2 TimescalesStages and timescales depend on mass (little bit of composition dependence). Massive stars evolve more quickly than light stars.Main sequence lifetime = fuel / consumption rate ~ Mass / Luminosity.
3 Stages The basic scheme is: Gas Cloud → Main Sequence → Red Giant → (Planetary Nebula or Supernova) → Remnant.
6 Stages Giant Molecular Cloud Giant Molecular Cloud--large dense gas cloud (with dust) that is cold!100,000's to few million solar masses of materialHas fragments of 10's to 100's solar masses that start collapsingReason(s) - shock waves, cool enough for gravity to take over, etc..
7 StagesProtostarProtostar - gas clump collapsing and heating up in center as it collapses.Gravitational energy being converted to heat.Lots of Infrared and Microwave radiation produced.Gets hot enough to glow red ( K), but gas/dust cocoon blocks visible light.IR and Microwave can pass through dust.
9 False-color infrared and radio maps of protostars False-color infrared and radio maps of protostars. (Courtesy Karen Strom, Mark Heyer, Ron Snell, FCAD and FCRAO.)
10 False-color images of a protostar and jet False-color images of a protostar and jet. Notice the Herbig-Haro objects (the blobs) in nearby gas.(Courtesy Patrick Hartigan, NOAO, and STSCI.)
11 Bipolar flow of gas ejected from a young star Bipolar flow of gas ejected from a young star. This is a radio picture in which the gas receding from us is shown in red, whereas the gas approaching us is shown in blue. (Courtesy Ronald Snell, FCRAO.)
12 Stages T-Tauri T-Tauri -- star like object visible to outside. Strong winds eject lots of material from young star (preferentially along rotational axes).Cocoon gas/dust blown away.Star starts fusion (H converted to He).
13 StagesMain SequenceMain Sequence--star is stable because of Hydrostatic Equilibrium.Fusing Hydrogen to Helium in core.Stars spends about 90% lifetime as main sequence.
16 Stages Sub-giant, Red Giant, Supergiant Sub-giant, Red Giant, Supergiant--Run out of core fusion fuel. Hydrostatic equilibrium upset.The Core shrinks. Fusion in shell around core starts. This Fusion is very rapid.The Luminosity (energy output) increases so gas envelope surrounding the core puffs out.
17 Stages Sub-giant, Red Giant, Supergiant At the bloated-out surface, the energy is spread out over a larger area so each square centimeter will be cooler giving the light a red color.Red giants can eject a lot of mass through “winds”’. Note: A Red Giant may be large in terms of linear size, but it is less massive than the main sequence star it came from!
18 The core of a star begins to shrink as a star uses up the hydrogen in its core. This compresses and heats the core.The heated core ignites the surrounding gas to make a shell source, and the outer layers of the star expand, turning it into a red giant.
19 StagesCore FusionCore Fusion -- core has shrunk enough to create high enough temperatures to start Helium (or heavier element) fusion (100 million K).In low mass stars the onset of Helium fusion can be very rapid, producing a burst of energy - helium flash.Eventually it settles down.Core Fusion is releasing more energy than main sequence stage, so star is bigger, but stable!
20 Stages Red Giant, Supergiant Red Giant, Supergiant -- core fuel runs out again. If massive enough, repeat core fusion (stage 5). Number of times to go through these stages depends on mass.Stellar nucleosynthesis of heavy elements occurs. Interplay of gravity and nuclear fusion.
22 StagesPlanetary Nebula or SupernovaPlanetary Nebula or Supernova -- outer layers ejected as core shrinks to most compact state.Low mass stars ( Msun) will go the planetary nebula route;High mass stars (5 - 50Msun) will go the explosive supernova route. Supernova explosion: the core has formed a very stiff neutron star and the in-falling outer layers rebound off it
23 StagesRemnantLow mass core (< 1.4Msun) shrinks to white dwarf. Electrons prevent further collapse. Size about that of Earth. Outer layers are planetary nebula.Higher mass core (1.4Msun - 3Msun) shrinks to neutron star. Supernova happens when neutron star is created. Neutrons prevent further collapse. Size about that of a large city.
