AST 104 TEST 2
Ideas to know for the exam:
1. What can color filters tell us about light from stars and nebulae? What does a star’s B-V color index mean? How can a false-color composite image be useful? a. Filters measure brightness through different filters (a hotter star will appear brighter through a B filter than a V filter).
b. Color index: the difference in the apparent magnitude of a star when viewed through a blue filter versus through a “visible” filter B V (the lower the index the hotter the star).
c. False color composites allow us to visualize the wavelengths the human eye does not see (near the infrared range).
2. What is the relationship between luminosity and absolute magnitude? What are the relationships between B-V color index, temperature, and spectral class? a. The absolute magnitude is a measure of the star's luminosity.
Don't forget about the age old question of mental imaging in which a speaker vividly
b. Color and spectral class is determined by temperature.
3. What are the axes of an H-R diagram? What are the different luminosity classes revealed in an H-R diagram constructed from a large variety of stars? How can you determine a star’s luminosity, luminosity class, or temperature using an H-R diagram? How do stars at different locations on an H-R diagram compare?
a. X- axis: B-V color index, temperature, and spectral class
Y-axis: luminosity and absolute magnitude
b. Ia) Brightest supergiants
Ib) Less luminous supergiants
II) Bright giants
IV) Subgiants (intermediate between giants and main sequence stars)
V) Main-sequence stars
c. By looking at the location on the HR diagram Don't forget about the age old question of cin 111
d. Top right = large cool stars (red giants); bottom right = small cool stars; top left = large hot star; bottom left = small hot stars (white dwarfs)
4. What can we learn by measuring the size and period of a binary star system’s orbit? a. We can work out their masses
D3 = (M1+M2) x P2
5. What is the main sequence? What is the relationship between mass, luminosity, temperature, and lifetime of stars along the main sequence?
a. Fuses H → He in the core
b. There is a strong relationship between a star’s mass and its luminosity (L ~ M4) If you want to learn more check out smallpoks
c. Cool and smaller stars burn longer than large hot stars because they do not fuse as fast.
6. What is the difference between a star’s “lifetime” and a star’s “age”? How can we use the H-R diagram for a star cluster to determine the age of the cluster? What does that tell us about the average age of the stars in the cluster?
a. Lifetime: a star's mass gives a measure of the amount of "fuel", and its luminosity gives a measure of the rate at which this "fuel" is consumed by nuclear burning, so a star's lifetime is proportional to its Mass divided by its Luminosity.
Age: astronomers determine the age of stars by observing their spectrum, luminosity and motion through space
b. By determining the mass of the main-sequence turnoff stars, we get the age of the cluster
c. A cluster is as old as its main sequence stars
7. How are apparent magnitude and absolute magnitude related to distance? How does a star’s apparent magnitude change with its distance? How can we use “spectroscopic parallax” to determine the distance to a star or star cluster?
a. A star’s absolute magnitude (M) is fixed (depends on luminosity), but a star’s apparent magnitude (m) changes with distance
c. The magnitude scales are defined so that…
i. If a star is 10 parsecs away, it’s apparent magnitude is equal to its absolute magnitude. Don't forget about the age old question of macromolecules notes
ii. a star’s apparent magnitude increases by 5 for each factor of 10 we move farther away in distance.
8. What are the key differences between star clusters? How are open clusters similar to each other? How are globular clusters similar to each other? How do open clusters and globular clusters differ from each other? Why are star clusters particularly interesting to study?
a. Open Clusters:
i. Loosely clustered groups of stars
ii. Generally contain a few hundred up to a couple thousand members iii. Typical diameters of about 10 parsecs
v. Blue and O, B, A stars
b. Globular Clusters:
i. Can contain more than 1 million stars
ii. Diameter of about 50 parsecs
iii. Always look like crowded spheres of stars, especially crowded in the center
v. Red and G, K, M stars
i. The stars in any given star cluster are all at (almost exactly) the same distance.
ii. The stars in any given star cluster all formed at the same time from the same cloud of stuff.
iii. There are distinctive similarities among open clusters and among globular clusters.
iv. There are distinctive differences between the two kinds of cluster. v. All of this gives us insights into the structure and the history of the universe.
