Astronomy 152 Review Sheet ~ Midterm PDF

Astronomy 152 Study Guide/ Review Chapter 1~ Introduction  Astronomers use the metric system because it simplifies calculations and use scientific notation for very large or very small numbers.  You live on a planet, Earth, which orbits our star, the sun...

Astronomy 152 Study Guide/ Review Chapter 1~ Introduction  Astronomers use the metric system because it simplifies calculations and use scientific notation for very large or very small numbers.  You live on a planet, Earth, which orbits our star, the sun, once a year. As Earth rotates once a day, you see the sun rise and set.  The moon is only one-fourth the diameter of Earth, but the sun is 109 times larger in diameter than Earth—a typical size for a star.  The solar system includes the sun at the center, all of the planets that orbit around it— Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune—plus the moons of the planets, plus other objects bound to the sun by its gravity.  The astronomical unit (AU) is the average distance from Earth to the sun. Mars, for example, orbits 1.5 AU from the sun.  The light-year (ly) is the distance light can travel in one year. The nearest star is 4.2 ly from the sun.  The Milky Way, the hazy band of light that encircles the sky, is the Milky Way Galaxy seen from inside. The sun is just one out of the billions of stars that fill the Milky Way Galaxy.  Galaxies contain many billions of stars. Our galaxy is about 80,000 ly in diameter and contains over 100 billion stars.  Some galaxies, including our own, have graceful spiral arms bright with stars, but some galaxies are plain clouds of stars.  Our galaxy is just one of billions of galaxies that fill the universe in great clusters, clouds, filaments, and walls—the largest structures in the universe.  The universe began about 14 billion years ago in an event called the big bang, which filled the universe with hot gas.  The hot gas cooled, the first galaxies began to form, and stars began to shine only about 400 million years after the big bang.  The sun and planets of our solar system formed about 4.6 billion years ago.  Life began in Earth’s oceans soon after Earth formed but did not emerge onto land until only 400 million years ago. Dinosaurs evolved not long ago and went extinct only 65 million years ago.  Humanlike creatures developed on Earth only about 4 million years ago, and human civilizations developed only about 10,000 years ago. Although astronomy seems to be about stars and planets, it describes the universe in which you live, so it is really about you. Astronomy helps you answer the question, “What are we?”  In its simplest outline, science follows the scientific method, in which scientists expect statements to be supported by evidence compared with hypotheses. In fact, science is a complex and powerful way to think about nature. Chapter 2 ~ Daily Cycles of the Moon and Sky  Astronomers divide the sky into 88 constellations. Although the constellations originated in Greek and Middle Eastern mythology, the names are Latin. Even the modern constellations, added to fill in the spaces between the ancient figures, have Latin names. Named groups of stars that are not constellations are called asterisms.  The names of stars usually come from ancient Arabic, though modern astronomers often refer to a star by its constellation and a Greek letter assigned according to its brightness within the constellation.  Astronomers refer to the brightness of stars using the magnitude scale. First-magnitude stars are brighter than second-magnitude stars, which are brighter than third-magnitude stars, and so on. The magnitude you see when you look at a star in the sky is its apparent  visual magnitude, mv, which includes only types of light visible to the human eye and does not take into account the star’s distance from Earth.  Flux is a measure of light energy related to intensity. The magnitude of a star is related directly to the flux of light received on Earth and so to its intensity.  The celestial sphere is a scientific model of the sky, to which the stars appear to be attached. Because Earth rotates eastward, the celestial sphere appears to rotate westward on its axis. The north and south celestial poles are the pivots on which the sky appears to rotate, and they define the four directions around the horizon: the north, south, east, and west points.  The point directly overhead is the zenith, and the point on the sky directly underfoot is the nadir.  The celestial equator, an imaginary line around the sky above Earth’s equator, divides the sky into northern and southern halves.  Astronomers often refer to distances “on” the sky as if the stars, sun, moon, and planets were equivalent to spots painted on a plaster ceiling. These angular distances, measured in degrees, arc minutes, and arc seconds, are unrelated to the true distance between the objects in light- years. The angular distance across an object is its angular diameter.  What you see of the celestial sphere depends on your latitude. Much of the southern hemisphere of the sky is not visible from northern latitudes. To see that part of the sky, you would have to travel southward over Earth’s surface.  Circumpolar constellations are those close enough to a celestial pole that they do not rise or set.  