Ijust discovered a 3.5 msun main-sequence star orbiting a 2.5 msun red giant. i'll bet that red giant was more massive than 3 msun when it was a main-sequence star.

When we have two stars orbiting each other, we can infer that they were born from the same amount of gas, nearly at the same time. Therefore, the two stars have the same age.

The evolution of a star depends on its initial mass: the more massive a star is, the faster it burns the hydrogen, the sooner it undergoes changes to keep the balance. 

Hence, if two stars are of the same age, but one is a red giant, while the other is still on the main sequence, we can infer that the red giant was more massive than the other before becoming a red giant.

Since the main sequence star has a mass M = 3.5 Msun, we can say that the red giant must have started as a star with M > 3.5 Msun.

Therefore, your bet is true.

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