APOD: Wide Field View of Great American... (2019 Jan 30)

[APOD Robot]

Wide Field View of Great American Eclipse

Explanation: Only in the fleeting darkness of a total solar eclipse is the light of the solar corona easily visible. Normally overwhelmed by the bright solar disk, the expansive corona, the sun's outer atmosphere, is an alluring sight. But the subtle details and extreme ranges in the corona's brightness, although discernible to the eye, are notoriously difficult to photograph. Pictured here, however, using over 120 images and meticulous digital processing, is a detailed wide-angle image of the Sun's corona taken during the Great American Eclipse in 2017 August. Clearly visible are intricate layers and glowing caustics of an ever changing mixture of hot gas and magnetic fields. Hundreds of stars as faint as 11th magnitude are visible behind the Moon and Sun, with Mars appearing in red on the far right. The next total eclipse of the Sun will occur on July 2 and be visible during sunset from a thin swath across Chile and Argentina.

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[Ann]

Great image.

There is a conspicuously blue star to the upper left of the Sun. Could that by any chance be Regulus?

Ann

[De58te]

Great image.

There is a conspicuously blue star to the upper left of the Sun. Could that by any chance be Regulus?

Ann

The chance that it is Regulus is most likely as the Stellarium program puts the Sun's position on August 21 right at the foreleg and chest of the constellation Leo.

[Nitpicker]

Yes it is Regulus. The author's page confirms it. Very impressed if you picked it from the APOD alone. Freakishly atune to the colour blue.

Amazing APOD.

[orin stepanek]

Seems like yesterday!

[BDanielMayfield]

Yes it is Regulus. The author's page confirms it. Very impressed if you picked it from the APOD alone. Freakishly[, expertly] atune to the colour blue.

Amazing APOD.

Yes, that's our Ann, (expertly insertion to soften the freak).

[Ann]

Yes it is Regulus. The author's page confirms it. Very impressed if you picked it from the APOD alone. Freakishly[, expertly] atune to the colour blue.

Amazing APOD.

Yes, that's our Ann, (expertly insertion to soften the freak).

Thanks for the compliment, Nit and Bruce, I really appreciate it!

Click to view full size image

Mars, Regulus and Comet Ison. Photo: Chris Schur.

This is how I identified Regulus. Regulus is bright star (mag +1.41) and in good photographs, it looks conspicuously blue. Also Regulus is located very close to the ecliptic, meaning that it will frequently pair up with objects in the Solar system. In the picture at left, it forms a striking conjunction with Mars. And because of its brightness and proximity to the ecliptic, why wouldn't it form a striking pair with the Sun during an eclipse?

I can think of no other blue star that meets all these criteria.

Ann

[neufer]

• https://apod.nasa.gov/apod/ap131017.html

<<Regulus is Latin for 'prince' or 'little king'. It is also known as Qalb al-Asad, from the Arabic قلب الأسد, meaning 'the heart of the lion', a name already attested in the Greek Kardia Leontos whose Latin equivalent is Cor Leōnis.

The spectroscopic binary Regulus A consists of a blue-white main-sequence star of spectral type B7V and its companion, which has not yet been directly observed, but is probably a white dwarf of at least 0.3 solar masses. The two stars take approximately 40 days to complete an orbit around their common centre of mass. Given the extremely distorted shape of the primary, the relative orbital motion may be notably altered with respect to the two-body purely Keplerian scenario because of non-negligible long-term orbital perturbations affecting, for example, its orbital period. In other words, Kepler's third law, which holds exactly only for two point-like masses, would no longer be valid for the Regulus system. Regulus A was long thought to be fairly young, only 50 – 100 million years old, calculated by comparing its temperature, luminosity, and mass. The existence of a white dwarf companion would mean that the system is at least a 1 billion years old, just to account for the formation of the white dwarf. The discrepancy can be accounted for by a history of mass transfer onto a once-smaller Regulus A.

