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191 terms

Astronomy Exam 1

STUDY
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According to the scientific method, a hypothesis that is proposed to explain a particular physical phenomenon is considered wrong if
it is in conflict with the results of just one reliable and repeatable observation.
The laws of physics developed in laboratories on Earth are generally assumed to be valid
everywhere in the universe.
Forms of radiation which are not visible
have allowed us to determine much of what we now know about the universe.
The idea of thermonuclear fusion, the release of energy following the conversion of hydrogen nuclei into a helium nucleus, was first suggested as the source of energy in
the central core of the Sun.
New stars appear to be formed in
nebulae.
The lifetime of a typical star (like our Sun) appears to be
a few billions of years.
The study of galaxies gives astronomers important clues about the
creation of the universe.
Astronomers sometimes announce they have discovered a new solar system beyond our own, or a new galaxy beyond our own. Which choice correctly describes the terms "solar system" and "galaxy"?
A solar system consists of planets and other objects orbiting around a star, whereas a galaxy is a system consisting of an immense number of stars.
The number of degrees in a full circle is
360
An arcsecond is a measure of
angle
One arcsecond is equal to
1/3600 degree.
One arcminute is equal to
1/60 degree.
The Crab Nebula shown in Fig. 1-8, Freedman and Kaufmann, Universe, 8th ed., has a diameter of about 10 light-years and is at a distance of 6500 light-years. What angle will this supernova remnant subtend in our sky?
5.3 arcminutes
The average distance from Earth to the Sun, 149,600,000 km can be written in shorthand notation as
1.496 ´ 108 km.
One astronomical unit, or one AU is defined as the
mean distance between the Sun and Earth.
One light-year is the
distance that light travels in one year.
The distance from Earth to the star Betelgeuse (in the constellation Orion) has been measured as 520 light-years. Expressed in parsecs, this is approximately
160 pc.
The Crab Nebula (Fig. 1-8, Freedman and Kaufmann, Universe, 8th ed.) is the result of a supernova explosion of a star that occurred at a distance of about 1.99 kpc from Earth. If people on Earth saw the explosion in the year 1054 A.D., when did the explosion actually occur?
about 5446 B.C.
The following distance units, arranged in size from smallest to largest, are
km, AU, ly, pc
An underlying theme of astronomy is that the
entire universe is governed by a single set of fundamental physical laws.
In modern astronomy, the constellations are
88 regions of sky, covering the entire sky.
The constellation whose stars are used as pointers to the north celestial pole in the northern hemisphere is
Ursa Major, the Big Dipper.
The phrase "diurnal motion" refers to the
gradual motion of the constellations from east to west across the sky each night, resulting in different constellations being visible at 4 A.M. than at 10 P.M. on any given night
Cygnus appears high in the sky at midnight in July. Andromeda appears high in the sky at midnight in September. (See Fig. 2-5, Freedman and Kaufmann, Universe, 4th ed.) Where is Andromeda at midnight in July?
It has moved to the western horizon.
The celestial equator is defined as the
line in the sky that is perpendicular to Earth's spin axis.
You are standing at a position north of the equator, but less than 45° north latitude, and you view the stars rising above the horizon. They will move in a direction which makes a slant with the horizon. The angle between this slanted line and the horizon is
more than 45° (but less than 90°).
To navigators in the northern hemisphere, their latitude in degrees is equal to
the angle between the north celestial pole and their northern horizon.
The celestial coordinates that together describe a star's position precisely and unambiguously are
right ascension and declination.
The ecliptic is defined as the
line traced in our sky by the Sun over one year against the background stars.
The ecliptic crosses the celestial equator at
two points, known as equinoxes.
At what average speed does the Sun appear to move across our sky with respect to the stars in order to move through one full circle in one year?
about 1° per day
What is the primary cause of Earth's seasons?
Earth's rotation axis tilts with respect to the plane of its orbit around the Sun.
The tilt of Earth's spin axis to the direction perpendicular to the ecliptic plane (known as the ecliptic pole) is
23.5°.
