Astronomy Concept Check – Flashcards
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Earth isn't really enclosed in a sphere with stars attached. Why then do astronomers find it convenient to retain the fiction of the celestial sphere? What vital piece of information about stars is lost when we talk about their position "on" the sky?
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The celestial sphere provides a natural means of specifying the locations of stars on the sky. Celestial coordinates are directly related to Earth's orientation in space but are independent of Earth's rotation. Distance information is lost
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Earth is actually farthest from the Sun during northern summer. Why, then, is it hottest in North America during this season?
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Because of the tilt of Earth's rotation axis, the Sun is highest in the northern sky during Northern summer, and the days are longest
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If Earth's distance from the Sun were to double, would you still expect to see total solar eclipses? What if the distance became half its present value?
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The angular size of the Moon would remain the same, but that of the Sun would be halved, making it easier for the Moon to eclipse the Sun. We would expect to see total or partial eclipses, but no annular ones. If the distance were halved, the angular size of the Sun would double, and total eclipses would never be seen-only partial or annular ones
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Why is elementary geometry essential for measuring distances in astronomy?
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Because astronomical objects are too distant for direct measurements, we must rely on indirect means and mathematical reasoning
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Can a theory ever become a "fact," scientifically speaking?
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A theory can never become proven "fact," because it can always be invalidated, or forced to change by a single contradictory observation. However, once a theory's predictions have been repeatedly verified by experiments over many years, it is often widely regarded as "true"
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In what ways did Galileo's observations of Venus and Jupiter conflict with the prevailing view at the time?
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The discovery of the phases of Venus could not be reconciled with the geocentric model. Observations of Jupiter's moons proved that some objects in the universe did not orbit Earth
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How do the geocentric and heliocentric models of the solar system differ in their explanations of planetary retrograde motion?
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In the geocentric view, retrograde motion is a real backward motion of a planet as it moves on its epicycle. In the heliocentric view, the backward motion is only apparent, caused by Earth "overtaking" the planet in its orbit
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In what ways did Galileo and Kepler differ in their approach to science? In what ways did each advance the Copernican view of the universe?
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Galileo was an experimentalist; Kepler was a theorist. Galileo found observational evidence supporting the Copernican theory; Kepler found an empirical description of the motions of planets within the Copernican picture that greatly simplified the theoretical view of the solar system
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Why is it significant that Kepler's laws apply to Uranus and Neptune?
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Because those planets were unknown to Kepler. The fact that the laws apply to the outer planets then constitutes a prediction subsequently verified by observation
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Why don't Kepler's laws tell us the value of the astronomical unit?
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Because Kepler determined the overall geometry of the solar system by triangulation using Earth's orbit as a baseline, all distances were known only relative to the scale of Earth's orbit-the astronomical unit (AU)
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Explain, in terms of Newton's laws of motion and gravity, why planets orbit the Sun.
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In the absence of any force, a planet would move in a straight line with constant velocity (Newton's first law) and therefore tends to move along the tangent to its orbital path. The Sun's gravity causes the planet to accelerate toward the Sun (Newton's second law), bending its trajectory into the orbit we observe
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What is a wave? What four basic properties describe a wave, and what relationships, if any, exist among them?
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A wave is a way in which energy or information is transferred from place to place via a repetitive, regularly varying disturbance of some sort. A wave is characterized by its wavelength, frequency, velocity, and amplitude: the product of the first two always equals the third
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What is light? List some similarities and differences between light waves and waves on water or in air.
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Light is an electromagnetic wave produced by accelerating charged particles. All waves on the list are characterized by their wave periods, frequencies, and wavelengths and all carry energy and information from one location to another. Unlike waves in water or air, however, light waves require no physical medium through which to propagate
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In what sense are radio waves, visible light, and X-rays one and the same phenomenon?
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All are electromagnetic radiation and travel at the speed of light. In physical terms, they differ only in frequency, although their effects on our bodies differ greatly
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Describe, in terms of the radiation laws, how the appearance of an incandescent lightbulb changes as you turn a dimmer switch to increase its brightness "off" to "maximum."
