20 and 25M$_\odot$ star, but the differences in the peak temperatures between the 15 and 20M$_\odot$ is caused, instead, by the amount of fallback. Although the initial density structures of these two stars are similar in the inner core (within 2.1M$_\odot$), much of this material falls back in explosions of the 20M$_\odot$. Its innermost ejecta (material that does not fall back) is much less dense (and hence lower temperature) than the 15M$_\odot$ star. To probe stellar structure, we must disentangle fallback effects from structure effects.

![image](Tpeak_15.pdf){width=".32\linewidth"} ![image](Tpeak_20.pdf){width=".32\linewidth"} ![image](Tpeak_25.pdf){width=".32\linewidth"}

![image](Rhopeak_15.pdf){width=".32\linewidth"} ![image](Rhopeak_20.pdf){width=".32\linewidth"} ![image](Rhopeak_25.pdf){width=".32\linewidth"}

### Probing Stellar Structure {#sec:probestruct}

All of the 15M$_\odot$ explosions produce low mass compact remnants (baryonic masses between 1.5-1.7M$_\odot$). This means that the innermost ejecta is low-entropy, high-temperature and high-density material that is near the launch of the supernova shock. The shock is strong and the ejecta is significantly heated during the explosion. As such, the 15M$_\odot$ star ejects a large amount of material near the core that has experienced extreme conditions. In most cases, it is this material that dominates the radioactive ejecta.

In contrast, much of this innermost ejecta falls back in our more massive progenitors. For these progenitors, burning layers of the star can reach high 