Learning from Light

Learning from LightStars And Their Light

StarsWe can learn a lot about stars from analyzing light.but what and how is this done?DistancesBrightness IntensityTemperatureCompositionOther Insights about our universe

Stars: Measuring Distance Using ParallaxHow can we find the distance to a star that is many light years away?Firstlets take a look at how things move while driving in a car.

Hold your finger out in front of your face at arms length. Alternate opening and closing each eye as you watch your finger.Note the distance your finger appears to shift in relationship to the background.Now, hold your finger closer to your face and complete the same task.What do you notice?

Stars: Measuring Distance Using ParallaxHow can we find the distance to a star that is many light years away?Firstlets take a look at how things move while driving in a car.Hold your finger out in front of your face at arms length. Alternate opening and closing each eye as you watch your finger.Note the distance your finger appears to shift in relationship to the background.Now, hold your finger closer to your face and complete the same task.What do you notice?

Stars: Measuring Distance Using ParallaxHow can we find the distance to a star that is many light years away?

Stars positions in the sky appear to shift through the year.

This is because our perspective changes as we orbit the sun.

Stars: Measuring Distance Using ParallaxHow can we find the distance to a star that is many light years away?

This shift in position is called a parallax shift.

It turns out that the angle the star moves in inversely proportionate to its distance from us

Stars: Measuring Distance Using ParallaxHow can we find the distance to a star that is many light years away?

The closer a star, the more it appears to shift across the sky during the year.

The further away a star, the less it appears to shift across the sky.

Example: If a star is twice as far way, parallax shift is twice as small.

Stars: DistanceOnce the parallax shift angle is found, the distance away from Earth can be calculated mathematically.

Stars: DistanceParallax angles are measured in arc-seconds1 arc-second = 1 = 1/3000 of a degreeDistances to stars measured in parsecs1 parsec = 1 pc = 3.25 light yearsStar 1 pc away would have a parallax angle of 1

Parallax AngleDistance (PC)Distance (LY)11 PC3.25 LY1/2 = 0.52 PC6.50 LY1/10 = 0.110 PC32.50 LY1/1000 = 0.0011,000 PC3,250 LY

Review QuestionWhat is the relationship that exists between the apparent shift (parallax shift) andHow far away an object is?The farther the object the smaller the apparent shiftThe closer the object the larger the apparent shift

The baseline from which the observations are taken?The larger the baseline the larger the apparent shiftThe smaller the baseline the smaller the apparent shift

The parallax angle?The larger the parallax angle the larger the apparent shiftThe smaller the parallax angle the smaller the apparent shift

How do we figure out the distances to stars that are farther than 1000 pc (3,250 LY) We cannot use parallax for stars further than 1,000 PC (3,250 LY) because the parallax angle is too small to measure.

Parallax Shift Explained Scientific American

Stars: Brightness (Luminosity)How can we measure the brightness of a star?A star can appear bright for a number of reasons.It is burning hotterIt is closer to your eyesIt is larger.What if a 100 watt light bulb is 75% further away than a 25 watt bulb?

Stars: Brightness (Luminosity)Apparent MagnitudeHow bright the star looks from EarthA small, cool burning star will look bright if it is close to EarthA hot burning star may look dim if its far away.

If two stars are the same distance away, what may cause one to look brighter than the other one?The brighter star may be bigger or hotter.

Stars: Brightness (Luminosity)Absolute Magnitude (ITS LUMINOSITY)The measurement of light intensity given off.Refers to the amount of energy being emitted from a light source.Like measuring the wattage of a light bulb.The smaller the star magnitude, the brighter it is.Example: (M -4.3) is brighter than (M7)

A stars absolute magnitude depends on how much light power it has and nothing to do with distance from Earth.

Stars: Analyzing the LightEven though stars are far away, we can study them through the light they produce.The process of analyzing starlight is called spectroscopy.The light given off from a star can be broken down into individual colors called a spectrum.

Stars: Analyzing the LightA stars atmosphere absorbs some of the light it gives off.When this happens, some of the rainbow of colors in the spectrum appear to be missing.

Spectra produced by looking at starlight are called absorption spectra.

Stars: Analyzing the LightDifferent types of gasses absorb different wavelengths of light.We can find out what gasses a star is made of by the wavelength of light it absorbs most.Each element has a unique pattern of dark lines (absorption lines) just like a finger print.

Stars: Analyzing the LightBased on the spectrum of galaxy X, which gas is galaxy x composed of?

Stars: Analyzing the LightBased on the spectrum of star W, which gasses is star W composed of?

Stars: Analyzing the LightThe light spectrum can also be described in terms of how much energy it releases.The light energy released from stars is in the form of waves.

Stars: Analyzing the LightThe wavelength of light absorbed depicts the temperature of a star.The spectrum is divided up into several spectral classes.Each spectral class represents a specific temperature.SPECTRAL CLASSES OF STARSOBAFGKM(LT)HOT STARSCOOLSTARS

Stars: Analyzing the LightThe thickness of the absorption line spectra of a star also gives indication to itsHigher TemperatureFaster its rotationHigher Internal Presssure

Stars: Inverse Square LawThe intensity of light weakens in proportion to the square of the distance.As our distance increases, the concentration of light decreases (INVERSE).This law can only be used on stars of the same spectral class.Equation: Luminosity (I) = Distance Squared (d) Squared x Brightness (b)

I = d2 x b

Review Question 1Stars "A" and "B" have the same luminosity. Star "A" is seven times farther away than star "B". How many times brighter will star "B" appear to be than star "A"?

Review Question 2Star "Q" and "P" have the same luminosity. If star "Q" is 4 lightyears away and has a brightness of 2, how far is star "P" if it has a brightness of 9?

Review Question 3Assume that Star S and T are the same class. Using the provided information (Star S brightness = 16 and its distance = 8; Star T has a brightness of 4) what is the distance of Star T?