Background: The livelihood of an open source ecosystem is important to different ecosystem participants: software developers, end-users, investors, and participants want to know whether their ecosystem is healthy and performing well. Currently, there exists no working operationalization available that can be used to determine the health of open source ecosystems. Health is typically looked at from a project scope, not from an ecosystem scope. Objectives: With such an operationalization, stakeholders can make better decisions on whether to invest in an ecosystem: developers can select the healthiest ecosystem to join, keystone organizers can establish which governance techniques are effective, and end-users can select ecosystems that are robust, will live long, and prosper. Method: Design research is used to create the health operationalization. The evaluation step is done using four ecosystem health projects from literature. Results: The Open Source Ecosystem Health Operationalization is provided, which establishes the health of a complete software ecosystem, using the data from collections of open source projects that belong to the ecosystem. Conclusion: The groundwork is done, by providing a summary of research challenges, for more research in ecosystem health. With the operationalization in hand, researchers no longer need to start from scratch when researching open source ecosystems' health.