SKYLIGHTS: Star of the Week

[MargaritaMc]

Star of the Week for week beginning Friday, August 1, 2014
from SKYLIGHTS

44 BOO (44 Bootis). Just when you think that star systems can get no odder, along comes something like 44 Bootis, ... Nothing about the star is very secure. At first look, a magnitude 5.2 solar type class G2 dwarf (but given as warm as F7) is in mutual orbit with a magnitude 6.10 K2 (G? K4?) dwarf with a period of 209.8 years ...
With a temperature of 5670 Kelvin, 44 Boo A shines with the total light of just 1.13 times that of the Sun, which leads to a radius of 1.1 solar radii, all consistent with a solar mass star of a bit more age than our own. 44 Boo B should then have a mass roughly equal to that of 44 A. But here comes the surprise, as 44 B is not only itself double, but is an eclipsing binary with an orbital period of a mere 6.43 hours, the little system varying continuously between magnitudes 5.8 and 6.4. The two stars, seemingly each lesser than the Sun, must be so close as to be in contact with each other, or at least nearly so, the little system shaped something like a peanut, which explains the continuous variation as the two whirl around presenting different faces to us. Mass is probably being transferred from one star to the other, indeed one star may be consuming the other. ...
Read on at
http://stars.astro.illinois.edu/sow/44boo.html

[MargaritaMc]

Skylights Star of the Week for week beginning Friday, August 8, 2014 http://stars.astro.illinois.edu/skylights.html

STAR OF THE WEEK: OMI AQL (Omicron Aquilae). Within the Milky Way, fifth magnitude (5.11) Omicron Aquilae sits in Aquila 1.5 degrees almost due north of first magnitude Altair, making it quite easy to find. A class F (F8) dwarf, Omi Aql is almost sunlike, though with some notable exceptions, one of major importance.
...
By far the most absorbing feature of Omicron Aquilae is that in 1979-80 it burst forth with two superflares in which it brightened by 0.09 magnitudes, nearly nine percent! The first flare could have lasted up to five days, while the second stayed up for nearly two weeks. Red dwarfs are supposed to behave this way, not those more or less like the Sun. Omi Aql thus joins an exclusive club that includes 5 Serpentis and Groombridge 1830 (HR 4550). Reasoning from ordinary solar and red dwarf flares, imagine what the X-ray intensity must have been. Imagine too what it would be like if the Sun were to join the group. Why such stars go off this way is a mystery, as is how frequently such flares go off. Without a planet, or so it appears anyway, there is nobody there to report back, which is probably a good thing, as it sure would be a dangerous place to be. 

[Ann]

Thanks again, Margarita, and how fascinating. I will allow myself two bits of speculation.

http://stars.astro.illinois.edu/sow/omiaql.html wrote:

Omi B's absolute brightness is consistent with it being classified as an M3 red dwarf, so the two stars are almost certainly partners.
...
The first flare could have lasted up to five days, while the second stayed up for nearly two weeks. Red dwarfs are supposed to behave this way, not those more or less like the Sun.

Well, Omicron B is a red dwarf. Is it absolutely certain that this red dwarf isn't responsible for the enormous flares?

All right. I guess not. However:

From these considerations, we get a total luminosity of 2.73 times that of the Sun and a radius of 1.47 times solar. Oddly, there is no measure of rotation speed.(My emphasis.)

My complete amateur guess is that Omicron Aquilae A is a really fast spinner. A dizzy rotation speed might tangle up the magnetic field lines around the star and lead to enormous flares when the field lines "untangle".

Ann

[MargaritaMc]

Star for the TWO WEEKS starting Friday, August 15, 2014 from Skylights by Professor Jim Kaler.
http://stars.astro.illinois.edu/skylights.html

HD 140283 (HD 140283 Librae)

And now (once again) for something completely different. ...

The very FIRST stars ... those formed right after the Big Bang, should have had no metals at all. We can't find them. Presumably, the first stars were all massive and therefore exploded, so there are none left to see. 

How far back in metal content, and thus toward the Galaxy's birth, indeed toward that of the Universe, can we go? Not visible to the naked eye, but a clear binocular object, seventh magnitude (7.21) HD 140283 (in the Henry Draper spectral catalogue) is certainly a stop along the way.
... From its temperature, luminosity (3.8 times that of the Sun), chemical composition, and theory, HD 140283 seems almost as old as the Universe itself, which dates at 13.8 billion years. Whatever the final word, the star is certainly among the oldest there are. While close to us now, HD 140283 is a visitor from the ancient Galactic halo, and is just passing through the orderly disk at a speed of 361 kilometers per second relative to the Sun, ... Whatever the details, HD 140283 is among the oldest things you can see other than the Universe itself that surrounds you.

