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121 Cards in this Set
- Front
- Back
hotter stars are ___, while cooler stars are ___. |
blue/white; red/yellow |
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astronomers can determine a star's surface temperature by: |
measuring the apparent brightness at different wavelengths, plotting a graph, and comparing it to theoretical blackbody curves to see which one matches the best |
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on a Balmer thermometer, what do the dark lines mean? |
they show that there are chemical elements present in the star's atmosphere (called absorption lines) |
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On a Balmer thermometer, different spectra mean different elements, which therefore means different ___. |
temperatures |
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On a Balmer thermometer, when do spectral lines for hydrogen become visible? |
they occur when the electron starts in n = 2 and moves up to higher levels by absorbing photons |
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Balmer lines of hydrogen are very ___. |
weak; thin, faint, hard to see. |
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Stars of intermediate temperatures have ___ Balmer lines |
thick, bold, easy to see; strong |
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Why are some lines stronger or easier to see than others on the Balmer thermometer? |
due to electronsa nd collisions between atoms, lines from other elements are strong or weak depending on the surface temp of a star. Cool stars = weak lines; hot stars = also weak lines; medium temp stars = STRONG lines b/c temperature just right (note: each chemical element has a different 'just right' middle point) |
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Why do cool stars have weak lines on a Balmer thermometer? |
collisions between atoms are weak, electrons are in n = 1, and not enough collisional energy is passed to electrons to get them into n = 2, so the n = 2 level isn't populated by electrons. |
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Why do hot stars have weak lines on a Balmer thermometer? |
the collisions between atoms are so strong that electrons are kicked right out, so few remain to get into n = 2 from n = 1 |
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Why do medium temperature stars create strong lines on a Balmer thermometer? How can different chemicals alter the admission lines? |
they are just the right temperature to get lots of electrons into n = 2 from n = 1, which create strong lines. Each chemical element has a different 'just right' middle point, so knowing what chemical element one is looking at is important to know how hot it is |
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spectral types or spectral classes are a ____ way to estimate surface temperatures of stars |
more accurate |
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In regards to spectral classes, the scales goes from __(hottest) to __(coolest) |
O - T |
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What are the different types of spectral classes? |
O B A F G K M L T (hottest to coolest) |
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What are the recently discovered 'cool objects' in the universe? (1995) Why aren't they classified as regular stars? |
brown dwarfs; they do not fuze hydrogen to helium in their cores, although some are hot enough to start the proton-proton chain, but only get to making 3He. |
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4 characteristics of brown dwarfs |
- do not fuze hydrogen to helium in their cores - they are objects between stars and jupiter-like planets - cool (1300 - 2000 K) - faint, small, hard to detect |
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How can we measure a star's colour? |
viewing it through transmission filters, which let light of a certain wavelength through, but blocks all others (e.g. only allowing blue light = B filter, etc.) |
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What is the UBV system in regards to analyzing star colours? |
u = ultraviolet, b = blue, v = visible (yellow) |
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To calculate the colours of a star, you must ____. |
observe the star in 2 different filters, then combine the results (i.e. observe the star thru the B filter, say, & calculate a magnitude using only the blue photons. Then, observe thru a different filter (V, for ex.), and calculate a magnitude thru it. - Note: adopt the letter of the filter as the symbol for the apparent magnitude, instead of using m. - define colour index as (B - V) = magnitude in B minus the magnitude in V - REMEMBER: the brighter the star, the LOWER the magnitude |
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the smaller the colour index, the ___ & ___ the star |
bluer, hotter |
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Suppose a star's magnitude thru the B filter is B = 11.1, and it's magnitude thru the V filter is V = 11.5. What is the colour index of the star? |
B - V: 11.1-11.5 = -0.4 |
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If B = 12.5 and V = 11.2, what is the colour index? |
B - V: 12.5 - 11.2 = 1.3 |
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If a star appears brighter in the red filter than the blue, then it is probably ___ in colour and ___ in temperature |
reddish; cool |
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What's the problem with assessing a star's size through a telescope? |
most stars appear as points of light even thru telescopes |
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We use information about luminosity and temperature to determine the ___ of stars. What is the equation? |
size; luminosity is proportional to radius squared x temperature to the fourth |
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The purpose of an HR diagram is to ___. |
plot stars' characteristics |
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How many stars are on the main sequence? |
80% |
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What are the three main names of groups of stars that do not belong on the main sequence? |
giants, supergiants, and white dwarfs |
