The observed value $Lambda_{rm obs}$ of the cosmological constant $Lambda$ is extremely smaller than theoretical expectations, and the anthropic argument has been proposed as a solution to this problem because galaxies do not form when $Lambda gg Lambda_{rm obs}$. However, the contemporary galaxy formation theory predicts that stars form even with a high value of $Lambda / Lambda_{rm obs} sim$ 50, which makes the anthropic argument less persuasive. Here we calculate the probability distribution of $Lambda$ using a model of cosmological galaxy formation, considering extinction of observers caused by radiation from nearby supernovae. The life survival probability decreases in a large $Lambda$ universe because of higher stellar density. Using a reasonable rate of lethal supernovae, we find that the mean expectation value of $Lambda$ can be close to $Lambda_{rm obs}$, and hence this effect may be essential to understand the small but nonzero value of $Lambda$. It is predicted that we are located on the edge of habitable regions about stellar density in the Galaxy, which may be tested by future exoplanet studies.
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