Download Chapter 10: The Stars

CHAPTER 10: THE STARS
S&T P. 491
• 7. Why do astronomers need to
know the apparent brightness and
luminosity of stars?
• The relationship between AB and
luminosity is used to determine the
distance to the star.
S&T P. 491
• 8. Why doesn’t stellar parallax work to find
distances to all stars?
• As the stars get further away, the parallax
angle gets smaller (and the apparent shift
gets smaller). At distances beyond a few
hundred light years, the angle and
apparent shift are so small we can’t
measure them from Earth.
STELLAR PARALLAX
Earth in January
Earth in June
Star A
Star B
Star C
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?
•𝐴𝐵 =
𝐿
4𝜋𝑑 2
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?
• 𝐴𝐵 =
𝐿
4𝜋𝑑 2
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?
• 𝐴𝐵 =
𝐿
4𝜋𝑑 2
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?
•𝐴𝐵 =
𝐿
4𝜋𝑑 2
FOR STARS THAT ARE CLOSE-BY
• We can use the Inverse Square
Law to calculate Luminosity
•𝐴𝐵 =
𝐿
4𝜋𝑑 2
Calculate
FOR STARS THAT ARE FAR AWAY
• We can use the Inverse Square
Law to calculate Distance
•𝐴𝐵 =
𝐿
4𝜋𝑑 2
Calculate
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?
• 𝐴𝐵 =
𝐿
4𝜋𝑑 2
COLORFUL CHARACTERISTICS P. 501
• How can EM radiation be used
determine important characteristics
of stars (composition, temperature,
color)?
COLORFUL CHARACTERISTICS P. 501
• Recall in Chapter 3 we used a
spectroscope to look at the
emission spectra of various light
sources.
COLORFUL CHARACTERISTICS P. 501
• The emission spectrum is created
because a gas is heated causing the
electrons to gain a quantum of energy,
then release the quantum of energy in
the form of light
• (spectral lines).
COLORFUL CHARACTERISTICS P. 501
• Astronomers use absorption spectra to
study starlight.
• How are absorption and emission
spectra related?
COLORFUL CHARACTERISTICS P. 501
• Absorption spectra are created when
light passes through a cold gas (like the
gas in the cooler outer layer of a star)
and that gas absorbs specific
wavelengths of energy
COLORFUL CHARACTERISTICS P. 501
• Scientists can determine which
elements are present in the outer layers
of a star by the wavelengths of light that
are absorbed .
COLORFUL CHARACTERISTICS P. 501
• Work with your team to complete P&P
#1-8 p. 501-507
• Complete R&C #1-3
COLORFUL CHARACTERISTICS P. 501
• P&P #2a: What are some
features of the spectra?
• There are dark lines, and
colors between them. The
top star has hydrogen
lines. There are a
different number of lines in
each spectrum.
COLORFUL CHARACTERISTICS P. 501
• P&P #2b: What are the
values in nm of the 5
main hydrogen lines?
• 390 nm, 397 nm, 410
nm, 434 nm, 486 nm,
656 nm.
COLORFUL CHARACTERISTICS P. 501
• P&P #2c: What are some
similarities and differences
among spectra?
• All spectra have lines. All
spectra have colors (red –
violet), some have more
lines than others, some
lines are darker and/or
wider.
SPECTRAL INTENSITY GRAPH
• Spectral Intensity Graph shows 2 main
things:
• The wavelength of peak intensity –
corresponds to the color of the star
• The absorption spikes – corresponds to the
elements present.
P&P #4 INTENSITY GRAPHS HANDOUT
• Star A  Star #2
• Star B  Star #3
• Star C  Star #1
• Star D  Star #4
P&P #5 MYSTERY STAR INTENSITY GRAPHS
HANDOUT
• Mystery Star A  Star #3 and intensity
graph B
• Similarities in the number of spikes
• Similar peak intensities (450 nm)
• Mystery Star B  Star #1/ graph C
• Very strong hydrogen lines
• Peak intensities at 400 nm
P&P #6 COLOR OF A STAR
• Astronomers can determine the color of a
star by the intensity graph.
• The peak intensity wavelength determines
the color of the star.
P&P #7 COLOR OF A STAR
• Star A (coolest) yellow color (600 nm peak)
• Star B – bluish-white color (451 nm peak)
• Star C – bluish color (402 nm peak)
• Star D (hottest) blue color (385 nm peak)
P&P #8 TEMPERATURE OF A STAR
• To determine the temperature of a star we
need to use Wien’s Law
2.9𝑥106
Units are in Kelvin
𝑇=
λ max 𝑛𝑚
(
)
R&C #1
• What can you learn about stars when looking at
their spectra?
• Can identify the elements present (absorption
spectra)
• Surface temperature (from peak intensity
wavelength)
• Color (from peak intensity wavelength
R&C #2
• The sun is a yellow-white star. What range of
surface temperatures would you expect for the
Sun?
• Use Wien’s law and the shortest wavelength for
a yellow-white star to find Tmax = 6000K
• Use Wien’s law and the longest wavelength for
a yellow-white star to find Tmin = 5000K
• So the range of surface temp is 5000-6000K
R&C #3
• Spectral analysis…
COLORFUL CHARACTERISTICS P. 501
• Race to the Stars!