Term
|
Definition
| great clouds of gas & dust p 487 |
|
|
Term
| with what chemical composition do most stars in our Milky Way begin their lives? |
|
Definition
| same as Sun; about 70& hydrogen, about 28% Helium, about 2% heavier elements p 487 |
|
|
Term
| on what do stars' apparent brightness depend? |
|
Definition
| distance, how much light it actually emits |
|
|
Term
| what is apparent brightness? |
|
Definition
| brightness of a star as it appears to our eyes p 487 |
|
|
Term
| how is apparent brightness defined? |
|
Definition
| amount of power (energy/ second) reaching us per unit area p 487 |
|
|
Term
|
Definition
| total amount of power that a star emits into space p 488 |
|
|
Term
| what is the difference between luminosity and apparent brightness? |
|
Definition
| Luminosity is a measure of power; apparent brightness is a measure of power per unit area p 487 fig 15.1 e.g. 100-watt bulb puts out same amount of light, but will dimmer the farther you are away from it p 488 |
|
|
Term
| what is the formula for the inverse square of law for light? |
|
Definition
| apparent brightness=luminosity/4π × distance^2 (p 488) |
|
|
Term
| what does the formula for the inverse square of law for light mean? |
|
Definition
| If we viewed the Sun from twice Earth's distance, it would appeared dimmer by a factor of 2^2=4; from 10 times the distance it would be dimmer by 10^2=100 (p 488) |
|
|
Term
| how does apparent brightness follow the inverse square of law for light? |
|
Definition
| Imagine layers of spheres around a star. The same amount of light must pass through each layer. Layer 1 is 1 AU and the light passes through 1 square. Layer 2 is 2 AU and the same amount of light passes thru 4 squares the size of the 1st square; Therefore each square receives only 1/4 of the light as sq 1. 3 AUs = 9 squares; each square receives 1/9 (p 488 Fig 15.2) |
|
|
Term
| Using the inverse square law of light, what do we need to calculate a star's distance if we know its apparent brightness? |
|
Definition
|
|
Term
| Using the inverse square law of light, what do we need to calculate a star's luminosityif we know its apparent brightness? |
|
Definition
|
|
Term
| what are the units of apparent brightness? |
|
Definition
|
|
Term
| in words, what is the formula for apparent brightness? |
|
Definition
| Apparent brightness is the star's luminosity divided by the surface area of the imaginary sphere p 488 |
|
|
Term
| in words, what is the formula for a star's luminosity? |
|
Definition
| star's luminosity is its apparent brightness multiplied by the surface area of its imaginary sphere (reworded from p 488) |
|
|
Term
| what is stellar parallax? |
|
Definition
| small annual shifts in a star's apparent position caused by Earth's motion around the Sun p 489 |
|
|
Term
| how do astronomer's measure stellar parallax? |
|
Definition
| comparing observations of a nearby star made 6 months apart; the nearby star appears to shift against the background of more distant stars because it is observed from two opposite points of Earth's orbit (6 months apart) p 489, also see Fig 15.3 |
|
|
Term
| The parallax angle of a star is equal to what? |
|
Definition
| half the star's annual back and forth shift, measured in arcseconds p 489 |
|
|
Term
| what is a parsec and where does its name originate? |
|
Definition
| 1 parsec is the distance to an object with a parallax angle of 1 arc second' parsec is a combination of the words parallax and arc second p 489 |
|
|
Term
| what is the formula to calculate distance in parsecs? |
|
Definition
| d (in parsecs) - 1 / p (in arcseconds) p 489 |
|
|
Term
| what is the distance to a star (in parsecs) for a star with a parallax angle of 1/2 arc seconds? |
|
Definition
|
|
Term
| Why couldn't ancient Greeks measure parallax? |
|
Definition
| Because even the nearest stars have parallax angles smaller than 1 arc second; the naked, human eye can only resolve ~ 1 arc minute p 489 |
|
|
Term
| What is the unit for 1,000 parsecs? |
|
Definition
|
|
Term
| What is the unit for 1,000,000 parsecs? |
|
Definition
|
|
Term
| 1 parsec = how many light years? |
|
Definition
| 3.26 p 489 (Xiaosheng sometimes says 3.3) |
|
|
Term
| how can we calculate distance in light years using parsecs? |
|
Definition
| d (in light years) = 3.26 X (1/ p in arc seconds) |
|
|
Term
| If we know the distance from parallax, what else can we calculate? |
|
Definition
| luminosity using inverse square law of light p 489 |
|
|
Term
| The dimmest stars have luminosities of... |
