Date: December 27, 2011
Title: The Stars: Metallicity and Populations
Podcaster: L Cate Kendal
Links: http://www.legofusker.net/astronomy.htm
Description: In this podcast we are going to talk about something known as the metallicity of stars, and how this can be used to classify stars into different populations.
Bio: L Cate Kendal is a science writer, theoretical physicist and amateur astronomer living in Scotland. She has written for the magazine Astronomy Now and does science outreach volunteer work with the Royal Observatory in Edinburgh.
Sponsors: This episode of “365 Days of Astronomy” is sponsored by Cosmic Vibrations. Bringing you experimental ambient music featuring sounds from the cosmos. Visit www.reverbnation.com/kiddscosmicvibrations
Transcript:
Hello. My name is L Cate Kendal and welcome to this podcast.
There are many different ways to classify stars, not just by size or by mass. In today’s podcast we are going to talk about something known as the metallicity of stars, and how this can be used to classify stars into different populations. This gives astronomers information on how our own galaxy has changed over time and how the regions of star formation have altered during the life of our galaxy. But it is not just the history of the Milky Way that can be deduced from the metallicity. We will also explore how the populations of stars themselves have changed over the lifetime of the universe, and how the stars that are being born today are different from the very first stars that were born.
Astronomers seem to face an almost impossible task when it comes to understanding stars. These objects are too far away for anyone to gain proper, first-hand experience of their properties, with the exception of our Sun.
Worse, each star lives for a very long time, even the shortest lived, most massive stars still exist for a few hundred thousand years, and that is far longer than we humans have been around to observe them. There are few stages that occur over short enough time scale to be observed in detail, supernova are one such class of event. But as these events are so very short in comparison with the ages of stars, they are extremely rare.
So, we see the stars as static, almost instantaneous snapshots of the different stages through which they pass during their lifetimes. And it is left to us to figure out the order of events and the connection between the different stages.
Astronomers have found a variety of clever ways to tease all the information they can out of a star light, but we are going to concentrate on just one of them at the moment. This is the star’s metallicity.
Stars generate energy by burning hydrogen to form helium, and the other processes in the star will generate a small amount of other elements as well. So there are always a small proportion of non-hydrogen and helium elements in stars.
Astronomers studying the chemical composition of stars take a very pragmatic view of the chemical elements. They divide the 92 naturally occurring elements into three groups : Hydrogen. Helium. And all the rest! It is this last part, “all the rest” that is quantified in the term metallicity, and it is defined as the proportion of the star’s material that is neither hydrogen nor helium.
The longer and faster a star has been burning its hydrogen and helium fuel in nucleosynthesis, the greater the amount of other elements that will have been created. So the metallicity of the star can give an estimate of its age, and so can be used to help classify stars into the different stages of stellar life.
However, the rate of nucleosynthesis and the age of the star are not the only factors that affect the metallicity. It also depends on the amount of other elements that were present in the star when it first formed. All stars, except the very first generation, contain material that has already been part of a star. Over time, the amount of other elements will increase, as more hydrogen and helium are processed. And so, over time with each successive generation of stars, the metallicity of the stars will increase.
This increasing metallicity is the key factor in defining the different populations of stars in the Milky Way.
Let’s start with the easy ones, with young stars such as our Sun. These are stars with relatively high metallicity. It is worth pointing out here that the term “relatively high” is just that. The actual amounts of these elements is still very small compared to the amount of hydrogen in the star, just a few percent, if that in the stars with the largest metallicity. These high metallicity stars are known as Population I stars (given the Roman numeral one).
Population I stars are all relatively young, less than about 10 billion years old and are most often found in the spiral arms of our galaxy. The stars in open clusters are all Population I stars.
Generally speaking, Population I stars have an ordered motion in the galaxy, and have close to circular orbits. Also, as these stars are associated with “other elements” that are not hydrogen or helium, they are more likely to have systems of planets around them.
Going further back in time, to look at older stars, the metallicity generally decreases. The stars with the lowest metallicity are Population II stars, designated with the Roman numeral II, and are pretty well the oldest stars that can be observed. These are found throughout the Milky Way, and often have elliptical and eccentric orbits. They’re concentrated in the centre of the galaxy and the galactic halo, and include RR Lyrae stars and sub-dwarf stars. The stars in globular clusters are Population II stars as well, implying that these clusters are very old.
There is no sharp dividing line between the two different populations, and metallicity varies between the two extremes, with various sub-populations being possible.
But there are still a clear pattern in the distribution of the two different populations of stars. So why are Population I stars not found in the centre? Why are Population II stars often found in globular clusters? The answers to these questions sheds light on the way the Milky Way has changed over time. As the rate of star formation has varied over time in the different parts of the Milky Way, so the variation in metallicity that we see now will reflect these differences.
And there is one other, interesting question that needs to be answered. Population II stars have very low metallicities. But where did these elements come from?
The theory of how the universe itself was created tells us that the initial material of the first stars had virtually no elements other than hydrogen and helium. The amounts are far below the levels seen in Population II stars. That means that the Population II stars are not the first generation of stars to be born, there was another generation before that. And it was from the debris of these first stars that the Population II stars formed.
These oldest stars are known as Population III stars. And these are thought to be the very first stars born out of the initial matter in the universe. They are hypothetical stars, there are no confirmed Population III stars visible today.
This being so, there is a lot of speculation about the nature of these stars. They may have been very hot and very bright, with very large masses, larger than the masses of the stars observed today. If they were, then the chances of ever observing them are quite small, as they would have relatively short lifetimes.
In this podcast, we have looked at the chemical composition of stars, and how the metallicity of a star can give us information on the age of the star and what sort of material the star formed out of. This in turn can give us an idea of how the material that makes up our galaxy has changed over time, and also how different parts of the Galaxy have changed at different rates.
Of the two different populations of stars that are observed, the younger, Population I stars are generally confined to the spiral arms and have relatively high metallicity. But the low-metallicity, Population II stars are often found in the centre of the galaxy and the halo.
The last population, the hypothetical Population III stars could still be out there, waiting to be observed by bigger and more powerful telescopes. These stars are the oldest possible stars, formed directly from the initial material in the universe.
I hope you have enjoyed this podcast about metallicity and stellar populations, and thank you for listening.
End of podcast:
365 Days of Astronomy
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