I85PREP
⌘ K
AST study materials
AST study resources

AST practice questions

One question at a time. Build your AST understanding.

First 50 of 200 questions. Each one lists the choices, the correct answer, and an explanation. The checker at the top of this page saves progress in this browser when it is available.

Continue in the practice tool above to load more questions, 50 at a time.

Question 1Astrophysics and Cosmology

Which quantity is conserved in a closed system undergoing any physical process?

  1. Total energy Correct answer.
  2. Total temperature
  3. Total volume
  4. Total entropy

Explanation

The first law of thermodynamics states that energy cannot be created or destroyed, only converted between forms, so the total energy of a closed system stays constant. Entropy, temperature and volume can all change during a process, so none of them is a conserved quantity in general.

Question 2Astrophysics and Cosmology

What is the primary energy source that powers the Sun's luminosity over its main-sequence lifetime?

  1. Gravitational contraction of the core
  2. Radioactive decay of heavy elements
  3. Chemical burning of hydrogen and oxygen
  4. Nuclear fusion of hydrogen into helium Correct answer.

Explanation

Main-sequence stars generate energy through core hydrogen fusion, chiefly the proton-proton chain in the Sun, converting mass into energy according to E = mc^2. Gravitational contraction and chemical burning supply far too little energy for the required timescale, and radioactive decay is negligible in comparison.

Question 3Astrophysics and Cosmology

A star's parallax is measured as 0.05 arcseconds. What is its approximate distance?

  1. 20 parsecs Correct answer.
  2. 5 parsecs
  3. 0.05 parsecs
  4. 50 parsecs

Explanation

Distance in parsecs equals the reciprocal of the parallax in arcseconds, so d = 1/0.05 = 20 parsecs. This reciprocal relation follows directly from the definition of the parsec as the distance at which one astronomical unit subtends one arcsecond.

Question 4Astrophysics and Cosmology

Which spectral class corresponds to the hottest normal main-sequence stars?

  1. Class O Correct answer.
  2. Class M
  3. Class K
  4. Class G

Explanation

The spectral sequence runs from hottest to coolest as O, B, A, F, G, K, M, so class O stars have surface temperatures above roughly 30,000 K. Class M stars are the coolest, with temperatures of only a few thousand kelvin.

Question 5Astrophysics and Cosmology

According to Hubble's law, what does a large recession velocity of a galaxy primarily indicate?

  1. A strong gravitational lens
  2. A large distance from the observer Correct answer.
  3. A large mass of the galaxy
  4. A young stellar population

Explanation

Hubble's law states that recession velocity is proportional to distance, v = H0 d, so larger velocities correspond to greater distances. The relation reflects the expansion of space rather than any property of the galaxy's mass or stellar content.

Question 6Astrophysics and Cosmology

Which process best explains why massive stars end their lives as supernovae?

  1. Sudden exhaustion of hydrogen in the envelope
  2. Collapse of an iron core that can no longer release fusion energy Correct answer.
  3. Slow mass loss through a stellar wind
  4. Fission of heavy nuclei in the core

Explanation

In massive stars, fusion proceeds until an inert iron core forms; further fusion would absorb energy rather than release it. Without pressure support from energy generation, the core collapses and rebounds, driving a core-collapse supernova.

Question 7Astrophysics and Cosmology

What distinguishes a galaxy's rotation curve from the pattern expected if all its mass were visible matter?

  1. It falls off rapidly beyond the visible disk
  2. It is identical to the expected curve
  3. It rises steeply as the square of radius
  4. It stays roughly flat at large radii Correct answer.

Explanation

Observed rotation curves remain approximately flat far beyond the visible disk, whereas Keplerian motion around the visible mass alone would decline with radius. This flatness is the classic evidence for extended dark matter halos surrounding galaxies.

Question 8Astrophysics and Cosmology

Which observation provided early evidence that the universe was once hot and dense?

  1. The redshift of nearby stars
  2. The existence of spiral arms
  3. The presence of binary star systems
  4. The cosmic microwave background radiation Correct answer.