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As the switch is turned and the temperature of the filament rises, the bulb's brightness increases rapidly, by Stefan's law, and its color shifts, by Wien's law, from invisible infrared to red to yellow to white
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What are absorption and emission lines? What do they tell us about the properties of the gas producing them?
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They are characteristic frequencies at which matter absorbs or emits photons of electromagnetic radiation. They are unique to each atom or molecule and thus provide a means of identifying the gas producing them
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In what ways to electron orbits in the Bohr atom differ from planetary orbits around the Sun?
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Electron orbits can occur only at certain specific energies, and there is a ground state that has the lowest possible energy. Planetary orbits can have any energy. Planets can stay in any orbit indefinitely; in an atom the electron must eventually fall to the ground state, emitting electromagnetic radiation in the process. Planets may reasonably be thought of as having specific trajectories around the Sun-there is no ambiguity as to "where" a planet is. Electrons in an atom are smeared out into an electron cloud, and we can only talk of the electron's location in probabilistic terms
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How does the structure of an atom determine the atom's emission and absorption spectra?
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Spectral lines correspond to transitions between specific orbitals within an atom. The structure of an atom determines the energies of these orbitals, and hence the possible transitions, and thus the energies (colors) of the photons involved
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How might the Doppler effect be used in determining the mass of a distant star?
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Measuring masses in astronomy usually entails measuring the orbital speed of one object-a companion star or a planet, perhaps-around another. In most cases, the Doppler effect is an astronomer's only way of making such measurements
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Why is it important for astronomers to analyze spectral lines in detail?
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Because, with few exceptions, spectral analysis is the only way we have of determining the physical conditions-composition, temperature, density, velocity, etc.-in a distant object. Without spectral analysis, astronomers would know next to nothing about the properties of stars and galaxies
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Why do the largest modern telescopes use mirrors to gather and focus light?
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Because large reflecting telescopes are easier to design, build, and maintain than refracting instruments
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Give two reasons why astronomers need to build very large telescopes.
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The need to gather as much light as possible, and the need to achieve the highest possible angular resolution
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Why is Earth's atmosphere a problem for optical astronomers? What can they do about it?
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The atmosphere absorbs some radiation arriving from space, and turbulent motion blurs the incoming rays. To reduce or overcome the effects of atmospheric absorption, instruments are place on high mountains or in space. To compensate for atmospheric turbulence, adaptive optics techniques probe the air above the observing sit and adjust the mirror surface accordingly to try to recover the undistorted image
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Cosmic radio waves are very weak, and the resolution of radio telescopes is often poor, so what can astronomers hope to learn from radio astronomy?
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Radio astronomy opens up a new window on the universe, allowing astronomers to study known objects in new ways and to discover new objects that would otherwise be completely unobservable. The faintness of the sources is addressed by combining the largest available telescopes with sensitive detectors. Resolution can be greatly improved by the use of interferometry, which combines the signals from two or more separate telescopes to create the effect of a single instrument of much larger diameter
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Describe some of the kinds of observations that astronomers make across the entire electromagnetic spectrum. Why are such observations necessary?
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The text presents multi wavelength observations of stars, star-forming regions, and galaxies. We must study astronomical objects at many wavelengths because most emit radiation across the entire spectrum, and much of what we know is learned from studies of invisible radiation
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List two scientific benefits of placing telescopes in space. What might be some drawbacks?
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Benefits: They are above the atmosphere, so they are unaffected by seeing or absorption; they can also make round-the-clock observations of the entire sky. Drawbacks: Cost/smaller size, inaccessibility, vulnerability to damage by radiation and cosmic rays
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Why do astronomers draw such a clear distinction between the inner and the outer planets?
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Because the two classes of planets differ in most every physical property-orbit, mass, radius, composition, existence of rings, number of moons
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Describe some basic similarities and differences between asteroids and the inner planets.
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Similarities: All orbit in the inner solar system, roughly in the ecliptic plane, and are solid bodies of generally "terrestrial" composition. Differences: Asteroids are much smaller than the terrestrial planets, are generally less evolved than the planets, and move on much less regular orbits
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In what sense are the comets we see very unrepresentative of comets in general?
