Limerick Astronomy Club: The sign of the archer

The Sagittarius constellation (which is recognised as being part of the Zodiac signs) is a comparatively large constellation which is primarily visible in the southern hemisphere. In the Northern hemisphere the constellation can be viewed low on the horizon from August to October. Similar to other zodiac constellations, Sagittarius was first classified by the Greek astronomer Ptolemy in the 2nd century.

In Greek mythology, Sagittarius embodies a centaur, being comprised of torso of a man and the body and legs of a horse. The centaur is portrayed as aiming an arrow toward the heart of the neighbouring constellation Scorpio, denoted by the red supergiant star Antares. According to the Roman author Hyginus, the constellation is named after Crotus, who is believed to have invented archery and lived on Mount Helicon. He had a close relationship with the Muses and it was at their request that Zeus placed him in the sky.

The constellation comprises of multiple objects including the Arches Cluster, the Quintuplet Cluster with the luminous Pistol Star, the Galactic centre as well as a number of well-known deep sky objects, such as the Sagittarius Dwarf Elliptical Galaxy, the Sagittarius Dwarf Irregular Galaxy, the Bubble Nebula, and as many as 15 Messier objects, among them the Sagittarius Star Cloud (Messier 24) and the Omega Nebula (Messier 17).

Sigma Sagittarii is the second brightest star within this constellation and lies almost 228 light years from Earth. It is a hydrogen fusing dwarf star with an apparent magnitude of 2.1. The star exhibits luminosity 3,300 times that of our Sun. Furthermore, it spins with a speed of about 200 kilometres per second, which makes it 100 times faster than our Sun. Delta Sagittarii is a multiple-star structure and has an apparent magnitude of 2.72 and is 1180 times more luminous than our Sun.

The Pistol Star is a blue variable star that is about four million times as luminous as our Sun and 120-200 times as massive, making it one of the brightest known stars. It can be found in the Galactic centre region and lies nearly 25,000 light years from our solar system. In spite of its distance it would be a fourth magnitude star, visible to the naked eye only for the interstellar dust in the way.

Limerick Astronomy Club meet the first Thursday of every month in room G10 in Mary I.