What is the speed of light in a vacuum?

About 30,000 km/s About 300,000 km/s (299,792,458 m/s) About 3,000,000 km/s About 150,000 km/s

How long does it take light from the Sun to reach Earth?

About 8 seconds About 1 hour About 8 minutes About 1 day

Proxima Centauri, the nearest star to our Sun, is about 4.2 light-years away. What does this tell us about the light we see from it tonight?

The light left Proxima Centauri 4.2 years ago — we are seeing it as it was 4.2 years in the past The light reaches us instantly because stars are special sources of light The number 4.2 tells us how bright the star is, not how far away it is The light left Proxima Centauri 4.2 million years ago

The Andromeda Galaxy is about 2.5 million light-years away. When we observe it with a telescope, what are we actually seeing?

Andromeda as it looks right now, because telescopes are very fast Andromeda as it looked 2.5 million years ago — before modern humans even existed on Earth Andromeda as it looked 2.5 billion years ago Only the outer parts of Andromeda — the center is too far to see

Why can nothing with mass travel faster than the speed of light?

Because our rockets are not powerful enough yet — future technology will break this barrier As any object with mass accelerates, it requires more and more energy to go faster, and reaching light speed would require infinite energy The speed of light limit only applies inside a solar system Objects would melt from the heat if they went any faster

Astronomers who study the most distant galaxies — over 13 billion light-years away — are also studying what?

Future galaxies — because distant light shows what galaxies will become Galaxies that do not actually exist — they are optical illusions The early universe, as it existed less than a billion years after the Big Bang Other universes beyond our own

A signal from Voyager 1 — the most distant human-made object, now over 23 billion km from Earth — takes about 21 hours to reach us. What type of signal is this?

A laser signal, which travels faster than radio A sound signal transmitted through space A radio signal, which travels at the speed of light A physical capsule ejected by the spacecraft

If a star 500 light-years away exploded as a supernova yesterday, when would we first detect the explosion from Earth?

Tomorrow — modern telescopes can detect events instantly In about 500 years from now — the light from the explosion has not reached us yet We would see it immediately through gravitational waves Never — we cannot see individual stars exploding

A light-year is best defined as which of the following?

The time it takes light to travel from Earth to the Sun One year as measured from the perspective of a photon of light The distance light travels in one year — about 9.46 trillion km The age of the universe divided by the speed of light

How does the fact that light has a finite speed serve as a kind of natural time machine for astronomers?

Astronomers can reverse the direction of light to see the past The farther away an object is, the older the light from it — so telescopes looking deep into space are also looking far back in time Telescopes slow down light so that it reveals information about the future This only applies to X-ray telescopes, not visible light telescopes