Astro Mid Term Sem. 2 (Book 2/7) Flashcards
if a M-S mass [M(sun)] is < 0.25, what is its evolution?
Red Giant -> He White Dwarf
if a M-S mass [M(sun)] is 0.25 - 0.8 what is its evolution?
Red Giant -> Planetary Nebula -> C White Dwarf
if a M-S mass [M(sun)] is > 8 (core < 3) what is its evolution?
Red Giant -> Supernova -> Neutron Star
if a M-S mass [M(sun)] is > 20 (Core > 3) what is its evolution?
Red Giant -> Supernova -> Black Hole
A star remains on the M-S as long as it can what?
As long as it can fuse hydrogen into helium in its core
The mass of a main sequence star determines its what?
It determines its core pressure and temperature
Stars with higher mass have higher core temperatures and more rapid fusion rates, making these stars what?
These stars will be both more luminous and shorter-lived
Stars with lower mass have cooler cores and slower fusion rates, making these stars what?
These stars will have smaller luminosities and longer lifetimes
What is the structure of an average, mature star? (like our sun)
- Hydrogen Fusion reactions occur in the core which releases;
- Gamma and X-ray Radiation. This radiation moves through the radiation zone from particle to particle, eventually heating gases at the bottom of the convection zone.
- Convection Cells carry energy to the surface, where it is emitted to space as Visible light, Ultraviolet Radiation, and Infrared Radiation
What is the life of the sun? (diagram in book 2 middle of page 2)
- Interstellar cloud; Protostar; Average, mature sun; Red Giant; Yellow Giant; Red Giant; Ejected Gas Shell w/central star; Planetary Nebula; White Dwarf is what is left
Thought Question: What happens when a star can no longer fuse hydrogen to helium in its core? A) Core cools off B) Core shrinks and heats up C) Core expands and heats up D) Helium Fusion immediately begins
Answer is B, the core shrinks and heats up
Thought Question: What happens as a star’s inert helium core starts to shrink?
A) Hydrogen fuses in shell around its core
B) Helium fusion slowly begins
C) Helium fusion rate rapidly rises
D) Core pressure sharply drops
Answer is A, hydrogen fuses in shell around its core
What happens as a star contracts?
Hydrogen (H) begins fusing very rapidly to helium (He) in a shell around the core.
Luminosity will also increase because the fusion rate is higher.
The size will increase: red giant phase
What happens as a star contracts?
Hydrogen (H) begins fusing very rapidly to helium (He) in a shell around the core.
Luminosity will also increase because the fusion rate is higher.
The size will increase: red giant phase
See middle of book 2 page 3 for slide
- Helium fusion requires higher temperatures than hydrogen fusion
- Helium fusion combines 3 He nuclei to make Carbon (C)
The expansion of a star to a giant or super giant size does what to the outer layers?
It cools the outer layers and moves towards the upper right in the H-R diagram
What is the evolution of a gas cloud onto the M-S? (as described in book 2)
- The star shrinks from a large cloud to a spinning ball of gas
- protostar heats up slightly
What happens to a new star once on the M-S? (as described in book 2)
- star burns hydrogen in its core
- star settles down onto the M-S
- slight increase in temp. and luminosity
What happens to a M-S star as it is becoming a giant? (as described in book 2)
- the hydrogen in the star’s core begins to run out
- the star leaves the M-S and swells to become a Red Giant
- the temp. drops
- the luminosity increases because the star is so much bigger
What happens to a M-S star as it is becoming a giant? (as described in book 2)
- the hydrogen in the star’s core begins to run out
- the star leaves the M-S and swells to become a Red Giant
- the temp. drops
- the luminosity increases because the star is so much bigger
What are the basic characteristics of a White Dwarf? (as described in book 2 bottom of page 5)
- The core of the star is left
- No more burning in the core
- The star will continue cooling until it can no longer be seen
What happens when the star ends fusion? (White Dwarf)
- C and O from core helium burning
- Thin layer of He (the product of shell hydrogen burning)
- Typical mass less than half a solar mass to more than 1 solar mass
- ## Same size as Earth
What happens when the star ends fusion? (White Dwarf)
- C and O from core helium burning
- Thin layer of He (the product of shell hydrogen burning)
- Typical mass less than half a solar mass to more than 1 solar mass
- Same size as Earth
- Compact (a spoonful of its matter weighs between 10-100 tons)
- In the final stages of the formation of a Planetary Nebula nuclear burning in the central star ceases
When a White Dwarf cools down enough, what does it form?
