Self-study
Quasars—celestial objects so far away that their light takes at least 500 million years to reach Earth—have been seen since 1963. ███ ████████ ████ ███ ████ ██ ██████ ████ █████ ███ ███ ███████ ███ ██ █████ ████ ██ ████ ████████ ██ █ ████ ████ ████████ ████ █████ ████ ██ ███████ ████ █████ ████████ ███ ███████ ████ █████ ██ █ ████ ████ ████████ ████ ████ █████ █████ █████ ███ ████ ████ █████ ███ ███████ ██████
Stimulus Summary
The stimulus gives us three facts.
Fact 1: Light from quasars takes at least 500 million years to reach Earth. So whenever we see a quasar, the light we're seeing was emitted at least 500 million years ago.
Fact 2: For anything as far away as quasars to appear from Earth the way quasars do, it has to burn at a rate producing more light than 90 billion suns. So when the light was emitted, the quasar was burning at this extreme rate.
Fact 3: Nothing burning at that rate can exist for more than about 100 million years.
Anticipation
A quasar we see today emitted its light at least 500 million years ago. While that light was being emitted, the quasar was burning so brightly that it couldn't have survived more than 100 million years. So, the quasar burned out at least 400 million years ago (500 million years for the light to travel, minus the 100 million years the quasar could possibly have continued to exist).
The quasar burned out long before its light reached us
Light's journey to Earth
≥ 500 million years
Quasar's max lifespan
≤ 100M yrs
quasar is gone for the rest of the journey
The quasar's lifespan fits inside the first chunk of the light's journey. The rest of that journey (hundreds of millions of years) happens with the quasar already gone.
So any quasar we've ever seen is long gone by now.
23.
If the statements above are █████ █████ ███ ██ ███ █████████ ████ ████ ██ ████ ██ ███ █████ ██ █████
a
Instruments in use ██████ ████ ████ ███ █████████ ██████ ██ ██████ ███████ ██ ██ █████
The stimulus tells us we've been seeing quasars since 1963 but doesn't tell us why we didn't see them before. Maybe the instruments weren't sensitive enough, but maybe astronomers hadn't yet pointed at the right patches of sky, or maybe they hadn't yet learned to recognize what they were seeing. The stimulus doesn't tell us. (A) doesn't have to be true.
b
Light from quasars █████ █████ ████████ █████ ██ █████
The stimulus says quasars have been SEEN since 1963, not that their light first arrived in 1963. Those are different claims. Light from a quasar could have been reaching Earth for thousands or millions of years before anyone was equipped to detect or recognize it.
c
Anything that from █████ ███████ ██ ██████ ██ █ ██████ ████ ████ ███████ ████ █████ ████ █████ ██ ████████ ██ ██ ███████ █████
(C) drops a key qualifier. The stimulus says that "for anything that far away to appear from Earth the way quasars do, it would have to burn" at the 90-billion-suns rate. The brightness condition applies specifically to objects at quasar distance. Something much closer to Earth could appear just as bright without burning anywhere near that rate, simply because it's nearby.
d
Nothing that is ██ ███ ████ █████ ██ ███████ ███ ███ ████████ ██ █████ ███ ████ ████ █████ ███ ███████ ██████
The 100-million-year limit applies to things burning at the 90-billion-suns rate. The stimulus doesn't say anything has to burn that brightly just because it's far away. A faint object at quasar distance could plausibly last much longer; it just wouldn't be visible to us as a quasar.
e
No quasar that ███ ████ ████ ████ ████ █████ ██████ ███ ███████
This must be true. Any quasar we see emitted its light at least 500 million years ago, and at the time, it was burning so brightly that it couldn't have lasted more than another 100 million years. So, the quasar died at least 400 million years ago. Whatever we see in the night sky labeled "quasar" is, in real time, long gone.