Astronomy 121 Test 2
For each question below, choose the item from a-e that fits best. -Microscopic grains -Star-forming region that can be recognized by its pink-glowing gas -Regions where stars form, and which contain H2 and CO -Emission given off by neutral Hydrogen
-Interstellar dust -HII region -Giant molecular clouds -21-cm radiation
What type of object is found at the center of this type of nebula (seen here in the optical)?
A white dwarf
Which of these is the best description of the orbital motion of a binary star system?
Both stars orbit around a common center of mass
Rank these star clusters from youngest to oldest.
C, B, D, A (globular cluster)
What are two primary characteristics of a main sequence star?
Core Hydrogen fusion and a stable balance of forces
What type of nebula is this?
Dark Nebula
On a clear autumn night, you spend some time examining the glowing HII regions in the galaxy M33, using a large reflecting telescope. If you were IN the galaxy M33, looking at these same objects, what would be another name for them?
Emission nebulae
Many stars in a typical open cluster are nearly as old as the universe
False
T or F) Planetary nebulae are sites of planet formation.
False
T or F) Supergiants are about as common as the sun
False
The red color we see on a lot of photographs of nebulae comes from which element?
Hydrogen
A star is born when...
Hydrogen fusion begins
When a star finishes `core hydrogen burning', and transitions to `burning' hydrogen in a shell around its core, what would we observe if we could look at the star from the outside?
It would get larger and cooler, moving off the main sequence toward the red giant branch of the H-R diagram.
Which of the following types of stars will spend the longest time (the greatest number of years) on the main sequence?
K
Which of these two clusters is older?
NGC 188
If you wanted to determine the composition of the interstellar medium, which of the following techniques would be the most effective?
Observe stars through the ISM, and look at absorption lines in the stars' spectra.
Gravity is what holds stars in a cluster together, what is the most important process that causes them to spread apart?
Random motion
Notice the bright star just above and to the left of the center of this image. Around it, we see nebulosity that is colored bluish-white. Which type of nebulosity is this?
Reflection nebulosity, where light from the star bounces off of tiny dust grains.
The astrophysicist who first calculated the highest mass that a dying star can have and still be a white dwarf was
S. Chandrasekhar
Notice the bright star just above and to the left of the center of this image. Above it in the image, we see finger-like portions of the nebula. What are these likely to be?
Star-formation pillars, like the `Pillars of Creation' in the Eagle nebula.
Astronomers have discovered pulsars spinning 500 x per second or more. How do astronomers think pulsars got to be spinning so outrageously fast?
Such fast-spinning pulsars have companion stars near them, which dump material on the pulsar and that spins it faster and faster
Which of the following statements about the mass of the Sun during its lifetime is correct?
The Sun will lose a significant amount of mass during and after its red giant phase
If you could travel several hundred million years back in time and observe these clusters, what would be different about their H-R diagrams?
The `turn-off points' would be higher up on the main sequence.
What happens when the electron in a hydrogen atom flips its direction of spin, from parallel to anti- parallel to that of the proton?
The atom emits 21-cm radio radiation.
The fact that this nebula (from the previous question) is emitting light tells us what about the object at its center?
The object is hot enough to emit significant amounts of ultraviolet radiation.
Why don't we see any of the cold black dwarfs that white dwarfs eventually cool to become?
The universe isn't old enough for this long cooling processes to have finished yet.
If we look at the light of a star as seen through the interstellar medium, we'll see a very narrow absorption line, since the ISM is very cold and thin.
True
T or F) Because more massive stars have more gravitational energy, they can fuse heavier nuclear fuels.
True
T or F) No known white dwarf has a mass greater than the Chandrasekhar limit.
True
The glowing orange gas in this picture is an example of an emission nebula.
True
How could we be fooled if we look at a main-sequence star through a cloud of interstellar dust?
We could think it's lower on the main sequence than it really is, because its light is reddened by the dust
How can dying stars cause new stars to form from giant molecular clouds (GMCs)?
When large stars end their lives as supernovae, the resulting shock waves can trigger the collapse of GMCs.
Einstein suggested that the regular change (advance) in the perihelion of the planet Mercury could be explained by:
a distortion in spacetime caused by the gravity of the Sun
In a supernova like SN1987A, once the crisis of iron fusion has begun, roughly how long does it take the star's core to collapse?
a few tenths of a second
When astronomers discuss a nebula, what are they talking about?
a giant cloud of gas and dust between or among the stars
What type of object is this?
a globular star cluster
The Orion Nebula is:
a large cloud of gas and dust illuminated by the light of newly formed stars within it
A member of the college football team wants to weigh as much as possible. Assuming he could somehow survive on all of them, at the surface of which object would he weigh the most?
a neutron star
An astronomer wants to observe a cloud of cold neutral (not ionized) hydrogen, far away from any stars. What would be an instrument that could help in this task?