The next star of the week will be on 29th August.

Margarita

[BDanielMayfield]

Star for the TWO WEEKS starting Friday, August 15, 2014 from Skylights by Professor Jim Kaler.
http://stars.astro.illinois.edu/skylights.html

HD 140283 (HD 140283 Librae)

And now (once again) for something completely different. ...

The very FIRST stars ... those formed right after the Big Bang, should have had no metals at all. We can't find them. Presumably, the first stars were all massive and therefore exploded, so there are none left to see. 

How far back in metal content, and thus toward the Galaxy's birth, indeed toward that of the Universe, can we go? Not visible to the naked eye, but a clear binocular object, seventh magnitude (7.21) HD 140283 (in the Henry Draper spectral catalogue) is certainly a stop along the way.
... From its temperature, luminosity (3.8 times that of the Sun), chemical composition, and theory, HD 140283 seems almost as old as the Universe itself, which dates at 13.8 billion years. Whatever the final word, the star is certainly among the oldest there are. While close to us now, HD 140283 is a visitor from the ancient Galactic halo, and is just passing through the orderly disk at a speed of 361 kilometers per second relative to the Sun, ... Whatever the details, HD 140283 is among the oldest things you can see other than the Universe itself that surrounds you.

Margarita

Margarita and Ann, this star HD 140283 is an old, and I do mean old, really, really old, friend of ours.

http://asterisk.apod.com/viewtopic.php?f=8&t=30787

[Ann]

Yes indeed, HD 140382 is old, Bruce!

Ann

[MargaritaMc]

Well spotted, Bruce! What a good memory you have. I had the feeling that there was something familiar nudging at the back of my mind, but I didn't find what it was. Thanks very much for making the link.

By the way, Ann, the hyperlink you made in your post above returned a 403 notice and wouldn't show me anything.
M

[Ann]

Margarita wrote:
By the way, Ann, the hyperlink you made in your post above returned a 403 notice and wouldn't show me anything.

It was an "old" smiley, Margarita! Nothing much to look at. I forgot... Geck has already provided us with a smiley like that.

Ann

[MargaritaMc]

Ah! Thanks for the ancient smiley Ann.
This star is a "sub dwarf". In my totally not humble opinion I think the various usages of the term "dwarf" in astronomy is unnecessarily confusing. Here is an outline of them from
Dr Kaler's Spectra website

The term "dwarf" is used in stellar astronomy in a variety of ways. Main sequence stars are commonly called "dwarfs." "White dwarfs," on the other hand, are a sequence of dead stars that have lost their outer envelopes and consist of little more than spent, old nuclear-fusing cores. There is also a set of stars that are similar to ordinary dwarfs except that compared to ordinary dwarfs they are somewhat too dim for their temperatures (or too hot for their luminosities, depending on how you look at them). They are therefore called "subdwarfs." On the HR diagram they run just to the left of the main sequence from about class G on down toward cooler stars. Like ordinary main sequence dwarfs, subdwarfs run off the energy generated by nuclear fusion, specifically the fusion of hydrogen into helium. Their distinctiveness is caused by a severe underabundance of metal atoms. A lower metal abundance makes the gases more transparent, which changes the stars' structures and the quality of the radiation they emit. Typically, subdwarfs contain only about a hundredth the iron of the Sun (relative to hydrogen), but at their most extreme the iron abundance (along with the abundances of other heavy elements) drops to only a ten-thousandth solar. Subdwarfs evolve into giants and white dwarfs just as do ordinary dwarfs.

But in a discipline which has magnitude going backwards and the x-axis of H-R diagram going in reverse to the way that graphs are usually orientated, I just have to accept it as one of the delightful eccentricities attached to a very, very old science!

A common "funny" that I've heard is that Sirius A is a dwarf which is white and Sirius B is a whit dwarf - and they are not the same thing!
M

[Ann]

Jim Kaler wrote:

There is also a set of stars that are similar to ordinary dwarfs except that compared to ordinary dwarfs they are somewhat too dim for their temperatures (or too hot for their luminosities, depending on how you look at them). They are therefore called "subdwarfs." On the HR diagram they run just to the left of the main sequence from about class G on down toward cooler stars.

