Astronomy Final

Planet

Earth is one of these

Star System

A system that may have more than one star.

Star Cluster

Collections of stars found within galaxies.

Galaxy

A collection of star clusters and independent stars that orbit a common center point.

Local Group Galaxy Cluster

A group of galaxies including the Andromeda, Milky Way, and Triangulum galaxies.

Laniakea Supercluster

A relatively small part of the entire universe.

Universe

The totality of all energy and matter that exists.

Astronomical Unit (AU)

A unit of astronomical distance based on the Earth-Sun distance.

Light Year (ly)

A unit of astronomical distance based on the distance light travels in one year.

Parsec (pc)

A unit of astronomical distance approximately equal to 3.26 light-years.

Look-back Time

The phenomenon where the further away we look, the further back in time we are seeing.

Celestial Sphere

The apparent sphere of the sky on which stars appear to be located.

Celestial Poles and Celestial Equator

Earth's poles and equator projected outward onto the apparent sphere of the sky.

Ecliptic

The path of the sun across the celestial sphere over a year, representing Earth's orbit around the sun.

Constellations

Imaginary patterns of stars along the celestial sphere.

Horizon

Where the sky appears to meet the ground from your point of view.

Azimuth

The direction along the horizon (north/south/east/west) where a star is located.

Altitude

How high above the horizon (in angular units) a star is located.

Circumpolar Stars

Stars that never set from your perspective on Earth's surface.

June Solstice

The longest day of the year in the northern hemisphere (summer).

December Solstice

The shortest day of the year in the northern hemisphere (winter).

Equinoxes

Points in time when there are 12 hours of day and night everywhere on Earth.

Synchronous Rotation

The moon spins on its axis in exactly the same amount of time it takes to orbit the Earth.

Meridian

Arc from due south on the horizon, to zenith, to due north.

Right Ascension (RA)

East/west position of a star, measured in hours, minutes, and seconds.

Scalar

A quantity that has a magnitude and a unit (e.g., mass, time, speed).

Vector

A quantity that has magnitude, unit, and direction (e.g., displacement, velocity, acceleration).

Linear Acceleration

Acceleration that occurs when speeding up or slowing down along a straight line.

Centripetal Acceleration

Acceleration that occurs when moving in a circle; the direction of velocity changes.

Newton's First Law of Motion (Law of Inertia)

Objects maintain a constant velocity unless acted upon by an outside force.

Newton's Second Law of Motion

F = ma (Force equals mass times acceleration).

Newton's Third Law of Motion

For every force that acts on one object, an equal yet opposite reaction force is exerted upon another object.

Law of Gravity

Every mass gravitationally attracts every other mass; the strength of the gravitational pull decreases as the distance between them grows (F = Gm1m2/r2).

Weightlessness in Orbit

Experienced due to falling around the Earth, not a lack of gravity.

Tides

Caused by the moon's gravitational pull being stronger on the near side of the Earth than the far side.

Spring Tide

Sun and moon work together to enhance the tides.

Neap Tide

Sun and moon work against each other to decrease tides.

Angular Momentum

Conserved quantity for a spinning object; momentum can only be transferred.

Wavelength

Distance from max-to-max or min-to-min of a wave.

Frequency

How many cycles a wave goes through in a given time interval, measured in hertz (Hz).

Electromagnetic Spectrum

In order of increasing energy/frequency: radio waves, microwaves, infrared, visible, ultraviolet, x-rays, gamma-rays.

Index of Refraction (n)

How much light is slowed down in a transparent material; n = c/v.

Mass Energy

The energy contained in physical objects.

Kinetic Energy

The energy of motion.

Thermal Energy

The energy of heat.

Radiant (Radiative) Energy

The energy of light.

Wein's Law

Hotter objects emit the most intense light at shorter wavelengths/higher frequencies (more blue light).

Spectra

Split light into its individual wavelengths to create a rainbow band.

Blueshift

Object is moving toward us (Doppler Effect).

Redshift

Object is moving away from us (Doppler Effect).

Major objects in the solar system

One star, 8 planets, 5 dwarf planets (minimum), 200+ moons, millions of asteroids (estimated), trillions of comets (estimated).

Inner solar system

Mercury, Venus, Earth, Mars, and the asteroid belt; Smaller, rocky or metallic objects; Inner planets orbit the sun relatively close to each other.

Outer solar system

Jupiter, Saturn, Uranus, Neptune, the Kuiper belt, and the Oort cloud; Larger, icy or gaseous objects; Outer planets orbit the sun increasingly far apart.

Retrograde motion

Objects that orbit 'the wrong way'.

