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.
- By studying the individual stars in a galaxy
...but this is limited only to nearby galaxies where we can resolve the
- By studying the integrated spectrum (combined light output)
of the stars in a galaxy
...this is less definitive, but we can apply this approach to
any galaxy in the observable universe.
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
- Stars radiate mainly in a small region of the EM spectrum:
the ultraviolet-optical-infrared region.
- Hotter stars are bluer; they emit more of their light at
short EM wavelengths.
- Cooler stars are redder; they emit more of their light
at long EM wavelengths.
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
The Hertzsprung-Russell (HR) diagram is the basic tool for
analyzing the temperature distributions of evolving stellar systems.
- In the HR diagram, we plot stellar temperature (increasing
to the left) against instrinsic stellar brightness or
luminosity (increasing upward).
- Most stars fall on the main sequence (MS), which runs
diagonally across the HRD. The Sun is an MS star (yellow in the
diagram). More massive stars on the MS are brighter and hotter (hence
- MS stars maintain themselves by burning hydrogen in their cores.
But when they run out of H fuel, they evolve off the MS (to the
right, or toward lower temperatures, in the diagram).
- More massive stars evolve faster.
- A single generation will have stars with a large range in mass.
This means its HR diagram will change as the more massive stars
evolve. Its MS will be sequentially "stripped" off as the generation
ages. The following animation shows how the distribution of stars
in the HR diagram changes with time. Labels by the dots indicate
the masses of individual stars (in solar units).
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.
- A "Young" (less than 100 Myr) generation contains massive stars.
These are hot and generate much BLUE LIGHT.
- An "Old" (more than 3 Byr) generation contains no massive, hot stars,
only cool low-mass stars on the main sequence and red giants. It is
therefore YELLOW-RED in color.
- A "Very Young" generation (less than 10 Myr) produces much
ionizing radiation, which creates "H II" regions in the
surrounding gas, whose spectra contain strong emission lines. The
dominant emission line of hydrogen ("H_alpha") gives such regions
a characteristic pinkish tint in color images.
- "Star formation rate": the total mass of gas converted into stars per year
(typically measured in solar masses per year).
- "Star formation history": a complete description of the star formation rate
over the lifetime of a galaxy (extending roughly 10 billion years into the
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:
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.