Starship Asterisk*

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

STAR OF THE WEEK: U 0RI (U Orionis) If you are looking for answers, "abandon hope, all ye who enter here." Perhaps it's not that bad. U Ori is a long period (or Mira Type) deep class M (M6.5) pulsating red giant variable (LPV). Don't confuse it with Upsilon Ori; "U" is a Roman letter variable star designation. Most of the time U Ori is out of sight as it reaches down to thirteenth magnitude (typical visual minimum is 12.6 but it can sink to 13.3) and a spectral class of M9.5, but for a brief time during its 372.4 day period, it can reach fifth magnitude (5.3, and at times as bright as 4.8). The star is way way north in Orion, in the Hunter's upraised arm and club, 12.8 degrees north of Betelgeuse, 0.3 degrees east and a bit south of Chi-1 Ori, and more in the territory of Taurus than Orion. Only three degrees south of the ecliptic, it's regularly occulted by the Moon.

Few of the star's properties seem definitive, including the amount of interstellar absorption. It's usually derived from how red the star appears relative to that found from its class, but U Ori appears too BLUE, perhaps from its surroundings shells.
... read more at the website

Jim Kaler: The sky this week begins:
The Moon starts off our week as a fat waxing crescent as it heads toward its first quarter on the morning of Monday, April 7th, after moonset. The night of Sunday the 6th it will be seen just shy of the phase, the following night just past it. It then brightens in the waxing gibbous phase, full Moon not reached until Tuesday the 15th, when there will be a total eclipse visible throughout North America. ...

This week's Star of the Week: Theta tucanae
Tucked (had to be said) into deep southern Tucana only 19 degrees from the South Celestial Pole, Theta Tuc is the most southerly star in the constellation with a Greek letter. It's hard to know what's more remarkable, Theta Tucanae's setting or its character. Dim to the naked eye (if visible at all), only sixth magnitude (6.13), it has a prominent place just to the west of the Small Magellanic Cloud. Easily visible though 200,000 light years away, the SMC is the second brightest of our two prominent satellite galaxies, the other (surprise) the Large Magellanic Cloud, which is somewhat closer (160,000 light years). Equally good, Theta Tuc is but a degree northeast of 47 Tucanae, one of the greatest globular clusters in our Galaxy. ...
read on at: Jim Kaler - SKYLIGHTS

Jim Kaler wrote:
The star is a whopper, a luminous class B0 dwarf (right at the edge of class O) at a most likely distance of 2960 light years. It's so far away, however, that parallax becomes unreliable, the formal uncertainties placing it as far as 3670 light years or as close as 2350. Upsilon may be part of the Orion OB1c association of hot massive stars, those in the region of Orion's Sword, but even at its statistically closest, it's notably farther than the Sword association's mean distance of 1650 light years. Given how far away it is, it's a bit surprising that Upsilon is dimmed just 0.13 magnitudes by interstellar dust. From its magnitude, the distance, and a temperature of 32,440 Kelvin needed to account for a lot of invisible ultraviolet radiation, we find a huge luminosity of 173,000 times that of the Sun, double that of Betelgeuse and Rigel, though not reaching the level of Theta-1 C. (At its likely closest, the luminosity drops to 116,000 Suns, still pretty impressive.) Temperature and luminosity then conspire to give a radius of 13.2 times solar. The projected equatorial rotation speed of just 15 kilometers per second is low, probably because the rotation pole is more or less directed at us, as for such stars we would expect more than ten times that speed. Theory tells of a huge mass of 30 Suns, with distance uncertainties allowing a range of 25 to 35. The great luminosity forces a wind that blows billions of times the strength of the solar wind. Now about halfway through its 5.5 million year dwarf lifetime, there is little question that Upsilon Orionis is going to blow up someday as a grand supernova, leaving behind a neutron star or even a black hole. Unfortunately, there seems to be no companion to interact with the proceedings and perhaps to tell us more.

