Chapter 9

luminous, faint, low

2 Your survey faces two difficulties: 1) The most ________ stars are so rare you find only a few in your survey region. In fact, there is not even one O star within 62 pc of Earth. 2) Lower-main-sequence M stars—sometimes called red dwarfs—and white dwarfs are so _____ they are hard to locate even when they are only a few parsecs from Earth. Finding every one of these stars in your survey sphere is a difficult task, but if you don't, your survey is incomplete. The star chart in the background of these two pages shows most of the constellation Canis Major; stars are represented as dots with colors assigned according to spectral class. The brightest stars in the sky tend to be the rare, highly luminous stars, which appear bright even though they are far away. Most stars are of very ____ luminosity, so nearby stars tend to be very faint red dwarfs. 2a Supergiants, giants, and O and B main-sequence stars are so rare their bars are not visible in this graph.

unaided

3 Luminous stars are rare but are also easy to see even at great distances. Most stars are very-low-luminous objects. Not a single white dwarf or red dwarf is bright enough to see with the ________ eye. See the H-R diagrams at right.

distance

9-1 STAR DISTANCES ________________: most important and difficult measurement in astronomy. Several different ways to find the distances to stars. Each way depends on a geometrical method that is like the method surveyors would use to measure the distance across a river they cannot cross. You can begin by reviewing that method and then apply it to stars.

baseline, triangle, angles, trigonometry,

9-1a The Surveyor's Triangulation Method To measure distance across a river- team of surveyors drive 2 stakes into the ground. The distance between the stakes is called the __________. They then choose a landmark on the opposite side of the river (tree), establishing a large _________ marked by the two stakes and the tree. Using instruments, surveyors sight the tree from the 2 ends of the baseline and measure the two angles on their side of the river. Knowing 2 ________ of the triangle and the length of the baseline side between them, they can find the distance across the river with _____________. If baseline= 50 meters and the angles= 66 degrees and 71 degrees, after calculating the distance from the baseline to the tree= 64 m.

parallax, venus

9-1b The Astronomer's Triangulation Method To find the distance to a star, astronomers use a long baseline the size of Earth's orbit. If you take a photograph of a star, wait 6 months, Earth will have moved halfway around its orbit. If you take another photograph of the star from different location, you will discover that the star is not in the same place. ____________ is the term that refers to the common experience of an apparent shift in the position of a foreground object due to a change in the location of the observer's viewpoint. Your thumb, held at arm's length, appears to shift position against a distant background when you look first with one eye and then with the other. In that case, the baseline is the distance between your eyes, and the parallax is the angle through which your thumb appears to move when you switch eyes. The farther away you hold your thumb, the smaller the parallax. __________ shows a parallax when observed from different locations on Earth.

apparent magnitude scale, earth, intrinsic brightness, distance

9-2 APPARENT BRIGHTNESS, INTRINSIC BRIGHTNESS, AND LUMINOSITY Your eyes tell you that some stars look brighter than others, and ____________ _____________ ____________ refers to stellar apparent brightness. The scale of apparent magnitudes only tells you how bright stars appear from _________. To know the true nature of a star you need to know its _____________ _____________, a measure of the amount of light the star produces. An intrinsically very bright star might appear faint if it is far away. Thus, to know the intrinsic brightness of a star, you must take into account its ______________ from you.

retinas, flux, square meter, wattage, square, distance, source, inverse square relation, intrinsic brightness

9-2a Brightness and Distance Looking at bright light your eyes respond to visual-wavelength energy falling on your _________, which tells you how bright the object appears. Brightness is related to the ______ of energy entering your eye. Astronomers and physicists define flux as the energy in joules (J) per second falling on 1 __________ _______. Joule is about the amount of energy released when an apple falls from a table onto the floor. A flux of 1 joule per second is also 1 watt. The ____________ of a light bulb tells you its intrinsic brightness. Compare that to apparent brightness of a light bulb, which depends on its distance from you. If you placed a screen 1 square meter near a light bulb, a certain amount of flux would fall on the screen. If you moved the screen twice as far from the bulb, the light that previously fell on the screen would be spread to cover an area 4x larger, and the screen would receive only 1/4 as much light. If you tripled the distance to the screen, it would receive only 1/9 as much light. Thus, the flux you receive from a light source is inversely proportional to the ___________ of the ____________ to the __________. This is known as the _________ _________ _________, Chapter 3 was applied to the strength of gravity. Apparent brightness of a star depends on its distance. If astronomers know the apparent brightness of a star and its distance from Earth, they can use the inverse square law to correct for distance and find the _____________ _____________ of the star

inverse square relation, brightness, 10 pc, absolute visual magnitude, V

9-2b Absolute Visual Magnitude Stars are scattered at different distances, and if you know the distance to a star, you can use the __________ __________ __________ to calculate the __________ the star would have at some standard distance. Astronomers use _______ as the standard distance and refer to the intrinsic brightness of the star as its ___________ ___________ ___________ ( M V ) , which is the apparent visual magnitude that star would have if it were 10 pc away. The subscript ___ tells you it is a visual magnitude, referring to wavelengths of light your eye can see. Other magnitude systems are based on other parts of the electromagnetic spectrum. The Sun's absolute magnitude is easy to calculate because its distance and apparent magnitude are well known. The absolute visual magnitude of the Sun is about +4.8. In other words, if the Sun were only 10 pc (33 ly) from Earth it would have an apparent magnitude of +4.8 and look no brighter to your eye than the faintest star in the handle of the Little Dipper. This path to find the distance to stars has led you to absolute magnitude, a measure of the intrinsic brightness of the stars.