Explanation

The cosmic microwave background is relic radiation from the epoch of recombination, when the universe cooled enough for photons to travel freely. Its near-perfect blackbody spectrum at about 2.7 K is a direct signature of a hot, dense early state.

Question 9Astrophysics and Cosmology

A star has a luminosity 100 times the Sun's and a radius 10 times the Sun's. What is its effective temperature relative to the Sun?

  1. Twice the Sun's
  2. Equal to the Sun's Correct answer.
  3. Ten times the Sun's
  4. Half the Sun's

Explanation

Using L = 4πR²σT⁴, the ratio L/Lsun = (R/Rsun)² (T/Tsun)⁴. With L/Lsun = 100 and R/Rsun = 10, the radius factor squared is 100, so the temperature ratio must equal one.

Question 10Astrophysics and Cosmology

Why do Cepheid variables serve as useful standard candles?

  1. Their brightness is constant over all time
  2. Their distances can be measured by parallax alone
  3. Their spectra show no absorption lines
  4. Their period is tightly related to their intrinsic luminosity Correct answer.

Explanation

The Leavitt period-luminosity relation links the pulsation period of a Cepheid to its intrinsic luminosity, so measuring the period yields the absolute magnitude. Comparing that with the apparent magnitude then gives the distance, which is why Cepheids anchor the extragalactic distance ladder.

Question 11Astrophysics and Cosmology

In the proton-proton chain, what is the net result of fusing hydrogen into helium?

  1. Two protons become one deuterium nucleus with no energy release
  2. One proton captures an electron to form a neutron only
  3. Four protons become one helium nucleus plus energy and neutrinos Correct answer.
  4. Three protons become one lithium nucleus

Explanation

The net reaction converts four protons into a helium-4 nucleus, releasing energy as gamma rays, positrons and electron neutrinos. The mass of the helium nucleus is slightly less than four protons, and the mass difference appears as energy.

Question 12Astrophysics and Cosmology

What does the equivalence principle in general relativity state?

  1. Light travels faster near massive bodies
  2. Gravity and acceleration are locally indistinguishable Correct answer.
  3. Mass and charge are always equal
  4. Energy and momentum are never conserved

Explanation

Einstein's equivalence principle holds that a uniform gravitational field is locally equivalent to uniform acceleration, so free-fall frames are locally inertial. This insight underlies the geometric description of gravity as spacetime curvature.

Question 13Astrophysics and Cosmology

A galaxy at redshift z = 1 is observed. Using a simple non-relativistic approximation, what recession velocity does this suggest?

  1. About 2c
  2. About 0.5c Correct answer.
  3. About 0.1c
  4. About c

Explanation

For small to moderate redshifts the approximation v ≈ cz gives v ≈ c for z = 1, but a proper relativistic treatment using the Doppler formula yields a lower value, roughly half the speed of light at z = 1. This illustrates why the linear Hubble relation must be applied with care at high redshift.

Question 14Astrophysics and Cosmology

Why does the main sequence appear as a band rather than a single line on the Hertzsprung-Russell diagram?

  1. The band is an artefact of observational selection only
  2. Stars have a range of masses and therefore a range of luminosities and temperatures Correct answer.
  3. Stars on the main sequence all have identical composition and age
  4. Main-sequence stars are all at the same distance

Explanation

Main-sequence stars are in hydrostatic equilibrium burning hydrogen, but their mass varies widely, and mass sets both luminosity and temperature. The spread of masses therefore produces a broad diagonal band rather than a single curve.

Question 15Astrophysics and Cosmology

What is the fate of a white dwarf that accretes matter beyond the Chandrasekhar limit?

  1. It ignites carbon fusion and explodes as a Type Ia supernova Correct answer.
  2. It becomes a neutron star quietly
  3. It expands into a red supergiant
  4. It collapses directly into a black hole without visible emission

Explanation

Electron degeneracy pressure supports a white dwarf up to about 1.4 solar masses. Beyond that limit, the core contracts and heats, igniting runaway carbon fusion that disrupts the star in a Type Ia supernova.

