Stars and Galaxies Exam 2
An electromagnetic wave is a _____ wave in which the oscillating electric and magnetic fields are _____ to the travel direction. (a) transverse, parallel (b) longitudinal, parallel (c) transverse, perpendicular (d) longitudinal, perpendicular (e) oblique, at a 45 degree angle
(b) longitudinal, parallel
Which of the following statements about photons is correct? (a) since light consists of photons, it has no wavelike properties. (b) photons cannot be divided into smaller units. (c) two photons with the same wavelength can have different energies. (d) visible light is the only form of radiation that comes in photons. (e) two photons with different frequencies can have the same energy.
(b) photons cannot be divided into smaller units.
The earth rotates on its axis from ____ making the celestial sphere appear to rotate from _____. (a) east to west, west to east (b) west to east, east to west (c) east to west, east to west (d) west to east, west to east (e) north to south, south to north
(b) west to east, east to west
For photons 1 and 2, if v1/v2 = 4 find E1/E2 (a) 1/4 (b) 1/2 (c) 4 (d) 1/16 (e) 16
(c) 4
For two stars with the following surface temperatures, calculate the ratio of wavelengths of peak emission λmax1/λmax2. T1 = 4000 K T2 = 16000 K (a) 1/4 (b) 2 (c) 4 (d) 8 (e) 1/2
(c) 4
Which of the following is true? (a) Light has only wave properties. (b) Light has only particle properties. (c) The interference pattern in the two slit experiment supports the wave nature of light. (d) The interference pattern in the two slit experiment supports the particle nature of light. (e) Light has neither wave properties nor particle properties.
(c) The interference pattern in the two slit experiment supports the wave nature of light.
Which of the following statements about constellations is NOT correct? (a) Astronomers have divided the entire sky into 88 constellations. (b) Every point on the sky lies within some constellation. (c) The stars in each constellation are always physically associated. (d) Constellations are apparent groupings of bright stars on the sky. (e) Different constellations are visible in summer and winter.
(c) The stars in each constellation are always physically associated.
According to Kirchoff's Laws, a hot, low-density gas produces (a) an absorption-line spectrum (b) a continuous spectrum (c) an emission-line spectrum (d) a spectrum with both absorption and emission lines (e) no spectrum
(c) an emission-line spectrum
A star with a surface temperature of 12,000 K appears _____ because ______. (a) white, all stars have the same color (b) red, it emits more red light than blue light (c) blue, it emits more blue light than red light (d) red, it emits no blue light at all (e) blue, it emits no red light at all
(c) blue, it emits more blue light than red light
The energy of an Hβ photon is ______ that of an Hα photon. (a) less than (b) equal to (c) greater than (d) either less than or greater than (e) none of the above; energies of these photons are too small to measure accurately.
(c) greater than
Which of the following is most responsible for different constellations becoming visible at different times of the year? (a) the tilt of the earth's axis (b) the moon's orbit about the earth (c) the earth's orbit around the sun (d) the earth's rotation on its axis (e) the sun's orbit in the Milky Way Galaxy
(c) the earth's orbit around the sun
In the classroom demonstration using the infrared camera, what material blocked the thermal infrared radiation? 1. plastic lenses in eyeglasses 2. glass 3. black plastic bag (a) 1 only (b) 2 only (c) 3 only (d) 1 and 2 only (e) 1, 2, and 3
(d) 1 and 2 only
What is the wavelength range of visible light? (a) 500 nm-900 nm (b) 300 nm-700 nm (c) 500 nm-800 nm (d) 400 nm-700 nm (e) 500 nm-1000 nm
(d) 400 nm-700 nm
The sun is composed mostly of (a) helium and oxygen (b) hydrogen and oxygen (c) hydrogen and carbon (d) hydrogen and helium (e) helium and lithium
(d) hydrogen and helium
As the frequency of a photon increases, the photon energy _____. As the wavelength of increases, the photon energy ______. (a) decreases, increases (b) decreases, decreases (c) increases, increases (d) increases, decreases (e) remains the same, remains the same
(d) increases, decreases
When an atom absorbs a sufficiently energetic photon to cause an electron to jump out of the atom, this is called: (a) convection (b) recombination (c) fusion (d) ionization (e) fission
(d) ionization
A photon of which of the following colors has the lowest energy? (a) blue (b) yellow (c) violet (d) orange (e) none of the above; all photons have the same energy.
