Regulus
Published on Nov 28, 2006 at 9:42 pm.
2 Comments.
Filed under stars.
In the heart of the constellation Leo is the star Regulus. (as seen in my crude sketch above). The name Regulus goes back to Copernicus, according to Richard Allen’s classic book about star names.  This star had earlier been called simply Rex (Latin for king), and it was called Regia by Pliny. Even in Arabic, it was Malikiyy (kingly). Quite a noble heritage! The constellation containing Regulus has been seen as a lion for a long time. And sometimes Regulus was also called Cor Leonis (Latin for the Lion’s Heart), or in Arabic as Al Qalb Al Asad (also meaning the Lion’s Heart). The Bayer designation for Regulus is Alpha Leonis, and it has a Flamsteed number of 32 Leonis.
Shining with a magnitude of 1.36, Regulus is the 22nd brightest star in the sky (21st if you don’t count the Sun). Regulus is a main sequence star with a spectral type of B7. It is blue hot with a maximum surface temperature of nearly 2.66 times that of the Sun (more on that later). Regulus is also bigger than the Sun, and it has about 3.5 times the Sun’s mass. It also shines with about 350 times as much light as the Sun. At a distance of only 78 lightyears, Regulus is the closest B type main sequence star, but getting farther from us at a rate of about 6 km/s.
Regulus is part of a multi-star system.  A pair of smaller stars orbit it with an orbital period of about 130,000 years. They are currently about 4200 AU (390 billion miles) from Regulus itself. These two stars together only amount to less than 30% of the mass of Regulus itself. The larger of the two is an orange dwarf star (spectral type K1) having about 80% of the Sun’s mass, and the smaller one is a very dim red dwarf (spectral type M5) with about 20% of the Sun’s mass. These two stars are themselves about 91 AU apart, and orbit one another about every 1000 years.
OK, so there you have some basic information about Regulus. But, none of that is really remarkable. What is rather remarkable, though, is that Regulus is rotating very rapidly. Harold McAlister of George State University’s Center for High Angular Resolution (CHARA) has made measurements that indicate that Regulus rotates with a period of only about 15.9 hours. Compare that with the Sun. The Sun takes about 25 days to rotate! Regulus is really ripping along! 
In fact, it is rotating so rapidly that its equator is bulging outward, as seen in the accompanying comparison of Regulus and the Sun.  The equatorial diameter is nearly 1/3 larger than the diameter from pole to pole.  At its equator, the gasses of the star are moving at a staggering 317 kilometers per second (compared with about 2 kps for the Sun’s equator).  Regulus is rotating so rapidly that it is almost tearing itself apart. It would only have to rotate about 10 to 15% faster for the gasses at the equator to fly off into space.  Gravity already has a tough time holding on to those gasses, which is why it is so oblate in shape.    Â
The odd oblate shape to Regulus results in several interesting things. The equator, being much farther from the core than the poles, is cooler. The temperature at the equator is only 10,300 K, rather than the 15,400 K of the poles. That means, while the poles are 2.66 times hotter than the Sun, the equator is only about 1.78 times hotter. The hotter that an object is, the greater its luminosity. That means that the poles of Regulus are 5 times brighter than its equator.Â
Quite an odd star, indeed.
-Astroprof
(Comparison image credit:Â H. McAlister, GSU)Â
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