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Re: SKYLIGHTS: Star of the Week

Posted: Sun Jun 01, 2014 2:39 pm
by Ann
BDanielMayfield wrote:
MargaritaMc wrote:I read the discussion about CU Virginis with interest, having only seen it when I posted this current week's Star of the Week.
Checking with Jim Kaler, he said that the version I posted a quote from has been superseded, as the term "white" causes confusion.
So then Ann's objection to this star being called white was right, which I don't find surprising. What does the updated version say as to CU Vir's color?
I just checked. This is what Professor Kaler says in his updated version:
CU Vir is too blue and, with a temperature of 12,800 Kelvin, too hot and blue for its A0 class, and B9p is probably more like it.
Let me briefly return to IQ Aurigae, the bluest of all A-type stars. This is what Bright Star Catalog says about the chemical composition, color, temperature and spectral class of IQ Aurigae:
Magnetic star showing chlorine. Silicon star. Helium and neon deficient. Bluest known peculiar A-star: color characteristic of about B4V. Classified A0p because He weak, but in other respects the color is more indicative of the temperature than the spectral class of Ap stars. One of the hottest Ap stars known, about 17000K.
It is the silicon-enhanced A-type stars that are blue. There are other A-type stars which show enhanced concentrations of other elements than silicon, but they are not unusually blue for their spectral class.

Ann

Re: SKYLIGHTS: Star of the Week

Posted: Mon Jun 02, 2014 12:25 am
by BDanielMayfield
Ann wrote:
BDanielMayfield wrote:
MargaritaMc wrote:I read the discussion about CU Virginis with interest, having only seen it when I posted this current week's Star of the Week.
Checking with Jim Kaler, he said that the version I posted a quote from has been superseded, as the term "white" causes confusion.
So then Ann's objection to this star being called white was right, which I don't find surprising. What does the updated version say as to CU Vir's color?
I just checked. This is what Professor Kaler says in his updated version:
CU Vir is too blue and, with a temperature of 12,800 Kelvin, too hot and blue for its A0 class, and B9p is probably more like it.
May you bask in the pleasing blue aura of sweet vindication. Your expertise when it comes to blue stars has been resoundingly confirmed. :clap: :clap: :thumb_up:

Bruce

Re: SKYLIGHTS: Star of the Week

Posted: Mon Jun 02, 2014 3:33 am
by geckzilla
It's too bad that she couldn't state why Kaler would have used the word "white" in this instance, which he did not say was wrong, just confusing. It would have greatly helped the situation if it were stated something like "Oh, he wrote 'white' because of [explanation]. But visually, this star is blue." Instead of that, it was implied that it was written completely in error. Vindicated? I know several people think that I am attacking Ann but these things bother me quite a lot.

Re: SKYLIGHTS: Star of the Week

Posted: Mon Jun 02, 2014 4:39 am
by Ann
geckzilla wrote:It's too bad that she couldn't state why Kaler would have used the word "white" in this instance, which he did not say was wrong, just confusing. It would have greatly helped the situation if it were stated something like "Oh, he wrote 'white' because of [explanation]. But visually, this star is blue." Instead of that, it was implied that it was written completely in error. Vindicated? I know several people think that I am attacking Ann but these things bother me quite a lot.
Good question, Geck. I have followed Jim Kaler quite closely for some years, although slightly less closely recently. I also own two of his books. In my opinion, Professor Kaler is not too interested in the B-V indexes of stars, nor even really of their visual appearances in the telescope, at least not when it comes to bluish stars. He goes straight for the spectral class and defines the color of the star from the spectral class. Therefore, a star of spectral class A is white to him. Similarly, a star of spectral class F is yellow-white and a star of spectral class G is yellow. He will normally call K-type stars orange and M-type stars red. O stars are blue to him, and B stars are blue-white.

Since Jim Kaler goes straight for the spectral class when he defines the color of a star, he calls Canopus a yellow-white star:
Canopus, the much grander star, is vastly farther away and is a rather rare class "F" yellow- white (7280 Kelvin) bright giant.
However, Canopus is remarkably bluish for an F-type star. It is not blue, of course, not so that it has a negative B-V index. But it has a remarkably "low" B-V index (for an F-type star) of about 0.15. That is clearly bluer than some A-type stars, for example Altair. (I don't have access to my software right now, but I'd say that the B-V index of Altair is about 0.27.) Yet Jim Kaler calls Altair white:
Though three of the stars of the Summer Triangle are all white in color and hotter than the Sun, all are also individuals. A class A (A7) hydrogen-fusing dwarf with a temperature of 7550 degrees Kelvin, Altair is the coolest of the three (with Vega and Deneb warmer at 9500 and 8400 respectively).
To me, the really interesting question is why A-type stars are "defined" as white. (See the link that Margarita sent.) My answer is that Vega has been used as the "perfect definition" of a white star, yet the visual appearance of Vega (through a telescope) is quite bluish. I have asked a number of people to look at Vega through a telescope and say what color it is. Everyone said that Vega looked blue.

