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OpenStem
@openstem · Joined Jul 2026
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Flashcards8 cards
What is the leading scientific explanation for how the Solar System formed?1 / 8
The nebular hypothesis — the Sun and planets formed together from a single collapsing cloud of gas and dust.
Astronomy · L2 · How the Solar System Formed
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Astronomy · L2 · How the Solar System FormedFlashcards7 cards
Why do astronomers put some telescopes in space instead of just building bigger ones on Earth?1 / 7
Earth's atmosphere blurs starlight and blocks many wavelengths, so a telescope above it gets a clearer, more complete view.
Astronomy · L2 · Space Telescopes
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Astronomy · L2 · Space TelescopesFlashcards8 cards
What are the two parts of a shadow cast in space?1 / 8
The umbra, the fully dark inner shadow, and the penumbra, the partially lit outer shadow.
Astronomy · L2 · Eclipse Mechanics: Umbra, Penumbra, and Types
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Astronomy · L2 · Eclipse Mechanics: Umbra, Penumbra, and TypesFlashcards7 cards
Where is the asteroid belt located?1 / 7
Between the orbits of Mars and Jupiter.
Astronomy · L2 · The Asteroid Belt and Kuiper Belt
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Astronomy · L2 · The Asteroid Belt and Kuiper BeltFlashcards9 cards
State Kepler's First Law.1 / 9
Every planet's orbit is an ellipse, with the Sun at one of the two foci — not at the centre of the ellipse.
Astronomy · L3 · Kepler's Laws & Orbital Mechanics
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Astronomy · L3 · Kepler's Laws & Orbital MechanicsFlashcards8 cards
State Newton's law of universal gravitation.1 / 8
Every mass attracts every other mass with a force proportional to the product of their masses and inversely proportional to the square of the distance between them:
Astronomy · L3 · Newtonian Gravitation
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Astronomy · L3 · Newtonian GravitationFlashcards8 cards
What does a star's spectral type (O, B, A, F, G, K, or M) actually measure?1 / 8
Surface temperature — O stars are the hottest (over 30,000 K) and M stars are the coolest (under 3,700 K). The letters run hottest to coolest.
Astronomy · L3 · Stellar Classification
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Astronomy · L3 · Stellar ClassificationFlashcards9 cards
What is a protostar?1 / 9
A dense, collapsing clump of gas and dust within a molecular cloud, heating up under its own gravity, but not yet hot enough at its core to sustain hydrogen fusion.
Astronomy · L3 · Stellar Evolution
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Astronomy · L3 · Stellar EvolutionFlashcards7 cards
What does the Big Bang model describe?1 / 7
That the observable Universe began roughly 13.8 billion years ago from an extremely hot, dense state, and has been expanding and cooling ever since — not an explosion into pre-existing space, but an expansion of space itself.
Astronomy · L3 · The Big Bang & Cosmology
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Astronomy · L3 · The Big Bang & CosmologyFlashcards7 cards
What is dark matter?1 / 7
An unseen form of matter, inferred from its gravitational effects, that neither emits nor absorbs light — it does not interact with the electromagnetic force the way ordinary matter does.
Astronomy · L3 · Dark Matter & Dark Energy
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Astronomy · L3 · Dark Matter & Dark EnergyFlashcards7 cards
What are the three main galaxy shapes in the Hubble sequence?1 / 7
Spiral, elliptical, and irregular — with barred spirals as a common variant of the spiral type.
Astronomy · L3 · Galaxy Types & the Milky Way
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Astronomy · L3 · Galaxy Types & the Milky WayFlashcards8 cards
How does the transit method detect an exoplanet?1 / 8
By watching a star's brightness over time and looking for a small, periodic dip caused by a planet passing directly in front of it, blocking a tiny fraction of its light.
Astronomy · L3 · Exoplanet Detection
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Astronomy · L3 · Exoplanet DetectionFlashcards7 cards
List the electromagnetic spectrum bands used in astronomy, from longest to shortest wavelength.1 / 7
Radio, microwave, infrared, visible light, ultraviolet, X-ray, and gamma-ray.
Astronomy · L3 · The EM Spectrum in Astronomy
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Astronomy · L3 · The EM Spectrum in AstronomyFlashcards8 cards
What is an absorption line, and what causes it?1 / 8
A dark line at a specific wavelength in an otherwise continuous spectrum, caused by atoms in a cooler outer layer of gas absorbing light of exactly the wavelengths their electrons can use to jump to a higher energy level.
Astronomy · L3 · Spectroscopy & Redshift
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Astronomy · L3 · Spectroscopy & RedshiftFlashcards7 cards
What is a neutron star made of?1 / 7
Almost entirely neutrons, packed together at nuclear densities — the collapsed core of a massive star, left over after a supernova, supported against further collapse by neutron degeneracy pressure.
Astronomy · L3 · Black Holes & Neutron Stars
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Astronomy · L3 · Black Holes & Neutron StarsFlashcards8 cards
What is the habitable zone of a star?1 / 8
The range of orbital distances from a star where a planet's temperature could allow liquid water to exist on its surface — not so close that water boils away, not so far that it freezes solid.
Astronomy · L3 · Habitable Zones & the Search for Life
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Astronomy · L3 · Habitable Zones & the Search for LifeFlashcards7 cards
What is stellar parallax?1 / 7
The tiny apparent shift in a nearby star's position against distant background stars, seen from two different points in Earth's orbit around the Sun (typically six months apart).
