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C13 : Stars and Galaxies
Section 1 : Stars
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 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
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
• The stars appear to complete one full
circle about 24 hours.
• Visible all year long due to their unique
• As Earth orbits the Sun, different
constellations come into view while others
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
• 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.
• 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
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
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
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
About 2000 km thick.
• Transition Zone – 2000 km to 10 000km
• 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
- 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
- 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
• 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
- 2 or 3 times each day during a sunspot
• 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.
The Sun – An Average Star
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.
Most stars (abt 90% of all stars) fit into a
diagonal band → main sequence
Bigger, hot, blue, bright stars → upper
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)
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
• 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
• 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
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
• Stars and planets can orbit around a black hole
Section 4 : Galaxies and the
• 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
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
• Smallest type of
• 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
• 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)