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Black holes are not cosmic vacuum cleaners. From a distance, a black hole pulls on things just as a star of the same mass would; only very close to it does its gravity become inescapable.

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Gravity depends on mass and distance, and a black hole is a very large mass squeezed into a tiny space. If the Sun were somehow crushed into a black hole of the same mass, it would be only about 6 km across, yet Earth would keep orbiting just as it does now; the days would simply turn dark and cold. The danger starts near the event horizon, the boundary beyond which nothing, not even light, can get back out. At the centre of our galaxy, about 26,000 light-years away, lies Sagittarius A*, a black hole of about four million Suns. Stars orbit it on steady paths, and tracking one of them, S2, was part of the work that won Reinhard Genzel and Andrea Ghez the 2020 Nobel Prize in Physics.

Türkçe oku
Kara delikler her şeyi içine çeken kozmik elektrikli süpürgeler değildir. Uzaktaki cisimleri, aynı kütledeki bir yıldız ne kadar çekiyorsa o kadar çeker; kütleçekiminden kaçmak ancak ona çok yaklaşınca imkânsız hâle gelir.

Kütleçekimi kütleye ve uzaklığa bağlıdır; kara delik ise çok büyük bir kütlenin çok küçük bir alana sıkışmış hâlidir. Güneş, kütlesi değişmeden bir kara deliğe dönüşseydi çapı yalnızca 6 km kadar olurdu; Dünya ise bugünkü yörüngesinde dönmeyi sürdürürdü. Değişen tek şey, günlerin karanlık ve soğuk olması olurdu. Tehlike, olay ufku denen sınıra yaklaşınca başlar: Bu sınırı geçen hiçbir şey, ışık bile geri dönemez. Galaksimizin merkezinde, bizden yaklaşık 26.000 ışık yılı uzakta, kütlesi Güneş'inkinin yaklaşık dört milyon katı olan Sagittarius A* adlı bir kara delik vardır. Yıldızlar onun çevresinde düzenli yörüngelerde döner. Bunlardan S2 adlı yıldızın izlenmesi, Reinhard Genzel ile Andrea Ghez'e 2020 Nobel Fizik Ödülü'nü kazandıran çalışmanın bir parçasıydı.

Sources and further reading

Background reading: Black hole
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