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one characteristic of main sequence stars |
they are fusing hydrogen to helium in their cores (like the Sun) |
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giants, supergiants, and dwarfs are all stars that are at different _____. |
stages of evolution |
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giants (2) |
- evolved stars 10 - 100x the size of our sun - brighter than our Sun |
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A medium sized star on the main sequence will evolve into a ____, while a massive star on the main sequence will evolve into a ____. |
giant; supergiant |
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dwarf |
a small star |
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our Sun is classified as a ___ star |
dwarf (surprisingly?) |
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red dwarf |
small, red, cool stars (spectral class M) |
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white dwarfs |
very small (about the size of Earth) very hot |
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If two stars are the same temperature, does that also mean they are at the same stage of evolution? |
No; two stars can be the same temp. but at different stages of evolution |
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luminosity |
the total energy output of a star per second |
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larger stars have ____ dense atmospheres and ___ spectral lines |
less dense; narrower |
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the more dense the gas of a star, the more ________, and the more pressure ____. |
collisions of atoms; broadening. |
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stars of the same temperature but of different physical size must be at _____ stage(s) of evolution |
different |
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the size of a star tells us how ___ it will be |
luminous |
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What are the 6 different luminosity classes? What do they stand for? |
Ia= bright supergiant Ib = supergiant II = bright giant III = giant IV = subgiant V = main sequence star |
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in star identification, one should use both the ___ class and the ___ class |
spectral; luminosity. Ex: GV2 means the star is on the main sequence (V) and has a sepctral type of G2 (surface temp of 5800 K) = the sun |
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what is spectroscopic parallax and how do you use it? |
- a new way to find distance - uses the HR diagram to plot spectral type and luminosity |
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how to use spectroscopic parallax |
1. get the star's spectral class (from pattern of absorption lines) and luminosity class (from width of lines) 2. measure the star's apparent magnitude (mv) 3. use spectral class and luminosity class to plot star on HR diagram, and read off Mv 4. calculate distance modulus (mv - Mv) 5. if you like, calculate the actual distance in pc. |
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what is the furthest distance we can calculate distance using spectral types? |
10,000 pc |
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the key to determining stellar masses is |
gravity |
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Are star mass and star size two different things? |
YES |
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mass |
how much 'stuff' a body contains |
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size |
how big or small something is in physical dimensions |
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binary stars |
two stars that orbit a common 'center of mass' like a balance point (i.e. think of two kids on a teeter totter) |
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What happens if two binary stars are identical in mass? What if star A is more massive than star B? |
same mass: they orbit the same distance away from the center of mass. different: the more massive star orbits closer to the center of mass, and the less massive star orbits further away from the c. of m. |
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what are the 3 different binary star types? |
visual binary, spectroscopic binary, and eclipsing binaries |
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visual binary |
can resolve both stars through a telescope |
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spectroscopic binary |
stars are so close together that through a scope, you only see one point of light |
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eclipsing binary |
probably the most useful. The stars are still close so we only see one point of light through the scope, but the angle of the orbit relative to Earth is such that periodically, the stars pass in front of each other, blocking some of the light (so we see the light dimming periodically) - plot a 'light curve:' the brightness of the whole system vs. time |
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most stars are ___ (4) |
- as hot as the Sun or cooler - as luminous as the Sun or cooler - the same size as the sun or smaller - as massive as the sun or less |
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intrinsically luminous stars are ___, but easy to see even though they may be far away |
rare |
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the brightest stars we see tend to be the most ___, not the most ____, while the nearest stars in space tend to be very ____, with only a few of them visible to the unaided eye |
luminous; nearby. faint |
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material between stars is called _____, and all of it is called the _____. |
interstellar matter; interstellar medium |
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what makes up the interstellar medium? |
2 parts: gas and dust |
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space is NOT a vacuum because of the existence of ____ |
interstellar medium |
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99% of interstellar medium is ____; the remaining 1% is ___. |
gas atoms and molecules (mostly H and a bit of He), predominantly concentrated in nebulae. Solid frozen grain/dust that may be soot-like or sand-like |
