Download The correct answers are written in bold, italic and underlined. The

Comins DEU 3e Ch 09 Quiz 1
The correct answers are written in bold, italic and underlined.
The most important questions to study for the exam are highlighted.
1. Which of the following methods has NOT been used to study the material of the
interstellar medium?
• Scattering of starlight
• Collection of dust and gas by spacecraft
• Absorption of light from more distant stars
No spacecraft has yet reached beyond our solar system into interstellar space. The
Pioneer and Voyager spacecraft are still exploring the far reaches of the heliosphere, the
space carved out in interstellar space by the solar wind.
2. The most abundant molecular constituent in interstellar space is molecular hydrogen,
H2, but it is difficult to detect. Which easily detected molecule, whose abundance ratio to
molecular hydrogen is reasonably constant, is found to accompany this molecule in
space, the measurement of which provides an estimate of the amount of molecular
hydrogen?
• CO, carbon monoxide
• CO2, carbon dioxide
• H2O, water vapor
CO emits strongly at 2.6 mm in the microwave region of the electromagnetic spectrum,
and there appears to be a well-defined relationship between the abundance of CO and the
abundance of H2.
3. Which of the following common molecules found in interstellar space contains
nitrogen atoms but no oxygen?
• Formaldehyde
• Methane
• Ammonia
Ammonia is NH3 and contains N but no O.
4. The best place to search for stars that are in the process of forming is
• in cold regions inside interstellar clouds.
• between interstellar clouds, where the density is low enough not to block our
view.
• inside large, hot regions of ionized hydrogen gas.
With the low temperature, the atoms and molecules are moving slowly enough that
gravity can pull them together and cause them to collapse into stars.
5. The typical mass of a molecular cloud appears to be
• between a few hundred and a few thousand solar masses.
• about 106 solar masses.
• approximately 1 billion solar masses.
Molecular cloud masses seem to be in this range, with no wide variation in mass.
6. The apparent reddening of light from stars after its passage through the interstellar
medium (ISM) is caused by
• the additional contribution to this starlight by emission from hydrogen gas in the
ISM.
• preferential scattering of blue starlight by fine dust grains.
• scattering of this light from rapidly moving material, this light being Dopplershifted toward the red end of the spectrum.
Fine particles of dust scatter blue light preferentially in a process similar to that which
operates in our own atmosphere to produce the blue sky.
7. Which of the following mechanisms is NOT thought to be significant in the triggering
and formation of new stars within the interstellar medium?
• The gravitational condensation of a single massive cloud
• The collision of two cold interstellar clouds
• The condensation of matter by the shock wave from a nearby supernova
In the slow condensation of a gas cloud, gas temperatures build up and the resulting
random atomic and molecular motions will disperse the cloud.
8. A protostar, formed in the center of a condensing mass of dust and gas, begins to glow
as it slowly warms. What process generates this heat?
• Radiation from nearby stars
• Nuclear fusion processes at its center
• The compression of the dust and gas by gravity
Condensation of matter will lead to heating, the source of the energy being gravitational
potential energy released as random kinetic energy of hot gas.
9. In a supernova, we are witnessing the violent death of a star. In what way will this
event also lead to the birth of a new star or stars?
• Intense radiation from the explosion will heat the surrounding gas, thereby
triggering the condensation of matter into new stars.
• Implosive forces acting inward at the center of the supernova will compress
matter into a new white dwarf star.
•
The shock wave produced by the expanding shell will compact the material and
stimulate new star birth as it impacts on a molecular cloud.
This process appears to be very effective. Such an explosion may have triggered the birth
of our solar system.
10. Which wavelength range in the electromagnetic spectrum has proven to be the most
useful in investigating star birth in dense molecular clouds?
• Infrared radiation emitted by warm regions and protostars
• Long wavelength radio waves
• Ultraviolet and X radiation
The peak emission for dense, warm objects will be at infrared wavelengths, and this
radiation can penetrate the dust and gas clouds.
11. What will be the influence of rotational motion on a dense molecular cloud of dust
and gas as it condenses into stars?
• Condensation will only take place if rotation is very slow because high rotation
will cause material to be spun out of the condensation.
• Slow rotation will permit a single star to condense, while faster rotation will
produce double or multiple stars or even planetary systems.
