ASTRO C10 Midterm #1

What is light?

Light is electromagnetic radiation—energy that travels as waves of electric and magnetic fields.

What are the main types of electromagnetic waves (from shortest to longest wavelength)?

Gamma rays → X

What determines a light wave’s color or type?

Its wavelength (or equivalently, its frequency).

What is the relationship between wavelength, frequency, and energy?

Shorter wavelength → higher frequency → higher energy.

What is a photon?

A particle of light that carries energy proportional to its frequency (E = h·f).

What is absorption?

When atoms absorb specific wavelengths of light, moving electrons to higher energy levels.

What is emission?

When electrons fall to lower energy levels and emit light at specific wavelengths.

What are absorption and emission spectra?

Absorption: dark lines in a continuous spectrum. Emission: bright lines at specific wavelengths. Each element has a unique spectral “fingerprint.”

What does a continuous spectrum come from?

Hot, dense objects (like incandescent solids or liquids) emitting all wavelengths.

What does an absorption spectrum come from?

A hot light source passing through a cooler gas that absorbs certain wavelengths.

What does an emission spectrum come from?

A thin, hot gas emitting only specific wavelengths.

What is the Doppler effect in astronomy?

The change in wavelength (or frequency) of light from a moving source.

What does redshift mean?

The object is moving away; wavelengths stretch longer (shift toward red).

What does blueshift mean?

The object is moving toward us; wavelengths shorten (shift toward blue).

What does the Doppler effect tell us about stars or galaxies?

Their radial velocity—how fast they’re moving toward or away from us.

What is thermal radiation?

The electromagnetic energy emitted by any object with a temperature above absolute zero.

What does Wien’s Law state?

Hotter objects emit light at shorter peak wavelengths (λmax ∝ 1/T).

What does the Stefan

Boltzmann Law state?

What happens to color as temperature increases?

Objects go from red → orange → yellow → white → blue as they get hotter.

What limits telescope resolution?

Diffraction (depends on wavelength and aperture) and atmospheric turbulence.

Why do astronomers use space telescopes?

To avoid atmospheric distortion and to observe wavelengths (like UV, X

What causes lunar phases?

The Moon’s changing position relative to Earth and the Sun; we see different portions of its illuminated half.

Order of the main lunar phases?

New Moon → Waxing Crescent → First Quarter → Waxing Gibbous → Full Moon → Waning Gibbous → Last Quarter → Waning Crescent.

What causes eclipses?

The alignment of the Sun, Earth, and Moon (in a straight line).

Why don’t eclipses happen every month?

The Moon’s orbit is tilted ~5° relative to Earth’s orbit, so alignment only occurs occasionally.

What are the three main phases of a solar eclipse?

Partial phase → Diamond Ring → Totality.

What is the corona?

The Sun’s faint outer atmosphere, visible only during totality.

What is the diamond ring effect?

A brief flash of sunlight seen just before or after totality when one last bright spot of photosphere is visible.

What is an annular eclipse?

When the Moon is too far from Earth to cover the Sun completely, leaving a “ring of fire.”

Why do total solar eclipses appear so perfect?

The Sun is about 390× larger than the Moon but also about 390× farther away—so they appear the same size.

How often does a total solar eclipse occur at a given location?

Roughly once every 300–400 years.

What causes Earth’s seasons?

The 23.5° tilt of Earth’s axis, not distance from the Sun.

What happens during an equinox?

Day and night are equal everywhere; occurs around March 22 and September 22.

What happens during a solstice?

One hemisphere is maximally tilted toward or away from the Sun, producing the longest or shortest day.

How does the Coriolis effect influence weather?

Causes rotating systems (like hurricanes) to spin counterclockwise in the Northern Hemisphere and clockwise in the Southern.

What is different about the sky between hemispheres?

Star rotation direction, Moon orientation, and visible constellations (e.g., Orion appears upside down).

What is retrograde motion?

The apparent backward motion of planets against the stars, caused by Earth overtaking them in orbit.

Describe Aristotle’s model of the solar system.

Geocentric; planets move in perfect spheres around Earth.

Describe Ptolemy’s model of the solar system

Geocentric; used epicycles and deferents to explain retrograde motion.

Describe Copernicus’ model.

Heliocentric; planets orbit the Sun, naturally explaining retrograde motion.

What is Kepler’s 3rd law (simplified)?

P^2 = a^3

What does Kepler’s 3rd law help determine?

The masses of celestial bodies (e.g., Sun, Earth, Jupiter).

How did the planets form?

Dust → pebbles → planetesimals → planets; leftover gas blown away by solar winds.

What causes tides?

Differential gravitational pull of the Moon (and Sun) across Earth.

Why are there two high tides per day?

One on the side facing the Moon and one on the opposite side.

What causes the Ring of Fire?

Tectonic plate boundaries with high volcanic and earthquake activity.

Why is the Moon heavily cratered while Earth isn’t?

Earth’s erosion and plate tectonics erase old craters; the Moon lacks both.

What are lunar maria?

Dark basaltic plains from ancient lava flows filling impact basins.

How do we determine the relative age of lunar surfaces?

More craters = older surface.

What is the Giant Impact Hypothesis?

