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C13 : Stars and Galaxies
Section 1 : Stars
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Demo : expanding universe (p. 369)
Q: What is a star ?
A: large, massive, hot ball of gas held
together by gravity and gives off light.
• Q: What is a Constellation?
A : a number of stars that appear to form
a pattern.
Constellations
• Constellations are not real
• The purpose is to help to locate stars.
• Stars in a constellation often have no
relationship to each other.
• Modern astronomy divides the sky into 88
constellations.
Circumpolar Constellations
• As Earth rotates, Ursa Major & Ursa Minor
and other constellations in the northern
sky circle around Polaris.
• They appear to move because Earth is
rotating.
• The stars appear to complete one full
circle about 24 hours.
• Visible all year long due to their unique
position.
• As Earth orbits the Sun, different
constellations come into view while others
disappear.
Absolute & Apparent Magnitudes
• When you refer to the brightness of a star,
you can refer to its absolute magnitude or
its apparent magnitude.
• Absolute magnitude – actual brightness of
a star
• Apparent magnitude – brightness of a star
that we see
Parallax
• It can be used to determine the distance of
the star from the Earth.
• Parallax is the apparent shift in the
position of an object when viewed from 2
different positions.
• Demo
• The nearer an object is to the observer,
the greater its parallax.
• Good to cal. distance
for near star.
d
3.26
p
• The parallax angle of
the closer object is
greater than that of
the farther object.
Properties of Stars
• Color: depends on temperature
from coolest to hottest : red, orange, yellow,
white, blue
• Size : depends on original mass of gas
cloud and age/lifestage of star
from smallest to biggest : white dwarf, mainsequence, giants, supergiants
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Brightness : depends on surface temp,
size, dist from Earth
• The light from a star comes from nuclear
reactions in center of a star
• A star must have a balance between :
1. press due to temp in core
2. Gravity
• Study the composition of stars by
observing their spectra.
Section 2 : The Sun
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It is the closest star to Earth.
Center of the solar system.
It is an enormous ball of gas.
It is yellow in color.
It is in the main sequence.
Brightness : medium
The size of the core ~ the size of Jupiter.
92 % hydrogen.
• The Sun has enough hydrogen to last 5
billion years.
• It produces energy by nuclear fusion
• Energy is transferred by convection and
radiation.
The Sun’s Atmosphere
• Photosphere – lowest
layer of the Sun’s atm.
The layer from which
light is given off.
6000 K ~ 5700 ºC.
• Chromosphere – the
layer on top of
photosphere.
About 2000 km thick.
• Transition Zone – 2000 km to 10 000km
thick.
• Corona – largest layer of the Sun’s atm.
- extends millions of km into space.
- 2 million K
- charged ptles escape here as
solar wind.
Surface Features
• Sunspots – dark color spots on the surface
becoz they are cooler than surrounding
areas.
- they are caused by intense magnetic
storms on the Sun.
- sunspots move because Sun rotates.
Faster at equator (25 days/rotation) and
slower at poles (35 days/rotation).
- sunspots are not
permanent.
- sunspot max : times
when many large
sunspots occur. Every
10 to 11 years.
- sunspot min : occur in
between sunspot max.
Prominences and Flares
• Prominence : the intensive magnetic fields
associated with sunspots causes
prominences.
• They are huge, arching columns of gas.
• Solar flares : gases near a sunspot
shooting outward at high speed.
• CMEs (Coronal mass ejections)
- occur when large amts of electricallycharged gas are ejected suddenly from
corona.
- 2 or 3 times each day during a sunspot
max.
• It can damage satellites in orbits.
• Interfere radio equipment.
• It causes auroras.
- high energy ptles contained in CMEs and
solar wind are carried past Earth’s
magnetic field → generates electric
current toward Earth’s poles → ionized
gases in atm →ions recombine with
electrons →produce ight →aurora.
Aurora
The Sun – An Average Star
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Middle-aged
Brightness is average
Yellow light
Light reaches Earth ~ 8 mins
Not close to any other stars
Closest star system to the Sun – The
Alpha Centauri System (triple star system)
Section 3 : Evolution of Stars
• (I) Classifying Stars
• High temps →brighter absolute magnitude
• Hertzsprung – Russell (H-R) diag.
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Most stars (abt 90% of all stars) fit into a
diagonal band → main sequence
Bigger, hot, blue, bright stars → upper
left
Smaller, cool, red, dim stars → lower
right (most stars found here)
Remaining 10% stars not found in main
sequence :
1. White dwarfs – hot but not bright
- small
- lower left of H-F diag
2. Red Giants – large
- bright but not hot
- upper right
- supergiants (largest giants)
Antares
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Supergiant
400 light years from Earth
3 500 K
16th brightest star in the sky
300 x diameter of Sun
11 000 times brighter than Sun
How do stars shine/generate
energy?
