Astronomy Exam 2
Star A is four times as luminous as star B. Star B's spectrum peaks at 300 nm, while star A's spectrum peaks at 150 nm. How large is the radius of star A compared to star B?
1/2 as large
Star A and Star B have different apparent brightnesses but identical luminosities. If Star A is 20 light-years away from Earth and Star B is 40 light-years away from Earth, which star appears brighter and by what factor?
Because Star A and Star B have identical luminosities, the difference in their apparent brightness is due solely to distance. Star B is twice as far from Earth as Star A. Star A is therefore 22 = 4 times brighter than Star B.
Arrange the following layers of the sun in order from most to least dense.
core, radiative zone, convective zone, photosphere
Which of the following properties decreases from the sun's core to it's outermost layer?
density
William Herschel
discovered castor was a binary star system
John Baptiste Riccioli
discovered first binary star
Convection in the sun can be observed through:
granulation
Which of the following aspects of sunspots varies regularly with the solar magnetic cycle?
latitude
The Hubble space telescope has detected galaxies with apparent magnitudes of about 30. Approximately how much fainter are these galaxies than stars with an apparent magnitude of 5?
10^10 times
A star has a parallax of 0.05 arcseconds. How far away from Earth is the star?
20 parsecs
Star A is twice as luminous as star B but has half the temperature of star B. How large is the radius of star A compared to star B?
2^5/2 as large
A sunspot has a temperature 80% as large as the rest of the sun's surface. How much less light does it produce per unit area than the rest of the sun?
40% as much
A G2 star has a luminosity 100 times that of the Sun. What kind of star is it? How does its radius compare with that of the Sun?
A G star with a luminosity of 100 LSun is a giant. Since it has the same temperature as the Sun but emits 100 times more energy, its surface area must be 100 times larger. Since the area of a sphere is given by A = 4pR2, the radius of the giant G star must be the square root of 100 or 10 times larger than the radius of the Sun.
Assuming an average sunspot cycle of 11 years, how many revolutions does the equator of the Sun make during that one cycle? Do higher latitudes make more or fewer revolutions compared to the equator?
According to the text, the Sun's equator has a period of 25 days. Divide 11 years by 25 days, and convert the units of one of them to find the ratio (no units). Since the higher latitudes have longer periods, they move more slowly and make few revolutions in the same cycle: 11 y/25 d × 365 d/y = 161 revolutions.
Which changes by the largest factor along the main sequence from spectral types O to M—mass or luminosity?
According to the text, the mass varies from only about 100 MSun to about 0.08 MSun, or by about a factor of 1000 along the main sequence, whereas the luminosity varies from 106 for the most luminous stars to less than 0.001 or a factor of more than 109 for the least luminous.
Joseph Fraunhofer
Analyzed solar spectrum of sun with spectroscope. Saw more that 800 dark lines on top of the continuous spectrum caused by absorbtion of some of the wavelengths by solar atmosphere. Called Fraunhofer lines
Describe how the mass, luminosity, surface temperature, and radius of main-sequence stars change in value going from the "bottom" to the "top" of the main sequence.
At the bottom, the mass, luminosity, surface temperature, and radius are all at their lowest values. As you head to the top of the main sequence, the values all increase and are at a maximum at the top. The values that change the most are luminosity and temperature. Radius has the least amount of change in value. This is based upon the values in Table 18.3 Characteristics of Main-Sequence Stars.
Explain how you would use the spectrum of a star to estimate its distance.
Begin by examining the detailed spectrum to estimate the star's spectral type and luminosity class. Use this information to find the place where the star belongs on the H-R diagram, and then read off the star's luminosity from the diagram. Finally, measure the distance by comparing the apparent brightness to the luminosity.
Summarize the evidence indicating that over several hundreds of years or more there have been variations in the level of the solar activity.
Counts of sunspots infer the overall magnetic field changes, which correlate with the level of the magnitude of the solar dynamo and hence solar activity. Astronomers have reliable data going back to 1750.
Annie Cannon
Created the spectral classification system based on surface temperature that astronomers use today. She also classified the spectra of around 500,000 stars in her career.
Kepler's Third Law
D^3=(M1+M2)P^2
John Flamsteed
Developed a star cataloguing system. Brighter stars got a number in each constellation in order of their location in the sky, or their right ascension.
Johann Bayer
Developed a system for naming stars that used Greek letters and Latin forms of the stars' constellation names
Willamina Flemming
Devised a system to clarify stars based on the strength of their hydrogen absorption lines.
