OpenStem

@openstem · Joined Jul 2026

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What supports a neutron star against gravitational collapse?1 / 8

Neutron degeneracy pressure — a purely quantum-mechanical effect from the Pauli exclusion principle: no two identical fermions (neutrons) can occupy the same quantum state. Packed to nuclear densities (~10¹⁷ kg/m³), neutrons are forced into high-momentum states, generating an outward pressure independent of temperature. This is the same mechanism (electron degeneracy) that supports white dwarfs, but neutron degeneracy can support a far more compact, massive object.

Astronomy

Astronomy · L5 · Neutron Star Physics

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Flashcards8 cards

What is gravitational-wave strain h, and how large is it for real detections?1 / 8

Strain is the fractional change in proper distance a gravitational wave induces:

Astronomy

Astronomy · L5 · Gravitational Waves

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Flashcards8 cards

What is the horizon problem in standard (non-inflationary) Big Bang cosmology?1 / 8

The cosmic microwave background is observed to be nearly isotropic (uniform temperature to ~1 part in 10⁵) across the entire sky, including regions that — tracing light-travel distances back through a purely radiation/matter-dominated expansion — could never have been in causal contact before recombination. Without inflation, there is no physical mechanism to have equilibrated their temperatures: the horizon problem asks why causally disconnected regions look identical.

Astronomy

Astronomy · L5 · Inflationary Cosmology

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Flashcards8 cards

How do galaxy rotation curves provide evidence for dark matter?1 / 8

For a spiral galaxy, Newtonian dynamics predicts orbital speed v(r) = √(GM(r)/r) should decline as r⁻¹ᐟ² beyond the visible disk (Keplerian falloff), once nearly all the mass is enclosed. Instead, observed rotation curves stay roughly flat (v ≈ constant) out to large radii, implying M(r) ∝ r — a mass distribution extending far beyond the visible stars and gas, in a roughly spherical 'halo'.

Astronomy

Astronomy · L5 · Dark Matter Candidates and Evidence

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What does 'hierarchical' structure formation mean, and why is it 'bottom-up'?1 / 8

In a Cold Dark Matter (CDM) universe, small-scale density perturbations grow and collapse first (they cross the non-linear threshold δ~1 earliest, since smaller scales have larger fractional fluctuations in a nearly scale-invariant primordial spectrum), forming small dark matter halos. These small halos then merge over cosmic time to build progressively larger structures — galaxies, groups, and eventually galaxy clusters. This 'small structures form first, then merge into larger ones' pattern is the opposite of a hypothetical 'top-down' scenario (large structures forming first and fragmenting), and CDM's bottom-up hierarchy is well supported by observations.

Astronomy

Astronomy · L5 · Hierarchical Galaxy Formation

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How does an absorption line form in a stellar spectrum?1 / 8

A star's continuum radiation is generated deep in the photosphere, where the local temperature is highest along the line of sight to that depth. As photons travel outward, atoms/ions in cooler, overlying layers absorb photons at specific transition wavelengths, then re-emit them isotropically (in essentially random directions). Along the observer's line of sight, this removes photons at the transition wavelength preferentially (since the re-emission depth is on average cooler and the process effectively scatters flux out of the beam), producing a dip — an absorption line — relative to the surrounding continuum.

Astronomy

Astronomy · L5 · Stellar Atmospheres and Spectral Line Formation

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What is the Eddington luminosity, and where does its formula come from?1 / 8

The Eddington luminosity is the maximum luminosity at which outward radiation pressure on ionised gas balances inward gravity for spherical accretion (or, more loosely, sets a characteristic scale for disk accretion). Balancing radiation pressure on free electrons (via Thomson scattering, cross-section σ_T) against gravity on the associated protons gives:

Astronomy

Astronomy · L5 · Accretion Disk Physics

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What is a kilonova?1 / 8

A kilonova is a roughly day-to-week-timescale optical/infrared transient powered by the radioactive decay of freshly synthesised, very neutron-rich heavy nuclei ejected during a neutron star merger (NS-NS or NS-BH). It is named for its peak luminosity — roughly 1,000× a classical nova, though still ~1,000× fainter than a typical supernova — and was predicted theoretically well before being confirmed observationally alongside GW170817 in 2017 (as AT2017gfo).

