Term
| What is the relation between the size of an electron orbit in an atom and the energy of the orbit? |
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Definition
The larger the orbit, the higher its energy. Thus, the smallest possible orbit corresponds
to the lowest energy level (i.e. the ground state). |
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Term
| What causes photons to be emitted and absorbed by atoms? |
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Definition
Photons are emitted by atoms when electrons drop to lower energy levels, releasing
energy. Photons are absorbed by atoms when electrons move to higher energy levels, which
requires energy input. |
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Term
How does the visible series of spectral lines of hydrogen (the Balmer series) arise? What are the
wavelengths and colors of the first three lines in the Balmer series? |
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Definition
Balmer photons are emitted when electrons drop from higher levels into the second
energy level of a hydrogen atom, i.e. the second smallest orbit. Balmer photons are absorbed
when an electron in a hydrogen atom moves from the second energy level into a higher level.
Line Color
H 656 nm Red
H 486 nm Blue-Green
H
434 nm Blue-Violet |
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Term
| What is the Doppler effect? |
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Definition
The Doppler effect is caused by the motion of a light source either toward or away
from an observer. When a light source moves toward or away from an observer, the observed
wavelength is shifted from the emitted wavelength by an amount proportional to the speed of the
object. Motion toward the observer results in a blue shift (shift to shorter wavelengths). Motion
away from the observer results in a red shift (shift to longer wavelengths). |
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Term
| How much larger is the Sun than the Earth in diameter? In volume? |
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Definition
The diameter of Sun is about 100x larger than that of the Earth. Thus, the volume of
the Sun is about 10^6 (one million times) larger than that of the Earth, i.e. if the Sun were hollow
it would take 10^6 Earth-sized objects to fill it. |
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Term
| What is the average density of the Sun? How does this compare with water? |
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Definition
The average density of the Sun is 1400 kg/m^3. This is similar to that of water
(1000 kg/m^3). |
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Term
| What is hydrostatic equilibrium? |
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Definition
Hydrostatic equilibrium is a balance between the outward gas pressure in the Sun and
the inward pull of gravity. |
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Term
| What are the central and surface temperatures of the Sun? |
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Definition
The central temperature of the Sun is about 1.5x10^7 K and the temperature in the
photosphere is about 5800 K. |
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Term
| What are the two most abundant elements in the Sun? |
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Definition
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Term
| What is the physical state of the material in the Sun? |
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Definition
Below the photosphere of the Sun, the hydrogen and helium in the Sun are in an ionized
state, i.e. the electrons move freely rather than being bound to a particular nucleus. |
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Term
| What is the energy source of the Sun? |
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Definition
The Sun produces energy by the thermonuclear fusion of hydrogen to helium in its core.
Through a series of reactions, four hydrogen nuclei are fused to produce one helium nucleus plus
energy. |
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Term
| About how much longer will the Sun’s present nuclear energy reserve last? |
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Definition
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Term
| What mechanisms transport energy in the Sun? |
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Definition
Energy is transported in the Sun by both radiation and convection. In radiation, photons
random walk outward in the Sun, carrying energy outward. In convection, hot gas rises and cool
gas sinks, also carrying energy outward. Radiation is important through most of the solar interior,
except for the outer 20% of the solar radius just below the photosphere, where convection is most
important. |
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Term
| What is the evidence that convection occurs in the Sun? |
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Definition
The observation of granulation in the Sun’s photosphere provides direct evidence of
convection. |
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Term
| List the layers of the solar atmosphere, in order of increasing distance from the Sun. |
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Definition
| photosphere, chromosphere, transition zone, corona |
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Term
|
Definition
| a jet of hot gas shooting upward in the chromosphere |
|
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Term
|
Definition
a looping region of hot gas, supported by magnetic field, that extends into the
corona |
|
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Term
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Definition
| a violent outburst of radiation and particles from a small area in the photosphere |
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Term
| Describe the properties of a sunspot. |
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Definition
Sunspots are regions in the photosphere that are somewhat cooler than the surrounding
area and thus emit less light. They are regions of strong magnetic field, which inhibits heat flow.
Sunspots tend to be found in groups of at least two. They last for about two months. |
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Term
| How often does a sunspot maximum occur? What is the underlying cause of the solar cycle? |
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Definition
| How often does a sunspot maximum occur? What is the underlying cause of the solar cycle? |
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Term
| What is the solar wind? What effect does it have on Earth? |
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Definition
The solar wind is a stream of charged particles, mostly protons and electrons, escaping
from the solar corona. Solar wind particles that become trapped by the Earth’s magnetic field
cause aurora. |
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Term
What is a neutrino? What produces neutrinos in the Sun? What happens to the neutrinos next?
Why are astrophysicists interested in detecting neutrinos from the Sun? |
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Definition
A neutrino is a very low mass particle that interacts very weakly with other matter.
