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The observable universe very likely holds more stars than there are grains of sand on Earth. One estimate puts all the sand on Earth's beaches and deserts at about 7.5 quintillion grains, a 75 followed by 17 zeros; estimates of the number of stars are more than a thousand times higher.

Astronomy

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Neither number can be counted. The sand figure, from researchers at the University of Hawaii, multiplies the grains in a teaspoon by a rough volume for the world's beaches and deserts. The star count multiplies the number of galaxies by the number of stars in a typical galaxy. Our own galaxy, the Milky Way, has between 100 and 400 billion stars. Deep images of the sky once suggested some two trillion galaxies; newer measurements point to a few hundred billion, which still gives somewhere between 10²² and 10²⁴ stars. On a clear, dark night the naked eye can see only a few thousand of them. Carl Sagan used the image of all the grains of sand on all the beaches of Earth in his 1980 television series Cosmos, and made the comparison famous.

Türkçe oku
Gözlemlenebilir evrendeki yıldız sayısı, büyük olasılıkla Dünya'daki kum tanelerinin sayısından fazladır. Bir tahmine göre Dünya'nın bütün kumsal ve çöllerindeki kum yaklaşık 7,5 kentilyon tanedir, yani 75'in ardından 17 sıfır. Yıldız sayısına ilişkin tahminler ise bunun bin katından da fazladır.

İki sayıyı da tek tek saymak imkânsızdır. Kum tahminini Hawaii Üniversitesi'nden araştırmacılar yaptı: Bir çay kaşığı kumdaki tane sayısını, dünyadaki kumsal ve çöllerin kabaca hesaplanan hacmiyle çarptılar. Yıldız tahmini ise galaksi sayısını, tipik bir galaksideki yıldız sayısıyla çarpar. Kendi galaksimiz Samanyolu'nda 100 ile 400 milyar arasında yıldız olduğu düşünülüyor. Gökyüzünün derin görüntüleri bir dönem 2 trilyon kadar galaksi olduğunu düşündürmüştü; daha yeni ölçümler birkaç yüz milyarı gösteriyor. Bu da yine 10²² ile 10²⁴ arasında yıldız eder. Açık ve karanlık bir gecede çıplak gözle bunların ancak birkaç binini görebiliriz. Bu karşılaştırmayı, 1980'deki Kozmos belgesel dizisinde evrendeki yıldızları Dünya'nın bütün kumsallarındaki kum taneleriyle kıyaslayan Carl Sagan ünlü yaptı.

Sources and further reading

Background reading: Observable universe
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