In today's episode, Phil looks at how gravity plays out across the universe.

Chapters:
Introduction
Understanding Mass
What is Gravity?
Different Types of Orbit
Escape Velocity
Why Are Astronauts Weightless?
Gravity Warps Space
Review

Massive stars fuse heavier elements in their cores than lower-mass stars. This leads to the creation of heavier elements up to iron. Iron robs critical energy from the core, causing it to collapse. The shock wave, together with a huge swarm of neutrinos, blasts through the star’s outer layers, causing it to explode. The resulting supernova creates even more heavy elements, scattering them through space. Also, happily, we’re in no danger from a nearby supernova.

Chapters:
Introduction: High Mass Stars
Core Fusion Creates Heavier Elements
Other Stages of High Mass Stars
Silicone & Iron Fusion
Core Collapse
Supernova Remnants
Explosive Nucleosynthesis
Review

Welcome to the first episode of Crash Course Astronomy. Your host for this intergalactic adventure is the Bad Astronomer himself, Phil Plait. We begin with answering a question: "What is astronomy?"

Chapters:
Introduction
What is Astronomy?
Who Studies Astronomy?
Origins of Astronomy
Astrology vs Astronomy
Geocentrism
Revolutions in Astronomy
Astronomy Today
Review

In today's Crash Course Astronomy, Phil takes a look at the explosive history of our cosmic backyard. We explore how we went from a giant ball of gas to the system of planets and other celestial objects we have today.

Chapters:
Introduction
Geocentrism vs Heliocentrism
Makeup of the Solar System
Is Pluto a Planet?
Our Solar System
How Our Solar System Formed
Planet Formation Depends on Distance to Sun
Review

Jupiter is the biggest planet in our solar system. The gas giant is NOT a failed star, but a really successful planet! It has a dynamic atmosphere with belts and zones, as well as an enormous red spot that’s actually a persistent hurricane. Jupiter is still warm from its formation, has an interior that’s mostly metallic hydrogen, and may not even have a core.

Chapters:
Introduction: Jupiter
Jupiter's Clouds: Zones & Belts
Jupiter's Great Red Spot
Jupiter's Structure
Is Jupiter a Failed Star?
Jupiter Radiates Heat
Jupiter's Strong Magnetic Field
Jupiter's Gravity
Review

Before moving on from Jupiter to Saturn, we’re going to linger for a moment on Jupiter’s moons. There are 67 known moons, 4 of which are the huge ones that we want to explore in greater detail. Ganymede is the largest - larger, in fact, than any other moon in the solar system and the planet Mercury! Callisto, orbiting the farthest out, is smaller but quite similar to Ganymede in many ways. Io, meanwhile, is most noteworthy for its tremendous volcanic activity. There’s also water on Ganymede and Europa!

Chapters:
Introduction: Jupiter's Moons
Ganymede: Jupiter's Largest Moon
Callisto: Jupiter's Second Largest Moon
Io: Volcanoes!
Europa: The Ocean Moon
The Habitable Zone
Jupiter's Other Moons
Review

In order to understand how we study the universe, we need to talk a little bit about light. Light is a form of energy. Its wavelength tells us its energy and color. Spectroscopy allows us to analyze those colors and determine an object’s temperature, density, spin, motion, and chemical composition.

Chapters:
Introduction
Light is a Wave
Electromagnetic Spectrum
How is Light Made?
Atomic Structure
Spectroscopy
Redshift vs Blueshift
Review

Today we are talking about the life -- and death -- of stars. Low-mass stars live a long time, fusing all their hydrogen into helium over a trillion years. More massive stars like the Sun live shorter lives. They fuse hydrogen into helium, and eventually helium into carbon (and also some oxygen and neon). When this happens they expand, get brighter, and cool off, becoming red giants. They lose most of their mass, exposing their cores, and then cool off over many billions of years.

Chapters:
Introduction: Low Mass Stars
Hydrogen Fusion
Life Cycle of Low Mass Stars
Larger Stars (Like Our Sun) Live Shorter Lives
Fueled By Fusion
Red Giants
White Dwarfs
The Fate of the Earth
Review

The fourth planet from the sun and the outermost of the terrestrial planets, Mars has long been a popular spot for missions and imagination. Phil walks you through the planet's topography, core, and features. We'll take a look back at Mars's past and makes predictions for its future, including the possibilities for human life.

Chapters:
Introduction: Mars
Mars is Red, Rusty, and Dusty
Surface Features of Mars
Mars's Polar Ice Caps
Martian Atmosphere
Martian Moons: Deimos and Phobos
Is There Water on Mars?
Was There Life on Mars?
Martian Missions
Review

Mercury is the closest planet to the sun. It has no atmosphere and is, as such, covered in craters. It's also incredibly hot but, surprisingly, has water ice hiding beneath its surface.

Chapters:
Introduction: Mercury
Mercury's Orbit
Mercury's Rotation - Orbit Ratio
A Weird Day on Mercury
Mercury's Craters
Mercury's Internal Structure
Deep Crater Water Ice on Mercury
Review

Today Phil helps keep you from ticking off an astronomer in your life by making sure you know the difference between a meteor, meteorite, and meteoroid. When the Earth plows through the stream emitted by a comet we get a meteor shower. Meteors burn up about 100 km above the Earth, but some survive to hit the ground. Most of these meteorites are rocky, some are metallic, and a few are a mix of the two. Very big meteorites can be a very big problem, but there are plans in the works to prevent us from going the way of the dinosaurs.

