Study Guide: Astronomy PDF

Study Guide: Astronomy Composition of the Sun’s Atmosphere The Sun, our star, has several layers beneath the visible surface: the core, radiative zone, and convective zone. These, in turn, are surrounded by a number of layers that make up the solar atmosphere. In order of increasing distance from...

Study Guide: Astronomy Composition of the Sun’s Atmosphere The Sun, our star, has several layers beneath the visible surface: the core, radiative zone, and convective zone. These, in turn, are surrounded by a number of layers that make up the solar atmosphere. In order of increasing distance from the center of the Sun, they are the photosphere, with a temperature that ranges from 4500 K to about 6800 K; the chromosphere, with a typical temperature of 104 K; the transition region, a zone that may be only a few kilometers thick, where the temperature increases rapidly from 104 K to 106 K; and the corona, with temperatures of a few million K. The Sun’s surface is mottled with upwelling convection currents seen as hot, bright granules. Figure 15.4 Parts of the Sun. This illustration shows the different parts of the Sun, from the hot core where the energy is generated through regions where energy is transported outward, first by radiation, then by convection, and then out through the solar atmosphere. The parts of the atmosphere are also labeled the photosphere, chromosphere, and corona. Some typical features in the atmosphere are shown, such as coronal holes and prominences. (credit: modification of work by NASA/Goddard)  1. The Sun’s core is extremely dense and is the source of all of its energy. Inside the core, nuclear energy is being. 2. Above the core is a region known as the radiative zone—named for the primary mode of transporting energy across it. The light generated in the core is transported through the radiative zone very slowly, since the high density of matter in this region means a photon cannot travel too far without encountering a particle, causing it to change direction and lose some energy. 3. The convective zone is the outermost layer of the solar interior. It is a thick layer approximately 200,000 kilometers deep that transports energy from the edge of the radiative zone to the surface through giant convection cells, similar to a pot of boiling oatmeal. The plasma at the bottom of the convective zone is extremely hot, and it bubbles to the surface where it loses its heat to space. Once the plasma cools, it sinks back to the bottom of the convective zone. 4. The photosphere is the layer where the Sun becomes opaque and marks the boundary past which we cannot see. 5. The region of the Sun’s atmosphere that lies immediately above the photosphere is called the chromosphere. 6. The increase in temperature does not stop with the chromosphere. Above it is a region in the solar atmosphere where the temperature changes from 10,000 K (typical of the chromosphere) to nearly a million degrees. The hottest part of the solar atmosphere, which has a temperature of a million degrees or more, is called the corona. Figure 15.17 Magnetic Field Lines Wind Up. Because the Sun spins faster at the equator than near the poles, the magnetic fields in the Sun tend to wind up as shown, and after a while make loops. This is an idealized diagram; the real situation is much more complex. The solar cycle is a nearly periodic change in the Sun's activity between the time where we can observe the most and least number of sunspots, and generally lasts around 11 years. Sometimes the surface of the Sun is very active with lots of sunspots, while other times it is quieter with only a few or even none. Also, at the peak of each solar cycle, the Sun's magnetic field changes polarity as its inner magnetic dynamo reorganizes itself. This can stir up stormy space weather around our planet. The cosmic particles from deep space that the field protects us from may also be affected, since when a magnetic field reversal occurs, it becomes more wavy, and can act as a better shield against them. What are some examples of solar activity? Solar activity associated with space weather that can affect the Earth includes phenomena such as: solar flares coronal mass ejections (CMEs) high-speed solar wind solar energetic particles CH.15 Vocab Active region an area on the Sun where magnetic fields are concentrated; sunspots, prominences, flares, and CMEs all tend to occur in active regions Aurora light radiated by atoms and ions in the ionosphere excited by charged particles from the Sun, mostly seen in the magnetic polar regions Chromosphere the part of the solar atmosphere that lies immediately above the photospheric layers Corona (of the Sun) the outer (hot) atmosphere of the Sun Coronal hole a region in the Sun’s outer atmosphere that appears darker because there is less hot gas there Coronal mass ejection (CME) a solar flare in which immense quantities of coronal material—mainly protons and electrons—is ejected at high speeds (500– 1000 kilometers per second) into interplanetary space Differential rotation the phenomenon that occurs when different parts of a rotating object rotate at different rates at different latitudes Granulation the rice-grain-like