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.

Crash Course Astronomy Poster: store.dftba.com/products/crashcourse-astronomy-poster

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Table of Contents
What are binary stars? 0:51
Merging peanut-shaped stars 6:52
Close binaries begin to flow into one another, sometimes blowing up the star 8:29

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PHOTOS/VIDEOS
Big Dipper www.deepskycolors.com/archive/2011/05/14/The-Big-Dipper.html [credit: Rogelio Bernal Andreo]
Sirius www.spacetelescope.org/images/heic0516a/ [credit: NASA, ESA, H. Bond (STScI), and M. Barstow (University of Leicester)]
Sirius A and B chandra.harvard.edu/photo/2000/0065/index.html [credit: NASA/SAO/CXC]
Clashing Winds (video) svs.gsfc.nasa.gov/cgi-bin/details.cgi?aid=11680 [credit: NASA's Goddard Space Flight Center]
The Radial Velocity Method (artist’s impression) www.eso.org/public/images/eso0722e/ [credit: ESO]
Mizar+Alcor commons.wikimedia.org/wiki/File:Thomas_Bresson_-_Mizar%2Balcor_(by).jpg [credit: Wikimedia Commons, Thomas Bresson]
Polaris imgsrc.hubblesite.org/hu/db/images/hs-2006-02-e-print.jpg [credit: NASA, ESA, and G. Bacon]
Does the Sun Have Long Lost Siblings? www.youtube.com/watch?v=IaWg2ACMspk [credit: SciShow Space]
Clashing Winds (image) svs.gsfc.nasa.gov/cgi-bin/details.cgi?aid=11680 [credit: NASA/C. Reed X-ray images courtesy of NASA/GSFC/S. Immler]
Artist’s impression of the pulsar PSR J0348+0432 and its white dwarf companion www.eso.org/public/images/eso1319c/ [credit: ESO/L. Calçada]
Artist’s impression of eclipsing binary www.eso.org/public/videos/eso1311b/ [credit: ESO/L. Calçada]
Artist’s impression of the yellow hypergiant star HR 5171 www.eso.org/public/images/eso1409b/ [credit: ESO]
Nova www.nasa.gov/centers/goddard/news/topstory/2008/keck_ophiuchi_prt.htm [credit: NASA, Casey Reed]
Artist's impression of RS Ophiuchi www.jodrellbank.manchester.ac.uk/news/2006/rsoph-radio/ [credit: David A. Hardy/www.astroart.org & PPARC]
An artist's impression of Sirius A and B www.spacetelescope.org/images/heic0516b/ [credit: NASA, ESA and G. Bacon (STScI)]
Artist's impression of vampire star www.spacetelescope.org/videos/astro_bn/ [credit: ESO/M. Kornmesser]
Type Ia supernova svs.gsfc.nasa.gov/cgi-bin/details.cgi?aid=10532 [credit: Walt Feimer, NASA/Goddard Space Flight Center]

1. Introduction to Astronomy
2. Naked Eye Observations
3. Cycles in the Sky
4. Moon Phases
5. Eclipses
6. Telescopes
7. The Gravity of the Situation
8. Tides
9. Introduction to the Solar System
10. The Sun
11. The Earth
12. The Moon
13. Mercury
14. Venus
15. Mars
16. Jupiter
17. Jupiter's Moons
18. Saturn
19. Uranus & Neptune
20. Asteroids
21. Comets
22. The Oort Cloud
23. Meteors
24. Light
25. Distances
26. Stars
27. Exoplanets
28. Brown Dwarfs
29. Low Mass Stars
30. White Dwarfs & Planetary Nebulae
31. High Mass Stars
32. Neutron Stars
33. Black Holes
34. Binary and Multiple Stars
35. Star Clusters
36. Nebulae
37. The Milky Way
38. Galaxies, part 1
39. Galaxies, part 2
40. Gamma-Ray Bursts
41. Dark Matter
42. The Big Bang, Cosmology part 1
43. Dark Energy, Cosmology part 2
44. A Brief History of the Universe
45. Deep Time
46. Everything, The Universe...And Life
47. Explore The Solar System:

In this Crash Course series, marvel at the wonders of astronomy with your host for this intergalactic adventure, the Bad Astronomer himself -- Phil Plait. In just 40 short lessons, you will learn the basics of the oldest science known to humanity.

Be sure to check out links to relevant Photos in the description for each video.