Download Objectives

Stars
Star Field as seen through the Hubble Space Telescope
2
Objectives
• Describe how a protostar becomes a star.
• Explain how a main-sequence star generates energy.
• Describe the evolution of a star after its main-sequence
stage.
Review
• Stars are essentially all made of the same material!
– Hydrogen and
– Helium
• Luminosity differences are due to the differences in the
temperatures of stars.
• Hotter stars appear Blue
• Cooler Stars appear Red
• Stars are divided into seven major spectral or temperature
classes
• O, B, A , F, G , K, M (Oh Be A Fine Girl (Guy) Kiss Me
• O – Hottest Stars
• M – Coolest Stars
One Exception
• Since 1995 Astronomers
have found new stars
with surface temps even
lower than spectral class
M.
– These are called
Brown Dwarfs, and
emit light in mostly the
infrared range.
•
Classifying Stars
• H-R Diagram
(Hertzsprung + Russel)
• In 1912 classification
scheme for stars invented
• Stars are plotted
according to:
• Luminosity (Absolute
Brightness) and
Temperature (Spectral
Class)
• Brightness increases as
you move up the y-axis
• Temperature decreases
as you move to the
right on the x-axis
Classifying Stars
• Notice that about 90% of stars
fall within a band which runs
diagonally through the middle
of the diagram. These are
Main Sequence stars.
• This band extends from the
hot, luminous blue stars to the
cool, dim red stars
– Ex- our Sun is a Main
sequence star
• Main sequence: the location
on the H-R diagram where
most stars lie; it has a diagonal
pattern from the lower right to
the upper left
Understanding the H-R Diagram
• Recall that two factors
determine the luminosity
(brightness) of a star:
• Temperature
• Size
• So, a cooler star may
still appear bright if it is
very large like….
• Giants
• Upper right hand side of
diagram
• Approximately 10 to 100
times larger than our Sun
• Ex- Aldebaran
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Relative Size of
some Well Known
Stars
H-R Diagram of
some Nearby stars
Star Formation
• A star forms in a dense,
cold cloud of dust and
gas made up of mostly
Hydrogen and Helium
called a Nebula.
– Gravity pulls particles
of the nebula closer
together.
– These regions spin,
shrink, and form a
flattened disk.
Star Formation
• The disk eventually forms
a central concentration of
matter called a protostar
• The protostar continues
to heat up due to further
contraction, begins to
glow, and develops
Bipolar Outflow 
• No Fusion Yet…heat
generated only by
contraction
Protostar
and
Bipolar
Jets
Protostar
14
Protoplanetary Disk- Photo taken by Hubble Space
Telescope
15
The Eagle
Nebula –
Possible
formation
of Many
stars.
16
Star Formation
• Eventually, contraction
produces a high enough
temperature at the core and
Nuclear Fusion Begins.
• At this time, Hydrostatic
Equilibrium is reached
• Hydrostatic Equilibrium:
when the outward force of
pressure caused by nuclear
fusion balances the inward
force of gravity
• ***Once fusion begins,
Hydrostatic Equilibrium is
reached and A MAIN
SEQUENCE STAR IS
BORN
Hydrostatic
Equilibrium –
The outward
pressure of
Nuclear Fusion
is EQUAL to the
inward Pull of
Gravity
Our Sun- A Main Sequence Star
Hydrogen Vs. Helium
Concentrations over the
Life of the SUN
Time Frame for Star Formation
• More mass = more heat =
faster star formation
• Less mass = less heat =
slower star formation
• Makes sense, right?
• Our sun probably took
about 50 million years to
form
Stellar Evolution
• More Massive Stars have to burn hotter and faster to resist
gravitational contraction and therefore use up their fuel
quicker.
• Are massive, hotter stars more or less luminous than others?
• Less massive stars burn cooler and therefore can last longer
• Our Sun will fuse hydrogen for about 10 billion years
• Once a star’s Hydrogen supply runs out, fusion stops and the
core begins to contract
• At this time, the outer layers of hydrogen fuse at an incredible
rate and the star expands to become a RED GIANT…. Demo
Red Giants
• Red Giants are very
luminous due to their large
size, but the heat is spread
out over a larger area so
stars in this stage are cooler
than they were before
burning through their supply
of Hydrogen
– That’s why it turns red!
• Low mass stars such as our
sun will become Red giants
• Higher Mass stars will
expand much further to
become Red Supergiants
Artist’s view of Earth and the Sun as a Red Giant Star
24
Leaving the Main Sequence
Reading Check
Where are giants and supergiants found on the H-R
diagram?
Giants and supergiants appear in the upper right part of the
H-R diagram.
Death of a Star
•
Once the hydrogen supply is
depleted, the Red Giant contracts,
heats up further, and begins
fusing helium at a very high rate.
•
When Helium runs out, fusion
stops, the core contracts further,
and the outer layers expand
again.
•
These outer layers of gas drift
away to form a Planetary Nebula:
a huge shell of gas and dust
illuminated by the very hot
exposed core of a star
Butterfly Nebula in Ophiucus
27
Eskimo Nebula in Draco
28
Cat’s Eye Nebula in Draco
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Final Stages of a Star
• As the planetary nebula
disperses, gravity causes the
remaining matter in the star to
collapse inward until it cannot
be compressed further.
• A hot, extremely dense core of
matter - a white dwarf - is left.
• White dwarfs
– very hot but not luminous
because of their small size
– shine for billions of years
before they cool completely
and become a black dwarf.
Sirius B is a white dwarf star
shown next to a much brighter
companion star, Sirius A.
Supernova
• Massive stars eventually
become hot enough to
fuse heavier elements in
their core
• These shrink very rapidly
and rebound with a
tremendous shock wave
which blows apart the
entire shell of the star in
an explosion called a…
Supernova!!!
Star field seen before supernova
Supernova
• The explosion causes a
dramatic increase in
brightness
• Energy released is more than
100 times what our sun will
radiate over its entire lifetime
• Supernovas outshine ALL the
stars in its own galaxy
COMBINED!!
• May even be visible on earth
during daylight hours
• very rare
After Supernova explosion
1987 Supernova in the Large Magellanic Cloud – Hubble
Space Telescope
33
Veil Nebula – Remnant of a supernova that exploded about
15,000 years ago
34
Crab Nebula- A Remnant of a Supernova Explosion
observed in 1054 AD which was bright enough to be
seen during the day for over three weeks and during
the night for 6 months
35
Supernova
• A white dwarf
which orbits a
Red Giant may
also become a
supernova
• The explosions
involved
completely
destroy the white
dwarf star and
may destroy
much of the red
giant.
White Dwarf and Companion Star will probably lead to a Supernova
Neutron Stars
• Neutron Star: a star which
has collapsed to the point
where electrons and protons
have compressed enough to
form neutrons
• Only massive stars can
become Neutron stars
• More massive (heavy) than a
white dwarf BUT only the size
of a large city!!!!!
• A paper clip made from a
Neutron star would outweigh
Mt. Everest
Final Stages of Massive Stars
Pulsars
A rapidly spinning neutron star which emits pulses of radio and optical
energy
Black Holes
• An object so
massive and
dense that even
light cannot
escape its gravity
• Occur when the
compression of a
star’s core is so
great that even
neutrons cannot
withstand the
weight of their own
gravity
Black Hole’s Effect on a Planet