It forms a huge crystal - a Degenerate Diamond
- This is the fate of the sun
How big can a White Dwarf be?
- White Dwarfs w/ the same mass as the sun are about the same size as the Earth
- Higher mass White Dwarfs are smaller
What are Planetary Nebulae?
- Gaseous shells surrounding the remnant carbon cores
- It is called a Planetary Nebula because the first ones discovered looked like the greenish-blue disk of a planet such as Uranus or Neptune
- However it has nothing to do with a planet
- It is composed on ionizing gases expelled by a dying star
What are Planetary Nebulae?
- Gaseous shells surrounding the remnant carbon cores
- It is called a Planetary Nebula because the first ones discovered looked like the greenish-blue disk of a planet such as Uranus or Neptune
- However it has nothing to do with a planet
- It is composed on ionizing gases expelled by a dying star
PLANETARY NEBULA: Look at slides in book 2 page 7
- The fuel in the star’s core becomes totally depleted
- The star begins to swell and shrink, throwing off its outer layers
- Gas is puffed out into space and surrounds the star in a shell, forming a planetary nebulae
- Luminosity stays about the same
- The very hot core becomes more visible as more and more layers are shed
- The temp. of the star increases dramatically
How does a star like our sun die?
It dies by shedding off its layers, creating a Planetary Nebula. It leaves only a White Dwarf behind
Sun-like Star Summary (see book 2 page 8)
1) M-S: Hydrogen fuses to Helium in core
2) Red Giant: Hydrogen fuses to Helium in shell around Helium core
3) Helium Core Burning: Helium fuses to Carbon in core while Hydrogen fuses to Helium in shell
4) Planetary Nebula leaves White Dwarf behind
What is the White Dwarf limit?
- A scientist (Chandrasekhar) calculated the mass-radius dependance in White Dwarfs
- Mass increases - Radius decreases
- Theoretical model: mass of 1.4 solar M, or larger, has radius 0 (zero) - nothing can stop collapse
What are Accretion Disks?
- Mass falling towards a White Dwarf from its close binary companion has some angular momentum
- The matter therefore orbits the White Dwarf in an Accretion Disk
What is a Nova?
- The temp. of accreted matter eventually becomes hot enough for hydrogen fusion
- Fusion begins suddenly and explosively, causing a NOVA
- The Nova star system temporarily appears much brighter
- The explosion drives accreted matter out into space
What happens during a Nova?
- Temperature and density high - nuclear explosion triggered - White Dwarf’s surface layer ejected intense brightening - NOVA meaning “new” (i.e. the star becomes visible across interstellar distances)
- Maximum brightness - few days - after several months returns to the pre-outburst level
- Mass transfer and accretion resumes
- Outbursts happen as long as the companion star is able to furnish fresh hydrogen-rich matter
After a Nova, the White Dwarf becomes?
- more massive than the sun
- consists predominantly of carbon and oxygen
- accretes matter from its companion relatively rapidly
- Nova outbursts relatively weak and eject only little matter
- Consequently, the White Dwarf grows in mass
Supernovae of Type Ia (Thermonuclear supernova) SEE SLIDE book 2 first slide of page 11
- Accretion has raised the White Dwarf’s mass to the critical mass of about 1.4 solar masses
- The density and temps. in the stars center become so severe that Carbon starts burning explosively
- Within roughly 1 second, the burning front moves all the way to the surface, making the entire White Dwarf one huge nuclear fireball
- The entire star explodes and destroys itself
- There is no stellar remnant
What do Supernovae mostly consist of?
They consist almost entirely of the heavier elements. There is no, or almost no, hydrogen
What are the characteristics of Supernovae Type Ia
- Intrinsically very bright - can be seen to great distances
- All have nearly identical intrinsic brightness: standard candle
- With their known intrinsic brightness allows astronomers to determine their distances
- Examining the wavelength distribution of the light from the supernovae tells how fast they are receding from us
- Distances and recession speeds give information about the expansion of the universe
