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how can astronomers determine distances

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in the far reaches of the universe

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a small galaxy close in looks similar to

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a large galaxy farther out

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this is a real challenge that

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researchers have found several solutions

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for

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one method uses something called a

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standard candle

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event that emits a specific known amount

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of light allowing scientists to find its

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distance with a straightforward formula

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this works because light sources

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appeared predictably dimmer the farther

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they are from an observer

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since astronomers know how much light a

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standard candle gives off

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they can determine its distance by

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measuring how dim it appears from earth

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since only very bright objects or events

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are visible in the far reaches of the

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universe the options for standard

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candles are limited some of the best and

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most reliable are exploding stars called

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supernovae

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there are a few different kinds of

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supernovae but the best for standard

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candles are type 1a

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these supernovae involve a white dwarf

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the leftover core of a dead star and one

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other star in a binary system

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some of the time it may be a white dwarf

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and a larger host star

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scientists think the white dwarf

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steadily accumulates material shed by

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the host star gaining mass in the

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process

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when it reaches a specific tipping point

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the white dwarf has gained enough mass

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to trigger a runaway reaction at its

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core

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and it explodes spectacularly sending

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out an expanding sphere of super hot

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material that glows from the energy of

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in other cases scientists think two

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white dwarf stars may form the binary

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either the stars finally merging

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together triggers the supernova

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or it happens as they spiral in closer

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and closer while the more massive of the

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two pulls material off its companion in

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before they merge it reaches the same

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mass tipping point in goes supernova

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always releasing a similar amount of

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because white dwarf explosions are all

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so similar the energy and light output

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of type 1a supernovae are easy to

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standardize

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type 1a supernovae are rare in any one

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galaxy

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occurring only once every 500 years or

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so in the milky way but because there

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are so many galaxies astronomers using

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current telescopes observe type 1a

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supernovae about 100 times a year

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by comparing the observed brightness

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with the intrinsic brightness

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astronomers can determine their

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distances within 6 percent

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the nancy grace roman space telescope

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set to launch in the mid-2020s will

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observe large patches of sky repeatedly

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increasing the opportunities to spot

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these supernovae

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scientists predict roman will see as

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many supernovae in one month as they've

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found in the last 20 years

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finding more of them will help

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astronomers refine the accuracy of this

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method contribute to an improved

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three-dimensional map of the universe

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and better understand

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how the universe has expanded and