(d) orange
When a photon is emitted from an atom, this indicates that an electron moved to a _____ orbit which corresponds to a move to _____ energy level. (a) larger, a higher (b) larger, a lower (c) smaller, a higher (d) smaller, a lower (e) larger, the same
(d) smaller, a lower
In science, if new and reliable observations disagree with a well-established theory, then the (a) observations should be archived for future reference and the theory retained as the best explanation of the phenomenon. (b) observations must be discarded. (c) theory must be discarded. (d) theory must be modified to account for the observations, and if this is not possible, then the theory must be discarded. (e) observations and theory must be discarded.
(d) theory must be modified to account for the observations, and if this is not possible, then the theory must be discarded.
What value is needed to directly find the surface temperature of the sun? (a) luminosity of the sun (b) radius of the sun (c) mass of the sun (d) λmax of the sun (e) intensity of sunlight
(d) λmax of the sun
A normal hydrogen arom contains _____ protons, _____ neutrons, and _______ electrons. (a) 1,0,0 (b) 1,1,1 (c) 1,2,1 (d) 2,1,2 (e) 1,0,1
(e) 1,0,1
The sun has a surface temperature of about 6000 K and a λ max value of 500 nm. A star with a surface temperature of 3000 K will have a λ max value of ______. (a) 125 nm (b) 750 nm (c) 5oo nm (d) 250 nm (e) 1000 nm
(e) 1000 nm
The sun is ______ times larger in volume than the earth. (a) 10 (b) 100 (c) 1000 (d) 10000 (e) 1000000
(e) 1000000
The outdoor temperature today is closest to ______. (a) 170 k (b) 370 k (c) 0 k (d) 30 k (e) 270 k
(e) 270 k
What happens to small pieces of water when a wave crosses a pond as a result of dropping a stone into the pond? (a) It travels the same direction as the wave. (b) It moves back and forth parallel to the direction of the wave only. (c) It travels in the opposite direction from the wave. (d) It moves up and down only. (e) It moves in a small circle (which combines up/down and back/forth motion)
(e) It moves in a small circle (which combines up/down and back/forth motion)
In the class demonstration of spectra produced by gas discharge tubes, the tube containing neon showed ______. (a) a spectrum with only one emission line (b) a spectrum with only one absorption line (c) a continuous spectrum (d) a spectrum with many absorption lines (e) a spectrum with many emission lines
(e) a spectrum with many emission lines
In the class demonstration in which a match was lighted, by a beam of electromagnetic radiation that was focused on it, what type of radiation was used? (a) ultraviolet (b) visible (c) radio (d) microwave (e) infrared
(e) infrared
In what wavelength range do humans at room temperature radiate energy most strongly? (a) visible (b) radio (c) ultraviolet (d) microwave (e) infrared
(e) infrared
Which of the following is arranged from low to high frequency? (a) microwave, infrared, radio, X-ray, ultraviolet (b) gamma-ray, ultraviolet, visible, infrared, radio (c) ultraviolet, X-ray, visible, radio, infrared (d) infrared, radio, X-ray, ultraviolet, gamma-ray (e) radio, infrared, ultraviolet, X-ray, gamma-ray
(e) radio, infrared, ultraviolet, X-ray, gamma-ray
Which of the following properties is the same for all types of electromagnetic radiation in a vacuum? (a) wavelength (b) frequency (c) amplitude (d) energy (e) speed
(e) speed
Explain how we can determine the distance of a star if we know both its luminosity and apparent brightness.
Answer: LuminosityL,apparentbrightnessB,anddistancedarerelatedbytheinversesquare law for radiation: B=L d2 This is an equation with three variables, B, L, and d. If we have values for any two of these then we can solve for the third one. In the case that we know L and B, we can solve for d. Note that you are not expected to work through a case like this. Rather, you are expected to understand the underlying concept that since B, L, and d are related by a formula, having values for any two of these allows us to find the value of the third one.
What is a "neutrino telescope"? Where are these located and why?
Answer: Neutrino telescopes use large amounts of matter—often large water tanks—which cap- ture a very small fraction of the total number of neutrinos that pass through. Neutrino telescopes are located deep underground, to shield out other subatomic particles that, unlike neutrinos, can- not pass directly through the Earth.