Question 16Astrophysics and Cosmology

Which quantity is used to describe the expansion rate of the universe today?

  1. The deceleration parameter
  2. The cosmological constant
  3. The critical density
  4. The Hubble constant Correct answer.

Explanation

The Hubble constant H0 is the present-day value of the expansion rate, expressed in kilometres per second per megaparsec. The cosmological constant and critical density are related parameters but do not themselves measure the current expansion rate.

Question 17Astrophysics and Cosmology

Why do astronomers infer the presence of dark matter from galaxy cluster dynamics?

  1. Cluster redshifts are always negative
  2. Cluster galaxies show no absorption lines
  3. Clusters emit too much X-ray radiation for their gas content
  4. Galaxy velocities in clusters exceed the value expected from visible mass alone Correct answer.

Explanation

Applying the virial theorem to the observed velocity dispersion of cluster galaxies yields a total mass far larger than the mass inferred from visible stars and gas. This discrepancy, first noted by Zwicky, is a key argument for dark matter on cluster scales.

Question 18Astrophysics and Cosmology

What happens to the temperature of the cosmic microwave background as the universe expands?

  1. It increases proportionally to the scale factor
  2. It remains constant
  3. It decreases inversely with the scale factor Correct answer.
  4. It oscillates periodically

Explanation

Cosmic expansion stretches the wavelength of the background photons, and a blackbody at temperature T redshifts to T proportional to 1/a, where a is the scale factor. The CMB therefore cools as the universe grows.

Question 19Astrophysics and Cosmology

Which nuclear burning stage immediately follows helium burning in the core of a massive star?

  1. Hydrogen burning
  2. Iron fusion
  3. Carbon burning Correct answer.
  4. Silicon burning

Explanation

After helium is exhausted, the core contracts and heats until carbon ignites, producing oxygen, neon and magnesium. The sequence then proceeds through neon, oxygen and silicon burning before an iron core forms.

Question 20Astrophysics and Cosmology

A star's spectrum peaks at about 300 nanometres. Using Wien's law, roughly what is its surface temperature?

  1. About 1000 K
  2. About 10,000 K Correct answer.
  3. About 100,000 K
  4. About 300 K

Explanation

Wien's law gives the peak wavelength as roughly 2.9 × 10^6 nanometre-kelvin divided by temperature. Dividing 2.9 × 10^6 by 300 nanometres gives about 9700 K, close to 10,000 K.

Question 21Astrophysics and Cosmology

Why is the cosmic microwave background so uniform across the sky, despite regions that were never in causal contact?

  1. Inflation stretched a small causally connected patch to a much larger size Correct answer.
  2. Gravitational lensing smoothed all fluctuations
  3. The universe was static before recombination
  4. The photons travelled faster than light in the early universe

Explanation

Cosmic inflation rapidly expanded a tiny, causally connected region to a size larger than the observable horizon, so the whole visible universe shares a common thermal history. This resolves the horizon problem that would otherwise make the CMB uniformity puzzling.

Question 22Astrophysics and Cosmology

Two galaxies have the same apparent magnitude, but one is a Cepheid-calibrated distance indicator at 10 Mpc and the other is a Type Ia supernova at 100 Mpc. What does this imply about their absolute magnitudes?

  1. The supernova is intrinsically about 100 times more luminous Correct answer.
  2. The Cepheid host is intrinsically more luminous
  3. The supernova is intrinsically about 10 times more luminous
  4. Both have the same absolute magnitude

Explanation

Apparent magnitude depends on both intrinsic luminosity and distance through the inverse-square law. For equal apparent brightness, a source ten times farther must be ten squared, or 100 times, more luminous intrinsically.

Question 23Astrophysics and Cosmology

In a flat universe dominated by matter, how does the scale factor evolve with cosmic time?

  1. It grows linearly with t
  2. It remains constant
  3. It grows exponentially with t
  4. It grows proportionally to t to the two-thirds power Correct answer.

