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After a protostar has progressed in its development, it will have a dense inner part and an outer envelope called a(n) ____.
Helium
A chemical element with symbol He, the second lightest element, known for being a colorless, odorless, tasteless, non-toxic, inert gas.
Open System
A system that exchanges both energy and matter with its surroundings, characteristic of most natural and biological systems.
Closed System
A physical system that does not exchange matter with its surroundings, though it may exchange energy.
Penetrating Radiation
Radiation that can penetrate through substances and materials, potentially causing ionization of atoms.
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