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nebula |
latin for 'cloud;' any cloud of gas and/or dust in space |
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hydrogen in ISM may be ___, ____, or ___. |
ionized (HII), neutral (HI), part of a molecule (H2) |
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HI |
"H one." Used for neutral hydrogen gas; not ionized |
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HII |
"H two." Ionized hydrogen; hydrogen that's lost its electron |
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H2 |
"H two." Two hydrogen atoms stuck together |
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soot-like dust |
lots of carbon; dark (soot, charcoal) |
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sand-like dust |
silicates (silicone & oxygen). Like sand on Earth; light |
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the typical size of dust particles in space are like the particles found in ____, not like the dust inside a house |
smoke (10s & 100s of nanometers big; comparable in size to the wavelength of blue light [400nm]) |
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since most of the gas in nebulae is ___, the HII region glows ___ |
hyddrogen; red (strong red hydrogen line in spectrum) |
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what are the 3 different kinds of nebulae? |
HII region (emission nebulae), reflection nebulae, and dark nebulae |
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HII region (emission nebulae) |
- hot gas around or near a hot star - the hot star's UV radiation ionizes the hydrogen gas around it - nuclei recapture the electrons into higher energy levels - electrons jump down those levels, and can produce lines (in spectrum) by emitting photons |
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reflection nebulae |
- thin dust scattering blue light from nearby stars, and light is scattered towards Earth - since blue wavelengths are comparable to the size of dust grains, the blue light is scattered, so the cloud appears to glow blue |
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dark nebulae |
denser clouds of dust that block the light from the stars behind it |
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the density of thin gas in nebulae is typically ____, as opposed to the air we breathe, which is ____. This means thin gas is extremely thin. |
one atom per cubic cm; 10000000000000000000(1x10 to 19) atoms per cubic cm |
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If interstellar matter is so thin, how can it still make an impact in what we see? |
a cumulative effect (e.g. a foggy day) |
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reddening |
more red light than blue light will pass through a particular ISM cloud, so cloud appears more red and skews the B-V colour index |
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extinction |
the dimming of star light as it passes through thin dust (dust scatters away the blue photons so we record fewer photons from the star than we would if the dust wasn't there) |
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cool clouds |
too cool (maybe 100 K) for there to be any high energy photons to kick out any electrons to show up in lines |
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HI clouds |
neutral hydrogen gas, no lines in visible spectrum |
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what are some properties of protons and electrons? |
mass, electric charge, spin (like a top; can spin clockwise or counterclockwise) |
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In HI clouds, what happens when both the proton and electron spin the same way? Or in opposite ways? |
- when both proton and electron spin in the same direction, the energy is a bit higher than if one is spinning the other way - if electron flips its spin so its opposite than that of the proton, the energy of the system decreases a bit, and a low energy, long-wavelength photon is emitted. In this, lambda = 21cm, which is in the radio part of the EM spectrum, so we find out they exist by detecting radiation. |
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molecular clouds |
- dense clouds of gas and dust (but still thinner than air) - giant molecular clouds are the birthplaces of stars - known molecules: hydrogen, ammonia, carbon monoxide, water, formaldehyde, acetylene, alcohols, acetic acid, etc. Over 120 molecules now known |
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giant molecular clouds spread over ____ pc across |
50 |
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in molecular clouds, the gas is ____; the cloud may develop a ____ |
cool and thin (10 - 20 K = freezing); dense core |
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triggers |
start the clumping process of molecular clouds. Various triggers put pressure on the cloud and cause it to clump up. Examples include shock waves and cloud collisions. |
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how do shock waves cause clumping in giant molecular clouds? |
causes compression and clumping of the cloud material; the clumps them collapse and form stars. E.g. supernova explosions; stellar wind from massive star nearby; spiral arms of our galaxy |
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how do cloud collisions cause clumping in giant molecular clouds? |
clouds are large and can interact w/ each other. The pressure of these interactions causes clumping |
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free-fall collapse |
when atoms can fall into the core concentration in the center as if no material is around them at all |
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Explain heating by contraction and how it creates protostars (4) |
- starting with giant molecular clouds where atoms free-fall collapse into the center of concentration - the heat generated by collisions between the atoms can escape easily at first (b/c the cloud is very thin and big) - core becomes opaque, dense, warmer than it was (still only about 100 K) - as cloud (dense core) collapses, the heat gets trapped by the material, provides an outward pressure, and the contraction slows down - central concentration of cloud now called a protostar, and it's shrouded in a cocoon of the leftover cloud still slowly collapsing in. The protostar itself is also still contracting |