The vernal equinox is that time of the year when the
Sun crosses the equatorial plane, or celestial equator, moving north.
At the summer solstice in the northern hemisphere, the Sun
reaches its highest angle above the southern horizon for the whole year.
Where would you have to be in the northern or southern hemispheres for the Sun to remain below the horizon for a 24-hour period for at least a part of the year?
above about 66.5° latitude
If you were standing on the equator, which of the following positions in the sky would pass directly over your head (i.e., through your zenith) at some time in one 24-hour period? (See Fig. 2-15, Freedman and Kaufmann, Universe, 8th ed.)
the vernal equinox, or the zero point of the right ascension on the celestial equator
Precession is
a very slow conical motion of Earth's axis of rotation.
A science fiction writer, writing a story about inhabitants on Earth in 14,000 A.D. who have survived a disaster that included the loss of modern navigational aids, describes them traveling due north across barren wastes by walking toward Polaris, the Pole Star. What is wrong with this situation?
Polaris will no longer be due north, because of Earth's precession.
Any star (except the Sun), when viewed from low and mid-latitudes, rises in the east
about 4 minutes earlier each evening.
How much of the total surface of the Moon is illuminated by the Sun when it is at crescent phase? (Hint: "total surface" - not just the part visible from Earth)
half
Earth's shadow falling on the Moon is the reason we see
lunar eclipses.
Why do we see different phases of the Moon?
The motion of the Moon in its orbit around Earth causes us to see different amounts of the sunlit side of the Moon.
When will the first quarter moon rise, approximately? (You may want to examine Figure 3-2 of Freedman and Kaufmann, Universe, 4th ed., and think about where you would need to stand on Earth to see the first quarter moon rising.)
noon
At what approximate time will the new moon rise?
sunrise
A full moon always occurs
when the Moon is farther from the Sun than is Earth.
The Moon is visible in the sky in the daytime from most places on Earth
about half the time, or for two weeks in every month.
If an observer on Earth sees the Moon to be full, than at the same time an observer on the Moon would see Earth to be at what phase?
new
The Moon rotates around its own axis in the same length of time that it takes to orbit once around Earth. This equality of rotation period and orbital period, which results in the same side of the Moon facing Earth at all times, is called
synchronous rotation.
In its orbit around Earth, the Moon
always keeps the same side toward Earth.
When viewed from a point directly above the plane of the planetary system, the Moon would appear to rotate on its axis
once per month, or once per revolution about Earth.
The term "synodic month" refers to the
time from new moon to new moon.
The term "sidereal month" refers to the
time over which the Moon completes one orbit around Earth, relative to the stars.
The fact that Earth-Moon system orbits the Sun (covering 30° per month) while the Moon orbits Earth means that the time between successive full moons, the synodic month, compared to one lunar orbital (sidereal) period is
about 2 days longer.
During a lunar eclipse the
Earth comes between the Sun and the Moon.
What is the phase of the Moon during a total lunar eclipse?
full
A solar eclipse occurs on Earth when the
Moon casts a shadow on Earth.
What is the phase of the Moon during a total solar eclipse?
new
There is about a 5° angle between the orbit of the Moon and the
plane of the ecliptic, or Earth's orbit.
Which of the following is a necessary condition for lunar or solar eclipses?
The Moon must be close to or crossing the ecliptic plane.
A lunar eclipse does not occur at every full moon because
the plane of the Moon's orbit is at an angle to the plane of Earth's orbit.
If the plane of the Moon's orbit were to be the same as the ecliptic plane, there would be a lunar eclipse
once a month.
The line of nodes of the Moon's orbit is the
line of intersection between the Moon's orbit and Earth's orbit (the ecliptic plane).
To witness an eclipse we must wait until which one of the following conditions is met?
the Moon must be on the ecliptic plain and the line of nodes must point in the general direction of the Sun
In a penumbral lunar eclipse
all parts of the Moon are partly (not totally) shaded from the Sun.
A total lunar eclipse is visible in principle (assuming clear skies everywhere)
to everyone in one hemisphere of Earth.
What is the maximum length of totality for a lunar eclipse?
