Astronomy: Stars, Planets, Galaxies, and Cosmology Key Concepts

What is a star?

A hot gaseous body mainly of hydrogen that emits electromagnetic radiation from nuclear fusion in its core; luminous.

What is a planet?

A spherical, non-luminous body in a stable orbit around a star.

What is a moon?

A natural satellite of a planet.

What is a solar system?

A star and all bodies held in orbit by its gravitational field—planets, moons, asteroids, comets, dust, and gas.

What is a galaxy?

A collection of billions of stars and their systems held together by gravity.

What is a nebula?

A cloud of gas (mostly hydrogen) and dust, often hundreds of light-years wide.

What is the universe?

Everything that exists: all space, time, matter, and energy.

What is an astronomical unit (AU)?

The mean distance from Earth to the Sun ≈ 1.5 × 10¹¹ m.

What is a light-year?

The distance light travels in one year; light speed = 3 × 10⁸ m s⁻¹.

What is parallax?

The apparent shift in a star's position used to determine its distance.

What is absolute brightness (luminosity)?

How bright a star would appear if it were 10 parsecs from Earth.

What is apparent brightness?

How bright a star appears from Earth.

State Newton's law of gravitation.

F = G m₁ m₂ / r²

What does each symbol represent?

F = gravitational force (N); m₁, m₂ = masses (kg); r = distance (m); G = 6.67 × 10⁻¹¹ N m² kg⁻².

Why are stars luminous?

They fuse hydrogen into helium in their cores, releasing energy.

What is plasma?

A state where electrons have enough kinetic energy to move freely, forming charged particles.

Describe the fusion process.

4 hydrogen nuclei → 1 helium nucleus + energy.

How is mass lost in fusion converted to energy?

E = m c².

What triggers star formation?

Turbulence in a nebula creates dense knots that collapse under gravity.

What is a protostar?

A forming star at the center of a collapsing nebula disk.

What is equilibrium on the main sequence?

Outward pressure from fusion balances inward gravity; stable for ≈ 90 % of the star's life.

How does a red giant form?

When core hydrogen runs out, the core contracts and outer layers expand and cool.

What is a white dwarf?

The hot, dense remnant after a red giant sheds outer layers; no fusion, cools over time.

What is a black dwarf?

A fully cooled white dwarf (none exist yet—universe too young).

What is a planetary nebula?

The ejected outer layers of a dying red giant.

What is a supernova?

A massive explosion when fusion stops in a large star, ejecting outer layers.

What is a neutron star?

A dense 20 km-wide core of neutrons left from a supernova (8-25 solar masses).

What is a black hole?

A collapsed core (> 25 solar masses) where gravity prevents light from escaping.

What is the stellar classification mnemonic?

O B A F G K M → "Oh Be A Fine Girl Kiss Me."

When and how did the universe begin?

≈ 13.8 billion years ago from a hot, dense singularity that expanded and cooled.

What is cosmological redshift?

Light from receding galaxies shifts to longer (red) wavelengths due to expansion.

How do absorption spectra show motion?

Each element has unique absorption lines; shifts toward red show galaxies moving away.

What is cosmic microwave background radiation (CMBR)?

Faint uniform radiation predicted by Gamow; detected 1963 matching expected wavelength, evidence of a hot early universe.

What is the relative abundance of light elements evidence?

Predicted by Big Bang nucleosynthesis—only light elements (H, He, Li) formed—matching observations.

What is spectroscopy?

Using a prism to split light into wavelengths to determine temperature, composition, and motion.

What are spectral lines?

Dark or bright lines where photons are absorbed/emitted as electrons move between shells.

What causes absorption lines in stars?

Outer layers absorb photons with energies matching electron transitions.

Why does each element have unique spectral lines?

Different energy gaps between electron shells produce distinctive patterns.