ASTRONOMY CH 16-17 QUIZ

Describe how coronal mass ejections may influence life on Earth.

A coronal mass ejection is a cloud of ionized gas that travels quickly from the surface of the Sun to Earth, where it will most likely be captured by Earth's magnetic field. Some of it can get through, however, and ionize the upper atmosphere. This can affect radio communication. The particles are swept along by the magnetic field and can induce large currents in electrical power grids, knocking them out. Earth satellites are particularly vulnerable to these electrical and magnetic storms because of their delicate electronics and exposed position above the atmosphere.

Why do we say that the solar cycle is 22 years long?

Activity in the Sun's magnetic field creates the cycle of sunspots seen on the Sun. The solar magnetic field reverses itself every 11 years, so it takes 22 years—two "sunspot cycles"—to go through a single cycle of magnetic reversals. If you are 22 years old, you are a sunspot cycle old.

What is the cause of sunspots, flares, and prominences?

All of these phenomena are caused by activity in the magnetic field of the Sun. Sunspots are caused by kinks or loops of magnetic field extending through the lower atmosphere. These areas of concentrated magnetic field repel hot material trying to rise up from the Sun's interior, so the section of the Sun underneath the knot cools off and darkens. Flares, by contrast, are areas where large amounts of energy are released in a short amount of time. Their origin is mysterious, but they are somehow connected to instabilities in the magnetic field. Prominences are caused by material ejected from the Sun's surface that follows along huge loops of magnetic field that carry the luminous gas far above the solar surface.

What is helioseismology, and what does it tell us about the Sun?

Helioseismology is the study of waves that ripple across the surface of the Sun. Some of these waves travel from deep inside the Sun. Their appearance on the surface provides information about the interior of the Sun that cannot otherwise be observed, such as temperature, density, and rotation speed. This is similar to the way seismic waves on Earth can tell us about Earth's interior.

How do astronomers measure stellar temperatures?

If we can determine the wavelength of maximum emission of a star's radiation, we can find its temperature using Wien's law. In addition, we can use the B and V filters to gather and count blue and yellow photons, respectively. Comparing the amount of blue light to the amount of yellow light can tell us the star's temperature.

How is parallax used to measure the distances to stars? What is a parsec?

Parallax is the apparent change in position of a nearby object due to the change in the viewing position of the observer. For example, astronomers can view a nearby star over the course of six months, from opposite sides of the Earth's orbit. The position of this star appears to change, relative to the more distant stars in the background. The farther the star, the smaller the shift. The inverse of this shift measured in arc seconds is equal to the distance measured in parsecs. If an object has a parallax shift angle of one arc second, its distance from the observer would be exactly one parsec. Conversely, two objects with an actual separation of one Astronomical Unit will appear to have an apparent angular separation of one arc second if they are at a distance of one parsec. The parsec is equal to 206,265 AU, or 3.26 light years.

What information is needed to plot a star on the H-R diagram?

The Hertzsprung-Russell (H-R) diagram is a plot of luminosity (increasing upward on the y-axis) vs. temperature (increasing to the left on the x-axis) for stars. We can determine a star's temperature by a number of methods, including finding spectral type or measuring colors. If we know the apparent magnitude of a star and its distance (e.g., by parallax), we can calculate the star's absolute magnitude. We can then find the luminosity from the absolute magnitude.

Name and briefly describe the main regions of the Sun.

(a) core—where energy is generated through fusion of hydrogen into helium;(b) radiation zone—a layer of thick gases that light from the core must travel through; (c) convective zone—where convection is the primary source of energy transfer; (d) photosphere—the "surface" of the Sun, where the gas is thin enough for light to escape; (e) chromosphere—a layer of low-density gas above the photosphere;(f) corona—the outermost atmosphere, which fades into the solar wind.

Explain two ways in which a star's real motion through space translates into motion that is observable from Earth.