Chapters:
Introduction: Meteors
Shooting Stars, Meteoroids, Meteors, or Meteorites?
Kinetic Energy
Compression & Ablation
Sporadic Meteors
Meteor Showers
Bolides
Classifying Meteorites
Very Big Meteorite = Very Big Problem
Review

Today we’re talking about our galactic neighborhood: The Milky Way. It’s a disk galaxy, a collection of dust, gas, and hundreds of billions of stars, with the Sun located about halfway out from the center. The disk has grand spiral patterns, formed by the traffic jams of stars and nebulae, where stars are born. The central region is shaped like a bar and is mostly old, red stars. There’s also a halo surrounding us of old stars.

Chapters:
Introduction: The Milky Way
What is the Milky Way Galaxy?
The Milky Way's Shape
Spiral Arms of the Milky Way
Mapping the Milky Way
Outer Halo of the Milky Way
Review

Join Phil for a tour of our capital-M Moon, from surface features, inside to the core, and back in time to theories about its formation.

Chapters:
Introduction: The Moon
How Big is the Moon?
Internal Structure of the Moon
The Moon's Surface: Highlands and Maria
How the Moon Formed: The Giant Impact Hypothesis
The Moon's Craters
Water on the Moon? H2O Yeah!
Review

In this episode of Crash Course Astronomy, Phil takes you through the cause and name of the Moon's phases.

Chapters:
Why does the Moon Have Phases?
New Moon
Waxing Crescent
First Quarter
Waxing Gibbous
Full Moon
Waning Gibbous, Third Quarter, and Waning Crescent
Earthshine
Review

Today on Crash Course Astronomy, Phil invites you to head outside and take a look at all the incredible things you can see with your naked eye.

Chapters:
Introduction: Naked Eye Observations
Classifying Stars by Magnitude
Star Colors
Constellations
Names of Stars
Light Pollution
Why Do Stars Twinkle?
Naked-Eye Planets
The Sky's Motion
Which Stars Can We See?
Review

Astronomers study a lot of gorgeous things, but nebulae might be the most breathtakingly beautiful of them all. Nebulae are clouds of gas and dust in space. They can glow on their own or reflect light from nearby stars. When they glow it’s usually predominantly red from hydrogen and green from oxygen, and when they reflect and scatter light it’s from massive hot stars, so they look blue. Stars are born in some nebulae and create new ones as they die. Some nebulae are small and dense, others can be dozens or hundreds of lightyears across.

Chapters:
Introduction: Nebulae
Emission Nebulae
Reflection Nebulae
Molecular Clouds
The Orion Nebula
Stars Form in Nebulae
Edges of Nebulae
Viewing Nebulae in the Infrared
Review

In the aftermath of an 8-20 solar mass star’s demise, we find a weird little object known as a neutron star. Neutron stars are incredibly dense, spin rapidly, and have very strong magnetic fields. Some of them we see as pulsars, flashing in brightness as they spin. Neutron stars with the strongest magnetic fields are called magnetars and are capable of colossal bursts of energy that can be detected over vast distances.

Chapters:
Introduction: Neutron Stars
Electron Degeneracy
Neutron Degeneracy
Neutron Star Characteristics
Pulsars
Magnetars
Review

Now that we’re done with the planets, asteroid belt, and comets, we’re heading to the outskirts of the solar system. Out past Neptune are vast reservoirs of icy bodies that can become comets if they get poked into the inner solar system. The Kuiper Belt is a donut shape aligned with the plane of the solar system; the scattered disk is more eccentric and is the source of short-period comets, and the Oort Cloud which surrounds the solar system out to great distances is the source of long-period comets. These bodies all probably formed closer to the Sun and got flung out to the solar system’s suburbs by gravitational interactions with the outer planets.

Chapters:
Introduction: Where Do Comets Come From?
Kuiper Belt, Scattered Disk, and Oort Cloud
Long-Period Comets come from the Oort Cloud
Short-Period Comets come from the Scattered Disk
Pluto, Plutinos, and other Kuiper Belt Objects
Oort Cloud Objects
Review

Saturn is the crown jewel of the solar system, beautiful and fascinating. It is a gas giant and has a broad set of rings made of ice particles. Moons create gaps in the rings via their gravity. Saturn has dozens of moons, including Titan, which is as big as Mercury and has a thick atmosphere and lakes of methane; and Enceladus which has an undersurface ocean and eruptions of water geysers. While we are still uncertain, it is entirely possible that either or both moons may support life.

Chapters:
Introduction: Saturn
Saturn's Atmosphere
Saturn's Hexagonal Storm
Saturn's Rings
Anomalies in Saturn's Rings
Titan: Saturn's Largest Moon
Enceladus’s Water Geysers
Saturn's Other Moons
Review

Last week we covered multiple star systems, but what if we added thousands or even millions of stars to the mix? A star cluster. There are different kinds of clusters, though. Open clusters contain hundreds or thousands of stars held together by gravity. They’re young and evaporate over time, their stars let loose to roam space freely. Globular clusters, on the other hand, are larger, have hundreds of thousands of stars, and are more spherical. They’re very old, a significant fraction of the age of the Universe itself, and that means their stars have less heavy elements in them, are redder, and probably don’t have planets (though we’re not really sure).

Chapters:
Introduction: Star Clusters
Determining the Age of Star Clusters
Open Clusters Evaporate
The Pleiades Star Cluster
Globular Clusters
Review