Huge galaxies can bend light and change how we see the stars behind them
Gravitational lens (noun, “Graah-vih-TAY-shun-ul LENZ” verb, “gravitational lensing”)
This effect occurs when gravity bends light from a distant object. If there is only empty space between the eye and, for instance, a faraway supernova (the huge explosion of a dying star) that supernova might be too faint to see. But if there is a big galaxy — a large cluster of stars — between the astronomer and that supernova, gravity associated with the galaxy’s mass might bend the supernova’s light. This could move that light around the galaxy — at the same time focusing it. By acting as a giant magnifying glass, this galaxy can make the distant supernova’s light bright enough for the Earth-bound viewer to see.
In a sentence
A gravitational lens can make a faraway object appear much closer, allowing us to see a supernova on the far side of the universe.
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astronomy The area of science that deals with celestial objects, space and the physical universe as a whole. People who work in this field are called astronomers.
galaxy A massive group of stars bound together by gravity. Galaxies, which each typically include between 10 million and 100 trillion stars, also include clouds of gas, dust and the remnants of exploded stars.
gravitational lensing The distortion of light by an intense gravitational force, such as what can be exerted by clusters of galaxies — the most massive things in the universe. The gravity can bend or focus light, making it appear brighter and in one or more different places in the sky.
gravity The force that attracts anything with mass, or bulk, toward any other thing with mass. The more mass that something has, the greater its gravity.
lens (in physics) A transparent material that can either focus or spread out parallel rays of light as they pass through it.
magnify To increase in apparent size or number of something.
mass A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.
quasar Short for quasi-stellar light source. This is the brilliant core of some galaxy (massive collections of stars) that contains a super-massive black hole. As mass from the galaxy is pulled into that black hole, a huge quantity of energy is released, giving the quasar its light.
star The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.
supernova (plural: supernovae or supernovas) A massive star that suddenly increases greatly in brightness because of a catastrophic explosion that ejects most of its mass.