Stellar Evolution Flashcards
What is ISM?
Interstellar Medium - gas and dust between stars.
Differentiate between ISM dust and gas and effect on light.
ISM gas does not significantly block light but it does scatter it.
ISM dust blocks it
What is a nebula?
An interstellar cloud.
What is dark nebula?
Made up of dust grains blocking any visible light coming from stars that lie behind it.
What is reflection nebula?
Haze around stars caused by fine grains of dust in a lower concentration than in dark nebula. Light from star is scattered but not blocked.
Why do reflection nebula have blue color?
Scatter blue light more efficiently than red.
What type of nebula appears red in colour?
Emission nebula
What is Interstellar reddening and extinction?
Extinction is the reduction in the intensity of light.
Reddening is scattering of bluer component.
What must happen for a star to form?
Interstellar material needs to condense to form a star so the force of gravity must overwhelm the internal pressure pushing the material apart.
What are Barnard objects? Give another name.
Extremely dense areas compared to ISM. Are dark nebulae where most star formations happen.
Bok Globules.
T or F. A dark nebula is dark at all wavelengths.
False.
T or F. A protostar shrinks to enter main sequence, due to thermal emission being the main energy source.
True.
Distinguish between layers of different stars depending on size.
Less than 0.4 M Consists of core, convective.
0.4 M to 4 M Consists of core, radiative and convective.
More than 4M Consists of core, convective and radiative.
What is a brown dwarf? What is the mass limited?
Failed star never reaches main sequence, can’t do fusion.
0.08 solar masses.
What is the upper limit for the mass of a successfully formed star?
200 solar masses.
HII regions are prominently red and are associated with what type of star?
Blue giant stars.
Is it possible to distinguish between a source redshifted and a source reddened by dust?
Yes spectroscopically the effects are different although visually the same.
T or F. The greater the mass of the protostar the faster it gets to the main sequence?
True.
What is stellar evolution?
Stellar evolution is a delicate dance between internal pressure and inward force of gravity.
When does a protostar become a main sequence star?
When hydrogen fusion begins.
Relationship between time on main sequence and mass.
1 / M^2.5
What are red dwarfs?
Stars with masses between 0.8 M and 0.4 M.
Explain the steps when core collapses.
More internal pressure
Outer layers expand
More Luminosity and less surface temperature
Explain the steps when core expands.
Less internal pressure
Outer Layers Contract
Less luminosity and more surface temperature
Explain what happens to star on main sequence.
Hydrogen fusion at core
Decreased number of particles in core
Less internal pressure
Core contracts
Increase density and temperature in core
Increase internal pressure
Expansion of outer layers
Increase luminosity
Decrease surface temperature
Repeat until Hydrogen at core is depleted
T or F. Once a star leaves the main sequence no hydrogen fusion occurs.
False.
There is still hydrogen fusing shell around core.
Explain red giant phase.
No core fusion
Core cools down and compress
Increase core temperature
Gravitational energy converted into thermal
Heat from the core expands to outer layers
Hydrogen fusion just outside the core
Hydrogen rich region expands
Helium generated falls into contracting core.
Core gains mass and temperature.
Luminosity increases
Increase of internal pressure
Outer layers expand dramatically
Surface temperature falls down to 3500K
Mass loss
T or F. In really large stars helium fusion begins dramatically.
False
Only in smaller stars is there a helium flash
What does AGB stand for?
Asymptotic Giant Branch
T or F. Stars shrink in the AGB phase.
False.
They grow in size dramatically
Differentiate between core of AGB stars for < 4M solar masses and 4M +.
<4M. Carbon-oxygen core
4M +. Iron core, big enough to fuse with carbon, oxygen.
T or F. If the outer layers of a star expand the surface temperature decreases.
True.
Why do stars become redder when leaving the main sequence?
Because they become bigger and hence cooler.
What is the primary source of energy for the star during the early giant phase?
Hydrogen fusion outside the core.
List order of possible fusions for a star.
Hydrogen
Helium
Carbon
Neon
Oxygen
Silicon
Iron
T or F. The core of AGB star of a massive star is smaller with respect to the core of AGB of less massive stars.
True.
What are white dwarfs?
The remnants of stars between 0.4-4M. No nuclear reactions at the core, the star cools down as it ejects material. Extremely dense core, initial high temperatures.
What are white dwarfs?
The remnants of stars between 0.4 and 4 solar masses.
T or F. White dwarfs have no nuclear reactions at the core.
True.
What is the mass-radius relation for a white dwarf?
The more massive a white dwarf is the smaller it is.
What is the Chandrasekhar limit?
The maximum mass of a white dwarf is 1.4 solar masses.
Why do white dwarfs solidify?
They cool down until they become solid. I.e. 0K
What are novae?
