ast201 term test 2

measuring surface temperature

• measure blackbody spectrum and use Wien’s law

measuring distance

• measure parallax

measuring luminosity

• measure apparent brightness and combine with distance

measuring chemical composition

• measure the star’s spectrum

  • spectral lines

balmer series

• series of absorption and emission lines beginning and ending in the n=2 level of hydrogen

• electrons must be in n=2 shell, ready to jump up in order for lines to show

  • heating up hydrogen promotes electrons to this shell

main sequence stars

• dwarf stars

  • not all small

• still fusing hydrogen in their cores

  • alive

  • share the same energy source

• come from other parts of the HR diagram, settle there, stay for a large fraction of their lives, then evolve away

• cannot travel up or down the sequence

stefan-boltzman equation

• amount of light emitted per square meter of surface

• L = SA * constant

• for blackbody radiation

red dwarfs

• M class main sequence stars

• actually small

• low mass, long lifespan

yellow dwarfs

• G class main sequence stars

• like the sun

blue dwarfs

• O class main sequence stars

• large

• high mass, short lifespan

white dwarfs

• actually small

  • about the size of earth

• below main sequence

• not fusing hydrogen in core

  • dead

star lifetimes

• lifetime = mass/luminosity

• mass: mass in core

• luminosity: burn rate, amount of energy being released per second

• large mass stars also release more energy

  • shorter lifespans

open clusters

• don’t have defined boundaries

• associated with clouds of gas

globular clusters

• globe-like, round balls of stars

• have more stars than open clusters

• ancient

main sequence turnoff

• determines the age of a cluster

• equal to the lifespan of the shortest-lived star still on the main sequence

  • most massive and luminous star

HII regions

• clouds of hot, ionized hydrogen gas

• pink areas in a galaxy (under visible light)

• singly ionized hydrogen: missing its only electron, proton only (H+)

molecular clouds

• clouds of molecular hydrogen (H₂)

• dust made up of tiny solid particles

• dark areas in a galaxy (under visible light)

  • bright areas under infrared light

• very cold: emit little visible light, but some infrared light

• engines of star formation

  • have clusters of blue stars in their cores that indicate origin of stars

infrared light

• passes through dust more easily than visible light

• can show stars forming deeply embedded in dusty gas clouds

molecular cloud core

• spherical ball of gas and dust

• can only remain stable for a short period of time

  • eventually its own gravity will take over and compress it

molecular cloud fragment collapsing

• conservation of energy: heats up

• gravitational potential energy converted to kinetic energy

  • atoms move faster, high temp

protostars

• formed at the core when atoms of a cloud fall inward and speed up

• not a star

  • not undergoing fusion reactions in core

  • on the way to become pre-main sequence stars

• core needs to reach 10 million K for the proton proton chain to begin

  • before then, mutual electric repulsion prevents collision

protostellar disk

• as molecular cloud core collapses, rotation causes it to flatten into a disk

• surrounds a protostar

• produces jets of material

pre-main sequence star

• fusion reactions in core create enough pressure to counterbalance star’s gravity

  • gravity crushing the star balances with energy being release

• gravitational star collapse slows and stops

• star lands on main sequence and becomes a zero-age main sequence star

  • stays like this for a really long time

hydrostatic equilibrium

• when a star is neither expanding nor contracting

• while a star is on the main sequence

• outward pressure from energy release due to hydrogen fusion exactly balances inward pull of gravity

high mass stars

• starting masses above 8 solar masses

• O and B stars

• do not die quietly

low mass stars

• starting masses below 8 solar masses

• A through M stars

• die quietly

• all become white dwarfs

hydrogen shell burning

• layer of hydrogen around helium core reaches temperature and density needed to start fusing hydrogen

• release of extra energy causes outer layers of star to slowly expand

helium flash

• helium core reaches right temperature for nuclear fusion

• occurs about a billion years after star leaves main sequence

• happens extremely fast (in a few minutes)

horizontal branch

• energy of helium flash expands core, hydrogen shell burning slows

• star shrinks and settles into new equilibrium

• don’t spend much time on this line

asymptotic giant

• star accumulated core of inert carbon and oxygen

  • dead now

• will never get hot enough to fuse carbon or oxygen

• will die permanently

degeneracy pressure

• core of inert helium ash isn’t producing enough new energy

• can’t sustain pressure needed to oppose gravity

• stops further contracting of core

  • prevents core from collapsing past white dwarf stage

  • doesn’t stop collapse of iron core

• you can only force electrons in stars to get so close to one another before they exert strong repulsion

  • electron degenerate matter is very dense, resist crowding

pauli exclusion principle

• each shell in an atom can only hold a fixed number of electrons

• i.e. 1s shell can hold only one spin up and one spin down electron

white dwarf

• exposed C/O core of a low-mass star that has expelled its outer layer

planetary nebula

• expelled outer layers of a dead low-mass star

  • energy release from helium fusing propels outer layers into space

binding energy

• how tightly bound the nucleus of an atom is

• changes from atom to atom

  • so does mass per nuclear particle

• iron has lowest mass per nuclear particle

changes in mass

• separating two objects that are attracted to one another by force requires that you supply energy

  • that energy is added to the mass of the objects

fusion

• if starting with lighter elements

  • hydrogen, helium, etc.

