Term
|
Definition
| : The Roche lobe is the boundary (surface) around two stars where the pull of gravity experienced by an object is balanced between the two stars. Inside the roche lobe, the gravity of the nearer object seems stronger; outside the roche lobes, the pull of gravity is dominated by the more massive object. |
|
|
Term
| inner Lagrangian point L1: |
|
Definition
| The inner Lagrangian point is the point at which the Roche lobes of two objects touch. It is a very stable place, if you are there it requires no energy to stay there, it feels equal effects of both stars |
|
|
Term
|
Definition
| Matter moving away from one star and passing through the point of equal gravity is captured by the other star; This flow can be caused by a stellar wind, or by evolution: as the star becomes a red giant, its surface swells up to fill the Roche lobe; Because it has (orbital) angular momentum, if the transferred mass lands on the companion star, it can cause it to spin (rotate) faster.consists of a 0.5 M! red giant and a Main Sequence star of about 2 or 3 M!. The star that is now a red giant was originally the more massive star, |
|
|
Term
|
Definition
As a higher mass star loses mass to its companion, its Roche Lobe shrinks; but the star is moving up the red giant branch, so it swells up even more. Something like this may have happened to Algol, when it started to become a red giant it transferred so much mass to its companion that the latter is now the more massive star!!
This is paradoxical because: The less massive star is already a subgiant, and the star with much greater mass is still on the main-sequence. Initially, this seems paradoxical as the partner stars of the binary are thought to have formed at approximately the same time and so should have similar ages. Thus the more massive star, rather than the less massive one, should have left the main sequence.
|
|
|
Term
|
Definition
| An accretion disk forms from the material falling onto an object. The mass transferred from the more massive star onto its companion star has (orbital) angular momentum, and as the companion star is small the transferred mass “misses” landing on it and instead orbits as an accretion disk around the star. (Inside the Roche lobe though, so it orbits the star due to its gravitational pull instead of flying off into space.) |
|
|
Term
|
Definition
| Matter streaming from one Roche lobe to the other, passing through the L1 point, carries a sideways (orbital) angular momentum, so it rotates around the other star instead of falling straight in. When large amounts of matter stream towards the smaller companion star suddenly a large “flare up” occurs. |
|
|
Term
|
Definition
| where the matter stream hits the accretion disk |
|
|
Term
| Interacting binaries containing white dwarfs: |
|
Definition
| When two stars are in a close binary system, they can potentially interfere with each other’s evolution. Any matter in the vicinity feels gravitational effects from both stars. Accretion disks formed around white dwarfs in binary systems emit radiation (UV and optical). Accretion disks feed mass to the white dwarfs |
|
|
Term
|
Definition
| Cataclysmic variables is any star that occasionally gets very bright and puts out material. Classical novae are a type of cataclysmic variable stars. The accretion disk can become unstable, and dump a large amount of mass onto a White Dwarf all at once, causing a severe flare-up in brightness. |
|
|
Term
| What is a classical nova: |
|
Definition
in a binary star system consisting of a white dwarf and a star which has filled its Roche lobe; on occasion, the accretion disk becomes unstable, causing a large amount of material to fall onto the white dwarf in a short time. This heats up the surface and causes a sudden onset of nuclear fusion (via CNO cycle) on the surface of the white dwarf. This emits large amounts of energy which `blasts' much of the surface layer back into space and causes the white dwarf to get significantly brighter temporarily. This does not destroy the WD, so after the system settles down, mass transfer can resume, and eventually another nova explosion can occur.
