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162 Cards in this Set

  • Front
  • Back
How far away is the nearest star beyond the Sun?

about 4 ly away

What is parallax

apparent shift in position of an object as the observer moves

The motion that is used to change the position of the observer in the most common parallax measurements of distances to relatively nearby stars is the

motion of Earth in its orbit around the Sun.

Stellar parallax appears because

Earth moves in space.

Apparent magnitude is a measure of a star’s

brightness, as seen from Earth.

A star of apparent magnitude +1 appears _____ thana star of apparent magnitude +2.


The statement that the apparent magnitude of a variable star has increased indicates that its

brightness has decreased.

Luminosity is measured in


The luminosity of a star is

its total energy output into all space.

An astronomer is measuring the brightness of a particular star through a telescope, using different filters in the visual(yellow-green), violet, and ultraviolet regions. What is the name of the technique being used by this astronomer?


For Balmer series lines to show up strongly in absorption in stellar spectra, significant numbers of hydrogen atoms have to have electrons in the n = 2 energy level. What then does the appearance of these lines in a stellar spectrum tell us about the temperature of the star’s surface?

The temperature must be reasonably high to excite the electrons to this level by collisions but not high enough to ionize the atoms.

Why is there a limited range of stellar surface temperatures around 10,000 K at which neutral hydrogen gas absorbs visible light in the Balmer series?

Electrons in hydrogen have to be at the n = 2 energy level to produce absorption in this series. If the gas is too cold, most atoms are in the n = 1 state; if it is too hot, most atoms are ionized.

The chemical makeup of the Sun’s surface can be determined by

solar spectroscopy

Which of the following spectral classification letters signifies the hottest stellar surface temperature?

D.) B

Which of the following four spectral classifications signifies the coolest stellar surface temperature?


The Hertzsprung-Russell diagram is a plot of
absolute magnitude (orintrinsic brightness) against temperature of a group of stars.
If you compare two stars, the one with the _____must have the greater luminosity.
smaller absolute magnitude

What are the two physical parameters of stars that are plotted in the Hertzsprung-Russell diagram?

luminosity and surface temperature

Compared with a star in the middle of the Hertzsprung-Russell diagram, a star in the lower left of the diagram is


Compared with a star in the middle of the Hertzsprung-Russell diagram, a star in the upper right of the diagram is



What is a dwarf star?


main-sequence star

What is a white dwarf star?


star of about the same size (diameter) as Earth

A typical white dwarf has a surface temperature about four times that of the Sun and a radius about 1% that of the Sun. What can you determine about the luminosity of a typical white dwarf from this information?

The white dwarf will be less luminous than the Sun

Where are the most massive stars to be found in the main sequence of a Hertzsprung-Russell diagram?

upper left end

What would be the mass of a main-sequence star that has a luminosity 1000 times greater than that of the Sun? (See Figure 11-14,Comins and Kaufmann, Discovering the Universe, 8th ed.)

5 solar masses

Whichof the following molecules is likely to be the most common in interstellar space

H2, molecular hydrogen

Which of the following common molecules found in interstellar space contains nitrogen atoms but no oxygen?

ammonia (NH3)

How is gas distributed in interstellar space

in clumps, concentrated in interstellar clouds

What is the typical mass of a giant molecular cloud?

100,000 to 1 million solar masses

The mass of a particular giant molecular cloud in interstellar space is 2 million solar masses. What is the mass of the helium in this cloud?

500,000 solar masses

What is the characteristic color of a reflection nebula?


Evidence of massive amounts of hydrogen gas surrounding some stars comes from

observation of emission of characteristic red Balmer light from nebulosity around them

Long-exposure color photographs of the night sky often show regions that glow red, such as the Rosette Nebula in Figure 12-8 of Comins andKaufmann, Discovering the Universe, 8th ed. This distinctive red color is caused by the

ionization and subsequent recombination of hydrogen atoms.

What is the ultimate fate of an open star cluster?

The stars in the cluster escape one by one until the cluster no longer exists.

