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Tom Mazanec
2015-Sep-11, 05:54 PM
Alpha Centauri A (and the sun!) is a Yellow Dwarf.
Alpha Centauri B is an Orange Dwarf.
Alpha Centauri C is a Red Dwarf.

But Sirius is not a White Dwarf! It is white and not a giant/supergiant, so what do I call it?

antoniseb
2015-Sep-11, 06:06 PM
I think you are using an older less precise naming convention. Usually we use spectral classifications, and identify whether it is off the main sequence or not, so Alpha Centauri A is a G star, Alpha Centauri B is K star, and Alpha Centuari C is an M star. Sirius is a A star. All four are on the main sequence, and are not listed as giants or variables. Sirius B, on the other hand, IS a white dwarf.

Hornblower
2015-Sep-11, 06:21 PM
This illustrates the sometimes sloppy use of jargon. I would be all for never using the term dwarf for any main sequence star as long as we use it for collapsed star remnants commonly called white dwarfs.

primummobile
2015-Sep-11, 06:25 PM
This illustrates the sometimes sloppy use of jargon. I would be all for never using the term dwarf for any main sequence star as long as we use it for collapsed star remnants commonly called white dwarfs.

Agree. The sun is larger than the majority of stars. It seems inconsistent to refer to it as a dwarf.

Jeff Root
2015-Sep-11, 06:27 PM
I haven't noticed that people stopped calling the Sun a
"yellow dwarf". I don't call it that, myself. I call the Sun
and Sirius "main sequence stars".

-- Jeff, in Minneapolis

George
2015-Sep-11, 06:40 PM
Alpha Centauri A (and the sun!) is a Yellow Dwarf.
Alpha Centauri B is an Orange Dwarf.
Alpha Centauri C is a Red Dwarf.
White dwarfs are not actually stars, but these are. The color is just an adjective that generalizes their spectral class and surface temperature. How these color assignments originally were given is an interesting story, but I charge extra for what little I have discovered. The Dwarf vs. Giant story is also interesting and not near as useful today as in the early 20th century, I think.

For the chromotologically oriented...
A has not a hint of yellow since it is white throughout its entire disk since it is analogous to the Sun (which ain't yeller).
B is too hot to be orange and is very likely just white given its high temperature (K1 class). Perhaps there is a hint of yellow along its limb.
C is possibly orange or orange-red but it is a small star that matches better the dwarf designation only because of its small size, not because of how it is grouped with other stars.

[Our atmosphere and other effects can alter their true color as we observe them.]

Sirius may have some bluish tint to it, especially near its hotter central region of its disk. Its surface temperature is almost 10,000K, but star temperatures vary across their disks since these are balls of gas and we can see deeper into the interior of stars when we look at their central regions, their limbs only reveal their cooler top regions of their photospheres.

Sirius B is definitely bluish white since its temperature is about 25,000K, but it is a true "white dwarf" and not a star since its days of fusion are done. It will cool down to a white color eventually.

Ken G
2015-Sep-11, 08:09 PM
Yes, the funny thing is, Sirius A should by all rights be a "white dwarf", while Sirius B should by all rights be a "blue dwarf", but if anything, we'd say it the other way around. That's just silly, so I agree with retiring the whole "main sequence stars should be regarded as dwarfs" idea. But then we'd also have to retire "subdwarfs", and so on. So the final result is, as usual, we just have to tolerate poor nomenclature, and try to avoid ambiguity. What else is new?

Tom Mazanec
2015-Sep-12, 12:36 PM
Actually, I believe "blue dwarf" is a term for a hypothetical type of star that will evolve in many millions of millennia as red dwarfs evolve off the main sequence.

Hornblower
2015-Sep-12, 01:07 PM
Actually, I believe "blue dwarf" is a term for a hypothetical type of star that will evolve in many millions of millennia as red dwarfs evolve off the main sequence.
No, they will never get as blue as Sirius B. As they use up their hydrogen, they will contract, get about as hot as the Sun and then start cooling.

