AS102 Exam 3

Chapter 18

White dwarf

• remaining cores of dead low-mass stars (Fe)

• not undergoing fusion

• very hot, but cools after time

• not very luminous

• small radius

• electron degeneracy keeps them from collapsing in

• higher-mass white dwarfs are smaller

White dwarf limit

• when a white dwarf’s mass is 1.4 Msun, the electrons are moving at nearly the speed of light

• The white dwarf mass limit is 1.4 M sun because nothing can move faster than the speed of light

White dwarfs in close binary systems

• higher mass star transfers mass to the lower mass star

• the star losing mass becomes a white dwarf

  accretion disk

• mass going to the nearby star has the same angular momentum

• matter orbits the white dwarf in an accretion disk

• disk heats and glows due to friction among the matter

  nova

• the new matter becomes hot enough for fusion and stars very suddenly

• this makes the star brighter temporarily

• excess accreted matter is shot into space in the explosion

• not as luminous as a supernova

• white dwarf is still intact

massive supernova

• the iron core of a massive star reaches the white dwarf limit and becomes a neutron star

• causes huge explosion

• can be seen on a light curve

white dwarf supernova

• carbon fusion begins suddenly

• in close binary systems when the white dwarf reaches the limit

• nothing is left after this explosion

• can be seen on a light curve

• white dwarfs don’t have H absorption lines

Neutron star

• ball of neutrons left from a massive star supernova

• degeneracy pressure of neutrons goes against gravity

• electron degeneracy pressure goes away because electrons combine with protons to make neutrons and neutrinos

• neutrons collapse to the center to make the neutron star

• discovered through seeing pulses of radio emission, coming from a pulsar (spinning neutron star)

• in a close binary system, an accretion disk is formed which increases angular momentum and increases its spin speed

• this gets hot enough that helium fusion starts suddenly and produces X-ray bursts, making it an X-ray binary

pulsar

• pulsars are neutron stars that beam radiation along a magnetic axis that is not aligned with the rotation axis, sweep through space like a lighthouse

• pulsars are always neutron stars

• pulsars spin fast because a stellar core’s spin speeds up as it collapses into a neutron star

• conservation of angular momentum

black hole

• gravity is so strong that not even light can escape

• escape velocity means that the kinetic energy must be equal to the work done against gravity

• the surface of a black hole is the radius at which the escape velocity equals the speed of light, known as the event horizon, or schwartzchild radius

• the mass strongly warps space and time in the vicinity of its event horizon

• gravity crushes all matter into a single point known as a singularity

• gravitational redshift is created because it takes light an extra long time to climb out of the hole

• time passes more slowly near the event horizon

• spaghettification caused by strong tidal forces

• some X-ray binaries contain black holes

neutron star limit

• neutrons in the same place cannot be in the same state

• neutron degeneracy pressure cant support a neutron star against gravity if its mass exceeds 3 Msun

• some massive star supernovae can create black holes if enough mass falls on the core

gamma-ray bursts

• coming from distant galaxies, so further back in time

• most powerful explosion in the universe

• could be the formation of a black hole

• some are produced by supernova explosions

• can be produced by collisions between neutron stars (merging)

neutron stars merging

• two stars orbiting close by will have strong gravitational waves

• the stars spiral together

• the energy release is greater than a massive star supernova

• a rare blend of elements is produced

black holes merging

• produce strong gravitational waves

• may be detected in the near future

the event that marks the end of a star’s evolutionary life before becoming a white dwarf is a planetary nebula

when a white dwarf accretes enough matter to reach the white dwarf limit it explodes

the gas in an accretion disk would orbit indefinitely if there were no friction

there could be neutron stars that appear as pulsars to other civilizations but not to us

according to the conservation of angular momentum, the neutron star’s rotation would slow down if a star orbiting in a direction opposite the neutron star’s rotation fell onto a neutron star

the escape velocity from an object increases if it is shrunk

the radius of the event horizon increases when you add mass to the black hole

if the sun were suddenly replaced by a solar-mass black hole, the earth would remain in the same orbit

