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This week’s new questions

How can crocodiles retain muscle strength when they are still for long periods? And how can an infinite universe have come from a finite original state?

Saltwater crocodile (Crocodylus porosus) swimming at water surface, split-level, New Guinea, Indo-pacific

How do crocodiles maintain their explosive power despite being motionless for long periods, while if I am sedentary, my muscles weaken?

Sarah Shuff, London, UK

The star-forming nebula W51 is one of the largest "star factories" in the Milky Way galaxy. "Star factories" like this one can operate for millions of years. The cavernous red region on the right side of W51 is older, evident in the way it has already been carved out by winds from generations of massive stars (those at least 10 times the mass of our Sun). The dust and gas in the region are swept around even more when those stars die and explode as supernovas. On the nebula's younger left side, many stars are just beginning to clear away the gas and dust.

The universe could be infinite in size, but how is it logically possible to attain an infinite size from a finite initial state?

Amir Malik, London, UK

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