Vega : The Zero Star
Vega, the Zero Star
Vega, the fifth brightest star within the sky, utilized in the past as a standard reference for the zero points on the magnitude scale, has an oblate form, because of its high rotation speed. This and alternative information have been obtained from studies that supported interferometry, a way that enables reaching a very high angular resolution, combining the photographs noninheritable by many telescopes.
ALFA Lyrae, the intense Vega, is 7.68 parsecs or twenty-five light-years off from the Solar System. Of a gorgeous none color, it belongs to the spectral sort A0 and is on the main sequence as the Sun. Visible from the northern hemisphere, it forms at the side of Deneb and Altair the so-called Summer Triangle, an asterism straightforward to identity with the eye even for the most inexperienced observers. Because of its brightness and color, Vega has been used for an extended time as a stand in the scale of stellar magnitudes, that is, it absolutely was the star on which to calibrate the zero-point of the scale.
Known and loved since precedent days, it's been studied over the centuries with all attainable instruments. However, the most vital and stunning progress had solely taken place in recent years, when, because of interferometry, it was discovered that it has an oblate form and that rotates so fast that is not far from being disrupted by its centrifugal force.
The most controversial points of the models created by the studies published up to now concerning the rotation period of Vega and the angle of inclination of its axis. In a trial to seek out higher solutions for these 2 values, a team of researchers led by John D. Monnier of the University of Michigan observed the star in 2007 and 2012 with the CHARA (Center for High Angular Resolution Astronomy) interferometer, a grid of six 1-meter telescopes placed on Mount Wilson in California.
CHARA has a baseline of up to 330 meters, the longest within the world for optical and near-infrared interferometry. It can reach a resolution of up to 0.0005 arc seconds, many times above that of the Hubble Space Telescope, albeit restricted to terribly bright and close objects. It is the equivalent of the ability to see the disk of a coin from a distance of 1,600 km. Solely because of interferometry, it's been possible to observe and measure Vega’s disk, which otherwise appears solely as a bright light spot.
The image of the star reconstructed by the CHARA interferometer shows a nearly circular disk as a result of it happens that Vega’s rotation axis is sort of aligned with our observation point. In short, from Earth, we glance at one of the poles of Vega. If our view were focused instead on the equator, the outline of the star would appear as an ellipse, because Vega is an oblate star, whose equatorial diameter is perceptibly larger than the polar diameter.
Deformation is inferred from a phenomenon called gravity darkening. Vega, in fact, emits more light from the poles than from the equator. It depends on its quick rotation that causes the star to expand at the equator. Therefore, matter in this region is less dense and undergoes a lower gravitational attraction than the poles, therefore it's less hot. Consequently, the rim of Vega, which corresponds to the equatorial circumference ascertained from one of the poles, seems to us less luminous than the center of the star disk, coinciding with the pole seen from the Earth, where instead matter is more compressed and hotter.
According to a study. published at the end of 2012, based on observations made with CHARA, the temperature distinction between the poles and the equator of Vega is over 1,000 degrees: 10,070 K at the poles, 8,910 K at the equator. The typical effective temperature for the whole stellar surface is 9,360 K, about 3,600 degrees more than the solar photosphere.
Data from CHARA conjointly show that Vega’s rotational velocity is less critical than had been calculated from previous interferometric observations. Other authors had found that the quantitative relation between the rotation speed and the breaking speed was between 0.90 and 0.93. Thus, according to them, Vega wasn't far away from reaching the breaking speed, beyond which the centrifugal force generated by rotation overrides the force with which gravity holds in place the stellar matter.
Monnier and colleagues instead calculated a more relaxed quantitative relation of 0.77 ± 0.05, according to which Vega rotates, at the equator, at about 195 km/s: a speed quite far from the breaking point, but still staggeringly higher than that of the Sun, that is barely about 2 km/s. An entire rotation of Vega lasts just over seventeen hours — a trifle, compared to the more than 25 Earth days that the Sun takes to make a full rotation.
The recalculation of the rotational velocity conjointly needed to redefine the age of Vega. Stellar evolution models show that stars’ rotational velocity varies according to age: the younger a star is, the faster it rotates. According to the evolutionary models adopted in the study of Monnier and colleagues, the rotational velocity of Vega suggests an age of 700 million years, that is, over two hundred million years more than the previous estimates. The distinction isn't little. Vega, in fact, is more than twice as massive as the Sun. That means it will burn its nuclear fuel about ten times faster than our star. So, at 700 million years recent, Vega is already well over half of its stay on the main sequence, while the Sun, at 4.6 billion years of age, has not even reached half its path.
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