when a star runs out of the hydrogen and helium that power it, they swell up really big. if our sun did this, it’d swallow up mercury and venus (but it won’t for ages and ages). the star collapses then and smashes back in to itself because of gravity. most of the time this makes a little star that stops shining because it’s run out of fuel. sometimes though, if the star was really big, it collapses and it’s so heavy that even light can’t escape it’s gravity, which is why they look black.
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there’s a black hole at the middle of the milky way. it’s the same size as the sun but weighs the same as 4 million suns, which is why its gravity keeps the entire galaxy in place!
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