-Classical novae reach much brighter flare-ups than the dwarf and recurrent novae. |
|
|
Term
|
Definition
| in a binary star system, which consists of at least a white dwarf that is about 1.4 M. The white dwarf gains a little bit of mass, which causes it to undergo a sudden burst of fusion, creating heavy elements (similar to a core collapse supernova, but producing far more iron and nickel). This fusion event generates so much energy that explosive nuclear fusion blows the WD apart. (The result of the explosion is similar to that of a core collapse supernova - an expanding shell of initially very hot material, followed by light from the radioactive decay of nickel and cobalt.) The initial mass must be about 1.4M_ (with little variation), so the result of the explosion is always the same. Since they are bright, we can see them in far distant galaxies, and since they are the same, we know their luminosity. With the combination of luminosity and apparent brightness we can determine their distance very accurately. |
|
|
Term
| “thermonuclear” supernova: |
|
Definition
| another name for a WD supernova, also called a Type Ia supernova (see above) |
|
|
Term
| Interacting binaries containing neutron stars: |
|
Definition
| similar to a WD binary system, falling matter forms an accretion disk which is hotter the closer to the star the disk is. Neutron stars have hotter accretion disks (than white dwarfs), which leads to additional X-ray emission (because of the increased heat). So a neutron star has X-ray, UV, and optical emissions, while a WD emits only UV and optical radiation |
|
|
Term
| making millisecond pulsars: |
|
Definition
| thought to be by accreting matter adding angular momentum to a neutron star, increasing its spin rate. (Forms similarly to a pulsar, but the added accretion matter accelerates the star so that its period is very fast - in the milliseconds) |
|
|
Term
|
Definition
| Mass, Net Electrical Charge, and Rotation are all physically measurable properties. |
|
|
Term
|
Definition
| the speed necessary for an object to completely escape the gravity of a large body; it is determined by the mass of the object to be “escaped” (black holes for our purposes) |
|
|
Term
|
Definition
| the theoretical boundary that marks the “point of no return” between a black hole and the outside universe; events that occur within the event horizon can have no influence on our observable universe. The boundary of the event horizon is the point where the escape velocity equals the speed of light. ( v_escape= c), which is measured using the Schwarzchild radius. |
|
|
Term
|
Definition
a measure of the size of the event horizon of a black hole;
R_s = 3.0 X M (in M_). So a 10M_ black hole has a R_s of 30km |
|
|
Term
| gravitational bending of light: |
|
Definition
| At a certain distance from the singularity (the Schwarzchild radius) gravitational bending of light is so severe that even light rays travelling directly outward from the BH are turned back in (ie, no light can escape) |
|
|
Term
|
Definition
| where the mass is located in a BH; there is zero radius, and thus infinite density. The place at the center of a black hole where, in principle, gravity crushes all matter to an infinitely tiny and dense point. |
|
|
Term
| tidal forces near a black hole: |
|
Definition
| a tidal force is a difference in the gravitational force at one end from the other. Tidal forces stretch things in the radial (inward, downward) direction and compress things in the crosswise direction (since radial lines converge). The “spaghettification” effect. |
|
|
Term
|
Definition
| a force that produces acceleration, but acts only between (any) two objects that possess mass (an interaction) |
|
|
Term
|
Definition
Einstein’s theory of gravity: looks at gravity as a geometrical effect: every mass “curves” the space around it, and other things (both objects
with mass, and massless things like photons) move within that curved space (differs from Newtonian view in that an object need not act upon another mass for the gravity to affect the space around it, gravity is instead a “field” in space-time). Explains why even light is affected (“bent”) even though it is mass-less. |
|
|
Term
|
Definition
| far from an object with mass, spacetime (on an embedding diagram) is nearly flat, but closer to a mass the space curves downwards and that curvature forms a “well” |
|
|
Term
| embedding diagrams (rubber-sheet diagrams): |
|
Definition
| where you can see the gravitational dip or “well” as a dimple in the surface around a massive object (ie, an object with mass, it need not be large). Denser objects curve space more sharply, so the dip in curvature has steeper “sides.” Black holes go to infinite density at the center, so they are very steep and “punch a hole” in space-time. |
|
|
Term
| gravitational time dilation: |
|
Definition
| Time passes more slowly for a person or object in a stronger gravitational field: for each second of time experienced by a person deeper in the gravity field (time appears to move normally for this person), several or many seconds pass for an observer outside the region of strong gravity (the object/person deeper in the gravity field appears to be slowing down until it reaches a dead stop). |
|
|
Term
|
Definition
Light moving away from a source of gravity redshifts. So an object moving towards a BH will appear to become dimmer and redder as it descends (gets closer to the source of the gravitational field). The inverse is also true,an object moving away from the BH (or remaining stationary while a probe or person moves away from the observer/mothership) will be blueshifted and look bluer.
