PBS_CrashCourseAstronomy

Now that weÕre done with the planets, asteroid belt, and comets, weÕre 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.

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.

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.

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.

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 objectÕs temperature, density, spin, motion, and chemical composition.

Today PhilÕs 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 starsÕ luminosity versus temperature, and most stars fall along the main sequence, where they live most of their lives.

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 weÕve even found planets that are the same size as Earth. We think there may be many billions of Earth-like planets in our galaxy.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

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.

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 a Big Bang, when the Universe was incredibly dense and hot.

As we approach the end of Crash Course Astronomy, it’s time now to acknowledge that our Universe’s days are numbered.

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