ASTR 112 - Review Questions Exam 2

The lower limit for a main sequence star is ______________ solar masses.

0.08

The luminosity (L) of a star is the total energy, the star radiates in _______________.

1 second

List two triggers that could start the gravitational contraction of a molecular cloud and subsequent star formation.

1) shock waves 2) collision of molecular clouds

Before a giant molecular cloud can contract gravitationally, it must overcome which four physical factors that resist compression?

1) thermal energy of the gas 2) magnetic field that fills the ISM 3) rotation in the Universe 4) interstellar medium turbulence

A star has a parallax angle of 0.019 arc seconds. How far away is the star from earth?

52.6 pc

Explain the process that creates a nova.

A nova occurs when a white dwarf, the dense core of a once-normal star, "steals" gas from its nearby companion star. When enough gas builts up on the surface of the white dwarf it triggers an explosion.

What is the helium flash?

An explosion that occurs if the gas in the helium core becomes degenerate, due to the pressure-temperature thermostat that controls the reaction rate no longer working.

List the two basic types of intrinsic variable stars

Cepheid, RR Lyrae

Briefly, discuss the gas-stars-gas cycle.

During a starʹs lifetime, it fuses hydrogen into helium and helium into carbon. If it is more massive, it also creates heavier elements, up to iron, by fusion, and even heavier elements, up to uranium, during a supernova event. When the star ends its life as a planetary nebula or in a supernova explosion, it disperses these elements into the ISM. The ISM thus gains these heavier elements and has a smaller proportion of hydrogen but a larger proportion of heavier elements than it did before. The ISM cools, forms molecular clouds, and then forms new stars, which are made out of the material of the ISM.

List the three types of nebulae in the interstellar medium

Emission, reflection, dark

T or F: The inter-cloud medium is believed to make up 90% of the mass of the interstellar medium.

False

T or F: The layer above the chromosphere is the photosphere.

False

T or F: the most accurate way to determine the surface temperature of a star is to study the pattern of emission lines from various atoms.

False

What occurs during nuclear fusion within the Sun, and how much energy is produced?

H nuclei fuse to produce He nuclei, releasing energy; 0.7% of the mass of each reaction is released as energy.

If hydrogen (H2) gas is the most common molecule found within the interstellar medium, then why do astronomers measure the amount of carbon monoxide (CO) gas to map molecular clouds?

Hydrogen (H2) is the most common molecule in the Universe, but it does not radiate strongly in the radio part of the electromagnetic spectrum. Conversely, carbon monoxide (C) is a much less common molecule in the Universe, but it is a very efficient radiator of radio energy. Radio astronomers measure the amount of CO and use it to infer H2 concentrations to help map a molecular cloud.

Describe Type II supernovae

Massive star develops an iron core and collapses; observed near star-forming regions. Have H emission lines.

Describe Type Ib supernovae.

Massive star loses outer envelope of H (super winds, or to binary companion) but then continues to evolve, develops iron core, and then collapses. No H emission lines (because H envelope gone).

Stellar spectra started with the amount of hydrogen found in the stars spectrum and started with the letters A through Q. What are the current letters used to classify stellar spectra?

O,B,A,F,G,K,M

Distinguish between pressure and density. How are they related to each other?

Pressure is the measure of force acting on a unit area. Density is the measure of how closely any given entity is packed.

What is the difference between the proton-proton chain and the CNO cycle?

Proton-Proton chain (low mass stars) uses only a high-density and high temperature hydrogen gas to fuse into helium. CNO Cycle (large mass stars) uses carbon, nitrogen, and oxygen in a much more complicated process of converting hydrogen into helium.

Why do sunspots occur darker than the rest of the Sun's surface?

Sunspots are cooler than photosphere, so look dark by comparison.

What is a planetary nebula?

The expelled outer atmosphere (usually in repeated surges) of an aging red giant star. Cause: when the star cools, its atmosphere becomes more opaque, light has to "push" against it to escape; at the same time, the helium-fusion shell becomes narrow and unstable, causing it to flare and also push the atmosphere outward. Has nothing to do with a planet.

