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Looking into deep space is also looking back in time, because light from distant stars and galaxies takes time to reach us.

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Light is fast but not instant: it covers about 300,000 km every second. We see the Moon as it was about 1.3 seconds ago, the Sun as it was about eight minutes ago, and Proxima Centauri, the nearest star beyond the Sun, as it was about 4.2 years ago. Astronomers measure such distances in light-years, the distance light travels in a year, about 9.5 trillion km. The Andromeda Galaxy, one of the most distant things visible to the naked eye, is about 2.5 million light-years away, so the light we see left it 2.5 million years ago. This delay lets telescopes study the young universe directly. In 2025 the James Webb Space Telescope photographed a galaxy called MoM-z14 as it was about 280 million years after the Big Bang. The oldest light of all, the cosmic microwave background, was released about 380,000 years after the Big Bang.

Türkçe oku
Uzayın derinliklerine bakmak, geçmişe bakmak demektir: Uzak yıldızların ve galaksilerin ışığı bize ancak uzun bir yolculuktan sonra ulaşır.

Işık hızlıdır ama anında ulaşmaz: Saniyede yaklaşık 300.000 km yol alır. Ay'ı yaklaşık 1,3 saniye, Güneş'i yaklaşık sekiz dakika, Güneş'ten sonra bize en yakın yıldız olan Proxima Centauri'yi ise yaklaşık 4,2 yıl önceki hâliyle görürüz. Gökbilimciler bu uzaklıkları ışık yılıyla ölçer: Işığın bir yılda aldığı yol, yaklaşık 9,5 trilyon km. Çıplak gözle görülebilen en uzak cisimlerden biri olan Andromeda Galaksisi yaklaşık 2,5 milyon ışık yılı uzaktadır; yani o galaksiden gördüğümüz ışık 2,5 milyon yıl önce yola çıkmıştır. Bu gecikme sayesinde teleskoplar genç evreni doğrudan inceleyebilir. James Webb Uzay Teleskobu 2025'te MoM-z14 adlı bir galaksiyi, Büyük Patlama'dan yaklaşık 280 milyon yıl sonraki hâliyle görüntüledi. Görebildiğimiz en eski ışık ise Büyük Patlama'dan yaklaşık 380.000 yıl sonra salınan kozmik mikrodalga arka plan ışımasıdır.

Sources and further reading

Background reading: Light-year
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