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Comets: Crash Course Astronomy #21

S1 E21 / 11m 54s
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 in the surface that release the gas as ice sublimates.
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The Oort Cloud: Crash Course Astronomy #22

S1 E22 / 11m 40s
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.
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Meteors: Crash Course Astronomy #23

S1 E23 / 11m 21s
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.
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Light: Crash Course Astronomy #24

S1 E24 / 10m 33s
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.
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Distances: Crash Course Astronomy #25

S1 E25 / 11m 20s
How do astronomers make sense out 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.
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Stars: Crash Course Astronomy #26

S1 E26 / 10m 40s
Today Phil's explaining the stars and how they can be categorized using their spectra. Together with their distance, this provides a wealth of information about them including their luminosity, size, and temperature. The HR diagram plots stars's luminosity versus temperature, and most stars fall along the main sequence, where they live most of their lives.
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Exoplanets: Crash Course Astronomy #27

S1 E27 / 11m 49s
Phil explains that YES, there are other planets out there and astonomers have a lot of methods for detecting them. Nearly 2000 have been found so far. 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.
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Brown Dwarfs: Crash Course Astronomy #28

S1 E28 / 11m 5s
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 as well called brown dwarfs. Brown dwarfs, have a mass that places them between giant planets and small stars.
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Low Mass Stars: Crash Course Astronomy #29

S1 E29 / 12m 2s
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. When this happens they expand, get brighter, and cool off, becoming red giants.
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White Dwarfs & Planetary Nebulae: Crash Course Astronomy #30

S1 E30 / 11m 9s
Today Phil follows up last week's look at the death of low mass stars with what comes next: a white dwarf. White dwarfs are incredibly hot and dense objects roughly the size of Earth. They also can form planetary nebulae: huge, intricately detailed objects created when the wind blown from the dying stars is lit up by the central white dwarf.
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High Mass Stars: Crash Course Astronomy #31

S1 E31 / 12m 2s
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 resulting supernova creates even more heavy elements, scattering them through space.
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Neutron Stars: Crash Course Astronomy #32

S1 E32 / 12m 42s
In the aftermath of a 8 – 20 solar mass star’s demise we find a weird little object known as a neutron star. Neutrons stars are incredibly dense, spin rapidly, and have very strong magnetic fields. Neutrons stars with the strongest magnetic fields are called magnetars, and are capable of colossal bursts of energy that can be detected over vast distances.
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Black Holes: Crash Course Astronomy #33

S1 E33 / 12m 11s
Stellar mass black holes form when a very massive star dies, and its core collapses. 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.
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Binary and Multiple Stars: Crash Course Astronomy #34

S1 E34 / 11m 46s
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. In some close binaries matter can flow from one star to the other, changing the way it ages.
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Nebulae: Crash Course Astronomy #36

S1 E36 / 12m 1s
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. Some nebulae are small and dense, others can be dozens or hundreds of light years across.
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Galaxies, part 2: Crash Course Astronomy #39

S1 E39 / 15m 20s
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.
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