by D. Vogt

Created on: March 29, 2010

It is currently impossible for scientists to count how many stars are in the universe. However, the current best estimate is that there are roughly 70 sextillion stars - or 70,000,000,000,000,000,000,000. More accurate counts would require a better way to detect small red dwarfs, as well as a better understanding of what lies outside the range of what astronomers refer to as the observable universe - the area of the universe from which light has reached observers on Earth.

- Counting the Stars -

One old saying has it that counting the stars in the sky is like counting the grains of sand on a beach. In this case, it's actually harder: there are many more stars in the universe than there are grains of sand on any beach on Earth. Our own Milky Way Galaxy, which is just one average-sized galaxy in an average-sized local cluster, is home to over one hundred billion stars.

In 2003, a team at Australia's Anglo-Australian Observatory decided to make an estimate, nevertheless. According to the head of the team, Simon Driver, they used a very time-consuming but relatively simple method: count the number of stars in the galaxies found in one section of the night sky, and then multiply that to cover the total sky, and thus the total visible universe. This amounted to counting the number of stars in about ten thousand galaxies, and then extrapolating to the number of stars present in several hundred billion galaxies. The number that the Australian team came up with was 70 sextillion.

- Beyond the Australian Star Count -

This is not the end of the story, however. As Driver admitted, his team's star count is almost certainly far short of the true total. There are two key reasons why they fell short.

First, it is surprisingly difficult to actually count all of the stars in a given slice of the sky. Some galaxies are hundreds of millions or even billions of light-years away; at such distances, powerful telescopes are necessary just to identify the galaxies, let alone count their constituent stars. Even closer to our own solar system, moreover, it is very difficult to count the number of smaller, cooler stars, known as red dwarfs. Even the closest red dwarf, Proxima Centauri, can only be identified with a telescope. The number of red dwarf stars could be far higher, or far lower, than scientists currently believe.

Second, there is still some uncertainty regarding what lies beyond the range of the visible, or observable universe. In theory, sufficiently powerful telescopes could detect light from about 14 billion light-years away - in other words, from places so far away from the Earth that light had to travel for fourteen billion years just to reach us. (In comparison, it takes light from the Sun just eight minutes to reach the Earth.)

This is the theoretical limit for observation because the universe is also judged to be about 14 billion years old: hence, if there are stars more than 14 billion light-years away, the light from them couldn't possibly have reached us yet. Whether or not there are stars beyond the 14-billion-light-year point is unknown. If there are, the real count could be many, many times larger than 70 sextillion. Unfortunately, there is likely no way we will ever know this. Even if the edge of the universe is still 14 billion light-years away, the light from a star shining on that edge today wouldn't reach us for 14 billion years, long after we will be dead and the Sun itself will have exhausted its hydrogen fuel supply.

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