How to align string after variable text length

Question

I'm looking to add padding after text to align a column of strings:

example output:

a:        ***
aa:       ***
aaa:      ***

I've tried adding adding a left and right adjust to format, but it seems to make the adjustment after the :

s = 'a: {:>10}\n' \
    'aa: {:>10}\n' \
    'aaa: {:>10}'
print(s.format('***', '***', '***')) 

which outputs

a:        ***
aa:        ***
aaa:        ***

Is there any way to account for the variable length of the a, aa, aaa