Starship Asterisk*

Ann wrote: ↑Thu Jan 31, 2019 7:59 am
Regulus is pretty darn oblate, too. Even though it is out-ovum-ed by Archernar.

• Regulus is an oblate spheroid:

neufer wrote: ↑Wed Jan 30, 2019 4:36 pm

https://en.wikipedia.org/wiki/Regulus wrote:
<<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:

neufer wrote: ↑Thu Jan 31, 2019 1:10 am

https://en.wikipedia.org/wiki/Spica wrote:
<<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.>>

https://en.wikipedia.org/wiki/Cassini_oval wrote:

---

<<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

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.

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.

Nitpicker wrote: ↑Wed Jan 30, 2019 8:01 pm
I was think Spica could easily be confused with Regulus.

https://en.wikipedia.org/wiki/Spica wrote:
<<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.>>

Nitpicker wrote: ↑Wed Jan 30, 2019 8:01 pm I was think Spica could easily be confused with Regulus.

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

https://en.wikipedia.org/wiki/Regulus wrote:
<<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.>>

BDanielMayfield wrote: ↑Wed Jan 30, 2019 3:03 pm

Nitpicker wrote: ↑Wed Jan 30, 2019 11:09 am 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).

Nitpicker wrote: ↑Wed Jan 30, 2019 11:09 am 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.

Ann wrote: ↑Wed Jan 30, 2019 5:40 am Great image.

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

Ann