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so

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first i will just talk briefly about

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habitable zones and how i'm defining

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them for this talk

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my and um

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basically there are obviously a lot of

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factors that could go into play when you

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discuss what

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defines a habitable zone

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it's like i feel like it's

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okay

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i'll choke

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all right

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is that better

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okay

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um basically we're approaching this idea

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of habitable zones from the

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astrophysical side so

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you know we have different stars that

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are

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different masses and different

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temperatures and so that's going to

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directly affect the location of a

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habitable zone around the star

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and

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four stars of different mass these

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habitable zones also

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expand outward at different rates based

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on the stellar evolution

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so we're trying to basically quantify

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how these

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stars change over time and thus how the

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habitable zones change over time

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so the two physical

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parameters that we can measure about

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stars are their mass

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and that affects its rate of hydrogen

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fusion

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and thus the main sequence lifetime so

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that is main sequence and so the more

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massive a star

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the shorter

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life span it's going to have on the main

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sequence and the main sequence for

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non-astronomers is basically just when a

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star is burning hydrogen in its core so

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it's about 90 of a star's lifetime it's

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really stable and long

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also composition so what the star is

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actually made of now stars are mostly

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hydrogen helium but they're also made up

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of

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other heavier elements

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and

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differing combinations of these elements

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actually affect the opacity in the star

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which

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affects the energy transport so if you

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have

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know photons being generated in the

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center of the star

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running into more material as they try

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to get out so if you have a higher

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opacity you're going to have

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less efficient energy escape

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so

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if you have less metal you're going to

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have lower opacity and more efficient

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energy escape and so two stars of equal

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mass if they have different compositions

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can actually you know change

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the lifetime of the main sequence

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significantly

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so basically this is just to show how

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the stars

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change over the main sequence

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so

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this is time in billions of years and

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this is luminosity so luminosity is just

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the brightness of the star so four five

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different compositions here

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black is just um solar composition you

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actually see that

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the lifetime can be significantly

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truncated

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versus you know a sun lives about 10

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billion years

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so something with um

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you know about half the oxygen of our

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sun let's say

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lives about half as long so

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really we're seeing and it's uh much

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brighter so there's a lot of different

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things that come into play here and i'll

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talk more about that

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so to model cellar evolution usually

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astronomers say

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okay there's metallicity in the star and

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they compare it to the sun so if you

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measure the iron abundance they just

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assume that all the other elements scale

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in the same proportions that are found

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in our own sun but that's actually

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not correct specific elemental

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abundances vary drastically

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between stars in particular oxygen we

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found significantly affects the stellar

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evolution

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so you know the location and lifetime of

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the habitable zones are dependent on the

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host star like i was saying

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so if we take into account this extra

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oxygen factor

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that's going to play a big role

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so what i've been doing is just use the

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stellar evolution code tycho

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i've been creating a big catalog of all

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these different kinds of stars

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evolutionary tracks

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um for different compositions i've made

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a grid of 376 so far

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mass is between 0.5 and 1.2 so kind of

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sun-like

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mass stars

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at different metallicities so that's

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that

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just

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solar

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abundance value scaled and then oxygen

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values i have two depleted values and

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two enriched values and those are based

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so we get something like this if i

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uh put something

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all together some of my output this is

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called a hertzsprung russell diagram

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it's just temperature versus brightness

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or the luminosity and we have my cool

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low mass stars on this side and my hot

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high mass stars

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on this side and even within each of

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these colors represents a different mass

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you can see that there's a large spread

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in the temperature and the brightness

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based on individual compositions so

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you have to look at both you can't just

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look at mass or composition to really

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get a full idea of

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how a star is going to evolve in its

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temperature and brightness over time

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so you know we care about temperature

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and brightness oh this is just my little

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test to show that this is the sun

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and it falls right on

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the solar

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track so that's a good test for our code

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make sure that it's outputting properly

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but we care about you know temperature

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and luminosity because

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at each time step taiko outputs these