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Most comets never come close enough to our Sun for us to see them
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Why are astronomers so interested in interplanetary matter?
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Because it is thought to be much less evolved than material now found in planets, and hence it is a better indicator of conditions in the early solar system
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Would you expect to find asteroids and comets orbiting other stars?
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Yes. If a star formed with a disk of matter around it, then even if it doesn't have planets like Earth, the basic processes of condensations and accretion would probably have occurred there, too
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Why is it not too surprising that the extrasolar planetary systems detected thus far have properties quite different from our solar system?
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Because the search techniques are most sensitive to massive planets orbiting close to their parent stars, which are exactly what have been observed
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In what ways do tidal forces differ from the familiar inverse-square force of gravity?
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A tidal force is the variation in one body's gravitational force from place to place across another. Tidal forces tend to deform a body rather than causing an overall acceleration, and they decrease rapidly with distance
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Why is the greenhouse effect important for life on Earth? What might be the consequences if the effect continues to strengthen?
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It raises Earth's average surface temperature above the freezing point of water, which was critical for the development of life on our planet. Should the greenhouse effect continue to strengthen, however, it may conceivably cause catastrophic climate changes on Earth
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Describe how scientists combine theory and observation to create models of Earth's interior. Give examples of some Earth properties that we actually observe and some that are derived purely from the models.
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Earthquakes and seismic waves are observed on Earth's surface. These data are combined with models of how waves move through Earth's interior to construct a model of our planet's composition and physical state. We measure Earth's mass, radius, and surface composition directly. However, the composition and temperature of our planet's interior are inferred from models
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How would our knowledge of Earth's interior be changed if our planet were geologically dead, like the moon?
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We would have far less detailed direct (from volcanoes) or indirect (from seismic studies following earthquakes) information on our planet's interior
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Describe the causes and some consequences of plate tectonics on Earth.
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Convection currents in the upper mantle cause portions of Earth's crust-plates-to slide around on the surface. As the plates move and interact, they are responsible for volcanism, earthquakes, the formation of mountain ranges and ocean trenches, and the creation and destruction of oceans and continents
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Describe three important ways in which the lunar maria differ from the highlands.
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The maria are younger, denser and much less heavily cratered than the highlands
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What does the existence of a planetary magnetic field tell us about the planet's interior?
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It tells us that the planet has a conducting, liquid core in which the magnetic field is continuously generated
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What single factor is most responsible for the evolutionary differences between Earth and the Moon?
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The Moon's small size, which is the main reason that the Moon has lost its initial heat, making it geologically dead today, and the insufficient gravity to retain an atmosphere
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How has the Sun's gravity influenced Mercury's rotation?
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The Sun's tidal force has caused Mercury's rotation period to become exactly 2/3 of the orbital period and the rotation axis to be oriented exactly perpendicular to the planet's orbit plane. A synchronous orbit is not possible because of Mercury's eccentric orbit around the Sun
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Describe some important differences between the atmospheres of Venus and Earth.
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Venus's atmosphere is much more massive, hotter, and denser than that of Earth, and is composed almost entirely of carbon dioxide
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How do scarps on Mercury differ from geological faults on Earth?
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Scarps are thought to have been formed when the planet's iron core cooled and contracted, causing the crust to crack. Faults on Earth are the result of tectonic activity
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Are the volcanoes on Venus mainly associated with the movement of tectonic plates, as on Earth?
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No. They are mainly shield volcanoes formed by lava upwelling through "hot spots" in the crust and are not associated with plate tectonics
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Why do astronomers think that the Martian climate was once quite different from today's?
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Observations reveal evidence from dried-up rivers, outflow channels, and what may be ancient oceans, strongly suggesting that the surface was once warm enough and the atmosphere thick enough for large bodies of liquid water to exist on the planet's surface. Today most of the remaining water is frozen, underground or at the poles
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Why do Venus and Mars lack magnetic fields?
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Venus probably has a large liquid metal core, but it rotates too slowly for a planetary dynamo to operate. Mars rotates rapidly buck lacks a liquid metal core
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If Venus and Mars had formed at Earth's distance from the Sun, what might their climates be like today?
