known if we

know their Classification. know their Classification.

• There Classification is completely revealed in their Spectra. There Classification is completely revealed in their Spectra.

• Comparing the spectra of nearby stars allows us to test this hypothis. Comparing the spectra of nearby stars allows us to test this hypothis.

• The Classification is known as The Classification is known as Spectral TypeSpectral Type

Spectral type is revealed in the Colors of Spectral type is revealed in the Colors of StarsStars

Stars come in many different colors.

The color tells us the star’s temperature according to Wien’s Law.

Bluer means hotter!

Spectral Type Classification Spectral Type Classification SystemSystem

O B A F G K M

Oh Be A Fine Girl/Guy, Kiss Me!

50,000 K 3,000 K Temperature

(L)

Spectral Types of StarsSpectral Types of Stars

Spectral Types of StarsSpectral Types of Stars

Spectroscopic parllaxSpectroscopic parllax If we know distance (d) from parallax If we know distance (d) from parallax

measurements and..measurements and.. If we know apparent visual magnitude (m) from If we know apparent visual magnitude (m) from

photometry or image size (apparent brightness photometry or image size (apparent brightness is also measured in this way).is also measured in this way).

Then we can calculate Absolute visual Then we can calculate Absolute visual magniutde (M). Luminosity is also measured in magniutde (M). Luminosity is also measured in this way. this way.

At the same time, we can obtain spectra and At the same time, we can obtain spectra and spectral type for all these nearby stars (about spectral type for all these nearby stars (about 10,000!)10,000!)

Now we know M from looking at the spectra! Now we know M from looking at the spectra!