absolute magnitudes, visible, temperature, luminosity

9-2c Luminosity The second goal: find out how much energy the stars emit. With the _______________ ____________ of the stars you can compare stars using our Sun as a standard. The intrinsically brightest stars have absolute magnitudes of about −8.3. If such a star were 10 pc away from Earth, it would be more than 25x as bright as Venus at its brightest. Such stars emit more than 100,000x as much light at visual wavelengths than the Sun. Absolute visual magnitude refers to _________ light, but you want to know the total output including all types of radiation. Hot stars emit a lot of UV radiation that you can't see, and cool stars emit plenty of infrared radiation. To add in the energy you can't see, astronomers make a mathematical correction that depends on the ______________ of the star. With that correction, astronomers can find the total electromagnetic energy output of a star, which they refer to as its __________________ (L).

color, spectral lines, photosphere, hotter, spectra

9-3 STAR TEMPERATURES Stellar spectral lines can be used as a star thermometer. With blackbody radiation you know that temperatures of stars can be estimated from their _____—red stars are cool and blue stars are hot. The relative strengths of various ___________ _______ give a greater accuracy in measuring star temperatures. For stars the term surface refers to the _______________, which is the limit of our vision into the star from outside, but is not an actual solid surface. Stars typically have surface temperatures of a few thousand or tens of thousands of degrees Kelvin. The centers of stars are much _______ than their surfaces but the ______ tell only about the outer layers from which the light you see departed.

surface, temperature, pattern, spectra

9-3a Temperature Spectral Types Strengths of spectral lines depend on the __________ (photosphere) ________________ of the star. From this you can predict that all stars of a given temperature should have similar spectra. Recognizing the __________ of spectral lines produced in the atmospheres of stars of different temperatures means there is no need to do a full analysis every time each type of spectrum is encountered. Time can be saved by classifying stellar spectra rather than analyzing each spectrum individually. Astronomers classify stars by the lines and bands in their ________, such as if it has weak Balmer lines and lines of ionized helium, it must be an O star

temperature, surface area, low, larger, diameters

9-4 STAR SIZES You can't see stars' sizes through a telescope; the images of the stars are much too small for you to resolve their disks and measure their diameters. There is a way to find out how big stars really are. 9-4a Luminosity, Temperature, and Radius The luminosity of a glowing object (star) is determined by its _________________ (Stefan-Boltzmann) and its __________ _______. Candle flame has a small surface area, but cannot radiate much heat because it is small; it has a ______ luminosity. However, if the candle flame were 12 ft tall, it would have a very large surface area from which to radiate, and, although it might be the same temperature as a normal candle flame, its luminosity would be higher. A star's luminosity is proportional to its surface area. A hot star may not be very luminous if it has a small surface area, but it could be highly luminous if it were _______. Even a cool star could be luminous if it had a large surface area. You can use stellar luminosities to determine the _____________ of stars if you can separate the effects of temperature and surface area.

main-sequence, giant , supergiant, less, density, broad, narrower, narrow, big, luminosity classes,

9-4b Luminosity Spectral Classification A star's spectrum contains clues as to whether it is a ______-_______________ star, a _______, or a _______________. The larger a star is, the ______ dense its atmosphere is. The widths of spectral lines are partially determined by the ____________ of the gas: If the atoms collide often in a dense gas, their energy levels become distorted, and the spectral lines are broadened. Example: in the spectrum of a main-sequence star, the hydrogen Balmer lines are _________ because the star's atmosphere is dense and the hydrogen atoms collide often. In the spectrum of a giant star, the lines are ___________ (Fig9-9), because the giant star's atmosphere is less dense and the hydrogen atoms collide less often. In the spectrum of a supergiant star, the Balmer lines are very __________. Astronomer can look closely at a star's spectrum and tell roughly how ______ it is. Size categories are called ________________ _________, because the size of the star is the dominating factor in determining luminosity. Supergiants are very luminous because they are very large.

gravity, matter, gravitational field, binary stars, far, close

9-5 STAR MASSES—BINARY STARS Your fifth goal is to find out how much matter stars contain, that is, to know their masses. Do they all have about the same mass, or is there a wide range, as there is for luminosity? __________ is the key to determining mass. _______ produces a gravitational field. Astronomers can figure out how much matter a star contains if they watch an object such as another star move through the star's ____________ _________. Finding the masses of stars involves studying _______ ________, pairs of stars that orbit each other. Many of the familiar stars in the sky are actually pairs of stars orbiting each other (Fig 9-10). Binary systems are common; more than half of all stars are members of binary star systems. But few can be analyzed completely. Many are so ___ apart that their periods are much too long for practical mapping of their orbits. Others are so ______ together they are not visible as separate stars.