The angular distance from the horizon to the north celestial pole always equals your latitude.  Precession is caused by the gravitational forces of the moon and sun acting on the equatorial bulge of the spinning Earth and causing its axis to sweep around in a conical motion like the motion of a top’s axis. Earth’s axis of rotation precesses with a period of  26,000 years, and consequently the celestial poles and celestial equator move slowly against the background of the stars.  The rotation of Earth on its axis produces the cycle of day and night, and the revolution of Earth around the sun produces the cycle of the year.  Because Earth orbits the sun, the sun appears to move eastward along the ecliptic through the constellations completing a circuit of the sky in a year. Because the ecliptic is tipped 23.4° to  the celestial equator, the sun spends half the year in the northern celestial hemisphere and half in the southern celestial hemisphere.  In each hemisphere’s summer, the sun is above the horizon longer and shines more directly down on the ground. Both effects cause warmer weather in that hemisphere and leave Earth’s other hemisphere cooler.  In each hemisphere’s winter, the sun is above the sky fewer hours than in summer and shines less directly, so the winter hemisphere has colder weather, and the opposite hemisphere has summer. Consequently the seasons are reversed in Earth’s southern hemisphere relative to the northern hemisphere.  The beginning of spring, summer, winter, and fall are marked by the o vernal equinox o summer solstice o autumnal equinox o winter solstice Earth is slightly closer to the sun at perihelion in January and slightly farther away from the sun at aphelion in July. This has almost no effect on the seasons. The planets move generally eastward along the ecliptic, and all but Uranus and Neptune are visible to the unaided eye looking like stars. Mercury and Venus never wander far from the sun and are sometimes visible in the evening sky after sunset or in the dawn sky before sunrise. Planets visible in the sky at sunset are traditionally called evening stars, and planets visible in the dawn sky are called morning stars even though they are not actually stars. The locations of the sun and planets along the zodiac are diagramed in a horoscope, which are the bases for the ancient pseudoscience known as astrology. According to the Milankovitch hypothesis, changes in the shape of Earth’s orbit, in its precession, and in its axial tilt can alter the planet’s heat balance and cause the cycle of ice ages. Evidence found in seafloor samples support the hypothesis, and it is widely accepted today. Scientists routinely test their own ideas by organizing theory and evidence into a scientific argument. Chapter 3 ~ The Moon  The moon orbits eastward around Earth once a month and rotates on its axis so as to keep the same side facing Earth throughout the month.  Because you see the moon by reflected sunlight, its shape appears to change as it orbits Earth and sunlight illuminates different amounts of the side facing Earth.  The lunar phases wax from new moon to first quarter to full moon and wane from full moon to third quarter to new moon.  A complete cycle of lunar phases takes 29.53 days, which is known as the moon’s synodic period. The sidereal period of the moon—its orbital period with respect to the stars—is a bit over two days shorter.  If a full moon passes through Earth’s shadow, sunlight is cut off, and the moon darkens in a lunar eclipse. If the moon fully enters the dark umbra of Earth’s shadow, the eclipse is a total lunar eclipse; but if it only grazes the umbra, the eclipse is a partial lunar eclipse. If the moon enters the partial shadow of the penumbra but not the umbra, the eclipse is a penumbral lunar eclipse.  During totality, the eclipsed moon looks copper-red because of sunlight refracted through Earth’s atmosphere.  The angular diameter of the sun and moon is about 0.5 degrees.  A solar eclipse occurs if a new moon passes between the sun and Earth and the moon’s shadow sweeps over Earth’s surface along the path of totality. Observers inside the path of totality see a total solar eclipse, and those just outside the path of totality see a partial solar eclipse.  When the moon is near perigee, the closest point in its orbit, its angular diameter is large enough to cover the sun’s photosphere and produce a total eclipse. But if the moon is near apogee, the farthest point in its orbit, it looks too small and can’t entirely cover the photosphere. A solar eclipse occurring then would be an annular eclipse.  During a total eclipse of the sun, the bright photosphere of the sun is covered, and the fainter corona, chromosphere, and prominences become visible.  Sometimes at the beginning or end of the total phase of a total solar eclipse, a small piece of the sun’s photosphere can peek out through a valley at the edge of the moon and produce a diamond ring effect.  Looking at the sun is dangerous and can burn the retinas of your eyes. The safest way to observe the partial phases of a solar eclipse is by pinhole projection. Only during totality, when the photosphere is completely hidden, is it safe to look at the sun directly.  