New stars are formed from

huge, cool dust and gas clouds.

How does the temperature of an interstellar cloud affect its ability to form stars?

Higher temperatures inhibit star formation.

Which of the following mechanisms is NOT considered to be away in which star formation is triggered or started?

heating of an interstellar cloud by radiation from embedded young stars

Which of the following mechanisms is thought to be ineffective and inefficient in the triggering of star birth in molecularclouds?

gravitational contraction of a hot gas cloud

Whatis a protostar?

sphere of gas after collapse from an interstellar cloud but before nuclear reactions have begun

At what stage in its life does a star pass through the protostar phase?

after condensation but before nuclear reactions begin in its core

Where are protostars most likely to form?

dark nebulae

Protostars are

very young objects still contracting before becoming true stars.

The source of a protostar’s heat is

gravitational energy released as the star contracts

What is believed to be the most important factor determining whether a collapsing region (dense core) in an interstellar cloud becomes asingle-star or a multiple-star system?

amount of rotation (spin)

Accretion of matter in an interstellar cloud leads to

a protostar.

Whatis a protostar called in the stage after it has finished accreting mass?

pre–main-sequence star

What point defines the end of the pre–main-sequence phase of a star’s life and the start of the main-sequence phase?

Nuclear reactions begin in its core.

In which region of the Hertzsprung-Russell diagram does a newly formed protostar first appear when it begins to shine at visible wavelengths?

right side; relatively large luminosity because of its size but cool

A main-sequence star can be no smaller than 0.08 solar mass.The reason is that

the temperature in a contracting protostar of less than 0.08 solar mass does not become high enough for nuclear reactions to start.

A brown dwarf is a(n)

object intermediate between a planet and a star, with not enough mass to begin nuclear reactions in its core.

TheT Tauri phase of a protostar is characterized by

strong spectral emission lines.

What is believed to be the maximum mass a star can have

about 150 solar masses

What is believed to prevent stars from being larger than a few hundred solar masses?

The temperature becomes so high that the excess mass is pushed back into space by radiation from the star.

Whatis the difference between an extrasolar planet and a sub-brown dwarf?

An extrasolar planet orbits a star;a sub-brown dwarf does not. Otherwise, they are the same.

Which of the following properties is NOT a characteristic of T Tauri stars?

nuclear reactions in the core

Which of the following facts referring to stars in a cluster is NOT particularly useful for interpreting the evolution of these stars?

The majority of the material in these stars is hydrogen.

The stars in an open cluster are useful for studying the early stages of stellar evolution because all the stars in a cluster have the same


Thedefinition of a main-sequence star is a star

in which nuclear fusion reactions generate sufficient energy to oppose further condensation of the star.

The stars that last longest are the stars

with the smallest mass

Which of the following statements about the rate of stellar evolution is true

The more massive the original star, the faster is the evolution

Low-mass stars can undergo two evolutionary phases called red-giant phases. What is the difference between them?

In the first, the primary productionof energy is from helium burning in the core. In the second, the primary production of energy is from helium burning in a shell around the core.

Nuclear fusion reactions of helium produce primarily

carbon and oxygen nuclei.

The structure of the deep interior of a low-mass star near the end of its life is a(n)

carbon-oxygen core, a shell around the core where helium nuclei are undergoing fusion, and a surrounding shell of hydrogen.

In a star’s evolutionary life, the asymptotic giant branch(AGB) is the

helium shell fusion phase.

A star ascending the red-giant branch for the second time in the asymptotic giant branch phase will have

no nuclear reactions in the core, but a helium-fusion shell outside the core, which itself is surrounded by a shell of hydrogen

In the process of helium shell fusion in a low-mass star near the end of its life, the star moves upward and to the right on the asymptotic giant branch of the Hertzsprung-Russell diagram. In this process,the star is

expanding, cooling, and becoming more luminous

What will be the mass of the Sun at the end of its asymptotic giant branch (AGB) phase, due to mass loss to space by its stellar wind?

about 0.5 solar mass

What happens to the surface of a low-mass star after the helium core and shell fusion stages are completed?

