Astronomy 103
Final review session
Dec 13 2007
What we’ll talk about
• Topics covered since last exam
• Strategies for studying for the final
• Date and location of the exam
Topics
• Since the last exam, we have essentially covered
three subject areas
– Stars and their evolution
– Galaxies
– Cosmology
So lets remind ourselves about each…
HR Diagram
• Useful for exploring properties of stars
• Can tell you about stellar evolution
• Compares temperature in K on x-axis to Luminosity
in solar units on y-axis
• Remember that temperature runs from hot to cool!
• Moving from lower right to upper left, stars increase
in radius
• Moving from upper right to lower left, stars decrease
in mass
Star formation
• Stars form from giant clouds of gas which collapse
under gravity
• Eventually, protostars form
• Once hot enough, hydrogen fusion can begin
• Contraction halts once star reached hydrostatic
equilibrium - balance of gas pressure and gravity
• At this point, star has reached the Main sequence
Main sequence stars
Main
sequence
Main Sequence stars cont.
• A useful definition of a main sequence star is
– A mass of gas burning hydrogen in helium in its core
– In hydrostatic equilibrium, and so therefore stable
• The most important factors in determining the future
evolution and fate of a star are
– Its mass (the available fuel)
– Its luminosity (the rate of fuel consumption)
Mass-Luminosity relationship
L = M3.5
Main sequence lifetime
• Stars live for different lengths of time depending on
their mass - the most massive stars live for the
shortest amount of time since they use up their
available fuel most quickly
• tMS = M/L
• Substituting in the mass-luminosity relation, we find
that tMS = 1/M2.5
Stars are like cars….
• Massive stars have higher luminosities, so burn their fuel more
quickly
• Low mass stars have low luminosities and so burn their fuel
more slowly
• Compare to gas mileage of cars!
Stellar Endpoints
Initial mass
remnant
composition
<0.5 Msol
white dwarf
helium
0.5 to 4 Msol
white dwarf
carbon/oxygen
4 to 8 Msol
white dwarf
oxygen/neon/magnesium
8 to 25 Msol
neutron star
neutrons
>25 Msol
black hole
?
(>50 Msol
neutron star?
neutrons)
Supernovae
• End point for massive stars, leaving behind neutron
star (pulsar) or black hole
• Also end point for white dwarfs in binaries which grow
in mass via accretion
• Very energetic stellar explosion
• Seeds elements into the interstellar medium
• Can be used as a standard candle since luminosity
known
• Can outshine host galaxy for a short time
Galaxies
•
•
•
•
•
Large gravitationally bound systems of stars
Contain billions of stars
Like stars, come in different masses and luminosities
Three types - spiral, elliptical and irregular
Galaxies are the basic building blocks of the universe
Quasars and AGN
• Many galaxies host supermassive black holes in their
centers
• Black hole mass many millions of solar masses
• Quasars are galaxies where the active galactic nuclei
is detected in the radio
• In optical, these looked somewhat like stars, hence
“Quasi-stellar radio sources”
• Now can resolve host galaxies
• More common in earlier stages of universe
Expansion of universe
• Edwin Hubble observed that galaxies are all moving
away from us
• We now know that this is due to the expansion of the
universe
• Hubble’s Law related recession velocity and distance
V = H0d
Age of universe
• Look at the board!
Big Bang
• Universe started as a singularity of infinite density
and infinite temperature
• Everything began at the big bang - time, space and
the laws of physics
• Universe has been expanding and cooling ever since
• Primordial elements which first galaxies and stars
formed from were also produced in big bang hydrogen, helium, lithium
Cosmic microwave background
• Afterglow of big bang - first light free to travel through
universe unimpeded by matter
• With expansion of universe, wavelength of this light
has been stretched out
• Longer wavelength - cooler temperature (Wiens Law)
• See structures in big bang - initial density fluctuations
which allowed structure to form in universe
Cumulative material
• Look at the board again!
–
–
–
–
–
–
Law of gravity
Stefan Boltzmann Law
Wiens Law
Inverse square law of light and standard candle
Energy of photon
The sun!
How to study!
• Look at your notes of what the professor discussed in
class
• Use practice questions to guide you to important
facts and concepts
• Use your notes from discussion
• Use the powerpoint slides to guide your reading
• Ask questions! Either of me, the professor or your
classmates
• Be aware of older concepts revisited in newer
material