Now that we’ve finished our tour of the planets, we’re headed back to the asteroid belt. Asteroids are chunks of rock, metal, or both that were once part of smallish planets but were destroyed after collisions. Most orbit the Sun between Mars and Jupiter, but some get near the Earth. The biggest, Ceres, is far smaller than the Moon but still big enough to be round and has undergone differentiation.

CORRECTION: In the episode, we say that 2010 TK7 is 800 km away. However, 2010 TK7 stays on average 150 million kilometers from Earth, but that can vary wildly.
Sorry about that!

Chapters:
Introduction: Asteroids
What are Asteroids?
Structure of the Main Belt
Ceres's Structure
Vesta and other Main Belt Asteroids
Rubble Piles
Why did the Asteroid Belt form?
Mars-crossing, Apollo, and Aten Asteroids
Trojan Asteroids & Lagrange Points
How Asteroids Get Their Names
Review

Thanks to observations of galaxy redshifts, we can tell that the universe is EXPANDING! Knowing that the universe is expanding and how quickly it's expanding also allows us to run the clock backward 14 billion years to the way the universe began - with a bang.

Chapters:
Introduction: The Origins of the Universe
Distant Galaxies Show a Redshift in Their Spectra
The Universe is Expanding
The Big Bang
Lookback Time
The Cosmic Microwave Background
What is the Universe Expanding Into?
Review

Double stars are stars that appear to be near each other in the sky, but if they’re gravitationally bound together we call them binary stars. Many stars are actually part of binary or multiple systems. If they are close enough together they can actually touch other, merging into one peanut-shaped star. In some close binaries, matter can flow from one star to the other, changing the way it ages. If one star is a white dwarf, this can cause periodic explosions, and possibly even lead to blowing up the entire star.

Chapters:
Introduction: Binary & Multiple Stars
Visual Binary Stars
Spectroscopic Binaries
Multiple Star Systems
Eclipsing Binaries
Contact Binaries
Stellar Novae
Review

We’ve covered a lot of incredible stuff, but this week we’re talking about the weirdest objects in space: BLACK HOLES. Stellar-mass black holes form when a very massive star dies, and its core collapses. The core has to be more than about 2.8 times the Sun’s mass to form a black hole. Black holes come in different sizes, but for all of them, the escape velocity is greater than the speed of light, so nothing can escape, not matter or light. They don’t wander the Universe gobbling everything down around them; their gravity is only really intense very close to them. Tides near a stellar mass black hole will spaghettify you, and time slows down when you get near a black hole — not that this helps much if you’re falling in.

Chapters:
Introduction
How Black Holes Are Formed
Misconceptions About Black Holes
Stellar Mass Black Holes
Spaghettification
Black Holes Warp Space-Time
Review

Thanks to the wonders of physics, astronomers can map a timeline of the universe’s history. Today, Phil’s going to give you an overview of those first few minutes (yes, MINUTES) of the universe’s life. It started with the Big Bang when the Universe was incredibly dense and hot. It expanded and cooled, going through multiple stages where different kinds of matter could form. It underwent a phenomenally rapid expansion called inflation, which smoothed out much of the lumpiness in the matter. Normal matter formed atoms between 3 and 20 minutes after the bang, and the lumps left over from inflation formed the galaxies and larger structures we see today.

Chapters:
Introduction: The History of the Universe
Giant Particle Colliders
Phase Changes
After the Big Bang: The First Minutes
Recombination
Inflation
Unanswered Questions in Astronomy
Review

While Jupiter is nowhere near massive enough to initiate fusion in its core, there are even more massive objects out there that fall just short of that achievement called brown dwarfs. Brown dwarfs have a mass between giant planets and small stars. They were only recently discovered in the 1990s, but thousands are now known. More massive ones can fuse deuterium, and even lithium, but not hydrogen, distinguishing them from “normal” stars. Sort of.

Correction: In the illustration at , the numbers listed after the star names are the year of discovery, not distance.

Chapters:
Introduction: Brown Dwarfs
L Stars & The Lithium Test
Discovering the First Brown Dwarfs
What Color Are Brown Dwarfs?
Physical Characteristics of Brown Dwarfs
Small Stars vs. Big Planets
Review

Today on Crash Course Astronomy, Phil explains comets. Comets are chunks of ice and rock that orbit the Sun. When they get near the Sun the ice turns into gas, forming the long tail, and also releases dust that forms a different tail. We’ve visited comets up close and found them to be lumpy, with vents on the surface that release the gas as ice sublimates. Eons ago, comets (and asteroids) may have brought a lot of water to Earth -- as well as the ingredients for life.

Chapters:
Introduction: Comets
Comets = Dirty Snowballs
Comets Have Two Tails
Short-Term vs Long-Term Comets
Where do comets come from?
Comets Up-Close
What Happens When Comets Hit Earth?
Review

This week we build on our naked eye observations from last week and take a look at the cyclical phenomena that we can see at work in the universe.

Chapters:
Introduction
Cycles in the Sky
The Zodiac
Planetary Movement
The Earth's Axis
Precession
Things We've Learned From Naked Eye Observations
Review

The majority of the universe is made up of a currently mysterious entity that pervades space: dark energy. We don’t know exactly what it is, but we do know that dark energy accelerates the expansion of space. We think this means the Universe will expand forever, even as our view of it shrinks while space expands faster all the time.

Chapters:
Introduction: The Expanding Universe
The Expansion of Space is Accelerating
What is Dark Energy?
Will the Universe Expand Forever? The Geometry of the Universe
The Cosmic Horizon of the Observable Universe
Review

Today on Crash Course Astronomy, Phil dives into some very dark matters. The stuff we can actually observe in the universe isn’t all there is. Galaxies and other large structures in the universe are created and shifted by a force we detect mostly indirectly, by observing its impact: DARK MATTER.

