ABOUT THIS IMAGE:

NGC 3621 is a spiral galaxy about 22 million light-years away in the constellation of Hydra (The Sea Snake). It is comparatively bright and can be seen well in moderate-sized telescopes. It appears to be a fine example of a classical spiral but it is in fact rather unusual: it does not have a central bulge and is therefore described as a pure-disc galaxy. This picture was taken using the Wide Field Imager on the MPG/ESO 2.2-meter telescope at ESO's La Silla Observatory in Chile. The data were selected from the ESO archive by Joe DePasquale as part of the Hidden Treasures competition.

This galaxy has a flat pancake shape, indicating that it hasn't yet come face to face with another galaxy as such a galactic collision would have disturbed the thin disc of stars, creating a small bulge in its center. Most astronomers think that galaxies grow by merging with other galaxies, in a process called hierarchical galaxy formation. Over time, this should create large bulges in the centers of spirals. Recent research, however, has suggested that bulge-less, or pure-disc, spiral galaxies like NGC 3621 are actually fairly common.

This galaxy is of further interest to astronomers because its relative proximity allows them to study a wide range of astronomical objects within it, including stellar nurseries, dust clouds, and pulsating stars called Cepheid variables, which astronomers use as distance markers in the Universe [1]. In the late 1990s, NGC 3621 was one of 18 galaxies selected for a Key Project of the Hubble Space Telescope: to observe Cepheid variables and measure the rate of expansion of the Universe to a higher accuracy than had been possible before. In the successful project, 69 Cepheid variables were observed in this galaxy alone.

Multiple monochrome images taken through four different color filters were combined to make this picture. Images taken through a blue filter have been colored blue in the final picture, images through a yellow-green filter are shown as green and images through a red filter as dark orange. In addition images taken through a filter that isolates the glow of hydrogen gas have been colored red. The total exposure times per filter were 30, 40, 40 and 40 minutes respectively

Notes:

[1] Cepheid variables are very luminous stars - up to 30 000 times brighter than our Sun - whose brightness varies at regular intervals over several days, weeks or months. The period of this variation in luminosity is related to the star's true brightness, known as its absolute magnitude. By knowing the absolute magnitude of the star, and measuring how bright it appears, astronomers can easily calculate its distance from Earth. Cepheid variables are therefore vital for establishing the scale of the Universe.