The Stars, Galaxies, and The Stars, Galaxies, and UniverseUniverse

Light: The Astronomer’s Light: The Astronomer’s Friend!Friend!

Light: Light: Maxwell’s Equations

This is what you just saw. The This is what you just saw. The prism refracts (bends) the light prism refracts (bends) the light

and divides it up into its and divides it up into its component parts.component parts.

Light can also be diffracted (split) Light can also be diffracted (split) by diffraction gratings, CDs, by diffraction gratings, CDs,

oily surfaces, and soap oily surfaces, and soap bubbles.bubbles.

Before we can study stars, we Before we can study stars, we must study the light that must study the light that

comes from them. For, it is this comes from them. For, it is this light that tells us everything light that tells us everything

that we want to know about a that we want to know about a star.star.

A nebula, the birthplace of starsA nebula, the birthplace of stars

Stars give off different types of Stars give off different types of light. We need to use these light. We need to use these

different types of light to study to study them. This involves using the them. This involves using the

electromagnetic spectrum..

Mr. T gives off a different type of Mr. T gives off a different type of light too. What kind of light is light too. What kind of light is

this?this?

Mr. T gives off a different type of Mr. T gives off a different type of light too. What kind of light is light too. What kind of light is

this?this?

Infrared! Visible

Infrared (FLIR) Camera

Visible Camera

Light is a form of electromagnetic Light is a form of electromagnetic radiation. It travels in waves, radiation. It travels in waves, similar to the waves on the similar to the waves on the

ocean.ocean.

Waves have certain Waves have certain parts to them. to them. The The crestcrest is the highest part, is the highest part, and the and the troughtrough is the lowest is the lowest

part.part.

The The amplitudeamplitude of a wave is the of a wave is the distance from the center of the distance from the center of the wave to the crest (or trough). It wave to the crest (or trough). It

is ½ of the wave height. is ½ of the wave height. Amplitude is usually measured Amplitude is usually measured

in the base unit of meters.in the base unit of meters.

Wave height

The The wavelengthwavelength of a wave is the of a wave is the distance from one crest to the distance from one crest to the next crest, or one trough to the next crest, or one trough to the

next trough.next trough.

The Greek letter lambda (The Greek letter lambda () ) means wavelength.means wavelength.

The The wavelengthwavelength of a wave is of a wave is measured in the base unit of measured in the base unit of

meters. Because the waves are meters. Because the waves are usually small, they are usually small, they are

measured in measured in μμm (micrometers) m (micrometers) or nm (nanometers). 1 or nm (nanometers). 1 μμm = 1 m = 1 X 10X 10-6-6 m and 1 nm = 1 X 10 m and 1 nm = 1 X 10-9-9 m m

The The frequency of a wave is a of a wave is a measure of how many wave measure of how many wave

crests pass a certain point in a crests pass a certain point in a certain time. The more waves, certain time. The more waves, the higher the frequency. The the higher the frequency. The

fewer waves, the lower the fewer waves, the lower the frequency.frequency.

The Greek letter nu (The Greek letter nu () means ) means frequency

The The frequency of a wave is of a wave is measured in units called hertz measured in units called hertz

(Hz). 1 hertz = 1 cycle per (Hz). 1 hertz = 1 cycle per second. second.

KROQ radio broadcasts on an FM KROQ radio broadcasts on an FM frequency of 106.7 MHz, KFWB of 106.7 MHz, KFWB

radio broadcasts on an AM radio broadcasts on an AM frequency of 980 KHz, and my frequency of 980 KHz, and my radio control airplane works on radio control airplane works on

a frequency of 72.670 MHz.a frequency of 72.670 MHz.

Electromagnetic waves are the result of Electromagnetic waves are the result of electricity and magnetism working working together. The Blue arrows show the together. The Blue arrows show the

electric field and the red arrows show electric field and the red arrows show the magnetic field. The fields are the magnetic field. The fields are

perpendicular to each other and to the perpendicular to each other and to the direction of the wave.direction of the wave.