|
Definition
| 10^-4 or 1/10,000 times the Sun p 490 |
|
|
Term
| The brightest stars have luminosities of... |
|
Definition
| 10^6 or 1 million times the Sun p 490 |
|
|
Term
| Stars have a wide range of luminosities. Where in this range is our Sun? |
|
Definition
| somewhere in the middle p 490 |
|
|
Term
| Which are more common? Dim stars or bright ones and how does our Sun compare? |
|
Definition
| Dim stars are more common. Even though the Sun has a mid-range luminosity, it is brighter than the vast majority of stars in our galaxy 490 |
|
|
Term
| why are apparent magnitudes called as such? |
|
Definition
| they compare how bright the different stars appear in the sky; originated with Greeks who only had naked eyes to measure brightness p 490 |
|
|
Term
| how are apparent magnitudes related to apparent brightness? |
|
Definition
| directly related, but the scale runs backwards; larger apparent magnitude = dimmer apparent brightness, e.g. star of mag 4 is dimmer than star of mag 1 p 490 |
|
|
Term
| each difference of 5 magnitudes = how many in brightness? |
|
Definition
|
|
Term
| a magnitude 1 star is how much brighter than a magnitude 6 star? |
|
Definition
|
|
Term
| a magnitude 3 star is how much brighter than a magnitude 8 star? |
|
Definition
|
|
Term
| Because of the relationship between apparent brightness and apparent magnitudes, what can happen to the measure of a star's apparent magnitude? |
|
Definition
| it can become fractionated; a few stars have apparent brightness <1. Eg, Sirius is our brightest star, but has apparent magnitude of —1.46 p 491 |
|
|
Term
| absolute magnitude is a modern way of describing a star's what? |
|
Definition
|
|
Term
| star's absolute magnitude = apparent magnitude it would have at what distance form Earth? |
|
Definition
| 10 parsecs or 32.6 light years from Earth p 491 |
|
|
Term
| The Sun's absolute magnitude is about 4.8, meaning its apparent magnitude would be 4.8 if it were 10 parsecs away form us. How would the Sun look? |
|
Definition
| Bright enough to be visible, but not conspicuous on a dark night p 491 |
|
|
Term
| How do we measure a star's temperature? |
|
Definition
|
|
Term
| which is cooler? A red star or a yellow star? |
|
Definition
|
|
Term
| why do stars come in different colors? |
|
Definition
| because they emit thermal radiation p 491 |
|
|
Term
| why does the Sun look yeller or white? |
|
Definition
| because its surface temp (5,800 K) causes it to emit most strongly in the middle of the visible portion of the spectrum p 491-2 |
|
|
Term
| how do astronomer's measure surface temperature? |
|
Definition
| by color and by spectral lines? p 492 |
|
|
Term
| how do astronomer's use color to determine a star's temperature? |
|
Definition
| by comparing a star's brightness in 2 different colors of light. Eg, if a star has more blue light than red light, the difference can be calculated using shape of themal read ion spectra p 491-2 see also Figure 5.19 |
|
|
Term
| why is using stellar spectra more accurate then using color to measure a star's temp? |
|
Definition
| because interstellar gas can affect the apparent colors of stars p 492 |
|
|
Term
| stars displaying spectral lines of highly ionized elements mean sweat? |
|
Definition
| a fairly hot star because because it takes a high temp to ionize atoms p 492 |
|
|
Term
| what are the letters representing the spectral type in order of hottest to coolest? |
|
Definition
|
|
Term
| In a spectral types subcategory, the large the number, the color or hotter the star? |
|
Definition
| The larger the number, the cooler, e.g. G@ is hotter than G3 but cooler than G1 p 493 |
|
|
Term
| The hottest stars, spectral type O, have temperatures exceeding how many degrees Kelvin? |
|
Definition
|
|
Term
| The coolest stars, spectral type M, have temperatures as low as how many degrees Kelvin? |
|
Definition
|
|
Term
| Why do spectral types follow the letters OBAFGKM? |
|
Definition
| 1st, astronomer's classified stellar spectra according to strength of their H lines, with O having weakest H lines. As they classified more stars, they found out that spectra fell in a natural order, but not alphabetical. O stars have weak H lines because nearly all H is ionized at their surface temp. Without electron jump btw levels (quantum), ionized H can neither emit nor absorb its usu wavelength p 493-4 |
|
|
Term
| Because of the required measurements for stellar mass, we can only measure the masses of what kind of stars? |
|
Definition
