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[Music throughout] How can astronomers determine

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distances in the far reaches of the universe? A small galaxy

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close in looks similar to a large galaxy farther out.

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This is real challenge that researchers have found several

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solutions for. One method uses something called a

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standard candle. A standard candle is a type of object or

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event that emits a specific, known amount of light, allowing

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scientists to find its distance with a straightforward formula.

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This works because light sources appear predictably dimmer

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the farther they are from an observer. Since astronomers know

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how much light a standard candle gives off, they can determine its distance

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by measuring how dim it appears from Earth.

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Since only very bright objects or events are visible in the

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far reaches of the universe, the options for standard candles are limited.

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Some of the best and most reliable are exploding stars,

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called supernovae. There are a few different kinds of supernovae,

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but the best for standard candles are Type Ia. These supernovae

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involve a white dwarf — the leftover core of a dead star — and

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one other star in a binary system. Some of the time

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it may be a white dwarf and larger “host” star.

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Scientists think the white dwarf steadily accumulates material shed by 

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the host star, gaining mass in the process.

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When it reaches a specific tipping point, the white dwarf has gained enough

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mass to trigger a runaway reaction at its core

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and it explodes spectacularly, sending out an expanding sphere

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of super-hot material that glows from the energy of the explosion.

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In other cases, scientists think two white dwarf stars may

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form the binary. Either the stars finally merging together

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triggers the supernova, or it happens as they spiral in

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closer and closer, while the more massive of the two pulls material

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off its companion in the final few minutes.

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Before they merge, it reaches the same mass tipping point and goes

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supernova, always releasing a similar amount of energy.

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Because white dwarf explosions are all so similar,

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the energy and light output of Type Ia supernovae are easy

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to standardize. Type Ia supernovae are rare in any one

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galaxy, occurring only once every 500 years or so in the

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Milky Way. But because there are so many galaxies, astronomers

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using current telescopes observe Type Ia supernovae about a

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hundred times a year. By comparing the observed brightness with 

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the intrinsic brightness, astronomers can determine their distances

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within 6 percent. The Nancy Grace

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Roman Space Telescope, set to launch in the mid-2020s,

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will observe large patches of sky repeatedly, increasing

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the opportunities to spot these supernovae.

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Scientists predict Roman will see as many supernovae in one month

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as they’ve found in the last 20 years.

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Finding more of them will help astronomers refine the accuracy of

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this method, contribute to an improved 3-dimensional map

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of the universe, and better understand how

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the universe has expanded and evolved

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throughout cosmic history.

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[NASA]

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[NASA]