Final Exam Flashcards
What are the interior layers on the sun?
- Core: where energy is generated
- Radiation zone: radiation carries energy outward
- Convection zone: churning plasma carries energy outward
What is the photosphere
“Surface” layer of the sun that is visible to the human eye; includes sun spots and granulations in which the hot spots will bubble up to the surface, cool, and sink down into the convention zone
What are the outer layers of the sun
- Chromosphere: thin layer containing hot gas
- Corona: extremely hot and emits mainly x-rays
- Solar wind: layer not actually on the sun but surrounding it in which particles are expelled into
What makes it so that the sun doesn’t explode/implode?
Hydrostatic equilibrium created by the balance between gravity and pressure
What is the main source of the sun’s energy
Nuclear fussion
What elements is the sun mostly composed of
Hydrogen and helium
When nuclei fuse what happens to their mass
sum of fused nuclei is less than the sum of initial masses. Lost mass is converted to energy
How does the energy from the core reach the sun reach the surface and how long does it take?
Gamma rays slowly work their way out from the core to the surface, exchanging energy with the solar matter along the way, turning them into visible wavelenghts (radiative diffusion); takes hundreds of thousands of years
What evidence is there that the sun goes through radiative diffusion?
- Sunquakes: measurement of the movement of the sun’s surface using doppler shift of visible light shows oscillation spots that are congruent with sun spots
- 2 neutrinos from the core emergy for each helium fussion reaction but are very hard to detect. Can be detected using underwater experiments which show observations consistent with expectations from nuclear fission
what are the 7 primary types of stellar spectra from hottest to coldest and what are the subclasses of each class
O B A F G K M
Oh Be A Fine Guy, Kiss Me
from 0 - 9 where 0 is the hottest and 9 is the coldest
What is a brown dwarf
- Too cool/small to initiate p-p fusion
- Has a mass .075x smaller than the sun
- undergoes deuterium fusion; still generates energy
What are stars mostly composed of
Hydrogen and Helium; mostly H
Why are there typically narrow spectral lines for bigger stars
- broad spectral lines are due to a high number of particle collisions. Larger stars have a greater volume thus, low pressure meaning particles are less likely to collide with each other
- Gas is more likely to stay ionized when pressure is low thus, in large stars there are a lot of ionized atoms which have different spectra than neutral atoms
In the H-R diagram, what is the relationship between luminosity and size of stars for a given temperature
Higher the luminostiy, bigger the star
On the H-R diagram where are the 4 types of stars that are not on the main sequence
White dwarfs bottom left, red giants mid way up to the right, red super giants top right and toward the center, and blue giants somewhat bottom right of the main sequence line
What stars are on the main sequence of the H-R diagram
Stars fusing H in their cores
What are star clusters
Stars that all formed about the same time, born from giant clouds of gas
If a neutral dust/gas cloud is between us and a star what will the absorption lines look like
Absorption lines of the star’s surface will be broad due to high pressure, there will be additional narrower lines from the dust cloud, if the star is in binary there will be broad lines that move back and forth due to star oscillation of each other
What are the first 4 steps that a star goes through when getting off of the main sequence
- H core is exhausted
- Left w/ contracting He core
- Heat from contraction raises temp of H layer until it goes through fusion
- creates more energy resulting in layer expansion - Star becomes red giant
- He core contracts and heats up
- He flash results from rapid onset of He fusion
- He core expands and star contracts - Red giant stabilized as core goes through fusion
- Once He core is exhausted, goes into another core collapse with C and O
What happens to an initially low mass star after it goes through a C and O core collapse
- Gravitational potential energy ignited fusion of He shell
- He flash occurs and H shell ignites P-P fusion - Star expands into 2nd red giant phase
- Star goes from red giant to white dwarf
- In doing so, star ejects shells of gas which form planetary nebulae
- Remnant star is very hot and dense
What happens to an initially high mass star after it goes through a C and O core collapse
- Left w/ an iron core
- Takes energy - Star goes into supernova explosion
- Shells stop producing energy resulting in increase in mass
- Core contracts but is stopped by degenerate electrons
- Electron save core but sudden contraction stop causes rest of star to be blown apart - Star becomes neutron star or black hole
- Black hole if star was initially > 40x the sun
- Neutron star if star was initially 10-40x the sun
Explain what the Chandrasekhar limit is
White dwarfs shrink as the mass in the star increases
