By day, they might work as ministers, musicians and construction managers. But at night, they pull out their telescopes and scan the starry skies searching for cosmic phenomena.
And sometimes these amateur astronomers happen upon discoveries that rock the scientific community.
The latest example was just last week, when an amateur from Australia noticed an abnormality near Jupiter that experts lauded as one of the most significant ever by an amateur
"When it's something that's accessible, there is a community of people who think that it is wonderful and want to participate and sometimes make important discoveries," said Roger Launius, a NASA space historian.
While he said that many amateurs have been formally trained in astronomy, others have learned along the way, driven by enthusiasm.
Suzanne Gurton, education manager for the San Francisco-based Astronomical Society of the Pacific, said professionals have more technical background to analyze data but amateurs play an important role in the collection of it.
"They all contribute to our knowledge of astronomy, to both the popularization and the science of astronomy," said Gurton.
Here are a few examples of those contributions.
Scientists around the world sat up and took notice last week when Anthony Wesley, the amateur Australian astronomer, reported to NASA that he was seeing something unusual in the southern hemisphere of Jupiter.
Something -- possibly a comet or meteoroid -- went slamming into the planet, was swallowed up in its atmosphere of ammonia and methane, and left a visible scar.
Scientists may never know the size of the object that hit Jupiter, and the planet itself will heal, but astronomers say the discovery is quite significant.
"The recent spot on Jupiter is one of the best accomplishments ever by an amateur astronomer ? it's of such importance," said Stephen Maran, press officer for the American Astronomical Society (AAS). In the mid 1990s, the Shoemaker-Levy 9 comet hit Jupiter and showed the world that planets, Earth included, were vulnerable to other space objects.
This recent finding, he said, shows that these occurrences don't just happen once in a lifetime but can occur regularly, even within a generation.
"It will be the public awareness that comes out of this finding that's all important," Maran said.
Earlier this week, Yale University announced that legions of volunteer astronomers from around the world helped scientists discover a group of rare galaxies called the "Green Peas."
Through the project Galaxy Zoo, volunteers and amateur astronomers help classify galaxies by sifting through an online database of images. The project was launched in 2007 by a team of astronomers in the U.K. and involves about 230,000 volunteers from around the world.
While poring over the images, Galaxy Zoo volunteers identified a number of unusual galaxies. Because they were so small and bright green, they called them "Green Peas."
"These are among the most extremely active star-forming galaxies we've ever found," said Carolin Cardamone, an astronomy graduate student at Yale and lead author of the paper, to be published in an upcoming issue of the Monthly Notices of the Royal Astronomical Society.