Introduction to Astronomy and Planetary Science

Star

A large, glowing ball of gas that generates heat and light through nuclear fusion.

Planet

Orbits a star, is large enough and more or less round, and has cleared most other objects from its orbit.

Asteroid

A relatively small and rocky object that orbits a star.

Solar system

The Sun and all the material that orbits it, including its planets and moons.

Galaxy

A great island of stars in space with hundreds of millions to a trillion or more stars, all held together by gravity and orbiting a common center.

Supercluster

A gigantic region of space where many individual galaxies and many groups and clusters of galaxies are packed more closely together than elsewhere in the universe.

Observable Universe

The portion of the entire universe that can be seen in principle from Earth.

Speed of light

300,000 kilometers per second.

Number of galaxies in the observable universe

Roughly 100 billion.

Light travel time from Sun to Earth

Approximately 8 minutes.

Astronomical unit (AU)

About 150 million kilometers, which is the average orbital distance of Earth from the Sun.

Cosmic calendar

A scale in which the history of the universe is compressed to one year.

Milky Way Galaxy mass contribution

The largest contribution to the mass is located in the halo made of Dark Matter.

Earth's rotation and orbit

Earth rotates once each day and orbits the Sun once each year.

Solar system orbit period

Completes one orbit of the galaxy in about 230 million years.

Speed of stars in the Milky Way

Each star orbits the center of the galaxy at a speed of the order of 70,000 km/hour.

Universe expansion

As the Universe expands, the average distance between galaxies increases over time.

Universe age

About 14 billion years old, limiting observation to light no more than 14 billion light-years away.

Celestial equator

Projection of Earth equator into space

The ecliptic

The Sun's apparent path through the celestial sphere (23.5° with respect to the equator)

Zenith

The point directly overhead

Horizon

All points 90° away from zenith

Meridian

Line passing through zenith and connecting N and S points on the horizon

North celestial pole

The point directly over Earth's North Pole

South celestial pole

The point directly over Earth's South Pole

Circumpolar stars

Stars near the north celestial pole that never cross below the horizon (never set)

Seasons

Caused by the tilt of the Earth's axis affecting sunlight distribution

Spring equinox

Occurs around March 21, when the Northern Hemisphere is tipped slightly toward the Sun

Fall equinox

Occurs around September 22, when the Northern Hemisphere first starts to be tipped away from the Sun

Winter solstice

The moment when the Southern Hemisphere is tipped most directly toward the Sun

Summer solstice

The moment when the Northern Hemisphere is tipped most directly toward the Sun

Tilt of Earth's axis

Remains close to 23.5°

Precession

The slow change in the direction the axis points in space over a 26,000-year cycle

High latitudes

Regions where summer Sun remains above the horizon all day and winter Sun remains below the horizon all night

Planetary retrograde motion

A planet appears to move backward relative to the stars when Earth passes it in its orbit

Altitude of the north celestial pole

The same as your latitude at any point on Earth

Direct sunlight

Received most directly by the Northern Hemisphere on the June solstice

Equinoxes

Moments when the Sun rises precisely due east and sets precisely due west, with equal sunlight for both hemispheres

Retrograde motion

Periods when planets appear to turn around and travel westward instead of eastward.

Photon energy

Decreases with wavelength; ultraviolet photons have less energy than gamma rays, and infrared photons have less energy than X-rays.

Speed of photons

All photons, regardless of their type, travel at the same speed.

Atoms

Made of protons, neutrons, and electrons; different chemical elements have different numbers of protons.

Nucleus

Nearly 100,000 times smaller than the atom but contains nearly all of its mass.

Color perception

An object appears green because it scatters green light and absorbs all other colors.

Blue filter

Transmits blue light and absorbs other colors, making everything look blue through it.

Wavelength

The distance between adjacent peaks in a wave.

Frequency

The number of times any piece of the rope moves up and down each second.

Absorption line spectrum

Produced when electrons in an atom step up to higher energy levels and photons are absorbed.

Emission line spectrum

Produced when electrons in an atom step down to lower energy levels and photons are emitted.

Types of spectra

Three basic types: continuous, emission line, and absorption line.

Doppler effect

Indicates that a star is moving away from us if its spectrum shows an emission line at a longer wavelength than in the laboratory.

Mass of the Sun

Over 99.8% of the solar system's mass and about 700-1000 times more massive than all the planets together.

Terrestrial planets

Small, rocky, with relatively high densities, and close to the Sun.

