Exam 2 Flashcards
What are the interior layers of the sun?
- Core: where energy is generated
- Radiation zone: radiation carries energy outward
- Convection zone: churning plasma carries energy outward
What is the phtosphere
The “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 convection 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 (mainlyl protons and electrons)
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 fusion
What elements is the sun mostly composed of
Hydrogen and helium
What is the strong force
Force with a very short range that only applies to the nucleus; causes nuclei to be pulled together (fusion) but only if there is a high enough temperature
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 of 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: We can measure movement of the Sun’s surface using doppler shift of visible light. This shows oscilation spots that are congruent with sun spots
- 2 Neutrinos from the core emerge for each helium fusion reaction but are very hard to detect but can be seen using underwater experiments; observations are consistent with expectations from nuclear fusion
What is the difference between luminosity and apparent brightness
Luminosity is the power put out by a star or what is seen at a fixed distance away from every star while apparent brightness is how bright the star appears from here on Earth
What does magnitude depend on
luminosity and distance
Given you know the star’s distance, how do you find its temperature, luminosity, and radius
Measure the apparent brightness to get the luminosity via 1/r^2 relationship. Measure the color, use equation: max wavelenght = (3x10^6)/ T to get temperature. Can find radius given that L=FA and F= sigma(T^4) where A=4(pi)R^2
When the nuclei of 2 light elements collide to undergo nuclear fusion what happens to some of their mass
Some of the mass is converted to energy
What are the 7 primary types of stellar spectra from hottest to coldest
O B A F G K M
Oh Be A Fine Guy, Kiss Me
What are the subclasses of each class
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
- Undergo deuterium fusion; still generate energy
What are most stars composed of
Hydrogen and Helium; mostly H
Explain why there are typically narrow spectral lines for big 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
How can we get the rotation of a star
Star’s approaching edge has light in that is shifted to shorter wavelenghts while star’s receding edge is shifted to longer wavelengths. Observed spectral lines are sum of these edges. If star is rotating rapidly, there will be a greater doppler shift and spectral lines will broaden
When stars in a binary system are too close to separate them visually how can we tell them apart
Look at spectral lines for system. Star moving towards Earth has wavelengths that are blue-shifted (decreased) while star moving away from Earth has wavelenghts that are red-shifted (increased)
What is proper motion
Motion of a star across the sky perpindicular to out line of sight
In the H-R diagram, what is the relationship between luminosity and size of stars fro a given temperature
Higher the luminosity, 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
In what way on the H-R diagram is increasing mass
Up and to the left
Stars with a higher mass have a _____ fusion rate and a ______ luminosity and _____ lifetime
Stars with a higher mass have a larger fusion rate and a higher luminosity and shorter lifetime
What are star clusters
Stars that all formed about the same time, born from giant clouds of gas
If a neutral dust/gas could 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 stars oscillation of each other
What are the ways that star formation can be “triggered” in a molecular cloud
- collision of two molecular couds
- impact of material in shockwave from a supernova explosion
- wave of pressure from spiral-arm of galaxy
- pressure from shockwave of gas from nearby newly-formed star
- In each case the molecular cloud is compressed, so its density increases, and gravitational collapse of “clumps” initiated
What are the first 4 steps that a star goes though 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 keeps contracting and heating up
- He flash results from rapid onset of He fusion
- He core expands and star contracts - Red giatn stabilizes as core goes through fusion
- Once He core is exhausted, goes into another core collapse with C and O
What happens to an initally 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 neulae
- 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
- After C and O fusion, becomes an iron core
- Doesn’t release energy but takes it - 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 stp 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 eventually happens to white dwarfs
Will cool to only a few hundred degrees where it is predicted to be a black dwarf in which it is effectively invisible but takes tens of billions of years
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 happens to a white dwarf in a binary system
- Other star transfers H to it, causing the dwarf to gain a new layer that suddenly reaches fusion in an explosive way such that the star becomes a nova; can happen numerous times
- Other star transfers mass at a much faster rate causing the dwarf to exceed the Chandrasekhar limit. Dwarf shrinks further and nuclear reactions occur until a carbon flash occurs casuing the enture dwarf to detonate into a Type Ia supernova
What are pulsars
beams of radiation from the magnetic poles of a neutron star that give rise to pulses of emission as the star rotates; as each beam sweeps over Earth, we see a short pulse of radiation
How did we figure out that pulsars are neutron stars
- Rapid pulse rate meaning it must be a very small object (even white dwarfs were too big and spinning as fast as these would cause it to fly apart)
- Pulsar was found in the Crab Nebula showing it must be a neutron star
What are starquakes
Occur when rapidly spinning obects bulge at the equator. When the object slows, the bulge become smaller but the crust is so tight that this change puts a strain on the crust and it snaps. Shift in size of the crust is tiny but causes the spinning object’s period to decrease momentarily resulting the object’s period to resume but from a lower point
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 nucleus, neutrinos, gamma rays, and positrons
How long does it take initally large and intially low mass stars to reach their end of life and what do they become
Takes intially 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 star have different spectal 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 main sequence for less time compared to smaller stars?
bigger stars have a higher temperature thus, higher rate of fusion meaning they use of the H in their cores faster
What are the ways we can determine the masses of two stars in a visual 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.
Periodic dips in light curve when one star passes in front of the other
What is the occurence of the following stars in our galaxy: Main sequence, red giant, white dwarf
Main sequence 90%
Red Giant 1%
White Dwarf 9%
What are two ways exoplanets have been identified by astronomers
- Periodic dips in light curve when planet is in front
- Periodic doppler shifts in star’s spectrum
What is a white dwarf
Star with no nuclear reactions going on in the core, which is primarily made of C and O
What is protostar
Star of uniform composition from center to surface that contains H but has no other rxns going on in core
What is a main sequence star
Star that is fusing H to He in core
What is happening in a red giant prior to He flash
No nuclear rxns going on in the core but is fusing H to He in a shell
What is happening in a red giant after He flash
Fusing He to C in the core and H to He in a shell
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
