AY 101 test 3


Terms in this set (...)

What is a star? How do stars compare to planets?
self luminous sphere of very hot gas whose luminosity is powered by nuclear fusion; more massive than any planet
Why are sunspots dark? What do sunspots tell us about solar rotation?
sunspots mean less light, not no light. they are cooler regions. tells us that the sun rotates, but not as a solid object
What is the evidence that convection operates just below the surface of the sun?
the surface of the sun is covered in small bright spots with darker borders; they are hot bubbles of rising gas (convection) dark borders is the cooler gas sinking back inside (like a lava lamp)
How does the appearance of the Sun change with wavelength?
it emits most of its light in the visible spectrum
What kind of spectrum does the Sun have?
when sunlight is dispersed, a rainbow spectrum of colors is seen interrupted by hundreds of dark absorption lines. they originate in the atmosphere of the sun
What powers the Sun's luminosity?
nuclear reactions, specifically fusion
structural layout of the sun
outer structure is seen directly, but inner can only be inferred from computer models.
outer to inner-corona, chromosphere, photosphere
What is the sunspot cycle? What phenomena are enhanced at sunspot maximum?
cyclical variation in the number of sunspots; about 11 years.
at sunspot maximum, we see enhanced solar activity, caused by sun's magnetic field. sun is loaded with conducting material (free electrons) that can be swirled by the rotation
What does it mean to say that the Sun is in gravitational equilibrium?
it is no longer collapsing, it is maintaining a fixed size and luminosity. Outward thermal gas pressure is exactly balancing the mass of the sun against gravity
What is "outward thermal gas pressure"?
outward gas pressure has to increase with increasing depth because the weight of the overlying layers increases. Temp and density of the gas must also increase with increasing depth.
visible surface of the sun: yellowish-white color 5800K. shows magnetically disturbed sunspots
colorful atmospheric gases just above photosphere. 10,000K
prominences plages filaments. most visible in hydrogren alpha filter
hot outer atmosphere of the sun; temp is 2,000,000K and source of solar wind
most visible in x ray
streamers, plumes, loops
how to determine the surface temperature of stars
how hot the photosphere is
determined by analyzing absorption lines in stellar spectra
how to determine the chemical compositions of stars
what the star is made of
determined by analyzing absorption lines

surface temperature determines how strong absorption lines appear
how to determine the luminosities of stars
we need 1) distance to this star and 2) the apparent brightness of star
how to determine the masses of stars
use binary stars (two stars in orbit around their COM) . observe orbit of fainter component relative to brighter
how to determine the sizes of stars
multiply angular size by distance to get actual size
which properties do or don't require knowledge of distance?
surface temp
chemical composition
radius (can only be inferred)
What is the chemical composition of a typical star near the Sun?
98% hydrogen and helium; 2% heavy elements
What is spectral classification? What is the spectral sequence OBAFGKM?
using relative strengths of spectral absorption lines to determine surface temperature.
divides stars into broad letter classes ordered by DECREASING surface temp
color also correlates with surface temp (cooler stars are reddish) (hot stars are bluish)

