Star Talk: Sagittarius points out center of the Milky Way galaxy

The constellation Sagittarius is usually shown with a bow pulled back ready to launch an arrow. This

Sagittarius, the half-man, half-horse constellation of the zodiac trots low over the southern horizon during the evening hours this time of year. A summer star figure, Sagittarius is usually shown with a bow pulled back ready to launch an arrow. This arrow also happens to point the way to the center of our galaxy.

In certain mythologies, Sagittarius was thought of as Enkidu, a friend of Gilgamech, who we now know as Orion the Hunter. One account has it that Orion was felled by the sting of a scorpion. To keep the hunter forever safe on the celestial sphere, Orion has been placed in the winter night sky opposite to the scorpion’s position of summer nights. The archer-centaur, Sagittarius, takes aim at the stars of Scorpius the scorpion as if to avenge his friend’s loss.

These days, the stars of Sagittarius are usually not imagined as a man’s head and torso attached to a horse’s body. The stars of this constellation are typically seen as a dot-to-dot teapot shape skimming above the southern horizon.

To see the teapot, picture its handle formed by imaginary lines between three fairly bright stars (on a star map, sigma, tau, and zeta Sagittarii) just over the south point around 9:30 p.m. this week. The middle star of this stellar handle is also the hand of the archer pulling back the string of his bow.

Three more stars (phi, lambda, and delta Sagittarii) slightly northwest of the handle form the lid of the teapot while its spout is shaped by connecting the westernmost star of the lid (delta Sagittarii) with the constellation’s westernmost bright star (gamma Sagittarii). This star also represents the archer’s arrowhead pointed west at the scorpion constellation.

Milky road

Visualizing the central region of Sagittarius as a teapot is useful because the spout of the teapot seems to be releasing a misty vapor that sweeps from the south to almost overhead and then into the northeastern sky during late August/early September evenings. This ashen glow is the combined light of hundreds of billions of stars that our sun is among; it is our Milky Way Galaxy, and the spout’s “steam” is revealed as stars when viewed though binoculars or telescopes.

This milky, celestial road or Via Lactia as the ancient Romans called the Milky Way, puzzled many people. Many thought of it as the by-way that souls would walk on their way to the other side. Galileo settled the debate with his telescopic observations showing that the nightglow of the Milky Way is the light of innumerable stars whose radiance melts together.

The Milky Way is a spiral-type, somewhat pinwheel shaped galaxy with a spherical core of older stars surrounded by a rather flattened disk of younger stars. Coincidentally, the arrowhead of Sagittarius points just below the center of our galaxy. The central part of the Milky Way is at such a great distance that it takes light almost 30,000 years to travel from there to us.

Visually, we can only see about one-fifth of the distance to our galaxy’s core because of obscuring material between the stars. Some of this interstellar “dust” can be seen as dark splotches against the Milky Way’s light. Radio waves from the center of our galaxy, however, can penetrate the dusty lanes and have revealed much information about the violent nature of our galactic core.

There, lurking in the center of the Milky Way, is believed to be a large black hole with a gravitational field that is more than 100,000 times as intense as our sun’s gravity.

September sky

Sagittarius is easy to find this summer because Jupiter, with its dazzling bright light, acts a signpost hovering above the handle of the teapot. Nothing is ever as bright as Jupiter under the nighttime starry dome except for the moon and twilight-hugging Venus. Named as the king planet by both the ancient Greeks and Romans, Jupiter’s current brightness in the night sky surely makes it king of the September stellar firmament.

A view through a telescope at Jupiter will reveal that this planet is wider at its midsection than from pole to pole. Its ovoid shape comes from its rapid rotation, going once around in less than 10 hours. Telescopes will also show Jupiter’s cloud belts stretching across the planet and nearby, its four largest moons displayed as tiny “stars,” the way Galileo saw them almost 400 years ago.

In contrast to Jupiter’s brightness, there is the subdued light of three planets — Mars, Mercury, and Venus — low in the western sky competing with the glow of sunset. To facilitate viewing the three, you will need a clear sky in the west, an unobstructed horizon, and binoculars to see Mars and Mercury in the twilight. Venus’ light can be seen without aid but it is also diminished by the twilight. The planetary trio is below the horizon within an hour after sunset.

The moon starts September as a slim crescent below the three planets, which makes the moon’s location on Sept. 1 excruciatingly close to the horizon after sunset. By Sept. 8, the moon has waxed to its gibbous phase and is found over the teapot’s spout.

The next evening, the moon skims just outside the pot’s handle and to the lower left of Jupiter. Full moon is on the 15th and will be the Harvest Moon. By the 27th, the moon has waned back to a thin crescent and will be positioned in the eastern sky at dawn, near the planet Saturn.

Categories: Life and Arts

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