9. How did Cecelia Payne determine that the Sun and all stars consist mainly of hydrogen and helium? How did this differ from the expectations at the time? a. When combining the temperature with the relative absorption strength, Payne worked out how much of each element was present in different stars b. As theorized at the time, she found similar amounts of heavy elements (carbon, silicon, etc.) to those present in the Earth.
c. However, she also found substantially more hydrogen and helium present in every star.
d. This idea that the simplest light gases (hydrogen and helium) were the most abundant elements in stars was so unexpected and so shocking that she assumed her analysis of the data must be wrong!
10. How does nuclear fusion power stars? Where does it take place within a star? How does the difference in mass between four hydrogen nuclei and a helium nucleus become heat?
a. Fusion is the process through which energy is released.
b. Main sequence: fuse in the core
Red Giants: fuse in the shell
c. Four hydrogens = 4 × 1.00794 u = 4.03176 u
One helium nucleus (two protons plus two neutrons) = 4.002602 u
4.03176 u – 4.002602 u = 0.02916 u (almost 1% is gone!) → turns into energy 11. What is the difference between nuclear fusion and nuclear fission? From what elements can energy be extracted through fusion? From what elements can it be extracted from fission? What is special about iron in this context?
a. nuclear fusion: the combining of two nuclei into a different, larger nucleus nuclear fission: the splitting of a large nucleus into multiple smaller nuclei b. Elements through fusion: Hydrogen → Iron
Elements through fission: Uranium → Iron
c. Neither fusion or fission release energy from iron because it has the lowest mass per nuclear particle.
12. In what way are the pressures exerted by gravity and energy released by fusion in a state of balance within a star? How does this balance change over the life of a star? How does the way in which fusion proceeds in a star change as it ends its time on the main sequence and transitions to a giant star?
a. Gravity exerts in while fusion exerts out. If gravity has a greater force than fusion it will condense to a neutron star or black hole. If fusion has a greater force than gravity it will expand to a red supergiant.
b. This determines the path the star will go through (high mass → type II supernova → neutron star or black hole; low mass → red giant → planetary nebula → white dwarf
13. What is the relationship between a star’s mass, luminosity, and lifetime? How do high-mass stars compare to low-mass stars in terms of luminosity and lifetime? a. The bigger, more luminous a star is the shorter its lifetime is
b. High mass stars are more luminous but liver short because fusion goes much more quickly due to the larger weight/pressure from the star’s larger mass. Low mass star are less luminous but live longer
14. What is the difference between an emission nebula (H II region) and a reflection nebula? In what ways are emission nebulae “stellar nurseries?”
a. emission nebula: cloud of gas and dust lying close to a hot star that ionizes the hydrogen in the cloud
reflection nebula: interstellar dust illuminated by stars
b. Infrared wavelengths reveal concentrations of interstellar gas.
These young star clusters in H II regions are found where this interstellar hydrogen is most dense
15. How do H II regions, young open clusters, middle-aged open clusters, and old open clusters differ?
a. Young star clusters in H II regions are found where this interstellar hydrogen is most dense
Middle-aged open clusters
Old open clusters
16. What are the stages of protostar formation? How does a protostar differ from a main sequence star? What is the “zero-age” main sequence?
a. 1. Dense cores form within a molecular cloud after a shock from some interstellar event (passing star, supernova, etc.).
2. A protostar with a surrounding disk of material forms at the center of a dense core, accumulating additional material from the molecular cloud through gravitational attraction.
3. A stellar wind breaks out but is confined by the disk to flow out along the two poles of the star.
4. Eventually, this wind sweeps away the cloud material and halts the accumulation of additional material, and a newly formed star, surrounded by a disk, becomes observable.
b. A protostar is the stage in a star's life before it is hot enough to fuse hydrogen c. zero-age main sequence - the time when a star stops contracting, settles onto the main sequence, and begins to fuse hydrogen in its core
17. What is a circumstellar disk? What is a protoplanetary disk?
a. circumstellar disk: a torus, pancake or ring-shaped accumulation of matter composed of gas, dust, planetesimals, asteroids, or collision fragments in orbit around a star.
protoplanetary disk: a rotating circumstellar disk of dense gas and dust surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star 18. What is a star’s habitable zone? How does a star’s habitable zone depend on the star’s luminosity?
a. habitable zone: the distance range from the star where we calculate that surface temperatures would be consistent with liquid water
b. A star's luminosity increases with time, both the inner and outer boundaries of its habitable zone move outward. Thus, a planet that is in the habitable zone when a star is young may subsequently become too hot.
