Astronomy Review Flashcards: Solar System, Stars, Galaxies, and Cosmology

A comprehensive set of practice questions (Question and Answer style) covering Solar System basics, planetary properties, comets, meteors, moons, the Sun, stellar physics, galaxies, cosmology, and related observational methods.

Which planets are visible to the unaided eye, and which additional planet is technically visible under perfect conditions?

Mercury, Venus, Mars, Jupiter, and Saturn are visible; Uranus is technically visible under perfect conditions but very faint without a telescope.

How can you distinguish planets from stars with the unaided eye?

Planets shine with steady light and appear to move relative to background stars over time; stars twinkle.

How do planets appear under a telescope compared with stars?

Planets appear as small disks; stars remain point-like sources of light.

What are the largest and smallest planets in the solar system, and which is nearest the Sun?

Largest: Jupiter; Smallest: Mercury; Nearest the Sun: Mercury.

Why do planets shine in the sky?

Planets reflect sunlight; they do not produce their own light.

Which planets have no satellites?

Mercury and Venus.

Where is the bulk of the solar system’s mass concentrated?

In the Sun; the Sun accounts for over 99.8% of the total mass.

On which planets would a person weigh less than on Earth, and on which would they weigh more?

Weigh less: Mars and Mercury. Weigh more: Jupiter, Saturn, Uranus, and Neptune (Venus similar to Earth).

If you were on Mars watching Earth with a telescope, what changes would you see in Earth’s appearance as it moves in its orbit?

Earth would go through phases, changing from crescent to gibbous to full as seen from Mars.

Why do planets shine but stars do not when viewed from Earth?

Planets reflect sunlight; stars emit their own light and appear as points with twinkling due to the atmosphere.

Do any planets have satellites (moons)? Which do not?

Mercury and Venus have no satellites.

Explain why a person would weigh less or more on certain planets compared to Earth.

Weigh less on Mars or Mercury due to weaker gravity; weigh more on Jupiter, Saturn, Uranus, and Neptune due to stronger gravity.

Why might Venus’ weight on its surface be almost the same as Earth’s?

Because Venus has a gravitational pull very similar to Earth’s.

What would you observe about Earth’s appearance from Mars as Earth orbits the Sun?

Earth would show phases, similar to the Moon, with varying illuminated portions.

How do comet orbits differ from planetary orbits, and why do comets have tails near the Sun?

Comet orbits are highly elliptical, unlike the nearly circular planetary orbits; tails form because sunlight sublimates icy material and solar wind pushes gas/dust away from the Sun.

Why do comet tails always point away from the Sun?

The solar wind and radiation pressure push the tail material away from the Sun.

What does the presence of stars through a comet’s head and tail imply about comets near Earth?

It implies comets are diffuse, not solid; Earth would encounter a cloud of dust/gas rather than a solid body.

Why are most meteorites found in Antarctica?

High visibility on white ice, ice flow concentrates meteorites in blue ice areas, and cold, dry conditions preserve them.

What is the Perseid meteor shower, and do the meteoroids have exactly 1-year orbits?

Perseid meteor shower occurs when Earth passes through Swift-Tuttle’s debris; meteoroids have a wide range of orbital periods, not exactly 1 year.

Why are iron meteorites overrepresented in museums compared to stony meteorites?

Iron meteorites are more durable and easier to identify and preserve; stony meteorites are more common but weathered.

If Earth had no atmosphere, would comets and meteors be visible?

Comets would still be visible (reflect sunlight), but meteors would not because they rely on atmospheric entry to produce light.

Why is there little evidence of ancient craters on Venus and what might explain resurfacing?

Lack of ancient craters is due to resurfacing from volcanism that renewed the planet’s surface about 300–600 million years ago.

List two reasons why Venus is hotter than Earth.

Proximity to the Sun and a runaway greenhouse effect from a dense CO2 atmosphere.

Why is Venus brighter in the sky as a crescent than as a full disk?

When crescent, Venus is closer to Earth, and its proximity increases apparent brightness despite a small illuminated area.

What explains why Mercury and Venus show phases like the Moon?

They have orbits inside Earth’s orbit, so the illuminated portion visible from Earth changes.

Why is Mars’ atmosphere thin, and how does this relate to meteoroid impact effects?

Mars has a very thin atmosphere, offering little protection from meteoroids, so impacts are more likely.

What is the distinction between asteroids and meteoroids, and where do asteroids mostly originate?

Asteroids are rocky bodies orbiting the Sun, typically larger; meteoroids are smaller fragments. Asteroids are remnants of early solar system, often due to Jupiter’s gravity.

Why are most asteroids not spherical?

They are not massive enough for gravity to pull them into a sphere; only the largest, like Ceres, are spherical.

What evidence supports that a large asteroid collision caused Earth’s dinosaur extinction?

The Chicxulub crater in Mexico is widely believed to be the impact site of the asteroid that caused the extinction about 66 million years ago.

