Astronomy Lectures 09–15: Light, Telescopes, and Solar System Formation

Essential vocabulary flashcards covering light, spectroscopy, telescope optics, planet types, solar system formation, and planetary interior structures from Astronomy Lectures 09–15.

Photon

A particle of light that carries a discrete quantum of energy given by $E = hf$.

Wavelength ($\lambda$)

The spatial distance between adjacent wave crests or peaks of an electromagnetic wave.

Frequency ($f$)

The number of complete wave cycles that pass a given point per second, measured in hertz ($\text{Hz}$).

Emission (light-matter interaction)

matter gives off light energy,

The Sun gives off light.

Absorption (light-matter interaction)

matter takes in light energy,

A black shirt absorbs sunlight and heats up.

Reflection / scattering

• Reflection: Light bounces off a surface.

• Scattering: Light spreads in different directions.

The Moon reflects sunlight,

Earth's atmosphere scatters sunlight, making the sky appear blue.

Transmission

light passes straight through matter

Sunlight passes through a clear glass window.

Plasma

The state of matter consisting of a high-temperature mixture of free electrons and positively charged ions, found in environments such as the solar corona.

Continuous spectrum

Hot, dense/opaque object emits light over a continuous range of

wavelengths.

Unbroken rainbow; examples include incandescent bulbs and the Sun’s visible light.

Emission-line spectrum

Hot, diffuse/low-density gas emits at specific wavelengths.

What it looks like / source

Bright lines on a dark background; example: a neon lamp.

Absorption-line spectrum

Cool, diffuse gas lies in front of a Dark lines missing from a continuous continuous-spectrum source and spectrum.

absorbs specific wavelengths.

Kirchhoff’s 3 laws

• Kirchhoff’s first law: hot, dense/opaque material produces a continuous spectrum.

• Kirchhoff’s second law: hot, diffuse/transparent gas produces bright emission lines.

• Kirchhoff’s third law: cool, diffuse gas in front of a continuous source produces dark absorption lines.

Wien's law

The relationship stating that the peak wavelength of thermal radiation is inversely proportional to temperature ($\lambda_{\text{peak}} \propto 1/T$), meaning hotter objects radiate at shorter, bluer peak wavelengths.

Stefan–Boltzmann law

the total thermal energy radiated per unit surface area of an opaque object is proportional to the fourth power of its temperature ($\text{Energy} \propto T^4$).

• Hotter objects emit much more energy per

  unit area.

Doppler shift

the apparent change in the frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source

Blueshift

moving toward us

Light waves get compressed to shorter wavelengths, shifting toward blue.

A Doppler shift toward shorter wavelengths and higher frequencies, indicating that an astronomical object is moving toward the observer along the line of sight.

Redshift

moving away

Light waves stretch out to longer wavelengths, shifting toward red.

A Doppler shift toward longer wavelengths and lower frequencies, indicating that an astronomical object is moving away from the observer along the line of sight.

Refracting telescope (refractor)

Uses lenses; refraction is bending of light as it passes between materials

such as air and glass.

Early example: Galileo’s telescope (1609). Large lenses are heavy and long tubes become impractical.

Reflecting telescope (reflector)

Uses mirrors to focus light.

Invented by Newton; can be built with much larger diameters; most modern large telescopes are reflectors.

Angular resolution

how well a telescope can tell two objects that are close together apart. Smaller angular resolution values mean finer detail.

• Angular resolution is inversely proportional to telescope diameter: doubling diameter gives about twice the

  resolution capability (a smaller minimum angle).

it improves (decreases in angle) with larger telescope aperture diameter.

Imaging

• take pictures of the sky; images in different filters can be combined into color images. Detectors can record wavelengths invisible to human eyes; false color may be used to show data.

Spectroscopy

separate light into wavelengths and analyze a spectrum to infer composition, temperature, and motion.

Timing

Measure how brightness changes with time. A light curve records brightness measurements over time; examples include variable events and a planet transiting a star.

space telescopes

avoid atmospheric turbulence and can produce sharper images; they also observe wavelengths blocked by the atmosphere.

