Binary Stars. Double Stars/Binary Stars Binary Star Double Star Earth.
Evolution of Stars - Physics and Astronomy at...
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Evolution of Stars 1. Formed from gas/dust cloud collapse from gravity 2. Fuse H to He on the Main Sequence. Then evolve off Main-sequence as they burn He and successive elements. 3.When nuclear fusion ceases, the stars either shed their atmospheres through winds or explosions. They return their “metal-polluted” gas to the galaxy. These heavy elements are then processed again and again in successive generations of stars. Universe began 13.7 billion years ago. At that time, only Hydrogen and Helium were present (trace amounts of Li, B, Be). At this point the metal mass fraction of the Universe was Z=0. Successive generations of stars have slowly increased the metal fraction in the Universe. As we study distant stars we find they have different metal mass fractions (the mass fraction when they were formed). Population III: stars with Z ~ 0. Population II: stars with some metals, Z~0.0001 to 0.001 Population I: stars with metals like the Sun, Z~0.01 to 0.03
Transcript of Evolution of Stars - Physics and Astronomy at...
Evolution of Stars1. Formed from gas/dust cloud collapse from gravity
2. Fuse H to He on the Main Sequence. Then evolve off Main-sequence as they burn He and successive elements.
3.When nuclear fusion ceases, the stars either shed their atmospheres through winds or explosions. They return their “metal-polluted” gas to the galaxy. These heavy elements are then processed again and again in successive generations of stars.
Universe began 13.7 billion years ago. At that time, only Hydrogen and Helium were present (trace amounts of Li, B, Be). At this point the metal mass fraction of the Universe was Z=0.
Successive generations of stars have slowly increased the metal fraction in the Universe. As we study distant stars we find they have different metal mass fractions (the mass fraction when they were formed).
Population III: stars with Z ~ 0.
Population II: stars with some metals, Z~0.0001 to 0.001
Population I: stars with metals like the Sun, Z~0.01 to 0.03
We can construct theoretical HR (color-magnitude) diagrams for stellar populations as a function of the cluster age. The model for a fixed time is an isochrone. This lets us determine the age of the star cluster and study stellar evolution (are our models correct ?!)
Kalirai et al. 2001, AJ, 122, 266
Color-Magnitude Diagram for NGC 6819
Models are for ages of t= 2, 2.5, 3.2, 4, 5 x 109 yrs.
Age of the Universe
Age of the Universe
Messier 3
Age of the Universe
Messier 3
Giants, Supergiants, & WDs
What happens when a star runs out of Hydrogen in its core?
Core contracts & gets hotter, which puffs up the envelope
Giants, Supergiants, & WDs
What happens when a star runs out of Hydrogen in its core?
Core contracts & gets hotter, which puffs up the envelope
Giants, Supergiants, & WDs
Slide
Evolution off the Main Sequence: Expansion into a Red Giant
Hydrogen in the core completely converted into He:
H burning continues in a shell around the core.
He Core + H-burning shell produce more energy than needed for pressure support
Expansion and cooling of the outer layers of the star → Red Giant
→“Hydrogen burning” (i.e. fusion of H into He) ceases in the core.
Slide
Red Giant Evolution
4 H → He
He
H-burning shell keeps dumping He onto the
core.
He-core gets denser and hotter until the next
stage of nuclear burning can begin in the
core:
He fusion through the
“Triple-Alpha Process”
4He + 4He → 8Be + γ8Be + 4He → 12C + γ
Slide
Outer layers expand due to radiation pressure from a hot core • Surface temperature drops by a factor of ~ 2• The radius increases by a factor of ~ 100• Luminosity increases ~ R2 T4 ~ 100-1000 times
Our Sun’s Evolution
Our Sun’s Evolution
Post Main Sequence: Solar Mass
Post Main Sequence: Solar Mass
How our Sun evolves off the Main Sequence to end up as a White Dwarf (degenerate Carbon-Oxygen core)
Post Main Sequence: Solar Mass
How our Sun evolves off the Main Sequence to end up as a White Dwarf (degenerate Carbon-Oxygen core)
Log10 T [K]
Log 1
0 (
L / L
⊙)
Evolutionary Stages:
#1-2. (Zero-Age) Main Sequence
#3-5. Core-contraction, H-shell burning, supergiant branch
#6-7. He core burning, Red-Giant Branch.
#8-10. Core-contraction, Carbon burning.
Log10 T [K]
Log 1
0 (
L / L
⊙)
Log10 T [K]
Log 1
0 (
L / L
⊙)
Post Main Sequence Tracks
Post Main Sequence Tracks
5 solar mass star
Post Main Sequence Tracks
5 solar mass star
Post Main Sequence Tracks
5 solar mass star
When He in the Core Runs Out
When He in the Core Runs OutShell-burning = Hydrogen and Helium burning shells(Carbon-Oxygen ash core)
When He in the Core Runs OutShell-burning = Hydrogen and Helium burning shells(Carbon-Oxygen ash core)
Pulsations due to double-shell burning eventually blow off the star’s outer layers to form a planetary nebula.
