Video pre-questions (and sample answers) from:

The Study of the Universe

NOTE: It will not be our usual practice to post questions from class here on the website; it is your responsibility to write them down during class (as you do for lecture notes). This set is posted to give you a feeling for the level of depth and detail that you should be able to achieve in answering them.

What is the astronomical unit and when is it used most conveniently?

The Astronomical Unit (abbreviated AU) is defined to be the average distance between the Earth and Sun. It is a convenient distance unit to use when discussing distances within the Solar System (such as between planets).

What is the light year and what is it used for?

A light year is the distance light travels in one year in a vacuum. (Since all forms of electromagnetic radiation travel at the same speed in a vacuum, it is also the distance that radio, infrared, ultraviolet, etc. travels in one year.) We use it to express interstellar distances.

What is a planet?

A planet is a small, non-luminous body that shines by reflected light from a star it orbits around.

What is a star?

A star is a globe of gas, held together by its own gravity, that generates energy by nuclear fusion.

What is a galaxy?

A galaxy is an enormous collection of stars, gas, and dust, held together by its own self-gravitation.

How are distances to things in the universe connected to our understanding of the history of the universe?

Since light (or any other form of electromagnetic radiation) doesn't travel infinitely fast, it takes time for light to get to us from things in the universe.  The farther away something is, the longer its light takes to reach us, and the farther back in time we see it.  For example, light from the moon takes about 1 1/4 seconds to reach us, so we always see it as it was 1 1/4 seconds ago.  Light from the nearest star takes about four years to reach us, so we are restricted to seeing it as it was four years ago.  Distant galaxies, billions of light years away from us, can only be seen as they were billions of years ago.  So, when we look far out into the universe, we are also looking far back in time.