Astronomy Midterm #2

The average temperature of the typical dark dust cloud is about a) 0 K. b) 2.73 K. c) 100 K. d) 3,000 K. c) 6,000 K.

100k

A hypernova creates a) a black hole. b) millisecond-duration gamma-ray bursts. c) a neutron star. d) a pulsar. e) Both B and C are correct.

A

Complex molecules in the interstellar medium are found a) primarily in the dense dust clouds. b) on the surfaces of the coolest class K and M stars only. c) scattered evenly throughout the universe, a product of the Big Bang itself. d) only around the supergiant stars like Betelgeuse that make their heavy atoms. e) uniformly throughout the disk of the Galaxy.

A

During the T-Tauri phase of a protostar, it a) may develop very strong winds. b) begins a period of reduced activity. c) lies on the main sequence. d) changes its spin direction. e) expands dramatically.

A

How long does it take an M-type star to reach the main sequence, compared to a solar-type star? a) about twenty times longer b) longer than the age of the Galaxy c) a tenth as long d) about twice as long e) about the same, 30 million years

A

Neutral hydrogen atoms are best studied from their energy given off as a) 21-cm waves in the radio region. b) red hydrogen alpha emission, at 656.3 nm. c) 121.3 nm as Lyman alpha emission in the UV. d) 0.2 nm as X-rays. e) Neutral hydrogen gives off no detectable radiation, since it is cold, not hot.

A

Neutron stars do NOT have a) rotation periods comparable to the Sun's. b) large surface gravities, compared to the Sun. c) masses greater than 1.4 solar masses. d) sizes comparable to large cities. e) strong magnetic fields.

A

Noting the main sequence turnoff mass in a star cluster allows you to determine its a) age. b) distance. c) total mass. d) radial velocity. e) number of stars

A

The Chandrasekhar limit is a) the upper-mass limit for a white dwarf. b) the temperature at which hydrogen fusion starts. c) the lower-mass limit for a Type II supernova. d) the point at which a planetary nebula forms. e) the temperature at which helium fusion starts.

A

The single most important determinant of the temperature, density, radius, luminosity, and pace of evolution of a protostar is its a) mass. b) molecular composition. c) chemical composition. d) magnetic field. e) spin.

A

What happens to light passing through even thin clouds of dust? a) It dims and reddens the light of all more distant stars. b) Its motion causes all light to be redshifted as it passes through these clouds. c) Its motion causes the light of stars beyond to twinkle. d) The light that passes through them is blueshifted due to the cloud's approach. e) Even a little can completely block all light, such as the Horsehead Nebula.

A

Which of these does NOT exist? a) a 1.5 solar mass white dwarf b) a 6 solar mass black hole c) a 1.8 solar mass neutron star d) a 0.06 solar mass brown dwarf e) a million solar mass black hole

A

Which of these would typically be among the brightest stars in an ancient globular cluster? a) K3 b) M5 c) O3 d) A4 e) B3

A

Who discovered the first four pulsars? a) Jocelyn Bell b) Carl Sagan c) Stephen Hawking d) Martin Schwartzschild e) Anthony Hewish

A

You would expect millisecond pulsars to be a) part of a binary system. b) collapsing rapidly. c) most common in open clusters. e) rotating slowly. isolated in space.

A

A cloud fragment too small to collapse into a main sequence star becomes a a) white dwarf. b) brown dwarf. c) T-Tauri object. d) pulsar. e) planet of another star.

B

A white dwarf has the mass of the Sun and the volume of a) Jupiter. b) Earth. c) Eros. d) Mars. e) the Moon.

B

Compared to a cluster containing type O and B stars, a cluster with only type F and cooler stars will be a) further away. b) older. c) younger. d) more obscured by dust. e) less obscured by dust.

B

Of the elements in your body, the only one not formed in stars is a) oxygen. b) hydrogen. c) carbon. d) iron. e) calcium.