The star is propelled slowly away from the core to form a planetary nebula.

Which of the following important components does a planetary nebula contribute to the interstellar medium?

nuclei of moderately heavy elements,major components of planets such as our own

A planetary nebula is created

over a few thousand years or more,in a slow expansion away from a low-mass star, driven by a series of thermal pulses from helium fusion.

What physical process provides the energy for the ejection of a planetary nebula from a low-mass star?

helium shell flashes in the helium fusion shell

Planetary nebulae are so named because

they were extended objects, often green-colored, that looked like planets when first seen by nineteenth-century observers through their telescopes

Usually, ideal gases increase their pressure and volume when heated and decrease their pressure and volume when cooled. Do these rules applyto stars?

No. White dwarfs are essentially degenerate matter in which the pressure is independent of the temperature.

Whitedwarfs radiate most strongly in the ultraviolet, with a peak wavelength of perhaps 300 nm. What would be the surface temperature of a white dwarf


Stars that have ejected a planetary nebula go on to become

white dwarfs.

The “star” that is seen at the center of a planetary nebulais

a small, hot, and very dense white dwarf star

A white dwarf star, the surviving core of a low-mass star toward the end of its life, can be found on the Hertzsprung-Russell diagram

below and to the left of the main sequence.

At which phase of its evolutionary life is a white dwarf star

very late for small-mass stars, in the dying phase

The Sun will end its life by becoming a

white dwarf.

How does a white dwarf generate its energy?

It no longer generates energy but is slowly cooling as it radiates away its heat.

Which type of dwarf is largest?

red dwarf

A white dwarf star is supported from collapse under gravity by

degenerate-electron pressure in the compact interior.

A white dwarf star, as it evolves, undergoes which of the following changes?

Luminosity and temperature decrease while its size remains constant

The mechanism that gives rise to the phenomenon of the novais

matter from a companion star falling onto a white dwarf in a close binary system, eventually causing a nuclear explosion on the dwarf’s surface.

A nova is an explosion involving a white dwarf. Can a white dwarf become a nova more than once? Why or why not?

Yes, A white dwarf can become a nova more than once if it continues to receive matter from a companion star.

What is the Chandrasekhar limit?

mass limit to the total mass of a white dwarf, beyond which the electron degeneracy pressure will be overcome and the core will collapse

A Type Ia supernova is the

explosion of a white dwarf in a binary star system after mass has been transferred onto it from its companion.

Type II supernovae show prominent lines of hydrogen in their spectra, whereas hydrogen lines are absent in spectra of Type Ia supernovae.Explain. (Hint: Think about the type of star that gives rise to each of the two types of supernova.)

Massive stars contain large amounts of hydrogen, whereas white dwarfs are mostly carbon and oxygen.

Can a white dwarf explode?

Yes, but only if it is in a binary star system.

From observations of supernova explosions in distant galaxies, it is predicted that there should be about five supernovae per century in our Galaxy, whereas we have seen only about one every 300 years from Earth. Explain.

Most supernovae occur in the galactic plane, where interstellar dust has hidden them from our view from Earth.

At the center of the remnant of a Type Ia supernova you would expect to find

nothing special

Which force induces the core to contract inward and get hotter in massive stars at the conclusion of each episode of nuclear fusion, such as the carbon-, oxygen-,and silicon- fusion cycles?


In which order do the stages of core nuclear fusion occur in the evolution of a massive star?

helium, carbon, neon, oxygen

Thermonuclear reactions release energy because the product (ash) nucleus

is more tightly bound than the original (fuel) nucleus

Which of the following statements does NOT describe a consequence of core collapse at the end of the life of a massive star?

The silicon core is converted to iron by fusion reactions

During its life, a massive star creates heavier and heavier elements in its core through thermonuclear fusion, leading up to silicon and iron. What is the fate of the iron that is created

The iron is torn apart by high-energy photons at the end of the star’s life.