Triton

The largest moon of Neptune and an example of a retrograde moon

Catastrophic encounter hypothesis

States that material was ripped loose from the sun by a close encounter with another star, eventually forming the planets.

Falsified Catastrophic encounter hypothesis

Predicts very few planets in random, chaotic orbits around other stars, neither of which is true.

Collapsing nebular theory of planet formation

The solar system began as a gas cloud that imploded.

Conservation of angular momentum

Will flatten the nebula into an accretion disk.

Frost line

Hydrogen-based compounds like water (H2O) could only freeze into solids at a minimum distance from the sun.

Planetesimal

The early 'seeds' that grew through collisions with other objects into the planets.

Earth’s moon

Formed through the collision between Earth and a Mars-sized planetesimal.

Radiometric dating of asteroids

Confirms the solar system is about 4.5 billion years old.

Inner planets in order of increasing distance from the sun

Mercury, Venus, Earth, and Mars.

Inner planets and Earth’s moon in order of increasing size

Earth’s moon, Mercury, Mars, Venus, and Earth.

Planetary interiors

Metallic core (densest materials), rocky mantle (mostly middle-density silicate rock), rocky crust (mostly lower density rocks).

Lithosphere

Upper-most layers of a planet, composed of solid rock; includes all of the crust and the upper mantle.

Earth

Mostly nitrogen and oxygen atmosphere

Venus and Mars

Mostly carbon dioxide atmosphere

Mercury and Earth’s moon

No significant atmosphere

Greenhouse effect

Gases such as water vapor and carbon dioxide slow the loss of heat from a planet’s surface, raising the average temperature.

Caloris basin

Very large impact crater on Mercury; the impact event caused earthquakes and warped terrain all the way on the opposite side of the planet.

Maria

Dark regions on the moon’s surface; large volcanic plains that fill in massive craters and low-elevation areas; darker than surrounding rock due to the higher concentrations of iron.

Phobos and Deimos

Two small moons of Mars that are captured asteroids.

Jupiter

Most massive of the planets, arguably approaches the mass of the smallest stars.

Asteroid Belt

Located between the orbits of Mars and Jupiter; consists of asteroids made of metals and/or carbon-rich rock.

Orbital Resonances (Asteroid Belt)

Set the inner and outer edges of the asteroid belt with Jupiter; gaps within the belt are also due to orbital resonances.

Jupiter's Gravity (Asteroid Belt)

Prevented a large, single object from forming in the asteroid belt.

Ceres

The only dwarf planet in the asteroid belt, containing 1/3 of the main belt's mass.

Ida

The first asteroid discovered to have its own moon.

Trojan Asteroids

Asteroids found in the L4 and L5 Lagrangian points of Jupiter, outside the main asteroid belt.

Kuiper Belt

Located beyond the orbit of Neptune; consists of icy objects similar in composition to comets.

Short-Period Comets

Originate in the Kuiper Belt (T < 200 years).

Orbital Resonances (Kuiper Belt)

Set the inner and outer edges of the main Kuiper Belt, due to interactions with Neptune.

Pluto

A dwarf planet in the Kuiper Belt with five moons, one of which (Charon) is half its size.

Eris

A dwarf planet slightly more massive than Pluto; its discovery prompted the creation of the dwarf planet classification.

Oort Cloud

A spherical distribution of icy objects at the extreme edge of the solar system; the origin of long-period comets.

Long-Period Comets

Originate in the Oort cloud (T > 200 years).

Comets (Composition)

Frozen lumps of ice that form comas and tails when close to the sun due to sublimation.

Coma Formation

Occurs when a comet is within 3-5 AU from the sun.

Tail Formation

Occurs when a comet is within about 1 AU from the sun.

Comet Tails

Comets have two tails: a plasma tail that points directly away from the sun and a dust tail that curves away from the sun.

Direct Imaging

A method of detecting extrasolar planets that can only be done for planets in wide orbits around nearby stars.

Astrometry

A method of detecting extrasolar planets that looks for the slight motion of a star in the sky as a planet pulls on it.

Doppler Shifts

A method of detecting extrasolar planets that looks for a blueshift/redshift of a star’s spectra as a planet pulls on it.

Transits

A method of detecting extrasolar planets that looks for a dip in a star’s luminosity when a planet passes directly in front of it.

Kepler Space Telescope

Found thousands of extrasolar planets using the transit method.

Habitable Zones

The range of distances around a star within which a planet with an atmosphere can support liquid water on its surface.

Brown Dwarf

An intermediate class of objects too massive to be a gas giant but of insufficient mass to be a star.

Rogue Planets

Objects that formed in orbit around a star but were later ejected and now orbit the center of the galaxy on their own.