Parallax
• Astronomers use the same principle
to measure the distance to planets
and nearby stars.
• When sighted from a different position,
an object seems to change position
since it is viewed from a different angle.
Proper motion
• This doesn’t mean stars don’t move
• Over a time period of many years, stars do change position
Big Dipper
Magnitude
• We define stellar magnitude as brightness:
– The brightest stars in the sky are classified as
magnitude 1, then 2, to 5 for the dimmest stars we
can see
– But are these brighter stars really brighter or just
closer?
• For this reason we separate:
– Apparent magnitude
• How bright a star looks in the sky
– Absolute magnitude
• How bright stars would look if they were all 10pc away
The distance ladder
How do we know the distances to
stars and galaxies?
• For close stars, we can use parallax
• For distant stars and galaxies we need
another method
• If only we knew the absolute magnitude of
some reference stars...
Cepheid Variables
• These stars’ gases are unstable and so
expand and contract
• We can find their distance since they follow
a well defined Period-Brightness
relationship
• The longer the period, the brighter the star
Stellar Evolution
All stars go through the same
basic phases of their “life”
• Conception
• Gestation
• Birth
• Middle age
• Mid-life crisis
• Retirement
Conception:
• A nearby supernova sends shock waves
through a nebula
• a region of a nebula starts to contract and
give off infrared radiation
The Birth of our Solar System
• From the ashes of a
dead star, gravity
starts to pull gases
together.
• To conserve angular
momentum, as the
swirling gases
contract, they begin to
spin
• Most of the mass is
drawn to the center,
with bands of smaller
mass circling around.
• Once enough mass and
pressure accumulates
at the center, the
nuclear furnace is
ignited
• A star is born!
• Collisions and gravity
decreases the number
of forming planets
Birth:
• light is emitted from the hot blue star
• The solar wind (about
100, 000 tons/s and up to
1000 km/s) clears out
any debris elements
close to the Sun.
Middle age:
• the main sequence, burning hydrogen
Mid-life crisis:
• The star swells to a giant when H starts to
run out
Retirement:
• Star’s atmosphere is blown off, becomes a
nebula
White Dwarf
• Like with people, the nature of retirement
depends on the star’s resources
(supernova?)
• Star’s dense core remains as a:
– White dwarf?
– Neutron star?
– Black hole?
Come on down to the
best Star lot in town!
Spica: The Ferrari of stars!
• Blue giant
• -3.2 Abs. Magnitude
• 11 Solar Masses!
• Buy now and get its
binary companion…
free!!
• Don’t wait! This star
will only last a few
hundred million years!
Wolf 359: economy model
• Red Dwarf
• Will last for a trillion
years on a single tank
of hydrogen!
• Only 7.5 ly away!
• It’s hard to find…a
deal like this!
What will it be?
• Economy model?
• Flashy sports star?
• Somewhere in between?
• Let’s choose “Sol”
– Medium mass star
• Fairly bright and warm so we don’t freeze
• Not too bright so we still have another 5 billion
years before it runs out of Hydrogen.
Classification of Stars
The temperature of a star
determines its color because of
the blackbody distribution
Spectral Thermometer
• We can use the color
of a star as a
thermometer.
• Blue stars are very hot,
while red stars are
cooler.
Hot stars also tend to be brighter
• If we graph brightness vs. temperature, we
find:
– most blue stars are very bright
– most red stars are very dim
– there are a few stars that do not follow this
trend: how can we explain this?
The H-R Diagram
Spectral Types • We can get more specific about color and give
letters for spectral type from blue to red hot
• For example the Sun is a “G” star: yellowish-white hot
• OBAFGKM
– “Oh, be a fine girl: kiss me”
– Old blue architects feed greedy kangaroo mothers
– Obsolete bears attack Friday’s good kangaroo moms
– Obese bald angry fat guy killed Mike
– Oh boy a fat guy killed me
– Oh boy a fat guy killed me
– Obese bartenders arrive for great kiwi mashing
– Oblong bat attack fairies grave klepto mother
– Open books are freakin’ great, killing minutes
– Oprah’s bookclubs are for getting Korean money
– Oprah’s books are flinging ginourmous killing
machines
– Oily bolts aid forming great killer machines
– Old basements are for growing king mushrooms