orbital motion, center of mass, closer, individual

9-5a Binary Star Orbits Key to finding the mass of a binary star is understanding of _______ ______. Each star in a binary system has its own orbit around the system's ________ __ _____ the balance point of the system. If 1 star is more massive than its companion, then the massive star is _____ to the center of mass and has a smaller orbit, while the lower-mass star has a larger orbit. The ratio of the masses of the stars in the binary system (Fig 9-11) is MA/MB, which equals rB/rA, the inverse of the ratio of the radii of the orbits. Example, 1 star in a binary system has an orbit twice as large as the other star's orbit, then it must be half as massive. Getting the ratio of the masses is easy, but that doesn't tell you the _________ masses of the stars. (As stars in a binary star system revolve around each other, the line connecting them always passes through the center of mass, and the more massive star is always closer to the center of mass.)

planets, orbits, masses, large, separate

9-5b Three Kinds of Binary Systems There are 3 types of binary stars are important for determining stellar masses. Studying binary stars is also preparation for knowing how to find _______ around stars other than our Sun, because a star with a planet orbiting around it is like a binary star system with one very small component. Each type of binary star system corresponds to a different technique for finding planets. In a visual binary system, the 2 stars are separately visible in the telescope and astronomers can watch the stars orbit each other over periods of years or decades, (Fig 9-12). From that, astronomers can find the orbital period and, if the distance of the system from Earth can be found, the size of the _____. That is enough to find the _______ of the stars. Many visual binaries have such ______ orbits their orbital periods are hundreds or thousands of years, and so we have not yet seen them complete an entire orbit. Many binary stars orbit close to each other they are not visible as _______ stars. Such systems can't be analyzed as a visual binary.

masses, hot, coolest

9-6a Mass, Luminosity, and Density The H-R diagram is filled with patterns that give you clues as to how stars are born, age, and die. When you add your data, you see traces of those patterns. If you label an H-R diagram with the _____ of the stars determined by observations of binary star systems, you will discover that the main-sequence stars are ordered by mass, (Figure 9-15). The most massive main-sequence stars are the ____ stars. As look down the main sequence, you will find lower-mass stars, and the lowest- mass stars are the ________, faintest main-sequence stars.

survey, relationships, computers

9-6b Surveying the Stars Want to know what the average person thinks about a certain subject, take a survey. If you want to know what the average star is like, you can _______ the stars. Surveys reveal important relationships among the family of stars. Over the years, astronomers have added their results to the growing collection of data on star distances, luminosities, temperatures, sizes, and masses. They can now analyze those data to search for survey __________ among these and other parameters. Modern astronomers are deeply involved in extensive surveys. Remember the earlier mention of the Hipparcos satellite that surveyed the entire sky measuring the parallax of more than a million stars? Powerful ________ to control instruments and analyze data make such immense surveys possible. For example, the Sloan Digital Sky Survey mapped a quarter of the sky, measuring the position and brightness of 100 million stars and galaxies. The Two Micron All Sky Survey (2MASS) mapped the entire sky at 3 near-infrared wavelengths. A number of other sky surveys are underway. Astronomers will "mine" these mountains of data for decades to come.

cooler, fainter, smaller, brown dwarfs, metal oxides, iron hydride, methane, water vapor, Y dwarfs

Astronomers continue to discover and define new types of spectral types. The L dwarfs, found in 1998, are ____________ and __________ than M stars. They are understood to be objects _____________ than stars but larger than planets and are called ___________ __________. The spectra of M stars contain bands produced by ________ _________, such as titanium oxide (TiO), but L dwarf spectra contain bands produced by molecules such as _______ __________ (FeH). The T dwarfs are an even cooler and fainter type of brown dwarfs than L dwarfs. Their spectra show absorption by __________ (CH4) and ________ _______. In 2011, astronomers using highly sensitive infrared detectors on space telescopes and large ground-based telescopes discovered a class of objects with temperatures below 500 K that are labeled __ ___________.

distance, multiplying, luminosity,

Astronomers know the luminosity of the Sun because they can send satellites above Earth's atmosphere and measure the amount of energy arriving from the Sun, adding up radiation of every wavelength, including the types blocked by the atmosphere. The ____________ from Earth to the Sun is necessary to calculate luminosity. The luminosity of the Sun is about 3.8× 10 26 watts (joules per second). Two ways to express a star's luminosity. For example, you can say that the star Capella is 150 times more luminous than the Sun. You can also express this in real energy units by ____________ by the ____________ of the Sun. The luminosity of Capella is about 5.7× 10 28 watts. When you look at the night sky, the stars look much the same, yet your study of distances and luminosities reveals an astonishing fact. The most luminous stars emit 10 billion times more energy per second than the least luminous. Clearly, the family of stars is filled with interesting characters.

