Astronomy 104: Stellar Astronomy Lecture 16: A Star is Born Spring Semester 2013 Dr. Matt Craig 1 1
Goals By the end of the next two to three lectures you should be able to: Describe the life cycle of a star. For each stage in the life of a star describe the relationship between pressure and gravity. Specifically, you should be able to identify which is larger at a certain stage: gravity, pressure, neither. Explain why a star changes the way it does during its lifetime in terms of the forces of pressure and gravity. List the life stages of a star in order. 2 2
Reading Read Chapter 11 11-1) Star Formation (Today) 11-2) Life on the Main Sequence (last time) 11-4) Late Stages in a Star's Life 11-5) Low Mass Remnants Chapter 12 will cover the remnants of High Mass stars 3 3
So Far... Main Sequence Summary The Main Sequence is a mass sequence of hydrogen-fusing stars... Stars which start with a higher mass have a higher temperature a much higher luminosity a much shorter lifetime. 4 4
Star life on HR diagram (Slides down blue line quickly, then stays at bo6om fusing Helium for a while) (takes 10 billion years ) 5 5
During Stage A The star is during this stage. a)growing wider b)staying the same width c) getting smaller (diameter) Cloud of gas fragments and contracts 6 6
During Stage A Pressure is gravity in this stage. a)larger than b)the same as c)smaller than Cloud of gas fragments and contracts 7 7
During Stage B The star is during this stage. a)growing larger b)staying the same size c)getting smaller (diameter) Like our Sun, in equilibrium 8 8
During Stage B Pressure is gravity in this stage. a)larger than b)the same as c)smaller than Like our Sun, in equilibrium 9 9
During Stage C The star is during this stage. a)growing larger b)staying the same size c)getting smaller (diameter) 10 10
During Stage C Pressure is gravity in this stage. a)larger than b)the same as c)smaller than 11 11
During Stage D The star is during this stage. a)growing larger b)staying the same size c) getting smaller (diameter) 12 12
During Stage D Pressure is gravity in this stage. a)larger than b)the same as c)smaller than 13 13
During Stage E The star is during this stage. a)growing larger b)staying the same size c)getting smaller (diameter) 14 14
During Stage E Pressure is gravity in this stage. a)larger than b)the same as c)smaller than 15 15
During Stage F The star is during this stage. a)growing larger b)staying the same size c)getting smaller (diameter) 16 16
During Stage F Pressure is gravity in this stage. a)larger than b)the same as c)smaller than 17 17
Summary Stage Diameter Pressure/Grav A/Formation shrinking Grav bigger B/Main Sequence steady balanced C/Red Giant growing Pressure bigger D/Helium flash shrink/steady balanced E/Red Supergiant growing pressure bigger F/Remnant depends depends 18
Stage Transitions (Slides down blue line quickly, then stays at bo6om fusing Helium for a while) (takes 10 billion years ) A B Formation ENDS and Main Sequence BEGINS when fusion starts in the core of the star. B C Main Sequence ENDS and Red Giant BEGINS when hydrogen in core of star runs out, ending fusion. 19 19
Stage Transitions (Slides down blue line quickly, then stays at bo6om fusing Helium for a while) (takes 10 billion years ) C D Red giant ENDS and Helium burning BEGINS when core is hot enough for helium fusion. D E Helium ENDS and Red Supergiant BEGINS when helium in core of star runs out, ending fusion again. 20 20
Stage Transitions (Slides down blue line quickly, then stays at bo6om fusing Helium for a while) (takes 10 billion years ) E F Red supergiant ENDS and Formation of Remnant BEGINS when all fusion has ended in core and any surrounding shells around the core. Transition can be smooth, quiet (low mass stars), OR a spectacular explosion (high mass stars) 21 21
Stage 0: Gas in space Orion Nebula http://www.assa.org.au/gallery/beales/orion.jpg 22
Clicker Question A cloud of gas can be observed (a)if it is hot enough to emit its own light (b)if it is near a bright object and reflects light (c)if it blocks the light coming from a bright object (d)any of the ways above 23 23
Trifid Nebula Reflection Absorption Emission http://www.aao.gov.au/images/captions/aat012.html 24
Stage 0: Gas in space Today stars form from clouds of relatively cool, dense gas Giant Molecular Clouds If too hot and can t cool off, high pressure outward... gravity can t compress the gas. If not very dense, then not enough material to generate enough gravity to compress cloud. Sometimes an external force can cause the cloud to collapse Collision with other gas cloud. Pressure from Supernovae Other? 25 25
Simulation of Star Formation Begins with a turbulent cloud containing 50 solar masses of gas. Denser portions of cloud collapse under self-gravity first, causing cloud to become fragmented. Fragments in which gravity overcomes pressure go on to become stars. A large cloud can make a whole cluster of stars. Simulation from Bate, Bonnell, and Brom (UK Astrophysical Fluids Facility) 26 26
Some Star Birth Facts Stars are born in litters. Open clusters Stars are born within clouds of gas. Many different masses of stars form at the same time. Many, many more low mass stars than high mass (today!) 27
Stellar Masses 28
What s the Supporting Evidence? Nebular hypothesis of star formation is nice... Consistent with our understanding of gravity and gas pressure. But is it correct? to be a theory a hypothesis need to make predictions and those predictions need to be verified. The hypothesis needs to be wellsupported to be a theory. 29 29
Star Birth Revealed http://hubblesite.org/newscenter/newsdesk/archive/releases/2000/19/image/a 30
Eagle Nebula Let's Zoom into here http://hubblesite.org/newscenter/newsdesk/archive/releases/1995/44/ 31
Eagle Nebula Star Formation Newborn stars are at tips of columns. Star above and to right of this image is evaporating all but the densest of the gas. 32
Detail of Tip 33
New Stars Clear Out Nearby Region Bubbles of hot gas are common Jets less common, but frequent Has something to do with rotation Detailed cause is not known 34
Star birth in process http://hubblesite.org/newscenter/newsdesk/archive/releases/2000/32/ 35
More Star Birth 36
Discs Disk http://hubblesite.org/newscenter/newsdesk/archive/releases/2000/32/ Jets 37
With Stars come Planets Stars form with disk of gas and dust around them. Dust in the disk sticks together to form clumps. The clumps collide to form planets. 38
Proto-Planetary Discs: Baby Pictures of Other Solar Systems 39
Another Disk 40
Summary: How Stars Form Where do stars form? Stars form in dark, dusty clouds of molecular gas. Why do stars form? Stars form in clouds that are massive enough for gravity to overcome outward pressure(s) (thermal or other). How does a cloud s rotation affect star birth? Conservation of angular momentum likely leads to the formation of disks and probably planets around protostars. 41 41
Star Life on Main Sequence Hydrogen burns in core while on main sequence. When hydrogen in core runs out the star goes from living to dying QUESTION: How has the Sun managed to stay completely stable for almost 5 billion years? ANSWER: It s automatic thanks to the solar thermostat! 42 42
Solar Thermostat and the Main Sequence Decline in core temperature causes fusion rate to drop, so core contracts and heats up Rise in core temperature causes fusion rate to rise, so core expands and cools down 43 43