Seeing red in distant galaxies

Seeing red: in distant galaxies that shine with a ruddy glow are stars that look older than the universe that begat them.

To an astronomer, color is just as important as it is to an interior designer--though in quite a different

way. To both, what the eye perceives as red is light of relatively long wavelength; the wavelength of the light the eye perceives as blue is relatively short. The designer, however, seeks the complex balance of wavelengths that, like the notes in a musical chord, gives a unified color tone to create a mood--a crimson, say, a scarlet, or a cardinal. The astronomer's colors are equally complex, but here it is the parts that are important, the individual, single-wavelength colors into which the spectrum of a distant star or galaxy can be analyzed.

The many colors of starlight, it turns out, can reveal a great deal about a star--including its age. Statistically speaking, long-lived stars emit more long-wavelength light than short-lived stars do. The most massive stars in the cosmos are also the bluest and brightest, and they tend to explode, as supernovae, after at most a few million years. Stars of lower mass and luminosity, however, glow dimly with red light for billions of years: the redder the star is, the older it is. So if, for instance, a large population of stars forms in a relatively short time, with a broad mix of stellar masses and luminosities, the combined light from those stars is relatively blue at first and then reddens gradually with age.

Astronomers have long exploited this correlation between age and color to study the ages of stellar populations in star clusters and galaxies. Time and again, such study has led to new and fascinating scientific puzzles. In the past few years, for instance, observations of numerous distant galaxies whose light dates to the earliest years of the universe have posed a bewildering paradox: some of these galaxies appear to be older than the universe that begat them. Yet there's no way these child galaxies can be older than their parent universe.

So why do we astronomers think these galaxies are so old? For one thing, their starlight is very, very red.

If the light emitted from a galaxy looks red, it's a safe bet that most of its stars are long-lived--and at least some of them are billions of years old. Moreover, if such a galaxy is also billions of light-years from Earth, the effects of cosmological redshift make the galaxy look redder still [see "A Desert No More," by Charles Liu, June 2004]. Such doubly red galaxies are known as "extremely red objects," or EROs.