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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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The Moon Zoo wants you!

A full disc view of the Moon

A new "citizen science" project will let you contribute to science by studying images from the Lunar Reconnaissance Orbiter spacecraft.

  • Public invited to explore Moon
  • Using hi-res images from LRO mission
  • Contribute to lunar science

More than 37 years after humans last walked on the Moon, planetary scientists are inviting members of the public to return to the lunar surface as “virtual astronauts” to help answer important scientific questions.

No spacesuit or rocket ship is required — all you need to do is go to and be among the first to see the lunar surface in unprecedented detail.

Moon Zoo logo

The Moon Zoo logo

New high-resolution images, taken by NASA’s Lunar Reconnaissance Orbiter Camera (LROC), offer exciting clues to unveil or reveal the history of the Moon and our solar system.

The Moon Zoo Web site is a citizen science project developed by the Citizen Science Alliance, a group of research organisations and museums, and builds on the team’s success with Galaxy Zoo, which has involved more than 250,000 people in astronomical research.

“We need Web users around the world to help us interpret these stunning new images of the lunar surface,” said Chris Lintott of Oxford University and chair of the Citizen Science Alliance. “If you only spend five minutes on the site counting craters you’ll be making a valuable contribution to science and, who knows, you might run across a Russian spacecraft.”

Counting craters

Scientists are particularly interested in knowing how many craters appear in a particular region of the Moon in order to determine the age and depth of the lunar surface (regolith). Fresh craters left by recent impacts provide clues about the potential risks from meteor strikes on the Moon and on Earth.

Craters on the Moon

An LRO image of lunar craters

“We hope to address key questions about the impact bombardment history of the Moon and discover sites of geological interest that have never been seen before,” said Katherine Joy of the Lunar and Planetary Institute and a Moon Zoo science team member.

NASA Lunar Science Institute (NLSI) scientists are contributing to the Moon Zoo efforts by providing science expertise. NLSI is also providing educational content and supporting outreach goals of the project.

“The NASA Lunar Science Institute is very excited to be involved with Moon Zoo and support lunar citizen science,” said David Morrison, NLSI director. “Science and public outreach are cornerstones of our Institute; Moon Zoo will contribute to the accomplishment of important science, while being a major step forward in participatory exploration.”

“The Lunar Reconnaissance Orbiter Project Science Office is excited to see LRO data being used for citizen science projects,” said Rich Vondrak, LRO project scientist from NASA’s Goddard Space Flight Centre.

“The Moon Zoo project provides an opportunity for everyone to participate in analysis of images from the LRO Camera and to make a significant contribution to scientific knowledge about the Moon.”

Adapted from information issued by NASA Ames.

Hubble wants you for the Zoo

The famous Sombrero Galaxy

The public can join Galaxy Zoo to help scientists categorise thousands of distant galaxies. This one is the famous Sombrero Galaxy.

As the Hubble Space Telescope achieves the major milestone of two decades on orbit, NASA and the Space Telescope Science Institute, or STScI, in Baltimore are celebrating Hubble’s journey of exploration with several online educational activities.

There are also opportunities for people to explore galaxies as armchair scientists and send personal greetings to Hubble for posterity.

NASA’s best-recognised, longest-lived and most prolific space observatory was launched April 24, 1990, aboard the space shuttle Discovery during the STS-31 mission. Hubble discoveries revolutionised nearly all areas of current astronomical research from planetary science to cosmology.

Over the years, Hubble has suffered broken equipment, a bleary-eyed primary mirror, and the cancellation of a planned shuttle servicing mission. But the ingenuity and dedication of Hubble scientists, engineers and NASA astronauts allowed the observatory to rebound and thrive. The telescope’s crisp vision continues to challenge scientists and the public with new discoveries and evocative images.

“Hubble is undoubtedly one of the most recognised and successful scientific projects in history,” said Ed Weiler, associate administrator for the Science Mission Directorate at NASA Headquarters in Washington. “Last year’s space shuttle servicing mission left the observatory operating at peak capacity, giving it a new beginning for scientific achievements that impact our society.”

A Hubble image of part of the Eagle Nebula

A Hubble image of part of the Eagle Nebula

Get involved with Hubble

Hubble fans worldwide are being invited to take an interactive journey with Hubble by visiting They can also visit to share the ways the telescope has affected them. Follow the “Messages to Hubble” link to send an e-mail, post a Facebook message, or send a cell phone text message. Fan messages will be stored in the Hubble data archive along with the telescope’s science data.

For those who use Twitter, you can follow @HubbleTelescope or post tweets using the Twitter hashtag #hst20.

The public also will have an opportunity to become at-home scientists by helping astronomers sort out the thousands of galaxies seen in a Hubble deep field observation.

STScI is partnering with the Galaxy Zoo consortium of scientists to launch an Internet-based astronomy project where amateur astronomers can peruse and sort galaxies from Hubble’s deepest view of the universe into their classic shapes: spiral, elliptical, and irregular.

Dividing the galaxies into categories will allow astronomers to study how they relate to each other and provide clues that might help scientists understand how they formed.

To visit the Galaxy Zoo page, go to

A screenshot from Galaxy Zoo

A screenshot from Galaxy Zoo

For educators and students, STScI is creating an educational website called “Celebrating Hubble’s 20th Anniversary.” It offers links to facts and trivia about Hubble, a news story that chronicles the observatory’s life and discoveries, and the IMAX “Hubble 3D” educator’s guide.

An anniversary poster containing Hubble’s “hall-of-fame” images, including the Eagle Nebula and Saturn, also is being offered with downloadable classroom activity information. Visit the website at

To date, Hubble has observed more than 30,000 celestial targets and amassed more than a half-million pictures in its archive. The last astronaut servicing mission to Hubble in May 2009 made the telescope 100 times more powerful than when it was launched.

Adapted from information issued by NASA / STScI.