Reference: Sky and Telescope, November 1997, p. 20.

Ken G
2015-Sep-12, 02:39 PM
Actually, I believe "blue dwarf" is a term for a hypothetical type of star that will evolve in many millions of millennia as red dwarfs evolve off the main sequence.
It just shows how limiting the nomenclature is. The Wiki on "blue dwarf" reveals it can either mean what you are talking about (what happens to a low-mass main-sequence star as it evolves toward higher luminosity), or about what I am talking about (a hot higher-mass main-sequence star). There is no way to tell the difference from the words alone, and the version I mean tends to be much hotter and bluer. The basic problem is, anyone can popularize a term, there's no real method for assuring consistent language. Also, astronomical objects tend to get named before we really understand the best ways to think about them.

George
2015-Sep-12, 03:39 PM
Several years ago, I cranked the numbers to see what temperature we need from a BB object to get a distribution similar to a blue sky... ~ 15 million K! Thus, star cores are blue. [Tyson mentioned this in a book sometime thereafter, so perhaps he's a lurker or a regular]

So blue is inaccurate for any star surface color yet it is still effective and not without merit. O class stars could be called blue and the B stars, perhaps A as well, would be better, I think, assigned cyan. F and G stars are white and the others I am unsure of their tint change as we get cooler.

Given the great success of astronomers in finding essentially every large animal in the jungle, why can't we reorganize the zoo with greatly improved taxonomy? Main sequence stars are regulars but deserve a label with more character besides "regular" and less cryptic than "main sequence". Giants and super giants are very useful with little or no contradiction. Perhaps dwarfs should be restricted to stars and something more clever for the toasted x- stars, which will descend from blue to red with enormous time. Of course, there are exotics like the T class that come with a variety of colors due to molecular composition.

I would bet the talent on-board here could offer some very nice advancement to our contradictory labels for all the glowing things out there.

Ken G
2015-Sep-12, 05:28 PM
Given the great success of astronomers in finding essentially every large animal in the jungle, why can't we reorganize the zoo with greatly improved taxonomy? Of course you're right, but it isn't going to happen. Remember, astronomers still refer to the late stages of evolution of a low-mass stellar envelope as a "planetary nebula", for crying out loud! Physics isn't always better-- there we have that the surface of the Sun is a "blackbody", of all things. No, get used to it, taxonomy and nomenclature are always going to make little sense, the important thing is to add whatever clarifying remarks are needed to avoid miscommunication.

korjik
2015-Sep-12, 07:38 PM
Several years ago, I cranked the numbers to see what temperature we need from a BB object to get a distribution similar to a blue sky... ~ 15 million K! Thus, star cores are blue. [Tyson mentioned this in a book sometime thereafter, so perhaps he's a lurker or a regular]

So blue is inaccurate for any star surface color yet it is still effective and not without merit. O class stars could be called blue and the B stars, perhaps A as well, would be better, I think, assigned cyan. F and G stars are white and the others I am unsure of their tint change as we get cooler.

Given the great success of astronomers in finding essentially every large animal in the jungle, why can't we reorganize the zoo with greatly improved taxonomy? Main sequence stars are regulars but deserve a label with more character besides "regular" and less cryptic than "main sequence". Giants and super giants are very useful with little or no contradiction. Perhaps dwarfs should be restricted to stars and something more clever for the toasted x- stars, which will descend from blue to red with enormous time. Of course, there are exotics like the T class that come with a variety of colors due to molecular composition.

I would bet the talent on-board here could offer some very nice advancement to our contradictory labels for all the glowing things out there.