| binary star systems p 494 |
|
|
Term
| what are binary star systems? |
|
Definition
| systems in which 2 stars continually orbit each other p 494 |
|
|
Term
| about how many stars are part of binary star systems? |
|
Definition
|
|
Term
| what are the 3 classes of binary stars? |
|
Definition
| visual binary, spectroscopic binary, and eclipsing binary p 495 |
|
|
Term
|
Definition
| pair of stars we can see distinctly (with a telescope) as the stars orbit each other p 495 |
|
|
Term
| If a star's companion is too dim to be seen, how can we know it is a binary star? |
|
Definition
| we observe the star slowly shifting in the sky p 495 |
|
|
Term
| what is a spectroscopic binary? |
|
Definition
| identified through observations of Dopler shifts in its spectral lines p 495 see also section 5.4 |
|
|
Term
| If one star is orbiting another, it periodically moves toward us and away form us in its orbit. What will its spectral lines show? |
|
Definition
| Alternating blue shifts and redshifts p 495 |
|
|
Term
| what is an eclipsing binary? |
|
Definition
| pair of stars that orbit in the plane of our line of sight p 495 |
|
|
Term
| when neither star is eclipsed, what do we see? |
|
Definition
| The combined light of both stars p 495 Fig 15.8 |
|
|
Term
| when one star eclipses the other, what happens to their combined light? |
|
Definition
| the star system's apparent brightness decreases because some of the light is blocked from our view p 495 Fig 15.8 |
|
|
Term
| The most dependable method of "weighing" a star's mass relies on which law, and what 2 things do we need to measure? |
|
Definition
| Kepler's 3rd law; measure both the orbital period, and the avg. orbital distance of the orbiting object p 494 |
|
|
Term
| how do we measure orbital period in a visual binary? |
|
Definition
| we simply observe how long each orbit take sp 495 |
|
|
Term
| how do we measure orbital period in an eclipsing binary? |
|
Definition
| we measure the time between eclipses p 495 |
|
|
Term
| how do we measure orbital period in a spectroscopic binary? |
|
Definition
| we measure the time it takes to shift back and forth p 495 |
|
|
Term
| why is determining average separation of binary stars usually much more difficult? |
|
Definition
| we can calculate the separation only if we know the actual orbital speeds, but Doppler shift only tells us the direction of the velocity (away form us or toward us) p 496 |
|
|
Term
| why are eclipsing binary stars easier to determine speed? |
|
Definition
| because they are in our line of sight, so their Doppler shifts tell us their true orbital velocities p 496 |
|
|
Term
| how can we determine stellar radii from eclipsing binary stars? |
|
Definition
| because we know how fast they are moving across our line of sight as one eclipses the other, we can determine their radii by timing how long each eclipse last p 496 see also section 13.2 |
|
|
Term
| how can we infer stellar mass from visual binary stars? (lecture) |
|
Definition
| measure period and measure orbit radius (lecture April) |
|
|
Term
| how can we infer stellar mass from eclipsing binary stars? (lecture) |
|
Definition
| measure period (lecture April) |
|
|
Term
| how can we infer stellar mass from spectroscopic binary stars? (lecture) |
|
Definition
| can measure period and velocity==> can infer stellar mass (lecture April) |
|
|
Term
| what is the overall range of stellar masses? |
|
Definition
| 0.08 Solar Mass to 150 Solar Mass |
|
|
Term
| what does the horizontal axis on an H-R diagram represent? |
|
Definition
| stellar surface temperature p 497 |
|
|
Term
| what does the vertical axis on an H-R diagram represent? |
|
Definition
| stellar luminosity in solar units p 497 |
|
|
Term
| stars on the upper left of an H-R diagram are what? |
|
Definition
|
|
Term
| stars on the upper right of an H-R diagram are what? |
|
Definition
| cool and luminous p 499-500 |
|
|
Term
| stars on the lower right of an H-R diagram are what? |
|
Definition
|
|
Term
| stars on the lower left of an H-R diagram are what? |
|
Definition
|
|
Term
| How can 2 stars have the same surface temp, but one is more luminous? |
|
Definition
| The more luminous one is larger in size p 500 |
|
|
Term
| how does an H-R diagram provide direct info about stellar radii? |
|
Definition
| because a star's luminosity depends on both surface temp and size p 500 |
|
|
Term
| Does stellar radii increase or decrease on the H-R diagram as e go from lower left to higher right? |
|
Definition
|
|
Term
