Or...
select - Colour Range
select Sampled Colours
slide fuzziness to 100
use the eyedropper to select the brightest star
adjust fuzziness slider up or down the scale until the it encompasses your desired selection.
Click on OK
Click on Select then Inverse
Click on Select then Modify - use the expand and contract controls to adjust the size of the selection if necessary.
Click on Select then Modify - Feather - for stars I usually us a feather radius of 2 pixels. (feathering softens the edges of the selection)