structure of the solar photosphere; granulation is produced by upwelling currents of gas that are slightly hotter, and therefore brighter, than the surrounding regions, which are flowing downward into the Sun Maunder Minimum a period during the seventeenth century when the number of sunspots seen throughout the solar cycle was unusually low Photosphere the region of the solar (or stellar) atmosphere from which continuous radiation escapes into space Plage a bright region of the solar surface observed in the light of some spectral line Plasma a hot ionized gas Prominence a large, bright, gaseous feature that appears above the surface of the Sun and extends into the corona Solar flare a sudden and temporary outburst of electromagnetic radiation from an extended region of the Sun’s surface Solar wind a flow of hot, charged particles leaving the Sun Sunspot large, dark features seen on the surface of the Sun caused by increased magnetic activity Sunspot cycle the semiregular 11-year period with which the frequency of sunspots fluctuates Transition region the region in the Sun’s atmosphere where the temperature rises very rapidly from the relatively low temperatures that characterize the chromosphere to the high temperatures of the corona Nuclear Fusion Nuclear fusion is the process by which two light atomic nuclei combine to form a single heavier one while releasing massive amounts of energy. Fusion reactions take place in a state of matter called plasma — a hot, charged gas made of positive ions and free-moving electrons with unique properties distinct from solids, liquids or gases. proton-proton chain series of thermonuclear reactions by which nuclei of hydrogen are built up into nuclei of helium Stars Perhaps the most important characteristic of a star is its luminosity—the total amount of energy at all wavelengths that it emits per second. Hipparchus did not have a telescope or any instrument that could measure apparent brightness accurately, so he simply made estimates with his eyes. He sorted the stars into six brightness categories, each of which he called a magnitude.  Star Color Approximate Temperature Example Blue 25,000 K Spica White 10,000 K Vega Yellow 6000 K Sun Orange 4000 K Aldebaran Red 3000 K Betelgeuse As we learned in The Electromagnetic Spectrum section, Wien’s law relates stellar color to stellar temperature. Blue colors dominate the visible light output of very hot stars (with much additional radiation in the ultraviolet). On the other hand, cool stars emit most of their visible light energy at red wavelengths (with more radiation coming off in the infrared) (Table 17.1). The color of a star therefore provides a measure of its intrinsic or true surface temperature. Vocab CH.17 Absolute magnitude is defined to be the apparent magnitude an object would have if it were located at a distance of 10 parsecs. So for example, the apparent magnitude of the Sun is -26.7 and is the brightest celestial object we can see from Earth. apparent brightness a measure of the amount of light received by Earth from a star or other object—that is, how bright an object appears in the sky, as contrasted with its luminosity brown dwarf an object intermediate in size between a planet and a star; the approximate mass range is from about 1/100 of the mass of the Sun up to the lower mass limit for self-sustaining nuclear reactions, which is about 0.075 the mass of the Sun; brown dwarfs are capable of deuterium fusion, but not hydrogen fusion color index difference between the magnitudes of a star or other object measured in light of two different spectral regions—for example, blue minus visual (B–V) magnitudes giant a star of exaggerated size with a large, extended photosphere luminosity the rate at which a star or other object emits electromagnetic energy into space; the total power output of an object magnitude an older system of measuring the amount of light we receive from a star or other luminous object; the larger the magnitude, the less radiation we receive from the object proper motion the angular change per year in the direction of a star as seen from the Sun radial velocity motion toward or away from the observer; the component of relative velocity that lies in the line of sight space velocity the total (three-dimensional) speed and direction with which an object is moving through space relative to the Sun spectral class (or spectral type) the classification of stars according to their temperatures using the characteristics of their spectra; the types are O, B, A, F, G, K, and M with L, T, and Y added recently for cooler star-like objects that recent survey have revealed Following Hertzsprung and Russell, let us plot the temperature (or spectral class) of a selected group of nearby stars against their luminosity and see what we find (Figure 18.14). Such a plot is frequently called the Hertzsprung–Russell diagram, abbreviated H–R diagram. It is one of the most important and widely used diagrams in astronomy, with applications that extend far beyond the purposes for which it was originally developed more than a century ago. Figure 18.14 H–R Diagram for a Selected Sample of Stars. In such diagrams, luminosity is plotted along the