The primary of Regulus A has about 3.5 times the Sun’s mass. It is spinning extremely rapidly, with a rotation period of only 15.9 hours, which causes it to have a highly oblate shape. This results in so-called gravity darkening: the photosphere at Regulus' poles is considerably hotter, and five times brighter per unit surface area, than its equatorial region. The star's surface at the equator rotates at about 320 kilometres per second, or 96.5% of its critical angular velocity for break-up. It is emitting polarized light because of this.>>

[drjhammond]

This is an amazing image of the solar eclipse. Interesting that I have not seen it earlier. This image shows the part of the corona which is also seen as the zodiacal light. The bright bands streaming to the left "below" Regulus and to the right "below" Mars are components of this part of the corona also called the F corona. "The F-corona, so-called because it shows the same dark absorption lines that Fraunhofer found in the on-disk spectrum..., is actually unrelated to the corona per se. It arises out of scattering of photospheric light by small dust particles in the ecliptic plane, and it may more accurately be thought of as the inner zodiacal light. thus, the F-corona is again not true coronal light but a reflection (literally) of the bright photospheric light." From "The Solar Corona , Second Edition", Leon Golub and Jay Pasachoff, p 5. The word "again" in this quote refers to the previous discussion of the K-corona which is light scattered by electrons of the coronal gas. The K-corona is the dominant part of the corona that we see during a total solar eclipse but is not light emitted by the sun's corona (the E-corona) which is fainter and dominated by shorter wavelengths in narrow emission lines. The Golub and Pasachoff book was the first place I read about the coronal light being photospheric light scattered by the electrons in the coronal plasma.

[Nitpicker]

I was think Spica could easily be confused with Regulus.

[Ann]

I was think Spica could easily be confused with Regulus.

Click to view full size image

Spica, Mars and Saturn. Photo: Luis Argerich.

You're right, Nit. It might easily have been Spica.

So in the end, I was just being "northern hemisphere-biased". I see more of Regulus than of Spica from my vantage point on the Earth, so when I see a photo of a blue star in conjunction with a planet, my first guess is that the star is Regulus.

But I also "like" Regulus because it is the Alpha star of a strikingly elegant constellation, Leo, whereas Spica's Virgo looks like "nothing" to me.

Oh, one more thing. The great American eclipse of 2017 happened in late summer on the northern hemisphere. Doesn't that make it more likely that the Sun would be close to Regulus than that it would be close to Spica?

Ann

[neufer]

I was think Spica could easily be confused with Regulus.

<<Spica, also designated α Virginis, is a spectroscopic binary star and rotating ellipsoidal variable; a system whose two stars are so close together they are egg-shaped rather than spherical, and can only be separated by their spectra. The primary is classified as a Beta Cephei variable star that varies in brightness over a 0.1738-day period. The spectrum shows a radial velocity variation with the same period, indicating that the surface of the star is regularly pulsating outward and then contracting. This star is rotating rapidly, with a rotational velocity of 199 km/s along the equator. The primary is one of the nearest stars to the Sun that has enough mass to end its life in a Type II supernova explosion.

Spica's components orbit each other every four days. Initially, the orbital parameters for this system were inferred using spectroscopic measurements. Between 1966 and 1970, the Narrabri Stellar Intensity Interferometer was used to observe the pair and to directly measure the orbital characteristics and the angular diameter of the primary, which was found to be (0.90 ± 0.04) × 10⁻³ arcseconds, and the angular size of the semi-major axis of the orbit was found to be only slightly larger at (1.54 ± 0.05) × 10⁻³ arcseconds.

Spica's apparent magnitude varies by 0.03 over an interval that matches the orbital period. This slight dip in magnitude is barely noticeable visually. Both stars rotate faster than their mutual orbital period. This lack of synchronization and the high ellipticity of their orbit may indicate that this is a young star system. Over time, the mutual tidal interaction of the pair may lead to rotational synchronization and orbit circularization.

The primary star is midway between a subgiant and a giant star, and it is no longer a main-sequence star. The evolutionary stage has been calculated to be near or slightly past the end of the main sequence phase. This is a massive star with more than 10 times the mass of the Sun and seven times the Sun's radius. The bolometric luminosity of the primary is about 20,500 times that of the Sun, and nine times the luminosity of its companion.

