Properties of Stars
“All men have the stars,” he answered,
“but they are not the same things for
different people. For some, who are
travelers, the stars are guides. For others
they are no more than little lights in the
sky. For others, who are scholars, they
are problems. For my businessman they
were wealth. But all these stars are
silent. You – you alone – will have the
stars as no one else has them.”
Antoine de Saint-Exupery (1900 – 1944)
from The Little Prince
WHAT DO YOU
THINK?
1.
How near to us is the closest star other than the
Sun?
2.
What colors are stars, and why do they have
these colors?
3.
How luminous is the Sun compared with other
stars?
WHAT DO YOU
THINK?
4.
Are brighter stars hotter than dimmer stars?
5.
Compared to the Sun, what sizes are other
stars?
6.
Are most stars similar to the Sun, one star with
planets, or in multiple-star groups?
A Snapshot of the
Heavens
• How can we learn about the lives of
stars, which last tens of millions to
hundreds of billions of years?
• we will never observe a particular star evolve
from birth to death
• so how can we study stellar evolution?
How can we Study the Life
Cycles of Stars?
• Key: All stars were NOT born at the same time
• stars we see today are at different stages in their lives
• we observe only a brief moment in any one star’s life
• by studying large numbers of stars, we get a
“snapshot” of one moment in the history of the stellar
community
How can we Study the Life
Cycles of Stars?
• We can draw conclusions just like we would with
human census data…we do stellar demographics!
A Snapshot of the
Heavens
• What two basic physical properties
do astronomers use to classify stars?
• What does that classification
tell us?
Classification of
Stars
• Stars were originally classified based on:
• their brightness
• their location in the sky
• This classification is still reflected in names of
the brightest stars…those we can see with
our eyes:
Classification of Stars
Order of brightness
within a constellation
Latin Genitive of
the constellation
 Orionis
 Geminorum
Classification of
Stars
• The old classification scheme told us little about a
star’s true (physical) nature.
• a star could be very bright because is was very close to
us; not because it was truly bright
• two stars in the same constellation might not be close to
each other; one could be much farther away
Classification of Stars
• In 20th Century, astronomers developed a more
appropriate classification system based on:
• a star’s luminosity
• a star’s surface temperature
• These properties turn out to depend on:
o a star’s mass and
o its stage in life
• measuring these=> reconstruct stellar life cycles
WHAT DO YOU THINK?
1.
2.
3.
4.
5.
6.
How near are stars?
DISTANCE
What colors are stars? TEMPERATURE
How luminous are stars? LUMINOSITY
Are brighter stars hotter? TEMP vs. LUMIN.
What sizes are stars? SIZE
Single or Multiple?
ORBITS  MASS
Step 1: Distance!
From Parallax
PARALLAX
•Determines distance based
on Earth’s Orbit around Sun
•SMALL angular shift!
•Even closest stars (4.3 light
years) show shift less than
1/4000th of a degree!
PARALLAX
•Example
http://www.solstation.com/stars/61cyg
ni2.htm
•Animation
http://ircamera.as.arizona.edu/NatSci1
02/NatSci102/lectures/otherstars.htm
•Current telescopes can
measure angles as small as
1/400,000th of a degree
(400+ light years)
Step 2: TEMPERATURES!
From Spectra!
COLORS of stars
lead to Surface Temperatures!
Pickering’s Team, led by
Williamina Fleming
Annie Cannon
Initial Spectral Type Classification
System
A B C D E F G H etc
Based on “strengths”
of Hydrogen Absorption lines
Revised Spectral Type Classification
System
O B A F G
K
M
Oh Be A Fine Girl/Guy, Kiss Me!
50,000 K
3,000 K
Temperature
Higher Temperature => Bluer
Higher Temperature => BRIGHTER
Step 3: BRIGHTNESS!
From distance and
apparent magnitude
How BRIGHT are stars?
• Bright because they are
nearby…
or…
How BRIGHT are stars?
• Bright because they are
really, truly bright?
Bright because they
are really, truly
bright?
Bright because
it is nearby…
Apparent Magnitudes
Ancient method for measuring stellar brightness from
Greek astronomer Hipparchus (c. 190 – 120 B.C.)
This scale runs backwards:
The bigger the number, the fainter the star
Brightest stars are #1, next brightest are #2, etc.