The zoo was reorganized quite a while ago. It is only when 'legacy' terminology like 'dwarf' is used that things get confusing. The Sun is a G2V star. That gives you nearly all the information you need to know the star's properties.

galacsi
2015-Sep-12, 08:06 PM
In another thread (Against the Mainstream / Newton Gravity force in spiral galaxy )
Dave Lee wrote :


http://en.wikipedia.org/wiki/Sirius
"the Sirius system is one of Earth's near neighbors. Sirius is gradually moving closer to the Solar System, so it will slightly increase in brightness over the next 60,000 years. After that time its distance will begin to recede."
"Sirius, known in ancient Egypt as Sopdet (Greek: Σῶθις Sothis), is recorded in the earliest astronomical records"
During this long time it had changed its color several times: (Red, Blue, white..)
"However, not all ancient observers saw Sirius as red. The 1st century AD poet Marcus Manilius described it as "sea-blue", as did the 4th century Avienus.[75] It is the standard star for the color white in ancient China, and multiple records from the 2nd century BC up to the 7th century AD all describe Sirius as white in hue."
This color change is an evidence that the star is adjusting its direction.
In the past it was moving away from the sun and now it is moving closer to the sun. In the future it will reverse its direction again and again.

Dave Lee think these changes of colors were caused by the doppler effect and as Shaula show him it is completely wrong.
In fact wikipedia talk about the "Red controversy"
Excerpt :

Around 150 AD, the Greek astronomer of the Roman period Claudius Ptolemy (https://en.wikipedia.org/wiki/Claudius_Ptolemy) described Sirius as reddish, along with five other stars, Betelgeuse (https://en.wikipedia.org/wiki/Betelgeuse), Antares (https://en.wikipedia.org/wiki/Antares), Aldebaran (https://en.wikipedia.org/wiki/Aldebaran), Arcturus (https://en.wikipedia.org/wiki/Arcturus) and Pollux (https://en.wikipedia.org/wiki/Pollux_%28star%29), all of which are clearly of orange or red hue.[71] (https://en.wikipedia.org/wiki/Sirius#cite_note-Holberg2007-157-75) The discrepancy was first noted by amateur astronomer Thomas Barker (https://en.wikipedia.org/wiki/Thomas_Barker_%28meteorologist%29), squire of Lyndon Hall (https://en.wikipedia.org/wiki/Lyndon,_Rutland) in Rutland (https://en.wikipedia.org/wiki/Rutland), who prepared a paper and spoke at a meeting of the Royal Society (https://en.wikipedia.org/wiki/Royal_Society) in London in 1760.[72] (https://en.wikipedia.org/wiki/Sirius#cite_note-Ceragioli1995-76) The existence of other stars changing in brightness gave credence to the idea that some may change in color too; Sir John Herschel (https://en.wikipedia.org/wiki/Sir_John_Herschel) noted this in 1839, possibly influenced by witnessing Eta Carinae (https://en.wikipedia.org/wiki/Eta_Carinae) two years earlier.[73] (https://en.wikipedia.org/wiki/Sirius#cite_note-Holberg2007-158-77) Thomas Jefferson Jackson See (https://en.wikipedia.org/wiki/Thomas_Jefferson_Jackson_See) resurrected discussion on red Sirius with the publication of several papers in 1892, and a final summary in 1926.[74] (https://en.wikipedia.org/wiki/Sirius#cite_note-Holberg2007-161-78) He cited not only Ptolemy but also the poet Aratus (https://en.wikipedia.org/wiki/Aratus), the orator Cicero (https://en.wikipedia.org/wiki/Cicero), and general Germanicus (https://en.wikipedia.org/wiki/Germanicus) as coloring the star red, though acknowledging that none of the latter three authors were astronomers, the last two merely translating Aratus' poem Phaenomena.[75] (https://en.wikipedia.org/wiki/Sirius#cite_note-Holberg2007-162-79) Seneca (https://en.wikipedia.org/wiki/Seneca_the_Younger), too, had described Sirius as being of a deeper red color than Mars.[76] (https://en.wikipedia.org/wiki/Sirius#cite_note-Whittet1999-80) However, not all ancient observers saw Sirius as red. The 1st century AD poet Marcus Manilius (https://en.wikipedia.org/wiki/Marcus_Manilius) described it as "sea-blue", as did the 4th century Avienus (https://en.wikipedia.org/wiki/Avienus).[77] (https://en.wikipedia.org/wiki/Sirius#cite_note-Holberg2007-163-81) It is the standard star for the color white in ancient China, and multiple records from the 2nd century BC up to the 7th century AD all describe Sirius as white in hue.[78] (https://en.wikipedia.org/wiki/Sirius#cite_note-Jiang1992-82)[79] (https://en.wikipedia.org/wiki/Sirius#cite_note-Jiang1993-83)
In 1985, German astronomers Wolfhard Schlosser and Werner Bergmann published an account of an 8th-century Lombardic (https://en.wikipedia.org/wiki/Lombardy) manuscript, which contains De cursu stellarum ratio by St. Gregory of Tours (https://en.wikipedia.org/wiki/Gregory_of_Tours). The Latin text taught readers how to determine the times of nighttime prayers from positions of the stars, and Sirius is described within as rubeola — "reddish". The authors proposed this was further evidence Sirius B had been a red giant at the time.[80] (https://en.wikipedia.org/wiki/Sirius#cite_note-Schlosser1985-84) However, other scholars replied that it was likely St. Gregory had been referring to Arcturus instead.[81] (https://en.wikipedia.org/wiki/Sirius#cite_note-McCluskey1987-85)[82] (https://en.wikipedia.org/wiki/Sirius#cite_note-VanGent1987-86)
The possibility that stellar evolution (https://en.wikipedia.org/wiki/Stellar_evolution) of either Sirius A or Sirius B could be responsible for this discrepancy has been rejected by astronomers on the grounds that the timescale of thousands of years is too short and that there is no sign of the nebulosity in the system that would be expected had such a change taken place.[76] (https://en.wikipedia.org/wiki/Sirius#cite_note-Whittet1999-80) An interaction with a third star, to date undiscovered, has also been proposed as a possibility for a red appearance.[83] (https://en.wikipedia.org/wiki/Sirius#cite_note-Kuchner2000-87) Alternative explanations are either that the description as red is a poetic metaphor for ill fortune, or that the dramatic scintillations (https://en.wikipedia.org/wiki/Scintillation_%28astronomy%29) of the star when it was observed rising left the viewer with the impression that it was red. To the naked eye, it often appears to be flashing with red, white and blue hues when near the horizon.[76] (https://en.wikipedia.org/wiki/Sirius#cite_note-Whittet1999-80)


