Ch 17

Stars

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

http://upload.wikimedia.org/wikipedia/commons/7/7c/Sun_red_giant.svg

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