In his book A View of the Universe, David Malin offered an explanation for astronomy's reluctance to acknowledge the existence of blue stars. I'm summarizing David Malin's claims from memory here, since I don't have access to his book right now, but what he said is that astronomy since antiquity has defined stars as white, yellow or red. When astronomers began studying double stars through a telescope, they saw blue stars (for example the fainter component of Albireo), but because the blue stars were members of binaries they were regarded as a curiosity. The idea that (single) stars can indeed be blue visually was mostly rejected, if only because the idea was so unfamiliar. Personally I have to admit that hardly any stars ever look blue to me without visual aids like a telescope.

But let's assume that David Malin was right in his claim that astronomy mostly rejected the idea that stars can "really" be blue. The only possible star colors would then be white, yellow and red. If you want to define the whitest possible star (and you live in the northern hemisphere, like most astronomers have done since antiquity), you might well pick the star that looks as perfectly non-red as possible. To me, that might very will be Vega, if only because Vega is bright and doesn't twinkle like a disco ball, like Sirius. Of course, I think that Vega is definitely very bluish when seen through a telescope, but if it was out of the question to define Vega as blue, then it might instead become the most perfect example of stellar whiteness.

And if Vega is perfectly white, and if it is white because it is an A-type star, then it really doesn't matter what it looks like through a telescope. It is white because it is white, and then all other A-type stars can be defined as white, too. And then F-type stars are yellow-white, and G-type stars are yellow, and K-type stars are orange and M-type stars are red. Only O- and B-type are "allowed" to be bluish. For that reason, a star like Rigel is often defined as bluish or blue-white. Jim Kaler defines Rigel, a B8-type supergiant, as blue or blue-white,, even though its B-V index is not all that impressive, around -0.03:
Like its class M reddish rival in Orion, Betelgeuse, Rigel (Beta Orionis) is a supergiant, though a contrasting blue one (actually more blue-white) of class B (B8).
So to summarize, I think that Jim Kaler simply defines the color of a star from its spectral class, although in a few cases he might underscore the actual color of them. That happens mostly when he talks about a few unusually red stars.

Ann

Re: SKYLIGHTS: Star of the Week

Posted: Mon Jun 02, 2014 5:09 am
by BDanielMayfield
MargaritaMc wrote:I read the discussion about CU Virginis with interest, having only seen it when I posted this current week's Star of the Week.
Checking with Jim Kaler, he said that the version I posted a quote from has been superseded, as the term "white" causes confusion.
Wikipedia is useful on the difference between conventional and apparent colours in star classification
The conventional color descriptions are traditional in astronomy, and represent colors relative to the mean color of an A-class star which is considered to be white. The apparent color descriptions are what the observer would see if trying to describe the stars under a dark sky without aid to the eye, or with binoculars
The table of the Harvard spectral classification scheme at Wikipedia attempts to reproduce these colours. This image is a copy of that table in an earlier form (the one at Wikipedia now includes classes L, T and Y.) M
So this conflict over the color of stars is due to the differences between "conventional color" and "apparent color", but what is more true, how a star actually appears, or holding on to a rigid, they're in that bin so that's what they're supposed to look like convention?

Re: SKYLIGHTS: Star of the Week

Posted: Mon Jun 02, 2014 5:58 am
by Ann
There might be a little more to it than that. The B-V index of Vega is 0.00, although I've also seen a tiny negative value for Vega, -0.01.

The B-V index measures fluxes of "visual" (yellow-green) versus "blue" light. For stars cooler than the Sun, there is a lot more "visual" than blue light. For the Sun, too, there is considerably more visual than blue light: the B-V index of the Sun is 0.656 ± 0.005. Even so, interestingly, the energy output of the Sun peaks in the green part of the spectrum, indeed not far from the blue part of the spectrum.