Recurrent events. A white dwarf in a binary system where accretion is happening, nuclear reactions will happen in the accretion disk but NOT THE CORE. But white dwarf survives.
What are planetary nebulae?
Stars around the size of the sun, suffer burst in luminosity that ejects shell of material into space. After several bursts all that remains is the core surrounded by glowing shells of ejected gas.
How long does it take a planetary nebule to reach full size?
10,000 years
Why are planetary nebulae greater than 50,000 years old not observed?
The shell has spread out so far that the gas does not glow.
Explain supernova type 1a.
A white dwarf in a binary system sucks in gas from its companion star. A runaway nuclear reaction ignites in the core of the white dwarf. Carbon and oxygen are converted to radioactive nickel. Within a few seconds the dwarf is destroyed.
Explain types 1b/c and 2 generally.
As a massive star nears its end, iron is produced which doesn’t undergo nuclear fusion. So the core becomes unable to generate heat, leading to a sudden collapse to nuclear density. A flood of neutrinos is released propelling the shock wave outward blowing it apart.
How far can type 1a supernovas be used to measure disance?
10^9 ly
Distinguish between type 1b, 1c and 2 supernovae.
1b - outer layer of hydrogen removed
1c - outer layer of hydrogen and helium removed.
2 - outer layers largely intact
T or F. We are made of star-stuff.
True without supernovae heavier elements could never have formed.
How big does a star have to be to from a black hole or neutron star upon its death?
Black hole - 25 M +
Neuron Star - 8M +
After hydrogen fusion stops in a massive star’s core, the core does what?
Collapses until helium fusion begins within it.
Is it possible to be warned that a massive star is about to explode?
Yes via neutrinos.
T or F. As a white dwarf cools down their size remains almost the same.
True.
T or F. The bigger the white dwarf the less massive they are.
True.
What is the most massive white dwarf known?
1.4 solar masses
What is the T Tauri phase of a protostar?
Strong stellar winds develop as star clears the area to the snow line of gas and dust.
What force keeps a main sequence star from collapsing on itself?
Radiation pressure.
T or F. A supernova can form a planetary nebula.
False.
T or F. A star the size of the sun will form a white dwarf immediately upon death.
False.
First forms a planetary nebula.
Explain the following.
- eclipsing binary
- astrometric binary
- mass transfer binary
- detached binary
- spectroscopic binary
Eclipsing binary involves two stars where one passes in front of the other. Variations in brightness
Astrometric binary involves two stars where the companions presence is deduced from its gravitational influence. We can’t directly observe the companion.
Mass Transfer binary involves a giant star transferring mass to a white dwarf, neutron star or black hole.
Detached Binary involves two stars far enough apart that they do not interact with one another.
Spectroscopic Binary involves two stars where the stars can be resolved individually using telescopes.
The core of a highly evolved high mass star is a little larger than:
Question 39 options:
our sun
Earth
our solar system
Jupiter
a neutron star
Earth
At what critical mass can a white dwarf become a type 1a supernova.
1.4 solar masses, the Chandrasekhar Limit.
Give the relationship between mass and main sequence life span.
mass is inversely proportional to main sequence life span * 100.
I.e. 10 solar masses has 1 / 1000th of Sun lifetime
Emission nebulae like Orion’s nebula (M42) or the region around the Horsehead nebula, occur only near stars that emit large amounts of what radiation.
Ultraviolet radiation.
The gas density in an emission nebula is typically about how many particles per cc?
Hundred
What two things are needed to create an emission nebulae?
hot stars and interstellar gas, particularly hydrogen
A typical protostar may be several thousand times more luminous than the Sun. What is the source of this energy?
gravitational energy as the protostar continues to shrink
What is characteristic of a main sequence star?
Question 59 options:
It has a mass less than the Sunʹs.
The rate of nuclear energy generated in the hydrogen to helium fusing core equals the rate radiated from the surface.
It has rapid rotation and a strong stellar wind.
Nuclear fusion in the core varies due to the amount of gravitational contraction that occurs and which heavy elements are produced.
All of the answers are correct.
The rate of nuclear energy generated in the hydrogen to helium fusing core equals the rate radiated from the surface.
Why are neutrinos from a type II supernova detected before photons?
Neutrinos escape from the star quickly because they hardly interact with matter; photons are delayed by interactions with matter.
Why can the lowest-mass stars not become giants?
They never use up all their hydrogen.
What supernova produces the most initial luminosity?
A type 1a supernova.
What effect do even thin clouds of dust have on light passing through them?
It dims and reddens the light of all more distant stars.
T or F. There is a mass-luminosity relation because all stars on the main sequence have about the same radius.
True.
After the core of a Sun-like star starts to fuse helium on the horizontal branch, the core becomes (a) larger, (b) cooler or (c) hotter
(c)
