Astronomy Exam 3

The evolutionary path of the Sun away from the Main Sequence is shown to the right. For each labelled stage (A-E) describe the energy

A: Hydrogen fusion B: H-Shell fusion and He core fusion C: H-Shell fusion and C core fusion D: Double-shell fusion E: Electron Degeneracy Pressure

During which of the following phases of the Moon is the Moon closer to the Sun than the Earth is? a) full b) new c) 1st quarter d) 3rd quarter

b

A red giant orbiting a black hole begins to spiral toward the black hole and you are observing it from a very far distance. (1) What would you observe to happen to the red giant as it approaches the event horizon? (2) Would you ever see the red giant pass through the event horizon? (3) Why does this behavior occur near a black hole? That is, how do astrophysicists describe the physics near a black hole? (4) What happens to the size of the black hole, that is, its Schwarzschild radius, as it swallows up the mass of the red giant?

1) You would see the red giant begin to stretch 2) The red giant would stay this way as it slowly turned red and then became invisible as it redshifted 3) The gravity at one end of the red giant is so much stronger which causes that end to be pulled and stretched 4) The Schwarzschild radius becomes bigger

Would a red supergiant be likely to have a planet around it with advanced civilizations, such as we have on Earth?

A supergiant would evolve much faster than the Sun, so life would not have had much time to evolve. In addition, rocky planets close in to the star would be burned when the star becomes a supergiant

A planetary nebula lasts forever

False

All planets move in elliptical orbits with the Sun at its center

False

The Sun will end its life as a red dwarf

False

The heaviest element produced in a supernova is silicon

False

Suppose that you observe a spaceship moving directly towards a Schwarzschild black hold. What would you observe happen to the spaceship? Would you ever see the spaceship cross the event horizon of the black hole? Explain why or why not

The spaceship would be observed to remain at the event horizon and be stretched. It would never be seen crossing the event horizon. Its color would shift towards red and then the infrared so that the spaceship would become invisible

What causes a star to become a Type II supernova? Please cite the types of stars involved, the final reactions, the reasons for the explosion, the role of the supernovae in a galaxy, and the possible remnants

The star must be an isolated, massive star. It fuses elements until the core comes Fe. Iron cannot create the outward pressure necessary to resist gravity because it absorbs energy. The inner core becomes protons and neutrons and the outer core collapses in and then bounces off the inner core creating a supernova. Heavier elements are produced by the neutrons and electrons colliding and these help form other stars or planets. The star becomes aa UD or NS.

Nothing can escape from a black hole

True

Describe the mechanism of a supernova. What types of remnants can be left behind after a supernova? What is the specific remnant in the Crab Nebula?

When the core of a massive isolated becomes iron, fusion cannot provide the outward pressure needed to support the star. The core is turned into neutrons and the outer core collapses and "bounces" off the inner core. The shockwave is observed as a supernova in which additional elements heavier than iron are formed

On which of the following quantities does the Schwarzschild radius depend? a) only the mass of the black hole b) whether the black hole was formed in a massive star supernova or in some other way c) the radius of the black hole, as measured by careful observations of its size d) both the mass and chemical composition of the black hole

a

Suppose a white dwarf is gaining mass because of accretion in a binary system. What happens if the mass someday reaches the 1.4-solar-mass limit? a) the white dwarf undergoes a catastrophic collapse, leading to a type of supernova that is somewhat different from that which occurs in a massive star but is comparable in energy b) a white dwarf can never gain enough mass to reach the limit because a strong stellar wind prevents the material from reaching it in the first place c) the white dwarf immediately collapses into a black hole, disappearing from view d) the white dwarf, which is made mostly of carbon, suddenly becomes much hotter in temperature and therefore is able to begin fusing the carbon. This turns the white dwarf back into a star supported against gravity by ordinary pressure

a

The energy emitted by a star increases as the fourth power of its surface temperature and a red giant star has a lower temperature than the Sun. Why then is the luminosity of a red giant about a thousand times larger than that of the Sun? a) Because it is bigger and therefore has a much larger surface area. b) Because the energy transfer from red giant's interior is much more efficient than it is in the Sun. c) Because more of its total output of energy is in the visible range than is that of the Sun and hence the red giant appears brighter. d) Because the red giant's nuclear furnace is producing much more energy than that of the Sun.

