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ATM-OBSERVERS-L  March 2011

ATM-OBSERVERS-L March 2011

Subject:

Re: Are there any green stars?

From:

"Howard L. Cohen" <[log in to unmask]>

Reply-To:

Howard L. Cohen

Date:

Wed, 23 Mar 2011 12:58:27 -0400

Content-Type:

text/plain

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Parts/Attachments

text/plain (133 lines)

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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