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Mu in Cepheus?

On Fri, Mar 25, 2011 at 6:35 PM, Howard L. Cohen <[log in to unmask]>wrote:

> Ivo,
>
> You are right -- you are wrong. The stars you mention are not truly red
> stars.
>
> Although many refer to some stars as red dwarfs, giants or supergiants,
> this is an exaggeration -- they are not quite red.
>
> So, yes, these stars not really red but yellow/orange as you say.  And they
> are visible at this time of year.  The star I am thinking about is "red" and
> not readily visible now but is visible in Gainesville at other times of the
> year.  (It will become visible in early morning in about a month from now.)
>  It is also much fainter than first magnitude though should be visible in
> very dark skies.
>
> Gamma Crucis is "redder" than either Arcturus or Aldebaran but again not
> quite "red."  Therefore, Gamma Cru is incorrect. (Gamma Cru is "technically
> visible from Gainesville though its maximum elevation would be only about 3
> degrees!)
>
> Howard Cohen
>
>
> At 06:20 PM 3/23/2011, Ivo Rabell wrote:
>
>> 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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>>
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>
>
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