Compare the lifetime of a red dwarf and a blue supergiant. What accounts for this difference in lifetime?
Answer: Red dwarf stars have long lifetimes, exceeding 1012 years. Blue supergiant stars have short lifetimes, about 2 × 107 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 (hydro- gen) than do red dwarfs, they nevertheless have much shorter lifetimes.
Sketch an H-R diagram. Label the axes (give the physical quantity corresponding to the axis; you do not need to put numbers on the axis). Use arrows to indicate in which direction the quantities increase. Sketch and label the locations of the main sequence, red giants, supergiants, and white dwarfs.
Answer: See Figures 11-7 and 11-8 on pp. 354 and 355 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.
Two stars have the same luminosity. Star 2 is 5× as distant as Star 1. How does the apparent brightness of Star 2 compare with that of Star 1?
Answer: Since both stars have the same luminosity, Star 2 must have a smaller apparent bright- ness than Star 1. Using the inverse square law, B = L/d2, we find that Star 2 has 1/25 the apparent brightness of Star 1.
Explain why a red giant star has a much higher luminosity than a main sequence star with the same surface temperature.
Answer: 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 pho- tospheres 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.
What is the definition of stellar parallax? What baseline is used in determining the parallax of a star and what is the length of this baseline? Draw a diagram showing how parallax is used in the determination of the distance of a nearby star.
Answer: 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. See Figure 11-1 on p. 343 of the text.
Describe the properties of a sunspot.
Answer: 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.
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?
Answer: 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 absorp- tion lines due to molecules which can only exist in the coolest stellar atmospheres. The Sun is a G star.
What is the Doppler effect?
Answer: 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).
Suppose that a spring is stretched between two points. If the spring vibrates so that its ends at mid-point remain stationary, and half the spring moves up while half the spring moves down, how many wavelengths are present on the spring? (a) 1 (b) 1/4 (c) 1/2 (d) 2 (e) 4
(a) 1
Of the following forms of cosmic electromagnetic radiation, which is (are) able to pass through the earth's atmosphere? 1. radio 2. X-ray 3. gamma ray (a) 1 only (b) 2 only (c) 3 only (d) both 1 and 2 (e) both 2 and 3
(a) 1 only
Consider an ultraviolet photon of wavelength λ1 = 250 nm and another ultraviolet photon of wavelength λ2 = 25 nm. The ratio of photon energies E1/E2 is: (a) 1/10 (b) 1/100 (c) 10 (d) 1/25 (e) 25
(a) 1/10
A nanometer (nm) is equal to _____. (a) 10^-9 m (b) 10^-3 m (c) 10^-6 m (d) 10^-4 m (e) 10^-12 m
(a) 10^-9 m
Suppose that ocean waves travel at 4 meters per second. If the frequency is 2 waves per second, what is the wavelength of the waves? (a) 2 meters (b) 4 meters (c) 1/2 meter (d) 8 meters (e) 1/4 meter
(a) 2 meters
Which constellation has "W" or "M" shape? (a) Cassiopeia (b) Ursa Major (c) Pegasus (d) Ursa Minor (e) Taurus
(a) Cassiopeia
Which constellation contains Polaris, the North Star? (a) Ursa Minor (b) Orion (c) Auriga (d) Ursa Major (e) Taurus
(a) Ursa Minor
As seen from Bloomington, the sky appears to rotate _____ about _____. (a) counterclockwise, Polaris (b) counterclockwise, the zenith (c) clockwise, Aldebaran (d) clockwise, the zenith (e) clockwise, Polaris
(a) counterclockwise, Polaris
Comparing black bodies of different temperature: (1) λ max_____ as temperature increases and (2) at any wavelength, a hotter black body has a ______ intensity. (a) decreases, larger (b) decreases, smaller, (c) remains the same, larger (d) increases, larger (e) increases, smaller
(a) decreases, larger
Comparing radio photons to ultraviolet photons, we can say that radio photons have _____ energy, _____ frequency, and _____ wavelength than do ultraviolet photons. (a) lower, lower, longer, (b) higher, higher, longer (c) higher, lower, shorter (d) lower, higher, shorter (e) lower, lower, shorter
(a) lower, lower, longer,
Which of the following has an absorption-line spectrum? 1. heated bar of iron 2 photosphere of the sun 3. neon sign (a) 1 only (b) 2 only (c) 3 only (d) 1 and 2 only (e) 2 and 3 only
(b) 2 only
100 km equals (a) 6 miles (b) 60 miles (c) 16 miles (d) 600 miles (e) 160 miles
(b) 60 miles
The Hα line has a wavelength of _____ and is in the _____ part of the spectrum. (a)122 nm, blue (b) 656 nm, red (c) 486 nm, red (d) 656 nm, blue-green (e) 486 nm, blue-green
(b) 656 nm, red
A professor shakes a spring stretched horizontally. As he shakes it faster (more shakes per second), the wavelength of the waves: (a) increases (b) decreases (c) stays the same (d) first increases and then decreases (e) first decreases and then increases
(b) decreases
Which type of photon behaves most like a particle? (a) ultraviolet (b) gamma ray (c) X-ray (d) visible (e) radio
(b) gamma ray
Orange light differs from blue light in that it (a) travels more quickly (through a vacuum) than blue light. (b) has a longer wavelength than blue light. (c) travels more slowly (through a vacuum) than blue light. (d) has a shorter wavelength than blue light. (e) has a higher frequency than blue light.
