Y12 Star Flashcards
Energy processes / transport within a star
For a star of 1Msun or less
From centre of the star to the outside:
Energy generation via nuclear fusion
Energy transport via radiation
via convection (circular “currents”)
Temp, density and pressure decreases from the centre.
What happens at the Core of a star
Nuclear fusion occurs at enormous temperatures
‘Shells’ of fusions of different layers (C-O core)
Gets hotter and denser over the life of the star
What is the Outer layer of a star made of and how does heat reach it?
Made of H
Heat from core to surface by convection currents
What does a star’s type depend on?
SIZE and AGE
Describe main sequence star
Fusing hydrogen to helium in the core
Includes red dwarves - 0.08-0.5Msun, fuses slowly
Red giant
He to C or higher
0.5-8 Msun
Super giants fusion and mass
He to C/O at first, then C to heavier elements later.
8-100 Msun
Why does a star stay the same size? Explain
Hydrostatic equilibrium
Gravity pulls material inwards
Thermal pressure of gas (kinetic energy of colliding particles) pushes outwards
Units:
Msun
AU
ly
Mass of the sun
Astronomical unit - distance between sun and earth
Light year and
Define Luminosity
TOTAL amount of energy emitted per sec compared to the sun
Lsun
Define absolute and apparent brightness
Absolute - how bright stars would be if compared directly - luminosity
Apparent - brightness of stars relative to earth
Depends on luminosity and distance from earth.
(Radiated light spreads, double distance means intensity drops by a quarter)
What are the spectral classes and what are they based on?
Hottest to coolest (blue to white to yellow to red):
O B A F G K M
‘Oh Boy An F Grade Kills Me’
Based on surface temperature
What are absorption lines and emission spectrum
Dark lines showing wavelengths of light absorbed by the elements in a star
Emission spectrum is the opposite - mostly dark with a few bands of light
What is a H-R diagram
Hertzsprung-Russel diagram
Compares Temperature (x-axis) and Luminosity (y-axis)
Temperature also shows corresponding colour, from OBAFGKM
Main features on a H-R diagram
Main sequence stars form a diagonal band, blue supergiants (class O) are hot and bright, down to red dwarfs (class M) which are cool and dim
Supergiants have higher luminosity (cluster in top right) because of high surface area and temp
White dwarves are under the main sequence band, bottom right corner
Describe the general life cycle of a star
Stella nebula
Average size star
Red giant
Planetary nebula
White dwarf
OR
Stella nebula
Massive star
Red supergiant
Supernova
Neutron star OR black hole
What is a Stella nebula
Giant Molecular Clouds (GMC) of dust and gas (mainly H2)
10s to 100s of ly across
How does a star form from a stellar nebula
An area of the cloud becomes denser (due to disturbances)
Gravitational collapse:
Collisions cause the gas to spin
Collapses and spins faster due to cons of angular momentum
Cloud spins and flattens into a disk (protoplanetary disk)
Protostar - centre emits infrared, surrounded by a protoplanetary disk
Increasing mass —> increasing density —> increasing friction —> increasing temp
Nuclear fusion of H to He begins in the core, pre-main sequence star surrounded by a planetary debris disk (which forms planetary systems)
How is mass of a star related to lifetime
Larger mass = greater core temp, burn H faster, shorter lives
Smaller main sequence stars = cooler core temp, burn H more slowly, longer lives
How is mass of a star related to life cycle
Sun like star (up to 1.5 Msun):
Red giant
Planetary nebula
White dwarf
Black dwarf
Huge star (1.5-3 Msun):
Red supergiant
Supernova
Neutron star
Giant star (3 Msun+):
Red supergiant
Supernova
Black hole
Describe main sequence to red giant star and supergiant star
H begins to run out
Thermal pressure drops
Core collapses due to gravity
Friction causes temp increase
100mill K, He fuses to C
(A shell of H fusion around core)
Outer gas layer expands, forming red giant
Huge surface area, cooler surface temp
Supergiant:
C and O formed are denser, so gravity pulls them in to create a new core
Gas layer expands due to thermal pressure, red supergiant 100x bigger than main sequence
Red giant after He begins to run out
He begins to run out
Core collapses
Temp rises
600mill K, C fusion begins, outer layer expands again
Shells of fusion reactions around the core
Fe in the centre, then Si etc to C, He, H on the outside (like layers of an onion)
Death of a star mass less than 0.1Msun
Not enough gravity, no nuclear fusion
Does not become a main sequence star
Glows due to friction
Failed star = brown dwarf
Death of star mass 0.1-0.5Msun
Red dwarf
Gas layer drifts away
Core cools, thermal pressure decreases so it shrinks
Cooling white dwarf
Black dwarf
Death of star 1-8Msun
He runs out, C and O core collapses
But NOT enough mass for C fusion
He shell causes Helium Flash
Outer gas layer blown away by stellar winds
Ionised gas emits light = planetary nebula
Remaining core becomes white dwarf
Black dwarf
White to black dwarf
White dwarf = small, dense, very hot surface temp
