Astronomy Midterm #2
Order the seven basic spectral types from hottest to coldest.
OBAFGKM
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).
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? Explain.
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.
Approximately 6000 stars are bright enough to be seen without a telescope. Are any of these white dwarfs? Use the information given in this chapter to explain your reasoning.
None of the stars visible to the unaided eye are white dwarfs. White dwarfs are hot but not very luminous since their surface area is so small. Just as we need telescopes to see low-luminosity, main-sequence stars, we need telescopes to see white dwarfs. (For the instructor, to be visible to the naked eye, a typical white dwarf would have to be at a distance of 3.26 × 10-3 light-years. The nearest star, Alpha Centauri, is at a distance of 4.3 light-years.)
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.
An astronomer discovers a type-M star with a large luminosity. How is this possible? What kind of star is it?
Since M stars are cool and emit very little energy per unit area (say per square meter), the only way that an M star can have a high luminosity is if it is very large (i.e., has a lot of square meters of surface area). This star is either a giant or a supergiant.
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.
How does activity on the Sun affect human technology on Earth and in the rest of the solar system?
Solar activity can affect satellite orbits, communication satellites, and the local power grids. It can also impact our spacecraft throughout the solar system, especially orbiters or landers on surfaces without an atmosphere.
How does activity on the Sun affect natural phenomena on Earth?
Solar activity can affect the aurora, weather, and climate.
Which is easier to observe at large distances—a spectroscopic binary or a visual binary?
Spectroscopic binaries are easier to observe at great distances, since the velocities of the stars about one another (and the Doppler shift in the spectrum) aren't impacted by the distance from the observer. A visual binary is harder to detect since, at a great distance, the separation of the two stars becomes too small to be detected—the light of the stars merges together at very great distances.
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.
What elements are stars mostly made of? How do we know this?
Stars are mostly made of hydrogen and helium. We know this by analyzing the relative strengths of absorption lines in their spectra.
What is the defining difference between a brown dwarf and a true star?
Stars have internal temperatures capable of sustaining hydrogen fusion. Brown dwarfs do not.
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. (In future chapters, a different criterion—the type of fusion each one experiences or doesn't experience—will be added to this list.)
Why do sunspots look dark?
Sunspots appear dark because they are cooler than the surrounding area of the Sun.
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.
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.
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.
How can the prominences, which are so big and 'float' in the corona, stay gravitationally attached to the Sun while flares can escape?
The material has to achieve or exceed escape velocity from the Sun. Energetic flares can reach high enough speeds to escape (about 600 km/s), but the prominences do not.
Two astronomy students travel to South Dakota. One stands on Earth's surface and enjoys some sunshine. At the same time, the other descends into a gold mine where neutrinos are detected, arriving in time to detect the creation of a new radioactive argon nucleus. Although the photon at the surface and the neutrinos in the mine arrive at the same time, they have had very different histories. Describe the differences.
The neutrino was generated in a fusion reaction in the Sun's core and made it out of the Sun in about 2 seconds and then continued through space until it arrived at Earth about 500 seconds later. Since this neutrino was detected by this process, we know it avoided oscillating and changing into a different kind of neutrino. The photon, which left the Sun's photosphere as sunshine, is "descended" from a gamma-ray photon that was created 100,000 to 1,000,000 years ago by fusion. That gamma ray was absorbed by a particle that reemitted a photon in a totally random direction. That process repeated itself over and over again until some of that energy emerged from the Sun's surface as a photon of visible light. That photon then also arrived at Earth a little more than 8 minutes later.
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 you want to determine the average educational level of people throughout the nation. Since it would be a great deal of work to survey every citizen, you decide to make your task easier by asking only the people on your campus. Will you get an accurate answer? Will your survey be distorted by a selection effect? Explain.
The people on a college campus have, on average, a higher level of education than does the population as a whole. Therefore, conducting the proposed survey on campus will not give a valid answer for the nation. Your sample suffers from the fact that you have preferentially selected people who have attended college (and, in the cases of faculty, graduate school).
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.
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.
Why would a flare be observed in visible light, when they are so much brighter in X-ray and ultraviolet light?
X-ray and ultraviolet light are absorbed by Earth's atmosphere, so those telescopes have to be in space and are relatively expensive. Less expensive visible-light telescopes can be used to observe the Sun since it's so close and the flares are still bright in visible light.
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).
Suppose you live in northern Canada and an extremely strong flare is reported on the Sun. What precautions might you take? What might be a positive result?
A strong flare may signal the ejection of charged particles from the Sun that can interfere with the operation of power stations and even cause the temporary loss of power on Earth. It might be appropriate to stock some candles, some wood for the fireplace, and some batteries to run devices so you can get information about when power might be restored. Since aurorae are often caused by these same charged particles, you might be treated to a fine display of the northern lights.
