#3 Study Guide:
guide is intended to highlight the important material you should know for the
third exam. It is intended that everything on the exam is covered in this
- Know what luminosity
is. It is the number of Watts output by a star. A Watt is the number of
Joules of energy per second.
- Know how the Sun gets its energy: hydrogen
fusion into helium by the pp-chain.
- Know E=mc2 and that it means that
mass and energy are related. The Sun gets its energy from conversion of
mass into energy. Over the course of its life, how long will the Sun fuse hydrogen to
helium? 10 billion years
- Know a few facts about the Sun
- Mass is 1.99x1030 kg
- Core - place where fusion takes place;
temperature is 15 million Kelvin
- radiation zone - takes 1,000,000 years for
photons to bump out
- photosphere - bright visible surface where
absorption lines form
- What determines the
luminosity of a star: size and photosphere temperature
- Know the most common spectral types and their order
by temperature: OBAFGKM.
- What determines the spectral type of a star?
Temperature and nothing else.
- What is the most common element in the Universe?
Hydrogen . How about the next most common? Helium. All stars are made up
of mostly Hydrogen and Helium.
- Know what a Hertzsprung-Russell diagram is and
what it is used for. What is plotted on a HR diagram?
- Know where the various luminosity classes of
stars are on the H-R diagram.
- Main Sequence dwarfs – V
- White dwarfs – WD
- Giants – III
- Supergiants – I
- Know what determines the luminosity class: the
size of the star
- Know the difference between Luminosity and
Apparent Brightness. Luminosity is the wattage of a star (energy per
second), and brightness is a measure of how bright the stars looks to us
on Earth (energy per second that falls on a square meter of area).
Apparent brightness varies by an inverse square law. If the distance to a
star is doubled, the brightness goes down by a factor of 4.
- Know that more luminous stars on the main
sequence are more massive. O stars have the most mass, M stars have the
- Lifetime of Stars
- M-types can live for 1000 billion years
- O stars live only a few million years
- G stars (our Sun) live 10 billion years
- Know what parallax is
and how it is used to measure the distance to stars. Parallax can be used
to measure distance out to about 300 LY from Earth.
- Know what is meant by
a standard candle and how that helps measure distance.
- Know how distance can
be determined from a stars spectral type and luminosity class. This is
called spectroscopic parallax. Sample questions:
- If two M5 stars are
observed in the nighttime sky to have the same brightness and star A is
a giant (III) and star B is a main sequence dwarf (V), which is closer
to the Earth?
- An O type star and a
G type stars appear equally bright in the nighttime sky. If we know both
are dwarfs, which is closer?
- Know the ingredients for star formation. Dust
and Molecular Clouds
- Stars form in dark dense molecular and dust
clouds. Massive stars form first and trigger additional star formation.
- Stars do not form in isolation, they seem to
form in clusters.
- Know the various steps in the evolution of a
mid-mass (a mass similar to the mass of the Sun) star. The CAPA problem:
hydrogen fusion, hydrogen shell fusion leading to a red giant, helium
fusion, planetary nebula, white dwarf
- Know what happens to high mass stars. More than
12 times the mass of the Sun explode in a type II supernova.
- Know the steps in the death of stars with mass
like the Sun.
- know what a planetary nebula is and how it is
- know what a white dwarf is and some of its
- size – about that of the Earth
- luminosity (and source of energy output -
small object cooling)
- what force balances gravitational collapse?
(degenerate electron pressure)
- What is the maximum mass for a white dwarf?
1.4 solar masses
- Know what Novae are and the model for Nova
- Type I and Type II are distinguished by the
presence of hydrogen lines and their light curves.
- Know what a type I supernova is (little
hydrogen) - model - collapsing white dwarf when mass > 1.4 solar
masses. These all have about the same peak output and serve as good
- Know what a type II supernova is: explosion of
a massive star
- When do you get a neutron star and when do you
get a black hole? <40 solar masses and > 40 solar masses
- Nucleosynthesis – the
atoms in our body were made in stars.
- Star clusters
- Two kinds
- Gobular clusters – old stars
- Open clusters – young stars
- Useful features
- stars formed at the same time
- all stars are about the same distance from the
- HR diagram plots of star clusters tell us about
- The mass of a main sequence star determines if
it will become a brown dwarf, white dwarf, neutron star or black hole.
- Less than 12 solar masses - WD
- 12-40 solar masses – supernova + neutron star
- greater than 40 solar masses – supernova +
- Know what a neutron star is. Mass of 1.4 to
3 solar masses.
- What balances gravity in a neutron star? Neutron
- Know what a pulsar is. They are rapidly rotating
neutron stars. We see the "pulse" when their magnetic field
lines point toward us.
