Object: alpha Leonis (Regulus)

Constellation
RA / DEC
Magnitude A / B
Separation
Position angle
Spectral class
Colour A / B
: Leo
: 10:08:22 / +11.58
: 1.4 /8.2
: 176.9”
: 307
: B7V / K2V
: Bright white / Yellow

Details sketch
Date / Time
Observing Location
Seeing / Transparency
Telescope
Eye-piece
Magnification / Field of View '
: 30/03/09 / 22:30
: Landgraaf
: 3 / 3
: Orion Optics UK 300mm
: 22mm Nagler Type 4
: 72 / 68
stacks_image_351CA644-97AE-43FA-9F17-19858389EA45
Observing report
In the 12-inch Dobson, even at the lowest magnification alpha Leonis is easily split, and can only be described as stunningly beautiful. In the 35mm Panoptic the extremely bright Regulus (alpha Leonis A) dominates the field of view with its sparkling appearance. The four large diffraction spikes amplify this effect dramatically. To the northwest I can see the fainter companion of Regulus, alpha Leonis B. It has a soft yellow colour.

In the 22mm Nagler, alpha Leonis looks at its best. This eyepiece, yielding a magnification of 72x and a field of view of 68', is also used for sketching Regulus.

Alpha Leonis is a wide double, separated by a few arc minutes. There is a large difference in magnitude between Regulus and its companion, at least 5 or 6 magnitudes, maybe even more. I do not see any other remarkable stars in the field of view, just some faint field stars. I see no nebulosity or glow of unresolved stars. Besides the yellow alpha Leonis B, I do not see stars that show any hint of colour. I tried all possible magnifications, even stopped down the telescope to 5.5 inches (off axis). This way I tried to see if the colour of Regulus maybe would shift a little to a more bluish tint, but despite all my experimenting, Regulus kept its bright white colour.
Notes
Regulus is a star of spectral type B7 V (Johnson & Morgan 1953) or B8 IVn (Grey et al. 2003), and is located at a distance of 24.3pc. In the Washington Double Star Catalogue I found three companions for Regulus. First there is the B-component, a K2V star at almost 3 arc minutes from Regulus, at a position angle of 307°. This is the yellow star in my sketch, to the northwest of Regulus. Spectroscopic measurements show that proper motion, radial velocity and parallax are consistent with the theory that the B-component is a true physical companion, and not just an optical companion of Regulus by chance alignment.

The B component has a very close, faint companion of magnitude 13.1, the C component. Both stars are separated only by a few arc seconds. They make a true physical binary, and are often referred to as the BC subsystem. Then there is a D component about 200 arcseconds from Regulus, but this star has only been measured once, which makes it very uncertain as a true physical member of the alpha Leonis system.
Rapid rotator
Alpha Leonis A is a well-known rapid rotator. In 2004 a team of scientist studied Regulus using the CHARA Array (H.A. McAllister et al). Their study revealed (among many others interesting results) some amazing facts. Alpha Leonis A is an oblate star, extremely flattened at the poles. The equatorial radius is 32% larger than the polar radius. Compared with our Sun, Regulus polar measurement is 3.15 wider than our Sun, but in equatorial measurement it is 4.15 times wider. The temperature at the poles is 15,400 K, whereas the equatorial temperature is only 10,300 K. The rotation period of Regulus is 15.9 hours, which means the rotation speed at the equator is 86% of the critical break-up velocity. Compare this to our Sun, with a rotational period of approximately 25 days! The study also showed that Regulus is gravity darkened at the equator.

stacks_image_814BFB2B-475F-49F0-8BCC-2F56BA2EF54F

Computer model of Regulus, with the Sun shown to scale. Credit: Wenjin Huang, CHARA, Georgia State University

Gravity darkening (also referred to as Gravity Brightening) is an astronomical phenomenon where a star rotates so rapidly that it has a detectably oblate shape. When a star is oblate, it has a larger radius at its equator than it does at its poles. As a result, the poles have a higher surface gravity, and thus temperature and brightness. Thus, the poles are gravity brightened, and the equator gravity darkened.

The star becomes oblate (and hence gravity darkening occurs) because the centrifugal force resulting from rotation creates additional outward pressure on the star. This means that equatorial regions of a star will have a greater centrifugal force when compared to the pole. The centrifugal force pushes mass away from the axis of rotation, and results in less overall pressure on the gas in the equatorial regions of the star. This will cause the gas in this region to become less dense, and cooler.

A close spectroscopic binary
The rapid rotation of Regulus is a bit puzzling, considering its age. Stars that are born as fast rotators slow down relatively quickly after their birth. Only at the conclusion of the core H-burning they achieve rapid rotation again. However, Regulus is a star that is still in the middle of its H-burning stage. There could however be another source for this rapid spin, an interacting binary.
In 2008, D.R. Gies et Al. conducted a study that led to the discovery that Regulus is in fact a spectroscopic binary. The companion could be a white dwarf. That means Regulus may be like Sirius, a bright star orbited by a faint white dwarf. The likely white dwarf companion of Regulus has a mass of 0.3 Solar masses and an orbital period of 40.11 days.
Regulus has a mass of approximately 3.4 Solar masses.


RESOURCES:

First Results from the CHARA Array. I. An Interferometric and Spectroscopic Study of the Fast Rotator α Leonis (Regulus) H. A. McAlister et al. (2004)

The past and future history of Regulus S. Rappaport1, Ph. Podsiadlowski2, and I. Horev. Draft version August 22, 2009

A Spectroscopic Orbit for Regulus
D. R. Gies1,2, S. Dieterich1, N. D. Richardson1, A. R. Riedel1, B. L. Team1, H. A. McAlister1, W. G. Bagnuolo, Jr.1, E. D. Grundstrom1,2,3, S. SĖ‡tefl4, Th. Rivinius4, and D. Baade5 (2008)

The Brightest stars
Fred Schaaf (John Wiley & Sons, 2008)