So You Wanna’ Be A STAR Characteristics of Stars Stellar Evolution.

So You Wanna’ Be ASo You Wanna’ Be A STARSTAR

Characteristics of StarsCharacteristics of Stars

Stellar EvolutionStellar Evolution

What is a Star?

• A body of gases that undergo fusion to create massive amounts of energy in the form of electromagnetic radiation

Characteristics of Stars

Astronomers look at stars through telescopes to analyze the light they emit and their relationships to other stars.

This can reveal to us a star’s:•Composition

•Color

•Temperature

•Mass

Composition of Stars

• A star is made of different elements in the form of gases.

• The gases in the atmosphere of a star absorb different wavelengths of light depending on which elements make up the gases.

• The light from a star indicates which elements make up that star.

• The Colors of Light A prism breaks white light into a rainbow of colors called a spectrum.

• An instrument called a spectrograph is used to break a star’s light into a spectrum. The spectrum of a star will vary depending on which elements are present.

Composition of Stars

There are 3 types of spectraThere are 3 types of spectra

Continuous: Where all Continuous: Where all colors are present.colors are present.

Emission: Where only Emission: Where only a few colors are a few colors are present.present.

Absorption: Where a Absorption: Where a few colors are few colors are missing.missing.

How a Continuous Spectrum is How a Continuous Spectrum is Created.Created.

If we pass white light If we pass white light trough a prism, it will trough a prism, it will be dispersed into a be dispersed into a spectrum spectrum corresponding to corresponding to ROYGBIV. ROYGBIV.

There are no colors There are no colors missing. That’s why missing. That’s why we call it a continuous we call it a continuous spectrum.spectrum.

How an Emission Spectra is How an Emission Spectra is Formed.Formed.

If the atoms in a gas become excited, it will emit If the atoms in a gas become excited, it will emit light.light.

If we pass that light through a prism, it will be If we pass that light through a prism, it will be broken down into its component colors.broken down into its component colors.

Just like the lab we’ll do today.Just like the lab we’ll do today.

Each Element Emits its Own Each Element Emits its Own Unique SpectraUnique Spectra

HydrogenHydrogen

HeliumHelium

MercuryMercury

IronIron

NitrogenNitrogen

Absorption Spectra is the Opposite Absorption Spectra is the Opposite of the Emission Spectraof the Emission Spectra

Instead of a hot gas emitting light…Instead of a hot gas emitting light… A cool gas A cool gas absorbsabsorbs light. light. That is why we call it an That is why we call it an absorption spectrum.absorption spectrum.

This creates an This creates an AbsorptionAbsorption spectumspectum

Where specific Where specific frequencies or colors frequencies or colors are absorbed and do are absorbed and do not show upnot show up

Emission and Absorption Emission and Absorption

The same colors The same colors emitted by a hot emitted by a hot gaseous element…gaseous element…

Are absorbed by that Are absorbed by that same cool gaseous same cool gaseous element element

How do we know what a star is How do we know what a star is made of?made of?

A star is too hot and A star is too hot and far away to take a far away to take a sample.sample.

Nothing would survive Nothing would survive this job.this job.

Not even fire-proof Not even fire-proof monkeys monkeys

So we use absorption spectraSo we use absorption spectra

The outer layers of the sun absorbs some light. The outer layers of the sun absorbs some light. We can determine what elements are in the We can determine what elements are in the

outer layer by examining the spectra.outer layer by examining the spectra. The missing colors correspond to the gases that The missing colors correspond to the gases that

absorbed them. absorbed them.

CompositionComposition

When the spectrum of a When the spectrum of a star is studied the star is studied the spectral lines act as spectral lines act as ““fingerprintsfingerprints.”.”

These lines identify the These lines identify the elementselements present and present and thus the star’s thus the star’s chemical chemical compositioncomposition..

Color & TemperatureColor & Temperature

A star’s color is an indication of its A star’s color is an indication of its temperaturetemperature

HOT stars emit SHORT-wavelength HOT stars emit SHORT-wavelength light and appear BLUElight and appear BLUE

COOL stars emit LONGER-wavelength COOL stars emit LONGER-wavelength light and appear REDlight and appear RED

Stars in the MIDDLE appear YELLOW, Stars in the MIDDLE appear YELLOW, like our sunlike our sun

Color & TemperatureColor & TemperatureStar ColorStar Color Surface Temp. Surface Temp.

(ºC)(ºC)ExamplesExamples

BlueBlue Above 30,000Above 30,000 10 Lacertae10 Lacertae

Blue-WhiteBlue-White 10,000-30,00010,000-30,000 Rigel, SpicaRigel, Spica

Blue-WhiteBlue-White 7,500-10,0007,500-10,000 Vega, SiriusVega, Sirius

Yellow-Yellow-WhiteWhite

6,000-7,5006,000-7,500 Canopus, ProcyonCanopus, Procyon

YellowYellow 5,000-6,0005,000-6,000 Sun, CapellaSun, Capella

OrangeOrange 3,500-5,0003,500-5,000 Arcturus, Arcturus, AldeberonAldeberon

RedRed Less than 3,500Less than 3,500 Betelgeuse, Betelgeuse, AntaresAntares

Stellar Brightness

• The measure of a star’s brightness is its magnitude

• The visibility of a star depends on its brightness and distance from the earth

Apparent Magnitude

The brightness of a star as it appears from Earth

Scaled from -30 to +29; the brighter the star, the smaller the number

The sun has an apparent magnitude of -26.8

The dimmest star visible with the unaided eye is +6

Absolute Magnitude

How bright a star actually isTwo stars with the same apparent magnitude do

not usually have the same absolute magnitude because one may be much further from us than the other

To determine absolute magnitude astronomers determine how bright a star would appear if it were seen at a distance of 32.6 light years

Most stars have an absolute magnitude between -5 and +15.

The sun has an absolute magnitude of +5

Measuring Distances in SpaceMeasuring Distances in Space

Due to the vastness of space, normal Due to the vastness of space, normal earth based measurements like kilometers earth based measurements like kilometers are too small for the scale of the universe. are too small for the scale of the universe. Because the distances are so great, all Because the distances are so great, all movement of stars and other objects in movement of stars and other objects in space appear to be the same (even space appear to be the same (even though they are not)though they are not)

A light-year is the A light-year is the distancedistance light travels in one yearlight travels in one year

300,000 km/s; 180,000 mi/s300,000 km/s; 180,000 mi/s About 9.5 x 10About 9.5 x 1012 12 or 9.5 trillion or 9.5 trillion

kilometers in one yearkilometers in one year Proxima Centauri, the Proxima Centauri, the

second closest star to Earth, second closest star to Earth, is about 4.3 light-years away is about 4.3 light-years away from thefrom the sun sun

Light-YearLight-Year

Parallax

•Parallax is the apparent shift in the position of an object when viewed from different locations. Measuring parallax enables scientists to calculate the distance between a star and the Earth.

Motions of Stars

• The Apparent Motion of Stars If you look at the night sky long enough, the stars also appear to move.

• The Actual Motion of Stars The apparent motion of the sun and stars in our sky is due to Earth’s rotation. But each star is also moving in space. Their actual movements, however, are difficult to see.

Apparent Motion of Stars

Actual Motion of Stars