small, stellar parallax, half, shape, size

Because the stars are so distant, their parallaxes are very _________ angles (in arc seconds). The quantity that astronomers call __________ ____________(p) is defined as _____ the total shift of the star, in other words the shift seen across a baseline of 1 AU rather than 2 AU. The parallax and the angles at the ends of the baseline measurements tell the same thing—the __________ and _________ of the triangle, thus the distance to the object in question. Measuring the parallax p is difficult because it is such a small angle. The star, Alpha Centauri, has a parallax= 0.76 arc second, and more distant stars have even smaller parallaxes. To see how small these angles are, imagine a dime 2 miles away from you. That dime covers an angle of about 1 arc second. Stellar parallaxes are so small that the first successful measurement of one did not happen until 1838, more than 200 years after the invention of the telescope.

census, 62 pc,

CONCEPT ART 9A THE FAMILY OF STARS 1 What is the most common type of star? What types of stars are rare? To answer these questions, you need to make a _______ of the stars. In doing so you collect information on their spectral classes, luminosity classes, and distances. Your survey of the family of stars produces some surprising demographic results. 1a You could survey the stars by observing every star within ____ of Earth. A sphere 62 pc in radius encloses a million cubic parsecs. Such a survey would tell you how many stars of each type are found within a volume of a million cubic parsecs.

constant radius, radius, downward, fainter

Calculations can be made to draw precise lines of __________ _________ across the H-R diagram based on the luminosities and temperatures at each point (Fig 9-8). Example, locate the line labeled 1‍ R ⊙ (1 solar radius) and notice that it passes through the point representing the Sun. Any star whose point is located along this line has a _______ equal to the Sun's radius. Notice that the lines of constant radius slope ____________ to the right, because cooler stars are always _________ than hotter stars of the same size, following the Stefan-Boltzmann Law. These lines of constant radius show dramatically that the supergiants and giants are much larger than the Sun. In contrast, white dwarf stars fall near the line labeled 0.01‍ R ⊙ . They all have about the same radius—about the size of Earth! Notice the great range of sizes among stars. The largest stars are 100,000x larger than the tiny white dwarfs. If the Sun- a tennis ball, the white dwarfs- grains of coarse sand, and the largest supergiant stars- as big as football fields.

hot, cool, intensity, wavelength, detail, curves, infrared, ultraviolet, spectrum

Fig 9-5 shows color images of 13 stellar spectra ranging from the hottest at the top to the coolest at the bottom. The spectral features change gradually from _____ to _______ stars. Astronomers rarely work with spectra as color images. They display spectra as graphs of ___________ versus ______________ with dark absorption lines as dips in the graph (Fig 9-6). Such graphs show more ______ than photographs. Notice the overall _________ are similar to blackbody curves. The wavelength of maximum is in the ___________ for the coolest stars and in the ___________ for the hottest stars. Compare Fig 9-5 and 9-6 and notice how the strength of spectral lines depends on temperature, as indicated in the previous discussion regarding Figure 9-4. It is straightforward to determine a star's temperature from the details of its ____________.

sun, few, balmer, weak, many, hot, 2nd, strong

Hydrogen Balmer absorption lines are produced by hydrogen atoms with electrons initially in the 2nd energy level. The strength of these spectral lines can be used to gauge the temperature of a star because scientists know from lab experiments with gasses and radiation, and also from theoretical calculations, that: A) If the surface of a star is as cool as the _______ or cooler, there are ____ violent collisions between atoms to excite the electrons, and most atoms will have their electrons in the ground (lowest) state. These atoms can't absorb photons in the _________ series. As a result, you should expect to find ________ hydrogen Balmer absorption lines in the spectra of very cool stars. B) In the surface layers of stars hotter than about 20,000 K, there are ________ violent collisions between atoms, exciting electrons to high energy levels or knocking the electrons completely out of most atoms so they become ionized. In this situation, few hydrogen atoms will have electrons in the 2nd energy level to form Balmer absorption lines. As a result, you should also find weak hydrogen Balmer absorption lines in the spectra of very ______ stars. C) At an intermediate temperature, roughly 10,000 K, the collisions have the correct amount of energy to excite large numbers of electrons into the ____ energy level. With many atoms excited to the second level, the gas absorbs Balmer-wavelength photons well and thus produces ________ hydrogen Balmer lines.

blocks, eclipsing binary, single, decreases, brightness, time

If the plane of the orbits is nearly edge-on to Earth, then the stars can cross in front of each other as seen from Earth. When 1 star moves in front of the other, it _______ some of the light, and the star is eclipsed. Such a system is called an _________ _________ system. From Earth, the 2 stars are not visible separately and looks like a ______ point of light. But, when 1 star moves in front of the other star, part of the light is blocked, and the total brightness of the point of light _________. Fig 9-14 shows a smaller star moving in an orbit around a larger star, first eclipsing the larger star and then being eclipsed as it moves behind. The resulting variation in the brightness of the system is shown as a graph of __________ versus ____, a light curve. (A small, hot star orbits a large, cool star and you see their total light. As the hot star crosses in front of the cool star, you see a decrease in brightness. As the hot star uncovers the cool star, the brightness returns to normal. When the hit star is eclipsed behind the cool star, the brightness drops. The depths of the 2 eclipses depend on the relative surface temperatures of the stars.)