Light travelling out of a region of strong gravity loses energy, only possible via the frequency becoming lower (recall that frequency and wavelength have an inverse relationship), which in turn causes the wavelength to get longer - a redshift . The reverse is also true.
|
|
|
Term
|
Definition
| See neutron stars in binary systems above. Just note that the same information about the accretion disk is also true for a BH (ie, produces X-ray, UV, and optical radiation). To determine if a BH or NS is at the center of the accretion disk |
|
|
Term
|
Definition
| the singularity becomes a ring instead of a point, and there is an outer, flattened boundary around the conventional spherical event horizon, called an ergosphere. In theory they are also possible energy sources if an infalling particle splits into two pieces, such that one will fall in and the other will fly out at high velocity, gaining energy from the BH in the process. |
|
|
Term
|
Definition
| inside the ergosphere, space-time is theorized to have a net rotation (a little like the co-rotating “magnetosphere” around a pulsar). |
|
|
Term
|
Definition
| on small scales in brief times, “virtual” pairs of particles/antiparticles are constantly being produced and annihilated. If this occurs near a BH, then one part of the pair falls in while the other escapes, taking mass and energy from the BH, causing the mass to “evaporate” away. As it loses mass, the radiation process occurs even faster, and eventually the entire BH “evaporates.” The lifetime of a BH is thus proportional to its mass. |
|
|
Term
|
Definition
| Is a textbook example of a system undergoing mass transfer from a neutron star to a binary system (the classic filling of the roche lobe, accretion disk, and the accretion disk spins and heats up until it emits x-rays) Her.X-1 is a perfect example of studying the companion star to know more information. The optical studies tell us the type of star, its evolutionary state, and the fact that it is filling its roche lobe. Coupled optical and xray studies were used to completely characterize the orbits of the two stars and to obtain a direct measure of their masses using Kepler’s Law. Her.X-1 is a classic case of a star simply losing its mass to its companion star through common envelope evolution. Quick fact: Her X-1’s x-rays are emitted in pulses (1.2 seconds apart). |
|
|
Term
|
Definition
| Walter Chroncite announced on the news in 1980 that scientists had discovered an object that was coming toward us and away from us at the same time, which was odd because it had two sets of emission lines that contrasted each other in regards to red and blue shifts (oscillation period of 64 days). Radio astronomers finally decided that the star was shooting out material in two opposite directions but somehow twisting around to throw the beams in one direction and then another. However, the jet streams aren’t pointed towards us, they are actually going side to side, but because they are not perfectly parallel to us, meaning they are pointed at us by at least one degree, they are visible from Earth. Since these jet streams are shooting out matter that we can see, we know that the opposing twin beams are shooting material out nearly at the speed of light. Another oddity is that the emitted material is not hot; SS 433 shows emission lines of neutral helium but none of ionized helium which would cause the temp. to be low. |
|
|
Term
|
Definition
A Galatic X-ray source. Scientists believe that it has a zcompanion star of about 30 solar masses that dumps mass into the (supposed) black hole from solar winds. Gravity sucks the matter in, heats it up, and creates an accretion disk which causes x-ray emissions. They also believe CX-1 was originally a 35 solar mass star that used up its hydrogen, leaving only a helium core (approximately 10 solar masses) that then burned up until it created iron, which resulted in core collapse and led to the formation of the theoretical black hole CX-1.
|
|
|
Term
|
Definition
| discovery by the Vela satellites, : These were deemed necessary by Colgate to help differentiate between nuclear threats and “supernova” explosions. He was concerned that opposing countries would detect these explosions and misinterpret them as nuclear attacks which would obviously lead to a nuclear war. The Vela 4 (1976)was the first to detect the x-ray bursts but it was unable to determine whether or not these bursts were actually coming from another part of the universe/galaxy or if they were coming from our Sun and the Earth. It wasn’t until the Vela 5 (1969) determined that the GRB’s weren’t from the Earth or Sun. |
|
|
Term
|
Definition
| GRBs appear to come from a kind of supernova explosion, in which a black hole forms. Energies higher than for “standard” SuperNovae, but if the radiation is beamed, the luminosities are less extreme. |
|
|
Term
|
Definition
| are thought to be the merger of two neutron stars, such as the binary pulsar. The gravity waves are most intense towards the very end of the “in-spiral,” as the stars begin to merge. |
|
|
Term
|
Definition
| similar to a pulsar, the “beams” are fast-moving jets that blast through what remains of the supergiant star (the pre-BH state), and if one of these jets is directed towards Earth (ie, along our line of sight/orbital inclination known to be nearly “dead on”) then at least one of the beams of a GRB is visible. |
|
|
Term
|
Definition
| has been detecting GRBs in γ-rays, X-rays, ultraviolet, & optical light (since 2004). |
|
|
Term
| collapsars (birth of black holes), |
|
Definition
|
|
Term
| merging neutron star binaries: |
|
Definition
| thought to be the origin of short-duration GRBs |
|
|
Term
|
Definition
|
|
Term
1. What is the significance of the Roche lobes in a binary star system? Under what circumstances does a Roche lobe correspond to the actual surface of a star?
