Term
Describe why a reflection nebula has a bluish color and why a star that lies behind a reflection
nebula appears to be redder than its true color. |
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Definition
Since blue light is more effectively
removed from the direct beam of light from the star, the star appears redder |
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Term
| Explain why Earth’s sky is blue. |
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Definition
The sky appears blue since air molecules scatter blue light more effectively than red
light. |
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Term
What are the approximate mass limits for main-sequence stars? What is the reason for these
limits. |
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Definition
The lower mass limit for main-sequence stars is 0.08M @. Below this mass, stars do not
undergo hydrogen fusion in their cores. The upper mass limit for main-sequence stars is about is
about 100 M . Above this mass, stars are blown apart by radiation pressure. |
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Term
| Beginning with the main sequence, what are the stages in the life of a 1 M @ star? |
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Definition
1. Main Sequence
2. Red Giant Phase
3. Helium Flash
4. Horizontal Branch
5. Second Red Giant Phase
6. Planetary Nebula Phase
7. White Dwarf Phase |
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Term
Draw an evolutionary track in an H-R diagram showing the path of a 1 M star from the main
sequence to the red giant phase. |
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Definition
| Figures 13-1 on p. 381 and 13-3 on p. 382 |
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Term
Describe in what ways the Sun will change when it becomes a red giant. What will be its energy
source during this phase? |
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Definition
When the Sun becomes a red giant, its radius will be larger, its luminosity will be
higher, and its surface temperature will be lower than at present. When the Sun becomes a red
giant, its energy source will be the fusion of hydrogen to helium in a shell around the core and
the gravitational contraction of the core. No nuclear fusion will take place in the core itself when
the Sun is a red giant. |
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Term
| What is the product of helium fusion? When does it begin in a 1 M@ star? |
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Definition
Three helium nuclei fuse to form a carbon nucleus with a release of energy; this is called
the “triple-alpha” process. Helium fusion begins at the upper tip of the red giant phase in a 1 M @
star. This stage is called the “helium flash.” |
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Term
| What type of fusion occurs in the core of a horizontal branch star? |
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Definition
The core of a horizontal branch star undergoes fusion of helium to carbon. Some of the
carbon fuses with helium to produce oxygen. |
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Term
| What is a planetary nebula? |
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Definition
the outer
layers of the stars into space where they form expanding gas shells. The stellar remnant left
behind is a small, hot, white dwarf. |
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Term
| What is the final state of the Sun? |
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Definition
After the Sun ejects its envelope during the planetary nebula phase, the remaining core
of the Sun will become a white dwarf. Thus, the final state of the Sun is a white dwarf that
gradually becomes cooler and fainter. |
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Term
What age stars are found in globular star clusters? What has become of the stars that were formerly
on the upper part of the main sequence in these clusters? |
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Definition
Globular star clusters contain only old stars, with ages of about 10 billion years (1010).
Thus, globular clusters only contain low-mass stars — less than about the mass of the Sun. The
higher mass stars that were originally present in globular clusters have already left the main
sequence and gone on to subsequent phases. |
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Term
| What is the maximum possible mass for a white dwarf? For a neutron star? |
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Definition
For a white dwarf, Mmax = 1.4 M@ .
