exam 3

TRUE/FALSE.

1.  All stars spend approximately the same amount of time on the main sequence.

FALSE

TRUE/FALSE.

2. Gravitational equilibrium means that the surface and the core of the Sun are at the same pressure.

FALSE

TRUE/FALSE

3.  Blue stars are always more luminous than red stars.

FALSE

TRUE/FALSE

4. The temperature of the Sun’s core is about 20,000 K.

FALSE

TRUE/FALSE.

5.  The more distant a star, the smaller its parallax.

TRUE

TRUE/FALSE.

6. No visible light can escape a black hole, but things such as gamma rays, X-rays, and neutrinos can.

FALSE

TRUE/FALSE.

7.  Although the Sun does not generate energy by gravitational contraction today, this energy-generation mechanism was important when the Sun was forming.

TRUE

TRUE/FALSE.

8.  Our Sun will end its life in a planetary nebula and become a white dwarf.

TRUE

TRUE/FALSE.

9.  Stars with high masses live longer than stars with lower masses.

FALSE

TRUE/FALSE

10. The apparent brightness of a star depends only on its luminosity.

FALSE

Which of the following methods has led to the most discoveries of extrasolar planet candidates?

•  A) Detecting a planet ejected from a binary star system

•  B) Detecting the shift of the star’s position against the sky due to the planet’s gravitational pull

•  C) Detecting the infrared light emitted by the planet

•  D) Detecting the gravitational effect of an orbiting planet by looking for the Doppler shifts in the star’s spectrum

•  E) Detecting the dip in measured brightness as the planet crosses our line of sight to the star

E) Detecting the dip in measured brightness as the planet crosses our line of sight to the star

A star’s luminosity is the:

•  A) Lifetime of the star

•  B) Surface temperature of the star

•  C) Total amount of energy that the star radiates each second

•  D) Total amount of energy that the star will radiate over its entire lifetime

•  E) Apparent brightness of the star in our sky

C) Total amount of energy that the star radiates each second

What do we mean when we say that the Sun is in gravitational equilibrium?

•  A) The Sun always has the same amount of mass, creating the same gravitational force

•  B) The hydrogen gas in the Sun is balanced so that it never rises upward or falls downward

•  C) There is a balance within the Sun between the outward push of pressure and the inward pull of gravity

•  D) The Sun maintains a steady temperature

C) There is a balance within the Sun between the outward push of pressure and the inward pull of gravity

Which of the following sequences correctly describes the stages of life for a low-mass star?
A) red giant, protostar, main-sequence, white dwarf
B) protostar, main-sequence, red giant, white dwarf
C) white dwarf, main-sequence, red giant, protostar
D) protostar, main-sequence, white dwarf, red giant
E) protostar, red giant, main-sequence, white dwarf

B) protostar, main-sequence, red giant, white dwarf

The core of the Sun is:
A) composed of iron
B) much hotter and much denser than its surface
C) constantly rising to the surface through convection
D) at the same temperature and density as its surface
E) at the same temperature but much denser than its surface

A) composed of iron

Why does stellar main-sequence lifetime decrease with increasing stellar mass?
A) It doesn’t; higher mass stars have more hydrogen available for fusion, and thus have longer lifetimes
B) Higher core temperatures cause fusion to proceed much more rapidly
C) Higher outward pressure prevents the core hydrogen from being replenished by the star’s outer layers
D) Strong stellar winds cause higher mass stars to lose mass quickly

B) Higher core temperatures cause fusion to proceed much more rapidly

Since all stars begin their lives with the same basic composition, what characteristic most determines how they will differ?
A) color they are formed with
B) luminosity they are formed with
C) mass they are formed with
D) time they are formed
E) location where they are formed

C) mass they are formed with

At the center of the Sun, nuclear fusion converts hydrogen into:
A) hydrogen compounds such as methane
B) radioactive elements such as uranium and plutonium
C) helium, gamma rays, and neutrinos
D) radiation and elements such as carbon and nitrogen
E) molecular hydrogen

C) helium, gamma rays, and neutrinos

What are the two primary methods by which planets have been found around other stars in our galaxy?
I) Direct images in visible and infrared light
II) Indirectly by detecting the motion of the host star
III) Indirectly by measuring the drop in brightness of the host star when the planet crosses our line of sight
A) I and III
B) I and II
C) II and III