Electromagnetic waves travel at Electromagnetic waves travel at the speed of light, which is the speed of light, which is measured to be 3.0 X 10measured to be 3.0 X 1088 meters/second or 186,000 meters/second or 186,000

miles/second or 669,600,000 miles/second or 669,600,000 miles/hour?miles/hour?

The speed of light (c) is equal to the frequency (

) times the waveegth ().c = x

How are energy, frequency, and How are energy, frequency, and wavelength related? Go wavelength related? Go here to to

find out.find out.

Here is the calculation for radio Here is the calculation for radio station KROQ at 106.7 MHz, or station KROQ at 106.7 MHz, or 106,700,000 Hz = 106,700,000 106,700,000 Hz = 106,700,000

cycles/second.cycles/second.

300,000,000 300,000,000 meters/second = meters/second =

106,700,000 106,700,000 cycles/second X 2.81 cycles/second X 2.81

metersmeters

The light that you see and the The light that you see and the radios stations described radios stations described beforehand are all part of beforehand are all part of

the the electromagnetic spectrum..

Here is a Here is a NOVA tour of the tour of the electromagnetic spectrum..

So, How do we know what stars So, How do we know what stars are made of is we can’t go are made of is we can’t go

to them to sample their to them to sample their chemistry?chemistry?

We use a method called We use a method called spectroscopy, where the , where the

spectra of a star is analyzed. spectra of a star is analyzed. We separate the light coming We separate the light coming

from a star by using a tool from a star by using a tool called a spectroscope.called a spectroscope.

Examples of Continuous and Examples of Continuous and Emission SpectraEmission Spectra

Continuous Spectra of Sunlight (photo courtesy of Michael Greenberg, 2012)

Emission Spectra of an iPhone screen (photo courtesy of Michael Greenberg, 2012)

We then analyze the spectra, We then analyze the spectra, or colors, that a star emits or colors, that a star emits

(gives off) or absorbs.(gives off) or absorbs.

The spectra below is referred to The spectra below is referred to as a as a continuous spectrumcontinuous spectrum. .

Light from all visible Light from all visible wavelengthswavelengths is being is being

emitted.emitted.

A continuous spectrum is emitted from A continuous spectrum is emitted from glowing solids (a light bulb), glowing glowing solids (a light bulb), glowing

liquids (molten iron), and the hot liquids (molten iron), and the hot compressed gases inside starscompressed gases inside stars

The second spectra below is referred The second spectra below is referred to as an to as an emission spectrumemission spectrum. Light . Light from only certain from only certain wavelengthswavelengths is is

being emitted.being emitted.

Each chemical element emits a different Each chemical element emits a different pattern of light. We can analyze these pattern of light. We can analyze these patterns to determine what chemical patterns to determine what chemical

element(s) is(are) in a star.element(s) is(are) in a star.

Go Go here for an example of for an example of various various emission spectraemission spectra for for

different gases.different gases.

The third spectra below is referred to The third spectra below is referred to as an as an absorption spectrumabsorption spectrum. Light . Light from only certain from only certain wavelengthswavelengths is is

being absorbed by the cooler gas it being absorbed by the cooler gas it passes through.passes through.

““By comparing emission and absorption By comparing emission and absorption spectra, scientists can determine what spectra, scientists can determine what elements are present in the cooler gas elements are present in the cooler gas

that is absorbing some of the light.”that is absorbing some of the light.”

We can also use absorption spectra to We can also use absorption spectra to determine the composition of a determine the composition of a

planet’s atmosphere. planet’s atmosphere.

““If the spectrum of the reflected light from If the spectrum of the reflected light from a planet contains dark lines not a planet contains dark lines not

contained in the sun’s spectrum, then contained in the sun’s spectrum, then these lines must be caused by these lines must be caused by

substances in the planet’s substances in the planet’s atmosphere.”atmosphere.”