Solar eclipses must occur at new moon, and lunar eclipses must occur at full moon. Because the moon’s orbit is tipped a few degrees from the plane of Earth’s orbit, most new moons cross north or south of the sun, and there are no solar eclipses in those months. Similarly, most full moons cross north or south of Earth’s shadow, and there are no lunar eclipses in those months.  The moon’s orbit crosses the ecliptic at two locations called nodes, and eclipses can occur only when the sun is crossing a node. Chapter 4 ~ Modern Astronomy  Archaeoastronomy is the study of the astronomy of ancient peoples. Many cultures around the world observed the sky and marked important alignments. Structures such as Stonehenge, Newgrange, and the Sun Dagger have astronomical alignments. In most cases, ancient cultures, having no written language, left no detailed records of their astronomical beliefs.  Greek astronomy, derived in part from Babylon and Egypt, is better known because written documents have survived. Classical philosophers accepted as a first principle that Earth was the unmoving center of the universe. Another first principle was that the heavens were perfect, so philosophers such as Plato argued that, because the sphere was the most perfect geometrical form, the heavens must be made up of spheres in uniform rotation.  This led to the belief in uniform circular motion.  Many astronomers argued that Earth could not be moving because they could see no parallax in the positions of the stars. Aristotle’s estimate for the size of Earth was only about one-third of its true size. Eratosthenes used the well at Syene to measure the diameter of Earth and got an accurate estimate.  The geocentric universe became part of the teachings of the great philosopher Aristotle, who argued that the sun, moon, and stars were carried around Earth on rotating crystalline spheres.  Hipparchus, who lived about two centuries after Aristotle, devised a model in which the sun, moon, and planets revolved in circles called eccentrics with Earth near but not precisely at their centers.  Retrograde motion, the occasional westward (backward) motion of the planets, was difficult for astronomers to explain.  About AD 140, Aritotle’s model was given mathematical form in Claudius Ptolemy’s book Almagest. Ptolemy preserved the principles of geocentrism and uniform circular motion, but he added epicycles, deferents , and equants. Ptolemy’s epicycles could approximate retrograde motion, but the Ptolemaic model was not very accurate, and it had to be revised a number of times as centuries passed.  Geocentric Universe  Copernicus devised a heliocentric universe. He preserved the principle of uniform circular motion, but he argued that Earth rotates on its axis and revolves around the sun once a year. His theory was controversial because it contradicted Church teaching. He published his theory in his book De Revolutionibus in 1543, the same year he died. Retrograde Motion  A hypothesis is a specific statement about nature that needs further testing, but a theory is usually a general description of some aspect of nature that has been tested. Some theories are very well understood and widely accepted. A natural law is a fundamental principle in which scientists have great confidence.  Because Copernicus kept uniform circular motion as part of his theory, his model did not predict the motions of the plants well, but it did offer a simple explanation of retrograde motion without using big epicycles.  One reason the Copernican model won converts was that it was more elegant. Venus and Mercury were treated the same as all the other planets, and the velocity of each planet was related to its distance from the sun.  The shift from the geocentric paradigm to the heliocentric paradigm is an example of a scientific revolution.  Although Tycho Brahe developed his own model in which the sun and moon circled Earth and the planets circled the sun, his great contribution was to compile detailed observations of the positions of the sun, moon, and planets over a period of 20 years, observations that were later used by Kepler.  Kepler inherited Tycho’s books of observations in 1601 and used them to discover three laws of planetary motion. He found that the planets follow ellipses with the sun at one focus, that they move faster when near the sun and that a planet’s orbital period squared is proportional to the semimajor axis, a, of its orbit cubed. Keppler’s First Law  The eccentricity, e, of an orbit is a measure of its departure from a perfect circle. A circle is an ellipse with an eccentricity of zero.  Kepler’s final book, The Rudolphine Tables (1627), combined heliocentrism with elliptical orbits and predicted the positions of the planets well.  Galileo used the newly invented telescope to observe the heavens, and he recognized the significance of what he saw there. His discoveries of the phases of Venus, the satellites of Jupiter now known as the Galilean moons, the mountains of Earth’s moon, and other phenomena helped undermine the Ptolemaic universe.  Galileo based his analysis on observational evidence. In 1633, he was condemned by the Inquisition for disobeying instructions not to hold, teach, or defend Copernicanism.  