24 Photographs of several planetary nebulas. (A) The Helix nebula Photographs of several planetary nebulas. (A) The Helix nebula. (Courtesy Anglo-Australian Observatory/photograph by David Malin.) (B) The Ring nebula. (Courtesy Hubble Heritage Team (AURA, STScI/NASA).) (C) The Butterfly Nebula (Bruce Balick, University of Washington. Vincent Icke, Leiden University (The Netherlands). Garrelt Mellema, Stockholm University/NASA.). Notice the central star in each. Other stars that look as if they are inside the shell are foreground or background stars.(A)(B)(C)Light and Atoms
28 Stages Remnant (continued) Highest mass core (> 3Msun ) shrinks to a point. On the way to total collapse it may momentarily create a neutron star and the resulting supernova rebound explosion.Gravity finally wins. Nothing holds it up. Space so warped around the object that it effectively leaves our space. Black hole!
36 Stellar Nucleosynthesis Stellar Nucleosynthesis--creating heavier elements (heavier than Helium) from lighter elements in stars.Lowest mass stars can only synthesize Helium. Stars around the mass of our Sun can synthesize Helium and Carbon.Massive stars with M > 5Msun can synthesize Helium, Carbon, Oxygen, etc; all the way to Iron.Elements heavier than Iron are made in supernova explosion.
37 Main Sequence Turnoff A star cluster’s H-R diagram changes with age. Main Sequence Turnoff-mass at that point tells you age of cluster.Assume that all stars in cluster form at about the same time.Stars slightly heavier than turnoff have already evolved away from main sequence.
38 The evolution of a low-mass star like the Sun plotted in an H-R diagram
40 Stellar Remnants Degenerate matter Degenerate matter: very dense matter in a state where the pressure no longer depends on temperature; due to quantum mechanical effects.Resist compression. Degenerate particles have no “elbow room”Gas like hardened steel!
41 Stellar Remnants White Dwarfs White Dwarfs--if core mass < 1.4 solar masses. Electrons are degenerate.Mass of Sun compressed to size of Earth.The density is about 1,000,000 g/cm3 (one sugar cube > 1 car!).White dwarf cools off from initial formation, temperature of about 100,000 K.
42 Stellar Remnants Neutron Stars Neutron Stars: core mass is between 1.4 and 3 solar masses.Compression so great that protons fuse with electrons to form neutrons. Neutrons are degenerate.About 30 km across! One sugar cube = mass of humanity!. Formed in supernova explosion.
44 Stellar Remnants Pulsars Pulsars--rapidly rotating neutron stars with STRONG magnetic fields (many times Sun's).Light flashes with period of milliseconds at start and lengthening over time.Lighthouse model-strong magnetic field creates electric field making charged particles flow out of the magnetic poles, producing a beam at the magnetic poles.If the beam sweeps past Earth, we see a flash of “light”.
45 Stellar Remnants Black Holes: core mass > 3 solar masses. Gravity finally wins, compressing core to mathematical point at center. Formed in supernova explosion.Surface gravity so strong that nothing can escape (not even light!) within a certain distance from mass point.
46 3 x core mass [in solar masses] km Stellar RemnantsBlack HolesBoundary is called the event horizon (or Schwarzchild radius)-no messages of events happening within radiusTo find the Schwarzchild radius of an object:3 x core mass [in solar masses] km
47 Stellar Remnants Black Hole Detection Mass of Companion in Binary: For binary, observe how the black hole moves visible companion around. Use Kepler's 3rd law to find masses.X-rays from Accretion DiskFor binary, look for X-rays produced in hot accretion disk--material pulled off visible companion spirals onto black hole.
48 Comparison of Stellar Remnants SizeDensity (1 tsp)White DwarfEarth5 tonsNeutron Star10 – 20 km100 million tonsBlack HolePointInfinite