Astronomy Test 4 – Flashcards

Use the equation: d = 1/p. d= 1/p d = 1/0.333 arcseconds d = 3.00 parsecs

The parallax angle would be larger because Pluto's orbit has a larger diameter than Earth's orbit.

It is bright because it is close to our Solar System.

It is bright because it is intrinsically luminous.

If a star has a large radius, then it has a large surface area. Since there is a larger surface area emitting light, more light will be given o (luminosity is larger). A hot star will be luminous because it is generating a lot of energy by nuclear fusion since it has a higher temperature. A cool star can be very luminous by have a very large radius (surface area).

Luminosity or Absolute Visual Magnitude on the vertical axis. Tempera- ture or Spectral class on the horizontal axis.

Main Sequence on the diagonal from upper left to lower right. Supergiants and giants upper center to upper right. White dwarfs on bottom left to bottom center.

Main Sequence

Roughly: Upper left: hot, bright, large; Upper right: cool, bright, large; Lower left: hot, dim, small; Lower right: cool, dim, small

Capella, because to have a luminosity 100 times that of the Sun but the same surface temperature, requires Capella to be larger than the Sun.

Capella, because it is cooler and so must be large to have the same lumi- nosity as Regulus a hotter star.

B spectral class star has a higher surface temperature than an F spectral class star.

Chapter 9

The interstellar medium is composed of about 75% hydrogen, 25% helium, 1% interstellar dust, with traces of heavier elements.

A nebula can be seen with visible light, whereas interstellar medium is generally not visible as it is not emitting or reecting visible light.

Gas pressure outward balances gravity inward. Turbulence and magnetic elds also help gas pressure balance against gravity.

The three processes that cause shock waves are (1) a blast of ultraviolet light from new born stars; (2) collisions between interstellar clouds; and (3) the blast from a supernova.

When a shock wave hits an interstellar cloud it compresses the cloud so that there is more mass in one area and the force of gravity is now greater than gas pressure. This causes the interstellar cloud to collapse. Since an interstellar cloud is not uniform in mass, there are several to many areas where the cloud collapses causing the cloud to break up into fragments.

No, it is not true. Interstellar clouds are huge and contain a great amount of mass. Depending on the size of the cloud, from 10 to 1000 stars may form in an interstellar cloud.

The source of energy in a protostar is gravitational potential energy. Gravitational potential energy is transformed into heat as the protostar contracts just like a nail heats up when a falling hammer falls on it.

Protostars are cool, faint objects. They are faint because most of the energy they emit is in the form of infrared light, not visible light. They are cool as their energy is generated as the protostar contracts and not from nuclear fusion. They are located in a disk-shaped cocoon of dust and gas. It is contracting and heating up. (See the solar nebula theory in chapter 16.)

For a protostar to move onto the main sequence, nucler fusion of hydrogen into helium must be occuring in its core and it has stopped contracting.

Gas pressure outward balances gravity inward. (Hydrostatic equilibrium)

Gas pressure depends on temperature, which in turn, depends on the rate of nuclear fusion. Gravity depends on how much mass the star has.

A main-sequence star is stable neither contracting or expanding - because hydro- static equilibrium is in eect. Nuclear fusion of hydrogen into helium is occuring in its core.

4 protons fuse together to form helium and energy in the form of gamma rays, positrons which annihilate with electrons to form gamma rays, and the motion of nuclei during the reaction. The proton-proton chain is Nuclear fusion.

The proton-proton chain takes the smaller hydrogen nuclei and fuses them to- gether to form the larger helium nucleus. Nuclear ssion takes a larger nucleus and splits it into smaller nuclei.

The mass of a helium nucleus is smaller than the masses of the hydrogen nuclei that are fused together. This dierence in mass was converted into energy (E = mc2 ).

The CNO cycle is a nuclear fusion cycle involving carbon, nitrogen, and oxygen. It is a more ecient way of fusing hydrogen into helium and is used by stars more massive than our Sun. You still have the basic equation as the proton-proton chain, but carbon acts like a catalyst in the reaction.

There are more protons in the carbon, nitrogen, and oxygen nuclei than in hy- drogen nuclei so these nuclei must be moving with higher speeds to overcome the electrical repulsion between the nuclei and come close enough for fusion to occur. Since the speed of a gas molecule depends on the temperature of a gas, higher speeds require higher temperatures.

Compare: Both reactions fuse 4 protons (hydrogen nuclei) in helium plus energy. Contrast: CNO cycle is more ecient than the proton-proton chain. Carbon is a catalyst in the CNO cycle, not in the proton-proton chain. CNO cycle requires higher temperatures than the proton-proton chain so that it can occur only in stars more massive than the Sun.

The more massive a star is, the more luminous it is because more mass leads to a larger gravitational force. More gravity requires more gas pressure to support the star's weight so the star stays in hydrostatic equilibrium. More gas pressure is acheived by a higher fusion rate which makes the star hotter. Hotter gases emit more electromagnetic radiation (are more luminous) than cooler gases.

The more mass a star has, the larger the star's gravitational force is. More gravity requires more gas pressure to support the star's weight so the star stays in hydrostatic equilibrium. More gas pressure is acheived by a higher fusion rate. The higher the fusion rate, the faster the star uses up the available fuel in its core.

A thermostat regulates the temperature in your home, say in the winter, by turning on the furnace when the house's temperature gets too cold and turning o the furnace when the house's temperature gets too hot. Hydrostatic equilibrium works in a similar fashion. If the rate of fusion gets too low, the star contracts (gravity > gas pressure) to increase the temperature of the core and increase the rate of fusion so more energy is given o. Then the temperature rises, gas pressure rises until it is equal to gravity. If the rate of fusion got too high, the star would expand because it is emitting more energy and at a higher temperature (gas pressure > gravity). As the star expands, the core cools and the rate of fusion decreases until once again gravity and gas pressure are equal.