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surface temperatures and luminosities

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and we put them in

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to

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habitable distances based on equations

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from ravi kaparapu's group

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and i won't go too much into this but

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basically i just wanted to show that

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these equations take in an effective

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temperature and a luminosity and that's

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what our code outputs at each time step

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of its evolution so we can actually

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calculate this d value this habitable

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zone distance for every

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point

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in a stellar evolution track based on

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these two parameters and so we

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ultimately end up with

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habitable zone limits the optimistic

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cases we don't really consider because

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they're optimistic we want to be more

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conservative so we take the runaway

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greenhouse to the maximum greenhouse

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cases

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and that's just um

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inner and outer habitable zone limits

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based on

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atmospheric properties of hypothetical

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planets that would be there so so if i

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plot my results based on those limit

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equations

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i get something like this so this is

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again age in billion years

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and distance along this axis

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and blue represents the sun or like

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solar composition so we have inner

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limits and outer limits

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so 1au is plot of reference so earth is

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about right there

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and so we're at the very inner limit of

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the habitable zone

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the red is for um

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about half the oxygen value

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so if you know if the sun had less

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oxygen we would actually be too close

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so this limit would be outward and we

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would be too close to the sun

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to

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maybe host liquid water on the surface

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so

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start problem which is that we don't we

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want to avoid planets that have recently

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entered the habitable zone because if

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something was frozen and then the star

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moved its boundaries outward and then it

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was at the right temperature to host

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liquid water on a planetary surface we

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don't know if that planet would have the

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impetus to maybe unfreeze itself

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and host liquid water so

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this prompts us to constrain the

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habitable zone with time criteria as

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well

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um

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so this was our first kind of iteration

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of this idea so we have the zero h main

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sequence and the terminal age main

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sequence that's just beginning and end

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of the main sequence

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and that's in blue and red

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and then the inner boundaries are solid

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the outer boundaries are dashed so this

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overlapping green shaded area is what

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would be called a continuously habitable

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zone so where

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the

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a planet in this green shaded area would

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remain habitable for its entire main

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sequence lifetime

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um

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however if we plot earth on this

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plot it doesn't

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quite fall into that region so we

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figured that that's not really a robust

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enough

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method of trying to understand

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habitability so instead we consider

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this uh continuously habitable zone of

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um a two billion year minimum so now the

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shaded green

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represents a planet at a distance

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in this green area would remain

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habitable for at least two billion years

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and so that's obviously an

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anthropomorphic

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boundary condition that we've imposed

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because it took earth about two billion

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years

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to

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biosignature

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but it's a starting place at least so we

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can now

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plot earth and it falls nicely in our

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little

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shaded area so basically this is just to

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how we're trying to constrain our

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habitable zone time limits

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now this is just a little overwhelming

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but it's basically the same thing as

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last plot

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except that this is for all of my 376

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stars

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so each line is a different data point

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and this is just again to reinforce the

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idea that composition really affects

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where your habitable zone distance is

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going to be so if you have a distance

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here

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and

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you have oxygen

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and okay so let's just look at the solid

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lines even

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there's a span in these elongated solid

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lines and that represents

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that spread and oxygen values that we're

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looking at so even a star of the same

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mass with different oxygen is going to

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have a habitable zone that's at a

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different distance

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and so that would affect where the

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planets could be habitable

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so

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um

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this i just wanted to point out that

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the fraction of time a planet would

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spend in the continuously habitable zone

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versus the entire main sequence

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is much higher

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for

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we actually don't see any

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because

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these stars are so short-lived that

301
00:11:02,790 --> 00:11:00,959
there's no continuously habitable zone

302
00:11:05,110 --> 00:11:02,800
for two billion years

303
00:11:07,670 --> 00:11:05,120
so that's one of the reasons why looking

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00:11:08,949 --> 00:11:07,680
at low-mass stars is really

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important

306
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and it's going to help you know further