Historians of science view Galileo’s trial as a conflict between two ways of knowing about nature, reasoning from first principles and depending on evidence.  The 99 years from the death of Copernicus to the death of Galileo marked the birth of modern science. From that time on, science depended on evidence to test theories and relied on the mathematical analytic methods first demonstrated by Kepler. Chapter 5 ~ Light and Optical Telescopes  Light is the visible form of electromagnetic radiation, an electric and magnetic disturbance that transports energy at the speed of light. The electromagnetic spectrum includes gamma rays, X-rays, ultraviolet radiation, visible light, infrared radiation, and radio waves. Electromagnetic Radiation  You can think of a particle of light, a photon, as a bundle of waves that sometimes acts as a particle and sometimes acts as a wave.  The energy a photon carries depends on its wavelength. The wavelength of visible light, usually measured in nanometers  (10−9 m) or angstroms (10−10 m), ranges from 400 nm to 700 nm (4000 to 7000 Å).  Radio and infrared radiation have longer wavelengths and carry less energy. X-ray, gamma- ray, and ultraviolet radiation have shorter wavelengths and more energy. Frequency is the number of waves that pass a stationary point in 1 second. Wavelength equals the speed of light divided by the frequency.  Earth’s atmosphere is fully transparent in only two atmospheric windows —visible light and radio.  An atom consists of a nucleus surrounded by a cloud of electrons.  The nucleus is made up of positively charged protons and uncharged neutrons.  The number of protons in an atom determines which element it is.  Atoms of the same element (that is, having the same number of protons) with different numbers of neutrons are called isotopes.  A neutral atom is surrounded by a number of negatively charged electrons equal to the number of protons in the nucleus. An atom that has lost or gained an electron is said to be ionized and is called an ion.  Two or more atoms joined together form a molecule.  The electrons in an atom are attracted to the nucleus by the Coulomb force. As described by quantum mechanics, the binding energy that holds electrons in an atom is limited to certain energies, and that means the electrons may occupy only certain permitted orbits.  The size of an electron’s orbit depends on its energy, so the orbits can be thought of as energy levels with lowest possible energy level known as the ground state.  An excited is one in which an electron is raised to a higher orbit by a collision between atoms or the absorption of a photon of the proper energy.  The size of an electron’s orbit depends on its energy, so the orbits can be thought of as energy levels with the lowest possible energy level known as the ground state.  Astronomical telescopes use a primary lens or mirror (also called an objective lens or mirror) to gather light and focus it into a small image, which can be magnified by an eyepiece. Short-focal-length lenses and mirrors must be more strongly curved and are more expensive to grind to shape. a. Refracting Telescope, b. Reflecting Telescope  A refracting telescope uses a lens to bend the light and focus it into an image. Because of chromatic aberration, refracting telescopes cannot bring all colors to the same focus, resulting in color fringes around the images. An achromatic lens partially corrects for this, but such lenses are expensive and cannot be made much larger than about 1 m in diameter.  Reflecting telescopes use a mirror to focus the light and are less expensive than refracting telescopes of the same diameter. Also, reflecting telescopes do not suffer from chromatic aberration. Most large telescopes are reflectors.  Light-gathering power refers to the ability of a telescope to produce bright images.  Resolving power refers to the ability of a telescope to resolve fi ne detail.  Diffraction fringes in the image limit the detail visible.  Magnifying power, the ability to make an object look bigger, is less important because it can be changed by changing the eyepiece.  Astronomers build observatories on remote, high mountains for two reasons. Turbulence in Earth’s atmosphere blurs the image of an astronomical telescope, a phenomenon that astronomers refer to as seeing. Atop a mountain, the air is steady, and the seeing is better.  Observatories are located far from cities to avoid light pollution.  In a reflecting telescope, light first comes to a focus at the prime focus, but secondary mirrors can direct light to other focus locations such as a Cassegrain focus or a Newtonian focus.  Interferometry refers to connecting two or more separate telescopes together to act as a single large telescope that has a resolution equivalent to that of a telescope as large in diameter as the separation between the telescopes.  For many decades astronomers used photographic plates to record images at the telescope, but modern electronic systems such as charge coupled devices (CCDs) have replaced photographic plates in most applications.  Astronomical images in digital form can be computer enhanced and reproduced as false-color images to bring out subtle details.  