The document discusses how astronomers can learn about stars by analyzing the light they emit. Key points include: - Parallax is used to measure the distance to nearby stars by observing how their positions shift relative to background stars over the year as Earth orbits the sun. Closer stars have larger parallax shifts. - Absolute magnitude indicates a star's intrinsic brightness, while apparent magnitude depends on both brightness and distance from Earth. Brighter or larger stars have smaller magnitudes. - Spectroscopy breaks light into a spectrum that reveals the chemical composition of a star by dark absorption lines that different elements produce. Temperature can also be inferred from spectrum. - The inverse square law states that a star's apparent brightness reduces with the square of

1. Learning from Light
Stars And Their Light

2. Stars
• We can learn a lot about stars from analyzing light….but what and how is this
done?
• Distances
• Brightness Intensity
• Temperature
• Composition
• Other Insights about our universe

3. Stars: Measuring Distance Using Parallax
• How can we find the distance to a star that is many light years
away?
• First…let’s 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?

4. Stars: Measuring Distance Using Parallax
• How can we find the distance to a star that is many light years
away?
• First…let’s 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?

6. Stars: Measuring Distance Using Parallax
How 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.

7. Stars: Measuring Distance Using Parallax
How 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…

8. Stars: Measuring Distance Using Parallax
How 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.

9. Stars: Distance
• Once the parallax shift angle is found, the distance away from
Earth can be calculated mathematically.

10. Stars: Distance
• Parallax angles are measured in arc-seconds
• 1 arc-second = 1” = 1/3000 of a degree
• Distances to stars measured in parsecs
• 1 parsec = 1 pc = 3.25 light years
• Star 1 pc away would have a parallax angle of
1”
Parallax Angle Distance (PC) Distance (LY)
1” 1 PC 3.25 LY
1/2“ = 0.5“ 2 PC 6.50 LY
1/10“ = 0.1” 10 PC 32.50 LY
1/1000“ = 0.001” 1,000 PC 3,250 LY

12. Review Question
• What is the relationship that exists between the apparent shift (parallax
shift) and…
• How far away an object is?
• The farther the object the smaller the apparent shift
• The closer the object the larger the apparent shift
• The baseline from which the observations are taken?
• The larger the baseline the larger the apparent shift
• The smaller the baseline the smaller the apparent shift
• The parallax angle?
• The larger the parallax angle the larger the apparent shift
• The smaller the parallax angle the smaller the apparent shift

13. 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.

14. Parallax Shift Explained – Scientific American

15. 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 hotter
• It is closer to your eyes
• It is larger.
• What if a 100 watt light bulb is 75% further away
than a 25 watt bulb?

16. Stars: Brightness (Luminosity)
• Apparent Magnitude
• How bright the star looks from Earth
• A small, cool burning star will look
bright if it is close to Earth
• A 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.

17. Stars: Brightness (Luminosity)
• Absolute Magnitude (IT’S 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 star’s absolute magnitude depends on how much
light power it has and nothing to do with distance
from Earth.

18. Stars: Analyzing the Light
• Even 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.

19. Stars: Analyzing the Light
• A 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.

20. Stars: Analyzing the Light
• Different 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.

21. Stars: Analyzing the Light
• Based on the spectrum of
galaxy X, which gas is
galaxy x composed of?

22. Stars: Analyzing the Light
• Based on the spectrum
of star W, which gasses
is star W composed of?

23. Stars: Analyzing the Light
• The 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.

24. Stars: Analyzing the Light
• The 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 STARS
O B A F G K M (L T)
HOT
STARS
COOL
STARS

25. Stars: Analyzing the Light
• The thickness of the absorption line spectra of a star also gives
indication to its
• Higher Temperature
• Faster it’s rotation
• Higher Internal Presssure

26. Stars: Inverse Square Law
• The 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

27. Review Question 1
• Stars "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"?

28. Review Question 2
• Star "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?

29. Review Question 3
• Assume 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?