The Moon formed from debris after a Mars

Why is Mercury’s temperature extreme?

It has almost no atmosphere to distribute or retain heat.

How is Mercury similar to the Moon?

Heavily cratered surface, no tectonic activity, no atmosphere.

Why is Venus the brightest planet?

It’s close to Earth and has highly reflective sulfuric acid clouds.

Why is Venus hotter than Mercury?

Thick CO₂ atmosphere traps heat via a runaway greenhouse effect.

What is the main gas in Venus’s atmosphere?

Carbon dioxide (96%).

What evidence suggests active volcanism on Venus?

Fluctuating sulfur dioxide levels and radar images showing possible lava flows.

Why doesn’t Earth experience a runaway greenhouse effect?

Earth’s CO₂ is mostly stored in oceans and rocks; natural carbon cycles regulate it.

What is the main driver of modern climate change?

Human

How does the greenhouse effect work?

Visible light heats Earth’s surface; infrared radiation is trapped by greenhouse gases, warming the planet.

What is light?

Light is electromagnetic energy that travels in waves at a constant speed (3 × 10⁸ m/s in a vacuum).

What are the main types of electromagnetic radiation?

Radio, microwave, infrared, visible, ultraviolet, X

What determines a light wave’s energy?

Its frequency — higher frequency means higher energy (E = hf).

What is wavelength?

The distance between wave peaks; longer wavelengths have lower energy..

How do astronomers use the Doppler effect?

To measure how fast stars and galaxies move toward or away from us and to detect exoplanets through small periodic shifts in a star’s light.

What is the Doppler wobble method?

Detecting a star’s slight “wobble” caused by an orbiting planet’s gravitational pull.

What is thermal radiation?

Light emitted by any object with a temperature above absolute zero.

What does Wien’s Law tell us?

Hotter objects emit light at shorter (bluer) wavelengths (λₘₐₓ = constant / T).

What does the Stefan

Boltzmann Law tell us?

What is a blackbody?

An idealized object that absorbs all light and emits a predictable spectrum depending only on temperature.

What determines a telescope’s light

gathering power?

What determines a telescope’s resolution?

The ratio λ/D — shorter wavelengths and larger diameters give sharper images.

What limits resolution on Earth?

Atmospheric turbulence blurs images (seeing conditions).

What is a refracting telescope?

Uses lenses to bend (refract) light to a focus; suffers from chromatic aberration.

What is a reflecting telescope?

Uses mirrors to focus light; avoids chromatic aberration and can be built larger.

What does Kepler’s First Law state?

Planets orbit the Sun in ellipses, with the Sun at one focus.

What does Kepler’s Second Law state?

A line from the planet to the Sun sweeps out equal areas in equal times — planets move faster when closer to the Sun.

What does Kepler’s Third Law state?

P^2 = R^3, the farther a planet is from the Sun, the longer its orbital period.

What does Newton’s First Law state?

An object in motion stays in motion unless acted upon by a force.

What does Newton’s Second Law state?

F = ma — force equals mass times acceleration.

What does Newton’s Third Law state?

Every action has an equal and opposite reaction.

What is the Law of Universal Gravitation?

F = Gm₁m₂ / d² — gravitational force increases with mass and decreases with distance squared.

How does gravity produce an orbit?

Gravity constantly pulls an object toward the planet, but its forward motion makes it “fall around” — it keeps missing, forming an orbit.

What are terrestrial planets?

Mercury, Venus, Earth, and Mars — small, rocky, dense, few moons.

What are giant (Jovian) planets?

Jupiter, Saturn, Uranus, Neptune — large, gaseous, low density, many moons and rings.

What is the “snow line”?

The boundary in the early solar nebula beyond which ices could form; separates terrestrial and giant planets.

What is the Roche limit?

The distance within which a moon or object is torn apart by a planet’s tidal forces (explains Saturn’s rings).

What are comets made of?

“Dirty iceballs” of rock and frozen gases that sublimate when near the Sun, forming tails that point away from it.

What is a periodic comet?

A comet that orbits the Sun repeatedly (returns at regular intervals).

Where do most comets come from?

The Kuiper Belt and the distant Oort Cloud.

What are asteroids?

Rocky remnants mostly found between Mars and Jupiter in the asteroid belt.

What is a meteor?

A “shooting star” — a meteoroid burning up in Earth’s atmosphere.

What is a meteorite?

A fragment of a meteoroid that survives passage through the atmosphere and lands on Earth.

What is an exoplanet?

A planet orbiting a star outside our solar system.

What is the Doppler wobble method?

Detects periodic shifts in a star’s light due to the gravitational pull of an orbiting planet.

What is the transit method?

Detects small dips in starlight when a planet passes in front of its star.

What can spectra during transits reveal?

The composition of an exoplanet’s atmosphere by analyzing which wavelengths are absorbed.

What did the Kepler Space Telescope discover?

Thousands of exoplanets — found that small, Earth

What is the “Goldilocks Zone”?

The habitable zone where conditions are just right for liquid water.

Why does the Sun look red at sunset?

Earth’s atmosphere scatters shorter blue light, leaving red/orange light.

What would happen if Earth had no axial tilt?

There would be no significant seasonal changes.