Look up: ‘the stars are still shining’
Summer nights are short, but for those who are patient (and sleep deprived), the late-night sky is full of celestial wonders, highlighted by our home galaxy, the Milky Way.
From a dark location, free from artificial light, our galaxy is luminous, with bright star clouds traversed by dark ribbons of intergalactic gas and dust.
While light pollution from nearby cities lessens its grandeur, the Milky Way can still be seen on clear, moonless summer nights from the New Milford area.
So, what are you seeing?
Our sun is one of hundreds of billions of stars, arranged in a massive, rotating spiral or pinwheel called the Milky Way, a name that dates back to classical antiquity.
Illuminated by starlight, it measures over 120,000 light years from tip to tip, although astronomers believe that the galaxy’s invisible reach may extend out 15 times as far.
The galaxy has a center or nucleus containing a supermassive black hole. Around the nucleus is a region densely packed with stars and dust, called the central bulge.
The central bulge is elongated in the Milky Way, forming a bar across the center of the spiral. From the bar originate several large arm-like structures of gas and dust, the star-forming regions of the galaxy, along with several minor arms and small spurs.
Our sun is located in one of these spurs, known as the Orion Arm or Orion-Cygnus Arm, about 28 thousand light years from galaxy center.
Our arm, or spur, is situated between two larger arms, Perseus (outbound) and Sagittarius (inbound). So, when we look towards the constellation Sagittarius in the summer sky, we are looking toward the center of our galaxy, although our view is hindered by regional clouds of gas and murky interstellar dust.
The arms travel in the plane of the galaxy, a disk-like region only 1,000 light years thick. At an average speed of 515,000 mph (828,000 km/h), it takes about 230 million years for our sun, and the solar system, to complete a circuit of the Milky Way, sometimes referred to as a “Galactic Year.”
To put that timescale in perspective, one Galactic Year ago, dinosaurs and small mammals were just beginning to appear following the most catastrophic loss of life in Earth’s history (more than 90 percent of marine species and 70 percent of terrestrial life died off in the Permian-Triassic mass-extinction approximately 250 million years ago).
Surrounding our galaxy is an immense halo of older stars, also rotating. The halo is home to about 150 globular clusters - densely packed spheroids populated with 100,000s of stars.
These clusters contain some of the oldest stars in the galaxy and are bright enough to be observed with binoculars and/or a small telescope.
Astronomers using NASA's Hubble Space Telescope and the European Space Agency's Gaia satellite have been analyzing the motion of these clusters as a means of measuring the total mass of the Milky Way, almost all of which (more than 90 percent) is comprised of a mysterious, invisible substance called Dark Matter (as it doesn’t emit light or appear to interact with normal matter).
The latest estimate from the Hubble/Gaia work is that our galaxy weighs about 1.5 trillion solar masses (one solar mass is the mass of our sun), including approximately 200 billion stars and one 4-million-solar-mass supermassive black hole.
In these unsettled times, take some time this summer and find a nice quiet place away from the incandescent glow of civilization and behold one of the most breathtaking sights in the sky - the Milky Way.
Our place in the galaxy only allows us to view the neighboring spiral arm, but that alone should provide some sense of the sheer enormity of our home — and the Milky Way is just one of potentially trillions of galaxies in the universe, all in motion.
Stay safe. Remember, the stars are still shinning.
Bill Cloutier is a volunteer at the John J. McCarthy Observatory in New Milford.