Libra Alpha - Zuben el Genubi

The bright stars located on the ecliptic have long interested astronomers more than all other stars. Why? There are many reasons. First, the stars in antiquity were considered fixed. Their position on the celestial sphere only needed to be determined once, and then they became reliable reference points for measuring the exact position of the planets and the Moon, which regularly passed near them. And besides, there was an opinion that some magic was associated with these stars, and when a planet passes by such a star, it takes over one or another magical power from it, which then affects us – this is already an astrological method of knowing the world. Now it seems that he is at least strange, but you had to start somewhere.

The main star of the constellation Libra – alpha – has an unexpected Arabic name “Zuben el Genubi”. Unexpected, because in translation it means “Southern claw”. It’s time to remember that the modern territory of Libra once belonged to the constellation Scorpio. Scorpio in ancient times occupied a much larger space in the celestial sphere, and all the stars that now belong to Libra then belonged to Scorpio.

The constellation Libra and its surroundings on the star map

And where were Libra?

And were they?

This is a rather complicated story, and it is better to consider it separately – in one of the following articles. For brevity, let me tell you that in Arabic and Greek astronomy, Libra appeared only closer to the beginning of a new era, and before that it was Scorpio. In Babylonian astronomy, Libra was indeed here. In Egyptian – Boat. In ancient Chinese astronomy, in the modern space of the constellation Libra, the priests saw the “God of Measure”, who balanced everything, assigned his measure to everything, which to some extent is consonant with the essence of Libra.

Modern asterism “Claws of Scorpio” – they do not affect the constellation Libra

In Roman astronomy, Alpha Libra had another name – “Kiffa Australis” (South Chalice), but the name is rather late, and refers to the time period in which the western part of Scorpio separated and gained independence in the form of the constellation Libra.

Alpha Libra is only slightly brighter than the 3rd magnitude, and even inferior in brilliance to nearby Beta (“Zuben el Shemali” – “Northern Claw”). The difference in brightness between these stars is only a tenth of a magnitude, and in the 16th century, when astronomers began to use Greek letters to designate stars on maps (in order of decreasing brightness), observers, of course, could not notice such a small difference.

In ancient times, the “Claws of Scorpio” covered the entire modern territory of the constellation Libra.

But be that as it may, Alpha Libra is a very interesting star, and deserves the right to be the first in the list of stars in its constellation.

First, it is a star quite close to the Sun. We are separated by 77 light years. Of course, modern spacecraft are not yet able to overcome such a distance in the foreseeable time, but in comparison with most of the stars visible to the eye, up to alpha Libra is “within a stone’s throw”. The brightest stars in Orion, for example, are located at an average distance of 1000 light years. And to the red-orange Antares, heading the neighboring constellation Scorpio, about 600 light-years. And the blue Spica in the constellation Virgo is also significantly further than alpha Libra – 260 light years from us.

The current configuration of the constellation Libra

Alpha Libra is not alone. Its companion is visible through binoculars – a yellowish asterisk of the 5th magnitude, 4.5 arc minutes away from the main star – the most vigilant people in favorable conditions can see both stars separately without optics. For astronomers, this distance between the components is considered very large. Usually double stars are located closer. And for a long time it was believed that this pair of stars is optical – not physically connected. Now astronomers admit the gravitational connection of this star system. The physical distance between the components is estimated at 5 thousand astronomical units (the Oort Cloud in the solar system is at least an order of magnitude larger). And the period of revolution of these two stars around a common center of mass can be 200 thousand years – for stars this is not so much.

This is what Alpha Libra looks like when viewed through a telescope with a small increase

In turn, the brighter and more massive component also consists of two stars. This pair – on the contrary – is so close that no modern telescope is able to separate it. And only spectral analysis irrefutably testifies to the duality of the system, which consists of two white stars comparable to Sirius (but Sirius, as we remember, is ten times closer to us, and therefore brighter). The distance between the components is comparable to the distance from the Sun to Mercury, and the period of complete revolution is several weeks.

It is not superfluous to mention that the described close binary system has the designation α2, while the far distant satellite of the 5th magnitude is designated as α1 (alpha-1). And this is not confusion, not a mistake – according to modern rules, the luminaries are designated in order of increasing Right Ascension – the main celestial coordinate. And a weaker star located in this priority system goes a little earlier.

If you do not know this rule, such a designation of stars may seem incomprehensible.

Relatively recently, it turned out that alpha-1 (a satellite of a bright star visible through binoculars) is also binary, and not just a binary, but an eclipsing binary – here in a close binary system one star periodically covers another star, and the total brightness of alpha-1 for a short time interval is weakening. There are such eclipses once every 16 years. And between eclipses, when the components diverge as far as possible (from the point of view of an earthly observer), they can be seen separately in very strong earthly telescopes, because the angular distance between the stars in this pair sometimes reaches half a second of arc (amateur telescopes cannot separate such a close pair, but it will not be difficult for professional interferometers to do this).

But that’s not all.

Located 2.5 degrees west of both alphas, the yellow dwarf KU Libra, which is a rotating variable star (the brightness of such stars is due to the presence of a large number of dark spots, similar to those on the Sun, only with a significantly larger relative area) is suspected of gravitational connection with the Alpha Libra system.

Alpha Libra and KU Libra

Pay attention to how the idea of interstellar connections and the very possibility of the existence of connected star systems developed. Galileo Galilei, the discoverer of double stars, considered the proximity of stars in the sky to be illusory, and did not allow physical communication between them. Later, orbital motion was discovered in some stellar pairs (such as Gamma Virgo – Porrima – where the rotation period of the components is 170 years, and changes become apparent during the life of one astronomer). So it was proved that stars can create gravitationally bound stable systems. But this was allowed only in relation to rather close pairs, and wide pairs were a priori considered optical. For centuries, the possible gravitational connection between Alcor and Mizar, between the components of the star Albireo, was denied, and Alpha Libra was also not considered as a physically connected system.

Now, astronomers are seriously studying the gravitational interaction of the 4-fold Libra alpha system with a yellow dwarf distant by 3 light years, somewhat similar to our Sun.

The astute reader may now ask the question:

“After all, the Sun also has neighboring stars nearby – both at a similar distance and with somewhat similar physical characteristics – the Alpha Centauri system. Yes – the distance is a little more – 4 light years. But what if there is a physical gravitational connection between the Sun and Alpha Centauri?”

Of course, there is no reason to consider the Sun as a member of the Alpha Centauri system, but the gravitational interaction of the Sun and the nearest stellar environment is now being actively studied, especially against the background of the fact that the Alpha Centauri system is approaching the Solar System quite quickly (23 kilometers per second), and calculations of future positions on The sky of the stars included in the Alpha Centauri system is now produced taking into account the mutual gravitational influence – it is quite significant.

But with regard to Alpha Libra and the yellow dwarf KU Libra, apparently, we are talking about gravitational connectivity, and not just about some mutual influence. So, most likely, Alpha Libra is a fivefold star system.

In addition to gravitationally bound satellites, this star system has some distant stellar environment with which Alpha Libra has a possible common origin. This is the so-called “Castor Stream” or “Castor Moving Group”, which, in addition to the title star from the Gemini constellation, includes Vega, Fomalhaut, Alpha Libra, Alpha Cephei and a dozen more faint stars that have only catalog numbers. To be precise, the existence of the “Castor Stream” is still only hypothetical, and is considered only on the basis of the revealed coincidence of the speeds of the mentioned stars – they all move with the same speed, and move in the same direction. It is possible that further research will reveal to us a more complete picture of the life of our nearest stellar environment.