ASTR 1230 (O'Connell)

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, or it can be dominated by a single generation.

We can analyze stellar populations in two main ways:

By studying the properties of the individual stars in a galaxy
By studying the integrated light (the combined light output) of the stars in a galaxy

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 -- made by combining many day's worth of exposure time with the Hubble Space Telescope. 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 going back as far in time as is possible. 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 mainly in bursts in others. In our galaxy the star formation rate (the total mass of gas converted into stars per year) is about 1 solar mass per year. On the other hand, some starbursts can reach over 1000 solar masses per year.

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.

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 bluer).
The main sequence phase of evolution is long-lived. MS stars maintain themselves by burning hydrogen in their cores. For a star like the Sun, that can continue for about 10 billion years because there are huge reserves of hydrogen in all MS stars. But eventually, those run out. When they run out of H fuel, stars evolve off the MS (to the right, or toward lower temperatures, in the diagram). The Sun will move toward the region marked "red giants" in the diagram.
More massive stars evolve faster. Even though they contain more hydrogen. they burn it much faster than lower mass stars.
The stars born in a given generation will have a large range in mass. The HR diagram of that generation will change quickly as its more massive stars evolve. Its main sequence will sequentially "burn down" as the generation ages and the most massive stars peel off to cooler temperatures. The mass of the stars near the MS "turnoff" become systematically smaller with age.
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). The three panels at the right show the structure at a given time of three stars of different mass: 2.6, 1.0, and 0.7 times the solar mass. The age is listed in billions of years. You can see how the mass of the stars at the "turnoff" point decreases in lockstep with the age of the generation.

D. THE INTEGRATED 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 in the EM spectrum.

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, keeping track, of course, of the fact that stars at the top of the diagram are much brighter than those at the bottom.

A predicted SED for ages between 10 million years and 15 billion years is shown in the animation below. The HRD of the single generation population is shown in the lower left panel, while its integrated SED is shown for the wavelength range 3000 to 7500 Å in the top panel. The colored regions in the top panel show the responses for three standard filters used by astronomers to determine the colors of stars. The "V" filter corresponds roughly to the response of the human eye. The lower right panel shows how the colors change with time (blue colors in lower left, red in upper right).

The dramatic change in the shape of the integrated SED with age is obvious in the animation.

A "young" (less than 100 million years age) population contains massive MS stars. These are hot and generate much blue light. The short wavelength SED even goes off the scale of the plot. Even though they may contain many cool stars, young populations are blue in integrated light
As age increases, the SED flattens. An "old" (more than 3 billion years age) population 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 million years age) produces much ionizing radiation in the extreme ultraviolet region of the SED (not shown). This creates regions in the surrounding gas whose spectra contain strong emission lines of hydrogen and other ionized gases. We call these "H II" (meaning "ionized hydrogen") regions. The dominant emission line of hydrogen (H-alpha) has a deep red color. The combination of this with the generally blue light of young stars produces a characteristic pinkish tint in color images of HII regions. .
The sharp dips evident in the integrated SEDs are produced by absorption by individual chemical species -- e.g. atomic hydrogen, calcium, sodium and iron or molecules like CN and TiO. The abundances of these species can be determined from the integrated SED.

The plot below contrasts two snapshots of the SED history: one of a very young population and another of an old one, but now plotted over a larger range of wavelength, extended into the ultraviolet band down to 1000 Å. The discriminatory power of the ultraviolet for study of the SED, especially of young populations, is exceptional, but because the Earth's atmosphere is opaque below about 3200 Å, this can only be exploited by telescopes in space.

Thus, we can use the integrated color of a stellar population to estimate its age, regardless of how distant its host galaxy might be. With suitable spectral resolution, we can also determine elemental abundances for the population.

We can also estimate the current star formation rate (SFR) of a galaxy by using the amount of blue light it generates or the strength of its ionized hydrogen emission lines. We do that by estimating the total mass in young stars and dividing by their age; the result is expressed in units of solar masses per year of new stars being formed.

E. THE STAR FORMATION HISTORY OF THE UNIVERSE

The SED analysis techniques we've described were developed in studies of nearby galaxies. After the amazing images of the distant universe were obtained by the Hubble Space Telescope, those techniques could be applied to the galaxies detected there in wholesale fashion.

Before that could be done, it was necessary to correct the SEDs for the effects of the redshift caused by the expansion of the universe. The more distant a galaxy is, the faster it is moving away from us. Because of the Doppler Effect on the spectrum of receding objects, the SED of a distant galaxy is shifted to longer (redder) wavelengths. The amount of the shift, and hence the distance of the galaxy, can be determined by the observed SED, and its colors can then be corrected for the shift.

Since 1996 many studies have been made of distant galaxies, using a combination of data from the HST, other space telescopes, and ground-based telescopes. We now have an solid understanding of the gross star formation history of the universe. That's displayed in the diagram below:

This shows the estimated average rate of star formation per unit volume on the vertical axis as a function of redshift on the horizontal axis. The scatter of the plotted points gives a measure of the uncertainty in the SFR determinations. The vertical scale is logarithmic, so it is greatly compressed. The upper horizontal scale is labeled in lookback time -- that is, the time it took light to reach us from each redshift epoch. Zero on the upper scale corresponds to the present time.

We see that the peak in the star formation density in the universe occurred at a redshift near 2 or at an epoch 10 billion years ago. Star formation has dropped smoothly, by a factor of 10, since that time. The earlier history is still sketchy because of the difficulty of detecting galaxies at redshifts above 4, but it's clear the SFR density was falling off above that point.

According to our best estimate, the universe is 13.7 billion years old, so the peak in the SFR density (sometimes called "cosmic noon") occurred some 3.7 billion years after the Big Bang. Much of the stellar mass that makes up the present-day universe was therefore formed relatively early but over an extended period rather than all at once in an intense, concentrated burst. Most galaxies formed their initial populations at that early time, but almost all, including ours, have continued to form stars, at a slower rate, since then.

It's amazing to realize that we have gone from not knowing that galaxies existed to being able to detect the first ones being formed --- probing distances of over 12 billion light years --- in only 80 years.

Web Links:

Background information on atomic physics and astronomical spectra (ASTR 1210)

Background information on stellar properties and evolution (ASTR 1230)

Information on the Hubble Deep Fields

Last modified December 2020 by rwo

Text copyright © 2003-2020 Robert W. O'Connell. All rights reserved. HR diagram animation copyright © by James Schombert (U. Oregon). Integrated SED animation copyright © by Chris Mihos (Case Western University). Cosmic SFR history from Piero Madau and Mark Dickinson (2014, ARAA, 52, 415). These notes are intended for the private, noncommercial use of students enrolled in Astronomy courses at the University of Virginia.