The origin, expansion, and large-scale structure of the universe. Individual stars and planets retain their existing subjects.
Test your knowledge with these top questions!
Why can images of the same lensed supernova show its explosion at different times?
Astronomers can use these arrival delays to help measure how fast the universe is expanding.
What shape can a distant galaxy appear to form through gravitational lensing with perfect alignment?
A second background galaxy on the same line of sight can produce a second ring, creating a cosmic bull's-eye.
Where is the mass acting as a gravitational lens relative to the distant object being viewed?
The lens need not shine brightly itself, so astronomers can use its effect on background light to study unseen mass.
What can gravitational lensing make one distant galaxy appear to have?
Light from one galaxy can follow different curved paths around a foreground mass before reaching Earth.
Halley's Comet's gas tail points in which direction?
The solar wind sweeps charged gas away from the Sun, so the tail can point ahead of the comet as the comet leaves.
In which year did Halley's Comet most recently pass closest to the Sun?
Spacecraft used that return to take the first close-up pictures of a comet's solid nucleus.
Halley's Comet supplies the dust behind which October meteor shower?
Earth also crosses Halley's dust trail in May, producing the Eta Aquariid meteor shower.
Which civilization recorded an appearance of Halley's Comet in 240 BCE?
Centuries of Chinese sky records help astronomers trace the comet's returns long before telescopes were invented.
What is the name for the point in the sky where the meteors in a shower appear to come from?
The radiant is only a perspective effect, like railroad tracks that seem to meet far away while parallel paths stay parallel.
Which galaxy cluster became famous for a collision that revealed dark matter?
When the clusters collided, hot gas slowed into a visible trail while dark matter sailed through, letting astronomers separate the two.