- Know what a black hole is and the evidence for
- What balances gravity
in a black hole? nothing
- Escape velocity greater than the speed of light
inside of the Schwarzschild radius.
- If the Sun were a black hole it would not suck
the Earth in
- Mass greater than 3 solar masses (not certain)
- Evidence for black holes comes from orbits of
companion stars and X-ray and gamma ray emission.
- gravity far outside a black hole is normal
- General relativity
- Acceleration in one direction is like gravity
in the other direction
- Mass causes space to curve, like a heavy person
laying on a bed.
- Time slows down near mass
- Mass can act like a lens to bend light
- Know that our galaxy is the Milky Way and we
think it is a spiral galaxy.
- Know the approximate properties of the Milky Way
- Size: 100,000 ly diameter
- Mass: 200 billion solar masses
- Know where the Sun is located in the Milky Way
(26,000 ly from the center in the spiral disk)
- Know the various parts of our galaxy: halo,
bulge, disk, and nucleus/galactic center.
- Know the features of the parts of our galaxy
- Halo and bulge (old stars with little gas and
dust) - globular clusters, high velocity stars, M type stars
- Disk - open clusters, large molecular clouds,
dust, and HI regions (21cm hydrogen emission is used to find these), OB
associations, O and B type stars.
- Know what dark matter is. Know the evidence for
dark matter (rotation curve)
- Be able to identify the three main types of
galaxies (spiral, elliptical, irregular).
- Know some characteristics
- Ellipticals, most common, no gas and dust, old
- Spiral, old and young stars, lots of gas and
- Irregular may be the result of collisions
- Know the distance to the nearest galaxies (like
M31). The nearest spiral galaxy like the Milky Way is M31 (Andromeda
Galaxy). It is about 2 million light years away.
- Know the characteristics of the galaxy types.
Table 15.1 has a summary.
- Spiral - like the Milky Way
- Elliptical - old stars, no gas and dust, little
rotation, star formation stopped 10 billion years ago.
- Irregular - may be due to collisions, show
intense star formation
- What does an active galaxy mean? They show
non-thermal spectra, e.g., they can emit a large number of radio wave
- What might cause an active galactic?
(supermassive black holes[most likely], collisions, or starburst
- Know the characteristics of the following
- Seyfert galaxies - very bright central part
with 10,000 times the luminosity given the number of stars
- starburst galaxies - show regions of intense
- Know what a radio galaxy is and some of its
- Know the characteristics of quasars
- large redshifts imply at least some are very
- recent photos show they are associated with
- large energy output; fast variation implies a
small size, but are very far away which means they must have a high
- can have red shifts corresponding to velocities
as high as 90% the speed of light
- Fast variations in X-ray output indicate they
must be only about 0.1 ly in size.
- What is a supermassive black hole? Black hole
with a mass of a billion solar masses.
- Know how the supermassive black hole model
explains Seyfert galaxies, quasars, and radio galaxies.
- Chpt 27:
- Know what the Hubble law is: velocity = Hubble
constant x distance.
- Know it is used to measure the distance to
objects very far away, like quasars and distant galaxies.
- What is redshift? Know that large redshift means
the object is far away. Redshifts corresponding to velocities of more
than 90% the speed of light have been observed. The largest observed
redshifts are around 5.
- How are distances measured in the Universe?
- Radar - distance to the Sun
- Parallax - distance to nearest stars
- Spectroscopic parallax - distance to stars in
- Cepheids, RR Lyrae, novae, supergiants -
distance to stars in nearby galaxies (know why RR Lyrae stars can be
used - same luminosity; know how Cepheids are used, i.e.
period-luminosity relationship; longer period means more luminous)
- Very bright objects, like Supernovae - Nearby
- Brightness of galaxies (Tully-Fischer law) -
distant clusters and superclusters
- Hubble law - distant objects like quasars
- Know what is meant by our local group of
galaxies (30 galaxies within 3 million ly)
- Know roughly the large-scale structure of the
Universe: clusters, superclusters, and voids.
- Know about the Hubble law and how it relates to
the age of the Universe: age of Universe = 1/H
- Know the big bang model and the current evidence
for the big bang
- 3 K background radiation
- light-element abundance (hydrogen, helium, and
- What is the critical density and why is it
- Muniverse> critical density-
closed universe and eventual contraction and big crunch
- Muniverse< critical density- open
universe and continued expansion
- Muniverse = critical density-
expansion stops at infinite time
- What was measured by COBE? (Microwave background
radiation and ripples) Why is it important? Confirmation of the big band,
shows early fluctuation in the Universe important for galaxy formation,
agrees with the presence of a lot of dark matter in the Universe.