Astronomy Test 3
first direct detection of gravity waves
- At a press conference on Feb. 11, 2016 it was announced that the LIGO instrument had directly detected gravity waves. They showed the exact signal expected for two merging black holes of about 30 M! each. It was seen on Sep. 14, 2015, but came from a location 1.3 billion light-years away. The energy equivalent of 3 M! was radiated in gravity waves. - - This was as big a deal as discovering the Higgs boson, maybe bigger: it proved that relatively massive black holes exist and merge, and introduced a new technique for astronomers to study the universe.
How can molecules form on grain surfaces?
- Atoms encounter and stick to grain surfaces •They "migrate" (move) on the surface and encounter other atoms and molecules •A new molecule forms by chemical reaction, releases energy, and is expelled as a free-traveling molecule
basic properties of black holes
- BH's have only three intrinsic physical and measurable properties: - Mass, Charge (electric), Rotation (spin on an axis) - They have no other distinguishing properties
gamma-ray bursts
- Bursts of gamma-rays from space were first detected in the 1960s by the Vela "spy" satellites - In the 1990s it was found that many of these bursts came from very distant galaxies, so they must be very luminous
What is the name of the largest asteroid or a "dwarf planet"? How big is it?
- Ceres - about the size of Texas
formation of the "protoplanetary" disk
- Conservation of angular momentum causes the shrinking disk to spin faster, and centripetal force acts to slow down contraction in the plane. - Gas is a fluid: it flattens into a disk smoothly. - solid objects collide with each other, and this "cancels out" vertical and non-circular motions, forcing the solid objects into flat ("co-planar"), circular orbits.
The four new "superheavy" elements
- Element 113: Nihonium, symbol Nh (RIKEN, Japan) - Element 115: Moscovium, symbol Mc (Dubna, Russia) - Element 117: Tennessine, symbol Ts (Oak Ridge, USA) - Element 118: Oganesson, symbol Og (after a scientist)
accretion disk around a compact object
- Friction heats the disk to high temperatures !it radiates in optical, UV light, or even X-rays, depending on the type of object. - From the accretion disk, matter falls ("accretes") onto the surface of the compact star. This can happen slowly or in sudden bursts or "gulps" of falling material.
formation of terrestrial planets
- In the warm inner disk, particles of rock and metal collided and stuck together by electrical forces; this process of accretion builds up planetesimals. •With help from gravity, the larger bodies mopped up the smaller ones, eventually forming four terrestrial planets.
General Characteristics of the Solar System: Small Bodies in Certain Regions
- Many small, rocky asteroids and icy comets are located in three main regions: -The Asteroid Belt -The Kuiper Belt -The Oort Cloud
"spinning up" a faded pulsar
- Mass being transferred from one star to the other in a binary system carries with it, orbital angular momentum. This gives it sideways moEon. - The matter will likely accumulate in a spinning disk around the other star. When material from the disk falls onto the neutron star's surface, it "yanks" it to spin faster.
a neutron star in an interacting binary
- Matter falling toward a neutron star forms an accretion disk, just as in a white-dwarf binary, but the disk is hotter, emits X-rays - The accreting matter adds angular momentum to a neutron star, increasing its spin rate and producing a "millisecond" pulsar.
How to know if the object is a neutron star or black hole?
- Need to measure the unseen object's mass •Use orbital properties of companion •Solve for the mass, using familiar methods! •Remember the issue of tilt; if you don't know the orbit's inclination, your answer is a lower limit to the companion's mass... •It's a black hole if it's not an ordinary star and its mass exceeds the neutron star limit (~3 M!)
Is the periodic table completed?
- No, there still must exist some very neutron-rich, very unstable isotopes of the super-heavies. And, according to theory, there should be "islands of stability" at even higher atomic numbers, perhaps 118, 120, or 126.
Where is the mass in a black hole?