- As a star increases in mass, electrons become more tightly packed resulting in a smaller radius
- Theoretical models show that the maxiumum mass that a star can end its life with and still become a white dwarf is 1.4x the sun
What does the end of life look like for main sequence star in a binary system
- One becomes a red giant, dumping mass onto the other
- This star grows in size, becoming a red giant
- The two stars will exchange mass back and forth until one becomes a white dwarf
What is a cephid variable
large yellow star that is anywhere from 10^3-10^5 time as luminous as the sun
Why are cephid star’s useful
Can be used to determine the distacne to stars close to them. Find period of star to get luminosity, use that to get apparent brightness, and then use the inverse square law (1/r^2) to get distance
What is the relationship between magnitude and brightness
lower magnitude = brighter star
What are the products of p-p fusion cycle
4He nuclei, neutrinos, gamma rays and positrons
How long does it take initially high and initially low mass stars to reach their end of life and what do they become
Takes initially low mass stars ~100 million years to become white dwarfs. Initially high mass stars end up as neutron stars or black holes
Why do different stars have different spectral lines
They are all mostly made of H and He but they have different surface temperatures which affect the ionization states of different elements thus, there are different absorption lines for different elements
Why do larger stars stay on the main sequence for less time compared to smaller stars?
Bigger stars have a higher temperature thus, higher rate of fusion meaning they use up the H in their cores faster
What are the ways we can determine the masses of two stars in a visually binary system
- measure period and semi-major axis visually
- Use Kepler’s 3rd law to get M1 and M2
- Find location of center of mass, relative to either star giving M1 and M2
what are the ways astronomers use to identify binary systems despite the two stars not being close enough to see their separate images?
Spectral lines of binary systems show periodic variation in double absorption lines as doppler shift of each star varies in its orbit (red shift=decrease and away, blue shift = increase and towards).
Periodic dips in light curve when one star passes in front of the other
What is the white dwarf
Star with no nulear reactions going on in the core, which is primarily made of C and O
What is a protostar
Star of uniform composition from center to surface that contains H but has no others rxns going on in the core
Arrange the following in order of their evolution: white dwarf, protostar, main sequence, red giant: after He flash, and red giant: before He flash
Protostar, Main sequence, red giant: before He flash, red giant: after He flash, and white dwarf
The force needed to maintain a body at constant speed in outer space is equal to _____
Zero. Speed is constant so there is no acceleration thus, no force needed
If the only force on an orbiting satellite is gravity, why doesn’t it fall to the Earth?
It is in “freefall” it is falling toward Earth but its velocity component perpindicular to the direction of the gravitational force means that it effectively “falls around the Earth”
If a star has a parallax angle of .05 arcseconds, how far away is it?
See study guide (1)
If the moon moved twice as far from Earth as it is now, how would the gravitational force between the Earth and the moon change?
See study guide (2)
what single piece of information is enough to determine whether a star is hotter than out sun?
Can look at the star’s color to determine the wavelength and use equation (max wavelenght is equal to 3x10 ^ 6/T)
Look at sketch on study guide (3), at which point in the planets orbit is the planet moving the fastest? At which point in the planet’s orbit is the planet’s speed increasing?
B is where the planet is moving fastest while A is where the planet’s speed is increasing. Know due to Kepler’s 2nd law (the closer a planet is to the sun during its orbit, the faster it moves)
What is Kepler’s 3rd law
The amount of time a planet takes to orbit the sun is related to the size of its orbit; implies that a planet with a larger distance from the sun, will take longer to orbit the sun
What is Kepler’s 1st law
Planets move in elliptical orbits with the sun at one focus of the ellipse
What are some advantages that a reflector telescope has over a refractor telescope
- Lenses absorb UV light
- Reflectors avoid the problem of chromatic abberation in the lens
- The device that collects the light is more easily supported in a reflector
- Flaws and bubbles in the material inside a thick mirror are not a problem, while flaws and bubbles in the material inside a thick lens are
Star A has a luminosity 4 times that of star B. The temperature of Star A and Star B are 3500 and 7000 kelvin. What is the radius of Star A compared to Star B?
See study guide (4)
Your weight on the surface of the Earth is 900 Newtons. The planet Vulcan has a mass 4x that of the Earth and a radius 3x that of the Earth. How much do you weigh on the surface of Vulcan?
