Ruby, 21 bytes

->(b){"*"*b.to_i(2)} 

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It goes from binary to a decimal integer, and becomes a factor by which an asterisk string is multiplied.

Ruby, 20 bytes

->(b){?**b.to_i(2)} 

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It goes from binary to a decimal integer, and becomes a factor by which an asterisk char is multiplied.

(Idea to switch to char from string was suggested by Travis. Thanks!)

Ruby, 21 bytes | (old)

->(b){"*"*b.to_i(2)} 

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Same as the upper one, but the upper one has been improved.