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Astronomy 102 Ch.16-20

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The main source of the tremendous amount of energy produced in the core of the Sun each second is
a chain of nuclear fusion reactions in which hydrogen nuclei (protons) combine to produce helium nuclei.
Which two parameters must be in balance within the Sun in order for the Sun to be in hydrostatic equilibrium?
the inward force of gravity and the outward force due to gas pressure
How is energy transported outward in the Sun's interior?
radiation in the core region and convection in the envelope
One new and innovative method for exploring the conditions within the solar interior is the measurement of
surface oscillations which are generated by sound waves passing through the solar interior.
A neutrino is
an uncharged subatomic particle with no or almost no mass and the ability to pass right through the Sun and the Earth without stopping.
When the number of neutrinos reaching the Earth from the Sun was first measured, it was only about a third of the number predicted theoretically. How has this discrepancy been resolved?
Neutrinos can change "flavor" as they travel from the Sun to the Earth.
Does the Sun have a solid surface, and where or why not?
No, it does not, because the Sun is entirely a gas, from its surface right to its center.
We see more deeply into the Sun at its center than at its limb because of the interaction of the light with atoms of the gas. What conclusion can be reached therefore if the Sun appears less bright at its limb than at the center?
Temperature decreases with increasing height in the solar photosphere.
The photosphere of the Sun is the
visible surface of the Sun, where granules and sunspots are found.
Spicules on the Sun are found
mostly along the edges of supergranular cells in the chromosphere.
One interesting and somewhat unexpected characteristic of the solar corona is its
extremely high temperature, greater than 106 K.
The solar wind appears to originate primarily
in coronal holes, low density, low temperature regions on the high atmosphere of the Sun.
The number of sunspots on the Sun
varies with a cycle of approximately 11 years.
The continuous spectrum (i.e., the blackbody portion of the spectrum) of a sunspot, compared to that of the quiet solar surface, appears
somewhat redder.
If the material at the Sun's equator is seen to rotate about the Sun's axis with a period of 25 days, what is the equivalent period of rotation for the solar polar regions?
about 35 days, much longer than that of the equator
In the year 2003, 2 years after solar maximum, the Sun's magnetic north pole was in the Sun's northern hemisphere. Of the following years, which is the earliest in which we would expect to find the magnetic north pole in the Sun's southern hemisphere?
2011 A.D., after the first solar minimum following 2003
Which of the following statements is a CORRECT description of sunspots on the Sun?
Their number varies over a period of about 11 years, but there can be periods of several decades at a time when no or almost no sunspots are seen on the Sun.
The one physical parameter that transforms a region of quiet solar surface into an active and violent region is an increase in
the intensity of the magnetic field.
A giant loop of gas arching out from the visible edge of the Sun is called a
prominence.
The energy (such as light and heat) released by the Sun is produced by nuclear reactions in the Sun's core, which convert hydrogen into helium. According to Einstein's equation, <I>E</I> = <I>mc</I><sup>2</sup>, the mass of one helium nucleus should be
less than the mass of four hydrogen nuclei.
How do astronomers measure the distance to a nearby star?
They observe the apparent change in its position with respect to other, more distant stars over a period of 1 year.
A particular star is found to have a stellar parallax of 1/8 arcsec. What is the distance to this star?
8 pc
The component of a star's velocity in three-dimensional space that is measured by observing the shift of lines in its spectrum compared to the positions of equivalent lines in the laboratory is its
radial velocity.
A star of magnitude +8 is
fainter than a star of magnitude +5.
A star of magnitude -1 is
brighter than a star of magnitude +1.
Astronomers have found that an important indicator of a star's surface temperature is
the ratio of apparent brightness for two different colors, bV/bB .
In the visible spectra of stars, the Balmer absorption lines of hydrogen are produced when atoms are excited from n = 2 to higher levels. If hydrogen absorption lines are very strong in the visible spectrum of a particular star, what is the gas temperature on this star's surface?
