as we are lifting all the star dust secrets to understand the cosmic origin of the

chemical elements. We're now going to look at the first chemical enrichment

event, and how the universe recycles matter. Imagine that this is the

primordial gas leftover from after the Big Bang. And as we already said, the

first stars formed from this gas. So here is the first star, and stars are not

static objects they actually evolve with time which is an interesting thing and

we're going to look into more detail at that later but for now, we're just going

to say that they evolve, for example into something that's called

a red giant. Actually it's going to get much bigger.

What happens is that already during this evolutionary phase here,

stars have strong stellar winds. They can lose mass from their surface, and

whatever is in that gas that's being lost gets put back into the reservoir

here. If this is a massive star which is a given in the case of a first star,

this star is going to keep evolving until it explodes as a giant supernova

-- so it's an explosion of the star, the star gets completely disrupted -- and

naturally everything from the outer portions as well as the inner portions

of the star gets spilled out and put back into the reservoir.

Here we now have all the new elements from the core of the stars that have

been put in into the reservoir. Sometime later, after the death of

these first stars, this gas cloud is chemically enriched. Then, the next

generation of stars forms from this enriched material, they evolve, the

massive ones contribute new elements, make new elements, and contribute them,

low-mass stars -- they don't explode as supernovae -- they just keep sitting there

happily ever after pretty much so they do not contribute to this chemical

evolution cycle but all the massive stars with every new generation

contribute to a successive buildup of all the elements with time. Now, an

interesting consequence of that is that old stars have a lower overall abundance

of these heavy elements because they simply formed at a time when this cycle

here had only gone round a few times. So old stars contain little of the heavy

elements (heavier than hydrogen and helium) and consequently in younger stars

starting with the Sun and even younger than that, they contain a relatively

larger amount, so they are are more enriched. We already had it, the Sun has 1.4%

of all these heavy elements, and a star that would be born today would

have 2%. These old stars, however, compared to the Sun contain only a

millionth of what the Sun contains. A millionth of 1% -- that's a really

really small number. That really makes old-style stand out.

The issue for us is that we need to figure out a way how to measure the

element composition of our stars so that we can figure out: are there older or

younger which really means have they're formed early on in this cycle here or

much later. We equate that to old age or younger age but we do so without an

actual age measurement. So it's an inferred quantity for a quantity for now

but various independent tests have shown that this is a pretty good assumption

and that starts with very little of all the elements really are old and formed as

some of these very early generations. Now, in terms of the

nomenclature, we have to introduce one important term namely all stars well as

I said we don't really have an each measurement we just infer that it's

formed soon after the Big Bang and so what astronomers use is the term

metal-poor because that actually describes what the Stars composition is

it is poor in heavy elements of metals as astronomer thing and it is poor

compared to the Sun the Sun is our reference star the Sun has 1.4 percent

of metal and our our all cells from the early universe contain only a tiny tiny

fraction of this year and so we call them metal poor and when you look for

the oldest stars or want to look for the older stuff what you actually have to do

is you have to search for the most metal poster

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