According to current theory, spiral galaxies begin as a giant rotating mass of gas and dust, which starts out in a roughly spherical shape before the edges flatten out into a disk and create the spiral arms. The original spherical shape lives on in an outer region of a galaxy known as the "halo" and, to a lesser extent, in the bulge.

Our own Milky Way is structured in this manner.

Using some of the first images from the 8-meter Gemini North telescope on Mauna Kea in Hawaii, the astronomers examined the innermost 260 light-years of M33 by combining images in three infrared wavelengths to form a false-color picture. The result shows bright red stars that represent an intermediate-age population, and bright blue stars that are young. Neither should be present in a bulge, said Andrew Stephens, a doctoral candidate in Ohio State's Department of Astronomy.

Instead, a bulge should be loaded with old stars.

The density of stars in the closely examined region looks more like a flat galactic disk than a bulge, Stephens said.

"This finding makes us question the role of a bulge in spiral galaxy formation," Stephens said. "If M33 doesn't have a bulge at all, then how exactly did it form? If it has young stars in its bulge, what triggered their formation?"

While a typical galactic disk is made up of stars of all ages, a bulge is thought to typically contain old stars dating from the time the galaxy formed. This is one reason that studying bulges can tell astronomers about how galaxies form and evolve, Stephens explained.

Stephens did the work with Ohio State astronomy professor Jay Frogel. The results were presented earlier this month at the meeting of the American Astronomical Society in Pasadena.

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