Chapter 10The Stars

Review/Discussion for Chapter Test

My Notes (+) from Dark SkiesBecause of Light Pollution, 70% of Americans cannot see the Milky

Way.

Flagstaff Dark Skies Coalition: Their mission is to “celebrate, promote, and protect the glorious

dark skies of northern Arizona”. Believe people should be able to experience “seeing the stars” Helping to set regulations for building lights

Fewer Glare-free Lower wattage

International Dark Sky Association: Developed to protect our views of the night-skies.

How are we doing in North Bend-Snoqualmie?http://www.blue-marble.de/nightlights/2010

Stellar Parallax

Fonzie Thumb Shift

Stellar Parallax

Earth in January

Earth in June

Star A Star B Star C

Why Does the Moon Follow You Around?

Driving down a road with mountains in the distance, you'll notice that the mountains don't seem to move much at all, while the farm houses closer to the road move much faster, but still a lot slower than the mailboxes blurring past you right beside the road.

This is the same situation as the moon ‘following you’. The Moon is simply a much bigger ‘mountain’, and it's much further away. This is called parallax. The angle between you and a nearby object changes much more rapidly as you pass them than the angle does for far away objects.

Take a flashlight into a dark room and stand next to a wall. Start wiggling the flashlight, and you'll see the bright spot on the wall moving a little. Back away from the wall, and keep wiggling the flashlight just as much as before. The spot will move faster and further as you increase your distance from the wall. If you get far enough away, you can try to hold the light still, but the spot will still wiggle a bit just because your hand is not perfectly steady.

Also remember this - the Moon is about a thousand miles wide, and you would therefore have to go 1000 miles just to have it move 1 of its widths across the sky. How long does it take you to drive a thousand miles?

My Notes – Monster UnitsLuminosity

Luminosity is the power output of a star (CORRECTION to the text: it is NOT a measure of energy output)

Sun luminosity ≈ 3.8 x 1026 Watts

Rather than use such large numbers, we can compare a star’s luminosity relative to the Sun.

The Sun has luminosity = 1 LsunProxima Centauri has luminosity = .000138 Lsun

Proxima Centauri is MUCH less luminous in addition to being 4.2 ly away. Even if it were only 1 AU, we would barely be able to see it because puts forth too little power.

* We will be using a similar comparison for the masses of celestial bodies, compared to the mass of the sun.

Springing Into Action~

Waves

Review: Amplitude = AWave height = 2APeriod = ΤFrequency = ƒWavelength = λ

Light Waves (electromagnetic waves) travel at about 300,000,000 m/s.This is true in a vacuum and is about the same in all gases.

But what about other types of waves? Obviously water waves don’t move the same speed as light. Or even as fast as sound.

So, wave speed is variable. But what makes it change? What factors affect the speed at which a wave travels through a medium?Do you have any guesses about this right now? MEDIUM!And ONLY Medium affects wave speed!

How Fast Do Waves Move?

If I give you a rope, a meter stick, and a stop watch, how could you measure the speed of a wave? Speed= Distance/Time-Speed=f times w.l.

Introduction to Wave SpeedDuring a storm, you will always see lightning before you hear the thunder. Why?

If the speed of sound is about 340 m/s, how much faster is the speed of light than the speed of sound?

Speed of lightSpeed of sound It is important to know the speed of a wave, because this is the speed at which the energy it carries is transferred from one place to another.

Much Ado about Waves p. 475

This type of wave is called a transverse wave because the direction of motion of the medium is perpendicular to the direction of motion of the wave.

Much Ado about Waves p. 475

Longitudinal waves (compressional waves) need matter through which to travel. They need a medium to compress, otherwise they cannot travel.

What direction does the piece of tape move in relation to the wave?

The tape moves left and right and the wave moves left and right

Much Ado about Waves p. 475

Longitudinal waves (compressional waves) compress matter as they travel through a medium

Much Ado about Waves p. 475

What happens when the frequency of the wave is increased (more waves per second)?

The wavelength gets smaller What happens when the frequency

is low (fewer waves per second)? The wavelength is longer

Much Ado about Waves p. 475

What is the relationship between frequency and wavelength?

Inverse relationship… when one goes up , the other goes down

Transverse & Longitudinal WavesTransverse Waves:

http://upload.wikimedia.org/wikipedia/commons/7/74/Simple_harmonic_motion_animation.gifThe disturbance is perpendicular to the direction the wave

travels.Stringed instruments like guitars, violins, banjosRadio waves, light waves, microwaves

 Longitudinal Waves aka Compression Waves (seismic “P” waves)

The disturbance is parallel to the direction the wave travels.

