Astronomy Final 9-15
A binary star system with an orbital period of 4 years is found to have an average separation between the stars (equal to the semimajor axis of the elliptical orbit) of 6 AU. What is the sum of the masses of the two stars?
13.5 solar masses.
How long after the Big Bang did the universe pass through the transition from a radiation-dominated to a matter-dominated universe?
30,000 years
What is the mass of a main-sequence star that has a luminosity 1000 times greater than that of the Sun?
6 solar masses
An important quantity in astronomy is the Chandrasekhar limit, which is equal to
1.4 solar masses, the maximum mass of a white dwarf.
In what way is the SETI@home program helping to search for radio signals from intelligent life-forms elsewhere in our universe?
A large number of computers analyze data from radio telescopes, sent to them from the University of California at Berkeley, when otherwise these computers would sit idle.
Which of the following statements CORRECTLY describes a typical quasar?
A quasar is an object whose luminosity is 100 times the luminosity of the entire Milky Way Galaxy, but whose diameter is about that of our solar system.
What makes a pulsar pulse?
A rapidly spinning, magnetized neutron star emits light and radio waves along its magnetic axis.
What is the most important characteristic of star clusters that makes them useful for understanding how protostars evolve onto the main sequence?
All stars in the cluster have approximately the same age.
Molecular hydrogen, H2, is thought to be the most abundant molecule in interstellar space but 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
In the search for intelligent life, scientists tend to assume that its biochemistry is more likely to be based on the element carbon than on any other element. Why is this?
Carbon is the most versatile element, combining with the largest number of other elements in many ways to produce long-chain molecules.
The Hubble classification for an elliptical galaxy that is almost circular is
E1.
The cosmic microwave background radiation, the remnant of the radiation produced in the original Big Bang explosion, appears to us to be not quite uniform in temperature or intensity in all directions, that is, it is not isotropic. Why is this?
Earth, as part of the solar system and the Milky Way Galaxy, is moving through this radiation and the Doppler effect produces this anisotropy.
Newton believed that the universe was static with the stars fixed in place for all time. In view of his theory of gravity, in which all objects attract all other objects in the universe, how did he explain the fact that these stars did not pull themselves toward each other under these mutual forces, causing the universe to collapse?
He proposed an infinite and uniform universe in which the gravitational forces on a star from neighboring stars were perfectly balanced by forces from other stars in every direction.
How is energy transported outward in the Sun's interior?
How is energy transported outward in the Sun's interior?
What causes a Type Ia supernova?
In a binary-star system, a giant star filling its Roche lobe dumps gas onto a white dwarf, causing the white dwarf's mass to exceed the Chandrasekhar mass limit.
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.
On which of the following objects in our solar system is it LEAST likely that life exists or even existed at an earlier time?
Io, a moon of Jupiter
What is the primary reason why a star of twice the mass of the Sun is considered an unlikely candidate for finding an extraterrestrial civilization?
Its main-sequence lifetime is too short.
Which of the following sequences of stellar classification types shows stars in order of increasing surface temperature?
K0, G9, G6, B2
What surprising conclusion about the distribution of mass in our Galaxy has resulted from a study of orbital speeds across the galactic disk?
Most of the mass in the Galaxy is dark and is detected only by its gravitational influence.
The radiation emitted into space by a pre-main-sequence star (its luminosity) robs it of energy, resulting in a slow gravitational contraction. What physical process halts this contraction and produces a stable, nonshrinking main-sequence star?
Nuclear fusion begins as the temperature rises, and releases energy into the core at the same rate at which the star is losing energy to space by radiation.
What solution has been proposed by cosmologists to explain the remarkable uniformity of temperature throughout our universe?
Space expanded by a very large factor during a very brief period soon after the time of creation.
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.
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.
What produces the rapid rotation rate of a young neutron star, or pulsar?
The core of the dying star spins up because it collapses to a very small radius.
Our present universe is dominated by dark energy, but at much earlier times the universe was matter-dominated. How did dark energy come to dominate matter?
The density of dark energy remains constant as the universe expands, whereas the density of matter in the universe decreases. Therefore, the density of matter eventually decreased below that of the dark energy.
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 fuses hydrogen fuel much faster than a low-mass star does.
According to our laws of physics, what is the greatest barrier to interstellar travel, whether by us or by others (UFOs?) who might want to visit us?
The time required to travel between stars
We can see only a certain distance out into the universe, and this distance is the same in all directions. Why?