- Since we are not aware of any mechanism - for example, kind of pressure - that can support an object with more than the neutron degeneracy pressure limit of about 3 M!, the mass must keep falling to the center. •In principle, the mass resides in a region of zero radius, zero volume, which makes the density infinite, because density = (mass)/(volume). •This region, actually just a point, is called the singularity.
binary stars and gravitational energy
- Surfaces of equal gravitational energy in a binary star system. The "Roche lobes" are the surfaces where the effects of the two stars balance. Anything at the "inner Lagrangian point," L1, has equal gravitational potential energy relative to both stars. - The L2 point is a location of stability. It is often used for space missions, where E1 and E2 are the Earth and Moon. The James Webb Telescope will go there.
Bombardment and the molten earth
- The Earth and inner Solar System were pelted with large and small planetesimals, which heated the Earth. (The most drastic of these was when our Moon formed.) As the Earth was heated inside, the rock and metal were liquified. - In liquid, it is possible for heavier material to sink through layers of less dense material, so the Fe and Ni in the proto-Earth settled to the center, creating Earth's iron core. This core is important because it generates a magnetic field that "shields" us from energetic particles coming from the Sun.
X-ray binary SS 433
- The accretion disk around the compact source "shoots" jets of relativistic particles out both poles. - Each jet is alternately red-shifted, blue-shifted, then red-shifted again; period of this wobble is 164 days. - There has long been a controversy over whether the central object is a neutron star or a black hole. Current evidence seems to indicate that it is a black hole.
evolution and fate of the binary pulsar
- The binary is losing energy via gravity waves, causing the period to shorten by 75 microseconds (10-6 sec) per year. This rate of energy loss is exactly what is predicted by general relativity. - Someday the two stars will collide, a truly catastrophic event! Gravita4onal wave emission will intensify towards the end of the "in-spiral." That has now been detected by LIGO! (for a different neutron star pair)
The "structure" of a black hole
- The event horizon is the hypothetical boundary of a region in space from which nothing, not even light, can escape. It's a kind of "one-way" membrane. Things can fall in, but never come out. - The mass is "located" at the central singularity, where the density is infinite.
Energy conversion: rotation to radiation in neutron stars
- The newborn neutron star rotates rapidly and has an intense magnetic field. It is an electro- magnetic dynamo, accelerating charged particles to high speeds & energies, so that they radiate profusely. The the neutron star uses its rotational energy as an energy source that powers its radiation. - As it loses rotational energy, the star rotates more slowly, so the period - interval between pulses - gets longer.
energy conversion: rotation to radiation
- The newborn neutron star rotates rapidly and has an intense magnetic field. It is an electromagnetic dynamo, accelerating charged particles to high speeds & energies, so that they radiate profusely. The (kinetic) rotational energy acts as an energy source that powers this radiation. - As it loses rotational energy, the star rotates more slowly, so the period - the interval between pulses - gets longer.
Pulsar slow downs
- The normal evolution (change) for an isolated pulsar is for it to slow down gradually. The Crab pulsar, for example, is slowing down at a rate of 10-15 seconds per second, which means that it will double in 30 million years. - However, sometimes a pulsar will undergo a "glitch" due to breaking of the neutron star's crust and resettling of its radius, which gets smaller. This causes a small but abrupt speed up of the rotation rate.
relativistic effects of strong gravity fields
- Time passes more slowly at stronger gravity, and distances in the direction of travel are foreshortened. - Light travelling away from a region of strong gravity is redshifted; light traveling into it is blueshifted.
Thermonuclear WDs #2: Supernovae Ia
- Type Ia supernovae occur when a white dwarf blows apart, which provides a fixed amount of energy (the internal gravitational energy of the WD). •The main uncertainty is whether the other star is a low-mass M.S. star, giant, or another white dwarf.
white dwarf with a strong magnetic field
- When the matter gets too close to the white dwarf, the magnetic field directs its flow. - Magnetic fields of typical WDs are about 106 G; for these "polars," the fields can be 60 times stronger. (Still not as strong as in pulsars.)
evolution of interacting binary stars
- When two stars are in a close binary system, they may interfere with each other's evolution; material in the vicinity feels forces from both stars. - One can draw a set of surfaces, each with different gravitational energy; the surface where the two stars have equal influence is called the Roche lobe.