What did Annie Cannon contribute to the understanding of stellar spectra?
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.
Based on their colors, which of the following stars is hottest? Which is coolest? Archenar (blue), Betelgeuse (red), Capella (yellow).
Archenar is hottest. Betelgeuse is coolest.
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.
Do stars that look brighter in the sky have larger or smaller magnitudes than fainter stars?
Brighter stars have smaller magnitudes than fainter stars.
Earth contains radioactive elements whose decay produces neutrinos. How might we use neutrinos to determine how these elements are distributed in Earth's interior?
By detecting the direction from which the neutrinos arrive at detectors in different locations, we can triangulate the location of potential sources within Earth. Note to instructors: Such detection of anti-neutrinos from decay processes within Earth is actually being done by the Kamioka Liquid Scintillator Antineutrino Detector.
There are fewer eclipsing binaries than spectroscopic binaries. Explain why.
Close to the Sun, there are more visual binaries than eclipsing binaries because it is possible to see two well separated stars at nearly any orientation, while eclipsing binaries have to be aligned just right. At large distances, the apparent separation between the two components of a visual binary decreases, and it becomes difficult to see the two stars separately. (If students want to consider all kinds of binaries, they could add that to detect a spectroscopic binary, it is only necessary to measure changes in wavelength of spectral lines due to orbital motion and the Doppler effect. Provided at least one member of the binary system is bright enough to measure spectroscopically, this orbital motion can be measured.)
Describe what a typical star in the Galaxy would be like compared to the Sun.
Cool, faint, low-mass stars located on the lower part of the main sequence are the most common, and therefore the most typical stars.
Describe the two main ways that energy travels through the Sun.
Energy is created through fusion of hydrogen into helium at the center of the Sun. This energy first enters the radiative region of the Sun, where photons are absorbed and reemitted in random directions until they make it about 2/3 of the way to the surface. Once the energy has traveled 2/3 of the way to the surface, it reaches the convection zone where the hot gas flows up and down (like water boiling in a pot) carrying the energy to the solar surface.
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.
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.
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.
The Sun is much larger and more massive than Earth. Do you think the average density of the Sun is larger or smaller than that of Earth? Write down your answer before you look up the densities. Now find the values of the densities elsewhere in this text. Were you right? Explain clearly the meanings of density and mass.
Mass is the total amount of material in a body, whereas density is the mass per unit volume. From Appendix E, the mass of the Sun is about 2 × 1030 kg, while the mass of Earth is 6 × 1024 kg. From Appendix F, the mean density of Earth is 5.5 g/cm3 or 5500 kg/m3. In an earlier chapter, we calculated the average density of the Sun to be 1.4 g/cm3.
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.
What is the ultimate source of energy that makes the Sun shine?
Matter that is converted into energy through the fusion of hydrogen into helium.
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.
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.
How does the mass of the Sun compare with that of other stars in our local neighborhood?
The Sun is more massive than the majority of stars in our neighborhood. Only a few other stars are more massive, whereas the vast majority are lower-mass stars.
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.
A friend who has not had the benefit of an astronomy course suggests that the Sun must be full of burning coal to shine as brightly as it does. List as many arguments as you can against this hypothesis.
The Sun is too hot to contain solid coal. Also, its density is lower than the density of coal. Even if the Sun were made of coal, burning coal produces energy so inefficiently that the Sun could keep shining at its present rate for only a few thousand years, yet we have geological and fossil evidence that Earth has been warm (and the Sun must therefore have been producing energy) for billions of years.
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.
Someone suggests that astronomers build a special gamma-ray detector to detect gamma rays produced during the proton-proton chain in the core of the Sun, just like they built a neutrino detector. Explain why this would be a fruitless effort.
The gamma rays produced in the proton-proton chain are quickly absorbed by surrounding atoms in the Sun and reemitted with slightly less energy. This process occurs so many times that by the time the photons are released from the Sun, their energy has dropped from gamma-ray level to visible-light or UV-photon level. Thus, it is extremely unlikely that any gamma rays from the core of the Sun would be detected.
The edge of the Sun doesn't have to be absolutely sharp in order to look that way to us. It just has to go from being transparent to being completely opaque in a distance that is smaller than your eye can resolve. Remember from the chapter on Astronomical Instruments that the ability to resolve detail depends on the size of the telescope's aperture. The pupil of your eye is very small relative to the size of a telescope and therefore is very limited in the amount of detail you can see. In fact, your eye cannot see details that are smaller than 1/30 of the diameter of the Sun (about 1 arcminute). Nearly all the light from the Sun emerges from a layer that is only about 400 km thick. What fraction is this of the diameter of the Sun? How does this compare with the ability of the human eye to resolve detail? Suppose we could see light emerging directly from a layer that was 300,000 km thick. Would the Sun appear to have a sharp edge?
