Topics for Today s Class

Transcription

1 Foundations of Astronomy 13e Seeds Chapter 11 Formation of Stars and Structure of Stars Topics for Today s Class 1. Making Stars from the Interstellar Medium 2. Evidence of Star Formation: The Orion Nebula 3. Young Stellar Objects and Protostellar Disks 4. Stellar Structure 5. The Source of Stellar Energy 6. The Pressure Temperature Thermostat Guidepost 11-1 The Formation of Stars In this chapter, you will consider how the interstellar medium condenses into stars and what conditions inside stars must be like How do stars form? What is the evidence that stars are forming now? How do stars maintain their stability? How do stars make energy? Stars are formed during the collapse of the cores of giant molecular clouds Clouds must contract and heat up to ignite thermonuclear processes Factors resisting the collapse of a gas cloud: Thermal energy (pressure), magnetic fields, rotation (angular momentum), and turbulence An external trigger is required to initiate the collapse of giant dense molecular clouds Shocks Triggering Star Formation, Part 1 Shock wave is induced and moves towards interstellar gas cloud Passes through and compresses gas cloud Motion of particles in the cloud continue post-shock wave Densest part of the cloud becomes gravitationally unstable Stars are born within the contracting regions of the gas Previous star formation can trigger further star formation through:1) Supernova Explosion: Massive stars die young => Supernovae tend to happen near sites of recent star formation Ancient Supernova Arc of compressed gas by shock wave from supernova Star formation triggered by compression 1

2 2) Ionization fronts of hot, massive O or B stars which produce a lot of UV radiation: Massive stars die young => O and B stars only exist near sites of recent star formation Roughly 40 young O and B stars are inflating a bubble of hot gas inside the nebula from which they formed. 3) Collisions of giant molecular clouds. Giant molecular clouds are very large and may occasionally collide with each other 4) Spiral arms in galaxies like our Milky Way: Spirals arms are probably rotating shock wave patterns. Protostars Protostars are Pre-birth state of stars: H He fusion not yet ignited Still enshrouded in opaque cocoons of dust => barely visible in the optical, but bright in the infrared. Heating by Contraction As a protostar contracts, it heats up due to free-fall contraction 11-2 The Orion Nebula: An Active Star- Forming Region Evolutionary track from birth to Main Sequence state Free-fall contraction (a) This H R diagram has been extended to very low temperatures to show schematically the contraction of a dim, cool protostar. 2

3 The Orion Nebula Pillars of Creation: Eagle Nebula (M16) Infrared image shows newly born stars, forming from regions of gas and dust. What Makes the Nebula Glow? A single very hot and short-lived O star that produces most of the ultraviolet photons that ionize the gas and make the nebula glow Infrared Visible Bok Globules Globules The visible nebula is only a small part of a vast, dusty molecular cloud Small dark clouds called Bok globules, named after astronomer Bart Bok, are found in and near star forming regions. The one pictured here is part of nebula NGC 1999 near the Orion Nebula. Typically about 1 light-year in diameter, they contain from 10 to 1000 solar masses. Evaporating Gaseous Globules ( EGGs ): Newly forming stars exposed by the ionizing radiation from nearby massive stars Young Star Clusters Ultraviolet radiation and strong stellar winds from young, hot, massive stars in open star clusters compress the surrounding gas resulting in new star formation. The hot, massive stars in the star cluster 30 Doradus Open Clusters of Stars a) An earlier generations of star out of frame on left triggered the formation of second generation of extremely massive stars. These stars are now triggering third generation of stars b) Super star cluster Westerlund 1 showing very massive stars formed about 5 million years ago. Star-forming region N11B Westerlund 1 3

