now.. here comes the problem.. as they say closest star is about 4.23 lightyears away from us.. a light year is 9.46 x 10 to 12th power.. so that is 40015800000000 whatever that number is km away from us.. so.. does this mean that the star in question is 400158000km large?
You can't actually see stars. As you note they are really far away, so the actual angular size of stars is measured in "mas" (milliarcseconds). What you are actually seeing is a point of light. When you take a photo of a star with your P900 at full zoom then the star is still far smaller than a single pixel. It just lights up that pixel because it's really bright (adjacent pixels are also illuminated because of sensor bloom, and the effects of the atmosphere and lens effects)
The sun (and approximately the moon) are 30 arcminutes (about half a degree) wide in the sky. That's 30*60*1000 mas, or 1.8 million mas. Betelgeuse is 50 mas, or one 36,000 the size of the moon in the sky. In terms of actual size it's 630 times the diameter of the sun.The following is a list of stars whose images have been resolved beyond a point source. Aside from the Sun, stars are tremendously small in apparent size, requiring the use of special high-resolution equipment to image. For example, the first star, other than the Sun, to be directly imaged was Betelgeuse. It has an angular diameter of only 50 milliarcseconds (mas).
Here's one pixel on the P900
The moon is about 2600 pixels wide there, so roughly Betelgeuse is 2600/36000 pixels wide, or less than a tenth the width of the pixel (and that's for something 630 times the size of the sun).
You can only "see" stars because they are so bright.