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Both might have climates quite similar to that on Earth, as the greenhouse effect would probably not have run away on Venus, and the water in the Martian atmosphere might not have frozen and become permafrost
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How did observations of Uranus lead to the discovery of Neptune?
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Uranus's orbit was observed to deviate from a perfect ellipse, leading astronomers to try to compute the mass and location of the body responsible for those discrepancies. That body was Neptune
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Why do observations of a planet's magnetosphere allow astronomers to measure the rotation rate of the interior?
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The magnetic field is generated by the motion of electrically conducting liquid in the deep interior and therefore presumably shares the rotation of that region of the planet
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List some similarities and differences between Jupiter's belts and zones and weather systems on Earth.
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Like weather systems on Earth, the belts and zones are regions of high and low pressure and are associated with convective motion. However, unlike storms on Earth, they wrap all the way around the planet because of Jupiter's rapid rotation. In addition, the clouds are arranged in three distinct layers, and the bright colors are the result of cloud chemistry unlike anything operating in Earth's atmosphere. Jupiter's spots are somewhat similar to hurricanes on Earth, but they are far larger and longer-lived
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Why are atmospheric features much less evident on Uranus than on the other Jovian planets?
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Uranus's low temperature means that the planets clouds lie deeper in the atmosphere, making them harder to distinguish from Earth
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Why is mathematical modeling essential to understanding the interior structures of the Jovian planets?
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We have no direct observations of the interiors of these worlds. Everything we know is derived from theoretical models of their interiors, coupled with observations of their masses, radii, and surface properties
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Why do all the Jovian planets have strong magnetic fields?
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They all combine the two essential ingredients needed to operate a planetary dynamo: Rapid rotation, in all cases faster than Earth's rotation, and conducting liquid interiors: metallic hydrogen in the cases of Jupiter and Saturn and "slushy" ice in the cases of Uranus and Neptune
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What is the source of energy for all the activity observed on Jupiter's Galilean moons?
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Jupiter's gravitational field, via its tidal effect on the moons
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Why was Voyage 1 unable to photograph the surface of Titan? Why do we now have a much better understanding of the moon's surface?
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The haze layers high in the moon's dense atmosphere are opaque to visible light. The Huygens probe landed on the surface, and Cassini's radar and infrared sensors can penetrate the haze
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What surface property of the medium-sized moons tells us that the outer solar system was once a very violent place?
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They are all very heavily cratered-a result of the heavy bombardment that marked the clearing of the comets from the outer solar system
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What is the Roche limit, and what is its relevance to planetary rights?
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The Roche limit is the radius inside of which a moon will be torn apart by tidal forces. Planetary rings are found inside the Roche limit, (most) moons outside
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What do Pluto and Triton have in common?
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They have similar masses, radii, composition, and perhaps also similar origins in the Kuiper belt
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Why is Pluto no longer regarded as a major planet?
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Pluto, like Ceres, Eris, and a few other bodies in the outer solar system, orbits the Sun and is large enough for its gravity to have caused it to assume an approximately spherical shape, but it is not massive enough to have cleared its orbital path of other solar system objects. In current terminology, it is called a dwarf planet, or a plutoid
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Why must we assume that the Sun radiates equally in all directions when computing the solar luminosity from the solar constant?
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When we multiply the solar constant by the total area to obtain the solar luminosity, we are implicitly assuming that the same amount of energy reaches every square meter of the large sphere
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Describe the two distinct ways in which energy moves outward from the solar core to the photosphere.
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The energy may be carried in the form of radiation, where energy travels in the form of light, and convection, where energy is carried by physical motion of upwelling solar gas
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Describe two ways in which the spectrum of the solar corona differs from that of the photosphere.
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The coronal spectrum shows emission lines of highly ionized elements, implying high temperature
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What do observations of sunspot polarities tell us about the solar magnetic field?
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There is a strong field, with a well-defined east-west organization, just below the surface. The field direction in the Southern Hemisphere is opposite that in the north. However, the details of the fields are very complex
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Why does the fact that we see sunlight imply that the Sun's mass is slowly decreasing?
