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Howard,

Thanks for clarifications about "green stars".

My first thoughts would have been Arcturus or Aldeberan but both stars  
I can see tonight and they are probably yellow/orange.

My guess would be a star I've never seen, Gamma Crucis in Crux  
(Southern Cross)

Ivo

> Interesting discourse about "green stars" but readers should note the
> following clarifications:
>
> I doubt these stars would look green to the eye even if bright enough
> for our amateur telescopes.
>
> The telescope used to obtain the image of the ultra cool brown dwarf,
> WISEPC JO45853.90+643451.9, only detects radiation in the infrared.
> The specific wavelengths are 3.4, 4.6, 12 and 22 microns.
>
> (A micron is one millionth of a meter and no longer often used but
> still sometimes convenient in some applications.)
>
> Note: The human eye, in contrast to WISE, can see radiation from about
> 0.4 to 0.7 microns, which we perceive as violet through red with green
> at roughly 0.5 to 0.6 microns.
>
> In other words, the WISE telescope (Wide Field Infrared Explorer) is an
> *infrared* space telescope (launched 14 Dec. 2009; decommissioned this
> past Feb. 17, 2011).
>
> Since the human eye cannot see infrared, the four spectral bands (3.4,
> 4.6, 12 and 22 microns) have been color coded blue, cyan, green and red
> respectively by those who have processed the image.
>
> I believe methane absorption in this star depresses the first two
> spectral bands (coded cyan and blue) and the star's light lacks
> radiation in the fourth spectral band (red).
>
> Note: The star does emit some radiation in the red but this is weak,
> contrary to what Ivo wrote -- a common mistake.  In any case, this is
> irrelevant here since WISE is blind to red (has no detector in the red
> spectral region).
>
> Therefore, WISE primarily "sees" radiation from this cool brown dwarf
> in the 12 micron spectral band that has been color coded green.
>
> Hence, this star "looks green" in WISE images.
>
> The choice of green is arbitrary but probably a good choice since it
> helps make these ultra cool brown dwarfs stand out from other stars in
> the WISE images, which would not necessarily appear green to WISE.
>
> And, yes, Ivo is probably right.  Further infrared surveys will
> probably detect many more of these "green stars" since they are
> probably very numerous.  Some astronomers, in fact, think some such
> stars may exist closer to us then the Alpha Centauri System.  Time will
> tell.
>
> Thanks, Ivo, for alerting readers about these extraordinarily cool stars.
>
> Now, who has seen a truly "red star."  (So called "red stars" as
> Betelgeuse or Antares are not really quite red, more orangy.)  Really
> red appearing stars are rare but such a breed does exist and at least
> one is naked eye.  What breed?  Which naked eye star?  (Unfortunately
> this is not the season to see it.)
>
> Howard Cohen
>
> At 10:13 PM 3/22/2011, you wrote:
>> Have you ever wonder if there are any green stars?  I have, so I did a
>> little research.
>>
>> I remembered seeing Hubble and NASA pictures of certain emission and
>> absorption nebulae with swaths of green colors and some of them with a
>> few small green stars.
>>
>> I also looked at NASA's WISE survey picture of a green color
>> star(scanning in infrared). NASA discovered this green ultra-cool
>> brown dwarf called WISEPC JO45853.90+643451.9 in Camelopardalis or The
>> Giraffe. The brown dwarf is position right on the neck of the Giraffe.
>>
>> This brown dwarf has methane in atmosphere, which absorbs blue light
>> and the dwarf is so faint that it does not give red light. Thus color
>> green.
>>
>> This week I decided to do some reading on subject. I found that some
>> Brown dwarfs (some astronomers think of them as failed stars) can
>> appear green.
>>
>> Brown dwarfs are cool and dim and keep contracting until a form of
>> pressure takes over-electron degeneracy pressure and these are called
>> T dwarfs. Other brown dwarfs called L dwarfs fuse deuterium for very
>> short periods of time.
>>
>> The spectrum of some of these T dwarfs shows absorption bands of
>> methane. Similar to spectra of Jupiter. NASA has detected tiny
>> crystals circling around 5 brown dwarfs. The crystals are thought to
>> be olivine (color green) thought to help seed formation of planets.
>>
>> The presence of methane and green olivine crystal means that a dwarf
>> star can appear to be green.
>>
>> The problem is, are these real stars or just large planets. Most T
>> dwarfs size is less than 13 Jupiter masses. Some are free-floating and
>> formed on their own through gravitational contraction out of a cloud
>> of gas-like normal stars.
>>
>> Some astronomers classify L dwarfs as stars and T dwarfs as planets.
>>
>> Gibor Basri UC at Berkeley proposed we call T dwarfs planemos
>> (planetary mass objects) if they are at least massive enough to be
>> spherical but not massive enough to be deuterium-fusing brown dwarfs.
>>
>> How do we know that some T dwarfs (electron degeneracy pressure) were
>> not L dwarfs (fused deuterium) at one time?
>>
>> It is my belief that with advance technology we will start seeing more
>> pictures of these green color dwarf stars.
>>
>> I wouldn't jump for joy if you think (as an amateur astronomers)
>> you'll ever see a green star through your telescope.  That technology
>> is way above our pay scale.
>>
>> Ivo Rabell
>
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