How Many Rings?

This infrared image from NASA's Spitzer Space Telescope shows the double-ringed galaxy Messier 94. Just outside the bright core, a burning ring of star formation glows brightly in the light of warm interstellar dust. Encircling it all is the faint blue gl

How many rings do you think you see in this image of the galaxy NGC 4736 (aka Messier 94)? At first glance you may believe you see several, but astronomers now believe there is only one. Historically, Messier 94 was thought to have two strikingly different rings: a bright, compact band encircling the galaxy’s core and a fainter band of stars falling outside the main disk.

Astronomers have recently discovered that the outer ring, seen here as a deep blue glow, is probably an optical illusion. A 2009 study combined infrared Spitzer Space Telescope data with those from other telescopes, including ultraviolet data from the Galaxy Evolution Explorer, now operated by Cal Tech, visible light data from the Sloan Digital Sky Survey, and shorter-wavelength infrared light from the Two Micron All Sky Survey (2MASS). This broad spectrum view of Messier 94 indicated that the outer ring is really two spiral arms.

The bright inner ring of Messier 94 is very real, a starburst ring caused by rapid star formation in the tight area.

Tucked in between the inner starburst ring and the outer ring-like arms are dusty arcs in the galaxy’s main disk that look like a collection of rings, but they actually are tightly wound spiral arcs.

Image Credit: NASA

Andromeda in UV

AndromedaGalex_900Andromeda Galaxy (aka M31) is just next door as large galaxies go, only about 2.5 million light-years. So close and spanning some 260,000 light-years, it took 11 images from the Galaxy Evolution Explorer (GALEX) satellite’s telescope to produce this portrait of the spiral galaxy in ultraviolet light. While its spiral arms stand out in visible light images, they look like rings in UV because the image is dominated by light from hot, young, massive stars. As sites of intense star formation, the rings have been interpreted as evidence Andromeda collided with its smaller neighboring elliptical galaxy M32 more than 200 million years ago.

Image Credit: NASA