Edward C. Pickering
Discovered a second class of binary stars
Describe how energy makes its way from the nuclear core of the Sun to the atmosphere. Include the name of each layer and how energy moves through the layer.
Energy is released as a result of nuclear reactions in the core of the Sun and travels upward (outward) in the form of light. It keeps doing that in the radiative layer, but as the temperature of the layer drops, the energy (wavelength) of the light drops as well. When the energy gets up to the convective layer, energy gets to the surface by moving the hot material of the Sun itself upward. The energy is released at the surface as light, cools the material, and the cooled material sinks back down again.
Suppose you have discovered a new cepheid variable star. What steps would you take to determine its distance?
First, measure the period of pulsation of the star. Then, use a period-luminosity relation to calculate the mean luminosity. Finally, compare the luminosity to the mean apparent brightness and calculate a distance.
How is a neutrino different from a neutron? List all the ways you can think of.
First, the neutrino's mass is much, much smaller; second, it hardly interacts with matter at all, where as a neutron interacts with other particles; third, it isn't one of the particles to make up an atom; fourth, it can "oscillate" (change from one type of neutrino to another between the Sun's core and Earth).
You go out stargazing one night, and someone asks you how far away the brightest stars we see in the sky without a telescope are. What would be a good, general response?
Generally, the stars that appear the brightest are also quite distant. Those stars are typically over 100 light-years away.
Sir William and Lady Margaret
Identified lines in the stellar spectra that were identical to those of elements on Earth.
Describe two ways of determining the diameter of a star.
In one method, the time for an object like the Moon to pass in front of a star can be measured to determine the diameter of a star. Since we know the speed of the Moon in its orbit, we can calculate the size of the star. For an eclipsing binary star, the time for one star to pass in front of another is dependent upon the relative diameters of each star. When the eclipses are aligned in such a way that they eclipse each other, we can measure the time for each star to eclipse the other. We can measure the speed of the stars from the Doppler shift in the spectrum. From knowing the time of eclipse and the speed, the size of each star can be determined.
Is the Sun an average star? Why or why not?
In some ways, the Sun is an average star. Its luminosity, mass, and temperature are close to the middle of the range of the extreme values seen for all stars. (You might say it is a "median" star, in this sense.) The Sun is also a main sequence star, as are about 90% of all the stars. In other ways, it is atypical: the vast majority of stars have a lower luminosity, lower mass, and lower temperature than does the Sun.
Friedrick Bessel, Thomas Henderson, and Friedrick Struve
Independently measured the parallaxes of the stars 61 Cygni, Alpha Centauri, and Vega.
What happens to a gamma ray photon produced in the sun's interior as it travels through the radiative zone?
It scatters every few centimeters and is transformed into many lower energy photons.
Describe the spectra for a spectroscopic binary for a system comprised of an F-type and L-type star. Assume that the system is too far away to be able to easily observe the L-type star.
L-type stars are very faint and cool, and have low mass. The spectrum of the F-type star would dominate and show the Doppler effect as it orbits slowly, although the amount of shift in the spectrum would be very small. No spectral features from the other star would likely be visible, so over time it would appear as if the spectral features for the F-type star would shift in a periodic manner.
Why is a higher temperature required to fuse hydrogen to helium by means of the CNO cycle than is required by the process that occurs in the Sun, which involves only isotopes of hydrogen and helium?
Like charges repel each other. The charge of a carbon nucleus is six times that of a proton. If a carbon nucleus and a proton are to interact, they must be moving at high enough speed to overcome the repulsive force of their positive charges. This repulsion is stronger than in the case of two protons. Since atoms move more rapidly at higher temperature, the CNO cycle requires a higher temperature than the fusion of hydrogen to form helium.
How to find luminosity with mass
Luminosity is equal to the objects mass to the 3.9th power
William Huggins
Made the first radial velocity determination of a star
How do objects of spectral types L, T, and Y differ from those of the other spectral types?
Many objects of spectral types L, T, and Y are brown dwarfs—that is, they are not massive enough to sustain nuclear fusion in their cores.
How to find mass with luminosity
Mass is equal to luminosity to the 0.25th power
What are the largest- and smallest-known values of the mass, luminosity, surface temperature, and diameter of stars (roughly)?
Mass ranges from more than 100 times the Sun's mass (up to 250 times the Sun's mass) down to 1/12 the Sun's mass. Luminosity ranges from a million times the Sun's luminosity down to 1/10,000 of the Sun's. Surface temperature ranges from nearly 40,000 K down to 2700 K. Diameter ranges from 1000 times the Sun's diameter down to 1/10 the Sun's diameter.