Sound waves (most of them) Both Transverse and Longitudinal Waves GIF

Neither Transverse or LongitudinalWater waves

http://upload.wikimedia.org/wikipedia/commons/b/be/Shallow_water_wave.gif 

What is a ‘Medium’?A medium is a substance or material that carries the wave. You might have heard the phrase ‘news media’. This refers to the various institutions (newspaper offices, television stations, radio stations, etc.) that carry news from one location to another. • The news moves through the media. • The media doesn't make the news and the media isn't

the ‘news’. • The news media is merely the thing that carries the news

from its source to various locations.

In a similar manner, a wave medium is the substance that carries a wave (or disturbance) from one location to another. The wave medium is not the wave and it doesn't make the wave; it merely carries or transports the wave from its source to other locations.

When a wave is present in a medium (that is, when there is a disturbance moving through a medium), the individual particles of the medium are only temporarily displaced from their rest position. There is always a force acting upon the particles that restores them to their original position.

In a slinky, each coil of the slinky ultimately returns to its original position.

In a water wave, each molecule of the water ultimately returns to its original position.

And in a stadium wave, each fan in the bleacher ultimately returns to its original position.

The particles of the medium (water molecules, slinky coils, stadium fans) simply vibrate about a fixed position as the pattern of the disturbance moves from one location to another location. 

A Wave Transports Energy

STAR LIGHT, STAR BRIGHT

“The main thing we can detect from stars is the EM radiation they produce.”

All matter absorbs EM radiation from its surroundings. Some of the radiation is reflected but at every wavelength, some of it is always absorbed. So, everything is sort of a “black body”… it absorbs every wavelength.

Conversely, all matter also emits electromagnetic radiation when it has a temperature above absolute zero. The radiation represents a conversion of a body's thermal energy into electromagnetic energy. Every object emits radiation of some kind, called Black Body Radiation. The types (wavelengths) of radiation are dependent upon the temperature. Therefore, we can look at the color and other types of radiation coming from any matter and make a pretty accurate estimation of the temperature!

Electromagnetic Spectrum

Black Body Radiation – good to know

•All objects emit radiation of some kind, called BBR

•The hotter the object, the higher the energy, the shorter the dominant wavelength of radiation. So, hotter objects are blue/violet.

•Blue stars are very hot. Yellow-white (like our sun) are cooler.

•Our eyes can fool us, so we use instruments to measure the spectrum and verify the radiation.

•The temperature is related to the peak of the intensity spectrum.•400 nm = 7000 Kelvin

•Wien’s Law:

What Does EM radiation tell us?From EM radiation, we can determine:

The color of the star

The surface temperature of a star

What elements are present

The percentages of each element presentAstronomers use this information to classify stars.More on this Tuesday once you’re well rested from the 3 day weekend!

Emission & Absorption SpectraRemember those Absorption and Emission lines we learned about during Ch. 3?

The emission lines are made when electrons jump from one level to another, releasing energy in the form of light. The energy spacing between ‘orbitals’ is different for each element. So, when an electron jumps from one level to another, the color / frequency is related to the change in energy levels, and is specific to each element present.

When astronomers look at the emission spectra for stars, they can tell what elements are present.

A Watt is a unit of power.Power = Energy / Time

Apparent Brightness – what we seeLuminosity = power output, in Watts.

Comparing brightness with luminosity gives information about how far away a star is located from Earth.

Stellar parallax does not work when stars are really far away… they don’t “move” enough! Instead, astronomers use a light-

meter to measure apparent brightness and compare (using math!) the relative brightness of closer stars to the “distant”

star.

Watt’s More – My Notes

Star Light, Star Bright (Day 2)

Exploration: Flash Light Decrease over

Distance

The Inverse Square Law What is the relationship

between Apparent Brightness and Luminosity (Power Output)?

It is an Inverse Square Relationship – as the distance increases, the AB decreases by the square of the distance.

The Inverse Square Law

What is the equation that relates Apparent Brightness and Luminosity (Power Output) of a star?

S&T p. 497 2. If the distance from the

source of energy increases 4 times, what happens to the apparent brighness, provided the luminosity remains the same?

S&T p. 497 3. If the distance from the

source of energy is cut in half, what happens to the apparent brightness, provided the luminosity remains the same?

Luminosity What is a Cepheid

variable star? Stars that cycle from dim

to bright in a very regular pattern (called a period)

Luminosity Why are Cepheid variable stars

important to astronomers? Henrietta Swan Leavitt discovered

that there is a relationship between a Cepheid’s period and its Luminosity. This is called the period-Luminosity Law.

Putting it all together

How can we use the Inverse Square Law for stars?

For stars that are close-by

We can use the Inverse Square Law to calculate Luminosity

Light meterStellar Parallax

Calculate

For stars that are Far AWAY

We can use the Inverse Square Law to calculate Distance

Light meter

Period-Luminosity

LawCalculate

Practice Question If the distance to Star A is 10 times

greater than the distance to star B, and the 2 stars have the same luminosity, how would their apparent brightness compare?

Try it! You’ll be glad you did!