size & structure of neutron stars
- a neutron star is the size of a small city: radius = 10-12 km - it is supported against gravity by neutron degeneracy pressure
2 examples of large carbon molecules in space
- aromatics - aliphatics
Stage 2 of cloud collapse: fragmentation
- clouds can fragment - which leads to new stars being born in the denser fragmented clumps - a typical cloud can break up into tens, hundreds, or even 1000s of fragments
growth by accretion
- dust to pebbles to planetesimals to planets - growth process works by accretion - gentle collisions, or gentle encounters between dust grains, process builds up to larger and larger sized objects through accreting more and more stuff
low mass x-ray binary
- ex. A0620-00 - The phrase "low mass" refers to the normal, companion star, which is a low mass star, often a K or M Main Sequence star. Some of these have black holes with quite large masses.
rapid or "r-process"
- floods Fe nuclei with neutrons - happens in merging neutron stars, maybe also core-collapse Supernovae from high mass stars
- Synthesis of elements in stars: - Hydrogen - Helium
- hydrogen = main sequence (pp chain or CNO cycle) - helium = carbon, oxygen, "triple a" after 1st red giant phase - Stars with ≥ 8 solar masses go on to produce many of the common middle-weight elements (O, Si, S, Fe). These are "alpha"reactions, adding He.
what happens at the beginning of cloud collapse?
- if a cloud is massive and dense enough, its self gravity will be large enough to overcome the internal gas pressure of the cloud - this happens in the cores of molecular clouds or dark dust clouds, where the density is highest and the temperature is very cold - these molecular clouds will collapse under their own weight
What is the other 0.5% of interstellar material in deep space? Where do they form?
- it consists of tiny solid particles called "astrophysical dust" or "grains." • The dust "formed" - condensed out of the gas into solids - mostly in the outer layers of Asymptotic Giant stars.
Nuclear fusion proceeds in a series of... Each reaction proceeds...
- nested shells, like an "onion" - faster than the previous one
slow or "s-process"
- neutrons captured one at a time - inside AGB stars - a life stage of lower mass stars
What did we learn from the 1987 supernova?
- only 170,000 light years away - burst of neutrinos seen, a neutron star was born
According to quantum theory, on small scales in brief times, pairs of what are doing what?
- pairs of particles and antiparticles are constantly being produced and annihilated (recall this in the early universe). - If this happens near a BH, it is possible that one of the particles falls in and the other escapes, because the event horizon has a non-zero, quantum "thickness."
What do tidal forces do?
- stretch things in the radial (inward, downward) direction... - and compress things in the cross-wise direction (since radial lines converge)
"Phases" of the ISM: neutral
- symbol: H I, H0 - temp in K: 100-1000 - location: far away from stars - how seen: radio line at λ = 21 cm
"Phases" of the ISM: Ionized
- symbol: H II, H+ - temp in K: 10,000 - location: near young, hot stars - how seen: optical lines such as Ha
"Phases" of the ISM: molecular
- symbol: H2 - temp in K: 10-100 - location: dense gas in MW plane - how seen: tracers such as CO
"Phases" of the ISM: coronal
- symbol: almost fully ionized - temp in K: 10^5-10^6 - location: diffuse gas above plane - how seen: x-rays
- Elements heavier than U are... - Why are their properties hard to determine?
- synthesized in accelerators on Earth; also made by stars - because they are unstable - "superheavy" elements (SHE's)!
Difference between the "chirp" in a black hole binary merger and a neutron star binary merger?
- the NS merger lasted much longer and was also seen in electromagnetic waves
When a supernova goes off...
- the light goes out in all directions - some may reflect off interstellar clouds "behind" the SN or off to the side
How do solids condense into a cooling disk?
- transition directly from gas (vapor) to solid phase - example on Earth: formation of snowflakes - solar nebula slowly cooled down, so condensation could eventually begin - regions nearest Sun were warmer than those farther away - pattern of condensation was determined by local temperature of the disk at that location
Why are Supernova Ia's bad news for white dwarfs?