B) I and II

After a massive-star supernova, what is left behind?
A) always a black hole
B) always a neutron star
C) always a white dwarf
D) either a white dwarf or a neutron star
E) either a neutron star or a black hole

E) either a neutron star or a black hole

What happens to the core of a star after it ejects a planetary nebula?
A) It becomes a white dwarf
B) It breaks apart in a violent explosion
C) It contracts from a protostar to a main-sequence star
D) It becomes a neutron star
E) None of the above

A) It becomes a white dwarf

Which of the following is closest in size (radius) to a white dwarf?
A) the Sun
B) a basketball
C) the Earth
D) a football stadium
E) a small city

C) the Earth

The age of small stars in a cluster can be determined by:
A) counting the number of stars in each spectral class
B) finding spectroscopic binaries in the cluster
C) finding pulsating variable stars in the cluster
D) determining the main-sequence turnoff point
E) fitting the position of the main sequence to the Sun

D) determining the main-sequence turnoff point

What eventually halts the gravitational collapse of an interstellar gas cloud that forms an object that is massive enough to become a star?
A) the crowding of electrons in the core
B) a critical fraction of the gas has been driven further into space
C) the central object becoming hot enough to sustain nuclear fusion in its core
D) nothing; all collapsing gas clouds become black holes

C) the central object becoming hot enough to sustain nuclear fusion in its core

What happens when a star like the Sun exhausts its core hydrogen supply?
A) Its core contracts, but its outer layers expand and the star becomes bigger and brighter
B) It expands, becoming bigger but dimmer
C) It contracts, becoming smaller and dimmer
D) Its core contracts, but its outer layers expand and the star becomes bigger but cooler and therefore remains at the same brightness
E) It contracts, becoming hotter and brighter

A) Its core contracts, but its outer layers expand and the star becomes bigger and brighter

What causes the radio pulses of a pulsar?
A) A black hole near the neutron star absorbs energy and re-emits juger it as radio waves
B) The neutron star’s orbiting companion periodically eclipses the radio waves that the neutron star emits
C) The vibration of the neutron star causes nuclear fusion that generates the radio pulses
D) As the neutron star spins, beams of radio radiation sweep through space. If one of the beams crosses the Earth, we observe a pulse
E) The neutron star spins

D) As the neutron star spins, beams of radio radiation sweep through space. If one of the beams crosses the Earth, we observe a pulse

Why does a star grow larger after it exhausts its core hydrogen?
A) The internal radiation generated by the hydrogen fusion in the core has heated the outer layers enough that they can expand after the star is no longer fusing hydrogen
B) The outer layers of the star are no longer gravitationally attracted to the core
C) Hydrogen fusion in a shell outside the core generates enough thermal pressure to push the upper layers outward
D) Helium fusion in a shell outside the core generates enough thermal pressure to push the upper layers outward
E) Helium fusion in the core generates enough thermal pressure to push the upper layers outward

C) Hydrogen fusion in a shell outside the core generates enough thermal pressure to push the upper layers outward

Which event marks the beginning of a supernova?
A) The sudden outpouring of X-rays from a newly formed accretion disk
B) The onset of helium burning after a helium flash in a star with mass comparable to that of the Sun
C) The beginning of neon burning in an extremely massive star
D) The expansion of a low-mass star into a red giant
E) The sudden collapse of an iron core into a compact ball of neutrons

E) The sudden collapse of an iron core into a compact ball of neutrons

Why do sunspots appear dark in pictures of the Sun?
A) They are extremely hot and emit all their radiation as X-rays rather than visible light
B) They actually are fairly bright, but appear dark against the even brighter background of the surrounding photosphere
C) They are holes in the solar surface through which we can see through to deeper, darker layers of the Sun
D) They are too cold to emit any visible light

B) They actually are fairly bright, but appear dark against the even brighter background of the surrounding photosphere

A star of spectral type G lives approximately how long on the main sequence?
A) 10 billion years
B) 1,000 years
C) 1 billion years
D) 10,000 years
E) 10 million years

A) 10 billion years