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"Hubble inside the Image / RS Puppis"

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This is an image of a star, a very special 
star, which is called a Cepheid variable.

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And the particular star we're 
looking at is called RS Pupis.  

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It's in a southern hemisphere constellation.

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It's not a naked eye star,

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but it almost is.

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And so it's bright enough that you can 
see it through even small telescopes.

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And it's been observed by amateur and professional 
astronomers for a very long time. We have,  

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you know, over a hundred years worth of 
data on the variability of this star.

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The star gets five times brighter 
at peak than from its trough.

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So RS Pup goes 40 or more days between peaks

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That happens on a 41 day time scale

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brightness from its dimmest to its 
brightest is about five times brighter.

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So that's a huge change.

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It's about 6500 light years from Earth. And 
that's a really important number for us to know.

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The distance, the precise distance

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It turns out it's a really important

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benchmark in how we understand 
the scale of the universe

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Cepheid variables have a very 
special relationship between  

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the period of variability, how fast 
it goes from one peak to the next,

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and their absolute brightness. So how 
much energy is coming out of that star?

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And if we know something's relative brightness

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and we can measure its absolute brightness from  

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something like the period luminosity 
relationship of the Cepheid variable,  

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then we can tell you what distance it must 
be at to look as bright as it looks to us.

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So this is a Cepheid variable.

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They're very important in our 
history of understanding our  

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scale of the universe and how it evolves.

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And this particular Cepheid variable

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is in our galaxy. So it's nearby enough 
that we can study it really well.

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this one is so special because it's near the tip

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of that period luminosity relation. So it really 
allows us to calibrate the entire relation

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and

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the fact that it's embedded in this 
beautiful cloud of dust. It looks amazing.

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a stunning Hubble image with 
all the filamentary detail.

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But there's also science embedded in this light,

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and that science allows us to get 
an extremely accurate distance  

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to this Cepheid variable within 
about 10% accuracy or even better.

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But this is a star that's about ten 
times more massive than the sun.

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More massive stars

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they live fast and they die young.

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and when that happens, the star 
starts to go through these changes.

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And so that's what is actually driving the period  

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luminosity relation is that the star 
is actually unstable during this time

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and
when it's got a lot of

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coming from the core, that core has to

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do something. When it expands, 
the radius gets much bigger,  

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the star gets brighter to a certain point where 
it cools down and that starts to contract again.

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Now, it doesn't have as much energy. 
It starts to fall back into gravity,

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where it gets close enough that 
it heats up again. That's actually  

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what's driving these period Luminosity relations.

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So even though the star is younger than our sun

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because it's ten times brighter, 
it's actually more evolved

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when you first look at it, it 
almost looks like the material  

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itself is spewing away from the star. 
But that's actually not what's happening.

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What's happening is that

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the light takes to get to the 
further out dust and gas is longer.

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So basically looking at almost like 
a snapshot through that gas and dust,

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reacting to the light.

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And that light echo allows us to make this 
extremely precise distance measurement.

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Now, due to improvements in our

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measurements of Cepheid variables like RS PUP

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we have now really narrowed down that window.

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What is the age of the universe?

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We now know it's 13 plus billion years old

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and we've seen that it's 
not just expanding linearly,  

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but it's actually an accelerating universe.

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it's all intimately tied to our understanding of 
Cepheid variables, and this one is the best studied.

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