- when so much mass accumulates on the white dwarf that its mass exceeds the Chandrasekhar limit of 1.4 M", the WD will explode. - very different phenomenon from a core-collapse supernova. Here, the WD is completely disrupted so the process can't repeat
How do neutron capture reactions work?
1. A neutron is captured 2. If the new nucleus is stable, nothing more happens 3. If the new nucleus is unstable, one of the neutrons changes into a proton plus an electron, converting the nucleus into the next higher chemical element
curvature of space near a black hole
1. a black hole sharply curves the spacetime around it 2. far from the black hole, spacetime is nearly "flat"; close to the black hole, the curvature forms a "well" that is infinitely deep 3. objects that venture too close to the black hole cannot escape from the "well"
Curvature of space near a large mass
1. a massive object curves the spacetime around us 2. far from the object, spacetime is nearly "flat"; close to the object, the curvature forms a "well" 3. in einstein's picture of gravity other objects sense the curvature and are drawn into the "well"
bending light: closing of the "light cone"
1. a supergiant star has relatively weak gravity, so emitted photons travel in essentially straight lines 2. as the star collapses into a neutron star, the surface gravity becomes stronger and photons follow curved paths 3. continued collapse intensifies the surface gravity, and so photons follow paths more sharply curved paths 4. when the star shrinks past a critical size, it becomes a black hole; Photons follow paths that curve back into the black hole so no light escapes
X Ray Binary containing a neutron star
1. the ordinary star has expanded to become a giant or supergiant, filling its Roche lobe: Some of its gas escapes 2. some gas from the ordinary star crosses the inner Lagrangian point and forms an accretion disk around the neutron star 3. The neutron star's magnetic field funnels gas onto the magnetic poles, forming hot spots 4. As the neutron star rotates, beams of X rays from the hot spots sweep around the sky
massive-star or "core collapse" supernova
A high-mass star reaches the end of its life and forms an iron core. This core collapses, the layers above it "bounce off" and explode into a supernova remnant. This is a Type II supernova.
elements made by the s- and r- processes
About half the nuclei heavier than Fe in the Solar System came from AGB stars, and the other half from binary neutron star mergers.
General Characteristics of the Solar System: Regular Patterns of Motion
All of the large bodies in the solar system orbit in the same direction and nearly the same plane, in nearly circular (low-eccentricity) orbits.
growth of planets in a protostellar disk
As "rolling stones" gather moss, the planetesimals sweep up material and clear out "gaps" in the disk
Runaway Mass Transfer
As a star fills its Roche lobe, it transfers mass to its companion. But because the donor star has lost mass, its Roche lobe shrinks. Therefore the star, which is becoming a red giant branch, loses mass more easily. The process "runs away."
the "chirp" of a black hole binary merger heard around the world
As the two black holes got closer to each other, they orbited faster - so the period got shorter. This is the equivalent of increasing frequency. The increase in frequency is often translated as a rising pitch in sound. The amplitude grew larger due to more energy in gravitational waves.
Molecular Clouds: cold, dense, dark
At optical wavelengths, molecular clouds do not emit, but the dust in them creates "shadows" against the background.
What are some common molecules in space?
CO, H2O, OH
outside the frost line
Cold enough for ices to form. This means there is a larger reservoir of matter to make the outer planets.
What is the concept of "escape velocity"?
Depending on its velocity at launch, the cannonball will either fall to Earth, end up in orbit, or escape entirely from Earth's gravity. For the last to happen, the actual velocity must exceed the escape velocity.
Explain the creation of matter from energy
Einstein's equation is a two-way street: energy can be converted to matter and anti-matter, in equal amounts. This is particle pair production. It happened in the early universe; recall the "particle zoo."
General Characteristics of the Solar System: Some Oddities
Exceptions to general patterns need to be explained: the Earth's "oversized" Moon, the odd tilt of Uranus, why Neptune has more mass than Uranus, ...
Neutron captures start with
Fe nuclei
rotating black holes
For a rotating black hole, the singularity becomes a ring instead of a point, and there is an outer, flattened boundary around the usual spherical event horizon. Between the two is the ergosphere or ergoregion.
What are some consequences of bombardment on the earth?