Spectrographs using prisms or a grating spread starlight out according to wavelength to form a spectrum revealing  hundreds of spectral lines produced by atoms in the object being studied. A comparison spectrum containing lines  of known wavelength allows astronomers to measure wavelengths in spectra of astronomical objects.  Astronomers use radio telescopes for three reasons: They can detect cool hydrogen and other atoms and molecules in space; they can see through dust clouds that block visible light; and they can detect certain objects invisible at other wavelengths.  Most radio telescopes contain a dish reflector, an antenna, an amplifier, and a data recorder. Such a telescope can record the intensity of the radio energy coming from a spot on the sky. Scans of small regions are used to produce radio maps.  Because of the long wavelength, radio telescopes have very poor resolution, and astronomers often link separate radio telescopes together to form a radio interferometer capable of resolving much finer detail.  Earth’s atmosphere absorbs gamma rays, X-rays, ultraviolet, and far-infrared. To observe at these wavelengths, telescopes must be located in space.  Earth’s atmosphere distorts and blurs images. Telescopes in orbit are above this seeing distortion and are limited only by diffraction in their optics.  Cosmic rays are not electromagnetic radiation; they are subatomic particles such as electrons and protons traveling at nearly the speed of light. They can best be studied from above Earth’s atmosphere. radio interferometer Chapter 6 ~ Stars  The agitation among the atoms and molecules of an object is called thermal energy, and the flow of thermal energy is heat. In contrast, temperature refers to the intensity of the agitation and can be expressed on the Kelvin temperature scale, which gives temperature above absolute zero.  Collisions among the particles in a body accelerate electrons and cause the emission of blackbody radiation. The hotter an object is, the more it radiates and the shorter is its wavelength of maximum intensity, λmax. This allows astronomers to estimate the temperatures of stars from their colors.  Density is a measure of the amount of matter in a given volume.  Kirchhoff’s laws explain that a hot solid, liquid, or dense gas emits electromagnetic radiation at all wavelengths and produces a continuous spectrum. An excited low-density gas produces an emission (bright-line) spectrum containing emission lines. A light source viewed through a low-density gas produces an absorption (dark-line) spectrum containing absorption lines. Atomic Spectra  An atom can emit or absorb a photon when an electron makes a transition between orbits.  Because orbits of only certain energy differences are permitted in an atom, photons of only certain wavelengths can be absorbed or emitted.  Each kind of atom has its own characteristic set of spectral lines.  The hydrogen atom has the Lyman series of lines in the ultraviolet, the Balmer series partially in the visible, and the Paschen series (plus others) in the infrared.  The strength of spectral lines depends on the temperature of the star. For example, in cool stars, the Balmer lines are weak because atoms are not excited out of the ground state. In hot stars, the Balmer lines are weak because atoms are excited to higher orbits or are ionized. Only at medium temperatures are the Balmer lines strong. Electron Orbit in the Hydrogen  The Doppler effect can provide clues to the motions of the stars. When a star is approaching, you observe slightly shorter wavelengths, a blueshift, and when it is receding, you observe slightly longer wavelengths, a redshift. This Doppler effect reveals a star’s radial velocity, Vr (p. 139), that part of its velocity directed toward or away from Earth.  Astronomers can measure the distance to nearer stars by observing their stellar parallaxes (p). The most distant stars are so far away that their parallaxes are unmeasurably small. Space telescopes above Earth’s atmosphere have measured the parallaxes of a huge number of stars.  Stellar distances are commonly expressed in parsecs. o 1 pc = 2.06 x 105 AU = 3.26 light years  Proper can give you a clue to distance because nearby stars tend to have large proper motions.  The amount of light received from a star, the light flux, is related to its distance by the inverse square law. Once you know the distance to a star, you can use the magnitude–distance formula to find its intrinsic brightness expressed as its absolute visual magnitude (Mv) — the apparent magnitude the star would have if it were 10 pc away.  A star’s distance modulus is the difference between its apparent and absolute magnitudes.  To find the energy output of a star, astronomers must correct for the light at wavelengths that are not visible to convert absolute visual magnitude into absolute bolometric magnitude, which can be converted into luminosity (L), the total energy radiated by the star in one second. 2 4 Quantitatively: L=4πR σT  The Hertzsprung–Russell (H–R) diagram is a plot of luminosity versus surface temperature. It is an important graph in astronomy because it sorts the stars into categories by size.  Roughly 90 percent of normal stars, including the sun, fall on the main sequence, with the more massive stars being hotter and more luminous.  The giants and supergiants, however, are much larger and lie above the main sequence.  