Modern astronomy, unlike ancient astronomy, does not mean a constellation of a group of stars that make up any memorable figure, but a certain section of the sky on an imaginary surface of the celestial sphere. Such an area includes not only stars, but also all other objects located within its borders.
In the first third of the 20th century, the International Astronomical Union finally approved the boundaries of 88 sections - constellations into which the entire earthly sky was divided. They differ significantly in the area occupied by the celestial sphere - from the tiny Southern Cross to the gigantic Virgin, Hydra and Ursa Major. One of the smallest areas in the sky is the constellation Arrows. The Latin, that is, international, name of the constellation is Sagitta (abbreviated Sge). Despite its modest size, it has been known since ancient times.
History and Legends
The group of stars that form the characteristic configuration of the constellation in the form of an arrow was familiar in ancient times to the inhabitants of the Middle East, Greece, and Rome. Claudius Ptolemy in the II century included Strela in his astronomical catalog, which, after being translated from Arabic, became known in medieval Europe under the name Almagest.
There are several mythological versions that explain the presence in the sky of an object so recognizable to the ancients. According to one of the most common legends about the constellation Strela, Hercules (Hercules) shot her with an eagle of Zeus, pecking a liver chained to a rock titanium Prometheus. Judging by the relative position of the indicated constellations, a formidable weapon flew past the target ...
Another myth conveyed by Eratosthenes claims that the Arrow was sent by Apollo in revenge for his son Asclepius to the Cyclopes. After all, it was these giants who forged Zeus lightning, with which he struck a god who gave people hope for immortality.
However, not always people saw in this combination of stars an arrow. We have reached such Latin names for the constellation as Dart, Reed stick (for writing), Pillar and some others.
Location in the sky and observation
The arrow is located in the northern hemisphere of the celestial sphere, covering an area of only about 80 square degrees (only Lesser Horse and Southern Cross occupy smaller areas of the sky). The small constellations Lysychka and Dolphin, the rather large and clearly visible constellation Oryol, and the huge Hercules are adjacent to it.
The arrow can be seen from almost the entire surface of our planet, with the exception of the Antarctic region south of –74 ° latitude. North of –68 ° it is characterized by full visibility. In the middle latitudes of the northern hemisphere, the constellation can be observed from late spring to mid autumn, and the best time for observation falls on July-August.
Finding the Arrow in the sky is easiest, relying on the location of the so-called Great Summer Triangle (Vega, Deneb, Altair), as soon as the stars become visible after sunset. Under the constellation Strela is the lower peak of the Triangle - Altair, and Vega and Deneb lie on imaginary lines close to the ends of Strela.
With the naked eye, you can distinguish about two dozen stars, four of which, forming an arrow-shaped figure, have a magnitude brighter than 5 m . However, a glance through binoculars already makes it possible to see a whole scattering of stars, since the strip of the Milky Way passes through the constellation Strela. The photo below was taken using a standard digital camera.
This small part of the sky is not rich in spectacular objects, but a lover of astronomy, armed with a small telescope or even simple binoculars, will still find something interesting on it.
The brightest stars
The brightest star here is the red giant Gamma Arrows, forming its "tip", the size of 3.51 m . Gamma is 260–275 light-years distant from the solar system; its luminosity apparently exceeds the solar by approximately 640 times. The surface temperature of this star does not exceed 4000 kelvin, the mass is 2.5 times the mass of the Sun, and in size this giant star is 55 times larger than our star.
The Arrow Delta, about 4 m in size, also having a reddish tint when viewed through binoculars, is a binary system of the red giant and the blue and white star of the main sequence. However, these components are not available for visual observation. They can only be resolved by spectroscopic methods.
Alpha Arrows, with its own name Sham, or Alsham, is a yellow giant of stellar magnitude 4.4 m . In the constellation of Arrows, Sham forms the upper part of the “plumage”. It is 4 times more massive than the Sun, 20 times larger than it, shines 340 times more powerful and is located at a distance of 470–475 light years.
Beta Arrows, respectively, the lower half of the “plumage” is a star, generally quite similar in physical parameters to Sham, however it is somewhat more massive and colder.
Interesting variables
The Arrow constellation includes a number of variable stars, among which there are also those that are available for observation using amateur equipment.
So, a very popular object is the eclipsing variable U Arrows, located to the right of the main figure of four bright stars. This is a binary system, the distance between the components of which is so small that one of them overshadows the other every 81 hours, 8 minutes, that is, 3.4 days. The orbital plane of the eclipsing companion practically coincides with the line of sight, so that the observer sees total eclipses, accompanied by a rapid decrease in brightness by 2.5 magnitudes. The duration of the eclipse is 1 hour 40 minutes.
In addition, the classical Cepheid S Arrows with a period of 8.38 days is of considerable interest. During this time, the brightness of the pulsating supergiant star changes from 5.2 m to 6 m .
Unusual cluster and planetary nebulae
In addition to variables that are attractive to the observer, as well as multiple stars, the constellation Strela also includes the globular cluster M71 with a magnitude of 8.3 m . It is located at 13,000 light years; its diameter is 27 light years. This is such a rarefied object that for a long time astronomers could not decide on its classification, taking it for an unusually dense and star-rich open cluster. However, it is now generally accepted that M71 is a globular star cluster, albeit slightly concentrated.
Owners of amateur telescopes can also please the constellation with two small blue planetary nebulae - NGC 6879 and NGC 6886. However, their observation is very difficult due to the faint brightness (13 m and 12 m ) and the small angular sizes of these stellar remnants.
Space call
Two of the stars of the constellation Strela, which are not very familiar at first glance, have an important feature: they are similar to the Sun in spectral class and are at a fairly close distance from it. Therefore, the stars of 15 Strela and HD 178428, 58 and 68 light-years distant from us, respectively, were chosen along with two other similar luminaries as the recipients of the interstellar radio message of Earthlings Cosmic Call ("Space Call").
An appeal to possible extraterrestrial civilizations was undertaken on the initiative of A. L. Zaitsev, a specialist in space radiolocation and an enthusiast for the search for reason in the Universe. It took place in 1999, and the Yevpatoriya RT-70 radio telescope was used to send it. In 2003, the "Space Call" was sent a second time, already to other stars.
The message contains a concise list of human knowledge in various fields of science, as well as brief addresses and wishes from many people from different countries. Stars 15 Arrows, it will reach in 2057, and HD 178428 - in 2067.
Modest charm
Of course, a small Arrow is not such a constellation that can immediately attract attention, like the bright, spectacular and rich in various spectacular objects Swan or Pegasus. However, the Universe is inexhaustible in almost any direction, and a true amateur astronomer will not miss this small piece of sky, having found there also cosmic phenomena that are interesting and exciting the imagination.