The expansion speed of Type Ia supernovae is measured by the rate at which [math]\text{Si}~\text{II}[/math] expands in the outer layers of the supernova photosphere. [math]\text{Si}~\text{II}[/math] velocities are typically between [math]11,000[/math] and [math]12,000~\text{km s}^{-1}[/math], but range as high as [math]30,000[/math] and as low as [math]5,000~\text{km s}^{-1}[/math] [see, e.g., 1]. read more