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 stars, which last millions to billions of years?of 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

How can we Study the Life Cycles How can we Study the Life Cycles of Stars?of Stars?

• A star can live for millions to billions of years.A star can live for millions to billions of years.• we will never observe a particular star evolve we will never observe a particular star evolve

from birth to deathfrom birth to death• The key is that all stars were not born at the same The key is that all stars were not born at the same

time.time.• the stars which we see today are at different the stars which we see today are at different

stages in their livesstages in their lives• we observe only a brief moment in any one star’s we observe only a brief moment in any one star’s

lifelife• by studying large numbers of stars, we get a by studying large numbers of stars, we get a

“snapshot” of one moment in the history of the “snapshot” of one moment in the history of the stellar communitystellar community

• we can draw conclusions just like we would with we can draw conclusions just like we would with human census data…we do stellar demographics!human census data…we do stellar demographics!

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

Orionis

Geminorum

Order of brightness within a constellation

Latin Genitive of the constellation

Classification of StarsClassification of Stars• The old classification scheme told us little The old classification scheme told us little

about a star’s true (physical) nature.about a star’s true (physical) nature.• a star could be very bright because is was very a star could be very bright because is was very

close to us; not because it was truly brightclose to us; not because it was truly bright• two stars in the same constellation might not two stars in the same constellation might not

be close to each other; one could be much be close to each other; one could be much farther awayfarther away

• In the 20In the 20thth Century, astronomers Century, astronomers developed a more appropriate developed a more appropriate classification system based on:classification system based on:• a star’s a star’s luminosityluminosity• a star’s a star’s surfacesurface temperaturetemperature• A star’s stage of lifeA star’s stage of life

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 and IntensityLuminosity and Intensity

• What is luminosity and how do we What is luminosity and how do we determine it?determine it?

• How do we measure the distance to How do we measure the distance to nearby stars?nearby stars?

• How does the magnitude of a star How does the magnitude of a star depend on its apparent brightness?depend on its apparent brightness?

Our goals for learning:

Luminosity of StarsLuminosity of Stars

• Apparent brightnessApparent brightness refers to the refers to the amount of a star’s light which reaches amount of a star’s light which reaches us us per unit areaper unit area..

• the farther away a star is, the fainter it the farther away a star is, the fainter it appears to usappears to us

• how much fainter it gets obeys an how much fainter it gets obeys an inverse square lawinverse square law

• its apparent brightness decreases as the its apparent brightness decreases as the (distance)(distance)22

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

App Bright = 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

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

Spectral Types of StarsSpectral Types of Stars

• Spectral types are defined by the:Spectral types are defined by the:• existence of absorption lines belonging to various elements, existence of absorption lines belonging to various elements,

ions, & molecules in a star’s spectrumions, & molecules in a star’s spectrum• the relative strengths of these linethe relative strengths of these line

• However, spectral type is not determined by a star’s However, spectral type is not determined by a star’s composition.composition.• all stars are made primarily of Hydrogen & Heliumall stars are made primarily of Hydrogen & Helium

Spectral type is determined by a star’s surface Spectral type is determined by a star’s surface temperature.temperature.• temperature dictates the energy states of electrons in atoms temperature dictates the energy states of electrons in atoms • temperature dictates the types of ions or molecules which temperature dictates the types of ions or molecules which

existexist• this, in turn, determines the number and relative strengths of this, in turn, determines the number and relative strengths of

absorption lines in the star’s spectrumabsorption lines in the star’s spectrum• this fact was discovered by Cecilia Payne-Gaposchkin in 1925this fact was discovered by Cecilia Payne-Gaposchkin in 1925

Spectroscopic parllaxSpectroscopic parllax Summary:Summary: If we know distance (d) from parallax measurements and..If we know distance (d) from parallax measurements and.. If we know apparent visual magnitude (m) from If we know apparent visual magnitude (m) from

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

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

We can obtain spectra and spectral type for all these We can obtain spectra and spectral type for all these nearby stars (about 10,000!)nearby stars (about 10,000!)

We can make a table that provides Absolute visual We can make a table that provides Absolute visual magnitude for stars given their spectral type. magnitude for stars given their spectral type.

With this table, we can find the distance to distant stars With this table, we can find the distance to distant stars simply by obtaining their spectra and apparent visual simply by obtaining their spectra and apparent visual magnitude. magnitude.

In a strange way, we have extended the parallax In a strange way, we have extended the parallax measurements out way beyond the one hundred parsec measurements out way beyond the one hundred parsec limit! limit!