The spectrum of the Sun has hundreds of strong lines of nonionized iron but only a few, very weak lines of helium. A star of spectral type B has very strong lines of helium but very weak iron lines. Do these differences mean that the Sun contains more iron and less helium than the B star?
No. The primary reason that stellar spectra look different is the stars have different temperatures. Most stars have compositions very similar to that of the Sun.
Order the seven basic spectral types from hottest to coldest.
OBAFGKM
Compare and contrast the four different types of solar activity above the photosphere.
Plages are regions of higher density and temperature than the surrounding material in the chromosphere. Prominences are huge loops of the Sun's ionized but cool material that are gently pushed by magnetic force from the chromosphere into the corona. Flares are brief, violent explosions that are hot and release a lot of energy. Coronal mass ejections happen when a flare is so violent that the flare material exceeds the escape velocity of the Sun and is ejected out into the solar system. All of these are physically related to sunspots.
The star Antares has an apparent magnitude of 1.0, whereas the star Procyon has an apparent magnitude of 0.4. Which star appears brighter in the sky?
Procyon appears brighter in the sky
What mechanism transfers heat away from the surface of the Moon? If the Moon is losing energy in this way, why does it not simply become colder and colder?
Radiation is the mechanism that transports heat away from the surface of the Moon. Since space is nearly a vacuum, the alternative mechanisms of conduction and convection cannot work. The Moon does become colder at night. It also becomes hotter and hotter during the day. Any location on the surface of the Moon is heated by the radiation from the Sun, which raises its temperature. When that location faces away from the Sun, it radiates heat away from the Moon's surface. Overall, the total amount of heat received from the Sun during the day equals the total amount of heat radiated away into space.
The star Sirius A has an apparent magnitude of −1.5. Sirius A has a dim companion, Sirius B, that is, 10,000 times less bright than Sirius A. What is the apparent magnitude of Sirius B? Can Sirius B be seen with the naked eye?
Remember that a difference in brightness of a factor of 100 corresponds to a difference of five magnitudes. We note that 10,000 = 100^2, so the difference would be 10 magnitudes. Therefore, the magnitude of Sirius B is -1.5 + 10 = 8.5. This is too faint to be seen by the naked eye.
Two stars have proper motions of one arcsecond per year. Star A is 20 light-years from Earth, and Star B is 10 light-years away from Earth. Which one has the faster velocity in space?
Since Star A is farther from Earth, its velocity in space must be greater than Star B's to produce the same proper motion (change of angle) as seen from Earth.
Suppose there are three stars in space, each moving at 100 km/s. Star A is moving across (i.e., perpendicular to) our line of sight, Star B is moving directly away from Earth, and Star C is moving away from Earth, but at a 30° angle to the line of sight. From which star will you observe the greatest Doppler shift? From which star will you observe the smallest Doppler shift?
Since Star B's motion is directly away from Earth, it will show the greatest Doppler shift. Star A, with its motion neither toward nor away from Earth, will show the least Doppler shift—no shift at all.
Suppose an (extremely hypothetical) elongated sunspot forms that extends from a latitude of 30° to a latitude of 40° along a fixed of longitude on the Sun. How will the appearance of that sunspot change as the Sun rotates?
Since higher-latitude regions on the Sun take longer to complete one full rotation, the sunspot will slowly tilt with respect to a constant line of longitude, with the part of the sunspot closest to the equator racing ahead of the part closest to the pole. In a few weeks, the spot would be elongated and pulled apart by these differences in the rotation period.
As seen from Earth, the Sun has an apparent magnitude of about −26.7. What is the apparent magnitude of the Sun as seen from Saturn, about 10 AU away? (Remember that one AU is the distance from Earth to the Sun and that the brightness decreases as the inverse square of the distance.) Would the Sun still be the brightest star in the sky?
Since the apparent brightness of an object decreases with the square of the distance to the object, the Sun's apparent brightness would be 102 = 100 times fainter from Saturn as viewed from Earth. Recall that a difference of a factor of 100 in apparent brightness corresponds to a difference of 5 in apparent magnitude. Therefore, the apparent magnitude of the Sun from Saturn is -26.7 + 5 = -21.7. Since the distances to the stars would not significantly change from Saturn, the apparent magnitudes of the stars would not change. Therefore, the Sun is still far brighter than Sirius, the brightest star in the sky (-1.5 apparent magnitude).
