Astro Bob: May flowers herald return of summer Milky Way
About the time you're thinking of going to bed, something grand is rising in the eastern sky — a ribbon of more than 100 billion stars.
There's always something to love about the sky no matter the season. And there are lots of seasons when it comes to stellar sights, not just the familiar quartet.
Every May, the luminous band of the Milky Way breaks over the eastern horizon at the same time spring wildflowers cleave the dirt and strain toward the sun. If you have a clear view to the east, you'll see a fluffy, star-studded "haze" along the bottom of the sky beginning around 10:30-11 p.m. local time. This month marks the start of the Milky Way's easy visibility during evening hours.
Vega, the brightest star in Lyra the Lyre and fifth brightest in the night sky, might catch your eye first when you face that direction. If so, let it lead you to the band, located 10° or one fist held at arm's length directly below it. Like a banner unfurled, the Milky Way reaches across the entire eastern sky from north to south. Take a moment to appreciate what you're seeing. No ordinary mist, stars and stardust create this vaporous glow.
With the naked eye you'll spot dozens of twinkly pinpoints along its length, but to get a better idea of the ribbon's starry richness, use binoculars or a telescope. I guarantee you'll be wowed. How many stars comprise our galaxy? No one knows the exact number, but it's estimated between 100 billion and 400 billion. If you could count stars at one a second it would take almost 32 years to reach a billion and 3,200 years to count to 100 billion. That's how many stars there are.
Like the solar system and numerous other galaxies, the Milky Way galaxy is fairly flat — it resembles a Frisbee viewed from the side. The sun, with its retinue of planets, orbits within the plane of this galactic Frisbee. As we look through the plane, the stars pile up over many light-years to form a dense band of softly glowing starlight . It looks hazy because most of the suns are too far and too faint to see separately. For that you need a telescope.
Our location in a flat galaxy is the reason we see a band and not stars spread evenly across the night sky. Now, if we gaze above or below the galactic plane the stars quickly thin out until we're we're looking into nearly empty space (see diagram below). Stars still hover there but far fewer. This sudden decrease further enhances and defines the ribbon-like appearance of the Milky Way band.
Across the year, we face different directions in space as the Earth orbits the sun. This allows us to see about two-thirds of the total Milky Way. There's always a part that remains invisible because it's cut off by the local horizon. Northerners can't see the far southern Milky Way, and southerners can't see the northern Milky Way. This time of year, from mid-northern latitudes, the Milky Way is visible from Cassiopeia (in the northeastern sky) to Sagittarius in the deep southern sky.
Of course, it doesn't end there, but plunges below the southern horizon through unfamiliar constellations like Norma, Centaurus, Crux and Carina before joining up with better-known groups like Puppis, Canis Major and Orion.
From Orion, the Milky Way continues north through Gemini, Auriga and Perseus before returning to where we started in Cassiopeia. Yes, a circle! The band of the Milky Way describes a circle — OK, more like a fat wreath — in the sky. And the only way to see the whole thing at once would be to leave the Earth and your spacecraft, too — safely tethered, of course!
As you twisted around to take in the view, you'd be able to trace the band for 360 degrees. For a more practical option, put a trip to the opposite hemisphere on your bucket list.
The brightest, widest and starriest section of that circle would be in the direction of Sagittarius, the location of the galactic center. Turning away from Sagittarius in either direction, your gaze would cross the star-and-nebula-rich spiral arms, marked by Cygnus and Carina.
With the center at your back you'd face the Milky Way's anticenter, located directly opposite Sagittarius in the constellation Auriga. In this direction, stars rapidly thin out and give way to the near-emptiness of intergalactic space. That's why the winter Milky Way (centered on Auriga) appears fainter than the summer one, where we face the galaxy's dense central region.
All of this is yours to enjoy without interference from the moon for the next two weeks. Why not make a drive to the country the next clear night? Stay up and see where you live in the context of something bigger — the universe.
"Astro" Bob King is a freelance writer for the Duluth News Tribune.