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  • Have you ever wondered how all the chemical elements are made? Then join me

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

  • chemical elements. Let's summarize what we've learned so far about the old stars

  • and how they can be used in our concept of stellar archaeology to understand

  • what happened soon after the Big Bang, in terms of chemical enrichment and

  • chemical evolution. We have old stars and we call them "metal-poor", and there are our

  • tool, our tool to study the early universe.

  • These stars are long lived. They have a low mass, something like 0.6 to 0.8

  • solar masses, and that means that they have lifetime of 15 to 20 billion years.

  • That means that they are still observable. That is very lucky for us,

  • and they are not just observable, there are actually easily observable

  • because they are now located in the Milky Way. Let's look at this again: so

  • this is a very quick drawing of our Milky Way. This is the bulge, the inner

  • part of our galaxy with a supermassive black hole in the center. And this here

  • are actually two disks. That is the disk, and we're about two thirds on the way

  • out. The bulge contains a lot of young stars, there's a lot of gas which means

  • you have formed a lot of stars which means you made a lot of elements and formed

  • more stars. So the bulge is very metal rich. The disk here is not quite

  • as metal-rich but still pretty enriched. Now, this is not the only

  • part of our galaxy. This is just the most visible part, namely the Milky Way band

  • on the night sky. That's from when you look into the spiral arms that make up the disk.

  • But we look in a different place for the oldest stars because they are kind of

  • located up here and below the disk: In

  • something that's called the halo. It's actually much larger than what

  • I'm drawing right now. That's called a halo of the disk. It's a

  • spherical envelope of this this disk here. All the old stuff is parked there.

  • It's bit of a junkyard, yeah, because when a galaxy forms, you have

  • small systems that actually come together and form bigger system, and then,

  • here, you have a bigger system, too, and then they come together and make an

  • even bigger one. That's called the hierarchical structure formation

  • paradigm. This is the Milky Way which means that these little guys

  • here kind of end up in the outskirts or at least a good amount of these little

  • guys end up in the outskirts but they are completely shredded apart. And what

  • is left are all the stars that have been spilled into the Milky Way. This is

  • how old stars actually get into the outer halo of the Milky Way.

  • I should mention here that little dwarf galaxies -- also actually in

  • in the halo of the galaxy -- they also pretty old, so these are entire little

  • systems here that are not completely shredded yet. They are just in the

  • gravitational field, here, of the Milky Way, and they are orbiting around the disk

  • and we also have globular clusters. These are clusters stars, actually clusters of stars with

  • up to a million stars, and they are also located here and down here, and they are

  • also really old. We don't really know where they come from but the halo

  • contains mostly three things: globular clusters, dwarf galaxies and lots of old

  • stars and so with our telescope we can peek from here and here up into the

  • halo, and here and observe all the old stars that are

  • in this range. All in all, our metal-poor stars are the local equivalent to what

  • we call the high-redshift universe. In a very complimentary way, both metal

  • poor stars and the furthest, most distant galaxies are used to study the early

  • universe. These faraway galaxies, when the light comes to us, we receive it from

  • this early time and this way we can figure out what this galaxy can tell us

  • about the early universe because it formed at that early time. Our metal-poor stars,

  • their light hasn't travelled for a long time. It has traveled maybe just

  • from here to us. That's a negligible amount of time

  • because these stars are today located in our Milky Way. But they are really old. We

  • see them as when they are old, not as when they were young as it's in the case

  • of these distant galaxies. But the fact that we see them all doesn't doesn't

  • matter to us. Because these stars don't get wrinkly or anything they just sit

  • there and they are just waiting for us to observe them. As we will see in

  • the following, these stars are really undisturbed and they just look like --

  • today they look just like what they did 13 billion years ago.

  • So that's a huge advantage for us, and of course we're going to

  • make use of it.

Have you ever wondered how all the chemical elements are made? Then join me

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B1 milky halo disk bulge galaxy metal

Ep. 5: Stellar Archaeology

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    林宜悉 posted on 2020/03/29
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