single, spectrum, spectroscopic binary system, doppler shift, blue, red, tipped, masses

If the stars in a binary system are close together, a telescopic view, limited by diffraction and atmospheric seeing, shows a _____ point of light. Only by looking at a ________, which is formed by light from both stars and contains spectral lines from both, can astronomers tell that there are 2 stars present. Such a system is a ____________ ________ _______. Familiar examples of spectroscopic binary systems are the stars Mizar and Alcor in the handle of the Big Dipper (Fig 9-10b). Fig 9-13 shows a pair of stars orbiting each other; the circular orbit appears elliptical because you see it nearly edge-on. If it were a true spectroscopic binary system, you would not see the separate stars. However, the _______ ____ tells you there were 2 stars orbiting each other. As the two stars move in their orbits, they alternately approach toward and recede from Earth and their spectral lines are Doppler shifted alternately toward _____ and then ____ wavelengths. Noticing pairs of spectral lines moving back and forth across each other would alert you that you were observing a spectroscopic binary. Spectroscopic binaries are very common, but are not as useful as visual binaries. Astronomers can find the orbital period easily, but they can't find the true size of the orbits b/c there is no way to find the angle at which the orbits are _______. Thus, can't find the true _______ of a spectroscopic binar, they can only find the lower limit to the masses. (Stars orbiting each other produce spectral lines with Doppler Shift. As the stars circle their orbits, the spectral lines move together. When the stars move perpendicular to our line of sight, there are no Doppler shifts. Spectral lines shifting apart and then merging are a sigh of spectroscopic binary. The size of the Doppler shifts gives information about the masses.)

Hipparcos, Gaia, map

In 1989, European Space Agency launched the satellite ________________ to measure stellar parallaxes from above the effects of Earth's atmosphere. This telescope observed for 4 years, and the data were used to produce two parallax catalogs in 1997. One catalog contains 120,000 stars with parallaxes 20x more accurate than ground-based measurements. The other catalog contains more than a million stars with parallaxes as accurate as ground-based parallaxes. Knowing accurate distances from the Hipparcos observations has given astronomers new insights into the nature of stars. 2013 European Space Agency launched the ______ spacecraft. Was designed to measure the parallaxes of a billion stars as faint as apparent visual magnitude +20. This will allow the first real three-dimensional _____ of our Galaxy; an catalog of results was released in 2016.

top, luminous, low, right, cool, left, hot, nuclear reactions

In an H-R diagram the location of a point representing a star tells you a lot about the star. Points near the ______ of the diagram represent very ___________ stars, and points near the bottom represent very-______-luminous stars. Points near the _________ edge of the diagram represent very _______ stars, and points near the ______ edge of the diagram represent very ______ stars. Figure 9-8 color is used to represent temperature: Cool stars= red and hot stars= blue. The main sequence is the region of the H-R diagram running from upper left to lower right. It includes roughly 80% of all stars, represented by a curved line with dots for stars plotted along it in Figure 9-8. The hot main-sequence stars are more luminous than the cool main-sequence stars. Look again at the classification diagram for cars in Figure 9-7. Vehicles not on the car main sequence have different kinds of engines than main-sequence cars. Stars not on the main sequence have different ___________ ____________ as their power sources than do main-sequence stars.

kinds, vertical, temperature, sun, luminosity, temperature, location, ages, motion

Like diagram, Fig 9-7 helps you understand the different kinds of autos, the H-R diagram can help you understand different ____ of stars. An H-R diagram, (Fig 9-8) has luminosity on the ___________ axis and ___________ on the horizontal axis. A star is represented by a point on the graph that tells you its luminosity and temperature. The symbol ⊙ refers to the _______. Thus L ⊙ refers to the ___________ of the Sun, T ⊙ refers to the temperature of the Sun, and so on. The H-R diagram in Fig 9-8 contains a scale of spectral types across the top. The spectral type of a star is determined by its _____________, so you can use either spectral type or temperature as the horizontal axis of an H-R diagram. Astronomers use H-R diagrams so often that they usually skip the words "the point that represents the star" and instead say that a star is located in a certain place in the diagram. Of course, they mean the point that represents the luminosity and temperature of the star and not the star itself. The __________ of a star in the H-R diagram has nothing to do with the location of the star in space. A star may move in the H-R diagram as it ______ and its luminosity and temperature change, but such motion in the diagram has nothing to do with the star's __________ in space.

luminosity classes, absolute magnitude, spectroscopic parallax, estimate

Most stars are too distant to have measurable parallaxes, but astronomers can find the distances to these stars if they can record the stars' spectra and determine their ________________ ____________. From spectral type and luminosity class, they can estimate the star's ____________ ___________, compare with its apparent magnitude, and compute its distance. Although this process finds distance and not true parallax, it is called ________________ ____________. Example, Betelgeuse is classified M2 Ib, and its apparent magnitude averages about +0.4. (Betelgeuse is somewhat variable.) You can plot this star in an H-R diagram where you would find that a temperature class of M2 and luminosity class of Ib (supergiant) corresponds to an absolute visual magnitude of about -6.0, which means a luminosity of about 30,000 L ⊙ . That information, combined with the star's apparent brightness, allows astronomers to estimate that Betelgeuse is about 190 pc. A combination of parallax measurements made by the Hipparcos satellite and radio telescopes yields a distance of 197 pc with an uncertainty of 24%. Spectroscopic parallax does give a good first ____________ of the distances of stars so far away that their parallax can't easily be measured.