A star's real space motion is observed as two components: the radial velocity and the transverse velocity. The radial velocity is just the star's motion toward or away from us; it can be determined using the Doppler effect. The transverse motion is perpendicular to this motion, and can be determined by observing the star's proper motion, an angular motion among the other stars measured in seconds of arc per year. If the distance to the star is known, proper motion can be converted into the true transverse velocity. To find the space velocity, the transverse velocity and radial velocity can be combined using the Pythagorean theorem: (space velocity)^2 = (radial velocity)^2 + (transverse velocity)^2.

What are the ingredients and the end result of the proton-proton chain in the Sun? Why is energy released in the proton-proton chain?

A total of six hydrogen atoms go into the proton<proton chain. What comes out is a helium nucleus, two neutrinos, two positrons (which are quickly annihilated by colliding with electrons), energy in the form of gamma rays, and two hydrogens. Thus, only four hydrogen atoms are consumed to make the helium. The mass of helium produced by the nuclear fusion is 0.7% less than the mass of the four hydrogens that were fused to make it. This small amount of "lost" mass is converted into energy. The amount of energy is easily calculated from E=mc2.

What is the main sequence? What basic property of a star determines where it lies on the main sequence?

About 90% of all stars plotted in the H-R diagram are found along a narrow S-shaped band running diagonally from upper left (hot and bright) to lower right (cool and dim). This is the main sequence. Stars along the main sequence all have a common source of energy, the fusion of hydrogen into helium, and all are in hydrostatic equilibrium. A star's position on the main sequence is uniquely determined by its mass: the most massive stars are in the upper left end while the lowest-mass stars are in the lower right end. The Sun is in about the middle of the main sequence.

Why do some stars have very few hydrogen lines in their spectra?

Extreme temperatures on either end of the scale can cause weak hydrogen lines in a stellar spectrum. If the star is too hot, most of the gas is ionized; without electrons to absorb light and make transitions from the second level, no hydrogen absorption lines will be created. If the star is too cool, all the electrons are in the ground state, and are unable to absorb visible light and create lines in the visible system.

In general, is it possible to determine the age of an individual star simply by noting its position on an H-R diagram? Explain.

High-mass stars spend very short amounts of time on the main sequence. Therefore, if one sees a hot, bright, massive star on the main sequence, it has only recently arrived there, and must be very young. However, low-mass stars use up their fuel very slowly. A low-mass, dim, cool star on the main sequence may have arrived there last year, or billions of years ago!

How do scientists construct models of the Sun?

Knowing basic facts about the Sun, especially the fact that it is made primarily of light gasses such as hydrogen and helium, and knowing how such gasses behave under conditions of high pressure and temperature, allows astronomers to model the entire structure of the Sun. The model is correct if it successfully predicts observed properties of the Sun, such as its luminosity, radius, and temperature. Some of the input information to the model is uncertain but the results suggest how correct this input data is. By making slight adjustments in the input parameters, the model is adjusted until its predictions are in agreement with all the observed properties. Once the model "works", astronomers are then able to learn from the model about the properties in the interior of the Sun. Models are used as a test to see whether we fully understand the structure and processes of objects. They are also then used to predict properties that may not be directly observable. Models also make predictions of observables that help us further test the validity of the model.

What would we observe on Earth if the Sun's internal energy source suddenly shut off? How long do you think it might take—minutes, days, years, or millions of years—for the Sun's light to begin to fade? Repeat the question for solar neutrinos.

Light waves can take millions of years to fight their way out of the thick gases in the Sun's interior, while neutrinos fly out in a straight line at almost the speed of light. Therefore, if we have a means of detecting neutrinos, we would know within minutes if nuclear fusion in the Sun were to shut down. If we only relied on visible light, however, it could take millions of years before the Sun would start to dim.

What is luminosity, and how is it measured in the case of the Sun?

Luminosity is a measure of the true brightness or total energy output of an object. For the Sun, it can be measured by experimentally determining how much solar energy is received by one square meter at the distance of the Earth from the Sun. This is then multiplied by the surface area of a sphere whose radius is the semi-major axis of the Earth's orbit.

Describe how astronomers measure stellar radii. List some characteristics of red-giant and white-dwarf stars.

