When we look up to the night sky it seems impossible to count the number stars visible to the naked eye even with a simple amateur telescope. And as telescopes go up in power, more and more stars are visible, only increasing the challenge. Is it possible for astronomers to figure out how many stars there are? If so, how?
What you might not expect is that it can be hard to define our universe in the first place. David Kornreich, an assistant professor at Ithaca College, New York State, and founder of Cornell University’s “Ask An Astronomer” service, says “I don’t know [the answer] because I don’t know if the universe is infinitely large or not.”
From what we can observe, it seems the universe goes back about 13.7 billion light years, but there could be vast amounts more. Some astronomers even think that we exist in a ‘multiverse’ where many similar universes cohabitate inside an even larger object.
A straightforward answer would involve estimating the number of stars in the average galaxy then multiply that by the supposed number of galaxies in the single, most comprehensive understanding of our universe. But even that is not as simple as it appears since some galaxies are easily seen, but others shine brightest in infrared. Plus, estimation comes with its own set of difficulties.
Using data gathered from the deep-field images delivered by the Hubble Telescope, an article in the October 2016 edition of Science posited that there could be as many as 2 trillion galaxies in our universe. That’s 10 times more than previously estimated. In an email to Live Science, Christopher Conselice, a professor of astrophysics at the University of Nottingham and the lead author of the article, suggested there are nearly 100 million stars in most galaxies.
But even the best telescopes may not see every star. In 2008, the Sloan Digital Sky Survey estimated that they found 48 million stars from their catalog of objects in only one-third of the sky. That’s about half the number that astronomer’s expected but when even stars similar to our own sun wouldn’t show up in this type of catalog, many astronomers prefer to use mass as an indicator of the number of stars in a galaxy. Even still, this presents it’s own unique difficulties when things like dark matter and galactic rotation have to be removed before calculations.
Some hope the Gaia mission, by the European Space Agency, and others like it, will provide more answers. The Gaia probe’s goal is to map the approximately 1 billion stars in our own Milky Way galaxy. It’s building on the work of a previous probe, the Hipparchus, which exactly mapped 100, 000 stars and only loosely located the full 1 million.
On their website, the ESA states:“Gaia will monitor each of its 1 billion target stars 70 times during a five-year period, precisely charting their positions, distances, movements, and changes in brightness. Combined, these measurements will build an unprecedented picture of the structure and evolution of our galaxy. Thanks to missions like these, we are one step closer to providing a more reliable estimate to that question asked so often: ‘How many stars are there in the universe?'”
Even with the narrowed definition of an observable universe, to estimate the number of stars, we must know the size of that universe. A knowledge difficult to uncover because space-time is curved and the universe is constantly expanding.
A deceivingly simple example and accounting for the fact that light from younger objects couldn’t travel in the early universe, the light given off by objects furthest away from Earth would take about 13.7 billion years to get here. Therefore the observable universe should have a radius of 13.7 billion light-years.
But does it really? Kornreich explains why this would cause some disagreement. “It’s a logical way to define distance, but not how a relativist defines distance,” he said. A relativist would measure distance along a device, like a yard stick.
Measuring the radius of the universe in this way would produce a different result and is sometimes said to be 48 billion light-years instead. This is where the curvature of space-time becomes an issue. While an observer measures with his ruler, the light continues to travel and changes the measurements even as he’s measuring it.
The easiest way to count stars is when they exist inside a galaxy as they have a tendency to cluster. To start this task, you’d require some sort of guess to the Galaxy amounts and average their populations.
Using the Milky Way as an example, some astronomers guess we have 100 billion ‘solar masses’ that is mass equal to 100 billion suns. Supposedly this would, on average, mean we have 100 billion stars in the Milky Way. However, as some stars are larger and others are smaller than the sun, this number could be different, meaning we could have 200 billion stars or more in our galaxy.
If that discrepancy between numbers is surprising, the discrepancies in our galaxy counts could be higher according to the images collected from the Hubble, which improve, changing the number of observable galaxies with each upgrade. With the latest updates in 2012, an area in the constellation Fornax was shown to have 5, 500 galaxies but in 2004 it showed 10, 000 galaxies, while a similar spot in Ursa Minor only had 3,000 in 1995.
Still, using an extremely rough guess of about 10 trillion galaxies in our universe, Kornreich multiplied that with the estimated number of stars from the Milky Way and came up with an absolutely huge number of possible stars: 1,000,000,000,000,000,000,000,000. That’s a “1” followed by 24 zeroes. He stressed that even a number that large is probably an underestimate with Kornreich suggesting there are even more stars and galaxies that will appear as we have more and more detailed glimpses of the universe.
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