Finally...A Green Star?

[Mr Q]

Saw this article and wanted to share it. My question is, thought there was no such thing as a green star? Could this photo be in a false color format?

Space telescope picks out 'green dot' in a field of stars - a star whose surface is room temperature, and cooler than human blood | Mail Online

[Macavity]

Certainly, visually, there are plausible reasons for there being "no green stars". As you doubtless know.

Why Aren't There Any Green Stars? - NASA Spitzer Space Telescope

Though it now seems relatively EASY to *image* green stuff generally - With narrow band filters and various (exotic) "pallets". Green almost seems to be "the new red"? LOL. I'm sure I have seen "green" stars in wide-field astronomy magazine images recently too - I thought it an artifact of the printing process. But maybe not...

Aside: I see the proliferation of additional "brown dwarf" spectral classes now includes magenta objects. Without atmospheric effects, our own sun's (palette) colour looks pinkish-white to me etc. etc.

[acey]

The star is not green, it's infra-red: it was imaged by an infra-red telescope, and the image shows it as green. The article says it has a surface temperature of 25 celsius so it has roughly the same colour as a completely darkened room: i.e. totally black to the eye, but detectable by an infra-red camera as a glow displayable on a screen as any colour.

The photograph is described like this at the NASA website: "This view shows three of WISE's four infrared channels, color-coded blue, green and red, with blue showing the shortest infrared wavelengths and red, the longest."

http://www.nasa.gov/mission_pages/WISE/multimedia/pia14721.html

There are a couple of "red" objects near the "green" one so they must be even cooler - don't know what they are.

[FraserClarke]

Precisely Acey -- this object looks green because of where it's emission primarily comes out colour bands used by WISE -- these are all in the mid-infrared (3--20 microns, I think). In the visible green (0.5 microns) it would be, well, practically invisible! (if you were sensitive enough to see it, it would probably look magenta/mauve coloured)

There are a couple of "red" objects near the "green" one so they must be even cooler - don't know what they are.

No, probably not. The "cooler objects are redder" relationship only works for black bodies -- brown dwarfs of this temperature are a very very long way from black bodies. Most of the flux from these cool objects is absorbed by molecules (H2O, CH4, NH3) in their atmospheres, and we only see light from a few narrow regions between the absorption. This leads to a series of what look like broad emission features (actually they are just the gaps between even broader absorption features) at points in the infrared spectrum. It's one of these features (and the lack of emission either side) that makes the really cool objects look green in the WISE bands.