Astro Bob: May flowers herald return of summer Milky Way

Milky Way Cygnus to Sagittarius June 12_2020_S.jpg — The bright star Vega in the constellation Lyra the Lyre precedes the band of the Milky Way as the two climb the eastern sky during late evening hours in mid-May. The late spring-summer portion of the Milky Way is much brighter and easier to see than the winter version. Viewed from the countryside it resembles a bank of puffy, star-studded clouds.

By Bob King

May 19, 2022 at 4:26 PM

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.

Milky Way in May — I've labeled several of the more prominent constellations and star in and near the Milky Way band. Cygnus is also called the Northern Cross. Notice that a dark lane cuts the bright Milky Way nearly in half. The swath, called the Great Rift, is caused by countless clouds of interstellar dust located between us and the distant background stars. The dust blocks their light the same way a cloud moving in front of the moon blocks its light. If the dust were removed, the Milky Way would shine more brilliantly.

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.

Milky Way core — Hubble reveals the dense, starry heart of our Milky Way galaxy to reveal more than a half million stars at its core. All told, the galaxy holds more than 100 billion suns.

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.

Milky Way in full — The composite of the entire Milky Way Galaxy shows just how flat our galaxy is. Stars are concentrated in a central bulge some 27,000 light-years away and in the flat plane. The sun and planets are located within that plane. When we look through it — no matter the direction — stars near and far line up to create the band of starlight we call the Milky Way. The galaxy is about 105,000 light-years across but just 10,000 light-years thick.

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.

Milky Way seasons — If we could see the Milky Way face-on, its spiral arms would be stunning (left). In summer, we face toward the center, located in Sagittarius, the reason the summer Milky Way appears bright and broad. In winter, we face out toward the galaxy's edge. When we look look above or below the plane (right), the stars thin out rapidly and give way to the modest sprinkling of suns we see dotting the sky away from 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.

Milky Way full circle — With no horizon in the way we would see the entire band of the Milky Way — a huge circle — that would loop around us from one end of the heavens to the other. This limited, 2D illustration shows it as a circle in the distance, but from a true outer space perspective, you'd see it wrapping around you across your entire field of vision.

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: 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.