19. What can we learn from observing a pulsating variable star’s apparent brightness? How does a Cepheid variable star’s luminosity relate to the period over which it brightens and dims? How can we use observations of these special stars to determine their distances from us?
b. Cepheid variables and noticed that those with longer periods were brighter than the shorter-period ones
c. By measuring the times of arrival of pulses from the same pulsar at different frequencies you can determine the distance to the pulsar, as long as you know the speed of radio waves through the interstellar medium at different frequencies.
20. What happens to stars after their time on the main sequence is over? How does fusion of heavier elements than hydrogen occur in giant stars as they evolve away from the main sequence? What happens when a star can no longer generate energy from fusion at all? How does the answer differ for low-mass stars versus high-mass stars? a. They become red giant stars
b. Low mass:
Hydrogen fuses to helium, and then helium fuses to carbon.
No fusion beyond carbon: not enough pressure builds up in the star’s core.
Hydrogen fuses to helium, helium fuses to carbon, then heavier
elements fuse (oxygen, neon, magnesium, and silicon), all the way up
c. Low mass:
Layer of helium hot enough to fuse
Layer of cooler helium
Layer of hydrogen hot enough to fuse
Cooler hydrogen beyond
High-mass stars can fuse elements heavier than carbon.
Hydrogen fusion is taking place in an outer shell.
Progressively heavier elements are undergoing fusion in the
higher-temperature layers closer to the center
21. What is a planetary nebula? What shape do they typically have and how do they appear from different perspectives?
a. Result from the outer layers of a star ejected as its fuel runs out
b. Clouds of dust and gas, bipolar planetary nebula shape, cluster of massive, hot young stars only a few million years old, an isolated star surrounded by a ring of gas that's perpendicular to the ring and on either side of it, there are two bluish blobs of gas
22. What is a white dwarf star? How is it different from a main sequence star? What is electron degeneracy pressure and what role does it play in a white dwarf star? a. High temperature, low luminosity and mass
b. Electron degeneracy pressure: the gravitationally-driven pressure is so great it squeezes atoms as close together as they can get
23. What happens during a Type II supernova? What happens in the core during the final stages of fusion? What elements are produced? What happens to the outer layers of the star and what happens to the core?
a. Star runs out of nuclear fuel and collapses under its own gravity The outer parts of the star are blown off into a “supernova remnant.” b. When fusion can no longer be sustained, the star collapses and crushes the core in a dramatic finish to the star’s life →
Forms a nickel-iron core →
The iron core reaches a critical mass unsustainable without the outward pressure of fusion and starts to collapse →
The inner part of the core is compressed into neutrons →
The infalling material from the rest of the star bounces off of this core collapse and forms an outward-propagating shock front →
The surrounding material is blasted away, leaving only a degenerate remnant of the core in the form of a neutron star
24. What is a neutron star? How is it different from a main sequence star? From a white dwarf star? What is neutron degeneracy pressure and what role does it play in a neutron star?
a. Electrons and protons combine to form neutrons. Neutrons run out of the room to move around. Neutrons prevent further collapse. Much smaller. b. A stellar application of the Pauli Exclusion Principle in which no two neutrons can occupy identical states, even under the pressure of a collapsing star → this pressure supports a neutron star
25. What is a pulsar? What did graduate student Jocelyn Bell observe in 1967 that led to their discovery? How are they related to neutron stars? What is responsible for their pulsing signals? How does an object so massive end up rotating so fast? a. A quickly rotating neutron star.
b. A precisely repeating radio signal
c. Neutron stars rotate very rapidly.
Their intense magnetic fields create beams of radiation.
This magnetic field doesn’t have to be aligned with their spin axis.
When the magnetic axis is not aligned with the rotation axis, the beams sweep through space like lighthouse beams.
