How Many Stars Are in the Universe?- The Surprising Number of Burning Masses

If you’ve ever stared into the night sky, it’s hard not to wonder how many stars are in the universe. Astronomers, musicians, poets, and children have pondered the same question for as long as humans have inhabited the earth. And even though we don’t know the exact number of stars in the universe, science gives us the basis for making good predictions.

How Many Grains of Sand Are There?

Counting the stars in the universe is similar to counting individual grains of sand on a beach. First, you can estimate grains by measuring the beach’s surface area. Next, determine the average depth of the sand layer. Then count how many grains are in a small representative amount of sand. Finally, multiply to estimate the total number of sand grains covering the beach.

Of course, while you are counting, the ocean’s waves have pulled sand from some regions and deposited more sand on other sections of the beach. And as visitors, we take sand away on the bottoms of our feet, shoes, clothes, and even in our hair. So getting a correct sand grain count proves extremely difficult. (Does it ever seem like you brought the entire beach home and redeposited it in your shower?)

Counting stars is similar to counting grains of sand. You can get a reasonable estimate, but you might never know the exact number of stars in the universe.

How Many Stars Are in a Galaxy?

Astronomers and scientists use galaxies to represent the “volume of sand” when counting stars. And since you can’t realistically count stars individually, researchers estimate the number of galaxies and their luminosity.

Scientists estimate the Milk Way Galaxy has 100 to 400 billion stars. But the difficulty in calculating how many stars there are lies partly in their size differences. Where grains of sand are similar in size, stars range drastically in their type and mass.

To calculate how many stars are in the universe, researchers estimate the galaxy’s mass and the percentage of mass coming from stars. However, deciding on the average star mass creates issues.

For example, if you calculate the number of stars using the size of the Sun, you’ll get a lower number than if you calculate using stars the size of brown dwarfs.

How Many Stars Are in the Universe?
Image: NASA

How Many Galaxies Are There?

There are a similar number of galaxies as stars in our universe. So, somewhere between hundreds of billions to trillions of galaxies exist.

So using the same simple calculations as with the numbers of grains of sand, there are potentially quadrillions of stars in the universe. That’s a complex number to comprehend! 

Written out, it looks like this: 1,000,000,000,000,000,000,000,000. So, yes, that’s 24 zeros behind the one. 

Of course, just like some areas on a beach have a deeper layer of sand and thus more grains, some galaxies have more stars while others have fewer. In the image below, you can see approximately 15,000 galaxies, each containing billions of stars.

And this image only contains one portion of the night sky. So you can immediately understand the challenges in counting exactly how many stars are in the universe. 

Stars in the galaxy
Image: NASA/ESA/M.Montes/P.Oesch

The primary way astronomers estimate how many stars exist in a galaxy is by determining the galaxy’s mass. First, they determine the mass by analyzing the galaxy’s light spectrum and rotation. Then, scientists try to figure out how much of the universe contains stars. 

While there is no single answer, astronomers know a few things about what comprises a galaxy.

  • Most of the mass (~90%) is dark matter which emits no light.
  • ~7% is made of cosmic dust and diffuse gas.
  • ~3% of a galaxy is stars in all sizes.

Herschel and Gaia: Star Counting Space Missions

The European Space Agency (ESA) has a couple of galaxy and star-counting missions in place. For instance, the space observatory Herschel used infrared to measure a galaxy’s luminosity. This new way of looking at stars lets Herschel count the number of galaxies during its mission.

Herschel Mission

The Herschel space-based telescope launched in 2009 and ran out of liquid coolant in 2013. So, even though it is now decommissioned, the telescope and infrared detectors confirmed the first finding of oxygen molecules in the Orion stellar nursery. 

Herschel also charted how quickly stars form throughout history. Estimating the formation rate helps you calculate how many stars are in the universe. Astronomers are still combing through the data that Hershel collected.

Gaia Mission

Gaia is ESA’s “billion-star surveyor,” and it’s been mapping stars since 2013. To date, Gaia has mapped 1.7 billion stars near the Sun. But even this dedicated star counter can’t see some of the faintest stars hidden behind cosmic dust clouds.

Gaia follows the Hipparchus mission that mapped about 100,000 stars with high precision and more than a million stars in total. Now Gaia targets those stars and new findings 70 times each during its mission. 

This space telescope creates three-dimensional star charts. And the data produced from mapping a thousand million stars will give scientists new views into how our galaxy formed and continues evolving. Gaia maps star luminosity, temperature, motion, and composition.

Gaia Mission
Image: ESA, NASA, K.Sahu, and W. Clarkson

You Can Help Count How Many Stars Are in the Universe

Would you like to count stars while acting as a citizen scientist? Gaia Vari is an ESA-funded program that invites citizens to contribute their time and skills to help classify Gaia’s dataset.

Astronomers need citizen assistance to browse data and create a variable object catalog. The automated classification system isn’t perfect. So the scientists know that some sources weren’t classified correctly. And they need human eyes to help with the classification.

Some discoveries are only possible with the human eye, no matter how elaborate the computer programs are. So Gaia Vari participants help discover objects hidden in data that might otherwise go unfound.

Gaia uses three ways of observing the universe.

  • astrometry (positioning and distance)
  • photometry (brightness and color)
  • spectroscopy (radial velocity and chemical composition)
European Space Agency Image
Image: European Space Agency

Gaia has mapped about 10 million variable objects (stars) whose brightness changes over time. Scientists believe that variability is an essential tool for studying astrophysics. For example, the changes in brightness help explain how stars form. They also aid in detecting exoplanets, invisible matter, and galaxy structures.

Citizen scientists in the Gaia Vari program help classify these variable objects based on how their brightness changes over time, the change periods, and the colors associated with the variables.

Final Thoughts: How Many Stars Are in the Universe?

Even though we don’t know the exact number of stars in the universe, scientists and astronomers continually make new findings that challenge what we think we know. For example, today’s space telescopes provide three-dimensional and infrared universe mapping.

The precise count of stars in the universe is difficult to determine, given its vastness and our limited observational capabilities, even with the latest technologies.

However, astronomers estimate that there are roughly 100 billion to 400 billion stars in our Milky Way galaxy alone. Considering that there are billions of galaxies in the observable universe, each containing billions to trillions of stars, the total number of stars is mind-bogglingly enormous. 

Some estimates put the count as high as 10^24 stars in the observable universe. Yet, it’s important to realize that this is still an estimation, and the actual number could be even higher. The sheer magnitude of stars in the universe underscores the vastness and complexity of our cosmic surroundings, leaving us in awe of the vast expanse that stretches beyond our comprehension.