Slide
What is left??
A stellar remnant: white dwarf, composed mainly of carbon and oxygen
Slide
Detecting the presence of an unseen companion, Sirius B by its gravitational influence on the primary star, Sirius A.
Wobbling motion of Sirius A
1850: a strange star was discovered
Slide
Slide
White DwarfsDegenerate stellar remnant (C,O core)
Extremely dense:1 teaspoon of WD material: mass ≈ 16 tons!!!
White Dwarfs:
Mass ~ Msun
Temp. ~ 25,000 K
Luminosity ~ 0.01 Lsun
Chunk of WD material the size of a beach ball would outweigh an ocean liner!
Sun’s Evolution into White Dwarf
Sun’s Evolution into White Dwarf
Low-Mass Star Maintains Gravitational EquilibriumWhat balances gravity? Electron Degeneracy!
Sun’s Evolution into White Dwarf
Low-Mass Star Maintains Gravitational EquilibriumWhat balances gravity? Electron Degeneracy!
White Dwarf
White DwarfPauli Exclusion Principle:No two electrons can be at the same place at the same time with the same energy.At high density, all the low energy states are occupied, leaving only high energy (high pressure) states.
Results in Degenerate Electron Gas: Pressure is independent of temperature Compression does NOT lead to heating
Works for stellar cores up to 1.4 solar masses.
Chandrasekhar Limit
Chandrasekhar LimitMass-Radius relation for White Dwarfs:More mass means smaller radius!
Chandrasekhar LimitMass-Radius relation for White Dwarfs:More mass means smaller radius!
but only up to the Chandrasekhar mass... then neutron degeneracy takes over
Log10 T [K]
Log 1
0 (
L / L
⊙)
Post-main sequence of higher mass stars.
Nucleosynthesis
NucleosynthesisCreating heavy elements (stars of at least 8 Msun)
NucleosynthesisCreating heavy elements (stars of at least 8 Msun)
Hydrogen: 10 MyrHelium: 1 Myr
Carbon: 1000 yearsNeon: 10 yearsOxygen: 1 year
Silicon: 1 day
Pulsating Stars
In 1596, David Fabricius (1564-1617, Lutheran Pastor) observed the star o Ceti (constellation Cetus). Over several months the
brightness of this star faded until it was not visible. After several more months the star returned to full brightness, he named this
star “Mira” (for miraculous).
Mira has an 11-month cycle.
Mira is a long-period variable star.
Pulsating Stars
Time (Period is 332 days)
Mag
nitu
de v
arie
s fr
om 3
.5 t
o 9.
Pulsating StarsIn 1784 John Goodricke (1764-1786) found that δ Cephei varied
with a period of 5 days, 8 hours, and 48 minutes.
This is a short-period variable star. Stars like it are called “classical cepheids”
Stebbins 1908, ApJ, 27, 188
Slide
Delta Cephei
By 2005, there are 40,000 cataloging pulsating stars.
More than 5% were discovered by Henrietta Swan Leavitt (1868-1921), who studied photographs of stars on different nights.
She discovered 2400 classical Cepheid stars with periods of 1-50 days, most of them in the Small Magellanic Cloud (a small galaxy orbiting the Milky Way Galaxy).
Pulsating Stars
Henrietta Swan Leavitt (1868-1921)
Leavitt observed that brighter Cepheids have longer periods
Cepheid Period-magnitude Relation
Large (left) and Small (right) Magellanic Clouds
Slide
Period-luminosity relationSince all cepheids in SMC are at the same distance from us, the same relationship should be between their periods and absolute magnitudes!
Slide
Period-luminosity relation A
vera
ge m
agni
tude
M<V
>
Period (log P)0.4 2.01.40.80.6 1.0 1.2 1.6 1.8
-2-7
-5-6
-4-3
Since all cepheids in SMC are at the same distance from us, the same relationship should be between their periods and absolute magnitudes!
Nearest Cepheid is Polaris (The North Star). In 1913 Ejnar Hertzsprung measured a parallax distance to this star of 200 pc. That allowed astronomers to place the
Cepheids on a Luminosity-Period relation:
〈MV〉= -2.81 log10 Pd - 1.43 or log10(〈L〉/ L⊙) = 1.15 log10 Pd + 2.47
Pulsating Stars
Sandage & Tammann, 1968, ApJ, 151, 531
Cepheid Stars
Cepheid StarsPeriod-Luminosity Relation: M = a + b log P
Cepheid StarsPeriod-Luminosity Relation: M = a + b log P
Cepheid StarsPeriod-Luminosity Relation: M = a + b log P
Cepheid StarsPeriod-Luminosity Relation: M = a + b log P
Pulsating Stars
Cepheid Stars
