Looking up into the night sky, it's challenging enough for an amateur astronomer to count the number of naked-eye stars that are visible. With bigger telescopes, more stars become visible, making counting impossible because of the amount of time it would take. So how do astronomers figure out how many stars are in the universe?
The first sticky part is trying to define what "universe" means, said David Kornreich, an assistant professor at Ithaca College in New York State. He was the founder of the "Ask An Astronomer" service at Cornell University.
"I don't know [the answer] because I don't know if the universe is infinitely large or not," he said. The observable universe appears to go back in time by about 13.7 billion light-years, but beyond what we could see there could be much, much more. Some astronomers also believe that we may live in a "multiverse" where there would be other universes like ours contained in some sort of larger entity.
Even if we narrow down the definition to the "observable" universe — what we can see — estimating the number of stars within it requires knowing just how big the universe is. The first complication is that the universe itself is expanding, and the second complication is that space-time is curved.
To take a simple example, light from the objects farthest away from us would take approximately 13.7 billion light-years to travel to Earth, taking into account that the very youngest objects would be shrouded because light couldn't carry in the early universe. So the radius of the observable universe should be 13.7 billion light-years since light only has that long to reach us.
Or should it? "It's a logical way to define distance, but not how a relativist defines distance," Kornreich said. A relativist would use a device such as a meter stick, measuring the distance along that device and then extending it as long as needed.
This produces a different answer, which some sources define as 48 billion light-years in radius. Sources vary on this number, however. That's because space-time is curved. As the observer does the measurement with the meter sticks, light travels at the same time and influences the measurements.
It's easier to count stars when they are inside galaxies, since that's where they tend to cluster. To even begin to estimate the number of stars, then you would need to estimate the number of galaxies and come up with some sort of an average.
Some estimates peg the Milky Way's star mass as having 100 billion "solar masses," or 100 billion times the mass of the sun. Averaging out the types of stars within our galaxy, this would produce an answer of about 100 billion stars in the galaxy. This is subject to change, however, depending on how many stars are bigger and smaller than our own sun. Also, other estimates say the Milky Way could have 200 billion stars or more.
The number of galaxies is an astonishing number, however, as shown by some imaging experiments performed by the Hubble Space Telescope. Several times over the years, the telescope has pointed a detector at a tiny spot in the sky to count galaxies, performing the work again after the telescope was upgraded by astronauts during the shuttle era.
A 1995 exposure of a small spot in Ursa Major revealed about 3,000 faint galaxies. In 2003-4, using upgraded instruments, scientists looked at a smaller spot in the constellation Fornax and found 10,000 galaxies. An even more detailed investigation in Fornax in 2012, with even better instruments, showed about 5,500 galaxies.
Kornreich used a very rough estimate of 10 trillion galaxies in the universe. Multiplying that by the Milky Way's estimated 100 billion stars results in a large number indeed: 1,000,000,000,000,000,000,000,000 stars, or a "1" with 24 zeros after it. Kornreich emphasized that number is likely a gross underestimation, as more detailed looks at the universe will show even more galaxies.