Welcome and rules, Section 6 COPYRIGHT
If you want to reference material somewhere else on the web, give a brief summary and link to the rest. People can go take a look at what you're talking about and then return to discuss it further. Please provide links and/or proper citations for all quoted material.

the Wikipedia article on Citation, which I excerpt from here
Citation has several important purposes: to uphold intellectual honesty (or avoiding plagiarism),[1] to attribute prior or unoriginal work and ideas to the correct sources, to allow the reader to determine independently whether the referenced material supports the author's argument in the claimed way, and to help the reader gauge the strength and validity of the material the author has used.

Margarita wrote:
Here is the link for the above quotation from Professor Kaler about Upsilon Orionis:

http://stars.astro.illinois.edu/sow/upsori.html

5 UMI
...
At fourth magnitude (4.25), number 5 in the Flamsteed list of numbered stars in Ursa Minor was obviously too faint for Bayer to make note of via Greek letter (though he and others indeed lettered fainter ones). Five Ursae Minoris may also be among the more prominent stars that everybody with a dark sky sees but pays no attention to. It's supremely easy to find and, given its position of only 14 degrees from the North Celestial Pole, it's visible from nearly everywhere north of 14 degrees north latitude any time of night. The front bowl stars of the Big Dipper, Merak and Dubhe (Beta and Alpha UMa), are famed as the Pointers to Polaris, the North Star. Similarly, the front bowl stars of the Little Dipper, Pherkad and Kochab (Beta and Gamma UMi), point right to 5 UMi. Only it's a lot closer, just 2 degrees north-northeast of second magnitude Kochab. It's so close that it might make a good addition to the Little Dipper as a drop of water flying off its lip. ...

Excerpt from: CHI CNC (Chi Cancri)

Click to view full size image

Photo by Jim Kaler. A full resolution unlabelled version can be found here: http://stars.astro.illinois.edu/sow/cnc-jup.jpg

Though only fifth magnitude (5.14), Chi Cancri nevertheless is part of the outline (at least that used here; others differ) of Cancer, the Crab, whose fame comes not from its bright stars but from its position in the ancient Zodiac,
... a class F (F6) hydrogen-fusing dwarf, it is not all that dissimilar from the Sun. ...
Chi Cancri is just under the "rotation break" at class F5, hotter than which stars rotate much more rapidly as a result of a decline in their outer convective layers, which generate magnetic fields that, coupled with the stars' winds, slow them down. Chi Cnc rotates with a projected equatorial speed of just 5 kilometers per second, giving it a rotation period under 13 days. Yet the star seems to have no magnetic activity, which may have died away as a result of age. Bright dwarfs like Chi Cnc are prime targets for planet searches. Yet the star seems devoid. ...
Not only are there no planets (at least found), there are no companions either, the star quite alone in a lonely spot in the Zodiacal Crab. 

Written byJim Kaler 4/18/14.

Jim Kaler wrote:
Too far away for parallax, the star's distance is estimated from its Galactic location and other considerations to be nearly 12,000 light years. A temperature of around 4800 Kelvin adds a bit of infrared radiation to the total, which with distance gives a luminosity of 470,000 times that of the Sun and a radius 0f 980 solar or 4.6 Astronomical Units, nearly 90 percent the size of Jupiter's orbit. Interferometer measures suggest that HR 5171A may be even larger, 1315 times solar (6.1 AU, 12 percent BIGGER than Jupiter's orbit), which with distance and a temperature of 5000 Kelvin yields a luminosity of closer to a million Suns. Any way we look at it, the birth mass seems to be around 39 Suns, placing it into the realm of the most massive stars in the Galaxy. Given mass loss through strong winds, the mass must now be a lot less (stars always ending their lives with much less mass than they started with). A high abundance of sodium in the HR 5171A's atmosphere suggests that it is in the later stages of core fusion (beyond that of helium, the sodium cycled upward) and that the star was once a true red supergiant that has evolved beyond that state. Perhaps it is becoming a "Luminous Blue Variable" like P Cygni or Eta Carinae, or a Wolf-Rayet star (one with a stripped core and thick envelope from mass loss) like that in Gamma-2 Velorum.

