Download The Stars

The Stars
How far away are they?
How bright and hot are they?
How are they similar to the sun?
How are they different from the sun?
Distances in Solar System
A beam of radar (or a laser) traveling at the speed of
light takes 2.58 seconds to go from the Earth to the
Moon and back. That’s 1.29 seconds one way.
1.29 x 186,000 mps = 240,000 miles.
Determining Distances
Beyond the Solar System
• The astronomic unit
works well for the Solar
System, but not
beyond.
• It takes too long for
radar to make the round
trip, so…
• To determine distances
in space, we use the
concept of Parallax.
The Parsec (parallax second)
• As the earth revolves around the sun, objects
in space shift due to parallax.
• A star is 1 parsec from earth if the shift is one
arcsecond (1/3600 degree).
D= 1
p
Suppose a star shifts 0.1
seconds of arc during the
earth’s revolution around the
sun. Then…
The star is 1/0.1 = 10
parsecs from Earth.
Distant stars
0.1 sec
Accuracy of parallax method
Such very small parallax shifts tell us
that stars are very far away from us.
Therefore, we cannot accurately
measure the distance of stars more
than 50 parsecs away, which is a shift
of only 0.02 arcseconds or 0.0000055
degrees.
Magnitude Revisited
• The brightness of the star is measured
by its magnitude.
• The brighter the star, the lower its
magnitude.
• The star Sirius has a magnitude of -1
and Polaris is +2. So Sirius is the
brighter star as we see it.
• This is called apparent magnitude.
Absolute Magnitude
• A star can be bright as
we see it for two reasons:
it actually is bright, or it is
close to us.
• Absolute magnitude
compares star brightness
as if they were 10
parsecs away.
A
B
Star A appears
brighter because it is
closer. When we
“move” them both to
10 parsecs, then B is
actually brighter. It
has the higher
absolute magnitude.
Are the stars moving?
• Stars appear to be
standing still (“fixed”),
but they are actually
moving fast.
• The great distances
make this proper
motion difficult to
detect.
• Barnard’s Star has the
greatest proper
(sideways) motion.
Big Dipper Animation
Are they getting closer or
farther away?
• Motion toward or
away from us is
detected by the
Doppler Effect.
• If the shift is toward
the red, it is
receding from us.
• If the shift is toward
the blue, it is
approaching us.
The larger the shift,
the faster the
motion of the star!
LUMINOSITY:
the energy radiated in a
unit of time
• Luminosity can be measured in watts (i.e. a
light bulb). With stars, we usually rate luminosity
with Sun = 1.
• Stars are very luminous if their absolute
magnitudes are low, and not luminous if the
absolute magnitudes are high.
• Star luminosity is dependent on the star’s
temperature and size (Stefan-Boltzman Law).
Stefan-Boltzman Law
R = 2 suns
R = 1 sun
T = 4000
°K
L = 4πR2σT4
• If the radius is
doubled, luminosity is
quadrupled.
• If the temperature is
doubled, luminosity
increases 16 fold.
T = 8000
°K
The spectra of stars changes
with temperature
HOT
star:
peak
is in
ultra
violet
L=
100
COOL star:
peak is in far
red
L = 0.1
SPECTRAL CLASSES
Temperature
O
30,000 - 60,000 K
B
10,000 - 30,000 K
A
7,500 - 10,000 K
F
6,000 - 7,500 K
G
5,000 - 6,000 K
K
3,500 - 5,000K
M
< 3,500 K
Blue stars
Blue-white stars
White stars
Yellow-white stars
Yellow stars (like the Sun)
Yellow-orange stars
Red stars
The commonly used mnemonic for the sequence of these
classifications is
"Oh Be A Fine Girl, Kiss Me".
Variety in Stars: size and color
For most stars, the hotter the star is the more massive it
is. Why is Capella an exception?
Variety in Luminosity
Due to Size
The Hertzsprung - Russell
Diagram
In the early
1900s, two
astronomers
independently
plotted the
temperatures of
stars vs. their
luminosities.
They found
most stars fell
along a diagonal
in the middle.
The Main Sequence
• This band is called
the main sequence.
• Generally, the hotter
the star, the more
luminous it will be.
• The main sequence
is not a straight line
but a band.
Why is the main sequence
curved?
• If all stars were the
same size as the
sun, the main
sequence would be
this red line.
• However, hot stars
are larger, and cool
stars are smaller
than the Sun.
LARGE
SMALL
The Red Giants
A minority of stars are
off the main
sequence.
Above the main
sequence on the HR
Diagram are the red
giants.
They are cool stars but
large, so they are
luminous.
The Supergiants!
• On the very top of the
HR Diagram are cool
but extremely bright
stars.
• These are the
supergiants, stars that
some day will
supernova.
• They eventually might
become black holes.
White Dwarf Stars
• In the opposite
corner of the HR
Diagram are small
hot stars, the white
dwarves.
• Their low luminosity
is due to their small
size, often less than
the Earth.
Mass-Luminosity
Relationship
• By studying binary star
systems, we can
determine the mass of
stars.
• For main sequence stars,
the greater the mass the
more luminous they are.
• In fact, the mass of a star
is its most important
property, determining its
temperature and the length
of its life.
For most stars,
you can
determine the
luminosity by this
formula:
Mass =
36 suns
L = 363 • √36
L = M3 • √M
L = 46656 • 6 =
279,936 suns
Quick Quiz!
• What is a parsec and how is it determined?
• What is the difference between apparent and
absolute magnitude?
• What two factors cause luminosity to increase?
• What are the spectral classes?
• Why is a blue star more luminous than a yellow
star of the same size?
• What does the H-R diagram show us about most
stars (main sequence stars)?
• What are red giants and white dwarf stars?
• What is the mass-luminosity relationship?