Explanation

For a matter-dominated flat universe, the Friedmann equation gives a scale factor proportional to t^(2/3). Exponential growth occurs in a dark-energy-dominated era, and linear growth would require a different equation of state.

Question 24Astrophysics and Cosmology

Why does the observed abundance of light elements such as deuterium and helium-4 constrain the baryon density of the universe?

  1. They are unaffected by conditions in the early universe
  2. Their primordial yields depend sensitively on the expansion rate and baryon density during nucleosynthesis Correct answer.
  3. They are produced entirely in stellar cores today
  4. Their abundance depends only on the age of the universe

Explanation

Big Bang nucleosynthesis yields depend on the neutron-to-proton ratio and on the expansion rate at the time of nucleosynthesis, both of which are sensitive to the baryon density. Comparing predicted yields with observed primordial abundances therefore pins down the baryon density.

Question 25Astrophysics and Cosmology

A quasar at high redshift shows a Lyman-alpha absorption trough blueward of the emission line. What does this indicate?

  1. The quasar itself is moving toward the observer
  2. Intervening neutral hydrogen clouds absorb photons shortward of Lyman-alpha Correct answer.
  3. The quasar has no supermassive black hole
  4. The absorption arises from the quasar's own photosphere

Explanation

Photons with wavelengths shorter than the Lyman-alpha transition can be absorbed by intervening neutral hydrogen at various redshifts, producing a series of absorption lines that merge into a trough. This Gunn-Peterson effect traces the distribution of neutral gas along the line of sight.

Question 26Astrophysics and Cosmology

Which quantity does the Stefan-Boltzmann law relate to a star's luminosity, radius, and effective temperature?

  1. The wavelength of peak emission
  2. The total energy radiated per unit surface area each second Correct answer.
  3. The rate of nuclear fusion in the core
  4. The gravitational binding energy

Explanation

The Stefan-Boltzmann law states that the power emitted per unit area of a blackbody surface is proportional to the fourth power of its temperature. Multiplying this flux by the star's surface area gives the luminosity, so the law links luminosity, radius, and effective temperature. It says nothing directly about peak wavelength or core fusion rates.

Question 27Astrophysics and Cosmology

A star's parallax is measured as 0.02 arcseconds. What is its distance?

  1. 50 parsecs Correct answer.
  2. 0.02 parsecs
  3. 2 parsecs
  4. 500 parsecs

Explanation

Parallax distance in parsecs equals the reciprocal of the parallax angle in arcseconds, so 1 divided by 0.02 gives 50 parsecs. This definition follows directly from the geometry of Earth's orbit and the small-angle approximation.

Question 28Astrophysics and Cosmology

Which property of a star primarily determines its position along the main sequence?

  1. Its magnetic field strength
  2. Its rotation period
  3. Its distance from Earth
  4. Its initial mass Correct answer.

Explanation

A star's initial mass sets its core temperature and pressure, which in turn fix its luminosity and surface temperature while it burns hydrogen in its core. Higher-mass main-sequence stars are hotter and far more luminous, giving the main sequence its mass-ordered spread. Rotation, magnetism, and distance do not set the fundamental track.

Question 29Astrophysics and Cosmology

Which statement best describes the Hubble classification of elliptical galaxies?

  1. They are classified by the strength of their central bars
  2. They are classified by the presence of an active nucleus
  3. They are classified by apparent flattening into types E0 to E7 Correct answer.
  4. They are classified by the openness of their spiral arms

Explanation

Elliptical galaxies are sorted by how flattened they appear, running from E0 for nearly circular to E7 for highly elongated. The number encodes the ellipticity derived from the axial ratio. Spiral arm openness and bars describe spiral galaxies, not ellipticals.

Question 30Astrophysics and Cosmology

A star has an apparent magnitude of 6 and an absolute magnitude of 1. Roughly what is its distance?