Week beginning March 2, 2014
109 VIR (109 Virginis). Bright fourth magnitude (3.73), part of the outline of the constellation Virgo, we might still tend to roll over 109 Virginis (best known by its Flamsteed number) as just another white class A hydrogen-fusing dwarf (A0 at that, same as Vega), its distance 134.5 light years, accurate to a mere one light year. Just two degrees north of the celestial equator and the second-most-easterly numbered star in Virgo, the star is beaten (as expected) only by 110 Vir, a dimmer (magnitude 4.40) ordinary class K (K0.5) orange giant 195 light years away. We tend to disparage such stars as overly common, but without them many of the constellation figures would dissolve; cut out the class B dwarfs too and there would not be much left at all. 109 Vir is also notable as an old photometric standard, one used to calibrate the spectral brightness of other stars. But there is much more.

Read about this "more" at
http://stars.astro.illinois.edu/sow/109vir.html

Week beginning March 9, 2014

STAR OF THE WEEK:
N VEL (N Velorum). Bayer could not see most of the stars of Argo. The Greek letters were assigned in the 1700s by that intrepid explorer of the southern skies, Nicolas de Lacaille. Breaking giant Argo into three parts, he distributed Greek letters among them, but then assigned Roman letters, first lower case and then upper case, within each of them. It's a testimony to the plethora of bright stars in Vela (the Sails: the other parts Carina, the Keel, and Puppis, the Stern) that third magnitude (3.13) N Velorum was placed so far down on the list. Anywhere else it would probably be a notable part of the constellation outline.

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

Jim Kaler wrote:
a CAR (a Carinae). Yes, a third magnitude (ok just barely, 3.44, but see below) star without a Greek letter, as applied across the board in Argo (the Ship) by Nicolas de Lacaille, not Bayer. Lacaille continued in each of Argo's parts with lower case Roman letters, so "a" Car (in Carina, the Keel of Argo) just barely missed Greek-letter status, as it directly follows brighter Omega Carinae. It might be less confusing to use the name from the Bright Star Catalogue, HR 3659, but out of respect for tradition we'll continue with "a Car." As bright as it is, the star is not a part of the constellation's outline. But that's really no surprise as it's just 0.7 degree west-northwest of much brighter second magnitude Aspidiske (Iota Car), which places it just within the boundaries of the False Cross made of Iota and Epsilon Carinae and Delta and Kappa Velorum. The False Cross can fool the observer into thinking it's the Southern Cross, which lies well to the east. Yet as a blue-white class B (B2) subgiant-dwarf (classed sort of in the middle) 447 light years away (give or take 16), a Car can easily hold its own. The star is clearly double, with a short orbital period of 6.7447 days. But the binary's nature is ambiguous. At the extremes, the two may be equal, or one star may provide essentially all the light. In either case, the binary is an "ellipsoidal" variable that changes between magnitudes 3.20 and 3.56, rotation and tidal forces distorting the stellar shapes so that as the stars orbit they present different angular sizes to us. With a temperature of 21,180 Kelvin, and allowing for a slight dimming of 0.21 magnitudes by interstellar dust, it or they shine with the light of 5060 Suns, most of it in the invisible ultraviolet. If we are seeing just one star, the radius is 5.3 times solar, and the rotation period (from an equatorial speed of at least 35 kilometers per second) is under 7.6 days, which is consistent with the orbital period and tidal locking (the rotation and orbital periods the same). Theory then shows that the mass is 8.5 Suns, that the star is about midway through its dwarf lifetime of around 30 million years, and that it's near the cusp above which stars explode as supernovae. Even if it becomes a massive white dwarf, it might get enough mass flowing from the companion that it will still collapse and blow up, as from Kepler's Laws, the two are just 0.14 Astronomical Units apart. Assuming equality between the two gives a rotation period of under 5.4 days (less than the orbital period), each then with masses of 7.5 Suns. They would produce a double white dwarf that if merged through tides and prior evolution could still cause a blowup. And all this for a seemingly innocuous star that one hardly notices within the great star clouds of Carina.