It is a relatively high temperature, about 10,000 K, so significant numbers of electrons are excited from the ground state, n = 1, to the first excited state, n = 2, but not too many of them have been ejected completely from the atoms
Which of the following sequences of stellar classification types shows stars in order of increasing surface temperature?
K0, G9, G6, B2
Which of the following descriptions most closely describes the changes that would be observed if a star were to increase its temperature slowly over some period of time (e.g., a year)?
The peak of the star's continuum spectrum would move toward the blue, the intensity would increase at all wavelengths, and some absorption lines would become less deep while others would become deeper.
A certain star is seen to have a relatively low surface temperature but a very high luminosity. What can we conclude from these observations?
The star must be very large.
A distant star of spectral type M is found to be very bright and easily visible from Earth. How would you describe it, and what would be its place in the Hertzsprung-Russell diagram?
a red supergiant; upper right-hand corner of the H-R diagram
A distant star of spectral type M is found to be very bright and easily visible from Earth. How would you describe it, and what would be its place in the Hertzsprung-Russell diagram?
a red supergiant; upper right-hand corner of the H-R diagram
If a star is found to evolve so that it moves from the main sequence to the red giant area of the Hertzsprung-Russell diagram (remember, red stars are cool), which of the following changes will have occurred?
The star will have expanded.
Two stars, one classified F7 V and the other K3 V, have the same apparent magnitude. From this information, we know that the K3 V star is
closer to the Sun than the F7 V star is.
Aldebaran, a star in the constellation Taurus, has a spectral-luminosity class of K5 III. This tells us that Aldebaran is
a cool giant.
How many of the stars in the vicinity of the Sun are members of a binary (or larger) star system?
About two-thirds of them are members of a binary (or larger) star system. The rest are single stars, like our Sun.
In a binary star system with stars of unequal mass, the center of mass, about which both stars orbit in ellipses, is always on the line joining both stars and
closer to the more massive star.
Suppose you photograph the spectrum of a very distant star and discover that the spectrum shows absorption lines of ionized helium (He II) and molecular absorption bands of titanium oxide (TiO). What is the most likely interpretation of this observation?
It is a binary star consisting of a hot star and a cool companion that are too close to each other to be resolved (i.e., seen as separate stars).
What condition is necessary in order for a binary star system to appear as an eclipsing binary?
The line of sight from the Earth to the binary star must be in or very close to the orbital plane of the binary star.
The radial-velocity curve of a star in a binary-star system is a plot against time of the
speed of the star toward or away from us.
If astronomers have only been observing stars for a very small fraction of the lifetime of a typical star (10-8 for a Sun-like star), how can they possibly have enough information to decide upon a star's evolutionary developments throughout its life?
They have enough information by observing stars at all ages and stages of development and fitting these observations to a model developed from laws of physics.
Interstellar matter shining with a reddish hue would be
hot, ionized gas.
What kinds of stars are responsible for the ionization of the gas in an H II emission nebula, which, when the electrons recombine with the ions, causes the nebula to emit Balmer H<SPAN style=FONT-FAMILY: Times New Roman; FONT-SIZE: 11pt;>&#945;</SPAN> and other atomic spectral lines?
hot O and B stars
Many young stars in new clusters appear to be surrounded by a blue, nebulous haze. The physical process that produces this blue nebulosity is the
preferential scattering of blue starlight by very fine dust grains in the interstellar medium.
Distant stars in our Galaxy tend to look redder than nearby stars. What causes this reddening?
scattering of starlight by interstellar dust between the Earth and the star
New stars are most likely to form
in cold, dense interstellar clouds of gas and dust.
As a protostar forms in the center of a condensing mass of dust and gas, it slowly gets warmer and begins to glow. What is the most important process that generates this heat?
the compression of the dust and gas by gravity
Which component of dense interstellar clouds obscures most of the visible light from the protostars within it?
dust
Which wavelength range in the electromagnetic spectrum has proven to be the most useful for investigating star birth in dense molecular clouds?
infrared radiation
What physical process occurs inside a pre-main-sequence star to stop the star from slowly contracting and shrinking, thus producing a stable, non-shrinking main-sequence star?