For a neutron star, Mmax 3 M @. |
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Term
What is the state of the core of a massive star immediately prior to the star undergoing a supernova
explosion? What event triggers the explosion? |
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Definition
the core of a massive star is composed of iron and
it does not undergo nuclear fusion. As the mass of the iron core increases, due to nuclear fusion in the surrounding shells, it eventually exceeds 1.4 M @. At this point, the core becomes unstable
and collapses. This core collapse is the event that triggers the explosion. |
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Term
Describe the events that lead to the ejection of the envelope of a massive star in a supernova
explosion. |
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Definition
As the iron core of a massive star collapses, the inner part of the envelope follows it
inwards. When the core collapses to a radius of about 10 km, it reaches nuclear density and
bounces back slightly. This bounce drives a powerful shock wave into the envelope, which provides
enough energy to completely eject the envelope into the space around the star. |
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Term
| What is left of a massive star following a supernova explosion? |
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Definition
The collapsed core of a massive star becomes a neutron star, with a mass of about
1.4 M@ and a radius of about 10 km. This is surrounded by a supernova remnant, a rapidly
expanding cloud of gas that once was the envelope of the doomed star. |
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Term
| Where do the elements heavier than helium originate? |
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Definition
Elements heavier than helium are produced in massive stars. These stars explode in
supernova explosions, ejecting these elements into the interstellar medium and enriching it. |
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Term
| What is the significance of the pulse of neutrinos observed from Supernova 1987a? |
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Definition
The neutrinos detected from Supernova 1987a were direct observational evidence that
the core of a massive star collapsed, triggering the supernova. It is a confirmation of our theories
of supernova production. |
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Term
Describe the possible final states of stars. How does the final state of a star depend on its initial
mass? |
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Definition
The possible final states of stars are: white dwarf, neutron star, and black hole. The final
state depends on how much mass is left in the core of a star when a star uses up all of its nuclear
fuel. If the initial mass of a star is less than about 10 M @, the core mass is less than 1.4 M @, and
the star ends as a white dwarf after the outer layers are ejected in the planetary nebula phase. If
the initial mass is greater than 10 M @, but less than about 25 M , then core mass is more than
1.4M @ but less than 3M @, and the star ends as a neutron star after the outer layers are blown off
in a supernova explosion. If the initial mass exceeds about 25 M @, then the core mass exceeds
3 M @, and it collapses to a black hole. |
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Term
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Definition
A pulsar is a rapidly rotating neutron star with a strong magnetic field that is beaming
radiation along its magnetic axis, which is not aligned with its rotation axis. We see a pulse of
radiation when one of the ends of the magnetic axis points at us. This is known as the “lighthouse
effect.” Most pulsars are detected in the radio part of the electromagnetic spectrum. Some pulsars
can also be seen in the optical and X-ray. |
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Term
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Definition
A black hole is a highly collapsed object with such strong gravity that nothing can
escape from its vicinity, not even light. |
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Term
| What is an event horizon? How is its size related to the mass of the black hole? |
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Definition
The event horizon of a black hole is a spherical surface around the black hole that separates
the hole from the exterior universe. Matter and radiation can pass through the event horizon
into the hole, but nothing can escape from inside the event horizon. Thus, external observers can
never see what occurs inside the event horizon. For a 3 M @ black hole, the radius of the event
horizon is 9 km. The radius of the event horizon is proportional to the mass of the black hole. |
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Term
What causes space to curve? What effect does spatial curvature have on light rays? What evidence
do we have for this effect in the solar system? |
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Definition
The presence of mass in the universe causes space to curve. The curvature is strongest
near a very dense mass concentration, particularly a black hole. Spatial curvature causes the paths
of light rays to bend. We observe this effect in the solar system, where light rays passing close to
the edge of the Sun bend by a small angle. |
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Term
| What is the evidence that the X-ray emitting binary star Cygnus X-1 contains a black hole? |
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Definition
The X-ray emitting binary star system Cygnus X-1 contains a highly collapsed object
with a mass of 5–10 M @. Since any collapsed object above 3 M@ is too massive to be supported
against gravity as a neutron star, the collapsed object in Cygnus X-1 must be a black hole. |
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Term
Where are black holes known to exist in the universe? What are the approximate mass ranges for
these black holes? |
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Definition
Black holes are found both in binary star systems (such as Cygnus X-1) and at the
centers of galaxies. Black holes in binary systems have masses similar to those of massive stars,
e.g. from about 3 M@ to a few tens of M@ . The black holes found at the centers of galaxies are
supermassive, i.e. in the range 10^6 - 10^10 M @. |
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Term
| What are the basic components of the Milky Way Galaxy? |
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Definition
the nucleus, the bulge, the disk,
and the halo. |
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Term
| How does the disk of the Milky Way Galaxy appear to us in the sky? |
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Definition
The disk of our galaxy appears to us a band of diffuse light around the sky, known as
the “Milky Way.” |
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Term
| Why do we see more stars near the band of the Milky Way than in other directions in the sky? |
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Definition
When we look in the direction of the Milky Way, our line of sight extends a long way
through the disk of the Galaxy where the star density is high. |
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Term
| Where in the Galaxy is the Sun located? |
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Definition
The Sun is located about 25,000 ly from the center of the Galaxy, in the galactic disk,
in a spiral arm. |
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Term
| What causes stars and gas clouds to orbit in the Galaxy? |
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Definition
Each object in the Galaxy feels the gravitational pull of every other object. This gravitational
attraction causes stars to orbit about in the Galaxy rather than to escape from the Galaxy. |
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Term
| How do stars orbit in the disk of the Galaxy? How do they orbit in the spheroid? |
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Definition
In the disk of the Galaxy, star follow nearly circular orbits in the same direction. Thus,
they have a common orbital shape, common orbital plane, and common orbital direction. The
orbits of disk stars are like those of planets in the solar system.