a

What prevents a neutron star from collapsing and becoming a black hole? a) in a neutron star the inward gravitational pull is balanced by an outward force due to neutron degeneracy b) in a neutron star the inward gravitational pull is countered by the centrifugal force due to their rapid rotation c) an outward force due to gas pressure, as in the Sun, balances gravity in the neutron star d) neutron stars are solid, and they like any solid sphere are held up by the repulsive force between atoms in the solid

a

In Type Ia supernovae, a white dwarf in a close binary accretes mass from its companion that causes the star a) to become a black hole b) to suffer carbon detonation in which it is totally destroyed c) to swallow up its companion and become a red supergiant d) to turn into solid neutrons and become a neutron star

b

Mars moves in an elliptical orbit around the Sun. The location of the Sun relative to their ellipse is a) at the exact center of the ellipse b) at the focus that is closer to the point where Mars moves the fastest c) at one end of the major axis of the ellipse d) at the focus that is closer to the point where Mars is moving the slowest

b

Observationally, how can we tell the difference between a white-dwarf supernova and a massive-star supernova? a) a Type II supernova happens only once, while a Type I can repeat periodically b) the spectrum of a Type II supernova shows prominent hydrogen lines, while the spectrum of a Type I supernova does not c) the light of a Type I supernova fades steadily, while the light of a Type II supernova brightens for many weeks d) we cannot yet tell the difference between the two types of supernovae

b

Referring to the figure, which object has the longest orbital period? a) B b) C c) D d) E

b

Suppose you drop a clock toward a black hole. As you look at the clock from a high orbit, what will you observe? a) time on the clock will run faster as it approaches the black hole, and light from the clock will be increasingly blueshifted b) time on the clock will run slower as it approaches the black hole, and light from the clock will be increasingly redshifted c) the clock will fall faster and faster, reaching the speed of light as it crosses the event horizon d) the clock will fall toward the black hole at a steady rate, so that you'll see it plunge through the event horizon within just a few minutes

b

The "star" that is seen at the center of a planetary nebula, like the Clown (or Eskimo) Nebula is a) a planet in the process of formation b) a small, hot, and very dense white dwarf star c) the accretion disk around a black hole d) composed almost entirely of neutrons, and is spinning rapidly

b

The figures below show two identical one solar-mass stars and four planets (A-D) of different masses in circular orbits of various sizes. Which situation has the *shortest* orbital period? a) A only b) A and B c) D only d) C and D

b

What conditions are required for a solar eclipse? a) the phase of the Moon must be new, and the Moon's orbital plane must lie in the ecliptic b) the phase of the Moon must be new, and the nodes of the Moon's orbit must be nearly aligned with Earth and the Sun c) the phase of the Moon must be full, and the nodes of the Moon's orbit must be nearly aligned with Earth and the Sun d) the phase of the Moon must be full, and the Moon's orbital plane must lie in the ecliptic

b

What makes a pulsar pulse? a) a white dwarf in a binary star system is periodically eclipsed by its companion star b) a rapidly spinning, magnetized neutron star emits light and radio waves along its magnetic axis c) a high-angular-momentum (rapidly spinning) black hole pulsates and sends out regularly spaced pulses of electromagnetic radiation d) a star vibrates

b

When a star leaves the main sequence and expands towards the red giant region, what is happening to the star? a) Helium burning is taking place in a spherical shell just outside the core; the core itself is almost pure carbon and oxygen. b) Hydrogen burning is taking place in a spherical shell just outside the core; the core itself is almost pure helium. c) Hydrogen burning is taking place in a spherical shell just outside the core; the core has not yet started thermonuclear reactions, and is still mostly hydrogen. d) Helium is being converted into carbon and oxygen in the core

b

The characteristics shared by all solitary white dwarf stars are that a) they have stopped generating thermonuclear energy but continue to shrink, thereby releasing gravitational energy as heat b) they have mass less than 1.4 solar masses c) they are generating thermonuclear energy but are maintaining a constant radius, and hence are not releasing gravitational energy d) they have ceased to generate energy by thermonuclear processes or gravitational contraction and are slowly cooling down

b or d

A black hole is so names because a) its spectrum has the same characteristics as a black body at a temperature of 150K b) it emits no visible light because it is cold, less than 10K c) no light can escape from inside its event horizon d) all their electromagnetic radiation is shifted into the infrared and so they are invisible

