Dear PF Forum, My friend just tell me about Kelt - 4A. Kelt - 4A orbits 3 stars. I think such thing is not uncommon in the universe for binary/ternary systems. It's a matter of probability. Probability means that for any system there can be many celestial objects. So how many percent do they have two celestial objects in a system where both are greater than the lowest possible mass of a star. Now, what I want to ask is this. 1: What is the lowest mass of a star? 10 Jupiter mass? 100 Jupiter mass? 2: What is the higest mass of a star? Last year, in PF a mentor/staff/advisor told me that a star can't be bigger than 200 (or 1000?) solar mass. Even in this scenario he/she said. If there's a nebula, say 1 million solar mass, then the star won't be 1 million solar mass. It is in 200 (or 1000?) solar mass. Because of the strong star wind the expell the material. Okay.. I can picture this. So there's a star inside the nebulae and the rest 999,000 solar mass is not part of the star. But what is the definition of a star anyway? A celestial object that fuses hydrogen to helium (in main sequence), and helium and carbon up to iron? So not all the material in that nebulae can be called a star because not all material undergo fusion? And if so, why we call the sun is 1 solar mass star? Because if I'm not mistaken, only the core of the sun fuses hydrogen. And its mass is only 30% of the sun. So what I want to ask in number question number 2 is this. If, there's a nebulae 1 million solar mass, why can't there be a star 1 million solar mass? All the material will be expeled by the explosion of the 1000 solar mas "star" (whatever we call it) inside it right? Isn't that the case of the sun. That the sun can't crunch to 12000 km because there's a fusion inside it that keeps the sun from collapsing. And STILL we call the sun is 1 solar mass star although it's the core that sustain it. Thanks for any reply.