Heating of the Earth due to its being bombarded from the outside, helped out by radioactive decays in the interior caused it to melt, enabling Fe and Ni to sink to the center. The still partially-molten Fe-Ni core of the Earth generates a magnetic field that protects us from harmful solar particles.
What is the most common molecule in space?
Hydrogen and then helium
Roche lobes and stellar radii
In the upper panel, both stars are clearly smaller than their Roche lobes. However, if the stars were closer together, the surfaces might start to get dangerously close to their own Roche lobes!
a relativistic effect: gravitational redshift
Light travelling out of a region of strong gravity loses energy. A physical object would slow down, but ....light can't slow down! Instead, the light is shifted to longer wavelengths, which have lower energy.
x-ray binaries with neutron stars or black holes
Mass flows from the companion star arrives near the NS or BH with high angular momentum (from the orbit); collects in a spinning "accretion disk," a holding tank for transferred mass. The disk is hot because of viscous forces (friction), so it emits optical, UV, and X-ray light.
flare-ups in accretion disks
Mass is not always delivered steadily and slowly onto the white dwarf. The accretion disk can become unstable, and dump a large amount of mass all at once, causing a severe flare-up in brightness. Types of such systems include dwarf and recurrent novae, etc., and are know to astronomers as "cataclysmic variables."
Why is it interesting to study the chemical composition in meteorites?
Meteorites can show us the composition of the early Solar System, which may include heating, cooling, crystallization, and/or irradiation. Some bits ("inclusions") predate the SS, are unchanged from when they formed in a cool star or supernova.
The solid phase: dust grains
Mixed with the gas are multitudes of tiny solid particles called "astrophysical dust" or "dust grains." These are heated by stars and motions, and radiate as thermal emitters, producing radiation mostly in the infrared range. At short wavelengths, they block light passing through.
A "post mass transfer" binary system
The "Algol paradox." A binary system composed of a Main Sequence star with a larger mass than its companion, which is a red giant. The star that is now a red giant was originally more massive, but it transferred mass to the formerly lower mass one.
accretion disk around a white dwarf
The accretion disk acts as a holding tank or reservoir of material. Its layers rub against each other (the technical word for this is "viscosity") and heat up, so that they glow in visible and ultraviolet light. The disk slowly feeds mass to the white dwarf, a process called accretion.
Why do molecular clouds absorb and emit radiation?
The dust in them absorbs short-wavelength light (including visible light) but radiates at longer, infrared wavelengths.
Einstein's theory: general relativity
The effect of any mass is to "warp" nearby space, creating a kind of undertow for passing objects. This is often visualized in 2D by an "embedding diagram," which illustrates the phenomenon of a gravitational "well."
High mass x ray binary
The phrase "high mass" refers to the normal, companion star, which is a high mass - O or B type - star; it does not mean that the black hole itself necessarily has a large mass.
Why are short period gamma ray bursts merging neutron stars?
The short-period GRBs may come from mergers of two neutron stars. During the final approach or "in-spiral," strong gravitational waves should be emitted.
What does the strength of gravity depend on?
The strength depends on both the mass of the object and how close you are to its center; the direction is always towards the center.
General Characteristics of the Solar System: Two Classes of Planets
The two classes are named after their largest members: Earth (Terra) and Jupiter (Jove)
The Interstellar Medium (ISM)
There are clouds of gas and dust in interstellar space. The dense ones, called "molecular clouds" because the gas in them is cool and mainly in the form of molecules, are where new stars will form. The nearest place where this is happening in a big away is the Orion Nebula.
What are Kuiper Belt Objects?
These are fairly large (> 1000 km) icy bodies on inclined, elliptical orbits in the Kuiper Belt region beyond Neptune. In retrospect, Pluto was just the first one to be discovered, mostly due to luck and the fact that it happened to be in the plane of the other planets when people were looking there.
Gravitational radiation or waves
They are "quadrupole" waves that squeeze objects in one direction and simultaneously stretch them in the perpendicular direction as they pass. They can be thought of as distortions or ripples in space-time.
What's the big deal about finding pulsars in binary systems?