B

Pulsars a) spin very slowly when they're young, and gradually spin faster as they age. b) spin very rapidly when they're young. c) generally form from 25-solar-mass stars. d) emit radio in all directions. e) are the cause of gamma-ray bursts.

B

The initial mass of a protostar generally determines the star's future evolution. But in some cases, what can alter this process? a) The star may drift away from the other stars in its formation cluster. b) The star may be in a spectroscopic binary system. c) The star may be isolated in space, far from other influences. d) The star may gain mass by passing through a dark cloud. e) The star may collide with another, unrelated star.

B

What explanation does general relativity provide for gravity? a) Gravity is inversely proportional to the radius of the body. b) Gravity is the result of curved spacetime. c) Gravity can affect only massive particles, not massless photons. d) Gravity is the opposite of the electromagnetic force. e) Gravity is directly proportional to the mass of the attracting body.

B

When a dense gas is said to be degenerate, it is because a) the electrons lack moral character b) electrons combine together due to intense gravity. c) Two electrons occupy the same space. d) further contraction would violate the Pauli exclusion principal.

B

Which of these would typically be the brightest star in a young open cluster? a) A2Ib b) B1V c) G2V d) K3II e) M3Ia

B

A photosphere appears on a protostar near the end of stage a) 1 b) 2 c) 3 d) 6 e) 7

C

A surface explosion on a white dwarf, caused by falling matter from the atmosphere of its binary companion, creates what kind of object? a) black dwarf b) brown dwarf c) nova d) Type I supernova e) Type II supernova

C

Neutron stars have a) very strong bipolar magnetic fields. b) weak or non-existent magnetic fields. c) monopolar fields that switch polarity every rotation. d) periods of days or weeks. e) no relation to pulsars.

C

Of the main sequence stars, those of type ________ have the longest main-sequence life spans. a) K b) M c) O d) B e) G

C

The Schwartzschild radius for a 12 solar mass star is a) 4 km. b) 15 km. c) 36 km. d) 100 km. e) 3000 km.

C

The most common molecule in a molecular cloud is a) carbon monoxide, CO. b) methane, CH4. c) molecular hydrogen, H2. d) water, H2O. e) ammonia, NH3

C

Three terrestrial-sized planets in orbits of a fraction of an AU have been found near a) Cygnus X-1. b) a white dwarf. c) a millisecond pulsar. d) Supernova 1987A. e) a magnetar.

C

What is the primary visible color of an emission nebula? a) blue due to ionized helium atoms b) red because of reddening by dust c) red due to the Hα line of hydrogen d) black, because of dust e) blue from the scattering of light off the tiny molecules

C

When a star's inward gravity and outward pressure are balanced, the star is said to be a) in thermal expansion. b) in gravitational collapse. c) in hydrostatic equilibrium. d) a stage 2 protostar. e) in rotational equilibrium.

C

Which is used observationally to determine the age of a star cluster? a) the total number of main-sequence stars b) the amount of dust that lies around the cluster c) the luminosity of the main-sequence turn-off point d) the number of white dwarfs e) the ratio of giants to supergiants

C

Which of these evolutionary paths is the fate of our Sun? a) supernova of Type II b) nova c) planetary nebula d) supernova of Type I e) brown dwarf

C

Which statement about dark dust clouds is true? a) Hydrogen and helium are the chief absorbing and scattering agents. b) They block the vast majority of radio waves from our Galaxy. c) They can be penetrated only with longer wavelengths such as radio and infrared. d) They can be penetrated only with shorter waves, such as UV and X-ray. e) They comprise the majority of the mass of the Galaxy.

C

Which statement about gamma ray bursters is NOT correct? a) In seconds, they radiate enormous amounts of energy. b) The beams may be bipolar ejections from the hypernova formation of black holes. c) They are scaled up X-ray bursters, with more massive objects involved. d) They seem to be coming from far beyond our own Milky Way. e) Millisecond flickering implies they are tiny in size.