• fusing them together lowers the mass per nuclear particle

  • releases energy

• i.e. the Sun

fission

• if starting with a heavy atom

  • uranium, lead, etc.

• splitting the nucleus lowers the mass per nuclear particle

  • releases energy

• i.e. nuclear power plant

iron core

• developed very quickly

• once developed, star can no longer produce energy

  • nuclear particles have lowest masses in iron nuclei

  • neither fission nor fusion can extract energy from nucleus

• can’t produce pressure needed to resist gravity

  • core begins to collapse

neutron degeneracy pressure

• kicks in after neutronization

• neutrons suddenly refuse to get any closer to one another

• neutrinos rush out

  • star explodes in supernova

supernova

• can outshine a galaxy

• releases a lot of energy

  • as much as the sun will in 10 billion years

type ii supernova

• originate from massive stars

• strong spectral lines of hydrogen

• core-collapse

chandrasekhar limit

• 1.4 solar masses is the maximum mass for white dwarf stars

• beyond that, electron degeneracy pressure can’t support against their own gravity

type ia supernova

• originates from low mass stars

• white dwarf that exceeds Chandrasekhar limit

  • can occur if it’s in a binary system with another star

    • needs to get close enough to gather material

• large portion of white dwarf immediately undergoes fusion reactions

  • explodes

supernova remnants

• shells of expanding gas left over by supernovae

  • some don’t leave any

  • some undergo core collapse afterward and form a black hole

• expand and dissipate over tens of thousands of years

  • rejoin gas and dust of interstellar medium

• elements heavier than hydrogen and helium may get incorporated into new stars and planets

final state of a dead star

• depends on initial mass and sequence of events that occur to it

• more difficult to predict for massive stars

  • lose large but uncertain fractions of their mass due to stellar winds

  • lose mass before becoming a supernova

failed supernova

• stars that collapse directly to form a black hole

  • no supernova

• can happen if outflowing neutrinos fail to drive off outer layers before they fall into collapsing core

neutron star

• supernova remnant between 1.5-3 solar masses

• if the ball of neutrons from type ii supernova doesn’t collapse to form a black hole

• spin much faster than the star they formed from

pulsar

• type of neutron star

• strong magnetic fields channel charged particles form the star into beams

  • sweep around the sky as neutron star rotates

• beams often misaligned with rotation axis

  • if you align with a beam, you detect pulses of radio waves

• rotate very quickly

  • collapsed a lot relative to original state as the core of a massive star

  • millisecond pulsars rotate more than 1000x per second

near-infrared view of milky way

• passes through dust easily

• can see thin, disk-like structure

far-infrared view of milky way

• can see mainly dust, no starlight

• cold dust

galactic disk

• thin, only a few thousand light years

• contains most of the dust and gas in the milky way

  • new star formation

  • younger, bluer stars

• contains open star clusters

• stars orbit in circles with the same orientation

  • except some up and down motion

starburst galaxy

• galaxy that is undergoing an exceptionally rapid burst of star formation

• star formation can drive a super wind that forces hydrogen out the galaxy

• often triggered by gravitational interactions among galaxies

• milky way may have once been one

galactic bulge

• less dust and gas

  • less new star formation

  • older, redder stars

• in most spiral galaxies, contains supermassive black hole

• stars have orbits with random orientations

galactic halo

• has very few stars over all

• contains globular clusters

  • i.e. omega centauri

• stars travel high above and far below the disk

  • on orbits with random orientations

• majority of dark matter

galaxy rotation curve

• orbital speeds of stars, nebulae, etc. inside galaxies

• speed increases as you move away from the centre, then levels off

  • flattens out at large radii

• counters expectation that speed would decrease at large radii

total mass of galaxy

• measured when we fit a galaxy’s rotation curve

  • actually distributed throughout galaxy, not concentrated by the centre

• must be much bigger than the combined masses of all their parts

  • baryonic matter: made of protons and neutrons

  • implies existence of dark matter

dark matter

• must model each galaxy as embedded in a sphere of non-baryonic matter in order to reproduce rotation curves

  • only explanation

• 90% of milky way

• unsure what it is made of, most likely a new form of matter

  • something that doesn’t interact with light but does produce a force of gravity

  • doesn’t emit, reflect, or absorb light

  • some portions may be non-baryonic matter we already know about

    • i.e. neutrinos or black holes

gravitational lensing

• amount of lensing in each case is more than can be explained by the luminous mass of the lensing object

• more proof of dark matter

colliding galaxy clusters

• i.e. bullet cluster

• can use gravitational lensing to find where most of the mass in the cluster is located

  • most of the mass passed through the collision and is still in the clusters

  • did not collide with itself, went through one another

• X ray imaging shows that the huge clouds of hot gas that make up most luminous mass in the pair is located between them

  • hot gas collided with itself via EMF interaction and slowed down

• dark matter is collision less: doesn’t interact with itself, except by gravity

weakly interesting massive particles (WIMPs)

• particles that only interact via gravity and maybe another weak force