What is the basic composition and structure of Jupiter, and why is it not rock with a solid core?

Jupiter’s low density (~1.33 g/cm3) indicates it is mainly hydrogen and helium, not rock and iron.

What is the nature of the Great Red Spot on Jupiter?

A persistent, high-pressure storm—like a huge hurricane or cyclone, long-lasting.

How do the main similarities and differences between Jupiter and Saturn compare?

Similarities: both gas giants, hydrogen/helium, many satellites, rings, fast rotation. Differences: Saturn is less massive and less dense and has a more prominent ring system.

What is Titan’s atmosphere like compared with other planetary satellites? Do other satellites have atmospheres?

Titan has a substantial atmosphere; several other satellites have atmospheres (e.g., Io has a thin atmosphere of sulfur dioxide) but none as dense as Titan’s.

Which planets besides Saturn have rings and what are these rings made of?

Jupiter, Uranus, and Neptune also have rings; rings are made of small particles of ice and rock, not solid sheets.

What are the chief constituents of the giant planets and how do Uranus and Neptune differ from Jupiter/Saturn?

Jupiter and Saturn: mainly hydrogen and helium. Uranus and Neptune are ice giants with water, ammonia, methane ices, plus hydrogen and helium.

What is the main difference between a planet and a dwarf planet?

A planet clears its neighborhood of other objects in its orbit; a dwarf planet has not cleared its orbit.

Do some dwarf planets have satellites? Give an example.

Yes. Pluto, a dwarf planet, has five known satellites including Charon.

How does Pluto compare in size to the Moon and to other planetary satellites?

Pluto is smaller than Earth’s Moon and smaller than several planetary satellites (e.g., Ganymede, Callisto, Io, Titan).

Why do we always see the same hemisphere of the Moon, and what is this phenomenon called?

Because the Moon is tidally locked to Earth; the same hemisphere always faces Earth.

What is wrong with the idea that the Moon provides illumination at night as the main source of light?

The Moon’s light is reflected sunlight, not its own light.

When does the Moon rise in relation to its phase, and how long is a lunar day?

A full Moon rises at sunset; a day on the Moon (sunrise to sunrise) lasts about two Earth weeks, as does the lunar night.

What is the approximate duration between New Moon and Full Moon?

About two weeks (half of the Moon’s 29.5-day cycle).

Which Moon feature is the largest, and what are the lunar maria?

Ganymede is larger than Earth’s Moon; the maria are large, dark, flat plains formed by ancient lava flows.

How do eclipses occur, and why don’t they happen every month?

Eclipses occur when the Moon’s orbit crosses the ecliptic plane during a new moon (solar) or full moon (lunar). They don’t happen every month because the Moon’s orbit is tilted about 5 degrees relative to the ecliptic.

What phase must the Moon be in for a solar eclipse? For a lunar eclipse?

Solar eclipse: new Moon. Lunar eclipse: full Moon.

What does the Moon’s interior suggest about its temperatures and tectonic activity?

The Moon is cooler and more rigid; it has weak or no active plate tectonics and limited internal heating compared with Earth.

What are the Moon’s maria and what caused the Moonquakes?

Maria are dark plains of solidified lava; Moonquakes are weaker and less frequent, indicating a cooler interior.

How is the Sun’s corona observed and what are its typical temperatures compared to the photosphere?

The corona is visible during total solar eclipses or with coronagraphs; its temperature exceeds 1,000,000 K, while the photosphere is ~5780 K.

Why is the photosphere the main source of the Sun’s radiation despite the hotter corona?

The photosphere is much denser, so it emits far more energy overall than the tenuous, hotter corona.

What is the basic process powering the Sun and where does fusion occur?

Hydrogen fuses into helium in the core, powering the Sun through nuclear fusion.

What is the approximate temperature of the Sun’s core, and why can’t the Sun have a solid core?

Core temperature about 15 million K; the pressure and temperature prevent a solid core from existing.

What is Proxima Centauri’s distance from the Sun?

About 4.24 light-years.

What is the significance of Cepheid variables in distance measurement?

Cepheid variables have a period-luminosity relationship that allows determination of their intrinsic luminosity and thus distance.

How can the Sun’s composition be determined?

By analyzing the Sun’s absorption lines in its spectrum and matching them to known elemental fingerprints.

What is Wien’s Law used for in stellar astronomy?

Determines a star’s surface temperature from the wavelength of its peak emission.

What is the Stefan-Boltzmann law and how is it used with stars?

L = 4πR^2σT^4; it relates a star’s luminosity to its radius and surface temperature, allowing diameter estimates from L and T.

What primarily causes sunspots to appear darker than surrounding areas?

Sunspots are cooler regions on the Sun’s surface, making them appear darker despite being very hot.

What causes the Sun’s 11-year cycle?