Chapters:
Introduction: Dark Matter
Discovery of Dark Matter
What is Dark Matter?
Axions
Gravitational Lensing and the Bullet Cluster
What Effect Does Dark Matter Have on the Universe?
Review

As we approach the end of Crash Course Astronomy, it’s time now to acknowledge that our Universe’s days are numbered. Stars will die out after a few trillion years, protons will decay and matter will dissolve after a thousand trillion trillion trillion years, black holes will evaporate after 10^92 years, and then all will be dark. But there is still hope that a new Universe will be born from it.

Chapters:
Introduction: The End of the Universe
Scientific Notation
The Five Ages of the Universe
The Primordial Era
The Stelliferous Era (You Are Here!)
The Degenerate Era
The Black Hole Era
The Dark Era
The Big Rip
Other Possibilities: Multiverses & The Cosmic Reboot
Review

How do astronomers make sense of the vastness of space? How do they study things so far away? Today Phil talks about distances, going back to early astronomy. Ancient Greeks were able to find the size of the Earth and from that the distance to and the sizes of the Moon and Sun. Once the Earth/Sun distance was found, parallax was used to find the distance to nearby stars, and that was bootstrapped using brightness to determine the distances to much farther stars.

Chapters:
Introduction
How did we calculate the Earth's Size?
THE Astronomical Unit (AU) = 149,597,870.7 km
Depth Perception & Parallax
Light Years & Parsecs
Brightness Indicates Distance
Review

Phil starts the planet-by-planet tour of the solar system right here at home, Earth.

Chapters:
Introduction: The Earth
Layers of Earth
Continental Plates
Why is the Earth's Core so Hot?
Earth's Magnetic Field
Earth's Atmosphere
Atmospheric Pressure
Liquid Water on Earth
The Greenhouse Effect: CO2 in the Atmosphere
Review

The big question in the comments last week was, "BUT WHAT ABOUT ECLIPSES?" Today, Phil breaks 'em down for you.

Chapters:
Introduction
How Do Solar Eclipses Work?
Umbra & Penumbra
The Sun's Corona
Solar Eclipse Totality
Annular Eclipse
Can You Look at a Solar Eclipse?
Lunar Eclipses
Size of the Earth and Moon
Review

Here it is, folks: the end. In our final episode of Crash Course Astronomy, Phil gives the course a sendoff with a look at some of his favorite topics and the big questions that Astronomy allows us to ask.

Chapters:
Introduction: Are We Alone in the Universe?
The Search for Earth-like Exoplanets
Is There Alien Life in the Universe?
SETI: The Search for Extraterrestrial Intelligence
Let's Explore the Universe!
Review

Today Phil explains that YES, there are other planets out there and astronomers have a lot of methods for detecting them. Nearly 2000 have been found so far. The most successful method is using transits, where a planet physically passes in front of its parent star, producing a measurable dip in the star’s light. Another is measuring the Doppler shift in a star’s light due to reflexive motion as the planet orbits. Exoplanets appear to orbit nearly every kind of star, and we’ve even found planets that are the same size as Earth. We think there may be many billions of Earth-like planets in our galaxy.

Chapters:
Introduction: Exoplanets
Reflexive Motion
Discovering the First Exoplanets
51 Pegasi b
Exoplanets in Transit
Detecting Exoplanet Transits
Photographing Exoplanets
Kinds of Exoplanets
Earth-like Exoplanets
Review

Take a 360-degree virtual tour of our Solar System, with the help of Crash Course Astronomy host Phil Plait!

The Milky Way is our neighborhood in the universe. It’s a galaxy and there are many others out there. Galaxies contain gas, dust, and billions of stars or more. They come in four main shapes: elliptical, spiral, peculiar, and irregular. Galaxies can collide, and grow in size by eating each other.

Chapters:
Introduction: Galaxies
Spiral Galaxies
Elliptical Galaxies
Galactic Collisions
Peculiar Galaxies
Irregular Galaxies
Review

Active galaxies pour out lots of energy, due to their central supermassive black holes gobbling down matter. Galaxies tend not to be loners but instead exist in smaller groups and larger clusters. Our Milky Way is part of the Local Group, and will one day collide with the Andromeda galaxy. Clusters of galaxies also clump together to form superclusters, the largest structures in the Universe. In total, there are hundreds of billions of galaxies in the Universe.

Chapters:
Introduction: Active Galaxies
Active Galaxy Structure: Central Black Hole
Active Galaxy Structure: Accretion Disks
The Milky Way's Supermassive Black Hole
The Local Group
Miklomeda: Andromeda and the Milky Way Collide!
Galaxy Clusters
Superclusters
How Many Galaxies Are There?
Review

Gamma-ray bursts are not only incredible to study, but their discovery has an epic story all its own. Today Phil takes you through some Cold War history and then dives into what we know. Bursts come in two rough varieties: Long and short. Long ones are from hypernovae, massive stars exploding, sending out twin beams of matter and energy. Short ones are from merging neutron stars. Both kinds are so energetic they are visible for billions of light years, and both are also the birth announcements of black holes.

Chapters:
Introduction
Gamma Ray Bursts and the Cold War
Where Do Gamma Ray Bursts Come From?
What Causes Gamma Ray Bursts?
Kinds of Gamma Ray Bursts: Long and Short
What Would Happen if a Gamma Ray Burst Hit Earth?
Review