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    which of these answers describes the fundamental assumption that is behind all of the methods that astronomers refer to as the distance ladder?

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    Cosmic distance ladder

    Cosmic distance ladder

    From Wikipedia, the free encyclopedia

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    For various definitions of distance in cosmology, see Distance measures (cosmology).

    Light green boxes: Technique applicable to star-forming galaxies.

    Light blue boxes: Technique applicable to Population II galaxies.

    Light Purple boxes: Geometric distance technique.

    Light Red box: The planetary nebula luminosity function technique is applicable to all populations of the Virgo Supercluster.

    Solid black lines: Well calibrated ladder step.

    Dashed black lines: Uncertain calibration ladder step.

    The cosmic distance ladder (also known as the extragalactic distance scale) is the succession of methods by which astronomers determine the distances to celestial objects. A real distance measurement of an astronomical object is possible only for those objects that are "close enough" (within about a thousand parsecs) to Earth. The techniques for determining distances to more distant objects are all based on various measured correlations between methods that work at close distances and methods that work at larger distances. Several methods rely on a standard candle, which is an astronomical object that has a known luminosity.

    The ladder analogy arises because no single technique can measure distances at all ranges encountered in astronomy. Instead, one method can be used to measure nearby distances, a second can be used to measure nearby to intermediate distances, and so on. Each rung of the ladder provides information that can be used to determine the distances at the next higher rung.

    Contents

    1 Direct measurement

    1.1 Astronomical unit

    1.2 Parallax 2 Standard candles 2.1 Problems 3 Standard siren 4 Standard ruler

    5 Galactic distance indicators

    5.1 Main sequence fitting

    6 Extragalactic distance scale

    6.1 Wilson–Bappu effect

    6.2 Classical Cepheids

    6.3 Supernovae

    6.3.1 Measuring a supernova's photosphere

    6.3.2 Type Ia light curves

    6.3.3 Novae in distance determinations

    6.4 Globular cluster luminosity function

    6.5 Planetary nebula luminosity function

    6.6 Surface brightness fluctuation method

    6.7 Sigma-D relation

    7 Overlap and scaling

    8 See also 9 Footnotes 10 References 11 Bibliography 12 External links

    Direct measurement[edit]

    shows the use of parallax to measure distance. It is made from parts of the Yale–Columbia Refractor telescope (1924) damaged when the 2003 Canberra bushfires burned out the Mount Stromlo Observatory; at Questacon, Canberra.[1]

    At the base of the ladder are distance measurements, in which distances are determined directly, with no physical assumptions about the nature of the object in question. The precise measurement of stellar positions is part of the discipline of astrometry.

    Astronomical unit[edit]

    Main article: Astronomical unit

    Direct distance measurements are based upon the astronomical unit (AU), which is defined as the mean distance between the Earth and the Sun. Kepler's laws provide precise ratios of the sizes of the orbits of objects orbiting the Sun, but provides no measurement of the overall scale of the orbit system. Radar is used to measure the distance between the orbits of the Earth and of a second body. From that measurement and the ratio of the two orbit sizes, the size of Earth's orbit is calculated. The Earth's orbit is known with an absolute precision of a few meters and a relative precision of a few parts in 100 billion (1×10−11).

    Historically, observations of transits of Venus were crucial in determining the AU; in the first half of the 20th century, observations of asteroids were also important. Presently the orbit of Earth is determined with high precision using radar measurements of distances to Venus and other nearby planets and asteroids,[2] and by tracking interplanetary spacecraft in their orbits around the Sun through the Solar System.

    Parallax[edit]

    Main article: Stellar parallax

    Further information: Parsec

    Stellar parallax motion from annual parallax. Half the apex angle is the parallax angle.

    The most important fundamental distance measurements come from trigonometric parallax. As the Earth orbits the Sun, the position of nearby stars will appear to shift slightly against the more distant background. These shifts are angles in an isosceles triangle, with 2 AU (the distance between the extreme positions of Earth's orbit around the Sun) making the base leg of the triangle and the distance to the star being the long equal length legs. The amount of shift is quite small, measuring 1 arcsecond for an object at 1 parsec's distance (3.26 light-years) of the nearest stars, and thereafter decreasing in angular amount as the distance increases. Astronomers usually express distances in units of parsecs (parallax arcseconds); light-years are used in popular media.

