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Summer and the Milky Way


darrell heath

Darrell Heath

Astronomy Columnist

On a late summer’s evening you can see a ghostly band of pale light arcing overhead from the southwest to the northeast, we call it the Milky Way. Various cultures from around the world have had their own takes as to just what that band of light is.
Many ancient peoples saw it as a river flowing across the sky and the Incans even said it’s where the god of thunder drew forth the water that falls as rain to Earth. To Native Americans it was the pathway upon which the souls of the dead traveled from this life into the next. More than one culture saw it as a stream of celestial milk. The Egyptians said that the milk was flowing from their cow deity named Bat (later called “Hathor”). In Greek mythology we learn that the milk is from the breast of Zeus’ wife Hera. According to the story Zeus had fathered the baby Hercules with a mortal woman and he tried to trick Hera into nursing the child. When Hera realized what was going on she got angry and tore the suckling infant away from her and the flowing milk became the band of light we see today.
All fine stories but what is it really?
First things first: our solar system is a family of planets and moons that orbit around the Sun and our Sun is just one of some 300 billion other stars that make up the Milky Way Galaxy. Our galaxy is one of an estimated 100 billion others, many of which are home to billions of other stars. There are several different categories of galaxy and our Milky Way belongs to the most beautiful of all, the spiral arm galaxy. This galactic variety is shaped like a flattened disk with a bright bulging center, home to a supermassive black hole. Rather than having stars scattered hither and thither we see them concentrated within majestic spiral arms. The arms emerge out of the galactic core and then fan out through the disk. Spiral arms are not like spokes in a bicycle wheel; they are actually waves that propagate throughout the disk. We don’t fully understand their nature but the waves sweep up clouds of gas and dust and form the arms. As a wave moves through an arm it squeezes these clouds of matter, which then triggers star formation. Looking at a Hubble Space Telescope image of a spiral galaxy you will see that the arms are full of hot, bright young stars, colored blue and white while gaps in between the arms contain older, red-colored stars.
Our Milky Way Galaxy is some 100,000 light years in diameter, we are located in a spiral arm about 27,000 light years out from the center: in the suburbs away from downtown galactic central where the highest concentration of stars reside. To put that into perspective let’s shrink the Galaxy down to a size we can wrap our heads around.
Let’s scale the Milky Way to some 2500 miles across, about the size of the United States. On this scale our solar system (the Sun and all the major planets) is the size of a quarter located somewhere around the Rocky Mountains. Just think about that for a moment. The Sun, which in reality is about a million miles across, is no more than a rice grain on this scale; the planets much smaller still; imagine how small you and I would be! All of the stars you see in the sky are other members of our galactic family and they are scattered like birdseed across our United States-sized galaxy.
Seeing the Milky Way in all its glory means observing it from as dark a site as possible: observing on a moonless night away from sources of man-made light pollution is best and after midnight, when it’s darkest, is ideal. During winter we face away from the Galaxy’s core, into an area with fewer stars but in summer we are looking into the core where there is an abundance of stars. The pale band you see is our galaxy viewed edge on and is the combined glow of billions of suns, too far and faint to be seen individually.
At local midnight the summer Milky Way emerges above the southwest horizon in the constellation of Sagittarius and arcs overhead through Cygnus and down to Cassiopeia in the northeast. As you look into Sagittarius you are looking in the direction of galactic central.
We can’t see the core because it is hidden behind a wall of gas and dust produced by stars gone supernova and older stars on their way out. Likewise, when you look overhead, in the constellation of Cygnus, you will see dark rifts where there appear to be no stars. This too is the result of clouds of gas and dust blocking the view beyond. But I’m not going to complain, the Milky Way is surely one of the most soul stirring sights the natural world has given us.

Darrell Heath works at the University of Arkansas at Little Rock and is a producer and host for the UALR Television show “The Night Sky.”