OpenStem

@openstem · Joined Jul 2026

7420 public items8 groups

Quiz4 questions

Why are millisecond pulsars, rather than ordinary (slower) pulsars, used as PTA clocks?

AThey are easier to detect at radio wavelengthsBTheir extraordinarily stable rotation allows timing residuals to be measured with ~100 ns precision over yearsCThey emit exclusively in gravitational wavesDThey have the strongest magnetic fields of any pulsar class

Astronomy

Astronomy · L5 · Pulsar Timing Arrays

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Quiz4 questions

What determines the mass of a primordial black hole at the moment it forms?

AThe mass of the progenitor star that collapsedBThe horizon mass at the time the perturbation re-enters the causal horizonCThe current Hubble parameter H₀DThe redshift of recombination, z ≈ 1100

Astronomy

Astronomy · L5 · Primordial Black Holes

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Quiz5 questions

Which of the following is NOT one of Sakharov's three conditions for baryogenesis?

ABaryon number violationBC and CP violationCDeparture from thermal equilibriumDConservation of lepton number

Astronomy

Astronomy · L5 · Baryogenesis

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Quiz5 questions

What does ξ(r) > 0 indicate about the distribution of galaxies at separation r?

AGalaxies are less clustered than a random (Poisson) distribution at that separationBGalaxies are more clustered than a random (Poisson) distribution at that separationCThe correlation function is undefined for positive valuesDThe galaxy density field is perfectly uniform

Astronomy

Astronomy · L5 · Large-Scale Structure and the Cosmic Web

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Quiz5 questions

Why does the Gunn-Peterson trough saturate so quickly, becoming insensitive once reionization is well underway?

ABecause quasars stop emitting light at high redshiftBBecause the optical depth for resonant Lyman-α scattering is so large that even x_HI ~ 10⁻⁴ produces total absorptionCBecause the CMB dominates the quasar spectrum at those wavelengthsDBecause reionization happens instantaneously

Astronomy

Astronomy · L5 · The Epoch of Reionization

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Quiz4 questions

What physically defines the tidal radius r_t of a star approaching a black hole?

AThe distance at which the star crosses the black hole's photon sphereBThe distance at which the black hole's differential gravity across the star exceeds the star's self-gravityCThe distance at which the star's orbital velocity equals the speed of lightDThe distance at which the accretion disk becomes optically thick

Astronomy

Astronomy · L5 · Tidal Disruption Events

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Quiz4 questions

What is the key empirical argument that a magnetar's emission is powered by its magnetic field rather than by rotation?

AMagnetars have no measurable spin periodBThe rotational spin-down luminosity L_sd = 4π²Iṗ/P³ falls well short of the observed persistent X-ray luminosityCMagnetars are not neutron stars at allDMagnetars rotate faster than millisecond pulsars

Astronomy

Astronomy · L5 · Magnetar Physics

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Quiz5 questions

What did the ~1.7 second delay between the GW170817 signal and GRB 170817A constrain?

AThe mass of the merging neutron starsBThe difference between the speed of gravity and the speed of light, to a precision of about 1 part in 10^15-10^16CThe distance to the host galaxy NGC 4993DThe total energy radiated in gravitational waves

Astronomy

Astronomy · L5 · Multi-Messenger Astronomy

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Quiz7 questions

What causes relativistic beaming of a jet's radiation?

AGravitational lensing by the central black holeBRelativistic aberration sweeping emission forward into a narrow cone around the direction of motionCMagnetic reflection off the accretion diskDScattering of the jet's own radiation by interstellar dust

Astronomy

Astronomy · L5 · Relativistic Jets: Doppler Beaming & Superluminal Motion

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Quiz7 questions

Why can't ordinary stellar fusion build elements heavier than iron-56 as an energy source?

AIron-56 nuclei are too small to fuse furtherBIron-56 has the highest binding energy per nucleon, so fusing beyond it requires a net energy input rather than releasing energyCStars run out of all nuclear fuel exactly at ironDIron-56 does not exist inside stars

Astronomy

Astronomy · L5 · Neutron-Capture Nucleosynthesis: the s-process and r-process

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Quiz7 questions

What is an ultra-high-energy cosmic ray?

AA photon emitted by a very distant quasarBA cosmic ray particle, usually a proton or nucleus, with energy above roughly 10^18-10^19 eVCA neutrino produced during the Big BangDAny particle detected by a ground-based telescope

Astronomy

Astronomy · L5 · Ultra-High-Energy Cosmic Rays & the GZK Cutoff

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Quiz7 questions

Why do different images of a strongly lensed quasar show the same brightness fluctuation at different times?