See study guide (5)
What is the speed of a 30 megahertz radio wave?
C = 3x10 ^8 m/s (all electromagnetic radiation has the same speed)
An electric stove burner on “high” radiates most strongly at about 2000 nanometers. What is the temperature?
See study guide (6)
What are the types of radiation wavelengths in order of lowest to highest frequency
Radio, microwave, infrared, visible, ultraviolet, X-ray, and gamma ray
A comet in our solar system has an orbit whose closest distance to the sun is 7 AU and farthest distance is 11 AU. what is the period of its orbit?
See study guide (7)
The size of a crater on the moon (at a distance of 400,000 km) is 1 km across what is the angular size of the crater observed from Earth, in arcseconds?
See study guide (8)
In yellow light (wavelength 500 nanometers) what is the minimum size of a telescope mirror on Earth that can resolve a crater with a minimum degree of .52 arcseconds
See study guide (9)
To go from a lower level in an atom to a higher level, an electron must _____
absorb a photon of energy
You are analyzing the radio spectrum of the outer part of a distant galaxy. A spectral line expected to be at 21 cm wavelength is instead measured to be at 20.95 cm. Is that part of the galaxy rotating towards you or away from you? How fast is it moving?
See study guide (10)
Is it ever possible for a cooler object to emit more energy (in a fixed time) than a warmer object. Explain
yes, the cooler object could be much larger
Why is an absorption spectrum especially useful in astronomers?
It has dark lines in it that allow astronomers to determine what elements are in the star
If the Earth was 4x more massive than it actually is, but had the same radius, how would the escape velocity for an object starting at the Earth’s surface can be changed?
see study guide (11)
If star A and star B are identical but star A is 100 times brighter which star is closer to us and by what factor is it closer?
Star A is 10x closer. Brightness = L/ (4Pid^2), 100 times brighter implies 10 times closer
If a 3 solar mass star and a 20 solar mass star are formed in a binary system which star is expected to
- evolve off the main sequence first?
- eventually form a carbon and oxygen rich white dwarf?
- eventually form a neutron star?
- 20 solar mass star
- 3 solar mass star
- 20 solar mass star
(larger mass burns faster thus, leaves main sequence first; initially low mass stars become white dwarfs; initially high mass stars become neutron stars or black holes)
The escape velocity from the surface of the Earth is 11 km/s. Imagine a planet whose mass is 8x that of the Earth, and whose radius is twice that of the Earth. What would the escape velocity be from the surface of that planet?
see study guide (12)
The Sun has a surface temperature of 5800 K. Consider star A which has a surface temperature of 11600 K and a radius which is 2x that of the Sun. What is the ratio of that Star’s luminosity to that of the sun?
see study guide (13)
What is Newton’s first law
Objects at rest tend to stay at rest while objects in motion tend to stay in motion
Put the following star colors in order of most hottest to least hottest: orange, yellow, and red
blue, yellow, orange, red
How is a black hole created?
When a neutron star exceeds about 2 solar masses it will collapse further. As it collapses, the escape velocity increases until it becomes the speed of light but nothing can exceed the speed of light so nothing can escape. The escape velocity will reach c when the object’s size is smaller than the Schwarzschild radius
What is gravitational redshift
As light leaves the graviational pull of Earth/star/blackhole, it loses “kinetic energy” and shifts to red. Evetually once a wavelength is equal to the Schwarzchild radius, no light gets out
What is gravitational time dilation
Clocks in a gravitational field run slower than clocks in free space. Once it becomes equal to the Schwarzchild radius, the clock frequency goes to zero and time stops
What is a black hole
object whose gravity is so strong that not even light can escape from it. Light emitted from outside the “horizon event” (Schwarzchild radius) can escape but light within it cannot
How can we detect black holes
Half are in binary systems. So identify binary systems via doppler shift but with only one visible star. Use Kepler’s law to show M > 3M of the sun thus not a neutron star. Look fro strong x-ray signal and if there, black hole candidate. Other half are likely at galactic centers as virtually every galaxy has a supermassive black hole in the center
Where is the roche limit in reference to an event horizon of a black hole and how will it affect objects falling into the event horizon
roche limit is well outside of the event horizon for a stellar mass black hole so objects will be pulled apart prior to hiting the event horizon but for a supermassive black hole the roche limit is well inside the event horizon so objects will be pulled apart only after passing the event horizon
Lable the parts of the milky way (14)
See study guide (15)
Once a black hole forms, the size of its event horizon is determine by
the mass inside the event horizon
What is gravitational lensing
When there is a large object between you and a light source, the light will be bent so that you can see it in two places. Can sometimes result in an “Einstein ring” around the object
What is Olber’s paradox
Idea that if the universe is infinitely large and old then the sky should be bright since there are stars everywhere. However, we don’t see stars everywhere thus, the universe is not static and is expanding in a way that prevents us from seeing very distant stars
What does the Milky Way’s evolution look like
- Initially two clumps, no spiral structure
- 0 - 2 billion years, cental bulge and fuzzy spiral arms develops
- 3 - 4 billion years, two well-defined spiral arms form
- > 8 billions years, multiple spiral arms
- more recently, central bar develops out of central bulge
How does the Milky Way rotate
Different parts rotate at different rates. Stars in the “thin disk” rotate faster than those in the “thick disk” and much faster than those in the spherical halo
What evidence do we have for dark matter
- Stars around in a galaxy are expected to move slower when farther away from the center but move much faster than Kepler’s 3rd law predicts; found to be due to matter that we can’t see.