Jim Kaler's Star of the Week, for the week beginning March 23, CU Virginis
How can we turn down a star with the name "CU" Virginis? Sadly, "CU" does not honor Champaign-Urbana, home of the University of Illinois (and of HAL in the seminal movie "2001: A Space Odyssey"), but is a variable star name, the fifth magnitude (nominally 5.01) star in eastern Virgo changing between 4.98 and 5.05 every 0.52 days. ... CU Virginis has been described as "one of the most enigmatic stars on the upper main (dwarf) sequence." A white, class A (A0p) hydrogen-fusing dwarf (the "p" standing for "peculiar") 258 light years away (give or take 5), CU Vir is a magnetic variable with greatly enhanced atmospheric silicon, and falls into the general class of "Alpha-2 Canum Venaticorum stars" (which occur among the F, A, and B dwarfs). They are similar to the "metallic-line stars," in which some elements are lofted to the surface by radiation, while others sink under the force of gravity. In stars like Alpha-2 CVn and CU Vir, there is also a strong magnetic field that concentrates enhanced elements into large, more or less polar starspots.

... The rest of the article, with useful hyperlinks, can be read at:
http://stars.astro.illinois.edu/sow/cuvir.html

Ann wrote:I'm sorry about being difficult here, but CU Virginis is not a white star, but a quite blue one.

Ann wrote:Oh, IQ Aurigae is bluer than CU Virginis. Both, however, are bluer than a lot of B-type stars. The bluer a star is, the more negative is its B-V index. The B-V index of IQ Aurigae is -0.167, which is very blue. CU Virginis is -0.118, which is really not shabby. Both these stars are classified as A0-type stars. Regulus, a B8 (or B7)-type star, is -0.087. Rigel, another B8-type star, though a supergiant one, is -0.030. Alkaid, a B3-type main sequence star, is -0.099.

If you take a look at that picture of IQ Aurigae that I sent a link to, you can see that IQ Aurigae is part of an asterism - a seeming cluster, which is however just a number of stars at different distances from us - you can see a star that is immediately to the right of IQ Aurigae, 17 Aurigae. This star is classified as a B-type star, although just barely - its spectral class is B9.5V. It does a negative B-V index, -0.043, but it is clearly less blue than A0pSi star IQ Aurigae.

Many B-type stars are less blue than both CU Viriginis and IQ Aurigae, the silicon-enhanced A-type stars.

Ann

DELTA PAV (Delta Pavonis). It's always fascinating to see among the stars one like the Sun, some kind of solar clone, and then wonder if there are any earths like ours. It's also fascinating to see one as bright as fourth magnitude (3.56, almost third), one prominent enough to be part of its constellation outline (Pavo, the Peacock), one deep in the southern hemisphere, the star but 24 degrees from the South Celestial Pole. Like the Sun it's a class G star, though cooler (G6-8, probably the latter), but unlike the Sun it's classed spectrally as a subgiant, which at first implies that it has just finished its career as a hydrogen-fusing dwarf or at least will soon. Of additional interest, it's not far from another star with solar similarities, Gamma Pavonis, an F6 dwarf. To be so visually bright for its class, the star has to be close to us, and at just 19.92 light years (with a probable error of only 0.02 light years) it most certainly is. Delta Pav's proximity also gives it a rapid motion across the sky relative to the background stars, a whopping (it's all relative) 1.66 seconds of arc per year, something that could be easily noted in a small telescope over a lifetime of observing.
Read more about the Star of the Week for the week beginning 30th March 2014 at:
http://stars.astro.illinois.edu/sow/deltapav.html

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

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.