... Visible in this image are dense knots of dark interstellar dust , bright stars that have formed in the past few million years, fields of glowing hydrogen gas ionized by these stars, and great loop s of gas expelled by dying stars. ... PAHs are thought to be created in the cooling gas of star forming regions, and their development in the Sun's formation nebula five billion years ago may have been an important step in the development of life on Earth. ... Generations of Stars in W5 . ...