- Found that galaxies in the Coma cluster were moving too fast for the cluster to be bound by visible matter thus it was proposed that the galaxies were held together by dark matter
- Most cases of gravitational lensing show much stronger lensing than can be accounted for by the visible mass of the foreground galaxies (found that the amount of dark matter is 5x the amount of luminous matter)
What is dark matter
Prevailing idea is that it is made up of weakly interacting massive particles such that they interact via gravity and have a large mass so that it can be cold but must only interact via the weak interaction or some new feeble interaction that is so weak it hasn’t been discovered
What are the two broad classes of stars and how do they differ
Population I stars
- in the disk, orbiting the galactic center
- typically young open clusters
- composed largely of heavy elements (composed of the remnants of other stars)
Population II stars
- mainly in the halo, follow elliptical orbits
- typically very old (11 - 13 billions years)
- Almost entirely H or He
What is the Tully-Fisher Law
Luminosity depends on the rotational rate. Spiral galaxies with higher rotational velocities tend to be more luminous. Can also be found by comparing luminosity to total mass of the galaxy; mass and rotational rate are related via Kepler’s 3rd law
What is hubble’s law and what are the implications of it
V = Hd where there is a positive relationship between distance and velocity. Implies that the universe is expanding (distance between points is getting larger)
What are Quasars and what is their importance
“Quasi-stellar radio source” that are found to be located at the same spot in the sky. Are incredibly red-shifted that they must be receding at a huge speed. So bright that they give us a window into galaxies in the early universe
What is good evidence that quasars are located inside galaxies
Nearby quasars show “fuzz” around them with the same reshift as the quasar
Why can we not see the visible light from the early stages of the birth of the universe
Its extremely red-shifted all the way into the microwave. Called the cosmic microwave background
What is the steady state cosmology
Idea of an expanding universe but with density and temperature staying constant. As such, matter would have to be continuously created everywhere in space (violates conservation of energy/mass)
What is the big bang cosmology
Universe is expanding as density and temperature change. Distance between points changes
What caused scientists to deny the steady state comology
Quasars were discovered to have a changing redshift over time, violating the isotropy and homogenity of steady state. The light released from the recombination stage should still be present today but has since been redshifted (CMB) thus, proving big bang
What is critical density
Overall density needed to “win” agaisnt gravity; depends on Hubble’s constant; faster exanding universe = the more density needed to have enough gravity to slow the expansion all the way down
What are the three scenarios for the future of the universe
- Closed (recollapsing universe) ends in a big crunch; gravity wins; density > 1
- Flat (critical universe) exactly the right amount of matter to balance; density < 1
- open (coasting universe) keep on expanding forever; gravity loses; density = 1
What allowed the discovery for the future of the universe
Found that the universe is open due to the presence of dark energy which is influencing Hubble’s constant
What is the recombination stage of the universe
Once universe started to cool after initially being hot and dense. Electrons combined with nuclei to form atoms, transparent to visible light: cosmic microwave background generated; 380,000 years
When looking back into the universe, when does known physics break down
10^-43 seconds