The temperature rises until nuclear fusion begins. The heat from nuclear fusion then balances the energy radiated to space, which maintains the internal pressure and stops the star from contracting further.
Which physical parameter uniquely fixes a star's location on the main sequence of the Hertzsprung-Russell diagram?
its mass
Which of the following objects are not associated with star formation?
red giants
Which of the following characteristics describes a T Tauri star?
variable light output, ejecting matter into space, and showing emission lines in its spectrum
Which of the following objects are directly related to the bipolar outflow of gas from young stars?
d) Herbig-Haro objects
The most important characteristic of star clusters that makes them useful for understanding how protostars evolve onto the main sequence is that all stars in the cluster
have approximately the same age.
When an astronomer plots the measured luminosities and temperatures of a large number of stars of a particular star cluster on a Hertzsprung-Russell diagram, she finds that the more luminous stars are on the main sequence but the stars of lower luminosity are to the right of the main sequence. What does this mean?
This is a young star cluster.
Molecular hydrogen, H2, is thought to be the most abundant molecule in interstellar space, but it is difficult to detect. Which easily detected molecule occurs in space along with hydrogen, the measurement of which allows us to estimate the amounts of molecular hydrogen?
CO, carbon monoxide
What are giant molecular clouds?
Large clouds of interstellar gas and dust in which temperatures are low enough and densities high enough for atoms to combine into molecules.
If supernovae are stars blowing up (self-destructing), how can a supernova be part of a star-formation process?
The explosion of the star creates a shock wave that compresses nearby interstellar clouds.
How does a main-sequence star's lifetime depend on its overall mass?
The higher the star's mass, the shorter its lifetime, because a more massive star burns hydrogen fuel much faster than a low-mass star.
A main-sequence star like the Sun is in hydrostatic equilibrium because of the balance between the inward pressure of gravity on each layer of gas and the
outward pressure of gases heated by thermonuclear fusion of hydrogen.
At the present time, the fraction of the mass of the Sun's core that is in the form of helium, a by-product of nuclear fusion, is
about two-thirds.
What happens after hydrogen fusion ceases in the core of a star?
The core contracts and the surface layers of the star expand outward.
What physical process begins to heat the core of an intermediate-mass star after the hydrogen "fuel" is depleted and thermonuclear fusion has stopped in that core?
the contraction of the core and release of gravitational energy
One might imagine that if energy generation by nuclear fusion were to increase in a star, this would heat the gas, which would in turn increase the rate of nuclear fusion, and so forth in a runaway cycle of increasing temperature. What process acts in a normal main-sequence star to prevent this behavior?
Increased energy generation causes a temperature increase that in turn causes expansion and hence cooling of the gas. This slows down the nuclear fusion.
What condition MUST hold in the core of a star before helium can begin to fuse, thereby releasing nuclear energy?
The temperature must reach about 100 million degrees (108 K)
The helium flash, in which helium fusion increases very rapidly in the core of a star, occurs because
electrons have become so crowded together that the fundamental characteristics of the gas change, and the pressure is no longer linked to the temperature.
Where in the Hertzsprung-Russell diagram do we find stars that have just finished the "burning" of hydrogen in their cores?
in a band parallel to the zero-age main sequence (ZAMS), extending from the high-luminosity, high-temperature region to the low-luminosity, low-temperature region
A globular cluster of stars is judged to be
old, because it contains no hot, blue, luminous main-sequence stars
When the luminosities and temperatures of stars in a particular star cluster are plotted on a Hertzsprung-Russell diagram, no stars are seen on the uppermost part of the main sequence, a few stars appear in the red-giant phase, and all other stars appear on the main sequence. How old is this cluster?
between 30 million and 100 million years
A low-mass star becomes more luminous while expanding and cooling as it evolves from the main sequence to the giant phase. In which direction will the star's position move on the Hertzsprung-Russell diagram?
up and to the right
The Hertzsprung-Russell diagram of a globular cluster contains stars along a line called the horizontal branch. These stars are
converting helium to carbon in their cores.