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Because the Sun shines by nuclear fusion, which converts mass into energy as hydrogen turns into helium
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Why can't astronomers use simultaneous observations from different parts of Earth's surface to determine stellar distances?
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Because stars are so far away that their parallaxes relative to any baseline on Earth are too small to measure accurately
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Two stars are observed to have the same apparent magnitude. Based on this information, what, if anything, can be said about their luminosities?
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Nothing-we need to know their distances before the luminosities (absolute magnitudes) can be determined
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Why does a star's spectrum depend on its temperature?
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Because temperature controls which excited states the star's atoms and ions are in, and hence which atomic transitions are possible
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Can we measure the radius of a star without knowing its distance?
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Yes, by using the radius-luminosity-temperature relationship, but only if we can find a method of determining the luminosity that doesn't depend on the inverse-square law (Sec. 10.2)
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Only a tiny fraction of all stars are giants. Why, then, do giants account for so many of the stars visible to the naked eye in the night sky?
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Because giants are intrinsically very luminous and can be seen on much greater distances than the more common main-sequence or white dwarfs
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Suppose astronomers discover that due to a calibration error all distances measured by geometric parallax are 10% larger than currently thought. What effect would this have on the "standard" main sequence used in spectroscopic parallax?
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All stars would be further away, but their measured spectral types and apparent brightness would presumably be unchanged, so their luminosities would be greater than previously thought. The main sequence would therefore move vertically upward in the H-R diagram. We would then use larger luminosities in the method of spectroscopic parallax, so distances inferred by that method would also increase
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What assumptions are we making when we apply spectroscopic parallax to measure the distance to a star?
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We are assuming that we can tell with some confidence whether or not an observed star lies on the main sequence, and that the radius of a main-sequence star of a given spectral type is fairly well determined. Both assumptions are reasonable, although significant uncertainties exist, especially with assumption
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How do we know about the masses of stars that aren't members of binaries?
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We don't. We assume that their masses are the same as similar stars found in binaries
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If space is a near-perfect vacuum, how can there be enough dust in it to block starlight?
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Because the scale of interstellar space is so large that even very low densities can accumulate to a large amount of obscuring matter along the line of sight to a distant star
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If emission nebulae are powered by ultraviolet light from hot stars, why do they appear red?
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Because the UV light is absorbed by hydrogen gas in the surrounding cloud, ionizing it to form the emission nebulae. The red light is H radiation, part of the visible hydrogen spectrum emitted as electrons, and protons recombine to form hydrogen atoms
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In mapping molecular clouds, why do astronomers use observations of "minority" molecules such as carbon monoxide and formaldehyde when these molecules constitute only a tiny fraction of the total in interstellar space?
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Because the cloud's main constituent, hydrogen, is very hard to observe, astronomers must use other molecules as tracers of the cloud's properties
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1) What distinguishes a collapsing cloud from a protostar, and a 2) protostar from a star?
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1) The existence of a photosphere, meaning that the inner part of the cloud becomes opaque to its own radiation, signaling the slowing of the collapse phase. 2) Nuclear fusion in the core, and equilibrium between pressure and gravity
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Do stars evolve along the main sequence?
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No. Different parts of the main sequence correspond to stars of different masses. A typical star stays at roughly the same location on the main sequence most of its lifetime
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If stars in a cluster all start to form at the same time, how can some influence the formation of others?
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Because stars form at different rates-high-mass stars reach the main sequence and start disrupting the parent cloud long before lower-mass stars have finished forming
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In what sense is the Sun stable?
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Hydrostatic equilibrium means that, if some property of the Sun changes a little, the star's structure adjusts to compensate. Small changes in the Sun's internal temperature or pressure will not lead to large changes in its radius or luminosity
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Why does a star get brighter as it runs out of fuel in its core?
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Because the non burning inner core, unsupported by fusion, begins to shrink, releasing gravitational energy, heating the overlying layers, causing them to burn more vigorously, and increasing the luminosity
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Will the Sun ever become a nova?