307
00:11:15,110 --> 00:11:12,320
quantify what kinds of stars we should

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actually focus on in the future with

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upcoming

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00:11:20,230 --> 00:11:18,720
missions you know to try to look at

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planets and

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search for possibly

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inhabited exoplanets

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so my next steps are basically just

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creating

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more

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data points to put in this catalog we've

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made this catalog

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available online

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and

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[Music]

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you can basically go on and

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put whatever tracks you want and it'll

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interpolate

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um different masses or different

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compositions and give you a habitable

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zone based on whatever input parameters

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you want i'm also going to do the same

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kind of

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calculations for carbon and magnesium

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because those are also really important

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for

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stellar evolution

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and we're also going to look at the late

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stage evolution beyond the main sequence

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because that should be really important

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as well

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and we're also going to expand the

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catalog to include stars of even lower

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mass so down to even

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like the brown dwarf limit because

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they're very abundant and they're very

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good

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they're very easy to detect plants

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around

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so those are really good candidates to

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look for

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habitable planets so

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uh thank you for your attention

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[Applause]

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all right

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00:12:43,910 --> 00:12:41,750
so um

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two questions what was the time

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00:12:50,629 --> 00:12:47,440
difference from your highest mass

355
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i guess entire main sequence lifetime

356
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to your lowest mass what was the the

357
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time difference

358
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uh so how long is the main sequence

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lifetime for like a 1.2 solar mass

360
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versus 0.5 solar mass yeah so it's on

361
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the order of like 5 billion years for

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the

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1.2 up to about 100 billion years

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for the 0.5

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and then my second point was that

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actually if you do look at the

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evolution of the solar luminosity with

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time

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there is some suggestion that after

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about a billion years the carbonate

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silicate feedback cycle on the earth

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will be insufficient to maintain

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uh clement conditions on the earth so

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that's why after you know the entirety

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of the main sequence lifetime of the sun

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the earth is not in the habitable zone

377
00:13:38,870 --> 00:13:36,880
right and yeah and that's i mean

378
00:13:40,470 --> 00:13:38,880
obviously it's a very complex problem

379
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and we're just trying to approach it

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kind of from the bottom

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like

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you know constrain it from the

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astrophysical point and then

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kind of collaborate with

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geophysicists and people who could tell

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00:13:54,949 --> 00:13:53,680
us more about the actual planet side as

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opposed to we're just coming at it from

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the star side

389
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but yeah it's

390
00:14:03,590 --> 00:14:00,160
there's a lot to consider with

391
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habitability so uh it's here uh really

