The Milky Way: A band of distant starlight
Anyone who has spent a significant period of time outside at night, far from city lights, has no doubt been intrigued by the irregular band of faint light stretching across the night sky above them. What they are seeing is the Milky Way or, more correctly, a section of the galaxy that is home to our solar system. The portion of the Milky Way we see from Earth is composed of a myriad of countless stars which, unlike those that we do see in the night sky, are so distant they cannot be resolved into individual stars with the naked eye. The Milky Way, in its entirety, is a massive barred, spiral galaxy, composed of over 400 billion stars, with a diameter of between 150,000 and 200,000 light years (lys). It is approximately 100,000 lys across, and rotates around its galactic centre at about 828,000 kilometres per hour. Until the 1920s, it was thought to contain all the stars in the universe. However, astronomers such as Harlow Shapley, Heber Curtis and Edwin Hubble helped prove that it is, in fact, only one of what we now know to be millions of distant galaxies in the universe. The Milky Way’s name comes from the Latin via lactea, which came from the ancient Greek for “milky circle.” Our sun is located roughly 27,000 lys from the galactic centre, and about 20 lys above the galactic plane. The Milky Way Galaxy has four major spiral arms (much like a child’s pinwheel), large formations of interstellar gas and dust that curve outward from the galactic centre in spirals (the spirals being shaped by the galaxy’s rotation). It also has two smaller arms or spurs. It is in one of these smaller spurs, the Orion — Cygnus Arm, that our solar system resides. Our Milky Way Galaxy is part of a subgroup of galaxies which, in turn, are part of a larger group of galaxies known as the Local Group, itself part of the larger Virgo Supercluster, part of the much larger Laniakea Supercluster. The Milky Way has a number of small satellite galaxies of its own, the largest and most famous being the Large Magellanic Cloud (163,000 lys distant) and the Small Magellanic Cloud (200,000 lys distant), both visible in the night sky of Earth’s southern hemisphere. Interestingly, the Milky Way Galaxy is believed to have been formed by the collision and merger of two large galaxies at some point in the distant past. The Milky Way is currently on a collision course with the Andromeda Galaxy, a collision and merger which, fortunately for us, won’t take place for millions of years. The Milky Way is prominent in the night sky in rural areas, far from city lights, when the moon is either low or absent. Depending on the time of night and the time of year, it may appear high or low in the night sky. It is inclined 60 degrees to the ecliptic plane (the apparent path of the planets, moon and sun across the sky), which itself varies in height in the sky depending on the time of year. The dark areas in the Milky Way are actually vast regions of interstellar dust which block the light of the more distant stars in the galaxy arm. If you look towards the constellation of Sagittarius (the Archer) in the southern night sky, you are looking towards the galactic centre where, it is believed, a massive black hole is located. The Milky Way is thought to be surrounded by an enormous halo of hot gases that stretch outward hundreds of thousands of light years into space.
THE LEGEND
In Greek mythology, the Milky Way is tied to the story of the birth of the famous Greek strongman and hero, Heracles (Hercules). Heracles was the demigod son resulting from the seduction of the mortal maiden Alcmene by the god Zeus (in the form of her husband, Amphitryon, who also lay with his wife after her fraudulent union with Zeus). Hera (wife to Zeus) was outraged by Alcmene’s pregnancy by Zeus, and ordered Ilithyia, goddess of childbirth, not to allow Alcmene to give birth. However, when Hera was distracted elsewhere, Ilithyia was tricked into assisting Alcmene when she went into labour and gave birth (over a two-day period) to Ipicles (mortal son of Amphitryon) and Alcides (demigod son of Zeus). Alcides’ name was subsequently changed to Heracles (meaning “for the glory of Hera”) by his parents to appease the angered goddess. Though Heracles was abandoned by his parents in a field (a common practice in those days to get rid of unwanted children), he was rescued by the goddess Athena, who took the abandoned baby up to Mount Olympus, where Hera, not knowing whose child it was, in a turn of motherly love, took the baby to her breast to nurse him. It seems Heracles was a little too rough at nursing, and when, in pain, Hera removed him from her breast, her mother’s milk splashed out into the heavens, thereby creating the Milky Way.
WHAT CAN YOU SEE NOW?
Mercury (magnitude -0.6) sits just over a hand and a half’s width (at arm’s length) above the SW horizon after sunset, setting shortly after 9 p.m. Venus shines brilliantly at mag. -4.12 high in the early evening southwest sky shortly before 6 p.m., setting just before 9 p.m. Mars (mag. -1.5) is in the pre-dawn southeastern sky shortly before 4 a.m., but will fade from sight as dawn breaks around 6:30 a.m. A brighter Jupiter (mag. -1.9) joins Mars in the pre-dawn southeastern sky shortly before 6 a.m., reaching a height of 10 degrees (a hand’s width at arm’s length) before fading in the light of the rising sun. Saturn is not currently visible. Until next time, clear skies.
EVENTS
Feb. 15: Last Quarter Moon