    Source : en.wikipedia.org

    Unit 22 Flashcards

    Start studying Unit 22. Learn vocabulary, terms, and more with flashcards, games, and other study tools.

    Unit 22

    11 studiers in the last hour

    The apparent brightness of our Sun is roughly 1000 watts per square meter. At a distance of 30 times the Earth-Sun distance, the apparent brightness of our Sun would be

    Click card to see definition 👆

    1.1 watts per square meter.

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    Cruising far from the Sun, we notice that the Sun's apparent brightness has dimmed to 0.1 watts per square meter. We know that the apparent brightness at a distance of 1au is 1000 watts per square meter. How far from the Sun are we?

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    100au

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    1/15 Created by ImBry

    Terms in this set (15)

    The apparent brightness of our Sun is roughly 1000 watts per square meter. At a distance of 30 times the Earth-Sun distance, the apparent brightness of our Sun would be

    1.1 watts per square meter.

    Cruising far from the Sun, we notice that the Sun's apparent brightness has dimmed to 0.1 watts per square meter. We know that the apparent brightness at a distance of 1au is 1000 watts per square meter. How far from the Sun are we?

    100au

    A star is observed to have an apparent brightness which is 10-6 times its absolute brightness. How far away is it?

    10,000 parsecs.

    A star at a distance of 1000pc should have an apparent brightness equal to its absolute brightness multiplied by

    10-4.

    Which of the following magnitudes corresponds to the brightest star?

    (A) +3.4. (B) +5.6. (C) +2.1. (D) -1.5. (E) +1.2

    -1.5 (smaller = brighter)

    A star whose apparent brightness is 10-6 times that of a first magnitude star would have magnitude

    16.

    Three 5 magnitude differences would give (1/100)×(1/100)×(1/100)=10^-6 times the brightness.

    A star with a distance modulus of zero is at a distance of

    10 parsecs.

    A star with an apparent magnitude of 5.7 and an absolute magnitude of -1.2 would appear in our sky as a star

    barely visible to the naked eye.

    A star with an absolute magnitude of 5.7 and an apparent magnitude of -1.2 would appear in our sky as a star

    of dazzling brightness.

    Suppose that the color and behavior of a star identify it as a type that we know has absolute magnitude 4.8. If the star's apparent magnitude is found to be 9.8, how far away is it?

    100 parsecs.

    A star is found to have absolute magnitude 4 and apparent magnitude 24. How far away is it?

    100,000 parsecs.

    Suppose that the color and behavior of a star identify it as a type that we know has absolute magnitude -3. If the star's apparent magnitude is found to be 2, how far away is it?

    100 parsecs.

    A star is found to have absolute magnitude 4 and apparent magnitude 19. How far away is it?

    10,000 parsecs.

    What astronomers refer to as a "standard candle" is defined as a light source whose

    absolute magnitude is known.

    Which of these answers describes the fundamental assumption that is behind all of the methods that astronomers refer to as the "distance ladder?"

    Distant objects are similar to nearby objects.

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    Verified questions

    ASTRONOMY

    What is the difference between mass and weight? Between speed and velocity?

    Verified answer ASTRONOMY

    Listed below are the distances, in kilometers (km), between the Sun and Earth for four months of the year. The drawing at the right shows four different locations of Earth during its orbit around the Sun. Note that for each location drawn, Earth is correctly shown with its rotational axis tilted at an angle of

    23.5^{\circ} 23.5 ∘ .

    \begin{matrix} \text{Month} & \text{Earth-Sun Distance}\\ \text{December} & \text{147.2 million km}\\ \text{June} & \text{152.0 million km}\\ \text{September} & \text{150.2 million km}\\ \text{March} & \text{149.0 million km}\\ \end{matrix}

    Month December June September March ​ Earth-Sun Distance 147.2 million km 152.0 million km 150.2 million km 149.0 million km ​

    Is the direction that Earth's axis is tilted changing as Earth orbits the Sun?

    Verified answer ASTRONOMY

    Compare the eye, photographic film, and CCDs as detectors for light. What are the advantages and disadvantages of each?

    Verified answer ASTRONOMY

    How would the strength of the force between the Moon and Earth change if the mass of the Moon were somehow made two times greater than its actual mass?