All right, but there are B-type subdwarfs too. I came across a few of them as I was looking up at least a thousand stars with blue colors and checking their apparent luminosities and Hipparcos parallaxes. There were a few incredibly faint blue stars there, among them HD 149382. This is a star whose spectral type is B5 and its B-V index is strikingly blue, far bluer than we would expect from a B5 type star, -0.28. But the V luminosity of this star is only 1.2 times the Sun! Clearly this is a very tiny star, perhaps in the process of becoming a white dwarf. (Or not.)

To be absolutely honest, I did not come across this particular star in my checking up of blue stars, because most of the stars I checked up had an apparent magnitude no fainter than eight. HD 149382 is a ninth magnitude star. But I did come across apparently unreddened early B-type stars (spectral classes B1 and B2) with reliable parallaxes that were only about thirty times as bright as the Sun in V light. As such, they were much fainter than normal early B-type stars.

In any case, the spectral class of subdwarf HD 140283 is not G or cooler. HD 140283 is an F-type star. So subdwarfs can apparently be hotter than Jim Kaler thought. (Or maybe Professor Kaler just thought that there are so few subdwarfs hotter than spectral class G that they are hardly worth mentioning. Or else, who knows, maybe the hotter subdwarfs are different from the cooler ones in some really fundamental way.)

Ann

[MargaritaMc]

Star of the week for week beginning Friday, August 29, 2014

From http://stars.astro.illinois.edu/skylights.html

STAR OF THE WEEK: NU HER (Nu Herculis)

The standard wisdom is that in his Uranometria of 1603, Bayer gave out Greek letters in order of decreasing brightness. However, he also used position, Hercules providing one of many examples. ... Nu Her is at the northwest corner of a small box formed also by Xi (southwest corner), Omicron (southeast), and 99 Her(northeast)and that contains the "Apex of the Sun's Way," the point near the Hercules-Lyra border toward which the Sun appears to be moving at some 20 kilometers per second amongst the brighter local stars (the exact position depending on your definition of "local"). ...

Aside from location, Nu Her quite nicely stands on its own as a rare class F (F2) yellow "bright giant," ...As it evolves, it will for a time turn into a pulsating Cepheid variable, such stars beloved of astronomers since they give a way (through the period-luminosity relation) of finding distance. ... Not massive enough to blow up, Nu Her will eventually lose its outer layers and die as a white dwarf of about 0.85 solar masses. 

Read more at: http://stars.astro.illinois.edu/sow/nuher.html

Margarita

[MargaritaMc]

Star of the Week for the week starting Friday, September 5, 2014.

STAR OF THE WEEK: EPS GRU (Epsilon Gruis), the last of the stars of third magnitude and brighter to be covered by the Star of the Week and included in the STARS website! ... Lying south-southeast of the main constellation (which really does look like a giant stalking bird) where it is barely noticed, Epsilon Gru is a white class A (A3) dwarf with a temperature of about 8600 Kelvin (the average of a fairly sizable spread). From temperature (needed to account for a bit of ultraviolet radiation) and a distance of 129 light years (give or take 2.5), we find a luminosity of 50.6 Suns and a radius of 3.2 times solar. The theory of stellar structure and evolution then yields a mass 2.4 times that of the Sun. The age is uncertain, but it seems to be more than halfway through its hydrogen-fusing lifetime of 600 million years ... Epsilon Gru's singular feature seems to be its high equatorial rotation speed, the observed values averaging 247 kilometers per second, which gives the star a rotation period of under 0.65 days. It's high enough to stir the surface gases so as to prevent separation of the elements, some falling under the effect of gravity, others lofted upward by radiation. The high speed will also make the star somewhat oblate, which then gives problems with temperature determination (the poles hotter, the equator cooler). Since we do not know the axial tilt, the speed may be even higher. ...
read more at http://stars.astro.illinois.edu/sow/epsgru.html

Margarita

[MargaritaMc]

Jim Kaler's Star of the Week for week beginning Friday, September 12, 2014 is

OMI HER (Omicron Herculis)
... Omicron forms a rough box that contains the Apex of the Sun's Way, the point among the local bright stars towards which the Sun is moving at a stately pace of 20 kilometers per second. 
... the distance of 338 light years (give or take 6) for this class B (B9.5, right on the edge of class A) dwarf, or in some circles, giant (the difference not great). The measured temperatures range from 8600 to 10,590 Kelvin, not so good. From the average of 9660, we can calculate the amount of ultraviolet radiation, which with the rest of the electromagnetic spectrum yields a total luminosity of 296 times that of the Sun and a radius of 6.2 times solar. The star stands out mostly for its equatorial rotation speed of at least 171 kilometers per second, which gives a rotation period of under 1.8 days (as opposed to the 25-day rotation period of the Sun). The high rotation stirs up the stellar gases, preventing any sort of diffusion of the elements that leads to odd chemical abundances at the stellar surface. It also makes the star somewhat oblate, which could account for the range in measured temperatures, as oblate stars are hotter at the poles and cooler at their equators (a phenomenon oddly called "gravity darkening"). Going along with the rapid rotation is a circumstellar disk that radiates emission lines, making Omi Her into a "Be star" whose icons are Gamma Cassiopeiae, Zeta Tauri, and Delta Scorpii. ...