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No. It does not have a binary companion to provide mass after the Sun becomes a white dwarf, so our star will never become a nova or a Type I supernova; it is too low in mass to become a Type II supernova
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Why does the iron core of a high-mass star collapse?
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Because iron cannot fuse to produce energy. As a result, no further nuclear reactions are possible, and the core's equilibrium cannot be restored
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How did astronomers know, even before the mechanisms were understood, that there were at least two distinct physical processes at work in creating supernovae?
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Because the two types of supernova differ in their spectra and their light curves, making it impossible to explain them in terms of a single phenomenon
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Why are observations of star clusters so important to the theory of stellar evolution?
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Because a star cluster gives us a "snapshot" of stars of many different masses-but of the same age and initial composition-allowing us to directly test the predictions of the theory
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Why is stellar evolution important to life on Earth?
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Because it is responsible for creating and dispersing all the heavy elements out of which we are made. In addition, it may also have played an important role in triggering the collapse of the interstellar cloud from which our solar system formed
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Are all supernovae expected to lead to neutron stars?
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No. Only Type II supernovae. According to the theory, the rebounding central core of the original star becomes a neutron star
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Why don't we see pulsars at the centers of all supernova remnants?
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Because a) not all supernovae from neutron stars, b) the emission is beamed and may not be observable from Earth, and c) pulsars spin down and become too faint to observe after a few tens of millions of years
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What is the connection between X-ray sources and millisecond pulsars?
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Some X-ray sources are binaries containing accreting neutron stars, which may be in the process of being spun up to form millisecond pulsars
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What are gamma-ray bursts, and why did they pose such a challenge to theory?
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They are energetic bursts of gamma rays, roughly uniformly distributed on the sky, occurring roughly once per day. They pose a challenge because they are very distant, and hence extremely luminous, but their energy comes from a region less than a few hundred kilometers across
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What happens if you compress an object within its own Schwarzschild radius?
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Its gravity becomes so strong that even light cannot escape, and it becomes a black hole
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How do Newton's and Einstein's theories differ in their descriptions of gravity? Why were scientists initially resistant to Einstein's theories of relativity?
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In Newton's theory, gravity is a force that exists between all massive bodies. In relativity, the acceleration we call gravity is actually our perception of motion through a curved spacetime, whose curvature depends on the local density of material. Special relativity makes some quite counterintuitive predictions about the universe-for example, time dilation, length contraction, and the loss of simultaneity-which scientists initially found hard to accept. However, all of its predictions have been repeatedly verified in laboratory experiments, and many predictions of general relativity have been verified by astronomical observations
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Why would you never actually witness an inflating object crossing the event horizon of a black hole?
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Because the object would appear to take infinitely long to reach the event horizon, and its light would be initially redshifted by the time it got there
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How do astronomers "see" black holes?
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By observing their gravitational effects on other objects, and from the X-rays emitted when matter falls into them
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Why do we see the Milky Way as a band of light across the sky?
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The Milky Way is the thin plane of the Galactic disk, seen from within. When our line of sight lies in the plane of the Galaxy, we see many stars blurring into a continuous band. In other directions we see darkness
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Can variable stars be used to map out the structure of the Galactic disk?
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No, because even the brightest Cepheids are unobservable at distances of more than a kilo parsec or so through the obscuration of interstellar dust
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Why do astronomers regard the disk and halo as distinctly different components of our Galaxy?
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They differ greatly in the spatial distributions of stars, the types and ages of stars found in each, the amount of interstellar gas they contain, and in the orbital motions of their component stars
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Why are there no young halo stars?
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Because all the gas and dust in the halo fell to the Galactic disk billions of years ago, halo star formation has long since ceased
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Why can't spiral arms simply be clouds of gas and young stars orbiting the Galactic center?
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Because differential rotation would destroy the spiral structure within a couple hundred million years
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In what sense is dark matter "dark"?
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Its emission has not been observed at any electromagnetic wavelength. Its presence is inferred from its gravitational effect on stars and gas orbiting the Galactic center
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What is the most likely explanation of the energetic events observed at the Galactic center?