Inverse Square Law for Light
– how distance relates to brightness…
Inverse
Square
Law
for Light
– how
distance
relates to
brightness
…
So what can we know
about stars?
• We measure distances of nearby stars with
PARALLAX
• We deduce actual brightness from distance
and the inverse-square law
and…
• We determine surface temperatures from
spectra
So where does knowing
absolute brightness
&
spectra get us?
Plot the two known
quantities
Surface temperature
Vs.
Luminosity (Absolute
Brightness)
against one another on
a graph
Plot all known stars!
Our sun
Plot all known stars!
Betelgeuse
Sun is much, much dimmer
than most “bright” stars
How does our
Sun compare
with other
stars?
Sun is brighter than most
“nearby” stars
The “HR”
Diagram
The “HR”
Diagram
90% of all stars
lie on the main
sequence!
WHY??
Stellar Lives!
BRIGHT
HOT
COOL
FAINT
Hypotheses to Explore
• Stars are born like children, cool and
small, and then heat up and grow
brighter over time.
Or…
• Stars are like candles, hot and bright,
and then cool off and get dimmer
over time.
Hypotheses to Explore
• Stars are born like children…
• Stars are like candles…
Or…
• Stars are born with different temps and
brightnesses, and change little over
90% of their lives.
Hypotheses to Explore
• IF…. Stars are born like children, cool
and small, and then heat up and grow
brighter over time.
• THEN… new clusters of stars should all
be cool M stars, and old clusters only
hot O stars.
Hypotheses to Explore
• IF…Stars are like candles, hot and
bright, and then cool off and get
dimmer over time.
• THEN… new clusters should show only
hot O stars, and older ones should
show cool M stars.
Hypotheses to Explore
• IF…. Stars are born like children, cool and
small, and then heat up and grow brighter
over time.
• IF…Stars are like candles, hot and bright,
and then cool off and get dimmer over
time.
• But “new” clusters like the Pleiades show all kinds of
stars!
The Pleiades
Step 4: HR Diagram!
From spectra and
absolute brightness
Diameter (Size) of Stars
• Calculated from known values:
o Luminosity
o Temperature
• Laws of Physics
oStefan-Boltzmann Law:
Luminosity ~ Surface Area x T4
• PingPong balls, volleyballs, and Stars…
Bigger
HUGE
TINY
Smaller
Determining Size
• Larger stars can be less dense at edges
• Less dense gas will change absorption lines
Luminosity
Classes of Stars
•Based on
Spectral Line
Shapes& Density
•Tied to physical
SIZE
•We’ll discover
they are
unstable…
Step 5: Mass!
From Binary Stars!!
Masses from BINARY STARS
•Doppler Shift of Spectra Lines tells us orbital velocities
•Velocities get us Orbital Sizes
•Orbits get us Mass
Eclipsing BINARY STARS
Mass from Light Curve
• Know periods from the light curve…
•  orbit distances from Kepler’s Laws
•  relative masses from Law of Gravity!
• Match mass to main sequence star type
• O-stars are more massive than M stars…
Mass-Luminosity
Relationship
for
Main Sequence
stars
Mass-Luminosity
Relationship
for
Main Sequence
stars
So… what is
going on?
• Why are only
SOME stars in the
supergiant
regions?
•Why are 10% in
the “giant” region
•What are the tiny
ones in the “white
dwarf” region?
Clusters of Stars as Key
Tests
• Look at large populations of stars
o Open clusters in Milky Way’s “disk”
o Globular Clusters around galaxy
• Assume all stars *about* same age
• Assume all stars *about* same distance
Open Clusters
• 100’s of stars
• 106 - 109 years old
• irregular shapes
• gas or nebulosity
is sometimes seen
Pleiades (8 x 107 yrs)
Globular Clusters
• 105 stars
• 8 to 15 billion
years old (1010 yrs)
• spherical shape
• NO gas or
nebulosity
M 80 (1.2 x 1010 yrs)
Pleiades H-R Diagram
Globular Cluster
H-R Diagram
Palomar 3
WHAT DID YOU
THINK?
• How near to us is the closest star other than the
Sun?
WHAT DID YOU
THINK?
• How near to us is the closest star other than the
Sun?
• Proxima Centauri is about 25 trillion mi (40 trillion km)
away. Light from there will take about 4 years to
reach Earth.