Sources of Continuous, Emission, and Sources of Continuous, Emission, and Absorption Spectra Absorption Spectra

ContinuousContinuous: From a light bulb or hot : From a light bulb or hot metalmetalEmissionEmission: From light emitted from a : From light emitted from a hot gashot gasAbsorptionAbsorption: From light that passes : From light that passes through a cooler gas through a cooler gas

The Doppler Effect. The Doppler Effect.

We can measure the amount of blue-shift We can measure the amount of blue-shift or red-shift in a star’s spectrum to or red-shift in a star’s spectrum to

determine if a star is moving towards determine if a star is moving towards us or away from us.us or away from us.

The Doppler Effect. The Doppler Effect.

You have experienced the Doppler Effect You have experienced the Doppler Effect before with police cars. If the police car before with police cars. If the police car

comes toward you, the higher pitch comes toward you, the higher pitch (shorter wavelength) you hear.(shorter wavelength) you hear.

The Doppler Effect. The Doppler Effect.

You have experienced the Doppler Effect You have experienced the Doppler Effect before with police cars. If the police car before with police cars. If the police car moves away from you, the lower pitch moves away from you, the lower pitch

(longer wavelength) you hear.(longer wavelength) you hear.

The Doppler Effect. The Doppler Effect.

If a star is moving towards us, we will see a If a star is moving towards us, we will see a blue-shift. This is because the blue-shift. This is because the

wavelengths of light are compressed wavelengths of light are compressed into the shorter blue wavelengths.into the shorter blue wavelengths.

The Doppler Effect. The Doppler Effect.

If a star is moving away from us, we will If a star is moving away from us, we will see a red-shift. This is because the see a red-shift. This is because the

wavelengths of light are expanded into wavelengths of light are expanded into the longer red wavelengths.the longer red wavelengths.

The Doppler Effect. The Doppler Effect.

The spectra of a star reveal which direction The spectra of a star reveal which direction a star is moving.a star is moving.

The Doppler Effect. The Doppler Effect.

A cosmological red-shift indicates that stars A cosmological red-shift indicates that stars and galaxies are moving away from us. and galaxies are moving away from us.

As Mr. Auld pointed out, this means As Mr. Auld pointed out, this means that our Universe is expanding, or that our Universe is expanding, or

moving outwards. WOW!moving outwards. WOW!

The Doppler Effect. The Doppler Effect.

Explain this sticker seen on the back of a Explain this sticker seen on the back of a car at Caltech. What does it mean?car at Caltech. What does it mean?

Stars and Their Stars and Their CharacteristicsCharacteristics

How many stars are in our galaxy? How many stars are in our galaxy? How many galaxies are in our How many galaxies are in our

Universe? The stars that you see Universe? The stars that you see with the naked eye are only within with the naked eye are only within

our Milky Way Galaxy.our Milky Way Galaxy.

What is the difference between What is the difference between astroastrollogy and astroogy and astronnomy?omy?

AstroAstrollogy is a ogy is a beliefbelief that the stars that the stars and planets can influence human and planets can influence human

life in a mystical fashion.life in a mystical fashion.

AstroAstronnomy is a science that attempts omy is a science that attempts to explain the Universe and to explain the Universe and everything within it. We are everything within it. We are

studying astrostudying astronnomy in this class.omy in this class.

We call groups of stars We call groups of stars constellationsconstellations. . These are names given to groups These are names given to groups of stars by ancient astronomers. of stars by ancient astronomers.

ConstellationsConstellations help us to find stars. help us to find stars. Go here for more information.Go here for more information.