They enable us to measure masses of actual neutron stars. This might help us learn the maximum mass a neutron star can have.
Mass transfer to a compact object
Things get interesting when the companion is a compact object. The latter object's mass is squeezed inside a small radius, leaving lots of space for the transferred mass to collect in a swirling accretion disk. The compact object may be a white dwarf, neutron star, or black hole.
non-relativistic effects of strong gravity fields
Tidal forces stretch things in the radial direction and compress things in the cross-wise direction .
a relativistic effect: time dilation
Time passes more slowly for a person or object in a stronger gravitational field. This is not a hypothetical statement! It was first measured in the 1970's on intercontinental flights, and is routinely taken into account for satellite communications.
inside the frost line
Too hot for H compounds to form ices
gravitational bending of light
Treating gravity as a field rather than a force explains why the path of light is bent even though photons have no mass: the light ray is bent because it travels through curved space.
How does the "lighthouse" model work?
When the beam is pointed at us, we see a brighter star image or radio "blip" than when it has swept past our line of sight.
Synchrotron
When the neutron core forms, the magnetic field is compressed to extreme strength. Electrons spiral around the magnetic field lines and are accelerated to very high energies. They then radiate a special kind of radiation - not thermal or blackbody-like - which is strong at low frequency, in particular, radio waves.
Why do neutron captures normally start with Fe nuclei?
because (1) they have large probabilities of capturing n's, and (2) because there is usually a lot of pre-existing Fe in the star from previous stellar generations.
meteor
flash of light seen when a small rocky object burns up as it passes thru the atmosphere. Exceptionally bright ones are called fireballs or "bolides."
Why could rotating black holes possibly be efficient energy producers?
if an infalling particle splits into two pieces, one will fall in and the other will fly out at high velocity, gaining energy from the BH's rotational energy reservoir. This may help BHs drive jets along the poles, which may explain certain phenomena including quasars.
Tidal force when approaching a black hole
it's strong when the local strength of gravity is changing rapidly with distance, as near an event horizon. This is not a result of general relativity!
nuclei beyond the iron peak are made by
neutron-capture reactions
Giant Molecular Clouds: where stars form
once stars begin to form, they illuminate the rest of the cloud
what happens when a particle pair is near a black hole?
one falls in, the other escapes! Since the energy came from the BH, the BH loses mass by this "evaporation" process.
What is another prediction of general relativity that has to do with radiation/waves?
that accelerating masses will produce travelling waves of gravitational energy, analogous to the electromagnetic waves produced by accelerating charged particles.
Newton's gravity - surface gravity
the acceleration due to gravity felt by an object on the surface of a planet, star, etc. as predicted by Newton's Law of Gravity. It depends on both the mass (M) and radius (R) of the object, hence on how concentrated (dense) it is:
Tidal force
the difference in the gravitational force across a length (e.g. your head to your feet). It is a property of any mass, and does not involve relativity.
What is the event horizon of a black hole?
the distance from the center at which the escape velocity, which increases as you get closer to the center, reaches the speed of light. That means anything within the event horizon, including photons trying to get out, will not escape
the most abundant elements are made by
the most common fusion reactions
meteorite
the rocky fragment that (sometimes) survives the trip and reaches the ground.
Newton's gravity - escape velocity
the speed needed for an object launched from the surface of a star to escape the gravity of the planet or star.
How do planetesimals become planets?
the terrestrial planets built up by collisions and by the accretion of planetesimals by gravitational attraction. The Jovian planets formed by gas accretion.
What are asteroids, comets and the Kuiper belt?
they are relics of the solar nebula, aka leftover planetesimals
late heavy bombardment
this is a period where the remaining planetesimals bombarded already-formed objects, leaving impact craters and tilting the rotational axes of several planets, including the Earth. We think this was caused by interactions among the giant outer planets.
