Nature of The System
At −0.27v visual magnitude, Alpha Centauri appears to the naked eye as a single star and is fainter than Sirius and Canopus. The next brightest star in the night sky is Arcturus. When considered among the individual brightest stars in the sky (excluding the Sun), Alpha Centauri A is the fourth brightest at −0.01 magnitude, being only fractionally fainter than Arcturus at −0.04v magnitude. Alpha Centauri B at 1.33v magnitude is twenty-first in brightness.
Alpha Centauri A is the principal member, or primary, of the binary system, being slightly larger and more luminous than the Sun. It is a solar-like main sequence star with a similar yellowish color, whose stellar classification is spectral type G2 V. From the determined mutual orbital parameters, Alpha Centauri A is about 10% more massive than the Sun, with a radius about 23% larger. The projected rotational velocity ( v·sin i ) of this star is 2.7 ± 0.7 km·s−1, resulting in an estimated rotational period of 22 days, which gives it a slightly faster rotational period than the Sun's 25 days.
Alpha Centauri B is the companion star, or secondary, of the binary system, and is slightly smaller and less luminous than the Sun. This main sequence star is of spectral type K1 V, making it more an orange color than the primary star. Alpha Centauri B is about 90% the mass of the Sun and 14% smaller in radius. The projected rotational velocity ( v·sin i ) is 1.1 ± 0.8 km·s−1, resulting in an estimated rotational period of 41 days. (An earlier, 1995 estimate gave a similar rotation period of 36.8 days.) Although it has a lower luminosity than component A, star B emits more energy in the X-ray band. The light curve of B varies on a short time scale and there has been at least one observed flare.
Alpha Centauri C, also known as Proxima Centauri, is of spectral class M5 Ve or M5 VIe, suggesting this is either a small main sequence star (Type V) or sub-dwarf (VI) with emission lines, whose B–V color index is +1.90. Its mass is about 0.123 M☉, or 129 Jupiter masses.
Together, the bright visible components of the binary star system are called Alpha Centauri AB (α Cen AB). This "AB" designation denotes the apparent gravitational centre of the main binary system relative to other companion star(s) in any multiple star system. "AB-C" refers to the orbit of Proxima around the central binary, being the distance between the centre of gravity and the outlying companion. Some older references use the confusing and now discontinued designation of A×B. Since the distance between the Sun and Alpha Centauri AB does not differ significantly from either star, gravitationally this binary system is considered as if it were one object.
Asteroseismic studies, chromospheric activity, and stellar rotation (gyrochronology), are all consistent with the α Cen system being similar in age to, or slightly older than, the Sun, with typical ages quoted between 4.5 and 7 billion years (Gyr). Asteroseismic analyses that incorporate the tight observational constraints on the stellar parameters for α Cen A and/or B have yielded age estimates of 4.85 ± 0.5 Gyr, 5.0 ± 0.5 Gyr, 5.2-7.1 Gyr, 6.4 Gyr, 6.52 ± 0.3 Gyr. Age estimates for stars A and B based on chromospheric activity (Calcium H & K emission) yield 4.4-6.5 Gyr, while gyrochronology yields 5.0 ± 0.3 Gyr.
Read more about this topic: Alpha Centauri
Famous quotes containing the words nature of the, nature of, nature and/or system:
“It is not women’s fault if we are so tender. It is in the nature of the lives we live. And further, it would be a terrible catastrophe if men had to live men’s lives and women’s also. Which is precisely what has happened today—to women.”
—Selma James (b. 1930)
“The nature of women’s oppression is unique: women are oppressed as women, regardless of class or race; some women have access to significant wealth, but that wealth does not signify power; women are to be found everywhere, but own or control no appreciable territory; women live with those who oppress them, sleep with them, have their children—we are tangled, hopelessly it seems, in the gut of the machinery and way of life which is ruinous to us.”
—Andrea Dworkin (b. 1946)
“What art can paint or gild any object in afterlife with the glow which Nature gives to the first baubles of childhood. St. Peter’s cannot have the magical power over us that the red and gold covers of our first picture-book possessed. How the imagination cleaves to the warm glories of that tinsel even now! What entertainments make every day bright and short for the fine freshman!”
—Ralph Waldo Emerson (1803–1882)
“Human beings are compelled to live within a lie, but they can be compelled to do so only because they are in fact capable of living in this way. Therefore not only does the system alienate humanity, but at the same time alienated humanity supports this system as its own involuntary masterplan, as a degenerate image of its own degeneration, as a record of people’s own failure as individuals.”
—Václav Havel (b. 1936)