(b) has a longer wavelength than blue light.
In a plot of a black body curve, the axes are (a) frequency and temperature (b) intensity and wavelength (c) temperature and wavelength (d) wavelength and frequency (e) intensity and temperature
(b) intensity and wavelength
An astronomical unit (AU) is a unit of (a) time equal to the time taken for light to travel from the sun to earth. (b) length of the average distance between the sun and earth. (c) mass equal to one star mass (d) length equal to the radius of the sun (e) length equal to the distance travelled by light in one year.
(b) length of the average distance between the sun and earth.
Where do we find low-mass stars on the main sequence? High-mass stars?
Answer: 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.
Define the quantities luminosity and apparent brightness. Explain how two stars with the same luminosity can have different apparent brightness. Explain how two stars with the same apparent brightness can have different luminosity.
Answer: Luminosity is the energy per time radiated by an astronomical object (e.g. a star) into space. Apparent brightness is the energy per time per area detected from an astronomical object at Earth. Luminosity L, apparent brightness B, and distance d are related by the inverse square law for radiation: B=L d2 Two stars with the same luminosity will have different apparent brightnesses if they are at different distances. For example, if one of the two stars in this case is 3× farther away from us, then it will appear to be 9× fainter than the other, even though the luminosities are the same. Two stars with the same apparent brightness will have different luminosities if they are at different distances. In this case, the more distant star has a higher luminosity.
The Hipparcos satellite was able to reliably detect parallaxes as small as about 1/150 arcsec. What is the greatest distance to which it can measure a parallax? Express your answer both in parsecs and light years.
Answer: d= 1 = 1 =150parsec p 1/150 Since 1 pc = 3.3 ly, 150 pc = 500 ly.
Define the following structures/events observed in the Sun's atmosphere: spicule, prominence, flare.
Answer: • spicule - a jet of hot gas shooting upward in the chromosphere • prominence - a looping region of hot gas, supported by magnetic field, that extends into the corona • flare - a violent outburst of radiation and particles from a small area in the solar atmosphere
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?
Answer: 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.
How often does a sunspot maximum occur? What is the underlying cause of the solar cycle?
Answer: A sunspot maximum occurs once every eleven years (11 yr). The cycle of solar activity is a result of interaction between the Sun's magnetic field and its rotation.
What is the physical state of the material in the Sun?
Answer: 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.
What mechanisms transport energy in the Sun? Describe each process. Where is each mechanism important?
Answer: 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 30% of the solar radius just below the photosphere, where convection is most important.
What are the two most abundant elements in the Sun?
Answer: Hydrogen and helium are the two most abundant elements in the Sun (as well as in the universe as a whole).
What is hydrostatic equilibrium?
Answer: Hydrostatic equilibrium is a balance between the outward gas pressure in the Sun and the inward pull of gravity. The Sun supports its own weight by its outward gas pressure, so that it is neither contracting nor expanding.
9. What is the energy source of the Sun? Describe the physical process that releases energy. Discuss where this process occurs in the Sun and why it occurs there.