Neutrinos are produced by nuclear reactions in the Sun, such as the fusion of two protons to form
a deuterium nucleus. Neutrinos that are created in the Sun escape immediately. Thus, the solar
neutrinos that we detect on Earth give us a direct measure of the nuclear reaction rate deep in the
core of the Sun. |
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Term
| What is a “neutrino telescope”? |
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Definition
Neutrino telescopes use large amounts of matter—often large water tanks—which capture
a very small fraction of the total number of neutrinos that pass through. |
|
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Term
What is the “solar neutrino problem”? What is thought to be the most likely solution to this
problem? |
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Definition
The solar neutrino problem is the finding that the observed number of neutrinos from
the Sun is about one third the number that had been predicted by standard solar models. A likely
solution to this problem is the finding that neutrinos oscillate between three different types as they
travel through space between the Sun and the Earth. |
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Term
| What is the definition of stellar parallax? |
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Definition
Stellar parallax is 1/2 of the annual shift in the apparent position of a nearby star, relative
to more distant stars, due to the orbital motion of the Earth about the Sun. The diameter of the
Earth’s orbit is used as the baseline, which thus has a length of 2 AU. |
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Term
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Definition
|
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Term
The Hipparcos satellite was able to detect parallaxes as small as 0.001 arcsec. What is the greatest
distance to which it can measure a parallax? |
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Definition
| 1 pc = 3.3 ly, 1000 pc = 3300 ly. |
|
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Term
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Definition
| the energy per time radiated by an astronomical object (e.g. a star) into space. |
|
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Term
|
Definition
the energy per time per area detected from an astronomical object at
Earth. |
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Term
| Brightness / luminosity equation. |
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Definition
|
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Term
| What do each of the quantities in the ideal gas law represent? |
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Definition
PV = NkT
Pressure
Volume
Number of particles
Boltzmann constant
Temperature |
|
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Term
Suppose that a helium balloon is moved from a warm room to the cold outdoors. Based on the
ideal gas law, what will happen to the balloon and why? |
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Definition
| The balloon will shrink somewhat |
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Term
What is the fundamental quantity that varies along the spectral sequence
(O B A F G K M)? What are the principal differences between the spectra of O and M stars?
What type of star is the Sun? |
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Definition
Surface temperature is the fundamental quantity that varies along the spectral sequence.
O stars are the hottest and M stars are the coolest. O star spectra show absorption lines due to
ionized helium, which indicates temperatures above about 30,000 K. M star spectra show absorption
lines due to molecules which can only exist in the coolest stellar atmospheres. The Sun is a
G star. |
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Term
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Definition
Figures 11-7 and 11-8 on pp. 329 and 330 of the text. The horizontal axis represents
either spectral class (O B A F G K M) or temperature (increasing to the left). The vertical axis
represents luminosity. |
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Term
Explain why a red giant star has a much higher luminosity than a main sequence star with the
same surface temperature. |
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Definition
Since the two stars have the same surface temperature, they both radiate the same
amount of energy per time per unit surface area. Thus, one square meter patches of the photospheres
of the stars have the same luminosity. However, since the red giant is much larger
than the main sequence star, it has a much larger surface area and thus has a much higher total
luminosity than does the main sequence star. |
|
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Term
| Where do we find low-mass stars on the main sequence? High-mass stars? |
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Definition
Low-mass stars are found in the lower-right part of the main sequence. These stars have
low luminosity and low temperature. High-mass stars are found in the upper-left part of the main
sequence. These stars have high luminosity and high temperature. |
|
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Term
Compare the lifetime of a red dwarf and a blue supergiant. What accounts for this difference in
lifetime? |
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Definition
Red dwarf stars have long lifetimes, exceeding 10^12 years. Blue supergiant stars have
short lifetimes, about 2x10^7 years. This large difference is due to the extremely high luminosity
of blue supergiants compared with red dwarfs. This causes blue supergiants to “burn” through
their nuclear fuel much faster. Thus, even though blue supergiants have more nuclear fuel (hydrogen)
than do red dwarfs, they nevertheless have much shorter lifetimes. |
|
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Term
| In what types of regions do stars form? |
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Definition
Stars form in cold, dense interstellar clouds. This type of region is known as a dark
nebula. |
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Term
| How is the energy lost by radiation from the surface of a protostar replaced? |
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Definition
Energy radiated from the surface of a protostar is replaced by energy released through
gravitational contraction. This keeps a protostar in thermal equilibrium. |
|
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Term
| Why can more be learned about protostars by observing them in infrared rather than visible light? |
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Definition
Protostars are surrounded by thick dust clouds that absorb more visible than infrared
light. Thus more of the infrared radiation from the protostars is able to escape from these clouds |
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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
Blue light is more effectively scattered by dust particles than is red light. Thus, we
see a blue haze from a cloud of gas and dust that surrounds a star. The source of the blue light
is photons from the star that were not originally travelling towards us but that were redirected
towards us upon being scattered by the dust in the cloud. Since blue light is more effectively
removed from the direct beam of light from the star, the star appears redder than the true color
that would be observed if the cloud were not present. |
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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. Thus, scattered blue light from the Sun lights the sky. If the Earth did not have an atmosphere,
the sky would appear black even when the Sun is above the horizon. |
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