1 hour 40 minutes
Which of the following factors makes it far more likely that a person will have seen a total lunar eclipse than a total solar eclipse?
A total lunar eclipse can be seen by people on most of the nighttime side of Earth, while a specific total solar eclipse can be seen only by people within a narrow strip of Earth's surface.
What is the maximum time of totality for any total solar eclipse observed from Earth's surface?
about 7.5 minutes
The total phase of a particular solar eclipse can be seen
only within a specific narrow strip across Earth's surface.
Which of the following parameters will dictate whether a particular solar eclipse appears as a total or an annular eclipse to an observer on the centerline of the Moon's shadow?
the distance of the Moon from Earth at the time of eclipse
What is the cause of an annular eclipse?
The Moon's position in its orbit is near apogee, its farthest point from Earth.
Which significant observation led the Greeks to accept the idea that Earth was a sphere?
The shape of Earth's shadow on the Moon during a lunar eclipse was always circular.
Eratosthenes measured the radius of Earth by
noting the different angles of the Sun at midday on the same day of the year at different positions on Earth.
In about 280 B.C., Aristarchus devised a method of estimating the relative distance of the Sun and the Moon from Earth by
measuring the angle between the Sun and Moon when the Moon is at first or third quarter.
The ancient Greek thinker whose model of the universe came to dominate the medieval world was
Ptolemy
A major theme of Ancient Greek philosophy was that stars and planets in the sky
followed patterns that could be described logically.
The correct order of "appearance" of the following "actors" on the "stage" of scientific discovery is
Ptolemy, Copernicus, Kepler, Newton, Einstein.
The word planet is derived from a Greek term meaning
wanderer
In the Greek era, it was almost universally believed that the
Earth was at the center of the universe.
An apparent eastward motion of a planet from night to night compared to the background stars (as viewed from Earth) is referred to as
direct motion.
An apparent westward motion of a planet from night to night compared to the background stars (as viewed from Earth) is referred to as
retrograde motion.
In the geocentric model of the solar system developed by Ptolemy, planets move
in circular epicycles while the centers of the epicycles move in circular orbits around Earth.
The primary purpose of describing planetary orbits in terms of epicycles and deferents was to account for the
pattern of direct and retrograde motion of a planet as it moved slowly against the background of stars.
The Almagest, a collection of early astronomical data and description of planetary position calculations, was written by
Ptolemy.
One unsatisfactory feature of the Ptolemiac description of the planetary system, particularly from a philosophical point of view was
the requirement of many unrelated parameters, such as epicycle and deferent sizes and speeds, with no unifying rules.
Retrograde motion of a planet when viewed from Earth is caused by
the relative motions of Earth and planet.
From Earth, we observe occasional retrograde motion in the motion of
all the planets, but not the Moon.
The early Copernican system for planetary motions is
Sun-centered, with planets moving in perfect circles around the Sun.
Which of the following statements correctly describes why Copernicus decided that the orbits of Mercury and Venus were smaller than the orbit of Earth?
Both planets stay fairly close to the Sun in our sky.
When Venus is at inferior conjunction,
it is at its smallest distance from Earth.
When Venus is at superior conjunction,
it is at its greatest distance from Earth.
When Saturn is at its farthest distance from Earth, it is at
conjunction
Where and when would Jupiter be seen from Earth when it is at opposition?
high in the south at midnight
The sidereal period of a planet is defined as the time between
two successive passages of the planet in front of a particular point in the sky (e.g., a star) as seen from the Sun.
The synodic period of a planet is the time between
successive alignments of Sun, planet, and Earth (e.g., time between successive oppositions).
A planet's sidereal year is different from its synodic year because the
Earth moves along an orbital path.
The greatest inaccuracy in Copernicus' theory of the solar system was that he
placed the planets in circular orbits.
The astronomer who used parallax to show that the supernova of 1572 occurred at a large distance from Earth was
Tycho Brahe.
Tycho Brahe
made accurate measurements of planetary positions, which Kepler later used to find the shapes of planetary orbits.
