Today
How do we measure properties of a star?
• Announcements:
– HW#8 due Friday 4/9 at 8:00 am.
• The size of the Universe (It’s expanding!)
• The Big Bang
• Video on the Big Bang
• NOTE: I will take several questions on exam 3 and
the final from the videos we watch. Sleep at your
own risk.
ISP209s10 Lecture 20
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How do we know what our
sun (and other stars) are made
of ?
How do we know the
temperature?
How do we know the size?
From the spectrum of the EM radiation
from the star (“Blackbody radiation”)
ISP209s10 Lecture 20
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Some Clicker Questions - #1
Some Clicker Questions - #1
What happens to a star if its surface temperature is increased and
its size remains the same?
A) It only gets brighter
B) It only gets more red
C) It gets brighter and more blue
D) It only gets dimmer
E) It gets dimmer and more red
What happens to a star if its surface temperature is increased and
its size remains the same?
A) It only gets brighter
B) It only gets more red
C) It gets brighter and more blue
D) It only gets dimmer
E) It gets dimmer and more red
Hint: recall how color correlates with temperature
ISP209s10 Lecture 20
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ISP209s10 Lecture 20
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A Stars Energy Source – nuclear fusion
The pp-chain in the Sun
The sun generates its energy by a set of fusion reactions
where 2 nuclei are “stuck” together and release energy.
(the strong force in action)
Conditions required for nuclear fusion (two things):
• High temperature: the central temperature of the sun is 15 million
Kelvin. This is necessary to overcome the repulsion between the
positively charged protons.
• High density: the probability of collisions must be high.
Note: the Sun is balanced just right. It does not burn too fast or two
slowly for us to have a potentially comfortable existence.
ISP209s10 Lecture 20
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A star is born
ISP209s10 Lecture 20
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Stars
1) Big clouds of gas (nebula) accumulate
by chance thanks to gravity
2) Atoms heat up as more and more material
falls towards the center under gravity as the
gas cloud collapses to a smaller size
3) Eventually, the gas is hot enough that
nuclear fusion reactions occur
4) Heat generated by fusion “balances” any
further collapse (thermal pressure)
ISP209s10 Lecture 20
time
-7-
• The mass of a star determines
most properties of a star: lifetime,
color, size, luminosity
• Massive stars are very bright and
hot, but they don’t last very long.
• Stars are a balance between
gravity and pressure from the
internal heat – hydrostatic
equilibrium
ISP209s10 Lecture 20
Mass
0.3 Msun
Lifetime
By
1000
1.0 Msun
10
3.0 Msun
0.35
10 Msun
0.025
60 Msun
0.002
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Relative Sizes of Stars
Blue – hot
Hertzsprung-Russell Diagram:How a star evolves
Red - cooler
ISP209s10 Lecture 20
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Evolutionary Path of our Sun: HR Diagram
ISP209s10 Lecture 20
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Evolutionary Path of our Sun: time line
Long parts:
Hydrogen
10 By
Helium
Helium
1 By
Hydrogen
ISP209s10 Lecture 20
White
Dwarf
many BY
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Heavier stars have different possible endings:
1) M = 10-30 Msun => neutron stars (only few km across!)
2) M > 30 Msun
=> star collapses into a black hole
ISP209s10 Lecture 20
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Some Clicker Questions - #2
Some Clicker Questions - #2
The line in the HR diagram
indicates the “main sequence”.
What determines if a star lies on
the main sequence?
A) It is blue
B) It is red
C) How it produces energy
D) Its temperature
E) What the surface is made out
of
Luminosity is relative to our Sun.
The line in the HR diagram
indicates the “main sequence”.
What determines if a star lies on
the main sequence?
A) It is blue
B) It is red
C) How it produces energy
D) Its temperature
E) What the surface is made out
of
Luminosity is relative to our Sun.
ISP209s10 Lecture 20
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ISP209s10 Lecture 20
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Some Clicker Questions - #3
Some Clicker Questions - #3
C
C
D
B
A
Luminosity is relative to our Sun.
ISP209s10 Lecture 20
Where do we find
White Dwarfs on
the HR diagram?
Hint 1: are white
dwarves hotter or
Colder than our
Yellow sun?
Hint 2: ask yourself
Where is our sun
Based on the -15luminosity
D
B
Where do we find
White Dwarfs on
the HR diagram?
Point A.
(our sun is at point B)
A
Luminosity is relative to our Sun.
ISP209s10 Lecture 20
White => hotter
than our sun
Dwarf => smaller
than our sun =>
-16Small luminosity
Some Clicker Questions - #4
Some Clicker Questions - #4
C
C
D
Where would we
find our Sun on
an HR diagram?
D
B
B
A
A
Luminosity is relative to our Sun.
ISP209s10 Lecture 20
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How do we measure big distances?
Luminosity is relative to our Sun.
ISP209s10 Lecture 20
Where would we
find our Sun on
an HR diagram?
Point B.
You could have
either 1) guessed
2) Memorized the
Temperature or
3) Made use that
Luminosity is rel.
To our sun. -18-
Stellar Parallax
Alpha Centauri is the next closest star to us.