Astronomy

Astronomy · L5 · Compact Object Mergers, Kilonovae, and r-Process Nucleosynthesis

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What is the CMB angular power spectrum C_ℓ?1 / 7

The CMB temperature map T(θ, φ) is decomposed into spherical harmonics, ΔT/T = Σ a_ℓm Y_ℓm(θ,φ). The power spectrum C_ℓ = ⟨|a_ℓm|²⟩ (averaged over m) measures the variance of temperature fluctuations at angular multipole ℓ, which roughly corresponds to angular scale θ ~ 180°/ℓ. Plotting ℓ(ℓ+1)C_ℓ/2π against ℓ gives the characteristic series of 'acoustic peaks' that encode a wealth of cosmological information.

Astronomy

Astronomy · L5 · The CMB Angular Power Spectrum

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Describe the core accretion model of planet formation.1 / 8

In core accretion, micron-sized dust grains in a protoplanetary disk collide and stick, growing through successive stages — grains to pebbles to planetesimals (km-scale) to protoplanets — via gravitational and collisional accumulation. Once a solid core reaches roughly 10 Earth masses, it can gravitationally capture a substantial gaseous envelope from the surrounding disk, triggering runaway gas accretion (if enough gas remains in the disk) and producing a gas giant; smaller cores simply remain as rocky/icy planets. This bottom-up process operates on a timescale of ~1–10 Myr, competing against the disk's own gas dissipation timescale.

Astronomy

Astronomy · L5 · Exoplanet Formation Theory and Demographics

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What is meant by a star's 'nucleosynthesis yield'?1 / 8

A stellar yield is the mass (or mass fraction) of each chemical element that a star of a given initial mass and metallicity returns to the interstellar medium (ISM) over its lifetime, via stellar winds and its terminal explosion or shedding event. Yields differ dramatically by mass and evolutionary channel — massive stars, AGB stars, and Type Ia supernovae each enrich the ISM with characteristically different mixes of elements, which is the basic input every galactic chemical evolution model needs.

Astronomy

Astronomy · L5 · Galactic Chemical Evolution

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State the (static, non-expanding) Jeans instability criterion and the Jeans length.1 / 8

In a static self-gravitating gas, pressure support competes with gravity; a perturbation collapses only if it is larger than the Jeans length, at which pressure forces can no longer respond fast enough to counteract gravitational collapse:

Astronomy

Astronomy · L5 · Cosmological Perturbation Theory

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What is a pulsar timing array (PTA)?1 / 7

A network of millisecond pulsars monitored for years to decades, used collectively as a galaxy-sized gravitational-wave detector. A passing low-frequency gravitational wave perturbs spacetime between Earth and each pulsar, producing tiny, correlated deviations in the precisely predicted arrival times of their radio pulses. PTAs (NANOGrav, EPTA, PPTA, CPTA, combined internationally as the IPTA) probe wavelengths of light-years, far beyond the reach of any ground- or space-based interferometer.

Astronomy

Astronomy · L5 · Pulsar Timing Arrays

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What is a primordial black hole (PBH), and how does its formation differ from a stellar-collapse black hole?1 / 7

A primordial black hole forms not from the collapse of a dying star, but from the direct gravitational collapse of an unusually large density perturbation in the very early universe, at the moment that perturbation's scale re-enters the causal horizon. Because they don't require a stellar progenitor, PBHs could in principle span an enormous mass range — from far below a solar mass up to, speculatively, supermassive scales.

Astronomy

Astronomy · L5 · Primordial Black Holes

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What is the baryon asymmetry problem, and how is it quantified?1 / 8

The observable universe is made almost entirely of matter, with essentially no primordial antimatter, despite the expectation that the hot early universe produced baryons and antibaryons in equal numbers. The asymmetry is quantified by the baryon-to-photon ratio:

Astronomy

Astronomy · L5 · Baryogenesis

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What is the overdensity field δ(x), and what role does it play in describing large-scale structure?1 / 8

The overdensity field is the fractional deviation of the local density from the cosmic mean:

Astronomy

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

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Outline the timeline from recombination to the completion of reionization.1 / 8

