Guiding Questions
1. How far away are the stars?
2. What evidence do astronomers have that the Sun is a typical star?
3. What is meant by a “first-magnitude” or “second-magnitude”
star?
4. Why are some stars red and others blue?
5. What are the stars made of?
6. As stars go, is our Sun especially large or small?
7. What are giant, main-sequence, and white dwarf stars?
8. How do we know the distances to remote stars?
9. How does our Sun evolve?
10. How can we find the temperature, power, and size of stars from
their spectra?
Careful measurements of the parallaxes
of stars reveal their distances.
• The brightness of a star is not a good
indicator of distance.
e.g., Polaris is closer than Betelgeuse but
Betelgeuse appears brighter.
• Distances to nearby stars can be measured
using parallax.
Parallax is the apparent change in the position of
an object do to a change in observing position.
Stellar Parallax
As Earth moves from
one side of the Sun to
the other, a nearby star
will seem to change its
position relative to the
distant background stars.
d=1/p
d = distance to nearby star in
parsecs
p = parallax angle of that star
in arcseconds
If a star’s distance is known, its Luminosity can
be determined from its brightness.
• A star’s luminosity can be determined from
its apparent brightness if its distance is
known.
L/L = (d/d)2 x (b/b)
Where L = the Sun’s luminosity
Luminosity
Function
As stars go, our Sun
is neither extremely
luminous nor
extremely dim.
It is somewhat more
luminous than most
nearby stars – of the
30 stars within 4 pc,
only three have a
greater luminosity.
Luminosity of Sun = L = 3.86 X 1026 W
Greater distances can be measured with
Cepheid variables
Cepheids compress, heat up, and
brighten
Periods reveals Luminosity of Cepheids
Measure a star’s Luminosity -> find its distance
from its apparent brightness.
• As you get farther and
farther away from a star, it
appears to get dimmer.
• Luminosity, L, doesn’t
change
• Apparent brightness, b,
does change following the
inverse square law for
distance.
b = L / (4pd2)
Intensity = Power/Area
Title
Title
Astronomers often use the magnitude
scale to denote brightness.
• Historically, the apparent magnitude scale runs
from 1 (brightest) to 6 (dimmest).
• Today, the apparent magnitude scale extends
into the negative numbers for really bright
objects and into the 20s and 30s for really dim
objects.
• Absolute magnitude, on the other hand is how
bright a star would look if it were 10 pc away.
Astronomers
often use the
magnitude
scale to
denote
brightness.
A star’s color depends on its surface
temperature.
Wien’s law: l(m) = 3 x 10-3 T(K)
The hotter the object, the shorter the wavelength of its brightest light
UBV photometry is the process of systematically looking at
intensity emitted by a star in three wavelength (color band) regions.
[U: ultraviolet, B: blue, V: visual]
The spectra of stars reveal their chemical
compositions as well as surface temperatures.
• In the late 19th Century,
Harvard astronomers obtained
spectra for hundreds of
thousands of stars.
• Annie Jump Cannon grouped
stellar spectra into a
classification scheme of
spectral types A through O.
• Today we recognize the spectral types O, B, A, F, G,
K, and M as running from hottest to coolest.
The spectra of stars reveal their chemical
compositions as well as surface temperatures.
The spectra of stars reveal their chemical
compositions as well as surface temperatures.
•
•
•
•
O BAF GK M
hottest to coolest
bluish to reddish
Further refined by attaching an integer, for
example: F0, F1, F2, F3 … F9 where F1 is hotter
than F3
• An important sequence to remember:
– Our Best Astronomers Feel Good Knowing More
– Oh Boy, An F Grade Kills Me
– Oh Be a Fine Girl (or Guy), Kiss Me
Strengths of absorption lines
(our Sun is a G2 and has strong FeII and Ca II lines)
Stars come in a wide variety of sizes
Stefan-Boltzmann law relates a star’s energy output,
called LUMINOSITY, to its temperature and size.
Flux = Intensity = Power/Area = sT4
LUMINOSITY = Power = Flux * Area = 4pR2 sT4
LUMINOSITY is POWER, or Energy/time, measured in joules per second
The Stefan-Boltzman constant, s = 5.67 X 10-8 W m-2 K-4
• Small stars have low luminosities unless they are
very hot.
• Cool stars must be very large in order to have large
luminosities (e.g. Red Giants).
HertzsprungRussell (H-R)
diagrams reveal
the different
kinds of stars.
HR DIAGRAM
Absolute magnitude vs
temperature
or
luminosity vs spectral type
Hertzsprung-Russell (H-R) diagrams
reveal the different kinds of stars.
• Main sequence stars
– Stars in hydrostatic
equilibrium found on a line
from the upper left to the
lower right.
– Hotter is brighter
– Cooler is dimmer
• Red giant stars
– Upper right hand corner
(big, bright, and cool)
• White dwarf stars
– Lower left hand corner
(small, dim, and hot)
Determining the Sizes of Stars from an HR Diagram
• Main sequence stars are
found in a band from the
upper left to the lower
right.
• Giant and supergiant stars
are found in the upper
right corner.
• Tiny white dwarf stars are
found in the lower left
corner of the HR diagram.
Details of a star’s
spectrum reveal whether it
is a giant, a white dwarf,
or a main-sequence star.
Luminosity classes
• Class I includes all
the supergiants.
• Class V includes the
main sequence stars.
• The Sun is a G2 V
Luminosity
increases with
mass, in mainsequence stars.
Bigger is brighter!
Luminosity
increases with
temperature, in
main-sequence
stars.
Bigger is hotter!
When core hydrogen burning ceases, a
main-sequence star becomes a red giant
.
• When all of the hydrogen
in the core has been
depleted, the interior can
no longer repel the inward
pull of gravity.
• The core heats under
pressure, causing the outer
layers to expand and swell.
• These outer layers get
farther from the hot core
and cool, resulting in a red
color.
H-R diagram shows
Sun’s evolution.
Guiding Questions
1.
2.
How far away are the stars?
What evidence do astronomers have that the Sun is a
typical star?
3. What is meant by a “first-magnitude” or “secondmagnitude” star?
4. Why are some stars red and others blue?
5. What are the stars made of?
6. As stars go, is our Sun especially large or small?
7. What are giant, main-sequence, and white dwarf stars?
8. How do we know the distances to remote stars?
9. How does our Sun evolve?
10. How can we find the temperature, power, and size of
stars from their spectra?