|
|
Definition
| Roche Lobes represents the theoretical boundary of two stars’ gravitational fields, including where the fields “touch” (at L1) if the stars are in a binary star system. roche lobes are where gravity is equal between two objects. They correspond to the actual surface when a star fills up a Roche Lobe, usually when one star (typically the more massive) evolves, usually into a Red Giant. After the larger star evolves and fills up its Roche lobe, matter may begin to be transferred through L1 to the smaller, companion star. |
|
|
Term
2. Why is it said that Algol, consisting of a red giant and a Main Sequence star that has a larger mass than the red giant, is a “paradox”? How can we explain/resolve the paradox?
|
|
Definition
| The current model of binary star systems says that Algol is odd because the larger star (the Red Giant) is believed to always be the one transferring mass, yet for Algol the Main Sequence star is larger. This paradox can be explained if we suppose that the Red Giant was at one time the larger of the two stars, but due to the mass transferred over time the Red Giant has decreased in size and the Main Sequence star increased until we reach “today” when we observe that the Red Giant is smaller than the Main Sequence star. (That is, “our model isn’t wrong, just blame mass transfer.”) |
|
|
Term
3. Specify, describe, and sketch the components of a binary star system in which mass is currently being transferred from a red giant to a white dwarf. What kind of emission
(radiation) does each object in the system produce?
|
|
Definition
| In this, we have a RG and a WD. The RG begins to leak some of its mass through the L point into the WD. Since the WD has such small SA, the mass carries and a sideways angle and misses the WD, forming an accretion disk. this is due to angular momentum. The friction of the accretion disk rubs together and heats up and glows in visible and ultraviolet radiation. |
|
|
Term
| 4. What happens to matter that passes through the inner Lagrangian point of a binary star system,and why? How does the latter depend on the nature of the star that is receiving the mass? |
|
Definition
| Matter moving from one star and passing throgh the point of equal gravity is captured by the other star. This is caused by either stellar wind or evolution. As star gets larger the roche lobe gets smaller. |
|
|
Term
5. There are several types of outbursts/explosions that can occur in a binary system that contains a white dwarf that is accreting material. Identify these events by name, explain what happens in each, and compare the outcomes for these different events.
|
|
Definition
| Classical Novae: accumulated matter on surface of WD gets so hot that nuclear fusion begins, explosion sends matter into space and star system gets temporarily brighter. The core of the white dwarf is left unharmed and the mass transfer still continues. Type Ia: The white dwarf is approaching the Chandrasekhar limit due to mass transfer and is blown apart by explosive nuclear fusion. The white dwarf goes up in flames.Core collapse (type 2) supernova: high mass star at end of life forms an iron core, the core collapses and the layers above bounce off exploding into a supernova. |
|
|
Term
6. What differences might you expect to find for the accretion disk around a neutron star as compared to an accretion disk around a white dwarf?
|
|
Definition
| Both are hotter at the points where the accretion disk are closer to the NS/WD, but the accretion disk around a NS is much hotter so it will emit X-rays while a WD’s AD will not. This is because the angular momentum of the mass transfer also speeds up the spin rate of the NS. WD’S will emit UV light. |
|
|
Term
7. Why is matter accreting onto a neutron star in an interacting binary star able – in at least some cases – to speed up the rotation rate of the neutron star? How might we see the result of this?
|
|
Definition
| the mass has angular momentum and the added angular momentum increases its spin rate. beams of x-rays are emitted from the hot spots. |
|
|
Term
8. What is the significance of the event horizon of a black hole? of the singularity? How does the structure of a black hole change if it is rapidly rotating?
