What Use Are 21 Cm Radio Waves to Galactic Astronomers

question 33

Multiple Choice

What use are 21 cm radio waves to Galactic astronomers?

• A) They cut through the dusty cocoons to let us watch star birth.
• B) We can reflect them off the core of the Galaxy.
• C) Their Doppler shifts let us map the motions and locations of gas in the spiral arms.
• D) They bounce off stars like our Sun to let us precisely measure their distances.
• E) They pick up the cool, dark matter much better than can optical telescopes.

---

Definitions:

Present Value

The current worth of a future sum of money, given a specific rate of return or discount rate.

Future Value

The value of an investment or cash at a specified future date, considering the interest rate or rate of return.

Term

A specific period or duration of time during which the conditions of a contract or agreement apply.

Annuity

A financial product that pays out a fixed stream of payments to an individual, primarily used as an income stream for retirees.