I imagine the root of your problem here is the use of "times". In English, "times" can be synonymous with the mathematical operation multiplication.
- 2 multiplied by 2 equals 4.
- 2 times 2 is 4.
Both of these mean the same. The latter is a more simplified way of expressing a mathematical equation. Children (in the UK, at least) are often taught to refer to multiplication tables as 'times tables'.
When comparing two things, you need a measure. In your example, that is brightness. You could say something is "twice as bright" as something else, but when you get into larger, or more complex ratios, you would need to express it mathematically.
Saying that some stars are "millions of times as bright as any other" means that their measured brightness is millions of multiples of the stars they are being compared to.