|
|
Definition
| at the event horizon, the escape velocity equals c, the speed of light. At the singularity, density is infinite and anything in the BH that reaches singularity is destroyed. If BH is rotating, an ergosphere(flattened outer boundary around event horizon) is formed. |
|
|
Term
9. Roughly what are the sizes of the radii of white dwarfs and neutron stars, and the event horizon of a black hole? Which sizes increase as the mass increase? Which of them decrease?
|
|
Definition
| WD: Less than 1.4 solar masses NS: Less than 2-3 solar masses BH: greater than 3 solar masses EH: three times the mass of a BH - things only change if the mass changes, the radius has no effect. If the mass of a BH increases then its event horizon also increases. The same goes with NS and its Roche Lobe. A NS and WD GET SMALLER if they’re mass increases, due to their degeneracy pressure; density must increase, which means the size must decrease! |
|
|
Term
10. Discuss the origin and effects of tidal forces near a compact star – in a region where the gravity is relatively strong and is changing rapidly with radial distance from the center.
|
|
Definition
| TF: the difference in gravity from one side of the planet to another. This causes spaghettification: stretching mass in the radial direction compressing the mass in the crosswise direction. It makes it look like spaghetti. It is stronger nearer the center of mass. |
|
|
Term
11. How does Einstein’s way of describing gravity differ from Newton’s theory? What different behavior do they predict for light (photons) traveling through a region near a large mass?
|
|
Definition
| According to Newton’s theory, gravity is a force so as light has no mass it will not be affected by proximity to a large mass (which is incorrect, as light actually “bends”). According to Einstein’s theory, gravity is instead a “field” so a large mass in space “curves” the space around it, causing anything close by (including the mass-less light) to be “bent” by its gravitational field. |
|
|
Term
12. Describe two (or three) relativistic effects that start to become noticeable, even dramatic, as you approach the event horizon of a black hole. (“relativistic” means that they are predicted by Einstein’s theory of general relativity, a theory about how gravity behaves).
|
|
Definition
Gravitational time dilation: time moves slower as you are closer to the center of gravity and Redshifts: think of throwing a ball in the air ,as the ball in going up it is losing energy (redshift) but as it is coming down it is gaining energy and speed(blueshift), see above for further detail.
|
|
|
Term
13. How might you be able to tell whether an X-ray binary system contains a neutron star or whether it harbors a black hole? What evidence is needed in order to settle this definitively?
|
|
Definition
| Gas is released from hot spots on NS. This causes a brief x-ray emission to be released. The inner accretion disk of a NS is hotter than a black holes. the width of emission lines from accretion disks can tell you the orbital speed of the disk, and from that, the mass of the central object. |
|
|
Term
14. How was the gamma-ray burst (GRB) phenomenon first discovered? What properties of these events made them hard to explain?
|
|
Definition
An American spy satellite detected gamma-rays from space when looking for them on Earth (from the U.S.S.R.). they were extremely luminous and not sure if they were close by or far away. |
|
|
Term
15. What is the “collapsar” theory for the “long-duration” (100 seconds, vs. a couple of seconds) GRBs? What do we think is produced in these events?
|
|
Definition
| when a supergiant with a mass of more than 30 solar masses reaches the end of its lifetime, the star’s core rapidly collapses to form a black hole. material around the black hole falls inward, forming an accretion disk and jets. the fast moving jets produce intense beams of gamma rays. the jets blast through what remains of the star, and if one of the jets and its beam is directed towards earth, we see a gamma ray burst. |
|
|
Term
16. What do we think is the explanation for the “short-duration” GRBs, and how might future measurements confirm (or disprove) this idea?
|
|
Definition
| The merger of two neutron stars is the cause of short-duration GRB. Measured by gravity waves, which are most intense at the end of the merge |
|
|
Term
17. What is “Hawking radiation” and how does it change the mass of a black hole?
|
|
Definition
| A BH’s mass is “evaporated” away by particle-pairs splitting up. One falls into a BH while the other escapes, taking with it some of a BH’s energy and mass. the positron falls into the black hole while the electron flies out. Losing mass causes the radiation rate to increase which in turn causes even more mass to be lost, so that eventually all of the mass in a BH is “evaporated” and nothing remains (in theory). |
|
|