The major contribution of Johannes Kepler to the development of modern astronomy was to
prove that planetary orbits are ellipses.
Kepler's first law states that a planet moves around the Sun in
an elliptical orbit, with the Sun at one focus.
To which point in a planetary orbit does the word "perihelion" refer?
the point closest to the Sun
To which point in a planetary orbit does the word "aphelion" refer?
the point farthest from the Sun
If an object's orbit around the Sun has an eccentricity of 0.1, then the orbit is
almost circular.
Kepler's second law states that a line joining a planet to the Sun
sweeps out equal areas in equal times.
Two of Kepler's Laws of Planetary Motion applied to Earth are that the radius vector from the Sun to Earth sweeps out equal areas in equal times, and that Earth's orbit is an ellipse with the Sun at one focus. One consequence of these is
Earth moves at different orbital speeds at different times of the year.
Kepler's third law of planetary motion relates the period P (in sidereal years) to the length of the semimajor axis a (in astronomical units) in which way?
P 2 = a3
Halley's Comet returns to the Sun's vicinity every 76 years in an elliptical orbit. (See Fig. 4-23, Freedman and Kaufmann, Universe, 8th ed.) What is the semimajor axis of this orbit?
17.5 AU
What did Galileo see when he observed Jupiter through his telescope?
four satellites (moons) orbiting Jupiter
The one significant observation Galileo made through his home-built telescope that convinced him that the planets revolved around the Sun was
that the appearance of Venus followed a cycle of phases, from crescent through quarter and gibbous phases to full phase.
To define an object's velocity, we need to specify
how fast it is moving and also in which direction it is moving.
Acceleration of a moving body is defined as the rate of change of
velocity with time.
An object orbiting the Sun in a circle can be said to be
always accelerating.
Acceleration due to gravity on the Moon is
significantly smaller than that on Earth.
The strength of gravity on Mars is about 40% of that on Earth. If your mass on Earth is 60 kg, what would your mass be on Mars?
60 kg
According to Newton's first law,
if no force is acting on an object, then the object's speed and direction of travel will both be constant.
According to Newton's second law of motion, an object acted on by a constant force
moves with constant acceleration.
Two spaceships are at rest in space. They have different masses but have rocket engines that can deliver identical force. If they fire their rockets at the same time, which ship will speed up more quickly (higher acceleration)?
the one with the lower mass
Which of the following statements is a correct version of Newton's third law?
Whenever object A exerts a force on some other object B, B must exert a force of equal magnitude on A in the opposite direction.
Which path would a planet (like Earth) take if the force of gravity from the Sun were to be suddenly removed?
The planet would move in a straight line tangential to its present orbit.
Earth exerts a force on you as you stand on its surface. What is the size of the force exerted on Earth by you, when compared to the above force?
the same
If the mass of the Sun were doubled, the gravitational force on Jupiter due to the Sun would
be twice its present value.
As a spacecraft such as Voyager 2 moves away from the Sun, the gravitational force on it F varies with distance R from the Sun in which mathematical way? (µ means "proportional to.")
F µ 1/R2
A person orbiting Earth in the Space Shuttle feels weightless because
only one force (gravity) acts on her, but gravity also accelerates the shuttle so that the shuttle does not push up on her to create the feeling of weight within it.
If Earth were to be moved to a distance of 10 AU from the Sun, how much stronger or weaker would be the Sun's gravitational force on Earth?
100 times weaker
Newton's law of universal gravitation is so named because it
holds for all objects in the universe.
Which objects are often found to follow parabolic orbits?
comets
A hyperbola is
an open curve of infinite length.
The Moon produces tidal disturbances on the oceans of Earth. In general, there are
two high and two low tides per day.
Spring tides occur
twice a month, at full and new moon.
Neap tides occur
twice a month, at first and third quarter moon.
Does the Sun or the Moon have a greater effect in causing the tides on Earth?
The Moon, because it has a larger difference between pulling on the near side and pulling on the far side of Earth.
How many "tidal bulges" are there on Earth, due to the Moon's gravitational pull?
two, one facing (almost) directly toward the Moon and one facing (almost) directly away from the Moon
The reason the Moon always keeps one face toward Earth is that
gravitational forces from Earth act on the tidal bulge of the Moon.