Most of the stars in the sky have no effective apparent size, even in our most powerful telescopes. Thus, direct determination of actual size is impossible. If the luminosity and temperature of a star are known, we can calculate the radius. Also, if two stars are eclipsing each other, we can measure their radii by timing how long it takes them to pass in front of each other. Giant stars can be dozens of times, perhaps hundreds of times, larger than the Sun. Many are called red giants because they have a surface temperature about half that of the Sun, with a red color characteristic of such temperatures. In terms of radius, white dwarfs are about the size of the Earth. Therefore, they have very low luminosities, despite having masses about the same as the Sun or a little more. They are white because they are many times hotter than the surface of the Sun, as a welding torch is.

Why are scientists so interested in solar neutrinos? What is the most likely solution to the solar neutrino problem?

Neutrinos are produced in the proton<proton chain, which occurs in the core of the Sun. The neutrinos pass unimpeded through the Sun at nearly the speed of light. So neutrinos, in a sense, allow astronomers to directly observe the core of the Sun and the processes that occur there, almost as they happen. For a long time, astronomers were puzzled because the Sun did not seem to be producing as many neutrinos as predicted. There were two possible explanations for the low number of solar neutrinos received on Earth: either the Sun was under-producing neutrinos, or something was happening to the neutrinos as they traveled to Earth. The first possibility was disturbing, as it would likely require the Sun's core to be cooling by 10%. (OMG.... !) But what could alter a neutrino across the void of space? Recently, we have discovered that there are different kinds of neutrinos, and that they can transform into each other during the trip to Earth through "oscillations." By creating neutrino detectors that can detect different kinds of neutrinos, we have confirmed that the Sun is producing all of the neutrinos we expect it to.

What fuels the Sun's enormous energy output?

The Sun's energy output is fueled by nuclear fusion of hydrogen into helium. In the process that takes place in the core of the Sun, four hydrogen atoms (really just protons) come together and fuse to form a heavier element, helium. In this process, a small amount of mass is "lost". That missing mass has been converted into energy. Einstein's famous equation, E=mc2, shows that a small amount of mass can become a large amount of energy.

Why does the H-R diagram constructed from data on the brightest stars differ so much from the diagram constructed from data on the nearest stars?

The brightest stars in the sky also happen to be intrinsically very bright stars. Although seen at relatively large distances, they still appear bright. Therefore, the brightest stars will tend to be clustered in the upper part of the H-R diagram, mostly giants and supergiants. The H-R diagram of the nearest stars is more reflective of the galaxy as a whole - low mass stars dominate and the highest mass stars are rare. The lower part of the diagram is therefore quite full in such a diagram.

What is the solar wind?

The corona of the Sun is so hot that some of the gas particles are traveling fast enough to escape the gravity of the Sun. The gas is mostly composed of the separated components of ionized hydrogen, namely protons and electrons. This flow of high-speed particles away from the Sun is known as the solar wind.

High-mass stars start off with much more fuel than low-mass stars. Why don't high-mass stars live longer?

The lifetime of a star depends not only on the amount of fuel available to it, but on how fast it uses that fuel, given by its luminosity. High-mass stars must produce large amounts of energy, just to support themselves against gravity. One star may have 10 times as much mass (fuel) as another star, but it also uses that fuel about 1000 times faster. The net result is the more massive star having a lifetime 100 times shorter than the low-mass star.

How are distances determined by spectroscopic parallax?

The main sequence represents a consistent relationship between temperature and luminosity for main sequence stars. Thus, if we know that a star is a main sequence star (by seeing pressure broadening in the spectral lines), and we have some way of determining temperature (spectral type or color), we can place that star on the main sequence of the H-R diagram. This will tell us the star's luminosity and absolute magnitude, which can then be combined with the apparent magnitude to find the distance.

How can stellar masses be determined by observing binary-star systems?