I found it interesting to read the links to the icon "Be stars", especially Delta Scorpii, or Dschubba, which I saw (knowingly) for the first time only a few nights ago, in a wonderful line-up of Antares, Dschubba, Mars, and Saturn.

Margarita

[MargaritaMc]

There is a typo in the above quote! The distance should read 338 light years.
M

[MargaritaMc]

Star of the week for week beginning Friday 19 September 2014

STAR OF THE WEEK: OMI SCO (Omicron Scorpii)
Fifth magnitude Omicron Scorpii (at 4.55 just over the border from fourth), in northern Scorpius, is just barely the westernmost star of the "Scorpion's Pentagon" ... Omi Sco (the second faintest) is a white class A (A5) "bright giant,"...Though second-faintest of the five, Omi Sco stands out as both the most distant, 880 light years (give or take 130), and thus not surprisingly as the most reddened and obscured by interstellar dust. Interstellar dimming becomes stronger with shorter wavelengths, which makes stars look "redder" than they actually are. Interstellar reddening (which can be measured by comparing the observed technically-defined color with that expected from the spectral class) is closely tied to the visual obscuration, which can then be found. Omicron Sco is dimmed by 2.27 magnitudes. Were the line of sight clear, the star would shine at a prominent second magnitude (2.28) and might even have a proper name. 

... Now just over 28 million years old, Omi Sco was born as yet another class B2 dwarf, or at least close, fitting it right into the other stars of the Pentagon, except of course for more massive Antares, which was born as a class O star. Omicrom Sco may become unstable as it cools and spend some time as a Cepheid variable, and will briefly also grace the sky as a red supergiant, though one not as grand that which dominates the Pentagon right now. 


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

M

[MargaritaMc]

Excerpt from Star of the Week for week beginning Friday, September 26, 2014

STAR OF THE WEEK: 1 DEL (1 Delphini). One might think that Number 1 in Delphinus, the Dolphin, would be right at the forefront of study. Number 1 and all that. But no, it's merely the westernmost naked eye (or close) star within the swimming constellation, Flamsteed numbers always going from west to east within a figure's rather flexible eighteenth century boundaries. Yet the star has an importance belied by its dim sixth magnitude (6.08) status. 1 Del is a class A (A1) "emission-line shell star," one with a surrounding equatorial disk that radiates the hydrogen spectrum. Most such stars are in class B, and constitute the populous set of "Be" stars epitomized by Gamma Cassiopeiae and Zeta Tauri. "Ae" stars on the other hand are rare. In a sense, they are the spillover into class A from hotter class B. But the bizarre world of astronomical nomenclature strikes again. Do not confuse them with Herbig Ae/Be stars like AB Aurigae, the class named after George Herbig, (1920-1913). While they too have equatorial disks, they are young protostars caught in the act of formation.
... Read more at http://stars.astro.illinois.edu/sow/1del.html

[MargaritaMc]

Excerpt from Star of the Week for week starting Friday, October 3, 2014.

XI AQR (Xi Aquarii). Aquarius (the Water Bearer) sprawls all over the place south of the celestial equator to the west of the Vernal Equinox in Pisces next door. The only really identifiable structure is the famed "Water Jar," a "Y"-shaped asterism near the constellation's northern border. Coming off the Water Jar to the west and southwest are the "lucky stars," Sadalmelik and Sadalsuud (Alpha and Beta Aquarii), oddly both third magnitude class G supergiants. Winding around to the southeast of Sadalsuud, near the border with Capricornus we find lonely fifth magnitude (but at 4.69 almost fourth) Xi Aquarii, a class A (A7) dwarf that sadly, but not surprisingly, lacks a proper name.

Read on at http://stars.astro.illinois.edu/sow/xiaqr.html

[MargaritaMc]

This week's Star of the Week is Kappa Delphini. It is a triple star, with Kappa A and a possible close companion plus Kappa C being the genuine components. There is also Kappa B but it seems to be a line of sight coincidence. You can read more about it here: http://stars.astro.illinois.edu/sow/kappadel.html

There are no quotes this week as the article about the various components of Kappa Del has defeated my attempts at creating a précis!