In 1675, Rømer measured the speed of light by
timing eclipses of Jupiter's satellites, which appeared to occur later when Earth was farther from Jupiter.
The first experiment in which the speed of light was measured precisely involved
reflecting light from a mirror rotating at a known speed and measuring the angle of deflection of the light beam.
Around 1670, Isaac Newton performed a crucial experiment on the nature of light when he
demonstrated that the colors that make up white light are intrinsic, not produced by the glass through which the light passes.
Radio waves travel through space at what speed?
at the speed of light, 3 ´ 108 m/s
Around 1801, Thomas Young in England showed that light behaves as a wave by
shining light through two closely spaced slits and observing the resulting pattern of light on a white screen.
Around 1800 William Herschel passed light through a prism and discovered that part of the non-visible radiation in the resulting spectrum would raise the temperature of a thermometer. This portion of the electromagnetic spectrum is
infrared
Visible light occupies which position in the whole electromagnetic spectrum?
between infrared and ultraviolet
Which one of the following statements is true?
Visible light takes up only a very small part of the total range of wavelengths in the electromagnetic spectrum.
Visible wavelengths of electromagnetic radiation have a range of wavelengths of
400 to 700 nm.
In terms of wavelengths, gamma rays are
the shortest-wavelength electromagnetic waves.
X-rays and light are
the same thing except that X-rays have a shorter wavelength than light.
In a radio wave transmitter (such as that used by a radio or TV station), when the frequency of the signals is increased, the
wavelength is decreased.
The temperature of a gas cloud in space is directly related to and representative of the
average speed of its atoms.
A scientist reports that his measurement of the temperature of the surface of a newly discovered planet is -20 K. What conclusion can you draw from this report?
The result is erroneous since one cannot have negative absolute temperature.
The temperature of a normal, healthy, human being (37°C) expressed on the Kelvin (absolute) scale is approximately
310 K.
An ideal blackbody in physics and astronomy is an object that
absorbs and emits electromagnetic radiation at all wavelengths.
A perfect blackbody is so named because it
absorbs all radiation falling on it and reflects none.
If all stars are considered to be perfect blackbodies, then it should follow that all stars
of the same temperature emit the same energy flux.
Figure 5-11 in Freedman and Kaufmann's Universe, 4th ed., shows that a blackbody with a temperature of 3000 K emits radiation that peaks at a wavelength much longer than wavelengths in the visible part of the spectrum. This means that
the object emits visible radiation, but not as intensely as at longer wavelengths.
We plot the intensity of the radiation emitted from a solid object at a certain temperature versus its wavelength. As we increase the temperature, the wavelength at which the spectrum peaks becomes shorter and shorter. This is an illustration of
Wien's Displacement Law.
We plot the intensity of the radiation emitted from a solid object at a certain temperature versus its wavelength. As we increase the temperature, the total amount of energy emitted all across the spectrum increases as the fourth power of the temperature. This is an illustration of
the Stefan-Boltzmann Law.
Cepheid-variable stars pulsate regularly in size. During the contraction part of the cycle, when the star's temperature is increasing, the peak wavelength of the emitted radiation
shifts from the visible to the UV part of the spectrum.
The "color" or wavelength of maximum emission of radiation for a hot, solid body (or a dense gas such as a star) when the body cools from a temperature of several thousand degrees
moves toward the red end of the spectrum.
You are asked to design a detection system for human beings (or animals) in darkness, using infrared detection. If human beings are at a temperature of about 310 K, what would need to be the wavelength of peak sensitivity of your equipment or cameras (1 µm = 10-6 m)? (Hint: Use Wien's law.)
9.35 µm
The energy flux F from a star is the
amount of energy emitted by each square meter of the star's surface each second.
The star Betelgeuse has a lower surface temperature than the Sun; therefore (with IR = infrared and UV = ultraviolet)
Betelgeuse emits less IR and less UV flux than the Sun.
