CONTENT:CHAP20:FIGURES:FG20_001.PCT Star In Equilibrium - In a steadily burning star on the main sequence, the outward pressure of hot gas balances the inward pull of gravity. This is true at every point within the star, guaranteeing its stability. ## CONTENT:CHAP20:FIGURES:FG20_002.PCT Change In Sunlike Star's Composition - Theoretical estimates of the changes in a Sun-like star's composition. Hydrogen (yellow) and helium (orange) abundances are shown (a) at birth, on the zero-age main sequence; (b) after 5 billion years; and (c) after 10 billion years. At stage (b) only about 5 percent of the star's total mass has been converted from hydrogen into helium. This change speeds up as the nuclear burning rate increases with time. ## CONTENT:CHAP20:FIGURES:FG20_003.PCT Hydrogen Burning Shell Inside Star - As a star's core loses more and more of its hydrogen, the hydrogen in the shell surrounding the nonburning helium ash burns ever more violently. ## CONTENT:CHAP20:FIGURES:FG20_004.PCT H-R Diagram Of Star Leaving Main Sequence - As the core of helium ash shrinks and the intermediate stellar layers expand, the star leaves the main sequence (stage 7). At stage 8, the star is on its way to becoming a red-giant star. The star continues to brighten and grow as it ascends the red-giant branch to stage 9, the top of the red-giant branch. As in Chapter 19, the diagonal lines correspond to stars of constant radius, allowing us to gauge the changes in the size of our star. ## CONTENT:CHAP20:FIGURES:FG20_005.PCT Evolution Of A Normal Sunlike Star - Diagram of the relative sizes and colors of a normal G-type star (such as our Sun) in its formative stages, on the main sequence, and while passing through the red-giant and white-dwarf stages. At maximum swelling, the red giant is approximately 70 times the size of its main-sequence parent; the core of the giant is about 1/15 the main-sequence size and would be barely discernible if this figure were drawn to scale. The length of time spent in the various stages-protostar, main-sequence star, red giant, and white dwarf-is roughly proportional to the length of this imaginary trek through space. ## CONTENT:CHAP20:FIGURES:FG20_006.PCT Helium Flash Stage On H-R Diagram - After its large increase in luminosity while ascending the red-giant branch is terminated by the helium flash, our star settles down into another equilibrium state at stage 10, on the horizontal branch. ## CONTENT:CHAP20:FIGURES:FG20_007.PCT Carbon Ash Core In Late Star - Within a few million years after the onset of helium burning, carbon ash accumulates in the inner core of a star, above which hydrogen and helium are still burning in concentric shells. ## CONTENT:CHAP20:FIGURES:FG20_008.PCT Star Entering Red Giant Branch On H-R Diagram - A carbon-core star reascends the giant branch of the H-R diagram-this time on a track called the asymptotic giant branch-for the same reason it evolved there the first time around: Lack of nuclear burning at the core causes contraction of the core and expansion of the overlying layers. ## CONTENT:CHAP20:FIGURES:FG20_09A.PCT H-R Diagram Of Old Star Cluster - The various evolutionary stages predicted by theory and depicted schematically in Figure 20.8 are clearly visible in this H-R diagram of an old star cluster-the globular cluster M3. The faintest main-sequence stars are not shown here because observa-tional limitations make it difficult to determine the apparent brightness of low-luminosity stars in the cluster. ## CONTENT:CHAP20:FIGURES:FG20_09B.PCT Wide-Angle Photo Of Globular Cluster M3 - Wide-angle photograph showing M3 as it appears in the night sky. The inset is a more detailed view of the cluster itself; its field is a few parsecs across. ## CONTENT:CHAP20:FIGURES:FG20_010.PCT Instability In Radius Of Red Giant Star - Buffeted by helium-shell flashes from within and subject to the destabilizing influence of recombination, the outer layers of a red giant become unstable and enter into a series of growing pulsations. Eventually, the envelope is ejected and forms a planetary nebula. ## CONTENT:CHAP20:FIGURES:FG20_11A.PCT The Ring Nebula - (1 of 3) - A planetary nebula is an object with a small dense core (central blue-white star) surrounded by an extended shell (or shells) of glowing matter. (a) The Ring Nebula in the constellation Lyra, a classic example of a planetary nebula, is about 1500 pc from us. It is about 0.2 pc in diameter-much larger than our solar system-but because of its great distance, its apparent size is only about 1/100 that of the full Moon, and it is too dim to see well with the naked eye. (See also the images on p. 52.) ## CONTENT:CHAP20:FIGURES:FG20_11B.PCT Cross Section Of Ring Nebula - (2 of 3) - The appearance of the planetary nebula can be explained once we realize that the shell of glowing gas around the central core is actually quite thin. There is very little gas along the line of sight between the observer and the central star (path A), so that part of the shell is invisible. Near the edge of the shell, however, there is more gas along the line of sight (paths B and C), so the observer sees a glowing ring. ## CONTENT:CHAP20:FIGURES:FG20_11C.PCT The Helix