If you ask me how these fluxes are measured, I must admit that I have no idea. What values you will get depends crticially on what exact wavelengths you measure, certainly when it comes to A-type stars. Let's assume, however, that the fluxes of B light and V light are indeed equal for Vega. Does that make Vega white?

In my opinion, it does not. To me, the Sun should be defined as white, for the simple reason that humanity has evolved in response to the light output of the Sun, and we should see sunlight and, more importantly, daylight, as white (or neutral). Yes, I know that our eyes are phenomenally good at adapting so that they almost always see the ambient light as neutal, or white. I don't think that that is a sufficent reason to define a star that is quite different from our own Sun as white, however.

In other words, I think that the B-V index of the Sun, 0.656 ± 0.005, should be the definition of stellar whiteness. Why not? Why should the light output of Vega be a better definition of it for us humans - particularly since you almost certainly have to define "B light" and "V light" in exactly the "right" way to come up with a B-V index of 0.00 for Vega?

Ann

Re: SKYLIGHTS: Star of the Week

Posted: Wed Jun 04, 2014 11:41 am
by BDanielMayfield
Ann wrote:To me, the Sun should be defined as white, ...

In other words, I think that the B-V index of the Sun, 0.656 ± 0.005, should be the definition of stellar whiteness. Why not?
Because, even when it is high over head, the Sun has a slightly yellowish tint to it. At least it does for me when I get past it's blinding glare by momentarily using the Clint Squint method.

Bruce

Re: SKYLIGHTS: Star of the Week

Posted: Wed Jun 04, 2014 11:57 am
by Ann
BDanielMayfield wrote:
Ann wrote:To me, the Sun should be defined as white, ...

In other words, I think that the B-V index of the Sun, 0.656 ± 0.005, should be the definition of stellar whiteness. Why not?
Because, even when it is high over head, the Sun has a slightly yellowish tint to it. At least it does for me when I get past it's blinding glare by momentarily using the Clint Squint method.

Bruce
I see what you mean, Bruce, and I like the font color you came up with for the Sun! I just think that the B-V index of the Sun should take the reddening of the sunlight caused by the atmosphere into account. Out in space, the Sun would not just be even more blindingly bright than it is from our vantage point here on Earth (provided you very carefully squinted at it from a distance of one A.U., or the distance between the Sun and the Earth), but it would also be very, very white. At least that is what I believe. I found Capella to be quite white when I looked at it through a telescope, and Capella is yellower than the Sun.

Ann

Re: SKYLIGHTS: Star of the Week

Posted: Sat Jun 07, 2014 8:00 pm
by MargaritaMc
This week's "Star of the Week" is actually THREE stars and the text is difficult to take excerpts from without risking breach of copyright. So I will simply give the link and brief details here.

The stars are in Virgo.

• Rho Virginis, class A0, is a Delta Scuti variable star, has a debris disc (so maybe planets) and is also a "Lambda Boötes star", depleted of iron.

• 27 Vir, only 0.2 degrees away from Rho Vir, is a class A7 star and also a Delta Scuti variable.

• 33 VIR, 1.3degrees from Rho, is a class K1 giant-subgiant, with a proper motion of more than 0.5 arc-seconds a year.

For the full details and all the informative hyperlinks, go to
http://stars.astro.illinois.edu/sow/rhovir.html

Margarita

PS. There is helpful basic info about the constellation at EarthSky

Re: SKYLIGHTS: Star of the Week

Posted: Sat Jun 07, 2014 8:19 pm
by Ann
Thanks, Margarita! A few additional details:

All three stars are close to ten times as bright as the Sun in visual light. Rho Virginis is about 12 times the luminosity of the Sun in V light, 27 Virginis is about 11 times as bright as the Sun, and 33 Virginis is a little more than 9 solar luminosities in yellow-green light. I find it interesting that these three stars are so similar in V luminosity. It is also interesting that Rho Vir and 33 Vir are both underluminous, although Jim Kaler has an explanation for the faintness of 33 Vir: It is most likely just starting to swell to become a red giant.

As for Rho Vir, it is worth comparing it with last week's Star of the Week, CU Virginis. CU Virginis is at least moderately bright (about 53 times the Sun), blue (B-V -0.118) and silicon-enhanced. Rho Viriginis is faint (12 times the Sun), rather non-blue (B-V +0.076) and silicon-depleted.

Of course, it is also quite interesting that Rho Vir appears to have a dust disk and just possibly planets, although none have been found yet.