C

Why are star clusters ideal "laboratories" for stellar evolution? a) Their stars are all about the same mass and temperature. b) The combined light of all the stars makes them easier to see. c) Their stars are all about the same age, composition, and distance from us. d) Like our Sun, they lie in the plane of the Milky Way. e) Their stars are all the same composition and stage in evolution.

C

A cluster with a lot of Type O and B stars, but no bright K or M stars, is very a) faint. b) densely packed. c) old. d) young. e) poor in heavy elements.

D

A recurrent nova could eventually build up to a a) core composed of iron. b) Type II supernova. c) helium flash. d) Type I supernova. e) planetary nebula.

D

A solar-mass star will evolve off the main sequence when a) it loses all its neutrinos, so fusion must cease. b) it expels a planetary nebula to cool off and release radiation. c) it explodes as a violent nova. d) it builds up a core of inert helium. e) it completely runs out of hydrogen.

D

After reaching its peak luminosity in hours or days, a nova declines in brightness over a period of a few ________ before returning to its pre-explosion luminosity. a) minutes b) hours c) days d) months e) years

D

Almost all of the complex molecules found in space are based on ________ atoms. a) iron b) oxygen c) hydrogen d) carbon e) silicon

D

At what core temperature does hydrogen begin to fuse to helium? a) 3,000 K b) 5,800 K c) 1 million K d) 10 million K e) 100 million K

D

In a very young star cluster, while the most massive stars are swelling up into giants, the least massive stars are a) also evolving off the main sequence as well. b) blowing off shells as planetary nebula instead. c) collapsing directly to white dwarfs. d) still on the zero-age main sequence. e) continuing to shine as stable main-sequence stars.

D

Interstellar gas is composed mainly of a) ammonia, methane, and water vapor. b) 10% hydrogen, 90% helium by numbers of atoms. c) only hydrogen. d) 90% hydrogen, 9% helium by weight. e) some hydrogen, but mainly carbon dioxide.

D

Of all forms of electromagnetic energy, ________ radiation gives us the least access to our entire Milky Way Galaxy. a) radio b) x-ray c) infrared d) visible light e) ultraviolet

D

The densely packed neutrons of a neutron star cannot balance the inward pull of gravity if the total mass is a) between 1.4 and 2.0 solar masses. b) Chandrasekhar's limit of 1.4 solar masses. c) not at least 25 solar masses. d) greater than Schwartzschild's limit of 3 solar masses. e) less than 1.0 solar masses.

D

The largest known black holes a) can be no bigger than the Earth, like white dwarfs. b) can be no more than 1.4 solar masses, according to Chandrasekhar. c) create the dark nebulae in the plane of the Milky Way. d) lie in the cores of the most massive galaxies. e) can be no bigger than a small city, just like neutron stars.

D

Two important properties of young neutron stars are a) search for their pulsar signal. b) search for radio waves from the accretion disk. c) locate a visible star that disappears when the black hole passes in front of it. d) look for its effects on a nearby companion. e) look for voids in the star fields.

D

Two-thirds of all known millisecond pulsars are found in what type of object? a) extremely distant galaxies b) open clusters c) giant molecular clouds d) globular clusters e) emission nebulae

D

What is the typical age for a globular cluster associated with our Milky Way? a) a few million years b) 200 million years c) a billion years d) 10-12 billion years e) 45 billion years

D

What temperature is needed to fuse helium into carbon? a) 5,800 K b) 100,000 K c) 15 million K d) 100 million K e) one billion K

D

Which of these is true of planetary nebulae? a) They are rings of material around protostars that will accrete into planets in time. b) They are the material which causes the eclipses in eclipsing binary systems. c) They are expelled by the most massive stars in their final stages before supernova. d) They are ejected envelopes surrounding a highly evolved low-mass star. e) They are the envelopes that form when blue stragglers merge.