The mass of a star can be determined by observing its gravitational effect on an orbiting companion body. The combined mass of the system can be calculated using Kepler's third law, if the period of the orbit and the semimajor axis can be observed. If the location of the center of mass of the system can also be determined, then the individual masses can be calculated. Star systems such as visual and eclipsing binaries are very valuable assets in helping us find stellar masses. Spectroscopic binaries are helpful too, but can provide only partial information on the masses.

Which stars are most common in our Galaxy? Why don't we see many of them in H-R diagrams? Which stars are least common in our Galaxy?

The most commonly occurring stars in the Galaxy are low mass main sequence stars of the M spectral type. About three-quarters of all stars are of this type. However, these stars have very low luminosities, and so these are not the stars that we commonly see with our eyes or even with telescopes. The most commonly seen stars are those with high intrinsic brightness, which can be seen over large distances. However, many of these most commonly seen stars are some of the rarest in the Galaxy, because of their huge masses. Although the most massive stars have more fuel to fuse, they must fuse that fuel quickly in order to support themselves against gravity. This is why they have such tremendous luminosities and can be seen across the Galaxy. Since they are so "short-lived," there aren't very many of them in the Galaxy.

How do astronomers go about measuring stellar luminosities? What is the difference between luminosity and apparent brightness?

The most straightforward way is to find out two pieces of information about the star: its apparent brightness, and its distance. Once we know these things, we can adjust the apparent brightness for distance and find the absolute magnitude, a measure of how bright the star would appear if it were 10 parsecs away. With the effects of distance thus removed, we can tell the star's luminosity. If distance is not known, we might be able to find luminosity from temperature and radius, if these are somehow known.

How do observations of the Sun's surface tell us about conditions in the solar interior?

The oscillations observed on the solar surface are similar to seismic waves observed on Earth, although they are different in origin. The patterns of the waves are influenced by the internal structure of the Sun. Models of the solar interior predict how the waves should behave; observed waves suggest how the models need to be modified until there is agreement between observations and models.

Briefly describe how stars are classified according to their spectral characteristics.

The patterns of the dark lines in the spectrum of a star depend strongly on the star's temperature, since temperature determines which elements in the star's photosphere are capable of absorbing light and creating absorption lines. Spectra are classified according to temperature in the order O, B, A, F, G, K, M, with O-type stars being the hottest. Within each of these types is a numerical subclassification ranging from 0 to 9, e.g., F0, F1, F2, .... F8, F9, G0, G1, .... Within a specific spectral type, 0 is the hottest and 9 is the coolest. To classify a star's spectrum, we identify the sources of the spectral lines and compare the patterns to the standard classes. A spectrum with strong ionized helium lines is type O; one with strong hydrogen lines is type A; and so forth.

How hot is the solar surface? The solar core?

The radius of the core is 200,000 km, the interior is about 300,000 km thick, convective zone is 200,000 km thick, photosphere is 500 km thick, chromosphere is about 2,000 km thick, and the corona extends about a few million km above the chromosphere. The photosphere is at a temperature of 5800 K and the core is at a temperature of approximately 15 million K.

Describe how energy generated in the solar core eventually reaches Earth.

The solar radiation is first produced in the core of the Sun, largely in the form of gamma rays. Because the gas in the core is totally ionized, it is transparent to radiation and so the radiation passes through it freely. But as we get closer to the surface, the temperature drops, and more and more of the gas is not ionized or only partially ionized. Such a gas is opaque to radiation. At the outer edge of the radiation zone, all of the radiation has been absorbed by the gas. This heats the gas and it physically rises, while cooler gas from the surface falls. This is the region of convection. The energy is transported by convection to the photosphere. Here, the density of the gas is so low that radiation can freely escape into space, and travel in a straight line to Earth.

Why does the Sun appear to have a sharp edge?

Virtually all the visible radiation we receive from the Sun comes from a thin layer called the photosphere. It is only 500 km thick; a small fraction of the Sun's radius. The agas below the photosphere is too thick for light to escape, and the gas above is too thin to absorb and emit significant quantities of light. Light can only escape from this narrow region, so the Sun has a very well-defined edge.