ASTR 1230 (O'Connell) Spring 2011


STELLAR POPULATIONS AND THE
HISTORY OF THE UNIVERSE


Stars are the building blocks of galaxies. Research on stellar populations is the study of the different generations of stars which make up a galaxy. This is the principal way in which we determine the life history of galaxies.

Astronomers use the term "stellar population" to refer to a single generation of stars characterized by a common age and chemical composition. A galaxy can be composed of a large number of individual populations.

We can analyze stellar populations in two main ways:

This lecture describes how astronomers are able to use integrated light to analyze galaxy histories.


A. MOTIVATION: GALAXIES IN THE DISTANT UNIVERSE

Large telescopes have provided images of thousands of galaxies at distances over 5 billion light years. The Hubble Ultra Deep Field is the best example of a deep galaxy survey. The extract from the HUDF below shows the strange kinds of galaxies that inhabit the distant universe. Click on the image to see the whole HUDF.


A major goal of studying very distant galaxies like these is to determine the star formation history of the universe. These galaxies are much too far away to detect individual stars.

But we can discover some of the characteristics of the stars which make them up by analyzing their integrated light.


B. THE ELECTROMAGNETIC SPECTRA OF STARS


C. THE EVOLUTION OF STARS

Stars are formed continuously in some galaxies, and only in bursts in others.

The combined light output of a stellar generation depends on the temperature distribution of its stars and how that changes with age.

The Hertzsprung-Russell (HR) diagram is the basic tool for analyzing the temperature distributions of evolving stellar systems.


D. THE SED/COLOR OF STELLAR POPULATIONS

From the HR diagram for a generation of stars at different ages, we can predict what its combined spectral energy distribution (SED) will be. The SED is simply the distribution of light energy over wavelength, or COLOR.

The animation below shows how the color of a generation changes with time in the HR diagram. As the population ages, the main sequence "burns down," and the remaining stars become concentrated to the right hand (redder) part of the diagram.


To predict the integrated SED of a generation at any time, we simply add up the light of all the stars in the HR diagram. The plot below shows the detailed SED's for two populations of different age:


The result is as follows:

Thus, the integrated color of a stellar generation (or "population") is a function of its age, and we can use this relation to age-date populations in distant galaxies.

Other terminology:


E. NEARBY GALAXIES IN COLOR









F. DISTANT GALAXIES CLOSE-UP WITH HST

Here are some high-resolution pictures of distant galaxies taken with the Hubble Space Telescope:




G. SPECTRAL "LINES" AND CHEMICAL ABUNDANCES

The gross color characteristics of galaxies, evident in the filtered images shown above, provide some information on their histories---but much more information is present in the high-spectral-resolution SED's (called "spectra"), which carry the signatures of individual types of atoms, ions, and molecules.

Studying these line spectra is how we gain much of our astrophysical insight into galaxies, especially their chemical abundances:





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Last modified March 2011 by rwo

Text copyright © 2003-2011 Robert W. O'Connell. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy courses at the University of Virginia.