The universe has a definite age.
What causes the cosmological redshift of light?
The wavelength of photons from a distant galaxy becomes stretched by the expansion of space while the photons are traveling toward us.
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 during which no or almost no sunspots are seen on the Sun.
What is the general distribution of galaxies in the universe?
They are grouped into clusters and superclusters, which in turn are distributed around huge, apparently empty volumes of space.
Where do white dwarfs come from?
They are the former cores of asymptotic giant branch stars that have pushed off their outer layers, leaving the core behind
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 six months.
What is believed to be the reason why some spiral galaxies contain a prominent ring of star formation and/or dust?
They suffered a collision from a smaller galaxy in the not too distant past.
Do we have any direct (although perhaps controversial) indication of life, past or present, elsewhere in the solar system?
Yes. Structures have been discovered in a Martian rock that resemble fossilized microorganisms.
The Local Group, the cluster of galaxies to which our Milky Way Galaxy belongs, is
a poor, irregular cluster of three to four dozen galaxies.
For an astronomer, what is the "water hole"?
a range of noise-free frequencies between the microwave emission lines of hydrogen and hydroxyl
The "central engine" of an active galaxy appears to be
a supermassive black hole at the center of an accretion disk, with jets of material being ejected perpendicular to the disk.
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
What is a positron?
an electron with a positive charge
The luminosities and temperatures of stars in a particular 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 or very close to the main sequence. How old is this cluster?
between 30 and 100 million years
In a W Ursae Majoris-type binary-star system,
both stars have expanded beyond their Roche lobes, so they are in contact and share a common atmosphere.
A star of magnitude -1 is
brighter than a star of magnitude +1.
Which element forms the chemical basis of life on Earth?
carbon
Which substances are thought to have been most important for forming complex prebiological compounds in early planetary atmospheres?
carbon dioxide, nitrogen, hydrogen, and water
An organic molecule is defined as any
carbon-based compound.
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.
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
Aldebaran, a star in the constellation Taurus, has a spectral-luminosity class of K5 III. This tells us that Aldebaran is a
cool giant.
Which component appears to contain the majority of the overall mass and energy of the universe?
dark energy, which causes the universal expansion to accelerate with time
When was most of the helium that exists in the universe today created?
during a period of intense nuclear reactions that ended about three minutes after the start of the Big Bang
In an X-ray burster, the source of the X rays is
explosive helium fusion reactions.
One interesting and somewhat unexpected characteristic of the solar corona is its
extremely high temperature, greater than 106 K.
A star of magnitude +8 is
fainter than a star of magnitude +5
Several objects believed to be black holes have been discovered in our Galaxy by
finding binary-star systems that emit X rays, with an unseen component that has a mass greater than 3 solar masses.
Penzias and Wilson were the scientists who
first observed the cosmic background radiation.
How many basic forces are there in the universe today?
four: the gravitational, electromagnetic, strong nuclear, and weak nuclear forces
Interstellar matter shining with a reddish hue consists of
hot, ionized gas.
The solar wind appears to originate primarily
in coronal holes, which are lower-density, lower-temperature regions in the high atmosphere of the Sun.
Where in the universe has evidence been found for the existence of supermassive black holes with masses of at least a million solar masses?
in the centers of many galaxies
Which wavelength range in the electromagnetic spectrum has proven to be the most useful in investigating star birth in dense molecular clouds?
infrared radiation
The arms of a grand-design spiral galaxy are formed when
interstellar matter piles up in denser regions that are stretched by differential rotation into long, well-defined spiral arms.
In which of the following atomic species will energy generation NOT occur when their nuclei undergo nuclear fusion?
iron, Fe
Which physical parameter uniquely fixes a star's location on the main sequence of the Hertzsprung-Russell diagram when it reaches this stage of its evolution?
its mass
The radius of the event horizon of a black hole—the Schwarzschild radius—is
larger the greater the mass of the black hole, with no known limit.
The energy (such as light and heat) released by the Sun is produced by nuclear reactions in the core of the Sun, which convert hydrogen into helium. The mass of the helium nucleus is
less than the mass of four hydrogen nuclei, the extra mass having been converted into energy.
Which of the following is NOT a consequence of coronal mass ejections and other surges of high-energy particles from the Sun striking Earth?
major tropical storms such as hurricanes and cyclones
The "spin-up" mechanism that has resulted in millisecond pulsars with abnormally high rotation rates is thought to be
mass transfer and directional streaming of matter from a companion star in a close binary system.