vertical axis. Along the horizontal axis, we can plot either temperature or spectral type (also sometimes called spectral class). Several of the brightest stars are identified by name. Most stars fall on the main sequence. Parallax The apparent change in the position of a nearby star as seen from Earth. This is caused by the Earth's revolution around the sun, which changes the direction in which we see the star relative to distant stars. Parsec A unit of length used to measure the distance to astronomical objects outside the solar system. One parsec is the distance at which an object has a parallax of one arcsecond. It's approximately equal to 3.26 light-years or 206,265 astronomical units (AU). Astronomers refer to all the material between stars as interstellar matter; the entire collection of interstellar matter is called the interstellar medium (ISM). Some interstellar material is concentrated into giant clouds, each of which is known as a nebula (plural Interstellar dust tiny solid grains in interstellar space thought to consist of a core of rocklike material (silicates) or graphite surrounded by a mantle of ices; water, methane, and ammonia are probably the most abundant ices Interstellar extinction the attenuation or absorption of light by dust in the interstellar medium Interstellar medium (ISM) (or interstellar matter) the gas and dust between the stars in a galaxy Figure 21.8 Formation of a Star. (a) Dense cores form within a molecular cloud. (b) A protostar with a surrounding disk of material forms at the center of a dense core, accumulating additional material from the molecular cloud through gravitational attraction. (c) A stellar wind breaks out but is confined by the disk to flow out along the two poles of the star. (d) Eventually, this wind sweeps away the cloud material and halts the accumulation of additional material, and a newly formed star, surrounded by a disk, becomes observable. These sketches are not drawn to the same scale. The diameter of a typical envelope that is supplying gas to the newly forming star is about 5000 AU. The typical diameter of the disk is about 100 AU or slightly larger than the diameter of the orbit of Pluto. Once a star has reached the main-sequence stage of its life, it derives its energy almost entirely from the conversion of hydrogen to helium via the process of nuclear fusion in its core (see The Sun: A Nuclear Powerhouse). Since hydrogen is the most abundant element in stars, this process can maintain the star’s equilibrium for a long time. Thus, all stars remain on the main sequence for most of their lives. Evolution out of Main Sequence into Red Giants Eventually, all the hydrogen in a star’s core, where it is hot enough for fusion reactions, is used up. The core then contains only helium, “contaminated” by whatever small percentage of heavier elements the star had to begin with. The helium in the core can be thought of as the accumulated “ash” from the nuclear “burning” of hydrogen during the main-sequence stage. Evolution of massive stars Massive stars evolve in much the same way that the Sun does (but always more quickly)—up to the formation of a carbon-oxygen core. One difference is that for stars with more than about twice the mass of the Sun, helium begins fusion more gradually, rather than with a sudden flash. Also, when more massive stars become red giants, they become so bright and large that we call them supergiants. Low-mass stars If the birth of a main-sequence star is defined by the onset of fusion reactions, then we must consider the end of all fusion reactions to be the time of a star’s death. As the core is stabilized by degeneracy pressure, a last shudder of fusion passes through the outside of the star, consuming the little hydrogen still remaining. Now the star is a true white dwarf: nuclear fusion in its interior has ceased. These stars die without an explosion, leaving behind a white dwarf and a planetary nebula: White dwarf: The core of a low-mass star becomes a white dwarf, a hot, dim object that's about the size of Earth and a million times denser than water. The white dwarf will slowly cool and fade over billions of years, eventually becoming a black dwarf. Planetary nebula: The outer layers of a low-mass star expand and shine as a planetary nebula. The nebula's spectrum includes a strong Halpha line, which can give the nebula a reddish color. High-mass stars These stars die in a supernova explosion, which is a spectacular event that releases an enormous amount of energy: Supernova: The core of a high-mass star collapses very quickly and the star's outer layers are blown off in a supernova. The supernova expels more than 90% of the star's mass, including newly formed heavy elements, into the galaxy. Remnants: The remnants of a supernova include a core of reduced mass, which could be a neutron star or a stellar black hole, and expanding gas and dust. After a type II supernova explosion fades away, all that is left behind is either a neutron star or something even more strange, a black hole. Neutron stars are the densest objects in the universe; the force of gravity at their surface is 1011 times greater than what we experience at Earth’s surface. The interior of