Margarita

[MargaritaMc]

The star of the Week for week beginning 17 October, 2014 was a little easier to take an excerpt from than last week's SOW.

FF AQL (FF Aquilae). Aquila is known for many things, among them a beautiful part of the Milky Way and the first-known Cepheid variable, Eta Aquilae, discovered by Edward Pigott in 1786. If things were fair, Cepheids (class F and G supergiants that typically vary by a magnitude or so over periods of several days) should be called Aquilids instead of being named after the second to be found, Delta Cephei (by John Goodricke shortly thereafter). ...

While fourth magnitude Eta Aql, eight degrees south of Altair, dominates Aquila's Cepheid scene, it's not the only one visible to the naked eye. In the far northwestern corner of the constellation lies the fifth magnitude (averaging 5.38) class F (nominally F8) supergiant FF Aquilae, known best by its two-letter variable-star name. The star's a bit of a curiosity as it varies by only a few tenths of a magnitude (5.20 to 5.55, the class going from F5 to G0) over a period of 4.471 days (increasing at a rate of 0.07 seconds per year), the size of the variation notably less than usual. While most Cepheids have light curves (plots of magnitude vs. time) characterized by a rapid rise in brightness followed by a longer fall, FF's curve is more sinusoidal, making it into an "s-Cepheid" that does not fit the usual period-luminosity relation. ...

Read more at: http://stars.astro.illinois.edu/sow/ffaql.html

Margarita

[MargaritaMc]

OOOPS! The last star of the week was for TWO weeks, and I didn't notice!
M

[MargaritaMc]

Star of the Week for the week starting Friday, October 31, 2014.

This is SUCH an interesting star, that I'm not going to attempt any excerpts. Go see!

PI-1 GRU (Pi-1 Gruis, plus a bit on Pi-2).

Within the southern triangle of modern Grus (the Crane), the binoculard eye might be drawn to what appears to be a naked-eye double, the constellation's "two pieces of Pi."

Margarita

PS. The paper that first described Stellar Class S is available here

[MargaritaMc]

Star of the "Week" for the two weeks beginning Friday, November 7, 2014.

PI CAP (Pi Capricorni). Enter here with brave caution unless you like uncertainty and confusion. Fifth magnitude (5.25) Pi Capricorni, in western Capricornus (the unlikely Water Goat), a few degrees south-southeast of (and pointed to by) brighter Alpha and Beta Cap, might better be known as the "if" star, as so much has to be assumed: "if this, than that," etc. Hardly anybody has observed it, the star getting a miserable 42 citations over the past century and most of these useless.
...
Read on at: http://stars.astro.illinois.edu/sow/picap.html

Margarita

[MargaritaMc]

Jim Kaler writes:

The next Skylights will appear Friday, December 5.

Another fortnight (necessitated by a move, Thanksgiving -- wishes for a happy one to all -- and other things) pretty much spans the waxing half of the lunar phase cycle.

The Star of the "Week" for these two weeks is GORGONEA SECUNDA (Pi Persei)


M

[MargaritaMc]

Jim Kaler writes in Skylights

The next Skylights will appear Friday, December 19.

It's yet another two-weeker and it's jammed full of good stuff...

The star of the two weeks is KAPPA PEG (Kappa Pegasi).

... a nifty very close triple that requires interferometry to resolve and study...

M

[MargaritaMc]

Jim Kaler writes:

The next Skylights will appear Friday, January 2, 2015. Thanks for your patience. Best wishes for a merry and happy holiday season

STARS OF THE WEEK: CHI AND PSI PEG (Chi and Psi Pegasi),
another two-for-one special. And this one is a natural for its rather remarkable symmetries. As Bayer neared the end of the Greek alphabet in his star-naming scheme for Pegasus (the Flying Horse), he arrived at Chi and Psi Pegasi, which straddle the eastern side of the Great Square (Alpheratz to Algenib), Chi to the south and east, Psi to the north and west, the two separated by six degrees (Chi almost on the Pisces border and notably closer to the Square's outline). The stars also straddle the equinoctial colure, the great circle that connects the equinoxes and the celestial poles, with Psi now the closer one of the pair to the colure. What is most remarkable is that that the two fifth magnitude stars (respectively 4.80 and 4.66) are both rather rare but quite similar class M (M2 and M3) red giants that appear to be in the same state of evolution...

Colure was a new term for me - I learn so much from Dr Kaler's sites!

Margarita