Chapter 19

Milky way galaxy

• gas clouds made of interstellar medium obscure the view because they absorb visible light

• primary features

  • disk

  • bulge

  • halo

  • globular clusters

• we see the galaxy from its side

• stars in the disk orbit in the same direction with a slight up-and-down motion

• stars in the bulge and halo have random orbits and orientations

galactic recycling

• star-gas-star cycle

  • atomic hydrogen clouds

  • molecular clouds

  • star formation

  • nuclear fusion in stars

  • returning gas

  • hot bubbles

• high mass stars have strong stellar winds that blow bubbles of hot gas

• lower-mass stars return gas to interstellar space through stellar winds and planetary nebulae

• X-rays from hot gas in supernova remnants reveal newly made heavy elements

• a supernova remnant cools and begins to emit visible light as it expands

• new elements made by a supernova mix into the ISM

• radio emission in supernova remnants is from particles accelerated to nearly light-speed

• cosmic rays probably come from supernova

• multiple supernovae create huge hot bubbles that can blow out of the disk

• gas cooling in the halo can rain back down on the disk

• atomic hydrogen gas forms as hot gas cools, allowing electrons and protons to join

• molecular clouds form next after the gas cools enough to allow atoms to combine into molecules, which then form stars

ionization nebulae

• found around short-lived high-mass stars

• active star formation

reflection nebulae

• scatters the light from stars

spiral arms are waves of star formation

1. gas clouds get squeezed as they move into spiral arms

2. squeezing of clouds triggers star formation

3. Young stars flow out of spiral arms

• created by spiral-density waves

halo stars are only old stars

disk stars are stars of all ages

how did our galaxy form

• cloud of intergalactic gas

• halo stars formed first as gravity contracted the gas

• remaining gas settled into a spinning disk

• stars continuously form in the disk as the galaxy grows older

• halo stars formed in clumps and merged later

there is a black hole at the center of our galaxy, with a mass near 4 million solar masses

the position of the sun in the Milky Way galaxy is best described as in the disk, slightly more than halfway out from the center

orbits of disk stars bob up and down because the gravity of disk stars pulls them toward the disk

the mass of our galaxy is best found by measuring the rotation of the galaxy

the oldest stars in the galaxy are usually low in heavy elements because they were formed before much chemical enrichment had taken place

the gas will be locked into white dwarfs and low-mass stars in 1 trillion years

gravitational scattering off molecular clouds changes the orbits of stars as they get older

the M star population is most likely to be part of the spheroidal population

since disk stars have higher metallicity, gas ejected from the halo stars enriched the material now in the disk stars

Chapter 20

a galaxy’s age, distance, and age of the universe are closely related

• spiral galaxy

• elliptical galaxy

• irregular galaxy

disk

• stars of all ages

• many gas clouds

spheroidal

• bulge

• halo

• old stars

• few gas clouds

barred spiral galaxy

lenticular galaxy

• disk like a spiral galaxy but much less ISM

spiral galaxies are often found in groups of up to a few dozen

elliptical galaxies are much more common in huge clusters of galaxies of hundreds to thousands

Measure the distances to galaxies

1. determine the size of the solar system using radar

2. determine the distances of stars out to a few hundred light years using parallax

3. the apparent brightness of a star cluster’s main sequence tells us its distance

4. because the period of cepheid variables stars (very luminous) tells us their luminosities, we can use them as standard candles

5. the apparent brightness of a white dwarf supernova tells us the distance to its galaxy is up to 10 billion light years

redshift of a galaxy tells us its distance through Hubble’s Law

distances of the furthest galaxies are measured from their redshifts

the expansion rate appears to be the same everywhere in space

the universe has no center or edge

the universe looks about the same no matter where you are within it

• matter is evenly distributed on very large scales in the universe

• it has no center or edges

• the cosmological principle has not been proven but it’s consistent

lookback time

expansion stretches photon wavelengths, causing a cosmological redshift directly related to lookback time

cosmological horizon

• marks the limits of the observable universe

• horizon in time rather than space

• the limit is the beginning of the universe