Neutrinos produced in the core of the Sun carry energy to its exterior. Is the mechanism for this energy transport conduction, convection, or radiation?
Since the majority of neutrinos produced inside the Sun do not interact with other particles as they leave the Sun, the energy they carry is transported as radiation.
Two stars orbit each other on circular orbits. The star with the larger orbital radius has a:
Smaller mass
How does activity on the Sun affect natural phenomena on Earth?
Solar activity can affect the aurora, weather, and climate.
You are able to take spectra of both stars in an eclipsing binary system. List all properties of the stars that can be measured from their spectra and light curves.
Spin and radial velocities, chemical composition, and radii/diameters.
Star X has lines of ionized helium in its spectrum, and star Y has bands of titanium oxide. Which is hotter? Why? The spectrum of star Z shows lines of ionized helium and also molecular bands of titanium oxide. What is strange about this spectrum? Can you suggest an explanation?
Star X is hotter. Looking at the Figure 17.5 Absorption Lines in Stars of Different Temperatures, we can see that ionized helium lines are strongest at high temperatures (> 30,000 K), whereas titanium oxide lines are strongest at lower temperatures (~3000 K). Therefore, Star X must be hotter than Star Y. Star Z's spectrum is strange because the presence of ionized helium and titanium oxide lines indicate both high and low temperatures at the same time. One explanation for this strange spectrum would be that Star Z is a hot star, like Star X, which creates the ionized helium lines, and that there is a cooler cloud of intervening material along the line of sight that creates the observed TiO lines. Another explanation might be that what appears to be a single star is actually two stars so close together we cannot separate them visually—a hot star with a cool companion.
How do we distinguish stars from brown dwarfs? How do we distinguish brown dwarfs from planets?
Stars have mass greater than 1/12th of the Sun's mass; brown dwarfs generally have between 1/100th and 1/12th the mass of our Sun; planets have masses less than that.
Starting from the core of the Sun and going outward, the temperature decreases. Yet, above the photosphere, the temperature increases. How can this be?
Temperature is related to the average kinetic energy of the material. The magnetic fields that erupt into the chromosphere and corona dump energy into the particles there. Since it's not as dense, the particles in the chromosphere and corona can get to high velocities and so have high temperatures.
Name five characteristics of a star that can be determined by measuring its spectrum. Explain how you would use a spectrum to determine these characteristics.
Temperature: Measure the relative strengths of spectral lines to determine a star's spectral class, for example, OBAFGKM. Spectral class corresponds to temperature. Composition: Use computer models and temperature to determine elemental abundances from relative strengths of absorption lines in a star's spectrum. Classify a star as a dwarf or giant: Measure the width of spectral lines. If the lines are narrow, the star's diameter is large. If the lines are wider, the star's diameter is smaller. Radial velocity: Measure the wavelengths of the lines in the star's spectrum. Compare the observed wavelengths to the known "rest wavelengths" of the lines to determine the Doppler shift. The Doppler shift of the star is determined by its radial velocity—its motion toward or away from Earth. Rotation: Measure the width of the star's spectral lines. The star's rotation creates a broadening of the spectral lines, which can be used to determine the star's rotation.
Why can only a lower limit to the rate of stellar rotation be determined from line broadening rather than the actual rotation rate?
The Doppler shift can only detect radial motion, that is, motion toward or away from the observer. If the axis of rotation is perpendicular to the line of sight, then the full rotational motion is radial. However, if the axis of rotation is parallel to the line of sight, then none of the rotational motion is radial. In most cases, the inclination of the rotational axis will be somewhere between these extremes, so part of the rotational motion will be radial, but not the full rotational motion. This means that the rotational motion detected by line broadening will usually not be the full rotational motion.
One method to measure the diameter of a star is to use an object like the Moon or a planet to block out its light and to measure the time it takes to cover up the object. Why is this method used more often with the Moon rather than the planets, even though there are more planets?
The Moon is a much larger-appearing object than any of the planets, and the chances of it passing directly in front of a star are greater than a planet, which would have to be very precisely aligned with the star.
What are the two sources of particles coming from the Sun that cause space weather? How are they different?
The Sun constantly throws off particles from the surface called the solar wind; this is constant and predictable over hundreds of years. The solar activity cycle can generate solar storms from coronal mass ejections, which are violent and hard to predict.
Describe the main differences between the composition of Earth and that of the Sun.