I have an other explanation for this alleged change of colour during the historical times :

A kind of nova event at Sirius B. Sirius B is not very far from Sirius A ,and with its enormous gravity accretes some gases and dusts coming from its neighbor.Then from times to times ,"Bang" a nuclear explosion occurs on its surface and expulses the reaction's products in space. A nebula is born ,expands and finally disapears.
This nebula is the cause of the red color of Sirius seen by some of our ancestors.
Because the light of Sirius A must go through it and so appears redish, like our sun when it is low on the horizon.

So what do you think of this idea ?

korjik
2015-Sep-12, 08:45 PM
In another thread (Against the Mainstream / Newton Gravity force in spiral galaxy )
Dave Lee wrote :

Dave Lee think these changes of colors were caused by the doppler effect and as Shaula show him it is completely wrong.
In fact wikipedia talk about the "Red controversy"
Excerpt :



I have an other explanation for this alleged change of colour during the historical times :

A kind of nova event at Sirius B. Sirius B is not very far from Sirius A ,and with its enormous gravity accretes some gases and dusts coming from its neighbor.Then from times to times ,"Bang" a nuclear explosion occurs on its surface and expulses the reaction's products in space. A nebula is born ,expands and finally disapears.
This nebula is the cause of the red color of Sirius seen by some of our ancestors.
Because the light of Sirius A must go through it and so appears redish, like our sun when it is low on the horizon.

So what do you think of this idea ?

It is far more likely that one guy got it wrong and called Sirius red by mistake than every other contemporary source got it wrong and never noticed the change in color.

Sirius B is too far away from A to accrete enough mass to have an outburst. The distance between them is 20 AU

galacsi
2015-Sep-12, 09:23 PM
It is far more likely that one guy got it wrong and called Sirius red by mistake than every other contemporary source got it wrong and never noticed the change in color.