Ann

Re: SKYLIGHTS: Star of the Week

Posted: Sat Jun 14, 2014 6:20 pm
by MargaritaMc
STAR OF THE WEEK: ASELLUS SECUNDUS (Iota Bootis). Way way up in the northwest corner of Bootes, five or so degrees to the northeast of Alkaid (Eta Ursae Majoris) at the end of the handle of the Big Dipper, lie three stars that that represent the outstretched fingers of the Herdsman, who follows the Great Bear around the pole. From east to west, they are Theta, Iota, and Kappa Boo. The first and third are fourth magnitude, Iota fifth (4.75). One would hardly expect them to have proper names when brighter stars lack them. But they do, again in order Asellus Primus, Asellus Secundus, and Asellus Tertius. Having nothing whatever to do fingers or any other part of Bootes, the names, from Latin, refer to first, second, and third donkeys, more specifically to donkey colts. According to Allen, they were applied by Bayer; one wonders what the great mapmaker had in mind.
...
But what about the star itself? Iota Boo is a class A (A9) hydrogen-fusing dwarf 94.8 light years away (good to half a light year) with a temperature of 7730 Kelvin. With little correction for ultraviolet or infrared radiation, "Secundus" shines with the light of 8.2 times that of the Sun, from which we derive a radius of 1.6 times solar. A relatively fast spinner, at least 137 kilometers per second at the equator (which keeps the chemical elements from separating into odd patterns), the star completes a rotation in under 0.6 days. Theory gives it a mass 1.7 times that of the Sun and suggests that the star is relatively young and has most of its hydrogen-fusing lifetime of 1.8 or so billion years left to go. Of special interest, it's a Delta Scuti type variable, oscillating in brightness by a percent or so with at least two separate pulsation periods of 38.22 and 30.55 minutes (the exact figures somewhat elusive)

read more (and the hyperlinks) at http://stars.astro.illinois.edu/sow/aselluss.html

Re: SKYLIGHTS: Star of the Week

Posted: Fri Jun 20, 2014 8:17 pm
by MargaritaMc
STAR OF THE WEEK: LAMBDA LUP (Lambda Lupi). For hot stars, look no farther than your nearest Wolf. The one in the sky. Stuck between Scorpius to the east and Centaurus to the west, Lupus's scattered stars are commonly connected in the form of a (as might be expected) loop, within which we find our star, Lambda Lupi, five degrees northeast of the second magnitude luminary, Kakkab (Alpha Lupi). Made of two hot class B (B3) dwarfs only a fraction of a second of arc apart, Lambda Lupi A (magnitude 4.47) and Lambda Lupi B (magnitude 5.27) combine to shine at mid-fourth magnitude (4.05) from a lucky distance of 777 light years, give or take around 120.
http://stars.astro.illinois.edu/sow/lambdalup.html

Re: SKYLIGHTS: Star of the Week

Posted: Sat Jun 28, 2014 10:16 pm
by MargaritaMc
The Star of the Week for week beginning 27th June 2014
GAMMA LIB (Gamma Librae). On the bright side of fourth magnitude (3.91), Gamma Librae anchors an eastern corner of the traditional figure of Libra, the Scales, which in ancient times held the autumnal equinox in its weighing pan. Gamma Lib, however, has the misfortune to be not only a common sort of star, a class G (G8.5) helium-fusing giant (some say a K0 giant, making it even more common), but has to compete with two of the most beloved stars, or at least star names, in the sky: Zubenelgenubi (Alpha Librae, to the west of Gamma) and Zubeneschamali (Beta Librae, to the northwest of Gamma), which in mythology represent the outstretched claws of Scorpius, the Scorpion, found to the southeast of Libra. Yet common as the star is, an air of mystery lingers about it...
read on at http://stars.astro.illinois.edu/sow/gammalib.html

Re: SKYLIGHTS: Star of the Week

Posted: Sat Jul 05, 2014 10:24 am
by MargaritaMc
"Fireworks (of a sort) in the Star of the Week" ( http://stars.astro.illinois.edu/skylights.html)
STAR OF THE WEEK: 5 SER (5 Serpentis). Near the southwest corner of Serpens Caput (the Serpent's Head, the constellation divided in two by Ophiuchus) lies seemingly anonymous fifth magnitude (5.09) 5 Serpentis (as numbered west to east in Flamsteed's catalogue). It's best known as that star next to the great globular cluster Messier 5, the two just a third of a degree apart. A close examination of the star, however, almost reverses the order, making M5 the guide to 5 Ser! 
... Five Ser has been observed to flare on three separate occasions. One in 1979-80 brightened the star visually by up to nine percent, the event lasting for as long as 25 days. Such energetic outbursts could not be good for any exposed life. Irregular variations suggest starspots going in and out of the field of view. While flaring is common among red dwarfs (such as Proxima Centauri), this kind of superflaring is highly unusual in solar type stars.