Astronomy · L3 · The Cosmic Distance Ladder
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Astronomy · L3 · The Cosmic Distance LadderFlashcards8 cards
What is the magnetosphere?1 / 8
The region of space around Earth dominated by Earth's magnetic field, shaped into a teardrop by the solar wind: compressed on the sunward side, stretched into a long magnetotail on the night side.
Astronomy · L3 · Aurora & Magnetosphere Physics
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Astronomy · L3 · Aurora & Magnetosphere PhysicsFlashcards8 cards
What is the giant-impact hypothesis?1 / 8
The leading explanation for the Moon's origin: about 4.5 billion years ago, a Mars-sized protoplanet collided with the young Earth at a grazing angle, ejecting debris that later accreted into the Moon.
Astronomy · L3 · Moon Formation & the Giant Impact
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Astronomy · L3 · Moon Formation & the Giant ImpactFlashcards8 cards
What is an open cluster?1 / 8
A loosely bound group of a few hundred to a few thousand stars, formed together from the same giant molecular cloud, found in the disk of the galaxy — for example, the Pleiades.
Astronomy · L3 · Star Clusters & Stellar Populations
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Astronomy · L3 · Star Clusters & Stellar PopulationsFlashcards8 cards
What are cosmic rays?1 / 8
Extremely high-energy charged particles — mostly protons, with some heavier atomic nuclei and electrons — travelling through space at speeds close to the speed of light.
Astronomy · L3 · Cosmic Rays & High-Energy Astrophysics
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Astronomy · L3 · Cosmic Rays & High-Energy AstrophysicsFlashcards8 cards
What causes gravitational lensing?1 / 8
Mass curves the spacetime around it, as described by general relativity, so light passing near a massive object follows a curved path — bending the apparent position, and sometimes the shape, of anything behind it.
Astronomy · L3 · Gravitational Lensing
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Astronomy · L3 · Gravitational LensingFlashcards8 cards
What are the three stages of impact crater formation?1 / 8
Contact and compression, excavation, and modification — the impactor first slams in and sends out shock waves, those shock waves excavate a transient crater, and the crater walls then collapse or rebound into their final shape.
Astronomy · L3 · Asteroid Impacts & Craters
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Astronomy · L3 · Asteroid Impacts & CratersFlashcards8 cards
What was Ptolemy's geocentric model, and how did it explain retrograde motion?1 / 8
A model, dominant for roughly 1400 years, in which Earth sits motionless at the centre of the universe and everything else orbits it. To explain planets occasionally appearing to move backward (retrograde motion), Ptolemy added epicycles — small circular loops layered on top of the main orbital circle.
Astronomy · L3 · History of Astronomy: Paradigm Shifts
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Astronomy · L3 · History of Astronomy: Paradigm ShiftsFlashcards8 cards
What is a 'hot Jupiter'?1 / 8
A gas giant comparable in mass to Jupiter or larger, but orbiting its star in only a few days — far closer than Mercury orbits the Sun, and hot enough that its outer atmosphere can reach thousands of kelvin.
Astronomy · L3 · Exosolar System Architecture
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Astronomy · L3 · Exosolar System ArchitectureFlashcards8 cards
Why does a collapsing molecular cloud core spin faster as it shrinks?1 / 8
Conservation of angular momentum. Even a slowly rotating cloud has some angular momentum L = I ω; as the cloud's radius shrinks, its moment of inertia I decreases, so ω must increase to keep L constant.
Astronomy · L3 · Solar System Formation: The Nebular Hypothesis
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Astronomy · L3 · Solar System Formation: The Nebular HypothesisFlashcards8 cards
What are right ascension and declination, and what do they measure?1 / 8
The two coordinates of the equatorial system, used to fix a position on the celestial sphere the way longitude and latitude fix a position on the ground. Right ascension measures east-west position around the celestial equator; declination measures north-south position above or below it.
Astronomy · L3 · Celestial Coordinates & Precession
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Astronomy · L3 · Celestial Coordinates & PrecessionFlashcards8 cards
What is a giant molecular cloud, and why is it the raw material for star formation?1 / 8
A giant molecular cloud is a vast, cold (roughly 10-30 K), dense region of interstellar gas and dust, made mostly of molecular hydrogen. It is cold and dense enough that gravity can eventually overcome the gas's internal pressure, making it the only kind of interstellar cloud dense and cold enough to collapse and form stars.
Astronomy · L3 · Star Formation & Molecular Clouds
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Astronomy · L3 · Star Formation & Molecular CloudsFlashcards8 cards
What are planetary rings actually made of?1 / 8
Not a solid sheet, but countless individual particles — ranging from dust grains to boulder-sized chunks, mostly water ice mixed with some rock — all independently orbiting the planet in roughly the same plane.
Astronomy · L3 · Planetary Ring & Moon Systems
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Astronomy · L3 · Planetary Ring & Moon SystemsFlashcards8 cards
What is a nebula, in the most general sense?1 / 8
An interstellar cloud of gas and dust. What distinguishes the different types is not the material itself but how — or whether — that material produces visible light.
Astronomy · L3 · Nebula Types
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Astronomy · L3 · Nebula Types