Nebula - (3 of 3) - The Helix Nebula appears to the eye as a small star with a halo around it. About 140 pc from the Earth and 0.6 pc across, its apparent size in the sky is roughly half that of the full Moon. (All the other stars visible in the photo are foreground or background objects, unrelated to the planetary nebula.) ## CONTENT:CHAP20:FIGURES:FG20_12A.PCT The Dumbbell Nebula - The Dumbell Nebula more clearly shows the shell-like structure of the expanding gases that make up a planetary nebula. ## CONTENT:CHAP20:FIGURES:FG20_12B.PCT The Cat's Eye Nebula - The Cat's Eye Nebula is an example of a much more complex planetary nebula. Intricate structures, including concentric gas shells, jets of high-speed gas, and shock-induced knots of gas are all visible. As usual, red indicates the presence of excited hydrogen. The nebula is about 1000 pc away, in the constellation Draco. It may have been produced by a pair of binary stars (unresolved at the center) that have both shed planetary nebulae. ## CONTENT:CHAP20:FIGURES:FG20_013.PCT Star's Transition To White Dwarf Stage On The H-R Diagram - A star's passage from the horizontal branch (stage 10) to the white-dwarf stage (stage 13) by way of the asymptotic giant branch creates an evolutionary path that cuts across the entire H-R diagram. ## CONTENT:CHAP20:FIGURES:FG20_014.PCT Photo Of Sirius B Dwarf Companion - Sirius B (the speck of light at right) is a white-dwarf star, a companion to the much larger and brighter star Sirius A. (The "spikes" on the image of Sirius A are not real; they are artifacts caused by the support struts of the telescope.) ## CONTENT:CHAP20:FIGURES:FG20_015.PCT Late Evolutionary Track Of Star Of Three Different Solar Masses - Evolutionary tracks for stars of 1, 5, and 15 solar masses (shown only up to the point of the helium flash in the low-mass cases). Low-mass stars ascend the giant branch almost vertically, whereas high-mass stars move roughly horizontally across the H-R diagram from the main sequence into the red-giant region. The most massive stars experience smooth transitions into each new burning stage. No helium flash occurs for stars more massive than about 4 solar masses. The loops in the tracks generally indicate the point at which a new burning stage begins. Some points are labeled with the element that has just started to fuse in the inner core. ## CONTENT:CHAP20:FIGURES:FG20_016.PCT Changing H-R Diagram For Hypothetical Star Cluster - The changing H-R diagram of a hypothetical star cluster. (a) Initially, stars on the upper main sequence are already burning steadily while the lower main sequence is still forming. (b) At 107 years, O-type stars have already left the main sequence, and a few red giants are visible. (c) By 108 years, stars of spectral type B have evolved off the main sequence. More red giants are visible, and the lower main sequence is almost fully formed. (d) At 109 years, the main sequence is cut off at about spectral type A. The subgiant and red-giant branches are just becoming evident, and the formation of the lower main sequence is complete. A few white dwarfs may be present. (e) At 1010 years, only stars less massive than the Sun still remain on the main sequence. The cluster's subgiant, red-giant, horizontal, and asymptotic giant branches are all discernible. Many white dwarfs have now formed. ## CONTENT:CHAP20:FIGURES:FG20_17A.PCT H-R Diagram For Double Cluster In Perseus - The "double cluster" h and c Persei. ## CONTENT:CHAP20:FIGURES:FG20_17B.PCT Double Cluster In Perseus - The H-R diagram of the pair indicates that the stars are very young-probably only about 10 million years old. ## CONTENT:CHAP20:FIGURES:FG20_18A.PCT Young Star Cluster - The Hyades cluster, a relatively young group of stars visible to the naked eye. ## CONTENT:CHAP20:FIGURES:FG20_18B.PCT Typical H-R Diagram For Young Cluster - The H-R diagram for this cluster is cut off at about spectral type A, implying an age of about 500 million years. ## CONTENT:CHAP20:FIGURES:FG20_19A.PCT Globular Cluster 47 Tucane - The southern globular cluster 47 Tucanae. ## CONTENT:CHAP20:FIGURES:FG20_19B.PCT H-R Diagram For Globular Cluster 47 Tucane - Fitting its main-sequence turnoff and its giant and horizontal branches to theoretical models gives 47 Tucanae an age of about 14 billion years, making it one of the oldest known objects in the Milky Way Galaxy. The inset is a high-resolution ultraviolet image of 47 Tucanae's core region, taken with the Hubble Space Telescope and showing many "blue stragglers"-massive stars lying on the main sequence above the turnoff point, resulting perhaps from the merging of binary star systems. ## CONTENT:CHAP20:FIGURES:FG20_020.PCT Roche Lobes In Binary System - Each star in a binary system can be pictured as being surrounded by a "zone of influence," or Roche lobe, inside of which matter may be thought of as being "part" of that star. The two teardrop-shaped Roche lobes meet at the Lagrange point between the two stars. Outside the Roche lobes, matter may flow onto either star with relative ease. ## CONTENT:CHAP20:FIGURES:FG20_021.PCT Comparison Of Types Of Detached/Contact Binary Systems - (a) In a detached binary, each