The Milky Way is estimated to be about 13.2 billion years old (the actual galactic disk, about 10-12 billion years old) and contains hundreds of billions of stars. Along with all other cosmic galaxies, it was formed as a result of the Big Bang process about 13.7 billion years ago. It takes its name from the Latin Via Lactea, from the Greek Kiklios Galaxios, meaning milky circle. The term is said to have originated from the Greek myth surrounding Hera, the Greek queen of the gods. Legend has it that the stars of the Milky Way are the milk gushing from Hera’s breasts when she awoke to find herself nursing, unknowingly, one of Zeus’ illegitimate children (the child was actually Heracles, the result of their union Zeus with a mortal maiden named Alcmene), was placed there by Zeus while he slept. When she tore the infant from her breast, some of the milk shot up into the heavens, thus forming the Milky Way. The Milky Way is metaphorically like an egg that has been cracked open in a frying pan. The central bulge in our galaxy corresponds to the yolk of the egg, while the surrounding, thinner arms of our Milky Way, seen when viewed in the night sky, correspond to the white of the egg. Our solar system, containing our planet Earth, lies within the white of the galactic egg, about 30,000 light years from the center of the egg yolk (1 light year equals about 9,460,000,000,000 kilometers). When we look at the band of the Milky Way galaxy arching across the night sky, we are essentially looking through the white of an egg. The regions of the sky on either side of the Milky Way belt, in which there are mainly fewer stars, correspond to the regions above and below the egg in the pan. When we look toward the bright, central region of the galaxy, in the direction of the constellation Sagittarius (the Hunter), we are looking toward the yolk of the egg. In this halo is the black hole Sagittarius A* (or Sag A*).
Our black hole
Sag A* (pronounced Sagittarius A-star, or Sag A-star) is a supermassive black hole at the galactic center of our Milky Way galaxy in the direction of the constellation Sagittarius. On May 12, 2022, astronomers using the Event Horizon telescope (a global network of radio telescopes) took the first picture of Sag A*. It wasn’t actually an image of the black hole itself, since black holes are so gravitationally strong that not even light can escape its pull, but, rather, the image of the accretion disk (formed by diffuse materials orbiting around from a massive central entity) of glowing dust and gas that is heated to millions of degrees as it is absorbed into the black hole. While a black hole gravitationally pulls surrounding stellar material into it, growing in size as it does so, theoretical research suggests that a black hole could never grow large enough to consume an entire galaxy. As a black hole consumes surrounding stars and gas, it releases radiation. At some point, the outward pressure of the radiation grows so large that it prevents the black hole from consuming any other material, and the black hole stops growing.
Collision course
While Sag A* may not pose a threat to our galaxy, a real threat does, however, lie in one of our celestial neighbors. The Andromeda Galaxy is located about 2.5 million light-years from the Milky Way. This galaxy is slowly moving towards the Milky Way, at a speed of 396,000 kilometers per hour, and it is expected that the two galaxies will merge together in about 4 billion years. The outer edges of both galaxies are already interacting gravitationally. The resulting merger will change the shape of both galaxies, resulting in a much larger and differently shaped (perhaps elliptical) galaxy, giving rise to billions of new stars, planets, star clusters, nebulae, etc. Interestingly, although the two galaxies will naturally merge, it is thought that, due to the vast distances between the stars in both galaxies, individual stars in both galaxies are unlikely to collide, although some may be ejected from the newly formed galaxy gravity. The black holes from each galaxy will also likely merge to form a much larger black hole. Given the enormous timescale over which the pending merger of the Andromeda and Milky Way galaxies will occur, humans are unfortunately not likely to be here on Earth (or even within the solar system) yet to marvel at the beauty and immensity of new light in the night sky. Perhaps our distant descendants will see the night sky from planets around distant stars. Our vast universe is in a state of constant flux, with the panorama of the night sky above us changing from night to night, season to season and year to year. Take a few moments on a clear moonless night to step outside and look up and marvel at the countless wonders of the Milky Way. It won’t be there forever.
This week’s night sky
Saturn (+0.3 mag, in Capricorn — the Sea Goat) becomes accessible around 8:10 p.m. when it reaches 12 degrees above the southeast horizon. It reaches its highest point in the night sky of 27 degrees above the southern horizon around 11:45 PM and disappears from view when it drops below 10 degrees above the southwestern horizon by 3:20 AM. Look for Saturn four degrees north of the moon on September 8. Jupiter (mag, -2.9, in Cetus – the sea monster) becomes visible around 9:25 PM, when it reaches seven degrees above the eastern horizon. It reaches a height of 44 degrees above the southern horizon at about 2:40 AM. and then disappears into dawn approaching 24 degrees above the southwestern horizon around 6:15 AM. Mars (mag -0.1, in Taurus – Taurus) rises in the east-northeast around 11:30 p.m., reaching a height of 63 degrees in the predawn, southern sky, before fading just after 6 a.m. Venus and Mercury are not visible this week. The September 10th full moon is most often referred to as the Harvest Moon, as September is the month most crop harvests take place here in the Northern Hemisphere. Until next week, clear skies.
Events:
September 7: Moon at perigee (closest to Earth) September 8: Saturn four degrees north of the Moon September 10: Harvest Full Moon
(Glenn K. Roberts lives in Stratford, PEI, and has been an avid amateur astronomer since childhood. He welcomes reader comments at [email protected])