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Observations of rapidly moving stars, gas, and variability of the radiation emitted suggest the presence of a roughly 3 million-solar-mass black hole
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In what ways are large spirals such as the Milky Way and Andromeda not representative of galaxies as a whole?
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Most galaxies are not large spirals-the most common galaxy types are dwarf ellipticals and dwarf irregulars
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What are some of the problems in measuring accurately the distances to faraway galaxies?
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Distance-measurment techniques ultimately rely upon the existence of very bright objects whose luminosities can be inferred by other means. Such objects become increasingly hard to find and calibrate the farther we look into intergalactic space
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How does the use of Hubble's law differ from the other extragalactic distance-measurement techniques we have seen in this text?
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It doesn't use the inverse-square law. The other methods all provide a way of determining the luminosity of a distant object, which then is converted to a distance using the inverse-square law. Hubble's law gives a direct connection between redshift and distance
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The energy emission from an active galactic nucleus does not resemble a blackbody curve. Why is this important?
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Because it means that the energy source cannot simply be the summed energy of a huge number of stars-some other mechanism must be at work
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How did the determination of quasar distances change astronomers' understanding of these objects?
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When quasars were discovered, they were thought to be faint, relative nearby stars, or starlike objects, although their unusual spectra posed problems for astronomers. Once astronomers realized that their odd spectra actually meant that they had very large redshifts, it became clear that quasars were actually among the most distant-and hence most luminous-objects in the entire universe
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How does accretion onto a supermassive black hole power the energy emission from the extended radio lobe of a radio galaxy?
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The energy is generated in an accretion disk in the central nucleus of the visible galaxy, then transported by jets out of the galaxy and into the lobes, where it is eventually emitted by the synchrotron process in the form of radio waves
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What assumptions are we making when we infer the mass of a galaxy cluster from observations of the spectra of its constituent galaxies?
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First, that the galaxies are gravitationally bound to the cluster. Second, and more fundamentally, that the laws of physics as we know them in the solar system-gravity, atomic structure, the Doppler effect-all apply on very large scales, and to systems possibly containing a lot of dark matter
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Why would we expect the evolution of galaxies in voids to differ from that of galaxies in clusters?
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Galaxies in voids are must less likely to encounter other galaxies, so their evolution will be dictated more by "internal" processes-star formation and stellar evolution-the galaxies in clusters will be more affected by interactions with other galaxies
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In what ways can astronomers test the predictions of the hierarchical merger scenario?
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The scenario makes predictions about how different types of galaxies were assembled over the history of the universe, and this has consequences for the types, compositions, and ages of the stars they contain. Astronomers can gauge the accuracy of the model by comparing its predictions with the properties of spiral and elliptical galaxies at different redshifts
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How does the existence of supermassive black holes in normal galaxies fit in with theories of active galaxy evolution?
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Astronomers think that the first black holes formed in young galaxies early in the history of the universe. The black holes grew by accretion of stars and gas from the surrounding galaxies, as well as mergers with other black holes when their parent galaxies merged. Eventually, they form the supermassive black holes observed in normal galaxies today. The galaxy mergers were often responsible for directing additional galactic gas onto the central black holes. The connection with active galaxies is that, during the periods of accretion, the black holes emitted large amounts of energy, becoming the active galactic nuclei we see
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Does every galaxy have the potential for activity?
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Probably not. There are may well be galaxies that do not harbor central black holes, and in any case, only galaxies in clusters are likely to experience the encounters that trigger activity
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What do observations of distant quasars tell us about the structure of the universe closer to home?
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Light traveling from these objects to Earth is influenced by cosmic structure all along the line of sight. Light rays are deflected by the gravitational field of intervening concentrations of mass, and gas along the line of sight produces absorption lines whose redshifts tell us the distance at which each feature formed. The light received on Earth thus gives astronomers a "core sample" through the universe, from which detailed information can be extracted
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In what sense is the universe homogeneous and isotropic?
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When viewed on very large scales-more than 300 Mpc-The distribution of galaxies seems to be roughly the same everywhere and in all directions
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Why do Hubble's laws imply a Big Bang?