Each time a beam sweeps toward Earth, we see a pulse of radiation. 26. What are the different fates of low- and high-mass stars? What is left behind by their outer layers? What is left behind of their cores? Under what circumstances does a star end as a white dwarf, a neutron star, or a black hole? What are the similarities and differences between these three types of objects?
a. Low Mass Stars
i. Hydrogen fuses to helium, and then helium fuses to carbon. No fusion beyond carbon: not enough pressure builds up in the star’s core.
ii. Stars move “upward” on the H-R diagram – to higher luminosity
without much change in temperature. Stars move down (when He
fusion starts,) then up again.
iii. Star gradually loses its outer parts via a stellar wind, forming a
beautiful “planetary nebula.”
iv. A white dwarf: The carbon core of the star, a million times denser than
ordinary matter. Starts hot, but cools. Very small, so luminosity is low.
Moves down and to the right in the H-R diagram.
b. High Mass Stars
i. Hydrogen fuses to helium, helium fuses to carbon, then heavier
elements fuse (oxygen, neon, magnesium, and silicon), all the way up
ii. Stars move “rightward” on the H-R diagram, toward cooler
temperatures without much change in luminosity. Stars move left and
right a few times, as fusion proceeds.
iii. Star loses its outer parts in a violent explosion, called a “Type II
iv. A neutron star or a black hole Neutron star Core mass is 1.4 times the
Sun’s mass, but compressed into 15 km diameter. Black hole Core
mass 3 MSun or more, in the form of curved space-time.
27. Gravity is weak, but why is it still so important in astronomy? What is escape
velocity, and how does it depend on an object’s mass and surface radius? What is a black hole? What is its “Schwarzschild radius” and its “event horizon?”
a. gravity is always attractive.
some objects in the universe have HUGE masses.
gravity extends forever.
b. speed at which something never comes back because the amount its
decelerating is less than how quickly gravity is weakening. the thing will go
on infinitely away from Earth → The smaller the radius, the larger the escape
velocity from the surface.
c. The gravity of a sufficiently massive core will overcome the neutron degeneracy pressure and continue to collapse indefinitely
d. Radius of a sphere such that, if all the mass of an object were to be compressed within that sphere, the escape velocity from the surface of the sphere would equal the speed of light. Also called the gravitational radius.
This is the “size” of a black hole. This is the region around the center of this incredibly dense object from which not even light
28. What is the difference between Newton’s description of gravity and Einstein’s updated description? How is spacetime distorted by the presence of mass? What are gravitational redshift, gravitational time dilation, and spaghettification? How do they depend on proximity to an extremely dense star like a neutron star or black hole?
a. Einstein: Spacetime is distorted by the presence of mass & energy. i. We say it becomes “curved” (which has a precise mathematical definition).
ii. The shortest paths through spacetime become curves…
iii. No longer a “straight line,” but a hyperbolic orbit or even a bound elliptical orbit.
b. gravitational redshift: light traveling away from the black hole becomes stretched to longer wavelengths
gravitational time dilation: as measured by an outside observer, time progresses more slowly closer to the event horizon
Spaghettification: objects are stretched and ripped apart as they approach the black hole
*These effects are true near any massive object because of its gravity! It’s only near neutron stars and black holes that they become dramatic.*
29. What are the differences between a Type Ia supernova and a Type II supernova? a. Type II supernovae result from the explosion of a massive star after its iron core collapses and rebounds.
b. Type Ia supernovae on the other hand result from the explosion of a white dwarf in a mass transfer binary system
30. What is a gravitational wave and how is it different from an electromagnetic wave? How are they similar? What are the sources of gravitational waves that have been observed so far in the 21st century?
a. Gravitational waves, similarly, are generated by the bulk motion of large masses and will have wavelengths much longer than the objects themselves. Electromagnetic waves, meanwhile, are typically generated by small movements of charge pairs within objects and have wavelengths much smaller than the objects themselves.
b. Gravitational waves are weakly interacting, making them extraordinarily difficult to detect; at the same time, they can travel unhindered through intervening matter of any density or composition.
c. Electromagnetic waves are strongly interacting with normal matter, making them easy to detect; but they are readily absorbed or scattered by intervening matter
d. Pairs of colliding black holes