Number Of Stars In The Sky

How many stars are there in the universe?

Tina S
Tina S
Jan 27, 2010
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Look into the sky on a clear night, out of the glare of streetlights, and you will see a few thousand individual stars with your naked eyes. With even a modest amateur telescope, millions more will come into view.

Stars are not scattered randomly through space, they are gathered together into vast groups known as galaxies. The Sun belongs to a galaxy called the Milky Way. Astronomers estimate there are about 100 thousand million stars in the Milky Way alone. Outside that, there are millions upon millions of other galaxies also! 

It has been said that counting the stars in the Universe is like trying to count the number of sand grains on a beach on Earth. We might do that by measuring the surface area of the beach, and determining the average depth of the sand layer. Discussing the number of stars in the Milky Way alone, astronomer William Keel, writing for the sci.astro Galaxies FAQ, claims that there are "about as many as the number of hamburgers sold by McDonald's."

One of the very first things that astronomers studied was the number of stars in the sky. From this, they hoped to get a mathematical picture of the shape and extent of the entire Milky Way galaxy.

About 3,000 stars are visible to the naked eye. With the use of better telescopes, more stars are visible than ever before. It is estimated that the 510-cm Mt.

There are probably about 400 billion stars in the Milky Way, although "a 50% error either way is quite plausible. As for the number of galaxies in the universe, well that's a whole separate mathematical puzzle.

Other star enumerators we located on the Web offer numbers ranging from more than 200 billion stars in our galaxy to 3 thousand million billion stars (3 followed by 16 zeroes), in the universe. NASA alleges there are zillions of uncountable stars.

Some of the stars that we can see look much brighter than others. Scientists call the brightness of a star its magnitude. A star with a magnitude of one is twice as bright as a star with a magnitude of two, a magnitude-two star is twice as bright as a magnitude-three star, and so on.

Stars with a magnitude of more than six can be seen only with a telescope. But telescopes can see stars with a magnitude of 21! A magnitude-one star is more than a million times as bright as a magnitude-21 star!

Over the decades, 'star count' sophisticated models have been created, and rendered into approximate mathematical functions that let us explore what we see in the sky.

For the Universe, the galaxies are our small representative volumes, and there are something like 101^1 to 10^12 stars in our galaxy, and there are perhaps something like 10^11 or 10^12 galaxies.

With this simple calculation you get something like 10^22 to 10^24 stars in the Universe. This is only a rough number, as obviously not all galaxies are the same, just like on a beach the depth of sand will not be the same in different places.

No one would try to count stars individually, instead we measure integrated quantities like the number and luminosity of galaxies. ESA's infrared space observatory Herschel, to be launched in 2007, will make an important contribution by 'counting' galaxies in the infrared, and measuring their luminosity in this range - something never before attempted.

Knowing how fast stars form can bring more certainty to calculations. Herschel[1] will also chart the 'formation rate' of stars throughout cosmic history. If you can estimate the rate at which stars have formed, you will be able to estimate how many stars there are in the Universe today.

Herschel is designed to view exactly the time in the evolution of the Universe, at the right wavelengths where it is thought the majority of the obscured star formation can be seen.

[1]Sir Frederick William Herschel,[1] KH, FRS, German: Friedrich Wilhelm Herschel (15 November 1738 – 25 August 1822) was a Hanoverian astronomer, technical expert, and a composer.

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