A planet that is not attached to a star but moves freely in space. When the temperature in the core is more than one million degrees. When nuclear reactions start occurring in its center. When its surface reaches a temperature of 10 million degrees. When we can see it with our telescopes.
Open clusters can have hundreds of stars, globular clusters at most a few.
Open clusters can have hundreds of stars, globular clusters hundreds of thousands. Globular clusters can have hundreds of stars, open clusters hundreds of thousands. Globular clusters can have hundreds of stars, open clusters at most a few. Plot the stars on an HR diagram and find the main sequence cutoff. Calculate the time taken by light to travel from the cluster to us.