Our Place in the Universe & Cosmic Scale (Lectures 1 & 2)

Planetary Science

The study of the creation and evolution of planetary bodies, moons, asteroids, and comets to investigate why life forms on some worlds and not others.

Biology

The study of the formation and evolution of life; currently limited to extrapolating Earth-based life to search for extraterrestrial life.

Goldilocks Zone (Habitable / "Splash" Zone)

The optimal orbital distance from a host star where temperatures are "just right" for liquid water to exist on a planet's surface (too close = blasted by radiation and water evaporates; too far = water freezes).

Star

A huge, glowing ball of mostly hydrogen gas that generates heat and light via nuclear fusion deep in its core (lower-mass stars live much longer than higher-mass stars).

Planet

A moderately large object that orbits a star and shines by reflected light; may be rocky, icy, or gaseous in composition.

Dwarf Planet

The classification given to Pluto and the family of icy objects orbiting beyond Neptune (Trans-Neptunian Objects / TNOs).

Moon (Satellite)

An object that orbits a planet.

Asteroid

A relatively small and rocky object (often non-spherical in shape) that orbits a star.

Comet

A relatively small and icy object that orbits a star; develops a coma of escaping gas, a plasma tail, and a dust tail ONLY when it comes close to the Sun.

Solar (Star) System

A star and all the material that orbits it, including planets, moons, asteroids, and comets.

Nebula

A huge interstellar cloud of gas (mostly hydrogen) and dust (tiny solid grains of carbon or silicon, similar to smoke particles).

Galaxy

An enormous "island of stars" held together by gravity and orbiting a common center; contains far more mysterious "dark matter" than visible stars (e.g., the Milky Way or Andromeda).

Cluster of Galaxies

A giant collection of galaxies held together by gravity.

Universe

The sum total of all matter and energy; everything within and between all galaxies.

Cosmic Address (Smallest to Largest)

Earth \rightarrow Solar System \rightarrow Milky Way Galaxy \rightarrow Local Group \rightarrow Local Supercluster \rightarrow The Universe.

Astronomical Unit (AU)

The average distance from the Earth to the Sun.

Atom

The microscopic building block of all chemical elements; hydrogen is the lightest and most abundant element in nature.

"Star Stuff" (Cosmic Origins of Elements)

The Big Bang produced only the lightest gases (Hydrogen and Helium); all heavier elements needed for life (like carbon and oxygen) were forged inside the cores of stars via nuclear reactions and expelled into space when stars aged and died.

Speed of Light

The absolute fastest speed anything can travel in the Universe; attempting to accelerate a particle near the speed of light causes its mass to increase.

Light-Year

A unit of physical distance (NOT time) equal to the distance that light travels in one year.

Gravitational Waves

Disturbances in the fabric of spacetime (not light or electromagnetic radiation) that travel at the speed of light; detected by the LIGO observatory from merging black holes.

Lookback Time

Because light takes time to travel through space, looking farther away into space means looking further back in time.

Observable Universe Limit

The boundary of how far we can see in the Universe; we cannot see beyond this limit because light from farther away would have had to be emitted before the Universe existed.

Grapefruit Scale Model of the Solar System

If the Sun is shrunk to the size of a grapefruit, Earth is the tip of a ballpoint pen, Jupiter is a marble, Pluto is a tiny speck of dust, and the nearest star is another grapefruit located across the country; demonstrates that the Solar System and Universe are mostly empty space.

Voyager Spacecraft

The most distant human-made object, which has left the planets behind and is traveling through interstellar space, though it has only covered a tiny fraction of the distance to the nearest star.

Spaceship Earth Motions

Earth is never sitting still: it rotates daily on its axis, revolves annually around the Sun, moves with the Sun around the center of the Milky Way Galaxy, and moves with the Local Group as the Universe expands.

Raisin Cake Analogy (Expansion of the Universe)

Galaxies outside our Local Group are moving away from us, and more distant galaxies move faster because space itself (the dough) is expanding uniformly. Locally, gravity within small galaxy groups overcomes this expansion.

Cosmic Calendar

Carl Sagan's scale compressing the entire history of the Universe into a single calendar year: the Big Bang happens at the start of January, the Milky Way forms in February, Earth and early life form in September, the Cambrian Explosion and dinosaurs occur in late December, and all of human civilization occupies only the final few seconds before midnight on December 31.

Universe Mass/Energy Composition

The vast majority of the Universe consists of Dark Energy, followed by Dark Matter, while normal atoms (mostly free Hydrogen and Helium, with a tiny fraction of stars and heavy elements) make up only a very small fraction of the Universe.