| what types of stars are on the lower left of the H-R diagram, and what are their characteristics? |
|
Definition
| white dwarfs; small in radius and appear white in color because of their temperatures p 500 |
|
|
Term
| what determines a luminosity class, e.g. I, II, III, etc., ? |
|
Definition
| its size, because of the region where it falls on the H-R diagram p 500 |
|
|
Term
| what are the sizes in the 5 stellar luminosity classes? |
|
Definition
| I Supergiants II Bright GIants III Giants IV Subgiants V Main-sequence p 500 |
|
|
Term
| which type of stars fall outside of the luminosity classes? |
|
Definition
| white dwarfs; are classified as "wd" p 500 |
|
|
Term
| what does it mean to say our Sun is G2 V? |
|
Definition
| G2 spectral type means it is yellow-white; V means it's a hydrogen-fusing, main-sequence star p 500 |
|
|
Term
| why do high-luminosity main-sequence stars have hot surfaces and low-luminosity main-sequence stars have cooler temps on the H-R diagram? |
|
Definition
| the stars position is closely related to its mass. All main-sequence stars are fusing H into He in cores, mass determines both surface temp and luminosity because it is the key factor in a star's rate of fusion p 500 |
|
|
Term
| why do more stars fall on the lower end of the main sequence of an H-R diagram? |
|
Definition
| because low-mass stars re more common than high-mass stars p 501 |
|
|
Term
| why is mass the most important attribute of a hydrogen-fusing star? |
|
Definition
| mass determines the balancing point at which energy released by hydrogen fusion in the core equals the energy lost from the star's surface p 501 |
|
|
Term
| why would the most massive main-sequence stars be thousands of times more luminous than our Sun, but only be about 10 times the Sun's radius? |
|
Definition
| Their surface temps must be significantly hotter p 501 |
|
|
Term
| what can we infer about any hydrogen-fusing, main-sequence star that has the same spectral type as the Sun (G2)? |
|
Definition
| it must have the same mass & luminosity as the Sun p 501 |
|
|
Term
| Do all stars follow this simple relationship between mass, temperature, and luminosity? |
|
Definition
| no, only main-sequence; not giants, supergiants, or white dwarfs p 501 |
|
|
Term
| what determines a main-sequence lifetime? |
|
Definition
| a star is born with a limited supply of core hydrogen and therefore can remain as a hydrogen-fusing main-sequence star for a limited time p 501 |
|
|
Term
| do more massive stars have shorter or longer lives than less massive? |
|
Definition
|
|
Term
| why do more massive stars have shorter lives than less massive? |
|
Definition
| its lifetime is determined by both its mass and its luminosity p 501 |
|
|
Term
| how does mass determine a star's lifetime? |
|
Definition
| mass determines how much fuel the star initially contains in its core p 501 |
|
|
Term
| how does luminosity determine a star's lifetime? |
|
Definition
| luminosity determines how rapidly the star uses up its fuel p 501 |
|
|
Term
| why do massive stars start with a larger supply of hydrogen but live shorter lives? |
|
Definition
| because they fuse hydrogen into helium so rapidly p 501 |
|
|
Term
| how long would the lifetime be of a 10-solar-mass star with a luminosity of 10,000 solar? |
|
Definition
| 10 times as much H as Sun being burned at 10,00 xs the rate of the Sun is expressed as 10/10,000 => 1/1,000 as long as the Sun's lifetime. Since the Sun is 10 b yrs old, expressed as 10 b / 1,000 = 10 million years (p 501) |
|
|
Term
| what are 2 reasons that are massive stars so rare? |
|
Definition
| 1) because their lifetimes are so short, and 2) because fewer massive stars are born to begin with p 501 |
|
|
Term
| the fact that massive stars even exist tells us what? |
|
Definition
| that stars must form continuously in our galaxy p 501 |
|
|
Term
| the massive, bright O stars in our galaxy formed only recently and will die before they have chance to do what? |
|
Definition
| complete one orbit around the center of the galaxy p 501 |
|
|
Term
| how long will 0.3 solar mass star with a 0.01 solar luminosity live? |
|
Definition
| 0.3/0.01 = 30 times the Sun; at 10 billion years (life of sun) = 300 billion years |
|
|
Term
| relative to the 14-billion-year-old universe, what does this mean? |
|
Definition
| even the most ancient of the small dim stars still survive and will live for 100s of billions of more years p 501 |
|
|
Term
| how are giants, supergiants, and white dwarfs different than main-sequence stars? |