What does a type like "K7" or "B1" mean?
b1 is hotter than K7
within their classes B1 is one of the hottest, K7 is cooler
How are distances to nearby stars determined?
trigonometric parallax-small shift of nearby star relative to background stars as earth orbits the sun
these shifts are very small; less than one arcsecond
1 arcsecond=1parsec in distance
When large numbers of stars are analyzed, what ranges do we find for their surface temperatures, luminosities, masses, and radii?
very wide range; sun is of intermediate luminosity
mass range .08Msun-150Msun
what is an H-R Diagram?
graph of luminosity versus surface temperature; brings attention to different subgroups of stars.
shows hidden information about stellar evolution and the life cycle of stars
red giants
contains large coal stars
white dwarfs
broader absorption lines (due to higher surface gravity)
very narrow absorption lines
main sequence stars
long phase of steady shining (most stars are in this phase)
line of constant radius
H-R diagram allows us to draw lines of constant radius-lines across diagram, all stars on one line have the same radius due to relationship between luminosity and size and temperature
What are luminosity classes? What do I, III, and V mean in a spectral classification?
breaks stars down into dwarfs, giants and supergiants based on spectrum and width of absorption lines.
what is spectral type and luminosity of the sun?
What is the significance of the main sequence? How does mass correlate with position on the main sequence? How does main sequence lifetime
depend on mass?
longest phase of stellar evolution
90% of ordinary stars are MS
all MS stars powered by core hydrogen fusion
more massive the MS star, the higher its luminosity and shorter its MS lifetime
What is meant by stellar evolution? What does the theory tell us?
essentially the life-track of stars
it's all the star balancing its weight against gravity, usually using outward thermal gas pressure due to random thermal motions.
why must stars evolve?
because when they exhaust their source of fuel, they must change to stay alive
How does stellar evolution account for what we see in an H-R diagram?
luminosity and surface temperature are always changing when evolving, giving us the output we see on the H-R diagram
Why is "balance" an important concept in the life cycle of a star?
it's all the star balancing its weight against gravity, usually using outward thermal gas pressure due to random thermal motions.
What is gravitational equilibrium?
outward push of pressure to combat gravity
How and where do stars form? What is a "protostar?"
gravitational collapse of interstellar cloud
protostar is continually shrinking mass that heats up as it contracts and radiates infrared radiation; core temp rises until it is hot enough for fusion
What does a star-forming region look like? How do we know that pink nebulae are stellar nurseries?
pink nebula, irregular in shape surrounds a cluster of young stars. pink due to hydrogen gas glowing by UV light (interstellar dust is mixed in)
What is the main sequence phase? What powers the star's luminosity during this phase? When a main sequence star runs out of fuel, what
when protostar hits 10-15 million K, fusion of HYDROGEN into HELIUM begins, halting collapse of star=MS. when fuel runs out, shell hydrogen fusion begins, evolution to giant phases begins
How is an H-R diagram used to study stellar evolution? What is meant by a "life-track"? What characterizes the life-tracks of low and high mass stars?
life track of SUN-LIKE stars:
main sequence -> first red giant -> yellow giant -> second red giant -> planetary nebula -> white dwarf (lame ending)

life track of HIGH-MASS stars:
same as low mass except evolve much more rapidly,
fuse not only core carbon and oxygen
instead of becoming red giants they become red supergiants
violent collapse as death
What are the "rules" of stellar evolution?
1. when star runs out of fuel, the core contracts to raise temp and turn "ash of previous phase into fuel
2. fusing ash always requires much higher temp
3. whether required temp is reached or not depends on mass
4. in order to power a star, fusion must release energy not take it away
5. high mass stars evolved faster than low mass stars in ALL phases of stellar evolution
What do we mean by a star's "core" and "envelope"? What happens to these regions after the main sequence phase ends?
core is center where all luminosity is produced
envelope is all gas around the core; does not power star
at the end of their lives they separate
What are the phases of stellar evolution of a sun-like star?
How do these phases map out onto an H-R diagram?
what powers a 1 solar mass star during (a)1st red giant phase, (b)yellow giant phase and (c)2nd red giant phase
first red giant phase-shell hydrogen fusion
yellow giant phase-core helium fusion
2nd red giant-double shell fusing phase
What is a planetary nebula? What is the nature of the dim central stars in these nebulae?
planetary nebula: former envelopes of huge stars in second red giant stage. dim central stars are former cores. colorful gases are due to UV light excitement

they are carbon-oxygen cores resulting from helium fusion
What balances a white dwarf's mass against gravity? What is degeneracy pressure?
electron degeneracy pressure
pauli exclusion principle-cause of degeneracy, two particles cannot have same quantum mechanical state of position/speed, causing some particles to have much higher speeds creating pressur
How do high mass stars evolve compared to low mass stars?
Why are there so many fusing shells after the main sequence phase?
high mass stars evolve much FASTER, can fuse not only carbon and oxygen
each exhaustion of core fuel leads to contraction and set of fusing shells leading to a core of 100% iron, which cannot power a star.
Why do supergiants occupy a roughly horizontal band across the top of the H-R diagram?
high mass stars must travel horizontally on H-R diagram to reach giant phase and go back and forth. low mass stars travel upward
What is a supernova? How does it relate to violent core collapse?
iron core means degeneracy pressure fails to balance weight of the star. leads to violent core collapse and violent ejection of the envelope leading to huge explosion known as supernova.
How does core collapse produce a neutron star?
pressure of collapse is so great that iron nuclei completely break leading to pure ball of neutrons. when the rest of the star collapses, it rebounds and a shock wave carries the envelope into space
Why is iron a death sentence for a massive star?
iron cannot power a star
How do supernovae enrich interstellar gases?
the released gases return to space enriched with heavy elements and mix with interstellar material
What is a supernova remnant?
the nebulae that return to space enriched with heavy elements
How does the correlation between rate of stellar evolution and stellar mass tell us the age of a star cluster?
the age of star cluster equals the main sequence lifetime of the stars at the "turn off" point of the main sequence
What is the "main sequence turn-off"?
stars seem to be turning off the main sequence towards giant regions. tells us age.
What are open and globular clusters?
open star cluster- a few hundred to a few thousand stars; brightest stars often bluish-tinted. upper main sequence stars; wide range of ages, mostly relatively YOUNG