• It will have no effect at all because gravitational attraction will easily overcome
the effects of cloud rotation.
Rotation determines whether the cloud condenses into a single star or breaks up into two
or more stars.
12. Which physical process generates the force inside a pre-main-sequence star to stop
the star from slowly condensing and shrinking by offsetting the force of gravity, thereby
resulting in a stable main-sequence star?
• No physical process can prevent this condensation until a black hole is produced
at the center of the star. Condensation and shrinking of the star continues slowly
throughout its lifetime.
• The additional heat generated by the nuclear fusion, starting when the
temperature reaches a certain limit, produces an increase in internal gas
pressure.
• Degeneracy pressure from electrons, caused by the quantum-mechanical Pauli
Exclusion Principle when the electrons are forced very close together, generates
extra pressure.
When the core temperature reaches 107 K, fusion begins to generate extra energy that
heats the gas to produce sufficient pressure to oppose the force of gravity.
13. An evolutionary track of a star is
•
•
•
the line traced out across a Hertzsprung-Russell diagram delineating the
luminosity-temperature values of stars that have just become pre-main-sequence
stars.
the line traced across the Hertzsprung-Russell diagram as luminosity and
temperature change during the star's evolution.
the line across the Hertzsprung-Russell diagram marking the luminositytemperature values for stars that have begun to generate energy in their interiors
by nuclear fusion.
These tracks follow the evolution of stars of different masses across the HertzsprungRussell diagram.
14. A brown dwarf star is
• a low-mass star that never achieves thermonuclear fusion in its core.
• the core of a red giant star, revealed when the outer layers are shed during the
planetary nebula phase.
• a white dwarf star that has cooled to a low temperature over its long lifetime.
Stars with masses less that 8% of that of the Sun remain as long-lived, dim brown
objects, never brightening like a main-sequence star.
15. Why have few, if any, stars with greater than about 120 solar masses ever been
detected?
• Because the mass of material within the original giant molecular clouds never
exceeds this mass
• Because protostars of this mass take a very long time to heat up and produce
visible light, and the universe has not existed for long enough for these stars to
become visible
• Because very massive protostars rapidly develop extremely high temperatures
and the resultant ultraviolet and visible light repels further gas and prevents
further build-up of mass onto the star
The initial condensation of a very large mass will be a violent and rapid process, and the
radiation pressure repels further mass, stabilizing the star at around 120 solar masses.
16. T Tauri stars are
• regularly pulsating post-main-sequence stars of about 2 solar masses.
• small low-mass red stars whose core temperatures will never become high enough
to induce nuclear fusion.
• highly variable, pre-main-sequence stars.
These rather unstable young stars appear to be ejecting significant mass as they settle
toward the main sequence.
17. A region of interstellar space shining with a reddish hue is
• a region of cool gas and dust.
•
•
a cold core of a giant molecular cloud.
a region of hot, ionized gas.
The region is ionized because ultraviolet light from hot stars has ejected the electrons
from hydrogen and other atoms. When the electrons recombine with protons to form
neutral hydrogen atoms again, they emit photons of red light (Balmer Hα photons).
18. Which stars are responsible for the ionization of the gas in an H II emission nebula,
resulting in the emission from Balmer Hα and other atomic transitions from
recombination of electrons and ionized atoms in this region?
• Hot O and B stars
• Very hot white dwarf stars
• Red giant and supergiant stars
The predominant emission from these hot stars is ultraviolet radiation that ionizes
hydrogen and other atoms. Recombination of electrons with the nuclei and ions produces
Hα and other atomic line emissions.
19. Main-sequence stars are different from all other stars because
• hydrogen is being converted to helium in a shell around the cores of these stars.
• they are all stars of low mass.
• hydrogen is being converted to helium in the core of these stars.
In fact, this is the definition of a main-sequence star.
20. Which of the following is true about open clusters?
• One star in an open cluster will eventually undergo a supernova explosion that
will quickly disperse the other stars.
• The motions of individual stars are such that all open clusters will eventually
disperse.
• They will all slowly condense into globular clusters, the stars having driven off
the remaining interstellar dust and gas.
Stars in an open cluster are not gravitationally bound into a stable configuration and will
eventually disperse.