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Because, tracing the motion backward in time, it implies that all galaxies, and in fact everything in the entire universe, were located at a single point at the same instant in the past. The cosmological principle does not allow that point to be special, leaving a Big Bang as the only possibility
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What are the two basic possibilities for the future expansion of the universe?
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If there is not enough matter in the universe, then gravity will halt expansion and reverse it, the universe will eventually re-collapse in a "Big Crunch." Otherwise, the universe will expand forever
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How is the curvature of space related to the size and density of the universe?
answer
A low-density universe has negative curvature and is infinite in extent; a critical-density universe is spatially flat (Euclidean) and is also infinite in extent; a high-density universe has positive curvature and is finite in extent
question
Why have astronomers concluded that dark energy is the major constituent of the universe?
answer
The observed acceleration of the expansion of the universe implies that some non gravitational force must be at work. Dark energy is our current best theory of the cause of that force. In addition, galactic and cosmological observations indicate that the universe is spatially flat, and hence has critical density, but that the density of matter cannot account for the total. The amount of dark energy needed to account for the cosmic acceleration is in good agreement with the required extra density-about 70% of the critical value
question
Why do astronomers think the universe will expand forever?
answer
There doesn't seem to be enough matter to halt the expansion, and in addition, the observed cosmic acceleration suggests the existence of a large-scale repulsive force in the cosmos that also opposes re-collapse
question
When was the cosmic microwave background formed? Why is it still a blackbody curve?
answer
It was formed at the time of the Big Bang. It is the electromagnetic remnant of the primeval fireball. It started off as a blackbody curve because it was the thermal emission of the hot, young cosmos. It has retained this form because the cosmological redshift has stretched all photons by the same factor, preserving the shape of the blackbody curve
question
How do we know that most of the dark matter in the universe is not of "normal" composition?
answer
The amount of deuterium observed in the universe today implies that the present density of normal matter is at most a few percent of the critical value-much less than the density of dark matter inferred from dynamical studies
question
Does the theory of inflation itself say anything about the composition of the universe?
answer
Not directly. Inflation implies that the universe is flat and hence that the total cosmic density equals the critical value. However, the matter density seems to be only about 1/3 of the critical value, and the density of electromagnetic radiation (the microwave background) is a tiny fraction of the critical density. Together, these statements suggest that the remaining density is in the form of "dark energy"
question
Why is dark matter important to galaxy formation? How do observations of the microwave background support this view?
answer
Because dark matter is largely unaffected by the background radiation field, it was able to clump earlier than normal matter, forming the structures within which luminous matter subsequently formed the galaxies we see today. The gravity of the early dark-matter clumps caused tiny fluctuations in the temperature of the cosmic microwave background. The ripples we see today in the microwave background are the beginning of galaxy formation long ago
question
Has chemical evolution been verified in the laboratory?
answer
Not yet. The formation of complex molecules from simple ingredients by nonbiological processes has been repeatedly demonstrated, but no living cell or self-replicating molecule has even been created
question
Which solar system bodies (other than Earth) are the leading candidates in the search for extraterrestrial life, and why?
answer
Mars remains the most likely site because of its generally Earth-like makeup and because several lines of evidence suggest that the planet was warmer the past and may have had liquid water on its surface. Europa and Titan also have properties that might have been conducive to the emergence of living organisms-liquid water under Europa's icy crust and an atmosphere and many complex molecules on Titan
question
If most of the factors are largely a matter of opinion, how does the Drake equation assist astronomers in refining their search for extraterrestrial life?
answer
It breaks a complex problem up into simpler "astronomical," "biochemical," "anthropological," and "cultural" pieces, which can be analyzed separately. This analysis itself helps identify the types of stars where a search might be most fruitful
question
What assumptions go into the identification of the water hole as a likely place to search for extraterrestrial signals?
answer
We assume that a technological civilization will opt to work in the radio part of the spectrum, where Galactic absorption is least, and will choose a region where natural Galactic background "static" is minimized. We also assume that the 21-cm line of hydrogen will have significance to them, since hydrogen is the most common element in the universe, and that hydroxyl will also have a special meaning if they evolved in a water environment as we did