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protostar |
shines with energy released by contraction. Not yet hot enough to begin fusion, so it cannot be classified as a star yet |
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why is the Orion nebula an 'excellent laboratory' for us to study on Earth? (2) |
- its relatively closeby (< 1500 LY) so features are resolved via telescope - star formation began as a wave several million years ago, so we can see several stages of star formation in one place |
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stellar wind |
stream of charged particles (e.g. protons and electrons) blowing off the 'surface' of a star |
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star formation pillar |
signpost of triggered star formation |
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birthline |
where protostars become visible |
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When do protostars begin fusion? |
- some never begin fusion - the timescale for collapse depends on the mass of the star |
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evolutionary track |
line on HR diagram showing how a star's (or protostar's) luminosity and temperature changes w/ time as it evolves
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young stellar object (YSO) [2] |
- once protostar is revealed, its name changes to YSO. Still not fusing yet. - YSOs often surrounded by protostellar disks; jets may be apparent |
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When does a YSO become a star? |
YSO continues to collapse and get hotter. When the central temperature gets high enough, H -> He fusion begins, and it is a star on the main sequence |
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disks |
formed from leftover material around protostars; material collapses down to form a disk around the protostar |
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an upper limit to stars' masses exist because of (2) |
1. mass loss due to stellar wind: massive stars are very hot and have strong winds that can blow away much of the material 2. cloud fragmentation: You would need a massive cloud to remain intact and form one massive star, but we don't see this; they usually fragment up to form multiple stellar clouds |
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hydrostatic equilibrium |
when a star is on the main sequence fusing H -> He, the pull of gravity towards the center is balanced by the outward gas pressure due to energy flow outwards and the temperature and density of the gas |
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stars at the lower end of the main sequence are ___ to study, but are very numerous. ex: ____ |
faint and difficult; brown dwarfs. |
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a star begins its life on _____, and is _____. |
the zero-age main sequence (ZAMS); stable (mostly) |
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As a star fuses H -> He in the core.. (4) |
- the # of nuclei gradually decreases, so core shrinks slightly - temperature and density of core increases, which means increased energy generation - more energy = more gas pressure outwards, which overpowers gravity in the outer layers of the star, so the outer layers expand a bit - the star therefore appears a bit cooler but more luminous |
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a star's lifetime on the main sequence depends on its ____. Regardless of mass, however, _____ of its life will be spent on the main sequence |
mass; 90% |
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what does an evolutionary track do? |
follows a single star through changes in temperature and luminosity as it ages |
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once hydrogen fuel is exhausted in a star's core, then no more ___ is generated; the core then mostly consists of ____. This is the beginning stage of a main sequence star becoming a ____. |
energy; helium; giant |
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Once the core of a star stops fusing H to He and consists predominantly of He, the core ____, which produces enough ___ to ____. This causes the shell around the core to begin ____. |
collapses; energy; heat the shell of hydrogen around the core; fusing H to He (called the turn-off point on the HR diagram) |
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After a main sequence star's outer shell begins fusing H to He (after its core stops and is predominantly He), ____ then causes the core to contract further |
gravity |
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burning in a star = ___ |
fusing |
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Once a main sequence star's outer-core shell starts fusion/burning, this new fusion energy causes the star to ____, and the outer layers to ____. This causes the star to appear _____, and is finally considered a ____. |
swell up; cool. brighter, redder; giant |
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the mass of a star on the main sequence dictates whether it will become a ___ or ___. |
giant; supergiant |
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Panli Exclusion Principle |
no two electrons in any level can have the same properties |
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degenerate matter |
a collection of free, noninteracting particles; gas is said to be 'degenerate' |
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Once a giant's temperature reaches 100 million K, the triple alpha process begins. What is this? |
- an alpha particle is a helium nucleus, so we have 3 helium nuclei fusing to form one carbon nucleus - This He to C fusion happens in the core via a series of reactions called the triple alpha process |
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As a giant ages, its core fuses __ to ___ first, then __ to ___, and then becomes a core of __ and __in its final stage |
H -> He; He -> C; C & O2 |
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nucleosynthesis |
the cosmic formation of atoms more complex than the hydrogen atom (e.g. the creation of carbon by a star's core) |