AEach image is actually a completely different, unrelated quasarBEach image's light follows a different path length and gravitational potential depth through the lensCThe quasar itself flickers independently in each directionDAtmospheric effects on Earth delay some images but not others

Astronomy

Astronomy · L5 · Strong-Lensing Time-Delay Cosmography & the Hubble Tension

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Note~334 words · 2 min

A 1,400-Year-Old Idea For most of recorded history, people believed Earth sat motionless at the center of the universe, with everything else — the Sun, Moon, planets, and stars — circling around it. This geocentric ('Earth-centered') model,

Astronomy

Astronomy · L2 · The Copernican Revolution

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Note~263 words · 1 min

A Thin, Cold, Rusty World Mars gets its nickname from iron oxide (rust) dust that coats its surface. Its atmosphere is a fraction of Earth's — only about 1% of Earth's surface pressure — and almost entirely carbon dioxide. That thin atmosph

Astronomy

Astronomy · L2 · Mars in Depth

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Note~148 words · 1 min

Our Sky Look up on a clear night and you will see the Moon and thousands of stars. During the day, the sky is lit up by one very important star — our Sun. On some nights the Moon looks like a full circle. On other nights it looks like a thi

Astronomy

Astronomy · L1 · The Sun, Moon, and Stars

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Note~208 words · 1 min

A Moon That Never Changes Shape The Moon looks different almost every night. Some nights it is a thin sliver. Other nights it is a bright, full circle. But here is the secret: the Moon itself never changes shape at all. The Moon is a giant

Astronomy

Astronomy · L1 · The Changing Moon

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Note~185 words · 1 min

Our Closest Star The Sun looks different from the stars we see at night, but it is exactly the same kind of thing — a star. It only looks so big and bright because it is much, much closer to us than any other star. The Sun is a giant ball o

Astronomy

Astronomy · L1 · The Sun, Our Star

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Note~187 words · 1 min

Pictures Made of Stars On a clear night, the sky is full of stars. For thousands of years, people have connected some of those stars into imaginary pictures called constellations — animals, heroes, and objects from their stories. The stars

Astronomy

Astronomy · L1 · Stars and Constellations

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Note~204 words · 1 min

A Star Built in Layers The Sun isn't a single uniform ball — it's built from distinct layers, each behaving differently as energy generated in the core works its way outward. The Corona's Strange Heat One of the Sun's biggest puzzles: the c

Astronomy

Astronomy · L2 · The Sun's Structure and Activity

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Note~207 words · 1 min

A Rivalry Reaches Orbit In the years after World War II, the United States and the Soviet Union became rival superpowers locked in the Cold War. Beyond weapons and politics, the two nations competed to prove technological superiority — and

Astronomy

Astronomy · L2 · The Space Race

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Note~218 words · 1 min

Exploration Didn't Stop with Apollo After the Apollo Moon landings ended in 1972, human and robotic space exploration didn't stop — it shifted focus. Today's missions range from a permanently crewed station in orbit to robotic rovers crawli

Astronomy

Astronomy · L2 · Modern Space Missions and Rovers

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Note~299 words · 1 min

Worlds Around Other Stars An exoplanet is any planet orbiting a star other than the Sun. The first one confirmed around a Sun-like star, 51 Pegasi b, was found in 1995. Since then, the count has exploded — astronomers have now confirmed mor

Astronomy

Astronomy · L2 · Exoplanets

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Note~180 words · 1 min

Leftovers From Building the Solar System Not everything in space is a planet, a moon, or a star. Space is full of smaller rocky and icy objects — leftover pieces from when the solar system was forming billions of years ago. How a Comet Grow

Astronomy

Astronomy · L1 · Asteroids, Comets, and Meteors

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Note~194 words · 1 min

A Pull You Can't See Gravity is an invisible force that pulls objects toward each other. The bigger and heavier something is, the stronger its pull of gravity. Earth is huge, so its gravity is strong enough to hold you, the oceans, and even

Astronomy

Astronomy · L1 · Gravity

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Note~191 words · 1 min

Pushing Against Gravity Getting to space is hard because Earth's gravity holds on tight. A rocket has to move extremely fast — thousands of kilometres per hour — to break free and stay in orbit or head further out. To reach that speed, a ro

Astronomy

Astronomy · L1 · Rockets

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Note~185 words · 1 min

Training for Space Becoming an astronaut takes years of hard training. Astronauts learn to fly spacecraft, fix equipment, do science experiments, and even go on spacewalks — all while preparing for a place with no air and almost no gravity.

Astronomy

Astronomy · L1 · Astronauts

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Note~161 words · 1 min

Eyes That See Farther Our eyes are amazing, but they can only collect a small amount of light. A telescope has a much bigger opening — a lens or a mirror — that gathers far more light, letting us see things that are too small, too faint, or

Astronomy

Astronomy · L1 · Telescopes

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Note~167 words · 1 min

The Right Planet in the Right Place Of all the planets we know about, Earth is the only one confirmed to have life. That's not an accident — Earth has a rare combination of things living creatures need. Earth sits inside the Sun's 'habitabl

Astronomy

Astronomy · L1 · Why Earth Is Special

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Note~178 words · 1 min

Four Seasons, One Tilted Earth Earth doesn't spin perfectly upright — its axis leans over by about 23.5 degrees. That single tilt is the reason we get four different seasons every year. As Earth travels around the Sun over the year, the til

Astronomy

Astronomy · L1 · The Four Seasons

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Note~137 words · 1 min

A Home That Circles Earth The International Space Station, or ISS, is like a giant laboratory and home built in space. It is big enough for astronauts to live and work for months, and it never lands — it just keeps circling Earth. A Day on

Astronomy

Astronomy · L1 · The Space Station

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