Red dwarfs lie at the bottom end of the main sequence. Some of the white dwarfs are very hot stars, but they fall below the main sequence because they are so small.  Observations made with interferometers confirm the sizes of stars implied by the H–R diagram and reveal that rapidly rotating stars are slightly flattened. HR Diagram  The large sizes of the giants and supergiants mean their atmospheres have low densities and their spectra have sharper spectral lines than the spectra of main-sequence stars. In fact, it is possible to assign stars to luminosity classes by the widths of their spectral lines.  Class V stars are main-sequence stars. Giant stars, class III, have sharper lines, and supergiants, class I, have extremely sharp spectral lines Chapter 7 ~ The Sun  The solar atmosphere consists of three layers of hot, low-density gas: the photosphere, chromosphere, and corona. Photosphere and Chromosphere  The granulation of the photosphere is produced by convection currents of hot gas rising from below  The edge or limb of the solar disk is dimmer than the center. This limb darkening is evidence that the temperature in the solar atmosphere increases with depth.  The chromosphere is most easily visible during total solar eclipses, when it flashes into view for a few seconds. It is a thin, hot layer of gas just above the photosphere, and its pink color is caused by the Balmer emission lines in its spectrum.  Filtergrams of the chromosphere reveal spicules, flame like structures extending upward into the lower corona.  The corona is the sun’s outermost atmospheric layer and can be imaged using a coronagraph. It is composed of a very-low density, very hot gas extending many solar radii from the visible sun. It has a high temperature — over 2,000,000 K.  Parts of the corona give rise to the solar wind, a breeze of low-density ionized gas streaming away from the sun.  The sun generates its energy through nuclear fusion, during which hydrogen nuclei fuse to produce helium nuclei.  Hydrogen fusion in the sun proceeds in three steps known as the proton–proton chain. The first step in the chain combines two hydrogen nuclei to produce a heavy hydrogen nucleus called deuterium. The second step forms light helium, and the third step combines the light helium nuclei to form normal helium. Energy is released as positrons, neutrinos, gamma rays, and the rapid motion of particles flying away. Proton-Proton chain  Fusion can occur only at the center of the sun because charged particles repel each other, and high temperatures are needed to give particles high enough velocities to penetrate this Coulomb barrier.  High densities are needed to provide large numbers of reactions.  Neutrinos escape from the sun’s core at nearly the speed of light, carrying away about 2 percent of the energy produced by fusion. Observations of fewer neutrinos than expected coming from the sun’s core are now explained by the oscillation of neutrinos among three different types (flavors).  Energy flows out of the sun’s core as photons traveling through the radiative zone and closer to the surface as rising currents of hot gas and sinking currents of cooler gas in the convective zone.  Sunspots seem dark because they are slightly cooler than the rest of the photosphere. The average sunspot is about twice the size of Earth. They appear for a month or so and then fade away, and the number of spots on the sun varies with an 11-year cycle. Early in a sunspot cycle, spots appear farther from the sun’s equator, and later in the cycle they appear closer to the equator. This is shown in the Maunder butterfly y diagram.  Astronomers can use the Zeeman Effect to measure magnetic fields on the sun. The average sunspot contains magnetic fields a few thousand times stronger than Earth’s. This is part of the evidence that the sunspot cycle is produced by a solar magnetic cycle.  The sunspot cycle does not repeat exactly each cycle, and the decades from 1645 to 1715, known as the Maunder minimum, seem to have been a time when solar activity was very low and Earth’s climate was slightly colder.  Sunspots are the visible consequences of active regions where the sun’s magnetic field is strong. Arches of magnetic field can produce sunspots where the field passes through the photosphere.  Arches of magnetic field are visible as prominences in the chromosphere and corona. Seen from above in filtergrams, prominences are visible as dark filaments silhouetted against the bright chromosphere.  Reconnections of magnetic fields can produce powerful flares, sudden eruptions of X-ray, ultraviolet, and visible radiation plus high-energy atomic particles. Flares are important because they can have dramatic effects on Earth, such as communications blackouts.  The solar wind originates in regions on the solar surface called coronal holes, where the sun’s magnetic field leads out into space and does not loop back to the sun.  Coronal mass ejections, or CMEs, occur when magnetic fields on the surface of the sun eject bursts of ionized gas that flow outward in the solar wind. Such bursts can produce auroras and other phenomena if they strike Earth.  Small changes in the solar constant over decades can affect Earth’s climate and may be responsible or the Little Ice Age and other climate fluctuations in Earth’s history.