a neutron star is composed of about 95% neutrons, with a small number of protons and electrons mixed in. In effect, a neutron star is a giant atomic nucleus, with a mass about 1057 times the mass of a proton. At least some supernovae leave behind a highly magnetic, rapidly rotating neutron star, which can be observed as a pulsar if its beam of escaping particles and focused radiation is pointing toward us. Pulsars emit rapid pulses of radiation at regular intervals; their periods are in the range of 0.001 to 10 seconds. The rotating neutron star acts like a lighthouse, sweeping its beam in a circle and giving us a pulse of radiation when the beam sweeps over Earth. As pulsars age, they lose energy, their rotations slow, and their periods increase. neutron star a compact object of extremely high density composed almost entirely of neutrons pulsar a variable radio source of small physical size that emits very rapid radio pulses in very regular periods that range from fractions of a second to several seconds; now understood to be a rotating, magnetic neutron star that is energetic enough to produce a detectable beam of radiation and particles Degeneracy pressure is a quantum mechanical effect that occurs when fermions are forced into higher energy states due to increased particle density: Explanation When the lower energy states are filled, additional fermions are forced into higher energy states, increasing the pressure of the fermion gas. This pressure is called degeneracy pressure. The escape velocity of a black hole is the speed at which an object must travel to escape its gravitational pull, and it's equal to the speed of light at the event horizon. Once an object crosses the event horizon, it can't escape the black hole's gravitational pull, no matter how fast it's moving. This is because the horizon is moving outward at the speed of light, so to escape, an object would need to travel faster than light. In modern terminology, we call an object from which light cannot escape a black hole. The best evidence of stellar-mass black holes comes from binary star systems in which (1) one star of the pair is not visible, (2) the flickering X-ray emission is characteristic of an accretion disk around a compact object, and (3) the orbit and characteristics of the visible star indicate that the mass of its invisible companion is greater than 3 MSun. A number of systems with these characteristics have been found. Black holes with masses of millions to billions of solar masses are found in the centers of large galaxies. Schwarzschild radius, the radius below which the gravitational attraction between the particles of a body must cause it to undergo irreversible gravitational collapse. This phenomenon is thought to be the final fate of the more massive stars (see black hole). Vocab CH.24 Accretion disk the disk of gas and dust found orbiting newborn stars, as well as compact stellar remnants such as white dwarfs, neutron stars, and black holes when they are in binary systems and are sufficiently close to their binary companions to draw off material Black hole A region in spacetime where gravity is so strong that nothing—not even light—can escape Equivalence principle concept that a gravitational force and a suitable acceleration are indistinguishable within a sufficiently local environment Event horizon A boundary in spacetime such that events inside the boundary can have no effect on the world outside it—that is, the boundary of the region around a black hole where the curvature of spacetime no longer provides any way out General theory of relativity Einstein’s theory relating gravity and the structure (geometry) of space and time Gravitational redshift an increase in wavelength of an electromagnetic wave (light) when propagating from or near a massive object Gravitational wave a disturbance in the curvature of spacetime caused by changes in how matter is distributed; gravitational waves propagate at (or near) the speed of light. Singularity the point of zero volume and infinite density to which any object that becomes a black hole must collapse, according to the general theory of relativity Spacetime system of one time and three space coordinates, with respect to which the time and place of an event can be specified Structure of the spiral system The Milky Way Galaxy’s structure is fairly typical of a large spiral system. (Spiral galaxies and other types of galaxies are described in the article galaxy.) This structure can be viewed as consisting of six separate parts: (1) a nucleus, (2) a central bulge, (3) a disk (both a thin and a thick disk), (4) spiral arms, (5) a spherical component, and (6) a massive halo. Some of these components blend into each other. Our galaxy, the Milky Way, has spiral arms, and our solar system is located on the inner edge of the Orion-Cygnus arm. Approximate sizes and shapes: Bulge: Roughly 6 kiloparsecs (kpc) across, with a somewhat flattened, boxy shape, containing mostly older stars and little gas or dust. Disk: Approximately 100,000 light-years in diameter and around 1,000 light-years thick, containing spiral arms and a mix of young and old stars, along with most of the galaxy's gas and dust. Halo: A large, spherical