392
00:14:08,550 --> 00:14:06,480
nice work i have two questions um i've

393
00:14:10,310 --> 00:14:08,560
seen some of the happy zone gets really

394
00:14:12,310 --> 00:14:10,320
close to central star when the stellar

395
00:14:14,310 --> 00:14:12,320
mass is small i was wondering do you

396
00:14:17,350 --> 00:14:14,320
need to worry about the irrigation of

397
00:14:20,310 --> 00:14:17,360
high energy photons in that situation

398
00:14:22,069 --> 00:14:20,320
yeah um we're actually part of our next

399
00:14:24,069 --> 00:14:22,079
steps that i didn't

400
00:14:25,990 --> 00:14:24,079
include on the slide was actually

401
00:14:28,470 --> 00:14:26,000
looking at um

402
00:14:31,110 --> 00:14:28,480
m dwarf age versus activity and so that

403
00:14:34,069 --> 00:14:31,120
would be the lower mass stars and then

404
00:14:35,750 --> 00:14:34,079
looking at how that activity

405
00:14:36,790 --> 00:14:35,760
you know changes over time and how that

406
00:14:38,949 --> 00:14:36,800
could really

407
00:14:41,430 --> 00:14:38,959
affect if there's a big flux of uv

408
00:14:43,509 --> 00:14:41,440
radiation or something

409
00:14:45,269 --> 00:14:43,519
even if the star lives a hundred billion

410
00:14:47,670 --> 00:14:45,279
years it's not going to be conducive for

411
00:14:50,150 --> 00:14:47,680
life if you have those big

412
00:14:52,150 --> 00:14:50,160
uv pulses happening all the time so that

413
00:14:54,790 --> 00:14:52,160
is something that we're going to

414
00:14:57,350 --> 00:14:54,800
consider cool so the second question is

415
00:14:59,910 --> 00:14:57,360
when you change the oxygen abandons

416
00:15:02,230 --> 00:14:59,920
does it affect your stellar evolution

417
00:15:04,230 --> 00:15:02,240
model or will that effect

418
00:15:06,949 --> 00:15:04,240
affect your um say

419
00:15:08,710 --> 00:15:06,959
planet atmosphere model as well

420
00:15:10,069 --> 00:15:08,720
sorry would it affect what um so you

421
00:15:13,030 --> 00:15:10,079
change the oxygen abundance in your

422
00:15:15,829 --> 00:15:13,040
model am i getting this right yes so so

423
00:15:17,750 --> 00:15:15,839
if we change the actual amount of oxygen

424
00:15:19,670 --> 00:15:17,760
and nothing else so everything else

425
00:15:22,150 --> 00:15:19,680
stays

426
00:15:24,949 --> 00:15:22,160
maybe solar yeah

427
00:15:27,670 --> 00:15:24,959
sorry sorry and then the major effect is

428
00:15:30,310 --> 00:15:27,680
on the stellar evolution

429
00:15:32,550 --> 00:15:30,320
yeah so the major effect is um

430
00:15:34,150 --> 00:15:32,560
the lifetime and the

431
00:15:35,750 --> 00:15:34,160
luminosity so

432
00:15:37,509 --> 00:15:35,760
and that's just due to what i was

433
00:15:40,230 --> 00:15:37,519
talking about with the opacity and the

434
00:15:42,470 --> 00:15:40,240
energy transport if you have more oxygen

435
00:15:44,470 --> 00:15:42,480
you're going to have more

436
00:15:46,230 --> 00:15:44,480
stuff bumping into the photons as it

437
00:15:48,310 --> 00:15:46,240
tries to escape so you're going to have

438
00:15:49,590 --> 00:15:48,320
less efficient energy escape and so the

439
00:15:51,509 --> 00:15:49,600
energy is going to stay in the star

440
00:15:52,870 --> 00:15:51,519
longer so it's actually going to let the

441
00:15:54,870 --> 00:15:52,880
star live

442
00:15:56,870 --> 00:15:54,880
longer than it would if it was just all

443
00:15:59,269 --> 00:15:56,880
radiating away really quickly yeah i was

444
00:16:00,949 --> 00:15:59,279
wondering would that change your planet

445
00:16:03,749 --> 00:16:00,959
atmosphere and the efficiency of

446
00:16:05,829 --> 00:16:03,759
trapping heat as well

447
00:16:09,509 --> 00:16:05,839
i guess i'm not sure i guess that would

448
00:16:11,350 --> 00:16:09,519
depend on the planet atmosphere right

449
00:16:13,110 --> 00:16:11,360
so i don't know if

450
00:16:15,590 --> 00:16:13,120
that would directly

451
00:16:18,069 --> 00:16:15,600
affect it would just from

452
00:16:19,269 --> 00:16:18,079
from this kind of analysis it would

453
00:16:22,310 --> 00:16:19,279
affect

454
00:16:23,509 --> 00:16:22,320
the temperature for sure so the surface

455
00:16:25,269 --> 00:16:23,519
temperature of the planet but then it

456
00:16:27,269 --> 00:16:25,279
would also depend on

457
00:16:29,910 --> 00:16:27,279
whatever atmosphere properties the

458
00:16:32,310 --> 00:16:29,920
planet had i i guess um i don't know if

459
00:16:33,110 --> 00:16:32,320
that answers your question

460
00:16:36,960 --> 00:16:33,120
but