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The Sun shines because hydrogen nuclei fuse into helium in its core, where the temperature is about 15 million °C. Every second it turns around 600 million tonnes of hydrogen into helium, and about 4 million tonnes of that mass is converted into energy.

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Each helium nucleus is built from four hydrogen nuclei, yet it has about 0.7 per cent less mass than the four had. Einstein's equation E = mc² explains where the missing mass goes: energy equals mass multiplied by the speed of light squared, so even a tiny amount of mass becomes an enormous amount of energy. The British astronomer Arthur Eddington suggested in 1920 that this kind of reaction powers the Sun, and in 1938 and 1939 the German-born American physicist Hans Bethe worked out the chains of reactions, work that earned him the 1967 Nobel Prize in Physics. The energy then takes a long time to escape through the Sun's dense interior, with estimates ranging from 10,000 to 170,000 years. Once it leaves the surface, sunlight reaches Earth in about 8 minutes and 20 seconds.

Türkçe oku
Güneş, merkezinde hidrojen çekirdekleri kaynaşıp helyuma dönüştüğü için parlar. Sıcaklığı yaklaşık 15 milyon °C olan bu merkezde her saniye 600 milyon ton kadar hidrojen helyum olur; bunun yaklaşık 4 milyon tonluk kısmı enerjiye çevrilir.

Her helyum çekirdeği dört hidrojen çekirdeğinden oluşur, ama kütlesi o dördünün toplamından yaklaşık yüzde 0,7 daha azdır. Kaybolan kütlenin nereye gittiğini Einstein'ın E = mc² denklemi açıklar: Enerji, kütle ile ışık hızının karesinin çarpımına eşittir; bu yüzden çok küçük bir kütle bile muazzam bir enerjiye dönüşür. İngiliz gökbilimci Arthur Eddington 1920'de Güneş'in enerjisini bu tür tepkimelerden aldığını ileri sürdü. Alman asıllı Amerikalı fizikçi Hans Bethe de 1938–1939'da tepkime zincirlerini ayrıntısıyla hesapladı ve bu çalışmasıyla 1967 Nobel Fizik Ödülü'nü aldı. Açığa çıkan enerjinin Güneş'in yoğun iç katmanlarından yüzeye ulaşması uzun sürer; tahminler 10.000 ile 170.000 yıl arasında değişir. Yüzeyden ayrılan ışık ise Dünya'ya yaklaşık 8 dakika 20 saniyede ulaşır.

Sources and further reading

Background reading: Nuclear fusion
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Next card in the archiveHydrogen nuclei in the Sun's core are all positively charged, so they repel each other, and by the rules of classical physics they almost never have enough energy to get close enough to fuse. Quantum tunnelling lets them slip through this barrier of repulsion now and then, and that is enough to keep the Sun shining.Read next →