This is CBSI in the world. I'm John Bachelors. The Milky Way are galaxy. Where did it come

from? What does it mean that it has in the middle of it a thick disc? Dr. Ken Croswell,

the author of Lives of Stars is here to introduce us not only to our Milky Way in its proportions,

but also new work that can date the Milky Way and can follow a chronology of how we got to

our special place in an arm of the Milky Way. Hint, there's a mystery here, a collision. Ken,

a very good evening to you. Your piece in science news is a wonder to me because it never

occurred to me to ask about the age of the thick disc and the thin disc and how the Milky Way

was constructed. But first, let's start with definitions. What is the thick disc? Where can we

look for it in the night sky? Good evening to you. Well, good evening, John. Well, let's actually start

with the thin disc because that's a lot easier to see. Almost all the stars you see at night are

part of the thin disc. The sun, which you see during the daytime, is also part of the thin disc.

The thin disc is a pancake shaped object that contains, as I say, most of these stars nearby,

all of the young stars, for example, Sirius and so forth. And its thickness from top to bottom

in our part of the galaxy is about 2000 light years. Now, the thick disc is an older disc. And we've

known about it since the 1980s. It was proposed back then, very controversial. And yet now everyone

accepts its existence. It's maybe 6000 light years in thickness from top to bottom. And the

stars there are all old. And so they're not nearly as prominent in the night sky. A possible

member of the thick disc is the bright star, Arcturus. It has a quite elliptical orbit around

the galaxy. And it's more metal poor than the sun is. And those are two characteristics of

thick disc stars often. So if you want to look at a thick disc star, probably Arcturus is your best

bet, although it's not 100% sure that it is a member of the thick disc. And then we also have

the stellar halo, which I did my PhD on. So the stellar halo is this vast collection of old stars

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