Strong‐lensing signatures of first‐star Lyα spheres. Sources at z = 30 with diameters ﹩d_{S}﹩ of 2.5^{﹩\arcsec﹩}, 5^{﹩\arcsec﹩}, 10^{﹩\arcsec﹩}, and 20^{﹩\arcsec﹩} are shown in purple, blue, red, and black, respectively, in the right panel. The corresponding images are shown in the left panel with the same colors. The lens is at redshift z = 0.36 with mass ﹩M_{\mathrm{vir}\,}﹩ = 1.5 × 10¹⁵ ﹩h^{-1}﹩ ﹩M_{\odot }﹩. The blue curves are critical curves, and the green curves are caustics. When the size of the source increases to cross the caustics, some separate lensed images merge together. For a sufficiently large source, all lensed images will merge and form a single irregularly shaped structure. This feature is distinct from ordinary strong lensing of galaxies. The resulting single lensed images will most likely be the first detected Lyα spheres.