The Constellation Orion

The positions of constellations change The positions of constellations change throughout the year because of throughout the year because of

Earth’s revolution around the sun.Earth’s revolution around the sun.The Constellation Orion is visible in Winter

The Constellation Scorpius is visible in Summer

Stars that are near the North Star Stars that are near the North Star (Polaris) are called Circumpolar (Polaris) are called Circumpolar

StarsStars

The The Apparent MagnitudeApparent Magnitude is how bright a is how bright a star appears from the Earth. The lower star appears from the Earth. The lower

the Star’s apparent magnitude, the the Star’s apparent magnitude, the brighter the star is. Negative brighter the star is. Negative

magnitudes are the brightest stars. For magnitudes are the brightest stars. For each decrease of 1 in apparent each decrease of 1 in apparent magnitude, relative brightness magnitude, relative brightness

increases by 2.5 times.increases by 2.5 times.magnitudmagnitudee

00 11 22 33 44 55 66 77 88 99 1010

Relative Relative brightnesbrightnesss

1010000000

40040000

16160000

630630 250250 100100 4040 1616 6.36.3 2.52.5 11

Star distances are measured in Star distances are measured in units called units called light yearslight years. A . A light light yearyear is the distance that light is the distance that light

travels in one year.travels in one year.

If light travels at 186,000 miles If light travels at 186,000 miles per second or 686,600,000 per second or 686,600,000

miles per hour, then how many miles per hour, then how many miles is one light year?miles is one light year?

Star Star ParallaxParallax, an apparent , an apparent shift in star position, is a shift in star position, is a

method of measuring method of measuring distances to the nearest distances to the nearest

stars.stars.

Depth Depth PerceptioPerception Parallax n Parallax CartoonCartoon

Graphic sent by Veronica Marshall, 2012

You use parallax every You use parallax every day. What do you use day. What do you use

it for?it for?

Distance in parsecs = 1 / parallax Distance in parsecs = 1 / parallax angleangle

1 parsec = 3.26 light years1 parsec = 3.26 light years

1 parsec = 3.26 light years1 parsec = 3.26 light years

Stars have different mass, size, Stars have different mass, size, and temperature.and temperature.

StarStar TypeType Solar Solar massesmasses

Solar radiiSolar radii

SiriusSirius Main seq.Main seq. 2.32.3 2.52.5

RigelRigel Blue superBlue super 2020 3636

BetelgeuseBetelgeuse Red superRed super 2020 1,0001,000

AldebaranAldebaran Red giantRed giant 55 2020

DenebDeneb Yellow Yellow SuperSuper

1414 6060

CapellaCapella Red GiantRed Giant 3.53.5 1313

PolluxPollux Red GiantRed Giant 44 88

AltairAltair Main seq.Main seq. 22 1.51.5

Relative Sizes of Stars and Relative Sizes of Stars and YouTube MovieYouTube Movie

Relative Sizes of StarsRelative Sizes of Stars

Relative Sizes of StarsRelative Sizes of Stars

Relative Sizes of StarsRelative Sizes of Stars

So…What is the Biggest Star?So…What is the Biggest Star?

Temperature and Color of StarsTemperature and Color of Stars

Temp. (C)Temp. (C) ColorColor ElementsElements ClassClass

> 30,000> 30,000 Bluish Bluish whitewhite

Ionized HeIonized He OOhh

9,500-9,500-30,00030,000

Bluish Bluish whitewhite

Neutral HeNeutral He BBee

7,000-7,000-9,5009,500

WhiteWhite Metals, HMetals, H AA

6,000-6,000-7,0007,000

Yellow Yellow whitewhite

Metals, HMetals, H FFineine

5,200-5,200-6,0006,000

YellowYellow Metals, HMetals, H GGirl (or irl (or GGuy)uy)

3,900-3,900-5,2005,200

Yellow Yellow orangeorange

Metals, HMetals, H KKississ

< 3,900< 3,900 redred TiOTiO22 MMee

The The LuminosityLuminosity of a star (shaped of a star (shaped like a sphere) is its actual like a sphere) is its actual

brightness.brightness.

LuminosityLuminosity only depends on only depends on size and temperature.size and temperature.