Shrink until ELECTRON DEGENERACY PRESSURE outwards stops gravity from pulling it inwards
Cools to become black dwarf
Supergiant to supernova
Supergiant forms
Fe core and layers
Critical mass reached, gravity forces so strong that iron core collapses, overcoming the electron degeneracy pressure
Rigid high density core formed
Falling material bounces off core
Shockwave, ejecting material = supernova
After supernova (1.4-3Msun)
Iron atoms collapse due to gravity
Protons and electrons join to form neutrons
Neutron star formed - extremely dense
Pulsar = fast spinning neutron star, light emitted from their poles
After supernova (3Msun+)
Gravitational force so great that neutrons are crushed to a ‘singularity’
Extremely dense, gravitational pull so strong that light cannot escape = black hole
Compare and contrast dwarf planets and planets
Similar: orbit the sun, spherical in shape
Differences:
Irregular pathway around the sun
Their pathway is not cleared out (asteroids and comets pass through)
Support the theory that planets of the solar system formed at the same time as the sun
Orbit the sun in the same direction (if captured by suns gravity they would be random directions)
Orbit in the same orbital plane - the ECLIPTIC
The ecliptic is perpendicular to sun’s axis of rotation
How are planets formed
Protoplanetary disc around protostar is vaporised when nuclear fusion begins
Remaining material blown out, cools and continues to orbit
Electrostatic attraction causes material to clump together
Gets bigger, gravity takes over = planetismals
Gravity and collisions cause them to sweep up more material and clear out pathways
Regions of the disk
Temperature gradient around the star
Inner: all components present in gaseous form
Rock/metal condensation line (0.3AU)
Heavier elements/compounds cool and condense
Intermediate: rocks and metals present and frozen flakes, H and He compounds present as gases
Frost line (3.5AU)
Lighter elements cool and condense
Outer region: rocks, metals and H compounds present as frozen flakes
H and He present as based
Describe the general make up of the solar system
Sun - 99.9% of mass
Mostly empty space
4 rocky inner planets
Asteroid belt
4 large gas planets
Kuiper Belt
Formation of four inner planets
Accretion off material into planetesimals
Planetesimals drawn in by gravity, collide and stick together
Increase size —> increase pressure and temp —> molten core
Gravity pulls planetesimal into sphere
Late heavy bombardment period - GPE of falling planetesimals converted to KE and heat, molten surface
Why are inner planets smaller
Less material available as lighter elements blown further away, only a small amount of heavy element material in GMC
Smaller orbits path, less change of gathering material
Formation of outer planets
Icy particles accrete, held together by gravity
Lots of icy planetesimals collide to form larger outer planets
Large = strong gravitational attraction, pull in huge volumes of H and He gas
Outer planets make up
Core of rock, metal, H compounds
Metallic H layer
Liquid H layer
Gaseous H layer
Visible clouds
Theory of Formation of our moon and evidence
Theory: moon too big to be a captured asteroid of formed by accretion
Mars sized body collided, causing material to be blasted into orbit around the earth (circumplanetary disc)
Material accreted over time
Evidence:
Similar isotopes of moon and earth rocks
Links of earth’s spin to moon’s orbit
Moon’s small core of iron
Three ways a moon could be formed
- Circumplanetary disc
Material pulled into orbit around planet to form orbiting disc of material, accreted over time to form moon - Collision
Large planetesimal collides, huge volume of material scattered into orbit - Capture
Asteroids travelling past pulled in by gravity
Number of moons around a planet depends on…
Size of planet - bigger planet can gather more material
Distance from frost-line
Closer to frost line, more material solidified
Explain regular and irregular moons
Regular:
Regular orbit (almost circular, planet near the centre)
Inclination - orbits in the same plane as the planet orbits the star
Prograde - orbits in same direction as planet spins
Irregular:
Orbit - egg shaped path, planet off centre
Inclination - orbits at an angle to orbital plane
Retrograde - orbits in opposite direction to planet spins
formation of asteroid belt solar system
between Mars / Jupiter
Jupiter’s gravity affected the planetismals that were beginning to form, so they became asteroids.
highly irregular in shape (except Ceres) because they are not large enough to become spherical
What is the Kuiper belt and how did it form?
a disk-shaped region past Neptune’s orbit, containing many small icy bodies
about 70,000 Kuiper belt Objects (KBOs) with diameter larger than 100km
what is the Oort cloud
extended region outside the Kuiper belt, contains icy objects that are remnants from the formation of the solar system
what are comets and where are they from, why do they have tails
objects from the Kuiper belt and Oort Cloud
made of ice and rock, travel around the sun
As comet nears the sun, solar radiation heats nucleus and ice sublimes
tail of gas / dust forms