|
Definition
| they have exhausted all the H in their cores, and can no longer generate energy in the same way as our Sun p 502 |
|
|
Term
|
Definition
| a giant star with mass similar to the sun that ran out of fuel and ejected its outer layers leaving behind a "dead" core p 502 |
|
|
Term
| why are white dwarfs still hot? |
|
Definition
| because they are essentially exposed stellar cores p 502 |
|
|
Term
| what are white dwarfs dim? |
|
Definition
| because they lack an energy source and only radiate their leftover heat p 502 |
|
|
Term
| how big is a typical white dwarf? |
|
Definition
| no large in size than Earth p 502 |
|
|
Term
| how massive is a typical white dwarf? |
|
Definition
| has a mass similar to our Sun p 502 |
|
|
Term
|
Definition
| any star that varies significantly in brightness with time p 502 |
|
|
Term
| why does a variable star vary significantly in brightness with time? |
|
Definition
| pulsating variable star. certain types of variable stars can't achieve the balance between power welling up form the core and the power being radiated from the surface. Sometimes, the star's upper layers are too opaque to allow much energy to escape. Pressure under the photosphere builds up and the star expands. This puffs up the outer layers until transparent enough to let energy escape. Underling pressure then drops, star contracts until energy-trapping resumes p 502 |
|
|
Term
| what is the range between pulses for a pulsating variable star? |
|
Definition
| several hours to several years p 503 |
|
|
Term
| what is the name of the region on the H-r diagram where most pulsating stars lie, and where is it? |
|
Definition
| the instability strip, btw main-sequence, and red giants p 503 |
|
|
Term
| what is the category of very luminous pulsating variable stars that lies on the upper portion of the instability strip? |
|
Definition
| Cepheid variable stars aka Cepheid stars aka Cepheids p 503 |
|
|
Term
| what re 2 signficant characteristics of Cepheids, and how have they helped us? |
|
Definition
| they are very bright and the their pulsating periods are closely related to their luminosities. Helped us establish distances to galaxies beyond the Milky Way p 503 |
|
|
Term
|
Definition
| many stars still congregate in the groups that they formed in inside the same interstellar cloud p 504 |
|
|
Term
| what are 2 reasons that a star cluster is useful to astronomers? |
|
Definition
| 1) all the stars lie in a cluster about the same distance from Earth, and 2) all the stars in a cluster formed at about the same time (within a few million yrs of each other) Therefore, astronomers can use clusters as a lab to compare stars, which enable us to use them as cosmic clocks p 504 |
|
|
Term
| what are 2 types of star clusters? |
|
Definition
| 1) open clusters, and 2) globular clusters p 504 |
|
|
Term
| in what 3 ways are open clusters and globular clusters different? |
|
Definition
| how densely they are packed with stars, their location, their ages |
|
|
Term
| where do open clusters reside relative to the galaxy and what is their typical age? |
|
Definition
| in the galactic disk; they are relatively young. p 504 |
|
|
Term
| typically how many stars do open clusters contain? |
|
Definition
| up to several thousand p 504 |
|
|
Term
| typically how wide across are star clusters? |
|
Definition
|
|
Term
| where do globular clusters reside relative to the galaxy and what is their typical age? |
|
Definition
| in the halo; among oldest in universe p 504 |
|
|
Term
| typically how many stars do globular clusters contain? |
|
Definition
| > a million concentrated in a ball; its central region can have 10,000 stars packed in a space that is just a few LY across p 504 |
|
|
Term
| typically how wide across are globular star clusters? |
|
Definition
|
|
Term
| how do we evaluate the age of a star cluster? |
|
Definition
| identify main-sequence turnoff points p 505 |
|
|
Term
| what is a main-sequence turnoff point? |
|
Definition
| precise point on H-R diagram at which a cluster's stars diverge from the main-sequence p 505 |
|
|
Term
| what is typical age of a globular cluster, as determined by main-sequence turnoff points? |
|
Definition
| > 10 billion years; more precise calculations put them at ~13 billion yrs, making them some of oldest known objects in galaxy p 506 |
|
|
Term
| what is typical age of an open cluster, as determined by main-sequence turnoff points? |
|
Definition
| very few are older than 5 billion years p 505 |
|
|