When a rod of metal is heated intensely, its predominant color
changes from red, through orange to white, and then to blue.
The important breakthrough in theoretical physics that was first suggested by Planck to explain the shape of the spectrum of a hot body was the
concept that electromagnetic energy was emitted in small packets or quanta.
A photon is
a bundle of pure energy.
In comparing photons of different wavelengths, we find that the energy carried by a photon
is larger if the wavelength is shorter.
In what way does a photon of blue light not differ from a photon of yellow light in a vacuum?
speed
The energy of a photon of X-rays, compared to the energy of a photon of visible light, is
much higher.
An electron volt (eV) is a unit of
energy
Spectral lines are of particular importance in astronomy because
each different element has a characteristic line spectrum.
Atoms in a hot, low-density gas (e.g., in a laboratory-type spectral source) emit a spectrum that is
a series of specific colors, unique to the type of atom in the tube, but fixed in position even when the gas temperature changes.
The gas in interstellar space between the stars is very tenuous ("thin"). Near a hot star, this gas is heated to a high temperature. Any such hot, tenuous gas emits light
only at specific wavelengths ("spectral lines"), and these spectral lines do not change in wavelength as the temperature changes.
The chemical makeup of a star's surface is obtained by
spectroscopy of the light emitted by the star.
If a certain gas is heated and observed through a grating, a bright line spectrum will be seen. If instead, a source of continuous spectrum shines through a cooler sample of this same gas, a dark absorption spectrum is observed. How do the positions of the lines in these two spectra compare?
The lines in the two spectra will be at the same frequencies. They will be the same spectra.
Chemical pollution often results in large numbers of very small particles being emitted into the atmosphere. What effect, if any, will this have on the color of the sunset?
It should make the sunset look more red.
The physical structure of an atom is
negatively charged electrons moving around a very small but massive, positively charged core.
The overall diameter of a typical atom is about
10-10 m, or 0.1 nm.
The New Zealand physicist Lord Rutherford and his colleagues in England demonstrated the existence of the very small but massive nucleus inside every atom in which crucial experiment?
deflection, and occasional reflection backward, of energetic nuclear particles from a beam aimed at a thin metal sheet
Most of the mass of ordinary matter resides in the
nuclei of atoms.
The diameter of the nucleus of a typical atom (as measured by Rutherford in the early 1900s) is
10-4 of the diameter of the atom.
The atomic number that designates the position of an element in the periodic table is equal to the
number of protons in the nucleus of the atom.
Isotopes of a particular element in the periodic table have which nuclear property in common?
same number of protons but different numbers of neutrons
The isotope 15N has an atomic number of 7. This isotope has
7 protons and 8 neutrons.
The specific colors of light emitted by an atom in a hot, thin gas (e.g., in a tube in a laboratory or a gas cloud in space) are caused by
electrons jumping to lower energy levels, losing energy as they do so.
When astronomers look for evidence of hydrogen gas in the spectra of the Sun, the planets, and nearby stars, the positions of the spectral features or "lines" due to hydrogen
are always in the same pattern, characteristic of hydrogen gas, as seen in the laboratory.
Electrons in atoms
occupy levels whose energies are fixed.
Ionization of an atom occurs when
an electron is removed from the atom.
An ionized hydrogen atom is simply
a proton.
The observed change in wavelength due to the Doppler effect occurs
only when the light source has a radial velocity (toward or away from the observer).
The spectrum of a star shows an equivalent set of dark absorption lines to those of the Sun, but with one exception: Every line appears at a slightly longer wavelength, shifted toward the red end of the spectrum. What conclusion can be drawn from this observation?
The star is moving rapidly away from Earth.
An astronomer photographs the spectrum of an object and finds a spectral line at 499 nm wavelength. In the laboratory, this spectral line occurs at 500 nm. According to the Doppler effect, this object is moving
toward Earth at 1/500 the speed of light.
A star looks predominantly yellow to an observer who is at rest with respect to the star. To an observer moving away from the star at 1% of the speed of light
the star will still look predominantly yellow, but each part of the spectrum will be shifted to slightly longer wavelengths.