#3 – Space

Flashcard maker : Lily Taylor
The space between stars is known to contain
gas, both atomic and molecular, and dust.
Which of the following objects is NOT an abundant physical component of the interstellar medium?
radioactive elements
Similar star clusters at different distances from the Sun appear to be fainter than expected by distance alone because
light is scattered and absorbed by interstellar dust and gas between distant clusters and the Earth.
Interstellar extinction is the
reduction of the apparent brightness of stars by scattering and absorption of their light by intervening interstellar clouds.
What do a reflection nebula surrounding a star and Earth’s atmosphere have in common?
The reflection nebula and Earth’s atmosphere both appear blue because of preferred scattering of this color of light.
The apparent reddening of light from stars after its passage through the interstellar medium (ISM) is caused by
preferential scattering of blue starlight by fine dust grains.
The light from a distant cloud of gas and dust looks distinctly red to the unaided eye. When a spectrum is taken, the red color is found to come from a single, bright spectral line. Thus the red color in this situation is due to
the Balmer spectrum of hydrogen.
How much of the visible mass of the Milky Way Galaxy is in the form of gas and dust spread out between the stars?
10%
What are the two most abundant elements in the universe?
hydrogen and helium
In which part of the electromagnetic spectrum are molecules most easily detected?
radio waves
What fraction of the mass of a typical interstellar cloud is hydrogen?
74%
What fraction of the mass of a typical interstellar cloud is helium?
25%
Which of 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?
blue
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. 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 a way 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 molecular clouds?
gravitational contraction of a hot gas cloud
What is 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 a single-star or a multiple-star system?
amount of rotation (spin)
Accretion of matter in an interstellar cloud leads to
a protostar.
What is 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.
The T 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.
What is 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
age.
The definition 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.
Because all the stars in a cluster begin forming at the same time, the stars arrive on the main sequence at
different times, more massive stars arriving first.
If we plotted the stars in a young star cluster on a Hertzsprung-Russell diagram, we would expect to see
the more massive stars on the main sequence and the less massive stars above the main sequence.
In the Hertzsprung-Russell diagram, how does the position of a typical star change while it is at the main-sequence phase of its evolution?
A star’s position on the main sequence is determined only by its mass, not its age, so stars do not move along the main sequence during evolution.

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