Asteroids often have ....
tiny moons
mass transfer in a binary system
• Matter moving away from one star that reaches the "inner Lagrangian point" of gravity balance between the two stars can be captured by the other star. • If the companion is a normal star, such as Main Sequence star, it is large enough to intercept the mass stream and collect the transferred mass.
indirect detection of gravity waves
• The strongest gravity waves will be produced by large masses experiencing high accelerations. • Stars in short-period, "tight" orbits will radiate gravity waves that carry away orbital energy, causing their orbits to shrink and periods to shorten. •We see the effects of this, in binary systems containing neutron stars.
black holes: concepts
•A "black hole" is the ultimate victory of gravity over pressure. •All matter in the star collapses into a point of infinite density: the "singularity." •At a certain distance from the singularity, gravitational bending of light is so severe, even light rays travelling directly "outward"are turned back in.
The Nebular Theory for the Formation of the Solar System
•A gas cloud - the solar nebula - begins to contract under gravity: a radially inward acceleration. •Gravitational potential energy is converted into kinetic, then thermal energy, so the nebula heats up. •While contracting, angular momentum is conserved, so the nebula spins faster and faster, and also flattens. •These processes are going on simultaneously.
the nature of pulsars
•A pulsar is a neutron star that beams radiation along a magnetic axis that is not aligned with its rotation axis. •The radiation beams sweep around like lighthouse beams as the neutron star rotates on its axis. •The star rotates extremely fast because its angular momentum was conserved when the core collapsed.
solar system formation: initial collapse
•An interstellar cloud begins to contract under its own gravity; acceleration increases as the cloud gets smaller. •Due to conservation of energy,the kinetic energy of collapse is converted to heat. •Due to conservation of angular momentum, the rotation speed of the cloud increases as it contracts.
Thermonuclear white dwarfs #1: novae
•Classical novae are much brighter than disk flare-ups. •They run nuclear fusion reactions up to the light alpha elements like Ne and Mg; they may also blast off some of the material from the white dwarf. •But they do not destroy the white dwarf. Therefore, after the system settles down, the mass transfer can resume and eventually another nova explosion can occur.
What causes a cloud to collapse?
•Just as for a star, a cloud will be stable against collapse if the thermal pressure counteracts gravity. •Dense, cold clouds have little pressure, low kinetic energy per particle, are more prone to collapse. •There is a critical size and mass, above which collapse will occur: this is called the "Jeans mass," and for the Milky Way, it's a few thousand solar masses. •Another possibility is that collapse begins after something - a supernova shock, passage through a spiral arm, or winds from nearby stars - compresses the cloud, pushing it over the critical density.
Mass transfer in Novae
•Mass accumulates on the surface of the white dwarf un?l the temperature and pressure are high enough for H fusion on its surface! •The nova star system appears much brighter, temporarily. •The explosion drives the accreted matter out into space.
How does a solar nebula become a protostar then become a disk?
•Material on the rotational axis is more strongly accelerated to the center than material on the equator, so a spinning protoplanetary diskforms. •Due to collisions between particles, up/down and inward/outward motions cancel and their orbits "circularize."
512 keV Electron-Positron Annihilation Line
•Near the surface of a neutron star, where magnetic fields can be as high as 1012 or even 1014-15 Gauss, electromagnetic energy densities are high enough to create electron-positron pairs. •When they annhilate, two photons with energies of 512 keV (λ = 2.4 x 10-12 m) are produced. This is an "emission line" in the X-ray spectrum. •This line has been seen in several neutron stars in the Galaxy.
Neutron Star Mass Limit
•Neutron degeneracy pressure is analogous to electron degeneracy pressure, but occurs at much higher densities (factors of 1 - 100 million times higher). •Neutron degeneracy pressure cannot support a neutron star against gravity if its mass exceeds about 2 - 3 Msun(this is akin to the Chandrasekhar limit for white dwarfs) •Some massive-star supernovae can make black holes, if enough mass falls onto core; astronomers now think that a certain sub-class of gamma-ray bursts are events that form black holes ("collapsar" model).
Where might some long gamma ray bursts come from? Where might short gamma ray bursts come from?
•Observations show that at least some gamma-ray bursts, that last a relatively long time, come from supernova explosions that may result in the formation of a black hole •Gamma-ray bursts that are relatively short are thought to come from collisions between neutron stars
What are magnetars?