region extending far beyond the disk, with a radius of around 65,000 light-years, primarily consisting of old stars and globular clusters. The Nucleus: At the very centre of the Galaxy lies a remarkable object—a massive black hole surrounded by an accretion disk of high- temperature gas. Spiral arms: Spiral arms are a defining feature of spiral galaxies, which are the most common type of galaxy in the universe Key points about each component: Bulge: Considered the center of the Milky Way, containing a high density of stars and thought to be a bar-shaped structure. Disk: Where most star formation occurs, with the spiral arms embedded within it. Halo: Contains the oldest stellar populations in the galaxy, with stars orbiting in a more random fashion compared to the disk. In astronomy, "Population I stars" are younger, metal-rich stars primarily found in the galactic disk, while "Population II stars" are older, metal-poor stars located in the galactic halo and globular clusters; this means that most active star formation happens within the galactic disk, where Population I stars are located. Old stars with few heavy elements are referred to as population II stars and are found in the halo and in globular clusters. Population I stars contain more heavy elements than globular cluster and halo stars, are typically younger and found in the disk, and are especially concentrated in the spiral arms. The three main types of galaxies are spiral, elliptical, and irregular, and they are categorized by their shape: Spiral These galaxies have a central bulge of stars surrounded by a disk with spiral arms. The arms are made of stars, gas, and dust, while the bulge contains older, redder stars. The Milky Way and Andromeda galaxies are spiral galaxies. Elliptical These galaxies are shaped like spheres or ellipses and are made up of mostly old stars. They have little interstellar matter and are often found in galaxy clusters. Elliptical galaxies are much fainter than other galaxies and are difficult to see without a telescope. Irregular These galaxies have no distinct shape and can appear chaotic. They can contain both old and new stars, and some may have dark patches of gas and dust. Irregular galaxies may have been once spiral or elliptical galaxies that were deformed by outside gravitational forces. Hubble's Law states that the galaxy's recession speed = Ho * distance, where Ho is known as the Hubble constant and is a measure of the slope of the line through the distance versus recession velocity data. The line goes through the origin (0,0) because that represents our home position (zero distance) and we are not moving away from ourselves (zero speed). To determine a galaxy's distance, we must rely on indirect methods. For instance, one assumption used by Hubble, and other early 20th century astronomers, is to assume all galaxies of the same type are the same physical size, no matter where they are. This is known as "the standard ruler" assumption. To determine the distance to a galaxy one would only need to measure its apparent (angular) size, and use the small angle equation: a = s / d, where a is the measured angular size (in radians!), s is the galaxy's true size (diameter), and d is the distance to the galaxy. Main sequence fitting involves the fitting of one cluster's main sequence to another cluster's main sequence. At first glance this may seem irrelevant, but it is the key to finding the distance to M3. In astronomy, a standard candle is a celestial object with a known luminosity, or the amount of light it emits per unit of time. Astronomers use standard candles to calculate the distance to celestial objects by comparing their known luminosity to their apparent brightness from Earth. Standard candles are important because they allow astronomers to measure the distances to galaxies, which is difficult because the apparent brightness of an object can be the same for both a distant, bright object and a closer, less bright object. Some examples of standard candles include: Cepheid variables: These pulsating stars are the first rung of the cosmic distance ladder. The luminosity of a Cepheid variable star is related to its pulsation period. Cepheids, also called Cepheid Variables, are stars which brigthen and dim periodically. This behavior allows them to be used as cosmic yardsticks out to distances of a few tens of millions of light-years. a uniform function of their brightness. This class of stars came to be known as classical Cepheid variables, and by comparing the apparent brightness measured from earth with the known actual brightness of the star, the distance to the star could be calculated with previously unmatched accuracy. RR Lyrae stars: These stars are also commonly used as standard candles. Type Ia supernovae: These stellar explosions are very bright and have a known luminosity. However, they are not always the same peak brightness, so they are considered more "standardizable" candles. A Type Ia supernova (read: "type one-A") is a type of supernova that occurs in binary systems (two stars orbiting one another) in which one of the stars is a white dwarf. The other star can be anything from a giant star to an even smaller white dwarf.