    Verified answer

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    The H–R Diagram and Cosmic Distances

    The H–R Diagram and Cosmic Distances

    LEARNING OBJECTIVES

    By the end of this section, you will be able to:

    Understand how spectral types are used to estimate stellar luminosities

    Examine how these techniques are used by astronomers today

    Variable stars are not the only way that we can estimate the luminosity of stars. Another way involves the H–R diagram, which shows that the intrinsic brightness of a star can be estimated if we know its spectral type.

    Distances from Spectral Types

    As satisfying and productive as variable stars have been for distance measurement, these stars are rare and are not found near all the objects to which we wish to measure distances. Suppose, for example, we need the distance to a star that is not varying, or to a group of stars, none of which is a variable. In this case, it turns out the H–R diagram can come to our rescue.

    If we can observe the spectrum of a star, we can estimate its distance from our understanding of the H–R diagram. As discussed in Analyzing Starlight, a detailed examination of a stellar spectrum allows astronomers to classify the star into one of the spectral types indicating surface temperature. (The types are O, B, A, F, G, K, M, L, T, and Y; each of these can be divided into numbered subgroups.) In general, however, the spectral type alone is not enough to allow us to estimate luminosity. A G2 star could be a main-sequence star with a luminosity of 1 LSun, or it could be a giant with a luminosity of 100 LSun, or even a supergiant with a still higher luminosity.

    We can learn more from a star’s spectrum, however, than just its temperature. Remember, for example, that we can detect pressure differences in stars from the details of the spectrum. This knowledge is very useful because giant stars are larger (and have lower pressures) than main-sequence stars, and supergiants are still larger than giants. If we look in detail at the spectrum of a star, we can determine whether it is a main-sequence star, a giant, or a supergiant.

    Suppose, to start with the simplest example, that the spectrum, color, and other properties of a distant G2 star match those of the Sun exactly. It is then reasonable to conclude that this distant star is likely to be a main-sequence star just like the Sun and to have the same luminosity as the Sun. But if there are subtle differences between the solar spectrum and the spectrum of the distant star, then the distant star may be a giant or even a supergiant.

    The most widely used system of star classification divides stars of a given spectral class into six categories called luminosity classes. These luminosity classes are denoted by Roman numbers as follows:

    Ia: Brightest supergiants

    Ib: Less luminous supergiants

    II: Bright giants III: Giants

    IV: Subgiants (intermediate between giants and main-sequence stars)

    V: Main-sequence stars

    The full spectral specification of a star includes its luminosity class. For example, a main-sequence star with spectral class F3 is written as F3 V. The specification for an M2 giant is M2 III. Figure 1 illustrates the approximate position of stars of various luminosity classes on the H–R diagram. The dashed portions of the lines represent regions with very few or no stars.

    Figure 1: Luminosity Classes. Stars of the same temperature (or spectral class) can fall into different luminosity classes on the Hertzsprung-Russell diagram. By studying details of the spectrum for each star, astronomers can determine which luminosity class they fall in (whether they are main-sequence stars, giant stars, or supergiant stars).

    With both its spectral and luminosity classes known, a star’s position on the H–R diagram is uniquely determined. Since the diagram plots luminosity versus temperature, this means we can now read off the star’s luminosity (once its spectrum has helped us place it on the diagram). As before, if we know how luminous the star really is and see how dim it looks, the difference allows us to calculate its distance. (For historical reasons, astronomers sometimes call this method of distance determination spectroscopic parallax, even though the method has nothing to do with parallax.)

    The H–R diagram method allows astronomers to estimate distances to nearby stars, as well as some of the most distant stars in our Galaxy, but it is anchored by measurements of parallax. The distances measured using parallax are the gold standard for distances: they rely on no assumptions, only geometry. Once astronomers take a spectrum of a nearby star for which we also know the parallax, we know the luminosity that corresponds to that spectral type. Nearby stars thus serve as benchmarks for more distant stars because we can assume that two stars with identical spectra have the same intrinsic luminosity.

    A Few Words about the Real World

    Introductory textbooks such as ours work hard to present the material in a straightforward and simplified way. In doing so, we sometimes do our students a disservice by making scientific techniques seem too clean and painless. In the real world, the techniques we have just described turn out to be messy and difficult, and often give astronomers headaches that last long into the day.

    Source : courses.lumenlearning.com

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