Far more likely ? Really ? I don't think so.


Sirius B is too far away from A to accrete enough mass to have an outburst. The distance between them is 20 AU

I don't know ,after all Sirius B is not only a very little star not bigger than a planet but also a magnetosphere which could be able to trap some plasma.

There is also the case of a comet or an asteroid crashing on its surface.

antoniseb
2015-Sep-12, 10:33 PM
Far more likely ? Really ? I don't think so. ...
My understanding is that the source is talking about seeing Sirius in the desert as it is rising, and so would appear red just as the Sun looks red when it is rising. So not a mistake except in modern interpretation.

Jeff Root
2015-Sep-13, 06:08 AM
Sirius is different from most stars in that it is bright enough
for its color -- whatever that color may be -- to be easily
visible to human eyes. If it was reported by some ancient
observers as being red, and that redness was due to Earth's
sky conditions, then I would expect other bright, non-red
stars to have also been reported as being red at the same
time and place.

I saw a bright red light in the sky in the middle of the night
while travelling from Iowa to the Twin Cities. The president
of the Minnesota Astronomical Society happened to be in
the passenger seat in front of me, so I asked him what he
thought it was. After a minute or more, he replied "Aircraft
on final." But we were nowhere near an airport. We kept
watching this thing for 20 or 30 minutes before we finally
decided what it was. It was slowly rising in the sky, slowly
becoming less red and more of a pale pink, and -- most
significantly -- I realized it was much later than I thought.
I had unconsciously assumed that it was about 1 or 2 AM.
In fact it was after 4 AM. Venus! We stopped at a rest
stop and put binoculars on it. It was too cold for me so
my hands shook, but my friend reported being able to
see the crescent shape. The eruption of Mount Pinutubo
in the Phillipines a few weeks before had put so much dust
into the upper atmosphere that Venus looked deep red
for many minutes after rising.

-- Jeff, in Minneapolis
.

Ken G
2015-Sep-13, 06:50 AM
Yes, I think the bottom line is, star colors are ambiguous enough that it would never be prudent to use ancient poetic descriptions of stars as evidence that they had undergone physical changes over those timescales. Stars can sometimes do radical things, but they spend the vast majority of their time not doing such radical things! It is only when the object is selected for having done something radical, like eta Carinae or something, that we can expect to see radical things. Sirius, being the brightest nighttime star in the sky, is certainly not selected for doing radical things, so we should look for more mundane explanations first.

galacsi
2015-Sep-13, 07:02 PM
Sirius is different from most stars in that it is bright enough
for its color -- whatever that color may be -- to be easily
visible to human eyes. If it was reported by some ancient
observers as being red, and that redness was due to Earth's
sky conditions, then I would expect other bright, non-red
stars to have also been reported as being red at the same
time and place.

I saw a bright red light in the sky in the middle of the night
while travelling from Iowa to the Twin Cities. The president
of the Minnesota Astronomical Society happened to be in
the passenger seat in front of me, so I asked him what he
thought it was. After a minute or more, he replied "Aircraft
on final." But we were nowhere near an airport. We kept
watching this thing for 20 or 30 minutes before we finally
decided what it was. It was slowly rising in the sky, slowly
becoming less red and more of a pale pink, and -- most
significantly -- I realized it was much later than I thought.
I had unconsciously assumed that it was about 1 or 2 AM.
In fact it was after 4 AM. Venus! We stopped at a rest
stop and put binoculars on it. It was too cold for me so
my hands shook, but my friend reported being able to
see the crescent shape. The eruption of Mount Pinutubo
in the Phillipines a few weeks before had put so much dust
into the upper atmosphere that Venus looked deep red
for many minutes after rising.

-- Jeff, in Minneapolis
.


Thinking about it , it is exactly my idea but transposed in our atmosphere instead in the vicinity of Sirius !