In the spheroid of the Galaxy (nucleus + bulge + halo), the stars have a much more random orbital
pattern. The shapes of the orbits vary from very elongated to nearly circular. There is no common
orbital plan nor is there a common orbital direction. |
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Term
Why did astronomers, prior to about 1920, incorrectly find that the Sun is located near the center
of the Milky Way Galaxy? |
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Definition
Prior to about 1920, astronomers did not realize that our view through the disk of our
galaxy is severely affected by the absorption of visible light by interstellar dust. Thus, our view
of distant stars in the disk is blocked. As a result, counts of stars in different directions around the
MilkyWay indicated roughly equal numbers, suggesting that the Sun is located near the center of
our galaxy. |
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Term
How did Shapley correctly determine the location of the center of the Milky Way Galaxy? In
what constellation is the galactic center found? |
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Definition
Shapley correctly reasoned that the system of globular clusters in our galaxy is centered
on the galactic center. Our view of most clusters is not blocked by dust, since the clusters are
found in the halo. Thus, absorption by dust in the galactic plane does not affect the determination
of the distances of most clusters. Shapley used cluster distances to locate the center of the galaxy,
which lies in the direction of the constellation Sagittarius. |
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Term
| At what wavelengths is the center of the Milky Way Galaxy best observed? |
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Definition
The center of our galaxy is best observed at infrared and radio wavelengths, since these
pass through the dust in the galactic disk. We also observe some X-ray and gamma-ray radiation
from objects located near the center of our galaxy. |
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Term
What is the best evidence that there is a concentration of 4 x 10^6 M @ at the center of the Milky
Way Galaxy? What is the nature of this concentration? |
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Definition
Careful measurements of the changing positions of stars near the center of the Milky
Way Galaxy show that these stars are orbiting a a mass 4 x 10^6 M@ at speeds up to thousands of
km/s. The mass is so concentrated that it must be in the form of a black hole. |
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Term
Compare the ages, types, and heavy element abundances of stars found in the disk of the Galaxy
with those of stars found in the bulge and halo. |
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Definition
The stars in the disk of the Galaxy range over the complete set of spectral types, and are
younger and have a higher heavy element abundance than do the stars in the bulge and halo. Only
G, K, and M stars are found in the bulge and halo. |
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Term
| How do we know that the Galaxy has a massive halo? |
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Definition
Although the halo of the Galaxy emits very little light, its presence is felt from its
gravitational effects. By measuring the speeds of stars or gas clouds in the other part of the disk
of the Galaxy, we can measure how much mass is in the halo. This tells us that the amount of mass
present in “dark matter” greatly exceeds the luminous material (e.g. stars) present in the Galaxy. |
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Term
What type of objects might make up the outer part of the halo of the Galaxy? What type of objects
cannot make up the outer part of the halo? |
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Definition
The outer part of the halo of the Galaxy may contain Jupiter-sized objects, brown
dwarfs, white dwarfs, neutron stars, black holes, or hypothetical subatomic particles called WIMPS
(weakly interacting massive particles). The outer part of the halo cannot be made of main sequence
stars, red giants, or supergiants, because these objects emit more light than is seen from
the halo. |
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Term
| What is a MACHO? How can a MACHO be detected? |
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Definition
MACHO stands for MAssive Compact Halo Object. This is a collapsed stellar remnant
(white dwarf, neutron star, or black hole) that might populate the halo of the Galaxy. MACHOs
can be detected as a gravitational lens. When a MACHO passes inbetween us and a more distant
star, the distant star appars brighter for a few days. By observing millions of distant stars, we have
a possibility of detecting a few MACHO events. |
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Term
What is a blue straggler in a globular star cluster? What is a theory that explains how these stars
are formed? |
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Definition
A blue straggler is a star that appears to be left behind on the main sequence when it
should have already gone on to its subsequent phases. The main theory for how these stars are
formed is that they are the products of stellar mergers. When two stars collide and merge, the gas
from the two stars is mixed together. This allows the new star formed by the merger to exist for
a time as a main sequence star, with hydrogen fusion occuring in its core. Thus, mergers of stars
can partially repopulate the upper main sequences of globular star clusters. Mergers will occur
most frequently in cluster cores, so this is where we expect to find the greatest number of blue
stragglers. |
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