Why is it believed that a planet like the Earth could not form around a Population II star, which has a composition similar to the material formed by the original Big Bang?
There would have been insufficient heavy elements to form the Earth
Cepheid variable stars are important because we can find the
distance to the Cepheid star after measuring only its period and apparent magnitude.
Compared to Cepheid variable stars, RR Lyrae stars have
shorter oscillation periods and are intrinsically fainter.
The star Mira, with a period of almost a year (332 days), is an example of a long-period variable. Where are long-period variables found in the H-R diagram?
the red giant region, with low temperatures
At what point in time does mass transfer begin in a binary star system, and why?
Mass transfer begins when one star expands and fills its Roche lobe, allowing matter to flow from it to its companion.
As their names imply, red giant stars and asymptotic giant branch (AGB) stars are both giant and red. Which one statement below correctly describes the fundamental difference between them?
Red giants have a hydrogen-burning shell, whereas AGB stars have both a hydrogen- and a helium-burning shell.
Carbon stars are
asymptotic giant branch stars with hydrogen-rich envelopes, in which carbon has been "dredged up" from the carbon-rich core by convection.
How does a planetary nebula form?
Episodes of thermal runaway in the helium-burning shell of a low-mass star push the envelope of the star off into space. The expelled envelope forms a planetary nebula.
Where do white dwarf stars come from?
They are the former cores of asymptotic giant branch stars that have pushed off their outer layers, leaving the core behind.
As a white dwarf star slowly evolves over billions of years,
its temperature and luminosity both decrease, but its size remains constant.
An important quantity in astronomy is the Chandrasekhar limit, which is equal to
1.4 solar masses, the maximum mass of a white dwarf star.
In which of the following atomic species will energy generation not occur when their nuclei undergo nuclear fusion?
iron, Fe
Which of the following will a high-mass star (say, 25 solar masses) NOT do at or near the end of its life?
eject its outer layers and become a white dwarf
During a supernova explosion involving a high-mass star, what happens to the iron in the core of the star?
High-energy photons break the iron nuclei apart into helium nuclei (photodisintegration).
What is believed to be the usual process that causes the actual explosion in a type II supernova (massive star exploding)?
The core collapses inward gravitationally and then rebounds outward. The combination of core rebound and neutrino pressure causes the envelope to explode outward.
The brightness of a supernova just after the explosion is equivalent to the total output of
all the stars in a typical spiral galaxy, up to 109 solar luminosities.
The progenitor star of supernova 1987A was a blue supergiant, whereas most progenitors of massive-star supernovae are red supergiants. What major observational effect did this have on the supernova?
It was not as bright as most other core-collapse supernovae, because it is harder to eject the outer layers from a blue supergiant than from a red supergiant.
Cerenkov light is produced whenever a charged particle
travels through a transparent medium faster than the speed of light in that medium.
Which of the following mechanisms is considered to be a possible cause of Type Ia supernovae?
In a binary star system, a giant star filling its Roche lobe dumps gas onto a white dwarf, putting the white dwarf over the Chandrasekhar mass limit.
Observations of supernovae in distant galaxies similar to our own Milky Way suggest that we should see about two supernovae per century in our Galaxy. However, the last recorded supernova in our Galaxy was observed by Kepler in 1604, almost 400 years ago. Why is this?
Our view is obscured by dense dust and gas clouds, preventing us from seeing the majority of supernova explosions that have occurred in the Milky Way.
What makes a pulsar pulse?
A rapidly spinning, magnetized neutron star emits light and radio waves along its magnetic axis.
What is the composition of a pulsar?
A pulsar is composed almost entirely of neutrons.
The energy released in a nova arises from
thermonuclear fusion of matter accreted onto the surface of a white dwarf star from its companion star in a binary system