What ionizes the gas in a planetary nebula (see the image below) and makes it visible? 
Definitions:
Demand Curves
Graphical representations showing how the quantity demanded of an item changes at different prices.
Market Entry
The act or process of beginning to compete in a new market, which may involve overcoming barriers and adapting to local conditions.
Competitive Price-searcher
An entity in a market that actively compares prices and seeks to set competitive prices for its products or services, acknowledging some level of market power.
Average Total Cost
The total cost of production divided by the total quantity produced, representing the cost per unit of output.
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