above, larger, giant stars, red dwarfs, low, small, white dwarfs

Size is important in determining the luminosity of a star. In the H-R diagram that some cool stars lie __________ the main sequence. Although they are cool, they are luminous, and that must mean they are _______ (more surface area) than main-sequence stars of the same temperature. These are called ________ _______, and they are roughly 10 to 100x larger than the Sun. The supergiant stars at the top of the H-R diagram are as large as 1000x the Sun's diameter. In contrast, ______ _________ at the lower end of the main sequence are not only cool but also small, giving them low luminosities. At the bottom of the H-R diagram are the "economy models," stars that are very _____ in luminosity because they are very _______. The ________ _________ lie in the lower left of the H-R diagram. Although some white dwarfs are among the hottest stars known, they are so small they have very little surface area from which to radiate, thus low luminosities.

mass, massive, luminous

Stars that do not lie on the main sequence are not in order according to _____. Some giants and supergiants are massive, while others are no more massive than the Sun. All white dwarfs have about the same mass, usually in the narrow range of 0.5 to about 1.0 solar mass. Because of the systematic ordering of mass along the main sequence, these main-sequence stars obey a mass-luminosity relation—the more _______ a star is, the more ________ it is (Fig 9-16). The mass-luminosity relation can be expressed as: Luminosity is proportional to mass to the 3.5 power. Example, a star with a mass of 4.0‍M⊙ can be expected to have a luminosity of about 43.5 or 128‍L⊙. Giants, supergiants, and white dwarfs do not follow the mass-luminosity relation. In the next chapters, the main sequence mass-luminosity relation will help you understand how main sequence stars generate their energy.

surface, medium, high, low, maximum, spectral lines, predicted

Strength of the hydrogen Balmer lines depends on the temperature of the star's __________ layers. Hot and cool stars= weak Balmer lines, but _________-temperature stars= strong Balmer lines. Figure 9-4 shows the relationship between gas temperature and strength of spectral lines for hydrogen and other substances. Each type of atom or molecule produces spectral lines that are weak at _____ and ____ temperatures and strong at some intermediate temperature, although the temperature at which the lines reach _____________ strength is different for each type of atom or molecule. Theoretical calculations of the type first made by Cecilia Payne can predict how strong various ___________ __________ should be for stars of different temperatures. A star's temperature can be determined by comparing the strengths of its spectral lines with the ______________ strengths. From stellar spectra, astronomers have found that these temperatures range from about 2300 K to 50,000 K. Compare these with the surface temperature of the Sun, which is about 5800 K.

separates, diameters

The Hertzsprung-Russell (H-R) diagram is a graph that __________ the effects of temperature and surface area on stellar luminosities and enables astronomers to sort the stars according to their _______________. You can plot a diagram like Figure 9-7 to show horsepower versus weight for various makes of cars. In so doing, you will find that in general the more a car weighs, the more horsepower it has. Most cars fall somewhere along the normal sequence of cars running from heavy, high-powered cars to light, low-powered models. You might call this the main sequence of cars. Some cars, however, have much more horsepower than normal for their weight—the sport or racing models—and the economy models have less power than normal for cars of the same weight.

star classification system, dropped, merged, reordered, 7, spectral sequence, hottest, decrease, 10 subclasses, temperature

The ________ ______________ ____________ was devised at Harvard during the 1890s and 1900s. Annie J. Cannon, personally inspected and classified the spectra of more than 250,000 stars. The spectra were first classified in groups labeled A through Q, but some groups were later ________, __________ with others, or ___________. The final classification scheme includes ___ main spectral classes or types that are still used today: O, B, A, F, G, K, and M. This set of star types, called the __________ ____________, is important because it is a temperature sequence. The O stars are the _________, and the temperature continues to _____________ down to the M stars, the coolest. Astronomers divide each spectral class into ___ _______________. For example, spectral class A consists of the subclasses A0, A1, . . . A8, A9. Next come F0, F1, F2, and so on. These finer divisions define a star's ________________ to a precision of about 5%. The Sun is not just a G star, but a G2 star, with a temperature of 5800 K. Generations of astronomy students have remembered the spectral sequence by using mnemonics such as "Oh Boy, An F Grade Kills Me," or "Only Bad Astronomers Forget Generally Known Mnemonics."