a radio telescope, tuned to a wavelength of about 21 centimeters
An astronomer wants to observe a cloud of dust in a relatively close part of the Galaxy. Unfortunately, this dust cloud is not located in the direction of a crowded region of stars. What instrument would be the most help in finding this cloud.
a sensitive infra-red telescope in orbit around the Earth
A group of graduate students, bored during a cloudy night at the observatory, begin to make bets about the time different stars will take to evolve. If they have a cluster of stars which were all born at roughly the same time, and want to know which star will become a red giant first, which of the following stars should they bet on?
a star of about the same mass as our Sun
Which of these stars will take the SHORTEST time to go from the earliest protostar stage to the main sequence?
a star ten times the mass of our Sun
Elements heavier than iron can be created during:
a supernova explosion
Which of the following stages will the Sun definitely go through as it gets older?
all of these choices
The event in the life of a star that begins its expansion into a giant is
almost all the hydrogen in its core that was hot enough for fusion has been turned into helium
Supposing we launched a very fast dart from the Space Shuttle, pointed in some direction away from any planet, so that it could travel beyond the solar system. What would it be most likely to hit first after traveling outward for a while?
an atom of interstellar gas
If an astronomer wants to find and identify as many stars as possible in a star cluster that has recently formed near the surface of a giant molecular cloud (such as the Trapezium cluster in the Orion Nebula), what instrument would be best for her to use?
an infra-red telescope (and camera)
You would find high mass stars in _______ and low mass stars in _______
an open cluster, both types of clusters
Which of the following statements about the life of a star with a mass like the Sun is correct?
as the star is dying, a considerable part of its mass will be lost into space
Why do all stars spend most of their lives on the main sequence?
because the fuel for energy production in this stage of the star's life is hydrogen; and that is an element every star has lots and lots of
Because white dwarfs are small, as their name implies, they are hard to see. What is a way astronomers have to find white dwarfs that distinguishes them from main sequence stars?
because white dwarfs get really hot, we can search for their ultraviolet radiation
According to the general theory of relativity, the presence of mass
causes a curvature (or warping) of spacetime
Astronomers now understand that the dark regions or rifts visible in parts of our Galaxy that are otherwise crowded with stars are caused by:
clouds with a considerable amount of dust which blocks the light of the stars behind them
After the core of a massive star becomes a neutron star, the rest of the star's material
explodes outward as a supernova
A grouping with 100 thousand stars would probably be a
globular cluster
If stars with masses like our Sun's cannot make elements heavier than oxygen, where are heavier elements like silicon produced in the universe?
heavier elements are made in the cores of significantly more massive stars than the Sun, which can get hotter in the middle
Which of the following statements about interstellar matter is FALSE?
if the matter were spread out evenly, it would be about as dense as the Earth's atmosphere
Which of the following statements about the Crab Nebula is FALSE?
inside, there are a number of newly formed massive stars (O and B type stars)
If you trace back the history of a carbon atom in your little finger through all of cosmic history, where did this atom most likely originate?
it was fused from 3 helium nuclei in the core of a red giant star long before the Sun existed
How long a main sequence star remains on the main sequence in the H-R diagram depends most strongly on
its mass
Astronomers believe that disks of material will form around protostars that are spinning. Which of the following observed phenomena is a good indication of the presence of a disk around a protostar?
jets and Herbig-Haro objects
Our Sun is an example of a...
main-sequence star
In figuring out the evolutionary tracks on the H-R diagram, astronomers
make model stars on a computer and then follow how their characteristics will change with time
To predict whether a star will ultimately become a black hole, what is the key property of the star we should look at?
mass
How are globular clusters distributed in our Milky Way Galaxy?
mostly in a large spherical halo (or cloud) surrounding the flat disk of the Galaxy
In a collapsing star of high mass, when electrons and protons are squeezed together with enormous force, they turn into a neutron and a:
neutrino
After it experiences a "helium flash" a star like the Sun will have a brief period of stability, fusing helium into carbon (and sometimes oxygen). During this brief stable stage, the star
none of these
A grouping with a hundred stars is probably a
open cluster
If you wanted to discover the youngest stars you could find in some grouping of stars in the Galaxy, which type of star group would be the best to search?
open clusters
If most stars are low-mass stars, and low-mass stars typically eject a planetary nebula, why then do astronomers see relatively few planetary nebulae in the sky?
planetary nebulae expand rapidly and soon become too faint to be visible
What kind of telescope did Jocelyn Bell use to discover pulsars in 1968?
radio
Astronomers observe a young cluster of stars, where stars with three times the mass of the Sun are still on the main sequence of the H-R diagram. Yet the cluster contains two white dwarfs, each with a mass less than 1.4 times the mass of the Sun. If we can show that the white dwarfs are definitely part of the cluster, how can their presence so soon in the life of the cluster be explained?