What are the basic physical properties of a black hole?
mass, angular momentum (or spin), and electric charge
The radio lobes observed on either side of a double radio source are caused by
material ejected from the nuclei of the galaxies.
Which wavelength range in the electromagnetic spectrum has been the most fruitful in the search for organic molecules in space?
microwave
The key piece of evidence showing that the expansion of the universe is accelerating is that the
most distant Type Ia supernovae are fainter than they would be if the expansion were not accelerating.
What did Einstein consider was the greatest blunder of his career?
not predicting that the universe is expanding
A globular cluster of stars is judged to be
old because it contains no hot, blue, luminous main-sequence stars.
Where in the universe does the "general" expansion occur?
only in the space that separates clusters of galaxies
A giant loop of gas arching out from the visible edge of the Sun is called a
prominence.
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?
red supergiant; upper-right-hand corner of the H-R diagram
Astronomers have found that an important indicator of a star's surface temperature is its
relative brightness at different wavelengths, or colors.
Measurements of the motions of stars and interstellar gas within the Milky Way Galaxy indicate that the rotational speed of the Galaxy at distances beyond 5000 light-years from the galactic center
remains approximately the same or increases slightly with increasing distance from there outward.
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.
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.
What are giant molecular clouds?
star-formation regions of up to about 300 light-years across that contain about a million solar masses of material
Although the interior of the Sun cannot be viewed directly by visible radiation, astronomers are able to explore this region by studying
surface oscillations that are generated by sound waves passing through the solar interior.
The Drake equation calculates the possible number of
technically advanced civilizations with whom we might communicate in our Galaxy.
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.
In spiral galaxies, the relationship between nuclear bulge and tightness of spiral arms seems to be that
the larger the nuclear bulge of the galaxy, the tighter the spiral arms are wound together.
According to the inverse-square law, if two stars have the same luminosity and if one star is nine times farther away than the other, then
the more distant star would be 81 times fainter.
The Hubble flow is a general recession of distant clusters and superclusters of galaxies away from us such that
the more distant they are, the faster they are moving away from our Galaxy.
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.
Scientists are now targeting Europa, one of Jupiter's moons, as a likely site for searching for elementary life-forms because
there is strong evidence for liquid water beneath the thick ice layer on its surface.
To find the distance to a Cepheid variable star, we need measure only
two quantities: its period and apparent magnitude.
What did Harlow Shapley use to find the distances to globular clusters, so he could use these to find the location of the center of our Galaxy?
variable stars
The number of sunspots on the Sun
varies with a cycle of approximately 11 years.
The characteristics of the radio source Sagittarius A* that indicate that this might be a supermassive black hole are
very high-speed motion of stars in elliptical orbits around this object, indicating a large concentration of mass, and very short X-ray flares, indicating small physical size.
The photosphere of the Sun is the
visible surface of the Sun, where granules and sunspots are found.
Which of the following was probably NOT a source of complex organic molecules, the building blocks of life, on early Earth?
volcanic eruptions
When matter collapses to form a black hole, what one fundamental characteristic determines whether the result is a Schwarzschild black hole or a Kerr black hole?
Rotation: A Schwarzschild black hole forms if the matter is not rotating, and a Kerr black hole forms if the matter is rotating
Jill is a navigator on a spaceship that is passing Mars at 99% of the speed of light. What does she see when she looks at a spaceport building on Mars that has a clock on it?
She sees the building look shortened in the direction that she is moving, and the clock on Mars appears to run slower than her own clock.
How does a planetary nebula form?
Episodes of thermal runaway in the helium-fusing shell of a low-mass star push the envelope of the star off into space. The expelled envelope forms a planetary nebula.
Does the Sun have a solid surface, and where or why not?
No, it does not. The Sun is entirely a gas, from its surface right to its center.
A particular planet is located in the habitable zone of a cool, M-type star, and is tidally locked to the star (in synchronous rotation, always turning the same side toward the star). The temperature on the warm, star-facing side is 20°C, which is close to room temperature on Earth. Would this planet be a likely place to look for life, and why or why not?
No. The star-facing side would be dry, and all the water would be in the form of ice on the dark side.
What important development occurred in the early universe at about 380,000 years after the Big Bang?
Protons and electrons combined to form neutral atoms, with the result that the universe became transparent for the first time.