Acycle in the number and size of sunspots, tied to magnetic activity reversals.

What is the Sun’s approximate surface temperature and core temperature?

Photosphere ~5780 K; core ~15,000,000 K.

What evidence supports Dark Energy’s existence and what fraction of the universe is thought to be dark energy?

Type Ia supernova observations suggest accelerating expansion; dark energy is about 68% of the universe’s total energy content.

What is Hubble’s law and what does it tell us about the universe?

v = H0 d; distant galaxies recede from us, indicating the universe is expanding.

What is the cosmic microwave background (CMB) and why is it important?

The leftover radiation from the early universe, a key evidence for the Big Bang; now observed as microwaves with slight anisotropies.

What is the difference between Population I and Population II stars?

Population I: younger, richer in heavy elements, found in the galactic disk; Population II: older, metal-poor, found in the halo and globular clusters.

Where is most interstellar gas located in the Milky Way and what is its main constituent?

Located in the thin disk, concentrated in the spiral arms; chiefly hydrogen (atomic and molecular forms).

What are the main kinds of radio-wave origins from space?

Thermal radiation from hot gases; synchrotron radiation from high-speed electrons in magnetic fields; molecular line emission from cool gas clouds.

What is the basic layout and scale of galaxies in the universe?

Galaxies are clumped into clusters and superclusters, with filaments and voids; they are not randomly distributed.

What evidence suggests dark matter exists in galaxies?

Measured rotation curves show outer stars moving faster than visible matter can explain, implying unseen mass.

What is the approximate age of the universe and Earth?

Universe ~13.8 billion years old; Earth ~4.6 billion years old.

What is the fate of very massive stars after the main sequence?

They become giants/supergiants and end their lives in a supernova; remnants may be neutron stars or black holes.

What causes a black hole to be black, and how can one be detected indirectly?

A black hole’s gravity is so strong that light cannot escape; detected via effects on nearby matter or X-ray emission from accretion disks.

What is an exoplanet and why is detecting them challenging?

Planets orbiting other stars; they are small and faint compared to their host stars, making detection difficult.

How did the planets likely form relative to the Sun?

The planets formed from a protoplanetary disk around the young Sun during solar system formation.

What is the main reason Earth’s oceans will not be present in a few billion years and what phase will Earth enter next?

As the Sun grows hotter and more luminous, Earth’s oceans will boil away; Earth’s future phase is expected to be increasingly hostile to life, possibly ending with a hot, inhospitable environment.

What are the three ways radio waves from space originate, as per the notes?

Thermal radiation, synchrotron radiation, and molecular line emission.

What characteristic makes Cepheid variables useful as standard candles?

Their period of pulsation correlates with intrinsic luminosity, allowing distance estimation.

What is the relationship between a star’s mass, temperature, and luminosity on the main sequence?

Higher mass stars are hotter and more luminous; they burn through fuel faster and have shorter lifespans.

Why is the Sun considered a typical star and what distinguishes some stars from being suitable long-term inhabitability candidates?

The Sun is a typical G-type main-sequence star; many stars share similar properties, but factors like mass, luminosity, and stability influence habitability potential.

Explain the concept of a black dwarf as it relates to stellar evolution.

A white dwarf that has cooled over a very long time; after cooling, it would become a black dwarf.

What is the main implication of the 3:2 spin-orbit resonance observed on Mercury?

It causes Mercury’s solar day to be much longer than its orbital period, resulting in unique day-night cycles.

What do the terms ‘red shift’ and ‘blue shift’ indicate in astronomy?

Red shift indicates objects moving away; blue shift indicates objects moving toward us; used to measure velocity along the line of sight.

What is the significance of tidal locking in the Moon’s phases and appearances?

Tidal locking causes the same lunar hemisphere to always face Earth, leading to consistent phases from our viewpoint.

What is the primary source of the Sun’s energy, and what is the basic nuclear reaction involved?

Hydrogen fusion in the core produces helium, releasing energy that powers the Sun.

What role do planetary rings play in a planet’s system, and which planets have rings?

Rings are made of small particles; Jupiter, Saturn, Uranus, and Neptune have rings; Saturn’s rings are the most prominent.

What is the current understanding of dark energy and its role in the universe’s expansion?

Dark energy is a mysterious form of energy driving the accelerated expansion of the universe; its exact nature is unknown.

How is the distance to a star cluster determined using its Cepheid variables?

By measuring the period of the Cepheids to determine their luminosity, then comparing to their apparent brightness to infer distance.

What is the difference between a binary star’s spectrum and its actual visual resolution?

Even if two stars can’t be visually resolved as separate without a telescope, their binary nature can be inferred from periodic Doppler shifts in their spectral lines.

What is the mass-to-radius trend for neutron stars and white dwarfs, and what does that imply about their densities?

Neutron stars and white dwarfs have very high densities due to extreme compression, with neutron stars being the densest known objects outside black holes.