In 1995, scientists picked out a tiny section of the night sky — the amount that would be covered by a tennis ball 100 m above you — that was unusually devoid of stars. To the naked eye, and even in a normal telescope, this region looked empty and black.
The scientists used the Hubble Telescope to take a 10—day long exposure of the empty region to find out what was out there deep in the blackness. They came back with an astonishing photo of over 10,000 galaxies, each one perhaps containing 100 billion or more stars. All in a pinpoint little square of the night sky.
Scientists used the info from this photo to postulate that the observable Universe contains over 100 billion galaxies. Today, that galaxy estimate has risen by 20 fold to 2 trillion, and it may continue to rise as we learn more. That suggests the total stars in the observable Universe to be somewhere between 10^33 and 10^25, or around 1 septillion stars.
To put that in perspective, people at the University of Hawaii spent an unreasonable amount of time calculating an estimate for the number of grains of sand on every beach in the world — 7.5 x 10^18 or 7.5 quintillion. That means that for every grain of sand on every beach on Earth, there are about 100,000 stars in the observable Universe.
Keishinrei 