Back To CourseBasics of Astronomy
28 chapters | 325 lessons
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When a chef is hard at work, he not only has a lot of ingredients to deal with but he also has a lot of utensils he needs to use. And every really good chef will tell you that it's not just about the ingredients themselves but their amounts and nuanced concentrations that can make or break a meal. So, I hope you enjoy cooking because this lesson is going to cook up a storm of ingredients in the form of details relating to our galaxy, the Milky Way, and its stellar composition.
Originally, our universe was essentially made up of 90% hydrogen and 10% helium. It's like flour and eggs to a baker, they're the base ingredients for seemingly everything.
There were very few, if any, metals in the beginning. Metals - to an astronomer - are atoms that are heavier than helium. They would be like a pinch of salt added to a huge bowl of soup or something - there was very little at first.
Therefore, the very first stars that were made in the universe had to be made almost entirely of eggs and flour - I mean, hydrogen and helium. The other chemical elements that came later, the metals, were made in a process called nucleosynthesis. For our lesson's purpose, nucleosynthesis refers to the process of making chemical elements heavier than helium by fusing atoms in stars or by way of supernovae explosions.
That was a delicious mouthful of words there, wasn't it? So let's break down the ingredients and cooking secrets of that definition. Stars make energy by nuclear fusion. When hydrogen is fused, helium is made. When helium is fused, carbon is made. And so on down the line. Therefore, the use of fuel by a star produces new things. It's like fusing eggs, milk, and flour to make dough.
When a star ages, it sort of takes off its coat in the form of a planetary nebula and spreads those newly made elements into space. It's like a chef taking off a coat that's coated with ingredients, which then fly all around in the kitchen.
A supernova explosion, on the other hand, is like a chef's stovetop exploding while cooking; it's going to leave very new and odd combinations of stuff splattered on the walls.
Anyways, the final course that I want you to munch on here is this: As stars live their life and die, they form small amounts of new elements, like sulfur, calcium, gold, platinum, and uranium, which are released into the interstellar medium where new stars are eventually formed.
Consequently, newer stars will have higher amounts of metals than older stars because they have such ingredients to work with, unlike their older brethren. Keep this in mind.
Astronomers have found that our galaxy has two general populations of stars. Population I stars are younger stars found in the disk of the galaxy that contain lots of atoms heavier than helium (metals). Population II stars are older, metal-poor stars found in a galaxy's nuclear bulge, halo, and globular clusters.
Now, don't get too excited about finding a goose that will lay golden eggs. Although population I stars are 'rich' in metals, you won't find one made entirely out of gold and platinum out in space. 'Rich' is a relative term, since I said before that only small amounts of metals are formed when stars make energy or explode.
To put this into numbers you can measure out in a cup, population I stars contain only 2-3% metal, but that's a lot compared to the 0.1% population II stars contain.
Population I stars that are very rich in metals are appropriately called extreme population I stars. They're found in the spiral arms of our galaxy. Intermediate population I stars are found throughout the disk. Our sun is one such intermediate population I star.
Intermediate population II stars are found in the central bulge, but the most metal-poor stars are the ones found in our galaxy's halo and the globular clusters. The latter are known as extreme population II stars.
Well, as with delicious sugar or fat, good things are best eaten in moderation. Similarly, I don't want to stuff you full of too much information, so we'll just boil everything down to this: our galaxy is made up of different populations of stars.
Population I stars are younger stars found in the disk of the galaxy that contain lots of atoms heavier than helium (metals), while population II Stars are older, metal-poor stars found in a galaxy's nuclear bulge, halo, and globular clusters.
The metals in those definitions, at least, to an astronomer - are atoms that are heavier than helium. The very first stars that were made in the universe had to be made almost entirely of hydrogen and helium.
The other chemical elements that came later, the metals, were made in a process called nucleosynthesis, which is the process of making chemical elements heavier than helium by fusing atoms in stars or by way of supernovae explosions. Logically then, newer stars will contain higher amounts of metals than older stars.
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Back To CourseBasics of Astronomy
28 chapters | 325 lessons