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Starts With A Bang

This Is Why Some Galaxies Have A Green Glow, Even Though There Are No Green Stars

Some rare galaxies exhibit a green glow thanks to the presence of doubly ionized oxygen. This requires UV light from stellar temperatures of 50,000 K and above. Oxygen is the 3rd most abundant element in the Universe: about 1% of all the atoms, by mass. (NASA, ESA, AND W. KEEL (UNIVERSITY OF ALABAMA, TUSCALOOSA), OF NGC 5972)

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.

Hanny’s Voorwerp, identified in 2011, was the first of some 20-odd objects now known to be a collection of green, glowing gas (because of ionized oxygen) that extends for tens of thousands of light years found outside of by nearby galaxies. It was initially discovered by citizen scientist Hanny Van Arkel, as part of the Sloan Digital Sky Survey’s “galaxy zoo” program. (NASA, ESA, W. KEEL (UNIVERSITY OF ALABAMA), AND THE GALAXY ZOO TEAM)

While sorting through some of the Sloan Digital Sky Survey’s 50 million galaxies, citizen scientist Hanny Van Arkel revealed a puzzle.

This close-up of Hanny’s Voorwerp, as imaged with the Hubble Space Telescope, shows a glowing green, irregular object with scant evidence for stars inside. For years, it was a mystery as to what, exactly, this object was. (NASA, ESA, W. KEEL (UNIVERSITY OF ALABAMA), AND THE GALAXY ZOO TEAM)

This weird phenomenon, Hanny’s Voorwerp, was unlike any other galaxy: distended, irregular, and green.

Modern ‘green pea’ galaxies have their doubly-ionized oxygen emission offset from the main galaxy; in the Subaru Deep Field, the galaxies themselves exhibit the strong emission. (NASA, ESA, AND Z. LEVAY (STSCI), WITH SCIENCE BY NASA, ESA, AND W. KEEL (UNIVERSITY OF ALABAMA, TUSCALOOSA))

Over the subsequent years, we’ve discovered perhaps 20 similar objects.

The Subaru Deep Field, containing thousands of distant galaxies exhibiting a signature of greenness in them: arising from doubly ionized oxygen. However, none of these galaxies appear green to the naked eye, as they are dominated by the light from stars, none of which are themselves green. (SUBARU TELESCOPE, NATIONAL ASTRONOMICAL OBSERVATORY OF JAPAN (NAOJ) IMAGE PROCESSING: R. JAY GABANY)

This posed a great puzzle for astronomy: no stars are green, so how does this galaxy shine?

The seven major star classes, organized by their colors. It turns out that these colors also correspond to a star’s surface temperature, and so O-stars are the hottest, while M-stars are the coolest. Note that stars range from blue to white to yellow to orange to red, but do not come in colors like green or pink. (E. SIEGEL)

Stars only range from red to light blue, dependent solely on their temperature.

This image from ESO’s Very Large Telescope shows the glowing green planetary nebula IC 1295 surrounding a dim and dying star located about 3300 light-years away. The green color arises from emission line transitions in the ionized gas surrounding the dim, dying star. (ESO / FORS INSTRUMENT)

But green glows do exist in space, arising from heated gas.

Hubble image of a small region of the Crab Nebula, showing Rayleigh–Taylor instabilities in its intricate filamentary structure. The green color arises from the presence of doubly ionized oxygen, whose transitions include a bright emission line in the green portion of our eyes’ visual spectrum. (NASA AND THE HUBBLE HERITAGE TEAM (STSCI/AURA))

When electrons transition between different energy levels, they emit light at specific, well-defined frequencies.

The strong green emission line (highest point) as shown in a sample of over 1,000 galaxies, spectrally stacked from the Subaru Deep Field. The other point “above” the curves is from hydrogen; the strong green oxygen line indicates incredibly intense radiation and temperatures in excess of 50,000 K. (MALKAN AND COHEN (2017))

Oxygen, the cosmos’s third-most abundant element, has a strong green emission line when it’s doubly ionized.

A number of possible atomic transitions in doubly ionized oxygen, as shown here, are peaked at “green” color frequencies: around 500 nanometer wavelengths. There is also a shorter-wavelength (bluer) optical transition, as well as a number of ultraviolet (coded in pink) and infrared (coded in red) transitions as well, though those are invisible to our eyes. (BERKLAS, CEPHEIDEN / WIKIMEDIA COMMONS)

With two electrons kicked off — occurring at temperatures of 50,000 K and above — it emits a spectacular green glow.

This spectacular image was created with composite Spitzer and Hubble data, and shows a tidally distorted galaxy, rich in gas and actively forming new stars, merging with an old, gas-free elliptical galaxy made up of older stars. The gas, illuminated here by stars, will not always be visible in a tidal interaction, not unless some other light source illuminates it. (NASA-ESA/STSCI/AURA/JPL-CALTECH)

When galaxies interact, gas often gets stripped away.

This galaxy has a ‘glowing heart’ because it’s a special type of active galaxy — a Seyfert galaxy — whose central black hole is presently devouring matter and emitting light at a higher temperature and luminosity than all the surrounding regions. When two galaxies interact, it can funnel matter into one of the supermassive black holes, creating a quasar or an active galactic nucleus. (SPACE SCOOP / ESA / HUBBLE & NASA, D. CALZETTI, UMASS AND THE LEGU.S. TEAM)

Simultaneously, one galaxy’s supermassive black hole activates, forming a quasar.

Astronomers found that Hanny’s Voorwerp is the only visible part of a 300,000-light-year-long gaseous streamer stretching around the galaxy. The greenish Voorwerp is visible because a searchlight beam of light from the galaxy’s core illuminated it. This beam came from a quasar, a bright, energetic object that is powered by a black hole. An encounter with another galaxy may have fed the black hole and pulled the gaseous streamer from IC 2497. (NASA, ESA, AND A. FEILD (STSCI))

As the quasar’s light strikes the gas, it ionizes oxygen, creating these Voorwerpjes.

A compilation of eight different Voorwerpjes, as imaged by a team using the Hubble Space Telescope, which were follow-up observations made of these unusual objects as they were discovered by citizen scientists sorting through data from the Sloan Digital Sky Survey. (NASA, ESA, AND W. KEEL ET AL., ARXIV:1408.5159)

Even before they’ve formed any stars, galaxies can still glow an eerie green.

One of these Voorwerpjes clearly shows that the green illumination is coming from gas well outside the main galaxy, which itself shows evidence of new star formation and activity near its core. There are many regions rich in this green emission that do not have identifiable stars in them. (NASA, ESA, W. KEEL (UNIVERSITY OF ALABAMA), AND THE GALAXY ZOO TEAM)

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.

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