Presentation on theme: "Life as a Low-mass Star Image: Eagle Nebula in 3 wavebands (Kitt Peak 0.9 m)."— Presentation transcript:
1Life as a Low-mass StarImage: Eagle Nebula in 3 wavebands (Kitt Peak 0.9 m).
2What are the life stages of a low-mass star? Low-mass: < 2 M_Sun; high-mass: > 8 M_Sun.
3A star remains on the main sequence as long as it can fuse hydrogen into helium in its core. main-seq_lifetime_and_mass.swf
4Thought QuestionWhat happens when a star can no longer fuse hydrogen to helium in its core?Core cools off.Core shrinks and heats up.Core expands and heats up.Helium fusion begins immediately.
5Thought QuestionWhat happens when a star can no longer fuse hydrogen to helium in its core?Core cools off.Core shrinks and heats up.Core expands and heats up.Helium fusion begins immediately.
6Life Track After Main Sequence Observations of star clusters show star becomes larger, redder, and more luminous after its time on the main sequence is over.
7Broken ThermostatAs core contracts, H begins fusing to He in a shell around core.Luminosity rises because core thermostat broken: increasing fusion rate in shell does not stop core from contracting.Shell burning is vigorous and adds more inert He to the core, increasing contraction and heating shell even more: red giant swells.
8He-fusion requires higher temperatures than H-fusion because larger charge leads to greater repulsion.Fusion of two He nuclei doesn’t work, so He-fusion must combine three He nuclei to make carbon (C).
9Thought QuestionWhat happens in a low-mass star when core temperature rises enough for helium fusion to begin?Helium fusion slowly starts up.Hydrogen fusion stops.Helium fusion rises very rapidly.Hint: degeneracy pressure is the main form of pressure in the inert helium core.
10Thought QuestionWhat happens in a low-mass star when core temperature rises enough for helium fusion to begin?Helium fusion slowly starts up.Hydrogen fusion stops.Helium fusion rises very rapidly.
11Helium FlashThermostat broken in low-mass red giant because degeneracy pressure supports core.Core temperature rises rapidly when helium fusion begins.Helium fusion rate skyrockets until thermal pressure takes over and expands core again.
12This is the “horizontal branch” (HB). Helium-burning stars neither shrink nor grow because thermostat is temporarily fixed.
13Life Track After Helium Flash Models show that a red giant should shrink and become less luminous after helium fusion begins in the core.
14Life Track After Helium Flash Observations of star clusters agree with those models.Helium-burning stars are found in a horizontal branch on the H-R diagram.
16Thought Question What happens when a star’s core runs out of helium? The star explodes.Carbon fusion begins.The core cools off.Helium fuses in a shell around the core.
17Thought Question What happens when a star’s core runs out of helium? The star explodes.Carbon fusion begins.The core cools off.Helium fuses in a shell around the core.
18Double Shell Burning Late in its life, a star like our Sun will have… An inert carbon core……surrounded by a shell of fusing helium……surrounded by a shell of fusing hydrogen.The star swells enormously in size, even bigger than before.But the core never gets hot enough to fuse carbon.Double shell burning never reaches equilibrium: fusion rate spikes periodically in a series of thermal pulses, dredging carbon up from the core.
19Only a white dwarf is left behind. A star like our sun dies by puffing off its outer layers, creating a planetary nebula.Only a white dwarf is left behind.Great planetary-nebula movies can be downloaded from Press Release (Helix Nebula) of the Space Telescope Science Institute (see hubble.stsci.edu)
20A star like our sun dies by puffing off its outer layers, creating a planetary nebula. Only a white dwarf is left behind.
21A star like our sun dies by puffing off its outer layers, creating a planetary nebula. Only a white dwarf is left behind.
22A star like our sun dies by puffing off its outer layers, creating a planetary nebula. Only a white dwarf is left behind.
23White DwarfNo fusion, and it cannot contract (due to degeneracy pressure).So a white dwarf just cools off forever, fading away…
27CNO CycleHigh-mass main- sequence stars fuse H to He at a higher rate using carbon, nitrogen, and oxygen as catalysts.A greater core temperature enables H nuclei to overcome greater repulsion.
28Life Stages of High-mass Stars Late life stages of high-mass stars are similar to those of low-mass stars…Hydrogen core fusion (main sequence).Hydrogen shell burning (supergiant).Helium core fusion (supergiant).High-mass stars become supergiants: luminosity doesn’t change much, but radius expands a lot.
29How do high-mass stars make the elements necessary for life?
30Big Bang made 75% H, 25% He — stars make everything else.
31Helium fusion can make carbon in low-mass stars.
32CNO cycle in high-mass stars can change C N & O.
33Helium CaptureHigh core temperatures allow helium to fuse with heavier elements.
41Core then suddenly collapses, creating supernova explosion. Iron builds up in core until degeneracy pressure can no longer resist gravity.Core then suddenly collapses, creating supernova explosion.death_seq_of_high-mass_star.swfDownload a good supernova explosion movie from the Chandra Science Center (chandra.harvard.edu)
42Supernova ExplosionCore degeneracy pressure goes away because electrons combine with protons, making neutrons and neutrinos.Neutrons collapse to the center, forming a neutron star.
43Side reactions involving neutrons can make nuclei with odd numbers of protons, etc. Energy and neutrons released in supernova explosion enable elements heavier than iron to form, e.g. Au, U.
48How does a star’s mass determine its life story?
49Role of MassA star’s mass determines its entire life story because it determines its core temperature.High-mass stars have short lives, eventually becoming hot enough to make iron, and end in supernova explosions.Low-mass stars have long lives, never becoming hot enough to fuse carbon nuclei, and end as white dwarfs.
50Low-mass Star SummaryMain Sequence: H fuses to He in core.Red Giant: H fuses to He in shell around He core.Helium Core Burning: He fuses to C in core while H fuses to He in shell.Double Shell Burning: H and He both fuse in shells.5. Planetary Nebula leaves White Dwarf behind.Not to scale!
51Reasons for Life Stages Core shrinks and heats until it’s hot enough for fusion.Nuclei with larger charge require higher temperature for fusion.Core thermostat is broken while core is not hot enough for fusion (shell burning).Core fusion can’t happen if degeneracy pressure keeps core from shrinking.Not to scale!
52High-mass Star Summary Main Sequence: H fuses to He in core.Red Supergiant: H fuses to He in shell around He core.Helium Core Burning: He fuses to C in core while H fuses to He in shell.Multiple Shell Burning: Many elements fuse in shells.5. Supernova leaves Neutron Star behind.Not to scale!
53How are the lives of stars with close companions different?
54Thought QuestionThe binary star Algol consists of a 3.7 MSun main-sequence star and a 0.8 MSun subgiant star…What’s strange about this pairing?How did it come about?
55Stars in Algol are close enough that matter can flow from the subgiant onto the main-sequence star.
56The star that is now a subgiant was originally more massive. As it reached the end of its life and started to grow, it began to transfer mass to its companion (mass exchange).Now the companion star is more massive.