avatarRobert Roy Britt


How Many Stars Are Really Visible?

Millions and billions… or at least a few dozen (or maybe only a handful).

Tucked into a sleeping bag under the stars in the mountains of Northern California as a kid, I marveled at the Milky Way, a magical yet real cosmic river of fuzzy brightness punctuated by points of light both faint and bright. I’d count the sparkling gems in a patch of sky through towering pines, invariably losing track after just a few.

I haven’t seen a sky like that in decades. But I remember thinking I must’ve been looking at millions of stars.

Not even close!

There are some 100 billion to 400 billion stars in our Milky Way Galaxy. But almost all are too dim to see with the naked eye. In fact, the visible light of the vast majority of those burning balls of hydrogen and helium, obscured by interstellar gas and dust, doesn’t reach even our most powerful telescopes.

The Milky Way is quite a sight, but don’t expect it to match a long-exposure photograph like this one. Photo: Pixabay/theartofsounds

Meanwhile, with all the light pollution on this good Earth — from skyscrapers to parking lots to porch lights — most earthlings will never see anywhere near all the stars I enjoyed on those summer weekends in the ’70s. It’s been estimated that 99% of people in the United States and Europe can no longer see that milky swath of the Milky Way.

“We’ve got whole generations of people in the United States who have never seen the Milky Way,” said NOAA scientist Chris Elvidge. “It’s a big part of our connection to the cosmos — and it’s been lost.”

Light pollution is so bad it affects sleep for people in cities and suburbs around the globe.

This view of the US at night is a highly processed image from NASA that involves stitching together satellite photos taken on multiple nights (to ultimately get a cloud-free view) and then filtering out natural light from such things as the moon and fires. Credit: NASA/EOS/Earth Observatory

Counting them out

So how many stars is it possible to see? Even our ancestors, before Thomas Edison, didn’t see millions.

From any single spot on Earth, there are perhaps 4,548 stars visible without aid of a telescope or binoculars, according to a high-end estimate from Sky & Telescope that counts on perfectly dark conditions with no moon, a view of the entire sky, and excellent vision. Other experts estimate 2,000 to 2,500 to be a more realistic max.

From a typical suburb, lighting drowns out dimmer stars, and haze on the horizon masks others, leaving maybe 450 visible — and some of those may be blocked by houses, trees or hills. In more dense suburbs, and nearer to cities, you’ll find fewer.

“A person living in a suburb of Boston probably would see more stars as opposed to someone living in a suburb of New York,” says Joe Rao, a meteorologist and skywatching expert who used to write for me when I was the editor of Space.com.

From a big city, figure 35 or so. And that’s if tall buildings aren’t blocking your view. Glance upward from Times Square in New York City and you’ll be lucky to see a single star.

Astronauts can see more stars than we do, because they’re above the blurring effects of Earth’s atmosphere. Here’s a view from the Space Station by astronaut Reid Wiseman. “The Milky Way steals the show from Sahara sands that make the Earth glow orange,” he said. Photo: NASA/Reid Wiseman

The magnitude of change

Curious how much things have changed over time, a few years back I turned to Rao for some context. Joe has served as an instructor and guest lecturer at New York’s Hayden Planetarium, and he’s been studying the night sky most of his life. He remembered visiting relatives about 50 miles north of Manhattan when he was a kid, not far from where he lives now. He thought back 50 years.

“There was absolutely no sign of any sky glow or light dome from New York City” from that distant spot, he told me. “The Milky Way did not appear like a hazy band of light, but with the naked eye actually had granularity and texture.”

That means the limiting magnitude — a measure of the dimmest stars visible — was close to 6.8, he explained. On this scale, larger numbers represent dimmer stars. Smaller numbers represent brighter stars, with several of the brightest being around 1st magnitude. Sirius, the brightest star in the night sky, has a negative magnitude (-1.5). The very brightest star in our sky is, of course, the sun, which during the day renders all other stars impossible to see with the naked eye (trust me, they are still there).

The scale is logarithmic, such that a 6th-magnitude star is 100 times dimmer than a 1st-magnitude star.

Graphic: Harvard/NASA/SAO

And what can be seen now?

“On my best nights, there is a diffuse glow somewhat resembling twilight, extending up to 45 degrees high in my southern sky — caused by Westchester and the glare put out by New York City,” Rao said. He figures the limiting magnitude overhead is about 4.8 now.

“That means that over the last half-century the sky brightness has increased so that I can no longer see stars that are six-times dimmer,” than what was visible back then, he said. “I can still see the Milky Way on summer and fall nights,” but as the Milky Way sinks closer to the southern horizon, it’s lost in “the murk,” he said.

(The International Dark Sky Association, which works to protect the night sky, offers extensive analysis on light pollution, along with suggestions for how governments and individuals can make change.)

The left photo was taken from Leamington, UT a few years ago when the population was about 217. The right image was taken from Orem, UT at the same time, when the population was around 400,000. The photographer, Jeremy Stanley, was aiming to show the difference light pollution makes. Credit: Via Wikipedia, CC by 2.0.

Treat yourself

If you’ve never seen the Milky Way, do try and find a dark place out in the country on some summer night and be amazed. Depending on where you live, you may need to trek deep into the mountains (on a moonless night, of course). National parks offer fair to very dark skies. The International Dark Sky Association has a list of places to consider.

Some urbanites and suburbanites can get a glimpse by simply driving out of town and finding a lonely road in the middle of nowhere. If you live near a city, the distance you’ll need to travel will vary by the size of the city. In the mountains about a 30-minute drive north of Phoenix, for example, the sky is impressive, even as the glow from the metro area drowns out dimmer stars to the south. Even way out there I enjoy maybe 70% of the detail I remember seeing as a kid.

On a side note: Wherever you’re able to find the darkest of skies, don’t expect to see a lot of colorful detail in the Milky Way like the photo at the top of this article. Photos like that involve time exposures and sensitive film or electronics to capture what the naked eye cannot see. But the Milky Way, or even a less-than-perfect star-filled sky, is still an amazing sight to the naked eye.

More Aha! Moments >>>

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