•Originally noticed as repeating bursts of γ-rays •Very strong burst in 1979 from a supernova remnant in the Large Magellanic Cloud •A 1998 burst partially ionized the Earth's atmosphere for a few hours! •Observations showed a variations with a 5-second period! - a neutron star!
formation of the Jovian planets
•Outside the frost line, ice as well as rock could form small particles; much more mass is available as ice, so larger planets were able to form. •The gravity of these larger planets was able to draw in and retain H and He. •This provided even more mass; Jupiter has about 300 times the mass of the Earth.
Pulsars and gravitational waves
•Pulsars are extremely accurate "clocks." •If a gravitational wave passes near one of them, the separation between it and other pulsars will change briefly; this will show up as a time delay (or the reverse).
products of the r-process
•Significant problem: reaction rates, even the masses, are often unknown for many unstable (short-lived) isotopes. •Fission reactions also occur, and compete with fusion.
What are millisecond pulsars?
•Some pulsars have extraordinarily short periods, measured in milliseconds = 0.001 seconds. •They are believed to be recycled or rejuvenated pulsars. •They were once normal pulsars born in a supernova, that lost rotational energy through radiation, slowed down, and faded as radio sources. •However, if they are in a binary system, mass can be transferred from the other star. The transferred material has angular momentum from the orbital motion, and speeds up the rotation of the neutron star when it lands.
Core collapse & neutron star formation
•The energy used in breaking apart the Fe nuclei is a net loss to the thermal pressure, leading to core collapse. •In the extremely dense core, electrons and protons are forced together into neutrons: "neutronization". •This process also releases a neutrino. •Collapse of the core is eventually stopped by neutron degeneracy pressure, at which point it has essentially become a neutron star. •The upper layers fall onto the core, rebound, & go flying into space•Details are still hazy.
Why are there two types of planets?
•The inner parts of the contracting solar nebula get hotter than the outer parts. • Metal and rock condense at higher T's than ice, so rocky bodies form in the inner disk, icy ones further out. • Since they are composed of the heavy elements and they are relatively rare, the inner planets have small masses.
Nuclear products of type Ia Supernovae
•The reactions in a Type Ia SN produce different proportions of elements than a supernovae from high mass star: they mostly make Fe (iron) and nearby elements such as Ni (Nickel). This is the "Fe group." (large bump, above right). •This adds new ingredients to the "cosmic soup."
hawking radiation
•There is a kind of "luminosity" characterized by a "Hawking temperature," which is inversely proportional to the BH mass. So, small BH's radiate more than big ones do. •This radiation represents a loss of mass-energy. As it loses mass, it radiates more and loses mass faster. Eventually it "runs away" and the BH evaporates! If BHs with masses of 1015 gm (a small asteroid) formed in the Big Bang, they would be dying now.
Prerequisites for neutron capture reactions
•There must be metal nuclei present, usually Fe but sometimes other elements too, products of earlier stars. These are called "seed nuclei." •You also need a supply of "free" neutrons to be captured by these seeds. This happens during certain advanced life stages of both low and high-mass stars. •Things work a bit differently depending on whether there is a trickle or a flood of neutrons. •The former is called the "slow" neutron capture or the "s-process" and the latter is the "rapid" neutron capture or "r-process." They produce different sets of trans-iron elements and isotopes.
the discovery of pulsars
•Using a radio telescope in 1967, Jocelyn Bell noticed regular pulses of radio emission from a place in the sky •When several similar sources were discovered, some in known supernova remnants (e.g. the Crab Nebula), it became clear these objects, dubbed "pulsars," were rapidly spinning (rotating) neutron stars.
Annihilation of matter and anti-matter
•When two identical particles of matter & antimatter collide, they annihilate (completely destroy each other) and their energy is converted to radiation: gamma rays •This happens during the first step of H-fusion in the Sun: a positron is produced along with deuterium, but soon meets a free electron and turns in photons •The vicinity of neutron stars is a place where this happens "out in the open" instead of buried in a star •For e+ and e−pairs, you get an emission line at an energy of 512 keV in the γ-ray spectral region