Which of the following phenomena is never a consequence of a supernova explosion?

formation of a planetary nebula

The core collapse phase at the end of the life of a massive star is triggered when

nuclear fusion has produced a significant amount of iron in its core.

Where would you expect to find a core-collapse supernova?

near a star-forming region

The explosion of a supernova appears to leave behind

a rapidly expanding shell of gas and a central neutron star.

What is a cosmic ray shower?

shower of particles produced when a cosmic ray strikes atoms in Earth’s atmosphere

Just before it exploded, the star that became supernova SN 1987A was a(n)

B3 I supergiant

The Crab Nebula is

a supernova remnant

A pulsar is a(n)

rapidly spinning neutron star

Neutron stars are believed to be created by

type II supernovae, i.e., explosionsof high-mass stars.

Which of the following sentences does NOT state a property of neutron stars?

Neutron stars contain strong gravitational fields but weak magneticfields.

What prevents a neutron star from collapsing and becoming a black hole?

Gravity in the neutron star is balanced by an outward force due to neutron degeneracy

The very strong magnetic field on a neutron star is created by

the collapse of the star, which significantly intensifies the original weak magnetic field of the star.

The pulsed nature of the radiation at all wavelengths that is seen to come from a pulsar is produced by

the rapid rotation of a neutron star that is producing two oppositely directed beams of radiation.

The interior of a neutron star is believed to consist of

neutrons in a superfluid state.

Which one of the following statements is NOT a consequence of the postulates of special relativity?

The wavelength emitted from a source moving with respect to an observer is different from the wavelength measured by an observer who is moving along with the source.

A Clock is moving across your line of sight with its face turned toward you.Which of the following statements about this clock, as seen by you, is NOT correct?

The clock will appear thinner, front to back, than it would if it were at rest.

Why does Einstein’s theory of special relativity carry the name special?

The theory deals only with objects moving in a straight line at a constant speed.

How must an object be moving for us to be able to use the theory of special relativity to describe the object?

The object must be moving at a constant speed in a straight line; how fast it is moving is not important.

In which of the following “frames of reference” would matter behave exactly as it would in a stationary frame of reference?

moving at a constant velocity

Which of the following is a correct and complete statement of Einstein’s first postulate of special relativity?

Your description of physical reality is the same regardless of the constant velocity at which you move.

In a TV tube, the picture is created by a beam of electrons that travels down the tube at a very high speed. What is the mass of one of these electrons, compared with an electron at rest?

The moving electron appears to have a greater mass.

Einstein’s theory of special relativity contains some very strange ideas such as time dilation (moving clocks run slow), length contraction (moving lengths areshorter), and lack of absolute simultaneity. What is the basis of these ideas?

The speed of light is the same for all observers in all reference frames.

Which statement best describes the “fabric” of space and time as outlined by the classical physics of Newton?

Space is perfectly uniform, filling everywhere like a fixed network, while time passes at a uniform rate for all observers.

In what way is the general theory of relativity more “general” (i.e., deals withmore situations) than the special theory of relativity?

The general theory includes gravitation and accelerated motion.

Supposea satellite were placed in orbit around (and very close to) a neutron star.Which theory would you need to use to describe how it moves?

general theory of relativity

How does a gravitational field affect the passage of time?

Clocks in a gravitational field run slower than clocks farther from the center of the field when viewed by an observer who is also farther from the center of the field.

In which of the following locations would a clock run at its fastest rate?

empty space

According to Einstein’s general theory of relativity, a clock that ticks at a regular rate far from a source of gravity will appear to tick

slower the closer it comes to the source of gravity.

An electromagnetic wave leaves the surface of a neutron star and travels outward. As the wave gets farther from the star’s surface, the _____ and the _____.

frequency decreases; wavelength increases

What happens to the wavelength of light as it travels outward through the gravitational field of a planet or star so that the field becomes less strong?

The wavelength increases

According to Newton’s law of gravity, why does Earth orbit the Sun?

The Sun exerts a gravitational force on Earth across empty space.

Accordingto general relativity, why does Earth orbit the Sun?