The radius of the Sun, according to Table 15.1 Characteristics of the Sun, is about 7 x 105 km. Since the photosphere is 400 km thick, this is only (400)/(700,000) = 0.0006 of the Sun's radius. In other words, the Sun goes from being completely transparent to completely opaque over a distance that is tiny compared to its radius, and the edge of the Sun looks sharp. If the photosphere were 300,000 km thick, it would span nearly half the Sun's radius. This means that the transition from transparent to opaque would happen gradually over this large distance and the Sun would appear to have a very fuzzy boundary, not a sharp edge.
If you were concerned about space weather and wanted to avoid it, where would be the safest place on Earth for you to live?
The safest place to live on Earth would be on the equator in a valley. Earth's magnetic field deflects the charged particles ejected by the Sun onto the magnetic poles of Earth. Those are currently located in high and low (extreme northern and southern) latitudes, so the equator would be the safest place. Earth's atmosphere absorbs at least some of the energy, so a lower elevation would also be safer.
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.
From Doppler shifts of the spectral lines in the light coming from the east and west edges of the Sun, astronomers find that the radial velocities of the two edges differ by about 4 km/s, meaning that the Sun's rotation rate is 2 km/s. Find the approximate period of rotation of the Sun in days. The circumference of a sphere is given by 2πR, where R is the radius of the sphere.
The solar radius (see Table 15.1 Characteristics of the Sun) is RSun = 6.96 × 108 m = 6.96 × 105 km, and the circumference c = 2πR = 4.37 × 106 km. The period, which is equal to the distance that a point on the surface of the Sun must travel to complete one full revolution divided by the velocity of rotation, is thus 4.37 × 106 km/(2 km/s) = 2.19 × 106 s = 25.3 d.
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? (Hint: Refer to the diagrams of eclipsing binary light curves.)
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.
How would two stars of equal luminosity—one blue and the other red—appear in an image taken through a filter that passes mainly blue light? How would their appearance change in an image taken through a filter that transmits mainly red light?
The two stars have equal total luminosity, but the blue star emits most of its energy at shorter wavelengths, whereas the red star emits most of its energy at longer wavelengths. Since a blue filter transmits only blue (shorter wavelength) light, the blue star will look brighter through the blue filter while the red star will look brighter through the red filter.
Explain how we know that the Sun's energy is not supplied either by chemical burning, as in fires here on Earth, or by gravitational contraction (shrinking).
The vast amount of energy produced by the Sun over the past 4.5 billion years exceeds the amount that could be supplied by burning or shrinking of the Sun by a significant factor. Chemical burning would only last a few thousand years, whereas gravitational contraction would provide energy for only about a hundred million years.
We discussed in the chapter that about half of stars come in pairs, or multiple star systems, yet the first eclipsing binary was not discovered until the eighteenth century. Why?
There are several reasons for this, but nearly all have to do with having access to technology that allows astronomers to measure motions and light variations that are not visible to the unaided eye. Spectroscopy is needed to detect motions, and high-resolution telescopes are needed to see the more distant visual binaries. Most eclipsing binaries produce light variations that are too subtle to be detectable without either careful observations or light-measuring equipment. Until this technology became available, the number of binary or multi-star systems was significantly limited.
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
Suppose you want to search for brown dwarfs using a space telescope. Will you design your telescope to detect light in the ultraviolet or the infrared part of the spectrum? Why?
Very low-mass stars or brown dwarfs are relatively cool, with temperatures of only about 2000 K. Such stars emit most of their light in the infrared and practically none in the ultraviolet.
Why do most known visual binaries have relatively long periods and most spectroscopic binaries have relatively short periods?
Visual binaries must be rather well separated to be detected as such. Thus, they generally have large semimajor axes, and by Kepler's third law, long periods and low orbital speeds. Spectroscopic binaries must have rather high orbital velocities so that the effect of this motion is clearly identifiable in the spectrum. Hence, they tend to have short periods. High orbital velocity occurs with long periods and large major axes only if the stars are very massive.
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.
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. Making some sketches may help answer this question.
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.
Make a sketch of the Sun's atmosphere showing the locations of the photosphere, chromosphere, and corona. What is the approximate temperature of each of these regions?
The order of the layers from the bottom up is: the photosphere (5800 K), the chromosphere (10,000 K), and the corona (1,000,000 K).
