Reports - 2011
Astronomy Benalla General Meeting Presentations - Wednesday 20th July 2011
Constellation of the month: SAGITARIUS.
Presenter: Jeff Knight
Rising behind Scorpius in the eastern July sky is the constellation Sagittarius,
otherwise known as The Archer.
It is immediately recognisable by the 'teapot' asterism, with the 'spout' (the arrow
of The Archer) pointing up toward the very centre and most dense part of our
Milky Way Galaxy.
The constellation as a whole depicts a mighty warrior, half man half horse, who is
drawing his bow and aiming his deadly arrow at the heart of mankind's enemy
(depicted in the night sky by the constellation Scorpius!).
Unlike most constellations, the listing of brightest stars by the Bayer designation
does not apply with Sagittarius. α Sgr (or 'Rukbat') is only the fifteenth brightest
star of the constellation at mag. 3.96, while ε Sgr (Kaus Australis) is the brightest
at magnitude 1.79. It appears the stars were designated by the significance of
their 'position' in this case. Their earliest Sumerian names tell us something of
what these stars were intended to represent.
The stars that lie in this constellation are a mix of single, double, and a variety of
multiple star systems - from giant stars that are colder than our Sun, to blue
dwarfs and super giants that outshine our Sun hundreds of thousands of times
over. For example, while Kaus Australis appears brightest to us, in reality it
would pale when compared to μ Sgr (Polis). The primary component in this
multiple star system, Polis A, has a radius over a hundred times that of our Sun,
yet is considerably further away from us (3600 light-years).
Sagittarius lies at the heart of our galaxy (the galactic centre) and contains numerous objects of interest. There are
fifteen Messier objects catalogued in the 18th Century, and a further fifty-seven deep-sky objects listed in the New
General Catalogue (NGC).
Of particular note are the Trifid, Lagoon and Swan nebulae (M20, M8, M17 respectively) which, when photographed
through different filters, reveal beautiful and breathtaking sights. Included among these are an almost endless display
of star clouds, globular clusters, and even neighbouring galaxies such as the irregular Barnard's Galaxy (NGC 6822).
What may be the most interesting object, however, may be one not visible through your telescope. From our
perspective it lies in line, and double the distance, from ε Sgr to γ Sgr (Alnasl), and is at the very core of the galaxy.
Lengthy studies suggest it has a mass 3,700,000 times that of our sun, and is slightly larger that the diameter of our
solar system. As a bright radio source, it has been named Sagittarius A* (pronounced a-star), and is the most likely
candidate for a supermassive Black Hole.
To date, twenty-two stars in this constellation are discovered to have planets, giving Sagittarius more planetary host
stars than any other constellation, but that is another topic for discussion!
Pictures from the Hubble Site