read more at http://stars.astro.illinois.edu/sow/5ser.html

Re: SKYLIGHTS: Star of the Week

Posted: Wed Jul 09, 2014 5:47 am
by Ann
Thanks, Margarita. Fascinating. I have to wonder what would make a mature, solar type star flare so violently.
http://stars.astro.illinois.edu/sow/5ser.html wrote:
The phenomenon makes us look a bit more suspiciously at our own Sun.
:ohno:

Ann

Re: SKYLIGHTS: Star of the Week

Posted: Wed Jul 09, 2014 7:09 pm
by MargaritaMc
That's why I so value Jim Kaler's star of the week: it gets me thinking!

Re: SKYLIGHTS: Star of the Week

Posted: Thu Jul 10, 2014 8:15 pm
by MargaritaMc
Professor Kaler has started to add information about the spectra of individual planetary nebulae to his pages on PNs.
http://stars.astro.illinois.edu/sow/pn.html
THE PLANETARY NEBULAE

From Jim Kaler's STARS

In which Hubble images are compared with Curtis's historic century-old set of Lick Observatory observations.

See the spectra of the Ring Nebula in Lyra, NGC 7009, and IC 418.

Re: SKYLIGHTS: Star of the Week

Posted: Thu Jul 10, 2014 9:07 pm
by geckzilla
Kaler wrote: Forbidden lines are not really forbidden, just difficult to produce from energy levels that do not readily interact with each other (making the transitions of electrons between them difficult). They are indicated by square brackets . . .
Indicated by square brackets. Finally. Now I know why we write it like [OIII]. It can't be that hard to find out if you ask the right question to the right person or search engine, but I never did.

Re: SKYLIGHTS: Star of the Week

Posted: Thu Jul 10, 2014 9:49 pm
by MargaritaMc
geckzilla wrote:
Kaler wrote: Forbidden lines are not really forbidden, just difficult to produce from energy levels that do not readily interact with each other (making the transitions of electrons between them difficult). They are indicated by square brackets . . .
Indicated by square brackets. Finally. Now I know why we write it like [OIII]. It can't be that hard to find out if you ask the right question to the right person or search engine, but I never did.
:lol2: Reading through Jim Kaler's site just after I first became interested in astronomy taught me so much!!

Re: SKYLIGHTS: Star of the Week

Posted: Thu Jul 10, 2014 9:54 pm
by MargaritaMc
In case you want to know more details...
http://www.daviddarling.info/encycloped ... _line.html
A forbidden line arises when an electron in an excited (energized) atom jumps from a metastable state to a lower energy level. Under normal circumstances, when particle densities are higher (greater than about 10^8 per cm^3), such an electron would almost immediately be knocked out of its metastable state by collision and not be given time to emit a photon. But in an environment like that of a planetary nebula, the time between collisions averages 10 to 10,000 seconds. Consequently, when ions such as O+, O2+ (singly and doubly ionized oxygen), or N+ (singly ionized nitrogen) go into metastable states by allowed transitions from higher states, they remain there undisturbed until they radiate spontaneously.

Re: SKYLIGHTS: Star of the Week

Posted: Fri Jul 11, 2014 2:16 pm
by MargaritaMc
I've just spent a satisfying hour looking at this:
http://stars.astro.illinois.edu/sow/n7009.html#spec
This historical slit spectrogram (broken into seven overlapping parts) was taken in 1961 by Lawrence Aller using the coude spectrograph of the 100-inch telescope at the Mt. Wilson Observatory. The weakest lines have strengths nearly a ten- thousandth that of the standard H-Beta line at 4861 Angstroms (which is not shown). Note the remarkable ionized oxygen (O II) spectrum, the confluence of the Balmer lines in the fifth panel, which are visible to wavelengths shorter than that of H30, and the Balmer continuum that runs to wavelengths shorter than the "Balmer limit" at 3646 Angstroms. (The Balmer lines and continuum are the set of electron transitions from higher orbits to orbit 2.) The lines marked "L.A." are from mercury vapor street lights in Los Angeles. Vastly more lines lie at both longer and shorter wavelengths; this spectrum is just a sample.