parsec, distances, atmosphere, parallax,

The distances to the stars are so large its inconvenient to use km or AU Units. When you measure distance via parallax use the unit of distance called a __________ (pc). The word parsec was created by combining parallax and arc second. One parsec equals the distance to an imaginary star that has a parallax of 1 arc second. A parsec is 2.06×10^5‍AU or 3.26 ly. Parsec units are used more often than light-years by astronomers because parsecs are more directly related to the process of measurement of star _____________. The blurring caused by Earth's ____________ makes star images appear to be about 1 arc second in diameter which makes it difficult to measure __________. Even if you average together observations made from Earth's surface, you cannot measure parallax with an uncertainty smaller than about 0.002 arc second. Thus, if you measure a parallax of 0.02 arc second from an observatory on the ground, your uncertainty will be about 10%. If you consider that 10% is the maximum acceptable level of uncertainty, then ground-based astronomers can't accurately measure parallaxes that are smaller than about 0.02 arc second. That parallax corresponds to a distance of about 50 pc (165 ly).

inference, masses, orbital period, orbital speed, radii, check, cross, diameters, 150,

The light curves of eclipsing binary systems contain information about the stars, but the curves can be difficult to analyze. Fig 9-14- idealized example of such a system. Once the light curve of an eclipsing binary system has been accurately observed, astronomers can construct a chain of ________ that leads to the ______ of the 2 stars. They can find the _______ _______ easily and can get spectra showing the Doppler shifts of the two stars. They can find the ______ ______ b/c they don't have to correct for the inclination of the orbits; you know the orbits are nearly edge-on, or there would not be eclipses. From that, astronomers can find the size of the orbits and the masses of the stars. Luminosity and temperature can be used to determine the _____ of stars, but eclipsing binary systems give a way to _____ those calculations by measuring the sizes of a few stars directly. The light curve shows how long it takes for the stars to ______ in front of each other, and multiplying these time intervals by the orbital speeds gives the ________ of the stars. There are complications due to the inclination and eccentricity of orbits, but often these effects can be taken into account, so observations of an eclipsing binary system can directly tell you not only the masses of its stars but also their diameters. From studying binary stars, astronomers have found that the masses of stars range from roughly 0.1 solar mass to nearly 100 solar masses. The most massive stars found in a binary system have masses of about ____ solar masses. A few other stars may be even more massive, but they are not members of binary systems, so astronomers can only estimate their masses from models based on their luminosities, temperatures, and other characteristics.

roman numerals, after, white dwarfs, distance

The luminosity classes are represented by the ___________ __________ I through V, with supergiants further subdivided into types Ia and Ib. Example, you can distinguish between a luminous supergiant (Ia) such as Rigel and a regular supergiant (Ib) such as Polaris. The star Adhara is a luminous giant (II), Aldebaran is a giant (III), and Altair is a subgiant (IV). The Sun is a main-sequence star (V). The luminosity class notation appears ______ the spectral type, as in G2 V for the Sun. __________ _________ don't enter into this classification because their spectra are very different from the other types of stars. The approximate positions of the main sequence, giant, and supergiant luminosity classes are shown on the H-R diagram (fig 9-8). Luminosity classification is subtle and not very accurate, but it is an important technique in modern astronomy because it provides clues to _________________.

large, luminous, low, M, distance, diameters, masses

The night sky is a beautiful carpet of stars. Some are giants and supergiants, and some are dwarfs. The family of stars is rich in its diversity. Pretend space is like a very clear ocean in which you are swimming. When you look at the night sky you are seeing mostly "whales," the rare very _____ and _________ stars, mostly far away. If instead you cast a net near your location, you would mostly catch "sardines," very-____-luminous __ dwarfs that make up most of the stellar population of the Universe. In this chapter, you explored the basic properties of stars. Once you found the _________ to the stars, you were able to find their luminosities. Knowing their luminosities and temperatures gives you their _________. Studying binary stars gives you their _______. These are all rather straightforward and simple data. But you have now discovered a puzzling situation. The largest and most luminous stars are rare, and the average stars are such small, low-mass things they are hard to see even if they are near Earth in space. Why does nature make stars in this peculiar way? To answer that question, you can explore the birth, life, and death of stars- begins in Chapter 10.

density, centers, diameter, mass, small

Though mass alone does not reveal any pattern among giants, supergiants, and white dwarfs, ________ does. Once you know a star's mass and diameter, you can calculate its average density= mass/volume. Stars are not uniform in density but are most dense at their _______ and least dense near their surfaces. A star's average density is intermediate between its central and surface densities. The Sun's average density is approximately 1 gram per cubic centimeter—about the density of water. Main-sequence stars have average densities similar to the Sun's density. As learned earlier about luminosity classification, giant stars are much larger in ________ than the main-sequence stars but not much larger in ____, so giants have low average densities, ranging from 0.1 to 0.01‍g/cm³. The enormous supergiants have still lower densities, ranging from 0.001 to 0.000001‍g/cm³. These densities are thinner than the air you breathe, and if you could insulate yourself from the heat, you could fly an airplane through these stars. Only near the center would you be in any danger; astronomers calculate that the material there is very dense. The white dwarfs have masses about equal to the Sun's but are very ______—only about the size of Earth. Thus, the matter is compressed to densities of 3‍million‍g/cm³ or more. On Earth, a teaspoonful of this material would weigh about 15 tons.

orbits, orbital period, stronger, 3rd, sizes, masses, determined, elliptical, tipped,