  1. About 5 parsecs
  2. About 100 parsecs Correct answer.
  3. About 1000 parsecs
  4. About 10 parsecs

Explanation

The distance modulus is the difference between apparent and absolute magnitude, here 6 minus 1 equals 5. Each 5 magnitudes corresponds to a factor of 10 in distance relative to 10 parsecs, so a modulus of 5 gives 100 parsecs.

Question 31Astrophysics and Cosmology

Why does the luminosity of a star on the horizontal branch differ from that of a red giant on the red giant branch at similar temperatures?

  1. Horizontal-branch stars are always more massive than red giants
  2. Horizontal-branch stars lack any nuclear fusion and simply cool
  3. Horizontal-branch stars are fusing helium in their cores and have smaller, hotter radii for their luminosity Correct answer.
  4. Horizontal-branch stars have much larger radii and lower surface gravity

Explanation

After the helium flash, core helium burning stabilizes the star at a smaller radius and higher surface temperature for a given luminosity, placing it on the horizontal branch. Red giant branch stars have inert helium cores, larger radii, and cooler surfaces. Mass is largely unchanged between the two phases.

Question 32Astrophysics and Cosmology

A supernova in a galaxy at 20 megaparsecs has a peak apparent magnitude of 15. Using a Hubble constant of 70 km/s/Mpc, roughly what is the galaxy's recession velocity?

  1. About 200 km/s
  2. About 1400 km/s Correct answer.
  3. About 700 km/s
  4. About 14000 km/s

Explanation

Hubble's law gives recession velocity as the Hubble constant times distance, so 70 times 20 equals 1400 km/s. This linear relation holds for relatively nearby galaxies where peculiar velocities are small compared with expansion.

Question 33Astrophysics and Cosmology

What does the observed flat rotation curve of a spiral galaxy imply about its mass distribution at large radii?

  1. Enclosed mass falls off steeply beyond the visible disk
  2. Enclosed mass grows linearly with radius, indicating a dark halo Correct answer.
  3. Mass is concentrated entirely within the central bulge
  4. The galaxy is rotating as a rigid solid body

Explanation

For circular orbits, orbital speed squared is proportional to enclosed mass divided by radius, so a constant speed means enclosed mass rises in proportion to radius. This continued mass growth beyond the visible disk points to an extended dark matter halo. A Keplerian decline would instead indicate little additional mass.

Question 34Astrophysics and Cosmology

What is the key difference between Type Ia and core-collapse supernovae that makes Type Ia useful for cosmology?

  1. Type Ia produce no observable light at maximum
  2. Type Ia occur only in elliptical galaxies and never in spirals
  3. Type Ia arise from the collapse of massive star cores and vary widely in brightness
  4. Type Ia arise from thermonuclear explosions of white dwarfs and have a narrow peak luminosity range Correct answer.

Explanation

Type Ia explosions occur when a white dwarf approaches the Chandrasekhar mass, so their peak luminosities cluster within a narrow range and can be standardized further using light-curve shape. Core-collapse supernovae come from massive stars and span a much wider luminosity range. This consistency makes Type Ia events valuable distance indicators.

Question 35Astrophysics and Cosmology

Which process is primarily responsible for the production of elements heavier than iron in the universe?

  1. Rapid neutron capture in neutron-star mergers and core-collapse environments Correct answer.
  2. Hydrogen fusion in the cores of main-sequence stars
  3. Cosmic ray spallation in the interstellar medium
  4. Helium fusion in red giant stars

Explanation

Iron is near the peak of nuclear binding energy, so fusion beyond iron releases rather than produces energy. Elements heavier than iron are built mainly by neutron capture, with the rapid process operating in violent environments such as neutron-star mergers and collapsing cores. Slow neutron capture in asymptotic giant stars contributes a smaller share.

Question 36Astrophysics and Cosmology

What does the Tully-Fisher relation allow astronomers to estimate for a spiral galaxy?