globular cluster- massive spherical cluster of a million stars, all red giants; all very OLD
What do the H-R diagrams of young and old star clusters look like?
younger star clusters turn off towards upper left end of MS.
older closer to lower right end of main sequence
What clusters actually show core helium-fusing ("horizontal branch") stars in their H-R diagrams?
globular clusters. so old they have large number of core helium fusing stars creating a horizontal branch
What have H-R diagrams of open and globular star clusters told us?
there are no young globular clusters in our galaxy
What is a white dwarf? Where do they come from? How big are they?
exposed cores of low mass, sun like stars that have shed outer layers. core made of degenerate matter, balanced against gravity by EDP. composition is carbon and oxygen nuclei. small and very dim
What powers the luminosity of a white dwarf? How will
the luminosity of a white dwarf change over time?
powered by stored heat; NO FUSION. as they cool off they will get fainter until they disappear
What is the white dwarf limit? What does the white dwarf limit tell us about the fate of most stars?
as it approaches 1.4Msun its electrons will be moving at nearly the speed of light.
fate of most stars are to become white dwarfs or stay on the main sequence almost forever (because they are below this limit)
How big are neutron stars? How do they relate to supernovae?
size of city; supernova is the collapse, leading to reactions that result in a neutron star
What are pulsars? How do pulsars connect to neutron stars? What major discovery linked pulsars to supernovae?
neutron stars discovered as pulsing radio sources; first appeared in graphs of radio brightness v time
pulsars come from supernova explosions and likely to be real neutron stars
What is the lighthouse model?
when you combine cons. of ang. momentum with rapid collapse you get a very rapidly rotating small object. you also get extremely powerful magnetic field, particles beam light from the poles
What is a black hole?
total gravitational collapse of core into a point of zero radius and infinite density = singularity
light cannot escape from it
How do black holes fit into the grand scheme of stellar evolution?
essentially the death of ONLY the most massive stars
What kinds of stars can become black holes?
most massive stars
What is meant by the singularity of a black hole?
zero radius and infinite density
What is the event horizon of a black hole?
imaginary spherical surface where escape velocity = speed of light
What is meant by "spacetime"?
space and time are connected.
general relativity interprets gravity as curvature of space; time can also be affected by gravity
If the Sun suddenly shrank into a black hole tomorrow, what would happen to the Earth?
nothing, it would continue in orbit
What conditions can lead to mass transfer in a binary star system?
the stars must be close together
What are Roche lobes and what is the significance of the connecting points between them?
roche lobes-invisible region defining gravitational domain of each star; relative size depends on masses
when one star fills it's roche lobe it can transfer some mass to the other star
What is an accretion disk, and why does one form?
transferred mask does not go directly into other star, settles into thin disk around it
How do you get a white dwarf in a binary?
The binary starts out with unequal mass stars.
The higher mass star becomes a red giant first and if the stars are very close together, can dump mass on the other star.
The higher mass star will eject its envelope as a planetary nebula and then leave behind a white dwarf. In the meantime, the other star speeds up its rate of evolution since it gained a lot of mass.
What accounts for the extreme phenomena which can occur when mass is dumped on a white dwarf?
when hydrogen gas accretes onto surface, fusion occurs on the surface. leads to intense brightening (NOVA)
What are a nova and a white dwarf supernova?
SUPERNOVA- occur when mass transfer brings white dwarf over white dwarf limit. carbon fusion detonates whole white dwarf like a bomb
What kinds of phenomena occur when a neutron star is in a close binary and receives mass from its companion? What are bipolar jets?
accretion disk forms, but its so hot that x rays are emitted.
x ray bursters: objects that intensely brighten x ray for brief period'
bipolar jets: material falls onto neutron star so fast that some gas is beamed out of poles
If black holes emit no light, how do we find them? What happens when mass is dumped onto a black hole?
look for bright source of x rays (could be part of very close binary)
How can you be sure that an X-ray source involves a black hole and not a neutron star?
measure mass of dead receiving star