To find the total mass of a binary star system, you must know the size of the ______ and the _______ ________—the length of time the stars take to complete one orbit. The smaller the orbits are and the shorter the orbital period is, the _______ the stars' gravity must be to hold each other in orbit. From the sizes of the orbits and the orbital period, astronomers can use Kepler's _____ law to figure out how much mass the stars contain in total. Combining that information with the ratio of the masses found from the relative sizes of the orbits reveals the individual masses of the stars. Finding the mass of a binary star system is easier said than done. One difficulty is the true _____ of the star orbits must be measured in units, like meters or AU, in order to find the _______ of the stars in units such as kilograms or solar masses. Measuring the true sizes of the orbits in turn requires knowing the distance to the binary system. Therefore, only stars whose masses astronomers know for certain are in binary systems with orbits that have been _________ and distances from Earth that have been measured. Other complications are that the orbits of the 2 stars may be ______; also, the plane of their orbits can be _______ at an angle to your line of sight, distorting the apparent shapes of the orbits. Astronomers must find ways to correct for these complications. Notice that finding the masses of binary stars requires a number of steps to get from what can be observed to what astronomers really want to know—the masses. Constructing such sequences of steps is an important part of science.

place, sample, biased, rare, red dwarfs, deceptive

What could you learn about stars from a survey of the stars near the Sun? The evidence astronomers have is that the Sun is in a typical ________ in the Universe. Therefore, such a survey could reveal general characteristics of the stars. Study Concept Art 9A, "The Family of Stars," and notice the following three important points: 1) Making a good survey is difficult because you must be sure you get a good _______. If you don't survey enough stars, or if you miss some kinds of stars, your survey might produce ________ results. 2) Luminous stars are ____. The most common types of stars are the low-luminous ___ ______. 3) A careful survey reveals that what you see in the sky is ________. Most of the bright stars in the sky are not typical stars but instead are highly luminous giants and supergiant stars that can be seen from great distances.

Y dwarfs

__ _____________- A substellar object with temperature below 500 K, having inferred properties intermediate between brown dwarfs and Jovian planets.

L dwarfs

__ _____________- Spectral classes of brown dwarf stars with lower surface temperatures and luminosities than M dwarfs.

T dwarfs

__ _____________- Spectral classes of brown dwarf stars with lower surface temperatures and luminosities than M dwarfs.

mass-luminosity relation

_____-_________ ________- The more massive a main-sequence star is, the more luminous it is.

light curve

_______ ________- A graph of brightness versus time commonly used in analyzing variable stars and eclipsing binaries.

visual binary system

________ _________ _________- A binary star system in which the two stars are separately visible in the telescope.

red dwarfs

________ ___________- A faint, cool, low-mass, main-sequence star.

main sequence

__________ _________________- The region of the H-R diagram running from upper left to lower right, which includes roughly 90 percent of all stars generating energy by nuclear fusion.

eclipsing binary system

___________ _________ _________- A binary star system in which the stars cross in front of each other as seen from Earth.

white dwarfs

___________ ___________- Dying star at the lower left of the H-R diagram that has collapsed to the size of Earth and is slowly cooling off.

giant

___________- Large, cool, highly luminous star in the upper right of the H-R diagram, typically 10 to 100 times the diameter of the Sun.

binary stars

____________ ____________- Pairs of stars that orbit around their common center of mass

brown dwarfs

_____________ _____________- A very cool, low-luminosity star whose mass is not sufficient to ignite nuclear fusion.

luminosity classes

______________ ______________- A category of stars of similar luminosity, determined by the widths of lines in their spectra.

stellar parallax (p)

______________ _______________- The small apparent shift in position of a nearby star relative to distant background objects because of Earth's orbital motion.

spectral sequence

______________ _________________- The arrangement of spectral classes (O, B, A, F, G, K, and M) ranging from hot to cool.

parsec (pc)

______________- The distance to a hypothetical star whose parallax is 1 arc second.

spectroscopic binary system

________________ ______ ________- A star system in which the stars are too close together to be visible separately. We see a single point of light, and only by taking a spectrum can we determine that there are two stars.

absolute visual magnitude ( M V )

________________ _________ _______________- Intrinsic brightness of a star. The apparent visual magnitude the star would have if it were 10 pc away.

intrinsic brightness

________________ _______________- A measure of the amount of light a star produces.

spectral classes

________________ __________________- A star's label in the temperature classification system based on the appearance of the star's spectrum.

intrinsic brightness

__________________ ____________________- A measure of the amount of light a star produces.

luminosity (L)

___________________- The total amount of energy a star radiates per second at all wavelengths.

supergiant

____________________- Exceptionally luminous star whose diameter is 100 to 1000 times that of the Sun.

spectroscopic parallax

_____________________ _______________- The method of determining a star's distance by comparing its apparent magnitude with its absolute magnitude as estimated from its spectrum.

Hertzsprung-Russell (H-R) diagram

_____________________-______________ ____________- A plot of the intrinsic brightness versus the surface temperature of stars. It separates the effects of temperature and surface area on stellar luminosity.