Detectable civilizations in our galaxy‏

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https://www.khanacademy.org/science/cosmology-and-astronomy/life-earth-universe/life-in-universe-topic/v/detectable-civilizations-in-our-galaxy-1

https://www.khanacademy.org/science/cosmology-and-astronomy/life-earth-universe/life-in-universe-topic/v/detectable-civilizations-in-our-galaxy-2

https://www.khanacademy.org/science/cosmology-and-astronomy/life-earth-universe/life-in-universe-topic/v/detectable-civilizations-in-our-galaxy-3

https://www.khanacademy.org/science/cosmology-and-astronomy/life-earth-universe/life-in-universe-topic/v/detectable-civilizations-in-our-galaxy-4

https://www.khanacademy.org/science/cosmology-and-astronomy/life-earth-universe/life-in-universe-topic/v/detectable-civilizations-in-our-galaxy-5