George
2015-Sep-14, 03:57 AM
Without atmospheric effects, what color is it? It is hot (~ 10,000K) and only ~ 1,000K cooler than bluish Rigel.

Terrestrially, do many consider it to be somewhat blue or is white? If white, is this a lower altitude issue? Is so, do we have any lower latitude observers who see some blue?

chornedsnorkack
2015-Sep-14, 12:19 PM
If white, is this a lower altitude issue? Is so, do we have any lower latitude observers who see some blue?

Where is latitude "lowest"?

Jeff Root
2015-Sep-14, 02:37 PM
At the equator. George is asking what people see when
Sirius is very high in the sky.

-- Jeff, in Minneapolis

chornedsnorkack
2015-Sep-14, 03:17 PM
Sirius is at its highest at 17 South, and gets as high at 34 South as it gets at equator.

Hornblower
2015-Sep-14, 07:52 PM
Sirius is the same color as any other "early" A star, a pale bluish white under conditions in which an F star looks neutral white and a G star pale yellowish white.

I am 100% confident that the "Red Sirius" of ancient times was nothing more than atmospheric reddening when near the horizon and/or the red sparkles I have seen when it is twinkling vigorously. Other stars are enough fainter that these effects are not as apparent to the naked eye.

George
2015-Sep-16, 08:13 PM
Since the "Sirian" stars were labeled as white by Father Secchi in his Type I classification (yellow Capella, and Sun for Type II), I am curious just how Sirius appeared to other astronomers during his time, as well as, those with experience like yours (Hornblower) in visual astronomy. Secchi's work was contemporary with Huggins (from Tulse Hill in London). Secchi, from Rome (about 41 deg. N; New York), would have an air mass of 2 with Sirius on the meridian (~ 30 deg. alt.) and Huggins with an air mass of 3. But I suspect both London and Rome may have had some serious reddening effects making a bluish white star white and a white solar star appear yellow-white. I realize you do see solar-like stars as yellowish white, IIRC, but I don't; 18 Sco being my proxy. Yet I have not, oddly enough, spent much time doing this because I chose to use their SEDs to establish "true" color, thus eliminating many color altering effects caused by our atmosphere. Unfortunately, this is still subjective since color modeling is not that accurate as I had hoped. [Input a 580nm spike in a model and you will likely not get a true clean yellow as found in the solar spectrum.] Google color temperature and you will only find one in 50, at best, somewhat correct, if you hold these to strict BB color renditions, though they are not. [The Asterochromograph will eliminate most subjective arguments, however, if it ever gets built.]

What is also missing, though implied, is the added city pollution, dust, pollen, etc. that profoundly alter the color and further explains what your are saying. Ancient times can often refer to desert regions. Pick any dusty, windy day in W. Texas and you can bet on a deep orange, or reddish orange, setting Sun. A setting sun for many eastern Californians may also being seeing a red Sirius soon if the fires are raging in the west. Ancient records likely emphasize the odd astro occurrences especially if astrology was influential, as it usually was. This is like the recent quips about the Tortoise and the Hare being a fable of only one race won by the tortoise and no reports of all the many loses.

Hornblower
2015-Sep-18, 01:29 AM
Yes indeed, really dusty or foul air can redden the stars significantly, and the great brightness of Sirius could make the reddish tint visible when fainter stars would fade to shades of gray.

Buttercup
2015-Sep-18, 01:31 AM
Last I knew, it was a "white dwarf."

Hornblower
2015-Sep-18, 02:05 AM
Last I knew, it was a "white dwarf."
That's Sirius B, the faint companion of the familiar Dog Star. It is sometimes called "the Pup."

Buttercup
2015-Sep-18, 02:10 AM
That's Sirius B, the faint companion of the familiar Dog Star. It is sometimes called "the Pup."

Oh. :lol: Duh...I should have known that!!! :( Sirius is SO BRIGHT. :doh:

I've been "off" all day. :wall:

Go on - laugh. I deserve it. :) [This morning it was me getting impatient with ATM for not returning my bank card fast enough...which was in my pocket again, already]