Answer: 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. The combined mass of the four hydrogen nuclei is slightly more than that of the resulting helium nucleus. The lost mass is converted to an equivalent amount of energy according to the relation E = mc2. Nuclear fusion occurs only in the core of the Sun, since it is only there that the temperature is sufficiently high (T > 107 K). High temperature is required for nuclear fusion, since nuclei must collide at high speed in order to overcome their electrical repulsion.
About how much longer will the Sun's present nuclear energy reserve last?
Answer: The Sun's present hydrogen energy reserve will last for another 5 × 109 years (5 bil- lion).
What is the average density of the Sun? How does this compare with water?
Answer: The average density of the Sun is 1400 kg/m3. This is similar to that of water (1000 kg/m3).
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?
Answer: The balloon will shrink somewhat, since the gas in the balloon will cool to the lower ambient temperature. The number of gas particles N in the balloon remains constant and the interior pressure P continues to be the same as the air pressure outside of the balloon. Thus, T and V are the only factors that vary in the formula in this case and it follows that a decrease in T produces a decrease in V , i.e. the balloon shrinks.
What are the central and surface temperatures of the Sun?
Answer: The central temperature of the Sun is about 1.6 × 107 K and the temperature in the photosphere is about 5800 K.
How much larger is the Sun than the Earth in diameter? In volume?
Answer: The diameter of Sun is about 100× larger than that of the Earth. Thus, the volume of the Sun is about 106× (one million times) larger than that of the Earth, i.e. if the Sun were hollow it would take 106 Earth-sized objects to fill it.
What do each of the quantities in the ideal gas law represent?
Answer: The ideal gas law can be written as: PV =NkT P is the pressure that the gas exerts on the walls of its container. V is the volume filled by the gas. N is the number of gas particles. k is Boltzmann's constant—a constant of nature. T is the temperature of the gas.
What is the evidence that convection occurs in the Sun?
Answer: The observation of granulation in the Sun's photosphere provides direct evidence of convection. The bright areas in the granulation pattern are the tops of rising columns of hotter gas. The dark areas are sinking columns of cooler gas.
What was the "solar neutrino problem"? What is the solution to this problem?
Answer: The solar neutrino problem was the finding that the number of neutrinos observed from the Sun by early neutrino detectors was was about one third the number that had been predicted by the standard solar model. The 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. Since only one type of neutrino was detected by early neutrino detectors, these experiments to measure the number of solar neutrinos being received found about one third of the number that was origi- nally predicted. Later generation neutrino detectors detect all types of neutrinos and confirm the predictions of the standard solar model.
What is the solar wind? What effect does it have on Earth?
Answer: 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. At periods of high solar activity, the resulting changes in the Earth's ionosphere can affect radio communications on Earth. In extreme cases, power distribution systems on Earth have shorted out as a result of changes in the ionosphere brought about by interaction with the solar wind.
Suppose that an astronomical object emits light with a rest wavelength of λrest = 400 nm that is observed to have a wavelength of λobs = 360 nm What is the speed of the object and in what direction is it moving?
Answer: UsetheDopplerformula: v = λobs − λrest c λrest v = 360nm−400nm =− 1 c 400 nm 10 The negative sign indicates motion toward the observer, i.e. the wavelength is blueshifted (ob- served wavelength is shorter than emitted wavelength). Thus, the object is approaching the observer at 0.1 c, i.e. 10% of the speed of light.
What are the parallaxes of stars at distances of: (a)1pc (b)3pc (c)15pc
Answer: d = 1/p, p = 1/d d = distance in parsecs (pc) p = parallax angle in seconds of arc (arcsec) Note that: 1 arc sec = 1/3600 degrees (a) d = 1 pc, p = 1/d = 1/1 = 1 arcsec (b) d = 3 pc, p = 1/d = 1/3 = 1/3 arcsec (c) d = 15 pc, p = 1/d = 1/15 = 1/15 arcsec
List the layers of the solar atmosphere, in order of increasing distance from the Sun. Discuss the temperature variation in the solar atmosphere.
Answer: layers: photosphere, chromosphere, transition zone, corona The temperature is about 5800 K in the photosphere, drops slightly going outward into the chro- mosphere, and then begins to rise sharply in the transition zone. The temperature reaches over 106 K in the corona.
The constellation Canis Major is also known as ______ and its brightest star is _____ "dog star"
Big Dog, Sirius
The constellation Auriga is also known as __________, and its brightest star is ______, "the goat star"
The Charioteer, Capella