4 11-3 Young Stellar Objects (YSOs) Higher-mass stars evolve more rapidly from protostars to stars than less massive stars The more massive a protostar is, the faster it contracts. A 1 -solar-mass star requires 30 million years to reach the main sequence. The dashed line is the birth line, where contracting protostars first become visible as they dissipate their surrounding clouds of gas and dust. The birth line: star emerges from the enshrouding dust cocoon => Ignition of P-P fusion reaction YSOs and Protostellar Disks Conservation of angular momentum leads to the formation of protostellar disks birth place of planets and moons The rotation of a contracting gas cloud forces it to flatten into a disk, and the protostar grows at the center. The scale of the top panel is much larger than that of the lower two panels. Protostellar Disks Radiation and winds from hot stars nearby are evaporating and distorting the disks. The disk above is seen silhouetted against the nebula, with light from the protostar that is embedded in the disk s center apparent at the upper edge. Although substantially larger than the present size of our Solar System, such disks are understood to be likely sites of planet formation. Protostellar disks observed in the Orion Nebula T Tauri Stars T Tauri Stars are young stars surrounded by gas and dust contracting towards the main sequence in HR Diagram. P-P fusion reaction has not yet started. The star cluster NGC 2264, embedded in the nebula pictured on this slide, is only a few million years old and the cluster contains many T Tauri stars Location of T Tauri Stars on HR diagram Protostellar Disks and Jets Herbig-Haro Objects Accretion disks that often lead to the formation of jets (directed outflows, bipolar outflows) Protostellar Disks and Jets Herbig Haro Objects Herbig Haro objects (named after the two astronomers who first described them, George Herbig and Guillermo Haro) are small nebulae that fluctuate in brightness. Evidently, they are produced by strong, variable jets of gas from newborn stars that collide with and excite the ISM. Herbig Haro Object HH34 At the center of this image, a newborn star is emitting a powerful jet to left and right. Where the jet strike the interstellar medium, they produce Herbig-Haro objects. The inset shows how irregular the jet is. Such jets can be over a light-year long and contain gas traveling at 100 km/s or more. 4

5 Herbig-Haro Object HH30 Arrows indicate knots of material that have moved noticeably in two years. The artist s impression on the right is to help interpret what can be seen in the Hubble Space Telescope images at left and center Stellar Structures What Keeps a Star Stable?: To analyze the structure of a star, it is helpful to divide its interior into concentric shells, much like the layers in an onion. This model is, of course, only an aid to your imagination. Stars are not really divided into such separable layers. Basically the same structure for all stars with approx. 1 solar mass or less Hydrostatic Equilibrium, Part 1 Imagine a star s interior composed of individual shells Within each shell, two forces must be in equilibrium with each other Outward pressure force must exactly balance the weight of all layers above everywhere in the star Gravity, i.e. the weight from all layers above One shell Outward pressure from the interior Hydrostatic Equilibrium, Part 2 Outward pressure force must exactly balance the weight of all layers above everywhere in the star. Energy Transport, Part 1 This condition uniquely determines the interior structure of the star. This is why we find stable stars on such a narrow strip (Main Sequence) in the Hertzsprung-Russell diagram. The three modes by which energy may be transported from the flame of a candle, as shown here, are the three modes of energy transport within a star. 5

6 11-5 The Source of Stellar Energy The CNO Cycle Recall from our discussion of the sun: Stars produce energy by nuclear fusion of hydrogen into helium. In the sun, this happens primarily through the proton-proton (PP) chain In stars slightly more massive than the sun, a more powerful energy generation mechanism than the PP chain takes over: The CNO Cycle. Energy Transport Structure Summary: Stellar Structure Inner convective, outer radiative zone Convective Core, radiative envelope; Energy generation through CNO Cycle Inner radiative, outer convective zone Radiative Core, convective envelope; Energy generation CNO cycle dominant PP chain dominant through PP Cycle Sun The Pressure-Temperature Thermostat How does the star manage to make just enough energy to stop contracting but not to start expanding? Answer in the next Slide 6

7 Acknowledgment The slides in this lecture is for Tarleton: PHYS1411/PHYS1403 class use only Images and text material have been borrowed from various sources with appropriate citations in the slides, including PowerPoint slides from Seeds/Backman text that has been adopted for class. 7