Suppose there are l0,000 civilizations broadcasting radio signals in the Milky Way GaIaxy right now. On average, how many stars would we have to search before we would expect to hear a signal? Assume there are 500 billion stars in the galaxy. How would the answer change if there were only 100 civilizations instead of 10,000?

Planetary Nebula Age. Suppose a planetary nebula is 1 pc in radius. If the Doppler shifts in its spectrum show it is expanding at 20 km/s, how old is it? (Note that 1 pc equals 3.1x1013 km, and 1 year equals 3.2x107 seconds, to two significant figures.) Please round your answer to two significant digits. At = years

I don’t understand how that it was estimated as length of a mouse is 4 to 15 cm in length. Is that a guess and the answer just varies?also where did 10^-1 come from

In this experiment, as a form of sheltering-at-home fantasy, we adopt a science=fiction scenario. It’s the year 2520 and you are an astronaut working for a private entity simply called The Company. The CEO of The Company is the 8th clone of Elon Musk. Elon 9 has provided you with a small interstellar spacecraft about a million times faster than anything we can conceive of today.             It is your job to check out the potential habitability of a few relatively near potentially habitable planets to see if human colonies can be established there to mine materials for the latest version of the Tesla automobile.             But there is a problem. Shortly before your launch, a solar-system-wide pandemic ground human economy to a standstill. So Elon 9 had to cut corners. The only device he could afford for you to measure gravity acceleration on the subject planets is a pendulum with a length of 100 cm. After landing, you will determine the gravitational acceleration at the surface of…

asap please

A new astronomical measurement suggests that the Hubble constant is 51 kilometers per second per Megaparsec. If this measurement is correct, what would the Hubble time be in units of years? Is this a plausible value based on other astronomical evidence? Why or why not?

why do scientists generally think that liquid water is necessary for the origin of life? (astronomy)

The isotope carbon-14 decays over time into nitrogen-14 with a half life of 5,730 years. Suppose you find a fossil that contains 1.25 grams of carbon-14 and 3.75 grams of nitrogen 14. How much carbon-14 was present in the organism at the time of death?