Astronomy

Astronomy · L5 · The Epoch of Reionization

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Define the tidal radius r_t, and state when a tidal disruption event (TDE) occurs.1 / 8

The tidal radius is the distance from a black hole at which its differential (tidal) gravity across a star exceeds the star's own self-gravity:

Astronomy

Astronomy · L5 · Tidal Disruption Events

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What distinguishes a magnetar from an ordinary pulsar?1 / 8

A magnetar is a neutron star with a surface magnetic field of order B ~ 10¹⁴–10¹⁵ G, roughly 100–1000× stronger than a typical pulsar's ~10¹² G. Crucially, its emission is powered mainly by the decay and dissipation of this magnetic field, not by rotational spin-down energy — the rotational energy-loss rate is orders of magnitude too small to power the observed persistent X-ray luminosity and bursts.

Astronomy

Astronomy · L5 · Magnetar Physics

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What are the four cosmic 'messengers' combined in multi-messenger astronomy?1 / 8

Astronomy

Astronomy · L5 · Multi-Messenger Astronomy

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What is relativistic beaming, and what causes it?1 / 8

The concentration of radiation from a source moving at a speed close to the speed of light into a narrow cone in the direction of motion, as seen by a stationary observer. It arises from the relativistic aberration of light: in the source's own rest frame, emission may be nearly isotropic, but transforming to the observer's frame sweeps that radiation forward into a tight beam of half-angle roughly 1/Γ, where Γ is the bulk Lorentz factor of the jet.

Astronomy

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

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Why can't fusion alone build elements heavier than iron-56 inside a star?1 / 8

Iron-56 has the highest binding energy per nucleon of any common nuclide, meaning it sits at the bottom of the nuclear binding-energy curve. Fusing nuclei together releases energy only up to iron; fusing beyond it would require a net INPUT of energy rather than releasing it, so ordinary stellar fusion cannot proceed past iron as an energy source.

Astronomy

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

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What defines an ultra-high-energy cosmic ray (UHECR)?1 / 8

A cosmic ray — almost always a proton or atomic nucleus — with energy above roughly 10^18-10^19 eV (an EeV, or exa-electronvolt), far beyond what any Earth-based particle accelerator can produce. The most extreme detected events reach energies over 10^20 eV, concentrated into a single subatomic particle.

Astronomy

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

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What is a strongly lensed quasar, and why does it produce multiple images with different arrival times?1 / 8

A quasar whose light passes close enough to an intervening massive galaxy to be strongly gravitationally lensed into multiple distinct images. Each image's light follows a different path length and passes through a different depth of the lens's gravitational potential (the Shapiro delay), so light from the same intrinsic brightness fluctuation in the quasar arrives at Earth through each image at a different time.

Astronomy

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

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What two parameters fully characterise a Kerr black hole, and what does the spin parameter represent?1 / 8

Mass M and spin angular momentum J, usually expressed through the dimensionless spin parameter a = J/(Mc), with 0 ≤ a ≤ GM/c² for a physical black hole. The spin parameter measures how close the hole is to its maximal ('extremal') rotation rate, a = GM/c², beyond which the horizon would cease to exist in the standard picture.

Astronomy

Astronomy · L5 · The Kerr Metric: Frame Dragging, the Ergosphere & the Penrose Process

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Quiz3 questions

How many planets are in our solar system?

ASevenBEightCNineDTen

Astronomy

Astronomy · L1 · Planets Quiz

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Quiz3 questions

What causes day and night on Earth?

AThe Sun moves around EarthBEarth spins on its axisCThe Moon blocks the SunDClouds cover the Sun

Astronomy

Astronomy · L1 · Day and Night

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Quiz4 questions

Why does the Moon seem to change shape during the month?

AThe Moon actually grows and shrinksBWe see different amounts of its sunlit halfCClouds cover part of itDThe Moon spins very fast

Astronomy

Astronomy · L1 · The Changing Moon Quiz

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Quiz4 questions

What is the Sun?

AA giant planetBA starCA moonDA comet

Astronomy

Astronomy · L1 · The Sun, Our Star Quiz

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Quiz4 questions

What is a constellation?

AA single very bright starBA pattern of stars that forms a picture in the skyCA type of planetDA cloud of dust

Astronomy

Astronomy · L1 · Stars and Constellations Quiz

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