  1. Its metallicity from its gas fraction
  2. Its age from its color gradient
  3. Its intrinsic luminosity from its rotation velocity Correct answer.
  4. Its central black hole mass from its bulge size

Explanation

The Tully-Fisher relation links a spiral galaxy's maximum rotation speed to its intrinsic luminosity, so measuring rotation via spectral line widths yields a distance when combined with apparent brightness. The relation reflects the connection between mass and rotation in disk galaxies. Metallicity and black hole mass involve different scaling relations.

Question 37Astrophysics and Cosmology

What does a measurement of the cosmic microwave background angular power spectrum primarily constrain?

  1. The geometry of space and the densities of baryons, dark matter, and dark energy Correct answer.
  2. The deuterium abundance in the solar photosphere
  3. The mass of the nearest supermassive black hole
  4. The rotation speed of the Milky Way's disk

Explanation

The positions and heights of the acoustic peaks in the CMB power spectrum depend on the sound horizon at recombination and on how the universe's contents alter the path of the light. Fitting the peaks therefore yields the spatial curvature and the densities of baryons, dark matter, and dark energy. Local galactic properties are unrelated to this cosmological signal.

Question 38Astrophysics and Cosmology

A distant galaxy is observed with redshift z = 3. Using the scale factor relation a = 1/(1+z), what was the scale factor when the light was emitted?

  1. 3.0
  2. 0.33
  3. 4.0
  4. 0.25 Correct answer.

Explanation

The scale factor at emission equals one divided by one plus redshift, so 1 divided by 4 gives 0.25. This means linear separations were one quarter of their present values when the light left the galaxy. Redshift itself is not the scale factor.

Question 39Astrophysics and Cosmology

Why does a star with more than about eight solar masses end in a core-collapse supernova while a lower-mass star forms a white dwarf?

  1. A massive core burns all its hydrogen before leaving the main sequence
  2. A massive core avoids nuclear fusion entirely until it collapses
  3. A massive core cannot be supported by electron degeneracy pressure and collapses after iron forms Correct answer.
  4. A massive core is supported by radiation pressure alone and never contracts

Explanation

In massive stars, successive fusion stages build an iron core whose degeneracy pressure cannot counter gravity once the core exceeds the effective Chandrasekhar limit. The core then collapses, producing a supernova and leaving a neutron star or black hole. Lower-mass stars shed their envelopes before reaching such conditions.

Question 40Astrophysics and Cosmology

In a universe containing matter and a cosmological constant, what happens to the expansion if the dark energy density remains constant while matter density falls?

  1. The expansion eventually accelerates as dark energy dominates Correct answer.
  2. The expansion reverses into a contraction once matter dilutes
  3. The expansion rate stays exactly constant at all times
  4. The expansion halts permanently when matter and dark energy are equal

Explanation

Matter density falls as the inverse cube of the scale factor, while a cosmological constant keeps a fixed energy density. As a result, dark energy eventually dominates the total energy budget, and its negative pressure drives accelerated expansion. Equality between the two marks a transition, not a stopping point.

Question 41Stellar Structure and Evolution

In the proton-proton chain, what is the net result of fusing four hydrogen nuclei into one helium-4 nucleus in the Sun's core?

  1. Conversion of helium into hydrogen with neutrino capture
  2. Release of energy with two protons converted to neutrons and positrons Correct answer.
  3. Absorption of energy with no change in particle number
  4. Production of carbon from three helium nuclei

Explanation

Four protons combine to form one helium-4 nucleus, and because helium-4 has slightly less mass than four protons, the mass difference appears as energy. Two of the protons become neutrons, emitting positrons and neutrinos in the process. This mass defect is the source of the Sun's luminosity.

Question 42Stellar Structure and Evolution

Which quantity is held constant along a line of constant luminosity in the Hertzsprung-Russell diagram?

  1. Effective temperature
  2. Total radiated power Correct answer.
  3. Radius
  4. Surface gravity

Explanation

A line of constant luminosity in the HR diagram represents stars emitting the same total power. Effective temperature varies along such a line because luminosity depends on both radius and temperature. Surface gravity and radius can differ among stars sharing the same luminosity.

Question 43Stellar Structure and Evolution

What does hydrostatic equilibrium in a star's interior require?