Star of the Week for week beginning July 11th

Posted: Sat Jul 12, 2014 9:43 am
by MargaritaMc
STAR OF THE WEEK: GMB 1830 UMA
(Groombridge 1830 Ursae Majoris, HR 4550 in the Bright Star Catalogue), which lies in southern Ursa Major roughly 17 degrees south of the bowl of the Big Dipper and eight or so degrees southwest of Chara (Beta Canum Venaticorum) in Canes Venatici. It's named after Steven Groombridge (1755-1832), who compiled a catalogue of accurate positions of circumpolar stars as seen from England. Just barely sixth magnitude (6.45, not quite seventh), this remarkable class G (G8) hydrogen-fusing dwarf ranks third in the list of high "proper motions" (angular speeds across the line of sight). Moving at a rate of 7.06 seconds of arc per year against the distant stellar background, it's behind only Barnard's Star in Ophiuchus (10.4 seconds) and Kapteyn's Star in Pictor (8.67). ... One might expect that, like Barnard's, it must be quite nearby to be zipping along so fast, but at a distance of 29.6 light years ... , it really IS moving fast, at an amazing 305 kilometers per second across the line of sight relative to the Sun. When combined with a speed toward us (the "radial velocity") of 98 km/s, we find the star to be clipping along at 320 km/s, some 20 times normal, making it among the fastest stars known. Equally remarkable is the chemical composition, which is very low in "metals" (to an astronomer everything but hydrogen and helium), the ratio of iron to hydrogen a mere four or five percent what it is in the Sun. The combination of speed and composition immediately tell that the star is a visitor from the Galaxy's extended and sparsely populated halo, which surrounds the Galactic disk that makes our Milky Way. ...

more at
http://stars.astro.illinois.edu/sow/gmb1830.html
Right ascension 11h 52m 58.7691s
Declination +37° 43′ 07.239″

The Wikipedia entry has the star marked on a star chart, but the compete chart won't link here.

Re: SKYLIGHTS: Star of the Week

Posted: Sat Jul 19, 2014 5:55 pm
by MargaritaMc
Star of the week for week beginning July 18th, 2014
83 UMA (83 Ursae Majoris). In a constellation known for its white class A stars and its orange class K giants, it's a surprise and a treat to come across something quite different. ... The prize in the set is 83 UMa, a red class M (M2) giant, which fits not at all with the rest of the gang.

More at http://stars.astro.illinois.edu/sow/83uma.html

Re: SKYLIGHTS: Star of the Week

Posted: Sun Jul 20, 2014 6:56 am
by Ann
Interesting, Margarita, and thanks again for posting this.
Source: https://www.noao.edu/outreach/press/pr03/sb0307.html
If UMa 83 had been inside a globular cluster (which it couldn't be: it is too young and metal-rich for that), it would have been called an asymptotic giant branch star, one that has "risen" towards the upper right part of the Hertzsprung-Russel diagram for the second time. Since UMa 83 certainly isn't a very metal-poor star, it won't have "moved very far to the left" along its own horizontal branch. It must have moved a little bit to the left, however, along its very short horizontal branch. Now it has exhausted its core helium, it is rising again, and, well, the end is near!

Ann

Re: SKYLIGHTS: Star of the Week

Posted: Sun Jul 27, 2014 3:41 pm
by MargaritaMc
The Star of the Week for week beginning Friday, 25th July, 2014, from Jim Kaler's Skylights
17 COM (17 Comae Berenices).
excerpts
... 17 Com A ... and ... 17 Com B are usually considered to be an easy "binocular"double. Merged together, they shine at magnitude 5.01. ... Both are given as class A (A0p) "peculiar" dwarfs, though 17 Com B is also listed as A2 to F2. ...

The Hipparcos parallaxes, however, call out a rather resounding "no," as the distance of 17-A is 238 light years, while that of 17-B is 223 light years. ... making it highly questionable that they constitute a binary system. The common motion is more likely to be the result of their both being members of the Coma Berenices open cluster(which is the major part of the larger modern constellation), whose stars are moving through space together while slowly orbiting some vague common center of mass and also interacting with one another. With a distance to the cluster of around 280-300 light years, 17 A and B are on the near side of it. We'll therefore consider them as separate stars that share a common name. ...
Read more at:
http://stars.astro.illinois.edu/sow/17com.html