Stars form in greatStars clouds form of gas in and great dust clouds of gas and dust Slide 1 Cosmic raw material Fig 20-CO, p.438 Chapter Opener The Eagle Nebula (M16) Stars form in great clouds of gas and dust, and this image shows a large region of such cosmic raw material. The gas is visible because, about 2 million years ago, the cloud produced a cluster of bright stars, whose light ionizes the hydrogen gas nearby, causing it to glow. The cluster can be seen just above and to the left of the darker columns of dust at the center of the image. The dark columns or “elephant trunks” of material are seen in much more detail in Figures 20.1 and 20.2. This false-color image was created by combining images taken through filters that select lines of hydrogen alpha (green), oxygen (blue), and sulfur (red). (T.A. Rector, B.A. Wolpa, and OAO/NRAO/AURA/NSF) 1 The Central Region of the Orion Nebula Slide 2 Fig 20-5a, p.443 Figure 20.5 The Central Region of the Orion Nebula The Orion Nebula harbors some of the youngest stars in the solar neighborhood. At the heart of the nebula is the Trapezium cluster, which includes four very bright stars that provide much of the energy that causes the nebula to glow so brightly. In these images, we see a section of the nebula in visible light (left) and infrared (right). The four bright stars in the center of the visible-light image are the Trapezium stars. Notice that most of the stars seen in the infrared are completely hidden by dust in the left image. (Left Image: Anglo-Australian Observatory; Right Image: 2MASS, IPAC & U. of Massachusetts) 2 The Trapezium cluster In Orion Nebula The Orion Nebula harbors some of the youngest stars in the solar neighborhood Slide 3 Fig 20-5b, p.443 Figure 20.5 The Central Region of the Orion Nebula The Orion Nebula harbors some of the youngest stars in the solar neighborhood. At the heart of the nebula is the Trapezium cluster, which includes four very bright stars that provide much of the energy that causes the nebula to glow so brightly. In these images, we see a section of the nebula in visible light (left) and infrared (right). The four bright stars in the center of the visible-light image are the Trapezium stars. Notice that most of the stars seen in the infrared are completely hidden by dust in the left image. (Left Image: Anglo-Australian Observatory; Right Image: 2MASS, IPAC & U. of Massachusetts) 3 The Rosette Nebula A cluster of stars formed recently in the center of this nebula. Stellar winds and pressure produced by the radiation from these hot stars have blown the gas and dust away from the cluster. Slide 4 Fig 20-6, p.444 Figure 20.6 Stellar winds and pressure produced by the radiation from these hot stars have blown the gas and dust away from the cluster so that the newly formed stars are easily seen in visible light. The nebula still contains many globules of dust. This star formation region covers an area on the sky that is six times larger than the area covered by the full moon. The colors in this image are not what your eyes would see; our picture was produced by combining images taken in the emission lines of hydrogen alpha (red), oxygen (green), and sulfur (blue). (T.A. Rector, B.A.Wolpa, M. Hanna, and NOAO/AURA/NSF) 4 Star formation can move progressively through a molecular cloud Triggered star-formation OB associations Groups of Different Ages, Sizes and Densities OB clusters Slide 5 Fig 20-7, p.444 Figure 20.7 Propagating Star Formation This schematic diagram shows how star formation can move progressively through a molecular cloud. The oldest group of stars lies to the left of the diagram and has expanded because of the motions of individual stars. Eventually the stars in the group will disperse and no longer be recognizable as a cluster. The youngest group of stars lies to the right, next to the molecular cloud. This group of stars is only 1 to 2 million years old. The pressure of the hot, ionized gas surrounding these stars compresses the material in the nearby edge of the molecular cloud and initiates the gravitational collapse that will lead to the formation of more stars. 5 The 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 c) A stellar wind breaks out, confined by the disk to flow out along the two poles of the star. d) Stellar wind sweeps away the cloud material and halts the accumulation of additional material. Slide 6 Fig 20-8, p.445 Figure 20.8 The 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 accreting envelope is about 5000 astronomical units. The typical diameter of the disk is about 100 AU or slightly larger than the diameter of the orbit of Pluto. (Based on drawings by F. Shu, F. Adams, and S. Lizano) 6 Hubble Images of a Gas Jets Flowing Away from a Protostar •One million years old •Light from the star itself is blocked by a disk •Thinner material, above and below the central part of the disk reflect light toward us •The material in these disks is flowing outward at speeds up to 960,000 km per hour Slide 7 Fig 20-9, p.447 Figure 20.9 Hubble Images of a Gas Jets Flowing Away from a Protostar Here we see a protostar, known to us as HH30, because it is a Herbig-Haro Object. The star is at a distance of about 450 LY and only about one million years old. Light from the star itself is blocked by a disk, which is over about 60 billion km in diameter and is seen almost edge-on. Thinner material, above and below the central part of the disk reflect light toward us. Jets are seen emerging in opposite directions and perpendicular to the disk. The material in these disks is flowing outward at speeds up to 960,000 km per hour. The series of three images show changes during a period of six years. Every few months a compact clump of gas is ejected, and its motion outward can be followed. The changes in the brightness in the disk may be due to motions of clouds within the disk that alternately block some of the light and then let it through. This image corresponds to the stage in the life of a protostar shown in Figure 20.8c. (A. Watson, K. Stapelfeldt, J. Krist, C. Burrows, & NASA) 7 Outflows from Protostars Slide 8 Fig 20-10, p.448 Figure 20.10 Outflows from Protostars These images were taken with the Hubble Space Telescope and show jets flowing outward from newly formed stars. The top image shows HH 47, a protostar 1500 LY away (invisible inside a dusty disk at the left edge of the image), which produces a very complicated jet. The star may actually be wobbling, perhaps because it has a companion. Light from the star illuminates the white region at the left because light can emerge perpendicular to the disk (just as the jet does). At right the jet is plowing into existing clumps of interstellar gas, producing a shock wave that resembles an arrowhead. The white bar is the size of 1000 astronomical units (1000 times the distance between Earth and Sun). The bottom image (HH 1 and 2) shows a classic double-beam jet emanating from a protostar (hidden in a dust disk in the center) in the constellation of Orion. Tip to tip, these jets are more than 1 LY long. The bright regions (first identified by Herbig and Haro) are places where the jet is slamming into a clump of interstellar gas. (C. Burrows, J. Morse, J. Hester, and NASA) 8 Disks Around Protostars Slide 9 Fig 20-11, p.449 Figure 20.11 Disks Around ProtostarsThese Hubble Space Telescope infrared images show disks around young stars in the constellation of Taurus, in a region about 450 LY away. In some cases we can see the central star (or stars—some are binaries). In other cases, the dark horizontal bands indicate regions where the dust disk is so thick that even infrared radiation from the star embedded within it cannot make its way through. The bright glowing regions are starlight reflected from the upper and lower surfaces of the disk, which are less dense that the central regions. (D. Padgett, W. Brandner, K. Stapelfeldt; IPAC/Caltech/JPL & NASA) 9 Evolutionary Tracks for Contracting Protostars Stars that lie above the dashed line would typically still be surrounded by infalling material and would be hidden by it. Slide 10 Fig 20-12, p.450 Figure 20.12 Evolutionary Tracks for Contracting Protostars Tracks are plotted on the H–R diagram to show how stars of different masses change during the early parts of their lives.