Space around the Sun is curved, and Earth follows a geodesic in this curved space.

Which of the following is not a test of general relativity but rather a test of special relativity?

The length of a moving object decreases when observed by a stationary observer

Ablack hole is so named because

it emits a perfect blackbody spectrum.

The escape velocity for material inside a black hole is

greater than the speed of light.

Ablack hole can be thought of as

a region with such a large mass density that even electromagnetic radiation cannot escape.

In general, how many fundamentally different types of blackholes are expected to exist?

two—those that rotate and those that do not rotate

What is a Schwarzschild black hole?

nonrotating black hole

What is a Kerr black hole?

rotating black hole

What property separates a black hole from the rest of the universe

event horizon

What is the event horizon of a black hole

“surface” from inside of which nothing can escape

What appears to happen to a clock as it approaches and reaches the event horizon around a black hole when viewed by a remote observer

Time appears to slow down and stop

If a person falls as far as the event horizon of a black hole, what would happento the person’s heart rate (apart from the effects caused by the person’s adrenalinelevel)?

The heart rate would appear to be zero; his heart would appear to have stopped

In the context of black holes, what is a wormhole

direct connection from a rotating black hole to another part of spacetime

The Milky Way Galaxy

is one of many billions of galaxies in the universe.

In1785 William Herschel proposed that the solar system was at the center of the Milky Way Galaxy. How did he reach this conclusion?

Herschel’s observations suggested that the stars in the Milky Way Galaxy were distributed uniformly around Earth.

The component of the Milky Way Galaxy accounts for interstellar extinction, the dimming of light from distant objects is


The spiral-arm structure of the Milky Way Galaxy has been measured and evaluated most effectively by observations of

21-cm radiation from interstellar hydrogen and the distribution of young stars.

Where are many of the older, metal-poor stars found in the Milky Way Galaxy?

in globular clusters in the galactic halo

Which of the following components of the Milky Way Galaxy outlines the spiral arms of the Galaxy?

young O and B stars, dust, and gas

What is the distribution of giant molecular clouds in the Milky Way Galaxy and other similar galaxies?

Giant molecular clouds occur primarily in the spiral arms.

Recent observations seem to indicate that, rather than being a spiral galaxy, the Milky Way may be

a barred spiral with a definite, straight bar across its center.

The dimensions of the disk of the Milky Way Galaxy are diameter _____, thickness_____.

100,000 ly; 2000 ly

Inthe Milky Way Galaxy, young, metal-rich stars are found

in the disk and spiral arms.

TheMilky Way is an example of which type of galaxy?


Where is the solar system located in the Milky Way Galaxy?

in the galactic disk

The Milky Way Galaxy appears to have a spiral structure with

four separate major arms.

If the Sun were to be at or close to the galactic center of the Milky Way, the intensity of starlight in the night time sky on Earth would be

extremely intense from the dense field of stars, equivalent to about 200 full Moons.

We have observed the central region of the Milky Way Galaxy in a number of wavelength ranges. With which one of the ranges is it most difficult to obtain information?


What is the significance of the object Sagittarius A* (“Sagittarius A-star”) in the Milky Way Galaxy?

Sagittarius A* appears to be the actual nucleus of the Galaxy.

What evidence now exists for a supermassive black hole at the center of the Milky Way Galaxy?

very rapid motion of matter close to the nucleus of the Galaxy, requiring a very massive body to hold it in orbit

The possible presence of a supermassive black hole at the center of the Milky Way Galaxy has been deduced from

the very high orbital speed of ionized gas clouds close to the galactic center.

19. Where inspace would you look for a globular cluster?

in the Milky Way galactic halo, orbiting the galactic centerin a long elliptical orbit around the galactic center

What is a galactic halo?

spherical distribution of stars and globular clusters centered on a nuclear bulge

Does the halo of the Milky Way Galaxy include other galaxies?

Yes. The halo region includes several small galaxies, among them the Magellanic Clouds, two small, irregular galaxies

The stars in the Milky Way Galaxy

move generally around the galactic center.