Nebulous space image of Messier 78

Minnesota Skies: May 2021

What can you find in the May night sky? How many stars are there? Answer these questions and more in our latest blog post!

Published04/29/2021 , by Thaddeus LaCoursiere

How many stars are there?

The answer to this simple question will take you on a whirlwind journey across space and time in our spring sky.

This is a simple enough question to ask and seems like it should be easy enough to answer. After all, if you walk outside on a clear night and spend enough time looking up, you can make an accurate count of everything you can see. Professional and amateur astronomers have done just that and come up with about 9,000 stars total that you could see from dark, non-light polluted skies, across the Earth. Yet there are more stars than just the ones you can see!

Look low to the east after sunset to spot the faint outer arms of the Milkyway. Although this part of our galaxy is not as prominent as the bright central core that will be visible later in the summer, taking a telescope and scanning along the galaxy will reveal that the haze our unaided eyes can see contains tens of thousands of stars, with an estimated 250 billion stars in total composing our galaxy. But wait, there are more galaxies than our own!

Looking high to the zenith, you’ll find the Big Dipper with its bucket upside down pouring water all over the Earth below. Trace out the handle of the Dipper and spot the star at the end, Alkaid. With a large pair of binoculars or a small telescope, look a few finger widths above Alkaid to find Messier 51, or the Whirlpool Galaxy. This grand-design spiral galaxy is a close neighbor to our own, a mere 31 million light-years away. It’s easy enough to see though because of the bright spiral arms, glowing with the light of over 100 billion stars! New stars are forming as you read this—spurred by the gravitational interaction between M51 and its companion galaxy M51b, visible as a small (just a few hundred million stars) bright object next to the larger galaxy. From just the three galaxies mentioned so far, we can see that there are quite a few more stars to count than we first thought! So our question becomes—how many galaxies are there?

Head back to the handle of the Big Dipper. Extend the curve of the handle downward, in a large arc a little longer than the distance of your hand, toward the next brightest star you can see: Arcturus.  Arcturus is part of the constellation of Boötes—the farmer, herdsman, or ice cream cone in the sky. The triangle of the ice cream cone will point you down another hand length in a straight line towards Spica, a bright star visible just another hand length above the horizon.

Spica is part of the constellation Virgo, the maiden, but if you connect the dots you might find a different shape, with the outstretched arms also looking like a snail’s antenna. Arms are more apropos, as held in between them you can find a treasure trove of galaxies. This is the Virgo Cluster, containing more than 2,000 galaxies, enough for a lifetime of observing! With this many galaxies, it’s hard to choose just one to focus on. If you have limited time, start with Messier 87.  With one trillion stars composing it, this supergiant galaxy is incredible to see through binoculars or a telescope.

  • Messier 51 (The Whirlpool Galaxy)
    Credits: NASA, ESA, S. Beckwith (STScI) and the Hubble Heritage Team (STScI/AURA)

  • Messier 87
    Credits: NASA, ESA and the Hubble Heritage Team (STScI/AURA)

  • NGC 4038-4039 (The Antennae Galaxies)
    Credits: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration

Our stellar count has jumped and we’re not done quite yet!

Nearby Spica, about a hand width to the southwest, is a bright “sail” of four stars, making up the constellation of Corvus, the Crow.  Looking a smidge to the west of the top right star—Gienah Ghurab—a medium-size telescope can resolve a fuzzy heart-shaped object. This is NGC 4038, or the Antennae Galaxies. The heart shape has been resolved by the Hubble Space Telescope into not one, but two galaxies, in the middle of a galactic collision! These two galaxies first started interacting a few hundred million years ago and now, millions of years later, we’re seeing the middle stage of this cosmic crash. The merger of these two galaxies is easy for us to spot—even at over 50 million light-years away—because as the gas and dust collide, regions collapse together into super star clusters, giving birth to billions of new stars. Looking at images from the Hubble Space Telescope, astronomers are able to estimate that even just the small bright spots are young clusters containing tens of thousands of stars.

As we observe the universe with better and better equipment we’ve found that galactic interactions are incredibly common. If you stick around long enough (just five billion years, give or take a few hundred million), the Milkyway and Andromeda galaxies will merge into a new super galaxy, jump-starting a new reign of star formation all around us. That’s not surprising, as we’ve also found that galaxies are incredibly common. Long-term deep sky surveys like the 2dF Galaxy Redshift Survey or the Two Micron All Sky Survey have enabled us to gain an unprecedented understanding of what is out there. The most detailed survey has been going on for almost 20 years with the Sloan Digital Sky Survey at Apache Point Observatory in New Mexico. This cosmic program uses a 2.5-meter telescope that has mapped over a third of the entire sky, finding over 1 billion galaxies, and allowing scientists to create the most detailed three-dimensional maps of the universe ever made. With SDSS data and other observations combined, our observable universe is estimated to contain upwards of two trillion galaxies. Each of these galaxies is a little bit different, but based on those observations, we can say that, on average, a galaxy has 100 billion stars in it.

So how many stars are there?  Multiple 2 trillion galaxies by 100 billion stars per galaxy… 2,000,000,000,000 x 100,000,000,000 = 200,000,000,000,000,000,000,000 stars!  (That’s 2e23, or 2 with 23 zeros after it!)

The number of stars is almost unimaginable, but for some comparison, the total number of grains of sand on the Earth is estimated to be 7,500,000,000,000,000,000 or 7.5 with 18 zeros after it. With rough estimates, there are more stars in the universe than grains of sand on the Earth. On the other hand, if you’re now feeling a little lost on the beach, contemplating the grand infinity of the universe, take a moment and drink some fresh water, and think—just a teaspoon of that water has the same number of molecules as there are stars in the sky.