  1. Convection carrying all energy outward
  2. The outward pressure gradient balancing gravity at each radius Correct answer.
  3. Radiation pressure exceeding gas pressure everywhere
  4. The core temperature remaining constant with radius

Explanation

Hydrostatic equilibrium means that at every layer the outward pressure force balances the inward pull of gravity. This balance sets the pressure gradient through the star's interior. It does not require radiation pressure to dominate or convection to carry all energy.

Question 44Stellar Structure and Evolution

A star's luminosity scales with radius and effective temperature as L proportional to R squared times T to the fourth. If two stars have the same luminosity but one has twice the radius of the other, how does the larger star's effective temperature compare?

  1. It is four times as large
  2. It is twice as large
  3. It is one quarter as large
  4. It is one half as large Correct answer.

Explanation

Setting L equal for both stars gives R1 squared T1 fourth equal to R2 squared T2 fourth. With R2 equal to twice R1, the temperature ratio is the square root of one quarter, which is one half. The larger star therefore has half the effective temperature.

Question 45Stellar Structure and Evolution

Why does the main sequence appear as a band rather than a single line in the HR diagram?

  1. Stars on the main sequence have a spread of masses, ages, and compositions Correct answer.
  2. Main-sequence stars all have identical radii
  3. The main sequence contains only pre-main-sequence objects
  4. Stellar rotation removes all temperature variation

Explanation

Main-sequence stars span a wide range of masses, so they occupy a band from hot luminous to cool faint objects. Metallicity and age also shift positions slightly. The band reflects real physical diversity rather than a single fixed structure.

Question 46Stellar Structure and Evolution

What primarily determines the position of a star on the main sequence?

  1. Its distance from Earth
  2. Its initial mass Correct answer.
  3. Its rotation period
  4. Its surface magnetic field strength

Explanation

Initial mass sets a star's core temperature, luminosity, and radius, so it fixes the main-sequence position. More massive stars are hotter and far more luminous. Distance and rotation affect observed properties but not the intrinsic main-sequence location.

Question 47Stellar Structure and Evolution

In a radiative stellar interior, energy is transported outward mainly by which process?

  1. Photon diffusion through repeated absorption and re-emission Correct answer.
  2. Neutrino streaming
  3. Bulk motion of hot plasma cells
  4. Sound waves carrying heat

Explanation

In radiative zones, photons random-walk outward, being absorbed and re-emitted many times. This diffusion is slow compared with the speed of light. Bulk plasma motion describes convection, not radiative transport.

Question 48Stellar Structure and Evolution

What happens to a star's core when hydrogen fusion ceases in the core of a sun-like star?

  1. The core contracts and heats while fusion continues in a shell Correct answer.
  2. The core expands and cools immediately
  3. The core stops contracting because pressure rises
  4. The star explodes as a supernova at once

Explanation

Without core fusion, pressure support drops and the inert helium core contracts, releasing gravitational energy and heating. Hydrogen fusion continues in a shell around the core. This leads to expansion of the envelope into a red giant.

Question 49Stellar Structure and Evolution

The Chandrasekhar limit is best described as which value?

  1. The minimum mass for hydrogen fusion
  2. The maximum mass of a neutron star
  3. The maximum mass a white dwarf can support by electron degeneracy pressure Correct answer.
  4. The mass at which a star becomes a red giant

Explanation

Electron degeneracy pressure can support a white dwarf only up to roughly 1.4 solar masses. Beyond that limit, the dwarf collapses. This limit is named for Chandrasekhar.

Question 50Stellar Structure and Evolution

A star has a luminosity of 100 solar luminosities and an effective temperature equal to the Sun's. What is its radius in solar radii?

  1. 100
  2. 1000
  3. 10 Correct answer.
  4. 1

Explanation

Since temperature equals the Sun's, luminosity scales with radius squared. A luminosity 100 times solar means the radius squared is 100, so the radius is 10 solar radii. The square root of the luminosity ratio gives the radius ratio.