What observations demonstrate the existence of an interstellar medium?

The interstellar medium can be observed with other wavelengths such as infrared, and can be observed with interstellar absorption lines to determine its chemical composition

What is the Chandrasekhar limit?

The upper limit to the mass of a white dwarf (approx. 1.4 solar masses).

T or F: Astronomers determine the internal structure of a star by using stellar models, since they cannot "see" into the interior of a star.

True

T or F: Convection is important in stars because it transports energy outward in the star.

True

T or F: Granules on the Sun's photosphere can be several thousand kilometers across.

True

T or F: HII regions are also called emission nebulae.

True

T or F: If two stars are emitting, the same amount of light, the star that is farther will appear dimmer.

True

T or F: Low-mass stars like the Sun transport energy near their surface using convection.

True

T or F: Sunspots are parts of active social regions dominated by magnetic fields that encompass all layers of the Sun's atmosphere.

True

T or F: Sunspots tend to occur in groups, or pairs, with the magnetic field around the pair resembling that around a bar magnet.

True

T or F: The Hertzsprung-Russell diagram is a plot for stars, using absolute magnitude versus temperature

True

T or F: all the elements heavier than helium on the periodic table, including the ones in your body, were created in the course of massive stars.

True

T or F: the age of a star cluster can be approximated by finding its turn off point in the distribution of data points that represent stars on the H-R diagram.

True

How does a gas cloud become hot enough to ignite nuclear reactions?

Under the continual collapse of the gas cloud due to gravity, the gas will increase in temperature. The gas in the cloud will be in free-fall and the gravitational potential energy will convert to heat. Nuclear fusion of H into He begins once it reaches 4 million Kelvin.

Why do Cepheids pulsate?

When a Cepheid is compressed, it becomes opaque. Photons are trapped inside, heating the gas and increasing its pressure. The high-pressure gas expands, becoming transparent. Photons escape, the gas cools, the pressure drops. As the pressure drops, the Cepheid is compressed by gravity.

Describe the process that causes a massive star to become a supernova.

When a massive star develops an iron core, nuclear fusion can no longer occur, and the core contracts, and grows hotter. The shells around the core burn, outward, leaving behind more iron, and further increasing the mass of the core. When the mass of the core exceeds the Chandrasekhar limit of about 1.4 solar masses, it collapses in a star-destroying explosion.

Describe Type Ia supernovae.

White dwarf exceeds Chandrasekhar limit and begins to collapse, ether because it gained mass from a companion star or two white dwarfs merged; carbon-oxygen core fuses because pressure-temperature thermostat not working ("carbon deflagration") -> white dwarf is destroyed. No H emission lines.

What forms of electromagnetic radiation from the Sun's emissions do we experience here on Earth? Illustrate how we know this to be the case with one practical example for each form.

X-ray & UV photons can interfere with radio communications; particles from flares can distort Earth's magnetic field & disrupt navigation systems; flares can cause surges in electrical power lines and damage to Earth's satellites.

Remember, Kepler's third law, which relates the period of revolution to the average distance to a planet. How do we modify that law to use of binary stars? a) Add the total mass of the two stars. b) Add the difference of their masses c) Add newtons law of gravitation d) Add the distance between the two stars e) Add Einstein's law of relativity

a) Add the total mass of the two stars

The most common type of star is a(n) ____________________ a) lower main sequence star b) Super giant c) giant d) White dwarf e) upper main sequence star

a) lower main sequence star

What's the four components of the interstellar medium - molecular clouds, intercloud medium, coronal, gas, H 1 cloud - with their respective description. You may use the choices more than once. a) clouds of gas and dust with density is ranging from hundreds to thousands of particles per cubic centimeter, found deep inside the ISM, where it is protected from UV photons b) the hot space of ISMs with the temperature of a few thousand K and the density of only 0.1 atom per cubic centimeter found between the cool H1 clouds c) the part of the ISM that can only be detected with x-ray telescopes that have temperatures of 10^6 K or higher d) ASM clouds of cool gas of only about 100 K and dust with densities ranging up to 100 atoms per cm³, and observed only at infrared and radio wavelengths e) a chaotic shape that is usually twisted, flattened, and tangled. f) highly ionized state g) the coolest temperature

a) molecular clouds b) inter cloud medium c) coronal gas d) H1 cloud e) H1 cloud f) coronal gas g) molecular clouds