Chapter 10

The mass of a star determines its lifetime. More massive stars are more luminous. More luminosity requires a faster rate of fusion. Faster fusion rate uses up the fuel in the core in a shorter time, which leads to a shorter lifetime.

Those main sequence stars with a small mass (spectral class M).

The mass of the core of the star determines whether the star will end as a white dwarf, neutron star, or a black hole.

All of the hydrogen in the core has been fused into helium.

All of the helium in the core has been fused into carbon and oxygen.

Core: Fusion has stopped, so gravity > pressure. Core is contracting and heating up. The core will contract until the matter becomes degenerate. Outer layers of the stars: Shell of gas around the core has been heated enough for hydrogen fusion to start. This heats up the outer layers of star so gas pressure > gravity. This causes the outer layers of the star to expand and cool.

It is a giant because the outer layers of the star have expanded. It is red because the outer layers of the star have cooled and the maximum visible light is emitted at the red end of the spectrum.

First time: see answer for #6. Second time: the gas shell that was fusing hydrogen into helium is now fusing helium into carbon and oxygen and the shell above that is fusing hydrogen into helium.

Degenerate matter is extremely high density matter in which pressure no longer depends on temperature. For stars, electrons are no longer able to move to dierent energy levels.

First, degenerate matter resists compression. Second, degenerate gas pressure does not depend on temperature.

Helium ash is the explosive fusion of helium that occurs in some stars about the size of our Sun when the core is hot enough to begin fusing helium into carbon and oxygen.

Core is fusing helium into carbon and oxygen. Layer just outside of core is fusing helium into hydrogen. Layer above this one is fusing hydrogen into helium. The outer layers are the same as in the star before.

Planetary nebula: an expanding shell of gas ejected from a star during the latter stages of its evolution. Caused by the expansion of the outer layers of the star due to heating by the shells of gas next to the core undergoing fusion.

No, 1.4 solar masses is the Chandrasekhar limit, which means that if the white dwarf's mass is above 1.4 solar masses it cannot support itself with dengenerate gas pressure and will continue to collapse.

At the bottom from left to center below the main sequence.

The star could not contract enough for the core to reach a high enough temperature to fuse carbon and oxygen. The core begins to cool so that gravity > gas pressure. The core contracts until it is made of degenerate matter. The degenerate gas pressure due to the electrons halts the contraction. The outer layers of the star have been expelled leaving the core exposed. This core is called a white dwarf.

The degenerate gas pressure due to the electrons balances the force of gravity.

The high-mass star has an onion-like structure with an iron core with several shells of gas fusing dierent elements (silicon, oxygen, neon, carbon, helium, and hydrogen). (See page 201, Figure 10-14 at the bottom.)

A Type Ia supernova is when a white dwarf in a binary star system gains more than 1.4 solar masses (greater than the Chandrasekhar limit) fuses the carbon-oxygen in its core in a very short time and the outer layers of the star are blasted away. The core is destroyed in the process. A Type II supernova results from the collapse of a massive star when its iron core collapses (as we discussed in class).

(1) When the iron nuclei in the core are disrupted they produce enormous amounts of neutrinos which carry large amounts of energy out of the core.

(2) This enormous amount of energy heats the gases in the core which expand outward taking the energy with them.

If the neutron star had a mass greater than the upper limit (2 - 3 solar masses), gravity would be strong enough to collapse the neutron star into a black hole.

Because of the conservation of angular momentum. Just like an ice skater bringing her arms closer to her body and having her rotational speed increase, a collapsing star will begin to speed up as its radius decreases while its mass remains the same.

Pulsars do not pulsate. The apparent pulsing is due to beams of radiation sweeping across the Earth as the pulsar rotates.

Beams of radiation sweeping out from the pulsar across the Earth, like beams of light from a lighthouse sweeping across the ocean, will be picked up by radio telescopes as bursts or pulses of radio signals.

A massive star with an iron core whose mass is greater than 1.4 solar masses and less than about 3 solar masses will supernova and the core will collapse. As the core collapses the nuclei in the core are broken apart by gamma rays. With this amount of mass the star's gravity is strong enough to force the free protons to merge with the free electrons to form neutrons.

Neutron stars are about 10 kilometers in diameter with a mass of about 1.4 to 2 solar masses which makes the neutron immensely dense. The neutron star is balanced between gravity and neutron degeneracy pressure.

Black holes are black because their escape velocity is greater than the speed of light. Nothing can escape from a black hole.

The speed of light is constant.

The point that the matter has contracted to is called a singularity. The Schwarzchild radius is the distance from the singularity to the point where the black hole's escape velocity equals the speed of light. The event horizon is the imaginary sphere centered on the singularity that has a radius equal to the Schwartzchild radius. It separates the region where information/matter can escape from the black hole from the region where information/matter cannot escape from the black hole.

Look for accretion disks; observe the effect of the black hole on matter orbiting it using the Doppler effect; in binary star systems that have a black hole, use the visible star to calculate the mass of the black hole using Kepler's third law; watch matter falling toward the suspected black hole, if it falls toward a neutron star a burst of energy will be detected when it hits the surface, if it crosses the event horizon of a black hole, no energy will be detected.

No, they still have to obey the force of gravity. Matter has to be on a deliberate course to cross the event horizon to go into a black hole.

The gas pressure can balance gravity when it is on the main sequence and the degenerate gas pressure can balance gravity when it is a white dwarf.

The gravitational force on the Earth would be the same so the orbit wouldn't change. (But, it would be a lot colder!)