The Sun is composed primarily of hydrogen and helium, and its elements exist in the form of gases because of this hot temperature. Earth, in contrast, is made mostly of heavier elements and includes many in liquid and solid form.
If the star Sirius emits 23 times more energy than the Sun, why does the Sun appear brighter in the sky?
The Sun is much closer to Earth than Sirius, so its apparent brightness is greater than that of Sirius.
What conditions are required before proton-proton chain fusion can start in the Sun?
The Sun must be dense and hot enough in the center for the motion of the protons to overcome their mutual repulsion, with a temperature of at least 12 million K.
What it the Zeeman effect and what does it tell us about the Sun?
The Zeeman effect is the splitting of spectral lines into several closely spaced lines due to the presence of a sunspot's magnetic field. The magnitude of the splitting tells us the strength of the local magnetic field on the Sun.
Two stars are in a visual binary star system that we see face on. One star is very massive whereas the other is much less massive. Assuming circular orbits, describe their relative orbits in terms of orbit size, period, and orbital velocity.
The larger-mass star would move in a smaller circle around the center of mass since it would be located closer to the center of mass, whereas the low-mass star would be found to have a large orbit farther from the center of mass. Both orbits would be concentric, located around the center of mass. The orbital periods would be the same for each object, similar to the objects on the seesaw—the distance from the center of mass doesn't influence the period. The velocity for the smaller-mass star would be faster than that of the more massive star since it has a larger orbit size compared to the massive star.
Explain why color is a measure of a star's temperature.
The light emitted by a star approximates a blackbody. The hotter a star's temperature, the shorter the peak wavelength of its spectral curve. Therefore, cool stars exhibit reddish colors, whereas hot stars exhibit bluish colors.
What two factors determine how bright a star appears to be in the sky?
The luminosity and distance of a star determine its apparent brightness in the sky.
What do measurements of the number of neutrinos emitted by the Sun tell us about conditions deep in the solar interior?
The neutrinos are being produced in the solar core by fusion reactions, and measuring their number gives us a sensitive probe into what is happening in the Sun's core. It helps confirm that there are enough proton-proton chain reactions (each of which produces a neutrino) going on in the Sun's core to explain the energy output of the Sun.
Which aspects of the Sun's activity cycle have a period of about 11 years? Which vary during intervals of about 22 years?
The number of sunspots goes from very few (maybe none) to about one hundred at one time (maximum) and back to very few (minimum) in cycles ranging from 9 to 14 years. During each cycle, the north or south magnetic pole of the sunspots leads. In the next cycle, the polarity reverses. So the overall magnetic activity of the Sun has an average cycle of 22 years.
Suppose the proton-proton cycle in the Sun were to slow down suddenly and generate energy at only 95% of its current rate. Would an observer on Earth see an immediate decrease in the Sun's brightness? Would she immediately see a decrease in the number of neutrinos emitted by the Sun?
The observer would see a decrease in the number of neutrinos almost immediately since neutrinos take only about 2 seconds to travel from the center of the Sun to its surface and another 8 minutes to reach Earth. Light, on the other hand, takes about 105 to 106 years to traverse the distance from the center of the Sun to its surface, so 105 to 106 years would elapse before an observer on Earth saw a decrease in the brightness of the Sun.
Explain how parallax measurements can be used to determine distances to stars. Why can we not make accurate measurements of parallax beyond a certain distance?
The parallax of a star is its shift in position during the course of half a year as measured against stationary background objects (distant stars or quasars). As a star gets more distant, the shift decreases, making more distant stars more difficult to measure.
Describe in your own words what is meant by the statement that the Sun is in hydrostatic equilibrium.
The pressure and gravity are in balance throughout the Sun, from the very center to the surface. This means the gas pressure at any depth within the Sun can support the weight of all of the gas pressing down upon it, due to gravity. So the Sun neither expands nor contracts but remains as it is; this is true at every point within the Sun as well as for the Sun overall.
What is the main reason that the spectra of all stars are not identical? Explain.
The primary reason that stellar spectra look different is that the stars have different temperatures. Each element (and ion) has a characteristic temperature at which the spectral lines it produces are strongest. Stars of different temperatures, therefore, exhibit different spectral lines.
Explain how the theory of the Sun's dynamo results in an average 22-year solar activity cycle. Include the location and mechanism for the dynamo.
The solar dynamo is thought to be generated by ions in the lower part of the Sun's convective zone. Moving charged particles (in this case, the ions) generate a magnetic field. The fact that the Sun's fluid layers spin at different speeds at different latitudes then causes these magnetic fields to twist and distort, reversing about every 11 years.