Adaptive optics

A ground-based telescope technology that that corrects for blurring caused by Earth's atmosphere, making telescope images sharper.

How it works:

• Earth's atmosphere makes starlight wobble and blur.

• The telescope uses flexible mirrors that adjust their shape to correct the distortion.

• The result is a much clearer image.

Examples of 4 space telescopes (for different radiatio

• Hubble: Observes mostly visible light; has a 2.4 m mirror.

• James Webb (JWST): Observes infrared light; has a 6.5 m mirror.

• Chandra: Observes X-rays, which can reveal extremely hot gas and black holes.

• Compton: Observed gamma rays, the highest-energy form of electromagnetic radiation.

Planets in order from the sun

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

Pluto = dwarf planet.

My Very Easy Method Just Speeds Up Naming

Terrestrial planets

The small, rocky, high-density inner planets of our solar system that possess metallic cores and solid surfaces: Mercury, Venus, Earth, and Mars.

Jovian planets

The large, low-density outer gas giant planets that feature deep gas-rich atmospheres and hydrogen compounds: Jupiter, Saturn, Uranus, and Neptune.

Astronomical unit ($\text{AU}$)

The average orbital distance between Earth and the Sun, defined approximately as $150\text{ million}\,\text{km}$.

Kepler's third law

The planetary orbital relationship formulated as $P^2 = a^3$, where orbital period $P$ is measured in years and semi-major axis $a$ is expressed in astronomical units ($\text{AU}$).

Motion of large bodies

planets orbit in the same direction and nearly the same plane; most planets spin and most moons orbit in the same general direction.

Flyby mission

A robotic spacecraft mission design that passes by an astronomical body once without entering orbit to conduct brief, flyby observations (e.g., Voyager 2).

Orbiter mission

A spacecraft designed to decelerate and enter a closed orbit around a target planetary body to carry out sustained, long-term scientific observations.

Lander / atmospheric probe

A space mission designed to touch down on a solid planetary surface or descend through a thick gaseous atmosphere to conduct direct in situ measurements.

Close encounter hypothesis

A discarded theory proposing that the planets condensed from gas ripped out of the Sun during a near-collision with another passing star.

Nebular hypothesis (nebular theory)

The accepted model stating that the solar system formed approximately $4.5\text{ billion years}$ ago from the gravitational collapse of a rotating interstellar gas and dust cloud.

Frost line

The specific boundary in the solar nebula (occurring at approximately $150\,\text{K}$) inside of which only rock and metals condense, and beyond which volatile hydrogen compounds can freeze into solid ices.

Planetesimals

Kilometer-sized solid rocky or icy bodies formed by the collision and electrostatic sticking together of microscopic solid grains in the protoplanetary disk.

Accretion

The gradual process of planetary growth occurring when planetesimals collide and gravitationally coalesce into larger protoplanetary cores.

Solar wind

A continuous stream of charged particles and radiation issuing outward from the young Sun that stripped leftover light gases (hydrogen and helium) from the inner solar nebula.

Asteroid

A rocky, metal-bearing planetesimal leftover from the formation era inside the frost line, most commonly located in orbits between Mars and Jupiter.

Comet

An ice-rich planetesimal leftover from planetary accretion beyond the frost line, stored primarily in distant reservoirs such as the Kuiper belt and Oort cloud.

Radioactive half-life

The duration of time required for exactly fifty percent of unstable parent radioactive isotopes in a rock sample to spontaneously decay into stable daughter isotopes.

Seismology

The scientific discipline that probes internal planetary layering and physical states by monitoring the propagation, speed, and refraction of earthquake-generated waves.

P-waves (pressure waves)

Longitudinal compressional seismic waves that travel by alternating compression and expansion, capable of propagating through solids, liquids, and gases.

S-waves (shear waves)

Transverse side-to-side seismic waves that propagate strictly through solid matter and are blocked completely by liquids such as Earth's outer core.

Differentiation

The gravitational sorting of molten planetary material by density, causing heavy metals (such as iron and nickel) to sink toward the interior while lighter silicates float upward.

Lithosphere

The rigid, mechanically strong outer layer of a rocky planet consisting of the entire crust and uppermost brittle portion of the mantle.