AST 309N - Exam 3

Explain the basic process of neutron-capture reactions and how they differ from the fusion reactions that produce elements lighter than Fe.

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 This differs from fusion reactions in that this begins with a metal nuclei (usually Fe). The elements produced by neutron capture are far less plentiful than those of fusion reactions.

What kind of event caused the first direct detection of gravitational waves by LIGO? What kind of event detected by LIGO also produced electromagnetic radiation?

A detection of gravity waves showed the exact signal expected for two merging black holes of about 30 solar masses each. It was seen on Sep. 14, 2015, but came from a location 1.3 billion light-years away. The energy equivalent of 3 solar masses was radiated in gravity waves.

What is a meteor, and what is a meteorite? Why is it interesting to study the chemical composition (elements) in meteorites?

A meteor is a 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. A meteorite is the rocky fragment that (sometimes) survives the trip and reaches the ground. Meteorites can show us the composition of the early Solar System, which may include "processing" (heating, cooling/crystallization, radiation). Some bits ("inclusions") predate the SS, are unchanged from when they formed in a cool star or supernova.

Where in our Solar System do we find most of the leftover planetesimals that did not get incorporated into major planets? (Hint: They are strongly concentrated in multiple regions.)

Asteroids are leftover planetesimals of the inner solar system, while comets are leftover planetesimals from the outer solar system. This also included the Kuiper Belt.

What (qualitatively) are the conditions inside giant molecular clouds? What are some of the most common molecules and why?

Giant molecular clouds are dense have cool gas that exists in the form of molecules. They are dark but cast a shadow against the background. As stars begin to form, the will be illuminated.

Where is the mass of a black hole located, what is this location called, and what is its density?

In principle, the mass resides in a region of zero radius, zero volume, which makes the density infinite, because density = (mass)/(volume).

Why are the inner four planets of our Solar System small and rocky, while the outer four planets are large and made of light materials including a lot of H and He gas?

Inside the frost line, it was too hot for H compounds to form ices. Metal and rock condense at these high temperatures, and they have smaller masses. Outside the frost line, it is cold enough for ices to form. This means there is a larger reservoir of matter to make the outer planets. More mass is available as ice. The gravity of these larger planets was able to draw in and retain H and He, adding even more mass.

What are the requirements for an interstellar cloud to start collapsing? How do the properties of molecular clouds meet these requirements?

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.

What happens in a classical nova? What happens in a "thermonuclear" or Type Ia supernova? What type of compact star is present in both, and what elements are made in each?

Mass accumulates on the surface of the white dwarf until the temperature and pressure are high enough for H fusion on its surface. The nova star system appears much brighter, temporarily, but the white dwarf is not destroyed. Type Ia supernovae occur when a white dwarf blows apart due to mass accumulation, which provides a fixed amount of energy (the internal gravitational energy of the WD). Type la supernovae make heavier elements (Fe and Ni).

Why do neutron stars have a maximum possible mass, and what is its value in solar masses?

Neutron degeneracy pressure cannot support a neutron star against gravity if its mass exceeds about 2 - 3 solar masses.

How did the heating of the early Earth lead to its present, multi-layer structure (Fe/Ni core, rocky mantle)? How is the presence of a molten Fe core helpful to life on Earth?

Radioactive decays in the interior caused the outside to melt, enabled iron and nickel to sink to the center. This generates a magnetic field that protects us from harmful solar particles

What causes the periods of most pulsars to gradually increase (get longer) with time?

Rotational energy is converted to radiation, so the pulsar will gradually slow down. This results in larger intervals over time.

What are the Roche lobes of a binary star system?

Surfaces of equal gravitational energy in a binary star system. The "Roche lobes" are the surfaces where the effects of the two stars balance.

Explain why the composition of the interstellar medium (ISM) is changing over time, in particular the fact that the concentration of elements heavier than H and He keeps increasing.

The ISM's composition changes because the phases are all gaseous, but the H atoms are in different chemical states, determined mostly by temperature. Stars cook elements by fusion reactions in their interiors. They disperse these products back into space, where heavy atoms become part of the ISM. New clouds form and create a new generation of stars. Each time around the cycle, the concentration of heavy elements increases.

Define the event horizon of a black hole in terms of Newton's theory of gravity. Describe it in terms of Einstein's theory of gravity(space-time curvature).How big is a 1 solar mass BH in km?

The event horizon is the radius at which the escape velocity reaches the speed of light. In terms of Newton's theory, this would apply to objects with mass. In terms of Einstein's theory however, gravitational bending of light is so severe, even light rays travelling directly "outward"are turned back in. A 1 solar mass BH has a 3 km horizon.

Why does the mass transferred from a star to a compact companion end up orbiting the compact object in an accretion disk? What kind of radiation does the disk emit?

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 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.

How does the s-process differ from the r-process, and where do each of these reactions take place, according to the latest information astronomers have?

The s-process describes slow neutron capture, where neutrons are captured one at a time. This occurs in AGB stars, a life stage of lower mass stars. The r-process describes rapid neutron capture, where Fe nuclei are flooded with neutrons. This happens in merging neutron stars, and maybe also core-collapse Supernovae from high mass stars. The two produce different sets of trans-iron elements and isotopes.

How do astronomers explain the existence of pulsars with periods of a few milliseconds?

The star rotates extremely fast because its angular momentum was conserved when the core collapsed.

What indirect evidence that gravitational waves exist did we know about before LIGO (the Laser Interferometric Gravitational-Wave Observatory) was built?

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. Our indirect detection of this is shortening of the period of the first binary pulsar (perhaps due to loss of energy through gravity waves).

Why does an object approaching the event horizon of a black hole feel a tidal force, and what unusual effects start to become apparent in this region of extremely strong gravity?

Tidal force is simply the difference in gravity from one end of the object to the other. Tidal forces are strong when the strength of the local gravity is changing rapidly with distance, as near an event horizon. Objects will be stretched along their length towards the hole, and they will be compressed in the crosswise direction.

How were pulsars discovered(what was observed)? Why did astronomers conclude that these sources must be neutron stars?

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.

What do astronomers think triggered an era of heavy bombardment of the inner Solar System (including the Earth) early in its history?

When Jupiter and Saturn fell into resonant orbits ("locked" to each other), Neptune and Uranus switched places, and many asteroids were flung outwards or inwards to the Sun. We think this was caused by interactions among the giant outer planets.

What causes the emission from pulsars to be seen as "pulses,"that is, as blips of emission that repeat at regular intervals? Typically, how long are the intervals between pulses(periods)?

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. The electrons radiate synchotron radiation, which is strong at low frequency, in particular, radio waves. The crab pulsar, for example, flashes every 0.033 seconds.

What are planetesimals, and how do they develop in the disk around a star that is forming?

Within the disk that surrounds the protosun, solid grains collide and clump together into planetesimals. Accretion turns dust and pebbles into planetesimals and planets.