Why do you think astronomers have suggested three different spectral types (L, T, and Y) for the brown dwarfs instead of M? Why was one not enough?
The surface temperatures of brown dwarfs range from under 700 K to around 2400 K. Brown dwarfs of different surface temperatures show distinct characteristic spectral lines. Class L dwarfs show lines for metal hydrides and alkali metals, T dwarfs show methane lines, and Y dwarfs show ammonia lines.
Which of the following energy transport mechanisms does not occur to a significant degree in the sun?
conduction
Name and describe the three types of binary systems.
The three types of binary systems are spectroscopic, visual, and eclipsing. A spectroscopic binary star is a binary star in which the components are not seen separately, but whose binary nature is indicated by periodic variations in radial velocity (changes in the Doppler shift of the spectral lines), indicating orbital motion. A visual binary is a binary star in which the two components are telescopically resolved (can be seen individually). An eclipsing binary star is a binary star in which the plane of revolution of the two stars is nearly edge-on to our line of sight, so that periodically, one star blocks the light of the other by passing in front of it.
The eclipsing binary Algol drops from maximum to minimum brightness in about 4 hours, remains at minimum brightness for 20 minutes, and then takes another 4 hours to return to maximum brightness. Assume that we view this system exactly edge-on, so that one star crosses directly in front of the other. Is one star much larger than the other, or are they fairly similar in size?
The two stars are fairly similar in size. Let star A be the star that is being eclipsed. In 4 hours, star A moves a distance equal to its own diameter. In 4 hours 20 minutes, star A moves a distance equal to the diameter of the star that is eclipsing star A. (Try drawing a diagram to confirm this statement.) Therefore, star A is 4 hours/(4 hours 20 minutes) = 92% as large as its companion.
If a visual binary system were to have two equal-mass stars, how would they be located relative to the center of the mass of the system? What would you observe as you watched these stars as they orbited the center of mass, assuming very circular orbits, and assuming the orbit was face on to your view?
The two stars would be at an equal distance from the center of mass, like an evenly balanced seesaw. They would orbit around the center of mass on opposite sides of a circular orbit—basically, a circle with the two objects always equally spaced apart.
How do we know the age of the Sun?
Through radioactive dating of rocks, we can determine the age of Earth, the Moon, and meteorites to be about 4.5 billion years. Our models of the formation of the solar system and observations of the formation of other stars with planets tell us that the Sun formed at the same time as the other members of our solar system.
Do neutrinos have mass? Describe how the answer to this question has changed over time and why.
Yes, they do have mass, and they have always had mass. Human just didn't know that at first. When neutrinos were first proposed by Pauli, physicists thought they were massless particles (all energy).
What quantities must be measured in order to calculate the luminosity of a star?
brightness and distance
What property of a star can best be used to determine it's temperature?
color
In which of the following ways are oxygen-16 and oxygen-17 the same?
number of protons
What properties of Cepheid variable stars are most often measured to determine their distance?
period and brightness
What causes a star to leave the main sequence?
the consumption of the stellar core's hydrogen
What was Henrietta Leavitt's most important contribution to astronomy?
the period-luminosity relation of Cepheid variables
Order these layers of the sun from the bottom up: photosphere, chromosphere, and corona. What is the approximate temperature of each of these regions?
the photosphere (5800 K), the chromosphere (10,000 K), and the corona (1,000,000 K).
What is the fate of positrons produced through fusion in the sun's core?
they are annihilated
Why are sunspots dark?
they are cooler than their surroundings.
Which method would you use to obtain the distance to each of the following?A. An asteroid crossing Earth's orbit B. A star astronomers believe to be no more than 50 light-years from the Sun C. A tight group of stars in the Milky Way Galaxy that includes a significant number of variable stars D. A star that is not variable but for which you can obtain a clearly defined spectrum.
A. Radar would be the best tool for measuring distances to objects in the solar system. B. A parallax measurement would be best for this nearby star. C. Cepheids or RR Lyraes would be useful for determining the distance to this cluster. D. The method using the H-R diagram and getting a spectrum to determine the luminosity class of the star.
Since the rotation period of the Sun can be determined by observing the apparent motions of sunspots, a correction must be made for the orbital motion of Earth. Explain what the correction is and how it arises.
It will take slightly more than the true (sidereal) rotation period of the Sun to bring a spot around the edge again because Earth moves farther along in the same direction as the Sun rotates.