It is 4 light-years away from us.
From the angle !,
we can figure
out the distance
from earth.
How do we measure such huge distances?
! !
• Radar – “nearby” things like the Sun
• Parallax – up to several hundred light-years
• Spectroscopic parallax – even bigger distances
! of 1 arcsec
corresponds to a
distance of 1
parsec (pc) =
3.24 ly
1 arcsec = 1/60 degree
ISP209s10 Lecture 20
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ISP209s10 Lecture 20
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Spectroscopic Parallax
The Pace of Science
• Astronomers have studied the sky for thousands of
years, but 90% of all astronomical information has been
obtained obtained since 1900. For example, in 1900
scientists believed that the universe:
L[Watts]
intensity =
4! d 2
–
–
–
–
–
If we know L the luminosity (HR diagram),
and measure the intensity, we can determine
d, the distance to the source
ISP209s10 Lecture 20
Was infinitely old
Was infinitely large
Contained only one galaxy (the Milky Way)
Did not change with time
Was uniform throughout (problem with Olber’s paradox)
• All of these are false!
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Edwin Hubble
ISP209s10 Lecture 20
The structure of our local set of galaxies
• In 1922 Edwin Hubble
measured the brightness of
variable stars in the Andromeda
galaxy.
You are here
• He discovered that the Andromeda galaxy was
about 2.5 million light years away.
• He was the first person to demonstrate the
finite size of the Universe and the the Milky
Way is not the only galaxy.
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(1pc = 3.24 light-year)
Close-up view
ISP209s10 Lecture 20
Zoomed out view
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Something Else!?! Hubble’s Law
Hubble Expansion
• The further away
an object is, the
faster it is
receding.
• Speed = H0 x d
• The Hubble
Constant: H0 = 77
km/s/Mpc
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The Universe
No matter where you are,
everything is moving away!
ISP209s10 Lecture 20
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How Did the Universe Begin?
• The Universe is not uniform.
• The 200 billion galaxies are clustered into
large clumps with voids in between
• Why?
• Answer: It must have started that way and
gravity is slowly pulling things together
ISP209s10 Lecture 20
• Because he was able to
measure distance, Hubble
observed that on average
all galaxies seem to be
moving away from us.
• The speed is related to
distance. Galaxies farther
away are moving faster.
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• It looks like the Universe started about 14
billion years ago and has been expanding
(space stretching) ever since.
• The model of what happened is called the Big
Bang.
• There is a lot we don’t understand. What came
before? What caused the big bang? Why is
there more matter than anti-matter in the
Universe?
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Evidence for the Big Bang
• The expansion of the universe: All galaxies
appear to be moving away from us.
• The abundances of the lightest elements
produced in the Big Bang: the universe is
mostly hydrogen and helium.
• The cosmic microwave background
radiation: It looks like we are in the middle
of a big oven with a temperature of ~3
Kelvin.
ISP209s10 Lecture 20
It looks like we are in the middle of an oven
The spectrum is
identical to that
of a blackbody at
3 Kelvin!
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Picture of the Universe at 300,000 years old
WMAP observatory
The splotches confirm the
Universe was non-uniform
In the beginning
ISP209s10 Lecture 20
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The History of the Universe
What we see as we
look away from the
Earth
We are effectively
looking back in
time.
ISP209s10 Lecture 20
The pattern indicates that the Universe is 13.7 billion years old.
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ISP209s10 Lecture 20
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Big Bang Timeline (the early moments)
Inflation of the Universe
The existence
of an unknown
scalar field
caused the
rapid inflation of
the Universe
First nuclei formed
Atoms
formed
Meters
Electro
Years since the Big Bang
ISP209s10 Lecture 20
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What about the future?
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Our Future (Details)
• Until very recently (past 5 years) we thought it
was possible that the Universe might end in a Big
Crunch.
• This would be the case if the mass of the Universe
were large enough to halt the expansion and bring
everything back together.
• In this model the Universe could be a neverending cycle of Big Bangs and Big Crunches.
• The microwave background measured by WMAP
points to an ever expanding Universe.
ISP209s10 Lecture 20
ISP209s10 Lecture 20
Space
stretched by
1050 times!
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3E+9 years from now, our Galaxy will collide
with the Andromeda Galaxy, either merging
Into 1 Super-Galaxy or ripping both apart.
In 5E+9 years our sun fries Earth when it turns into a Red Giant
In 1E+12 years the stellar era ends (Stars run out
of nuclear fuel).
In 1E+100 years, only remnant of stars remaining are black holes
Eventually, the black holes will “evaporate” due to quantum
Effects. All that remains in the universe is electrons, neutrinos,
ISP209s10 Lecture 20
-36Photons.
Why does time always move in one direction?
• Inflation during the Big Bang resulted in a
universe that had a very low entropy, much too
low for its size. It was like the Universe started
with all heads.
• Hence, everything in the universe moves toward
reaching the correct amount of entropy.
• Time has a direction because going back in time
would imply the entropy could be decreased. That
is very improbable.
• The universe tends toward increasing entropy.
• What is time?
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