Red Stars, Blue Stars, Old Stars, New Stars

Ever since astronomers, using the new large telescopes of the 1920's and 1930's, realized that galaxies were at vast distances from the earth, they have been using them to try to answer fundamental questions about the the structure and evolution of the Universe.

Our own Milky Way is a fairly typical spiral galaxy, containing about 100 billion stars in a giant pinwheel some 100,000 light years across that rotates once every 250 million years. The swirling spirals of such galaxies contain regions of young stars, mixed in with the dust and gas from which they formed. The Orion nebula, visible in binoculars on a clear winter night, is such a stellar nursery. At its current rate of star formation, it will be about 15 billion years before the Milky Way exhausts all the materials from which it can form stars. In a real sense, therefore, we are living in a galaxy still in the midst of formation.

In contrast, about 30% of the galaxies we see around us are just smooth balls of stars, some round like baseballs, others shaped more like rugby balls or footballs. The nearest example of such an elliptical galaxy is Messier 32, which can also be seen through binoculars if the sky is dark enough, right next to the great Andromeda galaxy. Elliptical galaxies are for the most part devoid of any star formation or the dust and gas from which stars form. They are in essence "passively" evolving, rather than actively forming.

This lack of star formation makes elliptical galaxies something of a rosetta stone for cosmologists. Astronomers in the 1960's showed that the elliptical structure of such galaxies could come about quite naturally if the gas that formed them collapsed and formed stars in a time very short compared to the current age of the universe. Thus we see before us a class of galaxies that are plausibly among the first that formed in the universe, each containing stars that are all roughly the same age.

If we can measure the ages of elliptical galaxies from the properties of the stars within them, we can estimate the age of the universe.

Furthermore, if we can identify and measure the sizes and colors of very distant elliptical galaxies and compare them to nearby ones, we can in principle measure the size of the universe, and test directly for the curvature of space predicted by Einstein's General Theory of Relativity.

It is the nature of astronomy that such grand goals rest upon seemingly small details. From ground-based observations, we know that the stars in elliptical galaxies are older than about 3 billion years. We can deduce this by comparing the colors and spectra of the galaxies to predictions from the theory of stellar evolution. Unfortunately, for ages older than this, changes in chemical composition and changes in age produce an almost identical effect on the optical colors and spectra. We are left with tantalizing and often contradictory estimates for the ages of the oldest galaxies, and we do not yet know how to correct the observations of distant galaxies for the passage of time since the light that we now receive was emitted from them.

Ultraviolet observations hold great promise for breaking this impasse. Because they consist of old, cool stars, it came as a great surprise some 25 years ago when the first space observatory showed that elliptical galaxies produce a faint ultraviolet glow. After two decades of controversy over what could be producing the glow, HUT observations from Astro-1, combined with advances in the theory of stellar evolution, essentially pointed the finger at "extreme horizontal branch" (EHB) stars. Horizontal branch stars are stars that have used up the hydrogen in their cores and are instead deriving their energy from the fusion of helium atoms. The term "horizontal branch" comes from the fact that such stars in our own galaxy form a horizontal line in a diagram of temperature vs. brightness. That is, they all have about the same brightness, but vary widely in temperature. "Extreme" horizontal branch stars are the hottest variety, having temperatures of about 25000 degrees Centigrade (the Sun's temperature is about 5500 degrees). The key to identifying these stars as the source of the UV glow came from HUT's sensitivity to light of shorter wavelengths than those that could be observed with either the Hubble Space Telescope, or its predecessor, the International Ultraviolet Explorer satellite.

The great promise of UV observations comes from the fact that the trends in the strength of the UV emission do not simply track changes in the optical colors. Something, be it age or chemical composition, is causing galaxies with nearly identical optical colors and spectra to have vastly different UV spectra. Theorists have been working busily over the past few years to try to figure out what that something is. It is clear from the work done so far that variations in the UV spectrum are extremely sensitive to both age and chemical composition (the helium abundance in particular). However, our state of ignorance is illustrated by two papers published nearly simultaneously last year that claimed, in one case, that the EHB stars represent an extremely old population of stars composed at birth of nearly pure hydrogen and, in the other case, that the EHB stars represent an extremely old population of stars composed at birth of about 40% hydrogen, 50% helium, and 10% iron. The implication in both cases is that elliptical galaxies must be at least 10 billion years old. However, the inferences for how elliptical galaxies formed and built up their chemical abundances are completely different. Furthermore, the recent detection of EHB stars in a cluster only about 8 billion years old in our own galaxy casts some doubt on the age estimates.

Observations of elliptical galaxies with Astro-2 will contribute immensely to our understanding of the UV glow. The first goal is confirmation that emission from the galaxies with the strongest UV emission indeed comes from EHB stars. To this end HUT will be trained on stars with a range of mass and temperature in our own galaxy to provide a direct calibration of the theoretical models that were used to analyze the Astro-1 observations. Second, HUT will spend nearly twice the time, with three times the sensitivity, observing a galaxy, Messier 60, with a very strong UV glow similar to that of the prime target on Astro-1. This will provide a much more precise estimate of the temperatures of the stars producing the glow, and allow for the first time an estimate of the chemical composition from the absorption lines in the spectrum. Finally, HUT will observe a half-dozen more galaxies with a range of UV strengths to measure in detail how the shape of the spectrum changes from galaxy to galaxy.

While it will probably be several years before the dust settles on theories for the UV glow, astronomers hope that if they can identify the mechanism that produces the hot stars and calibrate their ages, elliptical galaxies can fulfill their promise as a powerful probe of cosmic evolution.

Henry C. Ferguson, and the HUT team