D

Which statement about the stages of starbirth is false? a) At stage 1, only the cloud exists. b) The T-Tauri wind is prevalent in stage 5. c) By stage 3, the star has formed a photosphere. d) Nuclear reactions begin in the core by stage 4. e) By stage 7, the star has reached the main sequence.

D

Black dwarfs are a) very common, making up the majority of the dark matter in the universe. b) rare, for few binary systems are close enough for this merger to happen. c) rare, for collapsing cores of over three solar masses are uncommon. d) often made from very low mass protostars that never fuse hydrogen. e) not found yet; the oldest, coldest white dwarf in the Galaxy has not cooled enough yet.

E

Emission nebulae like M42 occur only near stars that emit large amounts of a) visible light. b) X-rays and gamma rays. c) microwaves. d) infrared heat. e) ultraviolet radiation.

E

For a white dwarf to explode entirely as a Type I supernova, its mass must be a) at least 0.08 solar masses. b) 1.4 solar masses, the Chandrasekhar limit. c) 3 solar masses, the Schwartzschild limit. d) 20 solar masses, the Hubble limit. e) 100 solar masses, the most massive known stars.

E

If light from a distant star passes close to a massive body, the light beam will a) continue moving in a straight line. b) accelerate due to gravity. c) slow down. d) change color to a shorter wavelength. e) bend towards the star due to gravity.

E

In the Lighthouse Model, a) the period of pulsation must speed up as the neutron star continues collapsing. b) the star literally turns on and off like a lighthouse beacon. c) all pulsars must have their poles pointed directly toward us. d) the period of pulsation slows down due to the drag of the remnant on its field. e) if the beam sweeps across us, we will detect a pulse of radiation.

E

The brightest stars in aging globular clusters will be a) blue stragglers. b) massive blue main-sequence stars like Spica. c) blue supergiants like Rigel and Deneb. d) core stars of planetary nebulae. e) red supergiants like Betelgeuse and Antares.

E

The mass range for neutron stars is a) 0.08 to 0.4 solar masses. b) 0.4 to 3 solar masses. c) 1.4 to 3 solar masses. d) 3 to 8 solar masses. e) 6 to 11 solar masses.

E

What would happen if more mass was added to a 1.4-solar-mass neutron star? a) It would erupt as a Type I supernova. b) It could eventually become a black hole, via a hypernova explosion. c) All of its protons and electrons would turn into quarks. d) It would grow larger, temporarily becoming a red giant again. e) It would blow off mass as an X-ray burster.

E

When a low mass star first runs short of hydrogen in its core, it becomes brighter because a) it explodes as a nova. b) helium fusion gives off more energy than does hydrogen. c) the helium flash increases the size of the star immensely. d) its outer, cooler layers are shed, and we see the brighter central core. e) the core contracts, raising the temperature and extending the hydrogen burning shell outward.

E

Which are the two most popular candidates for gamma-ray bursters? a) formation of uranium in the core of a supergiant, and collisions of white dwarfs b) mergers of two black holes, and merger of a neutron star and a white dwarf c) hypernova making pulsars, and mergers of two white dwarfs d) collisions between a white dwarf and a giant, and merger of two neutron stars e) hypernova making a black hole, and merger of two neutron stars

E

Dark nebulae are opaque to all wavelengths of electromagnetic radiation. (t or f)

False

Neutron stars are 100,000 times denser than white dwarfs. (t or f)

False

The "pulse" from a pulsar is due to the rapidly expanding and contracting outer shell of the star. (t or f)

False

The gas in the interstellar medium consists of mostly heavy elements. (t or f)

False

The least massive red main-sequence stars may have lifetimes of a trillion years. (t or f)

True

While more massive than most of its neighbors, our Sun is still technically a low-mass star. (t or f)

True

While most pulsars slow down over time, millisecond pulsars spin faster due to mass transfer from a close companion. (t or f)

True

Light from distant stars that must pass through dust arrives bluer than when it left its star (t or f)

false