The formula for luminosity is:The formula for luminosity is:L = 4L = 4rr22σσTT44

where L is Luminosity in Watts, where L is Luminosity in Watts, is 3.1416…, is 3.1416…, r is radius in meters, r is radius in meters, σσ is Stefan-Boltzmann’s is Stefan-Boltzmann’s

constant (constant (5.6703 x 105.6703 x 10-8-8 (W/m (W/m22KK44)), and T is , and T is Temperature in KelvinTemperature in Kelvin

The The Absolute Magnitude Absolute Magnitude of a star of a star is how bright it would be if it is how bright it would be if it

were a distance of 10 parsecs were a distance of 10 parsecs (32.6 light years) from Earth.(32.6 light years) from Earth.

This is different from apparent This is different from apparent magnitude because the magnitude because the

apparent magnitude does not apparent magnitude does not account for the account for the distancedistance to to

the star and the star’s the star and the star’s luminosityluminosity..

Apparent Magnitude vs. Absolute MagnitudeApparent Magnitude vs. Absolute Magnitude

StarStar Apparent Mag.Apparent Mag. Absolute Mag.Absolute Mag.

AldebaranAldebaran +0.87+0.87 -0.65-0.65

AlgolAlgol +2.09+2.09 -0.15-0.15

AntaresAntares +1.06+1.06 -5.38-5.38

BetelgeuseBetelgeuse +0.45+0.45 -5.09-5.09

CapellaCapella +0.08+0.08 -0.48-0.48

PolarisPolaris +1.97+1.97 -3.59-3.59

ProcyonProcyon +0.41+0.41 +2.62+2.62

RigelRigel +0.18+0.18 -6.75-6.75

SiriusSirius -1.44-1.44 +1.42+1.42

SpicaSpica +0.98+0.98 -3.55-3.55

Cepheid VariableCepheid Variable Stars can be Stars can be used to measure long distances used to measure long distances

to stars. Cycles of brightness to stars. Cycles of brightness range from 1 to 50 days. A star range from 1 to 50 days. A star with a cycle of 50 days would with a cycle of 50 days would be brighter than a star with a be brighter than a star with a

brightness range of 1 day. brightness range of 1 day. Astronomers can calculate long Astronomers can calculate long

distances by comparing a distances by comparing a Cepheid’s apparent and Cepheid’s apparent and

absolute magnitude.absolute magnitude.

Life Cycles of Stars are Life Cycles of Stars are shown with a shown with a

Hertzsprung-Russell Hertzsprung-Russell DiagramDiagramBlue Super

GiantsRed Super Giants

Main Sequence

White DwarfsRed Dwarfs

We gauge the Life Cycle of We gauge the Life Cycle of Stars using the Stars using the

Hertzsprung Russell (H-R) Hertzsprung Russell (H-R) DiagramDiagramBlue Super

GiantsRed Super Giants

Main Sequence

White DwarfsRed Dwarfs

Another H-R DiagramAnother H-R Diagram

Click on this link to Click on this link to investigate the different investigate the different life cycles of stars and life cycles of stars and

how they relate to the H-R how they relate to the H-R Diagram.Diagram.

The stability of a star The stability of a star depends on the balance depends on the balance between gravity pulling between gravity pulling inward and energy from inward and energy from

nuclear reactions pushing nuclear reactions pushing outwards.outwards.

Relativity and Black HolesRelativity and Black Holes

Albert Einstein was famous for his ideas on relativity.

The Stars Study QuizThe Stars Study Quiz

The Stars Pre QuizThe Stars Pre QuizPlease respond with Please respond with

“Agree,” “Disagree,” or “Agree,” “Disagree,” or “Don’t Know” on a half “Don’t Know” on a half

sheet of paper.sheet of paper.

1.1. We know about stars We know about stars by analyzing the light by analyzing the light

that we see.that we see.

2.2. Stars have life cycles, Stars have life cycles, which last many many which last many many

years.years.

3.3. The Universe is The Universe is becoming smaller.becoming smaller.

4.4. Stars are much closer Stars are much closer to us than the planets to us than the planets

are.are.

5.5. Hotter stars are red in Hotter stars are red in color.color.