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ATM-OBSERVERS-L  March 2011

ATM-OBSERVERS-L March 2011

Subject:

Re: Are there any green stars?

From:

Mike Toomey <[log in to unmask]>

Reply-To:

Mike Toomey <[log in to unmask]>

Date:

Sat, 26 Mar 2011 14:24:07 -0400

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (348 lines)

Last guess for me... Mira -- Omicron Ceti... even though wikipedia
says it's only 400 ly away.

See everyone at the star party tonight. Sure hope it clears!

M


On Sat, Mar 26, 2011 at 2:17 PM, Howard L. Cohen <[log in to unmask]> wrote:
> Ivo,
>
> Barnard's Star (V2500 Ophiuchi), as you say, is too faint (mag. +9.5 so not
> naked eye) to be the "red star" I am thinking about.  It is also now visible
> after about 2:00 a.m. EDT but the star I am thinking about is not.  And, its
> color is probably about the same as Betelgeuse -- probably more
> yellow/orange or orange to most eyes than really red.
>
> Bill Helms suggested Mu Cephei and Mike Toomey wrote that he liked this
> guess.  Mu Cep is a good candidate and likely one of the more "reddish"
> naked eye stars.  It is easily seen when near greatest magnitude but this
> huge, cool supergiant is a distant, irregular variable with periods
> typically from 2 to 2-1/2 years.  Magnitude variations are more than one
> magnitude, abt. +3.4 to +5.1.  Mu Cep is now visible in morning hours.  Mu
> Cep is certainly one of the "redder" stars.  Some might call this star
> "garnet colored."  Mu Cep is much redder than its cool temperature would
> imply due to absorption of the bluer colors by stellar and interstellar
> dust.
>
> However, I still know of a redder naked eye star, but it is now too close to
> the Sun in the sky to be easily seen.  Its red color makes it stand out from
> surrounding stars.  Though other stars can be found that are still redder, I
> think this "red star" is probably the reddest, naked eye star.  Like most
> stars of its type, its brightness varies by up to several tenths of a
> magnitude but it is still naked eye when dimmest.  It is a bright giant
> roughly 800 light year away.
>
> Howard Cohen
>
> At 08:54 AM 3/26/2011, Ivo Rabell wrote:
>>
>> Howard,
>>
>> Barnard Star is my only other guess and that's wrong because it's a
>> red dwarf and you can't see it with naked eye.
>>
>> Maybe Mike Toomey, being the most knowledgeable stargazer in our
>> group, would know answer to "red star... which can be seen with naked
>> eye... early morning in about a month from now".
>>
>> Ivo
>>
>>> 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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