some stars can lose a lot of mass on their way to becoming white dwarfs; thus the white dwarfs could have started out as quite massive stars
Astronomers were at first surprised to find complicated molecules in the interstellar medium. They thought ultra-violet light from stars would break apart such molecules. What protects the molecules we observe from being broken apart?
such molecules form in clouds that contain significant amounts of dust, and the dust in the cloud protects them from ultra-violet rays
As a cluster of stars begins to age, which type of star in the cluster will move off the main sequence of the H-R diagram first?
the O and B type stars
When two identical atomic clocks, one on the ground floor and one on the top floor, are compared,
the clock on the ground floor ran a tiny bit slower
You are an astronomy graduate student and you are observing the big Orion Nebula from an airplane that has a good-sized infrared telescope built into it (there really is such a plane.) On an infrared image of the Nebula, what would particularly stand out?
the clouds of the nebula that have a lot of dust in them
When a star first begins the long path toward becoming a red giant, a layer of hydrogen around the core begins to undergo fusion. If this layer was too cold to do fusion throughout the main sequence stage, why is it suddenly warm enough?
the core is collapsing under its own weight and heating up from the compression; this heats the next layer up
Astronomers believe that the many supernova explosions that happened in the Milky Way Galaxy could have played a role in the evolution of life over billions of years. How would they have influenced the development of life on Earth?
the cosmic rays produced by supernova explosions would have contributed to the rate of mutations over many generations
If observations of supernovae in other galaxies show that such an explosion happens in a spiral galaxy like the Milky Way on average every 25 to 100 years, why have astronomers on Earth not seen a supernova explosion in our Galaxy since 1604?
the disk of our Galaxy contains a great deal of dust, which tends to block the light of supernova explosions from more distant parts of our Galaxy
Which of the following is a characteristic of degenerate matter in a white dwarf star?
the electrons get as close to each other as possible and resist further compression
The region around a black hole where everything is trapped, and nothing can get out to interact with the rest of the universe, is called
the event horizon
If we look back to the first generation of stars made when the Galaxy was first forming, how do they differ from stars being formed today?
the first generation stars contain little or no elements heavier than helium
On an H-R diagram of a cluster of stars, which characteristic of the diagram do astronomers use as a good indicator of the cluster's age?
the point on the main sequence where stars begin to "turn off" -- to move toward the red giant region
Which of the following is one reason we do not detect a pulsar in many remnants of supernova explosions? (Careful! This one is tricky!)
the pulsar beam doesn't happen to point toward us in many cases
Astronomers use the term interstellar extinction to refer to:
the scattering and absorption of starlight by dust grains in space
Many names used by astronomers are misleading or outdated. A good example is the term planetary nebula, which astronomers use to refer to:
the shell let go by a dying low-mass star
According to Einstein's general theory of relativity, the stronger a star's gravity,
the slower time runs near it
Which of the following statements about the way the mass of a white dwarf affects spacetime is correct?
the white dwarf mass will curve spacetime; light has to follow that curvature
When stars become giants, which of the following does NOT usually happen?
their mass grows significantly as they incorporate planets and interstellar matter near the star
T-Tauri stars are dim in the visible spectrum because...
they don't fuse hydrogen
Your sweetheart gives you a piece of gold jewelry as a present to celebrate your passing your astronomy class. Where did the gold atoms in that gift originally come from (where were they most likely made)?
they were built up from smaller nuclei during a supernova explosion
The dust in the dust clouds in interstellar space consists of
tiny solid grains
A star whose temperature is increasing but whose luminosity is roughly constant moves in what direction on the H-R diagram?
to the left
When the outer layers of a star like the Sun expand, and it becomes a giant, which way does it move on the H-R diagram?
toward the upper right
An astronomer is observing a star which puzzles her. The lines in the star's spectrum indicates that the star is very hot and should therefore be blue. But the star looks reddish in photographs and in measurements of the continuous spectrum. What is one possible explanation of this puzzle?
we are seeing the light of the star through layers of interstellar dust
When neutron stars were first predicted theoretically, no scientist expected to be able to detect one of them across interstellar distances. What enabled astronomers to find neutron stars in the late 1960's?
we found strongly magnetic neutron stars whose whirling beams of energy were detected as pulsars
Astronomers studying regions like the Orion Giant Molecular Cloud have observed that a wave of star formation can move through them over many millions of years. What sustains such a wave of star formation in a giant molecular cloud?
when massive stars form, their ultraviolet radiation and later their final explosions compress the gas in the cloud and cause a new group of stars to form
Astronomers identify the "birth" of a real star (as opposed to the activities of a protostar) with what activity in the star?
when nuclear fusion reactions begin inside its core
A Herbig-Haro (HH) object is
where a jet from a star in the process of being born collides with (and lights up) a nearby cloud of interstellar matter
Which of the following stages can only occur in the life of a low-mass star (whose final mass is less than 1.4 times the mass of the Sun)?
white dwarf
