- July 21, 2017 - Planétarium
Nights in the month of July are the best time of year for admiring our galaxy, the Milky Way. Difficult to make out in skies above urban settings, it unveils all its wealth when seen in a sky not polluted by light.
Galaxies are immense archipelagos of stars. The Milky Way has a spiral shape, crossed by a bar at the center; its stellar disc has a diameter of about 120,000 light years (ly) and a thickness, on average, of 1,000 ly. We estimate that it contains over 200 billion stars. The Sun and its set of planets are located in the suburbs, some 27,000 ly from the Galactic Center. Note that the great majority of stars visible to the naked eye are located only a few hundred light years from us.
To the naked eye, the Milky Way appears in the form of a whitish band of light that, at this time of year, crosses the sky from the northeast to the southwest. Observed in the direction of the constellation Sagittarius, the Milky Way broadens, because we’re then looking towards the midpoint of the galaxy. And at the center of the Milky Way sits a gigantic black hole whose mass is estimated at more than four million times that of the Sun. But not to worry: we’re located much too far from this interstellar monster for there to be the slightest danger.
We can also see dark areas that seem to be lacking stars. Those holes are in fact clouds of gas and dust that obscure the light from distant stars.
If you get the opportunity, observe the Milky Way with binoculars or with a small telescope. The regions of the constellations Sagittarius and Scorpio in particular are packed with celestial objects like clusters of stars and gas on display in all their finery.
And all that’s left for me is to wish you a wonderful summer, and happy sky gazing.