c

A high-mass star near the end of its life undergoes successive cycles of energy generation within its core in which gravitational collapse increases the temperature to the point where a new nuclear fusion cycle generates sufficient energy to stop the collapse. This process does not work beyond the silicon fusion cycle that produces iron. Why? a) the pressure from high-energy photons and neutrinos at the very high core temperatures reached at this stage of development is finally sufficient to halt the collapse b) electrostatic forces between the highly charged iron nuclei are sufficient to overcome the collapse and stabilize the stellar core c) fusion of iron and nuclei into heavier nuclei requires energy rather than producing excess energy and therefore will not produce the additional gas pressure to halt the collapse d) iron nuclei are so large that they occupy all remaining space and so the collapse cannot continue

c

As massive stars evolve, they burn successively heavier elements (helium, carbon, oxygen, and silicon) in their cores. Why is a higher temperature needed in each succeeding stage? a) higher temperatures, and hence higher particle velocities, are needed in order that these heavier nuclei can avoid falling into a black hole in the center of each stellar core and can undergo nuclear fusion while remaining in orbit around it b) higher temperatures are needed in order to ionize the heavier atoms fully (i.e. remove all the electrons) so nuclear fusion can take place c) heavier nuclei have higher electric charges, so higher speeds are needed for the nuclei to overcome the electrostatic repulsion and undergo nuclear fusion d) none of the above reasons

c

The Clown (or Eskimo) Nebula was formed in the death of a ____ mass star and has a ___ ____ in the center, which is the _____ of the former star a) high, neutron star, core b) high, black hole, event horizon c) low, white dwarf, core d) low, neutron star, pulsar

c

The diagram shows the Earth and Sun as well as five different positions for the Moon. Which position of the Moon would cause it to appear like the picture below when viewed from Earth? a) A b) B c) C d) D

c

What can astronomers learn about a star from a track of its evolution on a HR diagram? a) how long ago it was born b) where it is located c) what surface temperature and luminosity it will have at each stage d) when it will die

c

What is the period of the orbit of object B shown in the figure? a) 1 year b) 2 years c) square root of 8 d) 4 years

c

Which of the following best describes why the Moon exhibits phases? a) the shadow of the Earth hitting the Moon causes phases b) the Moon actually orbits the Sun c) the relative positions of the Moon and Sun cause the phases d) the Moon's orbit is inclined by 5^0 so phases must occur

c

Which of the following statements about black holes is NOT true? a) if you watch someone else fall into a black hole, you will never see him or her cross the event horizon. However, he or she will fade from view as the light he or she emits (or reflects) becomes more and more red shifted b) a black hole is truly a hole in spacetime, through which we could leave the observable universe c) if the Sun magically disappeared and was replaced by a black hole of the same mass, the earth would soon be sucked into the black hole d) if we watch a clock fall towards a black hole, we will see it tick slower and slower as it falls nearer to the black hole

c

You discover a binary star system in which one star is a 15 MSun main-sequence star and the other is a 10 MSun giant. How do we think that a star system such as this might have come to exist? a) although both stars probably formed from the same clump of gas, the more massive one must have had its birth slowed so that it became a main-sequence star millions of years later than its less massive companion b) despite the low odds of finding a system with two such massive stars, there is nothing surprising about the fact that such systems exist c) the giant must once have been the more massive star but transferred some of its mass to its companion d) the main-sequence star probably is a pulsating variable star and therefore appears to be less massive than it really is

c

in a supernova we are witnessing the violent death of a star. In what way does this event lead to the birth of new stars? a) implosive forces acting inward at the center of the supernova will compress matter into new white dwarf stars b) intense radiation from the explosion will heat the surrounding gas, thereby triggering the condensation of matter into new stars c) the shock wave produced by the expanding shell, as it impacts upon clouds of gas and dust, will compress the material, enrich the cloud with heavy elements, and trigger new star formation d) the black hole remnant of the supernova swallows nearby gas and dust, thereby forming new stars

c

A neutron star's size is that of a) the Earth b) the Earth's orbit c) Texas d) a typical city

d

According to the heliocentric theory, which of the following objects can never transit (pass in front of) the Sun as seen from the Earth? a) The Moon b) Mercury c) Venus d) Mars

d

During a supernova explosion involving a high-mass star, what happens to the iron in the core of the star? a) iron is inert and does not react, so it remains as a dense iron core after the explosion b) the iron is thrown out into space, enriching the interstellar medium c) the iron remains intact inside the supernova remnant d) the iron nuclei are broken up by high-energy photons into their individual protons and neutrons (photodisintegration)

d

During helium burning in a star's later life, the chemical element produced by the combination of helium nuclei is a) the light isotope of helium, 3He b) heavy hydrogen, 2H c) beryllium, 8Be d) carbon, 12C

