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Can "tired light theory" explain the observed redshifts of galaxies? (Intermediate)
Could photons be dark matter? (Intermediate)
Does a photon observe other photons moving past it at the speed of light? (Intermediate)
Has there been an experiment that measured speed faster than the speed of light in vacuum? (Advanced)
How are photons created and destroyed? (Advanced)
How do photons from distant objects maintain enough energy to reach us? (Intermediate)
How do scientists slow light down? (Intermediate)
How do we feel heat? (Intermediate)
If light has no mass, then what draws it into a black hole? (Intermediate)
If photons have zero mass, why do they feel the effects of gravity? (Advanced)
Is light blueshifted when it is gravitationally lensed? (Intermediate)
Is light of different colors affected differently by gravity? (Intermediate)
What is the photoelectric effect? Why can't multiple low-frequency photons free an electron? (Intermediate)
Where did the matter in the universe come from? (Intermediate)
Where did the Universe come from? (Intermediate)
Why doesn't light from distant galaxies reach us instantaneously? (Intermediate)

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When unmanned spacecraft accelerate in space, what causes them to slow down? (Intermediate)

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If I were hypothetically wearing a spacesuit and sitting on one of the Voyager space probes at their current positions in space, how much light would I have? (Intermediate)
What makes certain lunar eclipses so special? (Beginner)
Why is Earth's core molten? (Intermediate)
What will happen to Earth's tides as the moon moves away from us? (Intermediate)
How can spiral galaxies keep their shape? (Advanced)
Does a black hole have a "surface"? (Intermediate)
How can you link telescopes to make a bigger "effective telescope"? (Intermediate)
What should I know about the upcoming Solar Eclipse (2017)? (Beginner)
Do galaxies that are receding from us faster than the speed of light disappear from our observations? (Intermediate)
Why are astronomers interested in gravitational waves? (Intermediate)

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What is the universe expanding into? (Intermediate)
What do I need to do to become an astronomer? (Beginner)
What's the difference between astronomy and astrology? (Beginner)
Why is looking out into space the same as looking back in time? (Beginner)
Can we find the place where the Big Bang happened? (Intermediate)

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What color is each planet? (Intermediate)
How are light and heavy elements formed? (Advanced)
What is the significance of the Tropic of Cancer, Tropic of Capricorn, Arctic Circle and Antarctic Circle? (Beginner)
How far is each planet from Earth? (Intermediate)
What is the evidence supporting the nebula theory of Solar System formation? (Intermediate)
How does the location of sunrise and sunset change throughout the year? (Advanced)
What is the difference between a "star" and a "sun"? (Beginner)
Why do the planets orbit the sun? (Beginner)
What are the requirements for being a planet? (Beginner)
Can artificial gravity be created in space? (Intermediate)

What color is each planet? (Intermediate)
What is the significance of the Tropic of Cancer, Tropic of Capricorn, Arctic Circle and Antarctic Circle? (Beginner)
How are light and heavy elements formed? (Advanced)
How far is each planet from Earth? (Intermediate)
Are aliens or UFOs real? (Beginner)
How does the location of sunrise and sunset change throughout the year? (Advanced)
Are there telescopes that can see the flag and lunar rover on the Moon? (Beginner)
What is the difference between a "star" and a "sun"? (Beginner)
When was the last time all of the planets were aligned? (Beginner)
What is the evidence supporting the nebula theory of Solar System formation? (Intermediate)

What color is each planet? (Intermediate)
How are light and heavy elements formed? (Advanced)
What is the significance of the Tropic of Cancer, Tropic of Capricorn, Arctic Circle and Antarctic Circle? (Beginner)
How far is each planet from Earth? (Intermediate)
What is the difference between a "star" and a "sun"? (Beginner)
How is it proved that the Universe is expanding? (Intermediate)
How does the location of sunrise and sunset change throughout the year? (Advanced)
Are there telescopes that can see the flag and lunar rover on the Moon? (Beginner)
When was the last time all of the planets were aligned? (Beginner)
Is the Moon moving away from the Earth? When was this discovered? (Intermediate)

What's going to happen on December 21st 2012? (Intermediate)
What color is each planet? (Intermediate)
At what speed does the Earth move around the Sun? (Beginner)
How far is each planet from Earth? (Intermediate)
Is the Moon moving away from the Earth? When was this discovered? (Intermediate)
How do you measure the distance between Earth and the Sun? (Intermediate)
Why do the planets orbit the sun? (Beginner)
What is the significance of the Tropic of Cancer, Tropic of Capricorn, Arctic Circle and Antarctic Circle? (Beginner)
How long does it take for the Sun's light to reach us? (Beginner)
When was the last time all of the planets were aligned? (Beginner)

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