WHAT DID YOU
THINK?
• How luminous is the Sun compared with other stars?
• The most luminous stars are about a million times
brighter, and the least luminous stars are about a
hundred thousand times dimmer than the Sun.
WHAT DID YOU
THINK?
• What colors are stars, and why do they have these
colors?
• Stars are found in a wide range of colors, from red
through violet as well as white. They have these
colors because they have different temperatures.
WHAT DID YOU
THINK?
• Compared to the Sun, what sizes are other stars?
• Stars range from more than 1000 times the Sun’s
diameter to less than 1/100 the Sun’s diameter.
WHAT DID YOU
THINK?
• Compared to the Sun, what sizes are other stars?
WHAT DID YOU
THINK?
• Compared to the Sun, what sizes are other stars?
• Stars range from more than 1000 times the Sun’s
diameter to less than 1/100 the Sun’s diameter.
WHAT DID YOU
THINK?
• Are brighter stars hotter than dimmer stars?
• Not necessarily. Many brighter stars, such as red
giants, are cooler but larger, than hotter, dimmer
stars, such as white dwarfs.
WHAT DID YOU
THINK?
• Are most stars isolated from other stars, as the Sun
is?
• No. In the vicinity of the Sun, one-third of the stars
are found in pairs or larger groups.
Summary of Key Ideas
Magnitude Scales
• Determining stellar distances from Earth is the first
step to understanding the nature of the stars.
Distances to the nearer stars can be determined by
stellar parallax, which is the apparent shift of a star’s
location against the background stars while Earth
moves along its orbit around the Sun. The distances
to more remote stars are determined using
spectroscopic parallax.
• The apparent magnitude of a star, denoted m, is a
measure of how bright the star appears to Earthbased observers. The absolute magnitude of a star,
denoted M, is a measure of the star’s true brightness
and is directly related to the star’s energy output, or
luminosity.
Magnitude Scales
• The absolute magnitude of a star is the
apparent magnitude it would have if
viewed from a distance of 10 pc. Absolute
magnitudes can be calculated from the
star’s apparent magnitude and distance.
• The luminosity of a star is the amount of
energy emitted by it each second.
The Temperatures of Stars
• Stellar temperatures can be determined
from stars’ colors or stellar spectra.
• Stars are classified into spectral types (O, B,
A, F, G, K, and M) based on their spectra or,
equivalently, their surface temperatures.
Types of Stars
• The Hertzsprung-Russell (H-R) diagram is a graph on
which luminosities of stars are plotted against their
spectral types (or, equivalently, their absolute
magnitudes are plotted against surface
temperatures).
• The H-R diagram reveals the existence of four major
groupings of stars: main-sequence stars, giants,
supergiants, and white dwarfs.
• The mass-luminosity relation expresses a direct
correlation between a main-sequence star’s mass
and the total energy it emits.
• Distances to stars can be determined using their
spectral types and luminosity classes.
Stellar Masses
• Binary stars are surprisingly common. Those that can
be resolved into two distinct star images (even if it
takes a telescope to do this) are called visual
binaries.
• The masses of the two stars in a binary system can
be computed from measurements of the orbital
period and orbital dimensions of the system.
• Some binaries can be detected and analyzed,
even though the system may be so distant (or the
two stars so close together) that the two star images
cannot be resolved with a telescope.
Stellar Masses
• A spectroscopic binary is a system detected from
the periodic shift of its spectral lines. This shift is
caused by the Doppler effect as the orbits of the
stars carry them alternately toward and away from
Earth.
• An eclipsing binary is a system whose orbits are
viewed nearly edge-on from Earth, so that one star
periodically eclipses the other. Detailed information
about the stars in an eclipsing binary can be
obtained by studying its light curve.
• Mass transfer occurs between binary stars that are
close together.
Key Terms
absolute magnitude
apparent magnitude
binary star
center of mass
close binary
eclipsing binary
giant star
Hertzsprung-Russell
(H-R) diagram
initial mass function
inverse-square law
light curve
luminosity
luminosity class
main sequence
main-sequence star
mass-luminosity relation
OBAFGKM sequence
optical double
photometry
radial-velocity curve
red giant
spectral types
spectroscopic binary
spectroscopic parallax
stellar evolution
stellar parallax
stellar spectroscopy
supergiant
visual binary
white dwarf