Magazine article Science News

Hubble Observations Back Merger Theory

Magazine article Science News

Hubble Observations Back Merger Theory

Article excerpt

Galaxies come in various shapes, including fuzzy footballs, elongated smears, hazy pinwheels, and glowing whirlpools. In the 1920s, astronomer Edwin Hubble resolved some of the confusion by classifying galaxies as either spiral (disk-like and compact) or elliptical (egg-shaped and diffuse). But the fundamental question remains: Why do galaxies look so different?

In recent decades, some astronomers have argued that spirals can merge to form larger, elliptical galaxies. Now, images from the Hubble space telescope provide some of the strongest evidence to date for the merger theory.

Astronomer Bradley C. Whitmore of the Space Telescope Science Institute in Baltimore and colleagues peered into the core of the elliptical galaxy NGC 7252, already suspected to be the product of a merger between two spiral galaxies, and saw something strange and unexpected. "Just for one terrible moment I thought, 'Oh my God, I gave them the wrong coordinates!'" Whitmore recalls.

Fortunately, Whitmore had indeed pointed Hubble in the right direction. And to his surprise, the telescope images revealed a pinwheel-shaped whorl of gas and stars in the galaxy's center. This "mini-spiral," as Whitmore calls it, measures 1/20 of the diameter of NGC 7252 and, in an unprecedented twist, rotates counter to the rest of the galaxy.

The astronomers also found at least 40 tightly packed, spherical knots of stars, called globular clusters, speckling the galaxy's central pinwheel. These young, blue clusters, previously detected with ground-based instruments but not seen clearly until now, provide a key piece of evidence for the merger theory. …

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