4.3 mil­lion so­lar masses: the weight of the black hole in the mid­dle of our Milky Way

Black holes are the most pow­er­ful preda­tors in the uni­verse, lurk­ing in the dark­ness of space for bil­lions of years. Nat­u­ral laws don’t ap­ply to them. Any­thing that gets too close to a black hole dis­ap­pears for­ever – be it an as­teroid, planet or even a su

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hen a star sud­denly­sudde seems in a hurry, it’s down to one of two rea­sons: first, it got too close to the ex­plo­sion of a neigh­bour­ing star and was vi­o­lently pushed away by the force of the im­pact. Barnard’s Star, a low-mass red dwarf, reached a record speed of over 500,000 kilo­me­tres per hour as a re­sult of this process.

Sec­ond, it got too close to the great­est preda­tor in the uni­verse – a black hole. These gi­gan­tic star-eaters lurk unseen in the deep­est dark­ness at the cen­tre of the uni­verse for bil­lions of years and mer­ci­lessly swal­low what­ever gets too close to them. This leads to the dra­matic end of a star’s life, with no chance of es­cape – even if it’s rac­ing along at no less than 1.4 mil­lion kilo­me­tres per hour. But where do the mon­strous top dogs of the uni­verse find the strength to tear en­tire so­lar sys­tems to pieces?


Black holes are true se­rial killers – they are al­ways de­vour­ing some­thing. Let’s take the mon­ster black hole at the cen­tre of the Milky Way as an ex­am­ple. Sagit­tar­ius A* is mas­sive, weigh­ing 4.3 mil­lion times the mass of our sun. And it has a huge ap­petite. It reg­u­larly gorges on a star – be­fore in­dulging in plan­ets, as­ter­oids and gas clouds for dessert. With an age of at least 11.4 bil­lion

years, Sagit­tar­ius A* has prob­a­bly gulped down hun­dreds of thou­sands of stars in its life­time.


In or­der to mur­der a star, a black hole sim­ply has to stay still – and wait. As soon as some­thing ap­proaches it, the ob­ject will be at­tracted to its unimag­in­ably large force of grav­ity – and swal­lowed in one gulp. An es­cape at­tempt is fu­tile. The max­i­mum speed in the uni­verse is lim­ited by the speed of light – 299,792,458 me­tres per sec­ond – and at a cer­tain dis­tance from the black hole, this is no longer fast enough to es­cape its jaws. This ef­fect al­lows the galac­tic can­ni­bals to ap­pear black, even though they’re ac­tu­ally full of light.


How­ever, there’s no need to panic – Earth doesn’t lie in the hunt­ing grounds of a black hole. The near­est star-eater, A0620-00 in the con­stel­la­tion of Mono­ceros, lurks at a dis­tance of around 3,500 light years – roughly 3,500 times 9.5 tril­lion kilo­me­tres – which is much too far for it to be a threat to us. But ex­actly how close would the Earth have to get to a black hole for it to be able to snap us up? The fig­ure varies be­cause it’s dif­fer­ent for ev­ery black hole – and de­pends on the den­sity of its mass and, there­fore, its grav­i­ta­tional force.

The most dense – and, con­se­quently, the small­est – known black hole is XTE J1650-500. Lo­cated in the Ara con­stel­la­tion, it is around 15,000 light years away from Earth, has a di­am­e­ter of just 24 kilo­me­tres and a static 3.8 so­lar masses. The den­sity of XTE J1650-500 is about one quin­til­lion kilo­grams per cu­bic me­tre. As a com­par­i­son, the av­er­age hu­man has a den­sity of around 1.06 grams per cu­bic cen­time­tre, while a tea­spoon of mat­ter from XTE J1650-500 weighs around 1,000 times as much as the Great Pyra­mid of Giza. Don’t get too close.

Hy­dro­gen clouds con­dense into a com­pact ball of gas and a star is born. The star be­gins to fuse hy­dro­gen with he­lium – and pro­duces en­ergy in the process. Stars, such as the sun, lose their outer lay­ers at the end of their lives due to the force of a su­per­nova ex­plo­sion.

If the star ex­ceeds the limit of 1.44 so­lar masses, its burn­ing pro­cesses change. The grow­ing giant be­gins to fuse he­lium and then car­bon.

Sun Neu­tron star White dwarf Black hole

etres kilom on milli 25

In the end, a white dwarf is all that re­mains of our sun. The star ex­pands and fuses heavy el­e­ments like neon and oxy­gen to meet its own en­ergy needs. Iron is the last el­e­ment to form dur­ing the nu­clear fu­sion. The core then col­lapses and un­leashes a su­per­nova. Neu­tron stars form when the dy­ing star has eight to 20 so­lar masses. Stars with more than 20 so­lar masses turn into a black hole.

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