star lies within its respective Roche lobe. (b) In a semidetached binary, one of the stars fills its Roche lobe and transfers matter onto the other, which still lies within its own Roche lobe. (c) In a contact or common-envelope binary, both stars have overflowed their Roche lobes, and a single star with two distinct nuclear-burning cores results. ## CONTENT:CHAP20:FIGURES:FG20_022.PCT Evolution Of The Algo Binary System - The evolution of the binary star Algol. (a) Initially, Algol was probably a detached binary made up of two main-sequence stars -a relatively massive blue giant and a less massive companion similar to the Sun. (b) As the more massive component (star 1) evolved off the main sequence, it expanded to fill and eventually overflow its Roche lobe, transferring large amounts of matter onto its smaller companion (star 2). (c) Today, star 2 is the more massive of the two, but it is on the main sequence. Star 1 is still in the subgiant phase and fills its Roche lobe, causing a steady stream of matter to pour onto its companion. ## CONTENT:CHAP20:SLIDES:SL20_001.PCT Egg Nebula - This image of the Egg Nebula, also known as CRL2688 and located roughly 3,000 light-years away, was taken by Hubble Space Telescope. The image shows a pair of mysterious "searchlight" beams emerging from a hidden star, criss-crossed by numerous bright arcs. The nebula is really a large cloud of dust and gas ejected by the dying red giant star, expanding at 115,000 mph. A dense cocoon of dust (the dark band in the image center) enshrouds the star and hides it from view. Starlight escapes more easily in directions where the cocoon is thinner, and is reflected toward earth by dust particles in the cloud. ## CONTENT:CHAP20:SLIDES:SL20_002.PCT Planetary Nebula 7027 - This Hubble Space Telescope image of planetary nebula NGC 7027 shows remarkable new details of the process by which a star like the Sun dies. New features include: faint, blue, concentric shells surrounding the nebula; an extensive network of red dust clouds throughout the bright inner region; and the hot central white dwarf, visible as a white dot at the center. ## CONTENT:CHAP20:SLIDES:SL20_003.PCT Cat's Eye Nebula - This Hubble Space Telescope image shows one of the most complex planetary nebulae ever seen, NGC 6543, nicknamed the "Cat's Eye Nebula." Hubble reveals surprisingly intricate structures including concentric gas shells, jets of high-speed gas and unusual shock-induced knots of gas. Estimated to be 1,000 years old, the nebula is a visual "fossil record" of the dynamics and late evolution of a dying star. ## CONTENT:CHAP20:SLIDES:SL20_004.PCT Hourglass Nebula - This is an image of MyCn18, a young planetary nebula located about 8,000 light-years away, taken with Hubble Space Telescope, which reveals the true shape of MyCn18 to be an hourglass with an intricate pattern of "etchings" in its walls. According to one theory the hourglass shape is produced by the expansion of a fast stellar wind within a slowly expanding cloud which is more dense near its equator than near its poles. ## CONTENT:CHAP20:SLIDES:SL20_005.PCT Commentary Knots In Helix Nebula - This colorful image from the Hubble Space Telescope shows the collision of two gases near a dying star. Astronomers have dubbed the tadpole-like objects in the upper right-hand corner "cometary knots" because their glowing heads and gossamer tails resemble comets. Although astronomers have seen gaseous knots through ground-based telescopes, they have never seen so many in a single nebula. ## CONTENT:CHAP20:VIDEO:VD20_001.MOV Evolution Of A 1-Solar Mass Star - Born from an interstellar cloud, a young star gradually sweeps away surrounding Dust and debris, revealing a genuine star like our Sun. Only in old age does the star change its appearance, first becoming a red giant and then leaving a white dwarf star at its core. ## CONTENT:CHAP20:VIDEO:VD20_002.MOV HR Diagram Tracks Stellar Evolution - A plot of luminosity and temperature allows us to follow the evolution of a 1-solar-mass star, from birth to death. Motion of the data point on the plot is roughly proportional to the time spent by the star at each stage. ## CONTENT:CHAP20:VIDEO:VD20_003.MOV Formation Of Helix Nebula - Planetary nebulae like the Helix Nebula form when a star late in its life ejects shells of gas into space. This "planetary nebula" formation happens in stages where, toward the end of the process, a faster moving shell of gas ejected off the doomed star collides with slower moving gas released several thousand years before the event. This creates a striking ring-like pattern of glowing gasses. ## CONTENT:CHAP20:VIDEO:VD20_004.MOV Formation Of Knots In Helix Nebula - This is a striking view of thousands of comet-like knots of gas in the Helix nebula. Standard models predict that the knots should expand and dissipate within a few hundred thousand years. However, dust particles inside each gas ball might collide and stick together, snowballing to planet-sized bodies over time. The resulting objects would be like Earth-sized copies of the frigid, icy planet Pluto. These icy worlds would escape the dead star and presumably roam interstellar space forever. ##