d

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? a) the nuclei are split apart by neutron bombardment, creating lighter elements such as carbon, oxygen, and neon b) it is locked up inside the star forever c) it is destroyed by later thermonuclear fusion reactions in the core that create even heavier elements such as lead, gold, and uranium d) it is torn apart by high-energy photons at the end of the star's life through a process known as photo disintegration

d

Electron degeneracy can stop the crushing force of gravity a) in neutron stars b) in the core of a star during and immediately after the red giant or super giant stage through the fusion of helium c) during the protostar stage of the formation of a star d) in white dwarf stars as long as the mass is less than 1.4 solar masses

d

If a person, or a star, where to travel near the event horizon of a black hole, it would experience what is known as spaghettification, in which a) the extreme curvature of spacetime would stretch and squeeze the object b) tidal forces would cause the object to be pulled into a thin shape c) gravity exerted from the singularity would be much stronger at one end of the object from the other causing extreme stretching d) all of the above are correct descriptions

d

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 a) contracting, cooling, and hence becoming less luminous b) expanding, heating up, and becoming more luminous c) contracting, becoming hotter, and becoming much less luminous d) expanding, cooling, and becoming more luminous

d

It's 3am and you can't sleep. You look out the window and see the Moon rising in the East. The phase is a) third quarter b) new c) first quarter d) waning crescent

d

On Earth, a paperclip made of neutron star material would weigh as much as a) any other paperclip b) a million paperclips c) a dozen elephants d) Mt. Everest

d

Stars are formed from interstellar matter. Why then are stars in open clusters metal-rich, while stars in globular clusters are metal-poor? a) Because open clusters contain very hot stars that produce metals by nuclear reactions in their outer layers b) Because globular clusters have "burned" their heavy elements over their longer lifetime c) Because globular clusters are too young for their stars to have produced any significant amount of metals d) Because open clusters are young and stars have formed from material that has been enriched in metals by supernova explosions of previous stars

d

The nova phenomenon, an occasional and sometimes repeated intense brightening of a star by a factor of about 10^6, is caused by a) a beam of radiation from a nearby pulsar, illuminating the surface of a red giant star and inducing rapid and intense heating b) the capture and rapid compression of matter by a black hole c) the explosion of a single massive star at the end of its thermonuclear fusion phases d) explosive hydrogen fusions on the surface of a white dwarf star after mass transfer from a companion star in a binary system

d

What is the physical reason for the apparent retrograde notion of the planets? a) as Earth passes another planet, its gravitational pull slows down the other planet so that it appears to be traveling backward b) apparent retrograde motion is an illusion created by turbulence in Earth's atmosphere c) when planets are farther from the Sun, they move slower than when they are nearer the Sun; it is during their slower period that they appear to move backwards d) as Earth passes another planet, the other planet appears to move backward with respect to the background stars, but the planet's motion does not really change

d

Who discovered that the orbits of planets are ellipses by analyzing data on Mars? a) Ptolemy b) Tycho Brahe c) Copernicus d) Johannes Kepler

d

Why does it require higher gas temperatures in the core of a star in order to produce nuclear fusion of helium compared to that required for hydrogen? a) He nuclei need to be moving faster in order to avoid the more numerous and faster H nuclei, with which they can combine with no energy generation. b) Higher speeds are needed between two He atoms in order to overcome the weak nuclear force. c) Higher atomic speeds are required in order to remove the two electrons from each helium atoms rather than one electron from each hydrogen before fusion can take place. d) Higher collision speeds are needed to overcome the greater electric repulsion between doubly charged He nuclei

d

The star Sirius is actually a binary star in which the brighter, more massive star (Sirius A) is an A-type main-sequence star and the fainter, less-massive star (Sirius B) is a white dwarf. Students are asking how is this system most likely formed. Four sample responses are given below. Which explanation, if any, is correct and what are your reasons? a) Sirius A formed as a single star and later captured a passing white dwarf which become Sirius B b) Sirius B formed first and evolved to become a white dwarf. The capture of gas and dust from interstellar clouds then resulted in the formation of Sirius A c) Sirius B was initially more massive than Sirius A and evolved faster; it then became less massive due to mass loss to space and mass transfer to Sirius A d) Sirius A was always the more massive of the two and became a red giant; then mass transfer to Sirius B accelerated the evolution of Sirius B and caused it to become a white dwarf

d. Astronomers believe that mass transfer can occur in close binaries and, as a result, the evolution of the stars is altered. An example is Algol