The secondary member of this system is one of the few stars whose spectrum is affected by the Struve–Sahade effect. This is an anomalous change in the strength of the spectral lines over the course of an orbit, where the lines become weaker as the star is moving away from the observer. It may be caused by a strong stellar wind from the primary scattering the light from secondary when it is receding. This star is smaller than the primary, with about 7 times the mass of the Sun and 3.6 times the Sun's radius. Its stellar classification is B2 V, making this a main-sequence star.>>

[Ann]

Click to view full size image

Regulus is pretty darn oblate, too. Even though it is out-ovum-ed by Archernar.

Wikipedia wrote:

The primary of Regulus A has about 3.5 times the Sun’s mass. It is spinning extremely rapidly, with a rotation period of only 15.9 hours, which causes it to have a highly oblate shape.[26] This results in so-called gravity darkening: the photosphere at Regulus' poles is considerably hotter, and five times brighter per unit surface area, than its equatorial region.[13] The star's surface at the equator rotates at about 320 kilometres per second (199 miles per second), or 96.5% of its critical angular velocity for break-up. It is emitting polarized light because of this.

Click to play embedded YouTube video.

You may or may not enjoy this video. The person who made it thought that Regulus is yellow-white.

Ann

[Ann]

Click to view full size image

The star Kepler 11145123 is the roundest natural object
ever measured in the universe. Illustration: Mark A. Garlick.

And now that I'm spinning wildly off topic...

Astro Bob wrote:

Prof. Laurent Gizon of the Max Planck Institute in Germany and his colleagues selected a slowly rotating star named Kepler 11145123. This hot and luminous star is more than twice the size of the Sun and rotates three times more slowly than the Sun. Naturally, the slower a star rotates, the less flattened it becomes. Some stars like Altair in Aquila the Eagle (in the Summer Triangle) rotate so quickly — 10 hours in this case — they look more like footballs than globes.

...

NASA’s Kepler mission kept tabs on the star for more than four years. Its instruments were so sensitive they could detect different pulsations depending on the latitude (distance north or south of the equator) they examined. Think of a plucked string, which can vibrate at many different frequencies or modes. The technique of determining what goes on beneath the surface of stars using their pulsations is called asteroseismology.

Comparing the frequencies of low-latitude regions vs. those of high latitudes, they were able to show that the difference in the star’s radius — the distance from center to edge — is only 1.86 miles (3 km) with a precision of 0.6 miles (1 km). Wow, now that’s a sphere! Well, almost. Euclid would still furrow his brow at this sliver of imperfection, I suppose.

Ann

[neufer]

Regulus is pretty darn oblate, too. Even though it is out-ovum-ed by Archernar.

• Regulus is an oblate spheroid:

<<The primary of Regulus A ... is spinning extremely rapidly, with a rotation period of only 15.9 hours, which causes it to have a highly oblate shape. This results in so-called gravity darkening: the photosphere at Regulus' poles is considerably hotter, and five times brighter per unit surface area, than its equatorial region. The star's surface at the equator rotates at about 320 kilometres per second, or 96.5% of its critical angular velocity for break-up. It is emitting polarized light because of this.>>

• Spica is egg shaped like a Cassini oval:

<<Spica, also designated α Virginis, is a spectroscopic binary star and rotating ellipsoidal variable; a system whose two stars are so close together they are egg-shaped rather than spherical.>>

---

<<A Cassini oval is defined as the set (or locus) of points in the plane such that the product [not the sum] of the distances to two fixed points is constant. This may be contrasted with an ellipse, for which the sum of the distances is constant, rather than the product. Cassini ovals are the special case of polynomial lemniscates when the polynomial used has degree 2. Cassini ovals are named after the astronomer Giovanni Domenico Cassini who studied them in 1680. Cassini believed that the Sun traveled around the Earth on one of these ovals, with the Earth at one focus of the oval some seven decades after Kepler's Copernican ellipses

No hen ever laid an oblate spheroid egg.