Cepheid variables can be used to determine distances because their _____________ can be determined from their _________________.

absolute magnitude, period

Matter swirling into a compact object forms a(n) __________________ disk.

accretion

How did the suns vertical position on the HR diagram and its radius compared to those of giant stars? a) the sun is higher on the HR diagram and has a smaller radius b) the sun is lower on the HR diagram and has a smaller radius c) the sun is lower on the HR diagram and has a larger radius d) the sun is higher on the HR diagram and has a larger radius

b) the sun is lower on the HR diagram and has a smaller radius

Match the following events or labels to the given H-R diagram. The figure represents the pre main-sequence evolution of a low mass star. A, B and C refer to the axes of the diagram. Use one letter for each question.

birth line - I radius - C main sequence stars - H protostar - D brightness - A fusion of H into He - G temperature - B giants - E supergiants - J white dwarfs - F

Why are the hydrogen bomb are lines of spectral type O stars, weaker than those of the sun? a) type O stars contain less hydrogen than the sun b) the strength of Balmer lines is inversely proportional to surface temperature c) most of the hydrogen in spectral type O stars' outer layers is ionized d) type of stars are cooler than the sun

c) most of the hydrogen in spectral type O stars' outer layers is ionized

How many nuclear reactions occur in a proton-proton chain? a) four b) one c) three d) five e) two

c) three

High mass stars evolve differently than medium mass stars because they are able to fuse _______________.

carbon

Energy flows by radiation or convection inside stars but almost never by ________________ because radiation or convection is much more efficient.

conduction

Which component of the interstellar medium has the greatest temperature?

coronal gas

The images produced by ______________ reveal streamers in the corona that follow magnetic lines of force in the Sun's magnetic field.

coronographs

The emission lines from 0 III are called _______________, because they are not visible in laboratories on earth due to the much greater pressure of the gas on earth. a) "fathom" lines b) unresolved lines c) unseen lines d) forbidden lines e) unique lines

d) forbidden lines

When a gas is so dense that most of its electrons are not free to change their energies, it is called ___________________.

degenerate matter

The corona can reach temperatures of up to _______. a) 10,000K b) 5800 K c) 500,000 K d) 1,000,000 K e) 2,000,000 K

e) 2,000,000 K

What causes granulation on the photosphere? a) bursts of energy b) differential rotation c) solar cycle d) magnetic fields e) convection

e) convection

H II emission nebula regions are found around _________________.

giant molecular cloud (or) high temperature stars

The two elements a one-solar-mass star uses over the course of its entire evolution are ___________ and ______________.

hydrogen; helium

When a large gas cloud starts to collapse in a free-fall due to gravity, the temperature of the gas will __________________.

increase

The __________________ is a high temperature, low density region in the interstellar, medium around the sun.

local bubble

When the sun becomes a white dwarf, its interior will be composed of mostly carbon and _________________ nuclei.

oxygen

Many medium-mass stars will expel their outer atmospheres to form a ________________ nebula as they age.

planetary

The instability strip on the H-R diagram is the location for what type of stars?

pulsating stars, Cepheid variables, RR Lyrae variables

The least common stars are _______________ stars.

upper main sequence

The interstellar reddening does not shift the ________________ of spectral lines, as does the Doppler effect.

wavelength

Hydrostatic equilibrium is the balance between ___________ and radiation _________________.

weight, pressure