• Core of Sun → high temp → H atoms fuse into
He → releases huge amt of energy
• 4 H nuclei combine to create 1 He nucleus
• Mass of 1 He nucleus < 4 H nuclei → mass is
lost → energy is produced
• E = mc2
Eg: m=1 kg
E = 1 x (3x108)2 = 9 x 1016 J
→ 2.8 million US hours / year
Demo
• P. 382 “make a model”
Evolution of Stars
• When hydrogen nuclei (protons) depleted
→ star loses its main sequence status
• Brightest stars → less than 1 million years
to deplete
• Dimmest stars → many billions of years
• Sun – main sequence
- life span ~ 10 billion years
- 5 billion years left
• What is a Nebula?
large and massive cloud of gas and dust
particles
How a star is formed?
• Due to supernova or collisions of 2 clouds of
nebula → ptles of gas and dust exert
gravitational force on each other → nebula
contracts → instability within nebula → nebula
breaks apart into smaller pieces → ptles in
smaller pieces of nebula move closer → T
increases (ptles collide more as they are closer)
→ protostar → core reaches 10 million K →
fusion begins → star is formed and is stable →
stars in main sequence
Star is stable in main sequence becoz heat
from fusion causes Press increases →
Press balances the gravity → when
hydrogen in the core is depleted → gravity
> press → core contracts → T inc in core
→ outer layer expands (more He) →
outerlayer temp dec → core temp reaches
100 million K → He nuclei fuse to form
Carbon → giant
How a white dwarf is formed?
• After the star’s core uses much of its
helium (this stage is after giant. The
hydrogen fuel has used up → does not
produce any new helium nuclei → helium
combine to form carbon) → contracts even
more (no more fusion → gravity pulls in)
→ outer layers escape into space →
leaves behind the hot, dense core.
• Size of Earth
• Eventually the white dwarf will cool and
stop giving light
Supergiants and Supernovas
• Stars more than 8x massive than Sun →
evolution occurs more quickly and more violently
• Massive stars → core heats up to higher temps
→ heavier elements form by fusion (becoz
higher temp is needed to fuse bigger elements.
Eg : He → C needs higher temp) → star
expands into supergiant → iron forms in the core
→ iron atomic structure does not release energy
through fusion → core collapses (exploded) →
shock wave
Neutron Stars
• If the collapsed core of a supernova (after
the explosion) is ~ 1.4 → 3 times the mass
of Sun → shrink to ~ 20 km in diameter →
only neutrons can exist in the dense core
→ neutron star
Black Holes
• If the collapsed core of a supernova is > 3 times
the mass of Sun → continue to collapse → until
it becomes a point → gravity near this mass is
so strong that nothing can escape from it, not
even light → black hole
• Black hole is not like a giant vacuum cleaner
sucking in distance objects
• Only if the object crosses it → pulled into the
hole
• Stars and planets can orbit around a black hole
Section 4 : Galaxies and the
Universe
• Galaxy : a large group of stars, gas, and dust
held together by gravity. (also includes planets)
• Milky Way Galaxy : our galaxy
• Galaxies are separated by huge distances –
millions of light years apart.
• Galaxies are grouped together into clusters
• Stars (gp) → galaxies(gp) → clusters
• Milky Way belongs to Local Group Cluster (~45
galaxies)
3 Major types of Galaxies
1. Spiral Galaxies
• Whirlpool – like
• Lots of dust in spirals
• Closer to center
→faster it orbits
• Milky Way
2. Elliptical Galaxies
• 3-D ellipses : football
• Round : soccer ball
• No spirals
• Very little dust
3. Irregular Galaxies
• No recognizable
shape
• Smallest type of
galaxy
• 2 irregular galaxies
orbit the Milky Way →
Clouds of Magellan
The Milky Way Galaxy
• ~ 1 trillion stars
• Stars (including Sun) orbit around the core.
It takes 225 million years for the Sun to
make 1 round around the core.
• Has a supermassive black hole at its
center. It is about 2.5 million times as
massive as the Sun.
Origin of the Universe
1. Steady State Theory – the universe
always has been the same as it is now.
The universe always existed and always
will.
• Evidence indicates that the universe was
different in the past.
2. Oscillating Model – universe began with
expansion → expansion slowed →
universe contracted →process began
→oscillating back and forth.
3. Big Band Theory – started with a big band
→expanding ever since.
Doppler Shift
• If a star is moving toward Earth, its
wavelength of light are compressed →light
from the star is shifted to the blue end of
the spectrum.
• If a star is moving away from Earth, its
wavelength stretched →light from the star
is shifted to the red end of the spectrum.
• Red Shift → Universe is expanding
• Quick Demo (p.389)