Jovian planets

Large, with relatively low densities, gas-rich, and far from the Sun.

Nebular theory

Explains that our solar system formed from the gravitational collapse of a great cloud of gas.

Heating in solar nebula

The temperature increases as the solar nebula collapses due to conversion of gravitational potential energy to kinetic energy.

Spinning of solar nebula

The solar nebula rotates faster as it shrinks in radius, similar to an ice skater pulling in her arms.

Flattening of solar nebula

Occurs as a natural consequence of collisions between particles in a spinning cloud.

Frost line

The boundary in the solar system beyond which hydrogen compounds can form ices.

Accretion

The process by which gravity assembles planetesimals into larger bodies.

Asteroids

Rocky leftovers from the accretion process, found inside the frost line.

Comets

Icy leftovers from the accretion process, found outside the frost line.

Water-bearing planetesimals

Planetesimals that carried water and impacted Earth and other terrestrial planets.

Giant impact hypothesis

The theory that the Moon formed from debris resulting from a giant impact between a Mars-size planetesimal and the young Earth.

Exceptions of solar nebular theory

Phenomena such as the extreme axis tilt of Uranus, the large metallic core of Mercury, the backward rotation of Venus, and the formation of Earth's Moon explained by giant impacts.

Triton's orbit

An exception to the solar nebular theory that cannot be explained by a giant impact event.

Rock age determination

The process of analyzing the proportions of various atoms and isotopes to determine the age of a rock.

Meteorites

Rocks that have fallen to Earth, providing samples that have not melted or vaporized since the solar system's formation.

Earth rocks

Rocks on Earth that are generally younger than meteorites due to melting and reforming processes.

Kuiper belt

A region of the solar system beyond Neptune, where Pluto and Charon are considered large members.

Earth's interior layers

Divided into three layers: Core (highest density, nickel and iron), Mantle (moderate density, minerals with silicon and oxygen), and Crust (lowest density, granite and basalt).

Lithosphere

The outer layer of a planet, defined by the hardness and rigidity of the rock, comprising the crust and part of the mantle.

Convection cycle in the mantle

The slow movement of rock within the mantle, taking approximately 100 million years for one complete cycle.

Temperature gradient in a planet

The increase in temperature with depth inside a planet, causing hot rock to rise and cooler rock to fall.

Ductility of rocks

The property of rocks becoming softer and more deformable when warm, allowing them to flow over long timescales.

Differentiation

Gravity pulls high-density material to center while lower-density material rises to surface, resulting in material separated by density.

Geological activity

Primarily driven by internal heat, powering processes such as mantle convection, plate tectonics, volcanism, and earthquakes.

Sources of internal heat

1. Accretion: Incoming planetesimals collided at high speed, depositing energy that turned into heat. 2. Differentiation: Gravitational potential energy into thermal energy as dense materials sank to the core. 3. Radioactivity: Radioactive materials decay, releasing heat into planetary interiors.

Radioactive decay

Currently the most important heat source for Earth, where radioactive materials decay and release heat.

Convection in the mantle

The process in which hot, solid rock rises and cooler rock sinks within Earth's mantle, driven by temperature differences.

Magnetic fields

Generated by the movement of molten iron and nickel in the liquid outer core, combined with Earth's rotation.

Heat loss from Earth

Occurs through convection, conduction, and radiation.

Convection

Movement of hot material within the mantle, transporting heat as hot material rises and cool material falls.

Conduction

Transfer of heat through solid materials.

Radiation

Emission of energy from the surface into space.

Geologically dead planets

Smaller planets and moons cool off faster than larger planets, resulting in geological inactivity.

Impact cratering

Craters are about 10 times wider than the objects that made them.

Volcanic outgassing

The process by which Earth's atmosphere and oceans were formed from gases released from the interior.

Tectonic forces

Stresses in the crust created by convection of the mantle, leading to mountain ranges and valleys.

Greenhouse gases

Gases that absorb and reemit thermal radiation, heating the lower atmosphere.

Global average temperature

On the surface, it is 57 F (14 C) due to the greenhouse effect.

Martian winds

Strongly affected by extreme seasonal changes, with one-third of the total carbon dioxide moving seasonally between polar caps.

Martian geology

Offers evidence that Mars had a warmer and wetter past, showing features of water erosion.

Opportunity rover

Provided strong evidence for abundant liquid water on Mars in the distant past, with rocks containing tiny spheres of hematite.