In 2011, a bizarre, green object was discovered. At long last, we finally understand why.
One of astronomy’s biggest surprises came in 2007, when a mysterious green object was found.
While sorting through some of the Sloan Digital Sky Survey’s 50 million galaxies, citizen scientist Hanny Van Arkel revealed a puzzle.
This weird phenomenon, Hanny’s Voorwerp, was unlike any other galaxy: distended, irregular, and green.
Over the subsequent years, we’ve discovered perhaps 20 similar objects.
This posed a great puzzle for astronomy: no stars are green, so how does this galaxy shine?
Stars only range from red to light blue, dependent solely on their temperature.
But green glows do exist in space, arising from heated gas.
When electrons transition between different energy levels, they emit light at specific, well-defined frequencies.
Oxygen, the cosmos’s third-most abundant element, has a strong green emission line when it’s doubly ionized.
With two electrons kicked off — occurring at temperatures of 50,000 K and above — it emits a spectacular green glow.
When galaxies interact, gas often gets stripped away.
Simultaneously, one galaxy’s supermassive black hole activates, forming a quasar.
As the quasar’s light strikes the gas, it ionizes oxygen, creating these Voorwerpjes.
Even before they’ve formed any stars, galaxies can still glow an eerie green.
Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.