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Puzzle of the “Green Pea” galaxies

"Green Pea" galaxies

A selection of "Green Pea" galaxies discovered by "citizen scientists" of the Galaxy Zoo team.

Galaxies come in many different shapes and sizes. There are spiral galaxies, like our Milky Way; elliptical types, like the Andromeda Galaxy, and irregular types, like the Small and Large Magellanic Clouds, visible to stargazers in the Southern Hemisphere.

But galaxies that look like green peas floating in space?

That’s exactly what a bunch of “citizen scientists” found in 2007, as part of a worldwide online effort to categories millions of galaxies photographed by a project called the Sloan Digital Sky Survey, or SDSS.

The SDSS had imaged the northern sky in great detail. But astronomers didn’t have a hope of categorising all the galaxies captured by the SDSS images—there just weren’t enough hours in the day.

So, through an online project called Galaxy Zoo (followed by Galaxy Zoo 2), they enlisted the aid of recreational astronomers around the world to sort through the vast depository of night sky images.

Categorising and counting galaxy types is both important to learn about the evolution of the Universe. But it can also be difficult because of the ambiguous shape of many of the galaxies.

"Green Pea" galaxies

Another bunch of Green Pea galaxies. Astronomers think their strange colour comes from a lack of heavy metals.

And so it was that in 2007, some of the citizen scientists spotted galaxies that just didn’t fit into any of the standard categories. They were small, round and green…hence the name, “Green Pea” galaxies.

Alerted to the enigmatic objects, professional turned their attention to them, and soon came up with an explanation.

The Green Peas appear to be compact, low-mass galaxies undergoing a brief burst of intense star formation. They also seem to be “metal-poor”…metals in this astronomical sense meaning any element heavier than hydrogen and helium.

It seems the heavier element gases have been either blasted out of the galaxies by “winds” produced by supernovae (exploding stars), or have been sucked out the gravitational pull of nearby galaxies. Maybe both explanations are right.

Astronomer Ricardo Amorin says, “This Green Pea discovery is a fabulous example of how normal citizens, ‘astronomy lovers’, can help scientists with their collective efforts.”

“Discovering Green Pea galaxies has opened a new window to investigate galaxy evolution and star formation in the early Universe.”

The latest incarnation of Galaxy Zoo uses data provided by the Hubble Space Telescope, to peer deeper into the Universe than before. Perhaps even more citizen science discoveries are just around the corner.

Adapted from information issued by JENAM / SDSS / Richard Nowell.

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