Number the steps of stellar evolution in the correct order

A star is a plasma ball held together by its gravity and compressed by a nuclear fusion at some stage in its development. The closest star on Earth is called the Sun. Several other stars can be seen in the night sky, but none of them is comparable to the Sun due to their very large distances to Earth. Traditionally, stars have been grouped into different constellations.

The stars are formed mainly by the collapse of gas vapors consisting of hydrogen due to gravity. Once the star core is formed, it slowly begins to convert hydrogen to helium in the nuclear fusion. The energy generated in the process flows from the center of the star to its surface and later radiates to outer space. The star is in equilibrium because its internal radiation pressure prevents gravity from collapsing further. When a star's hydrogen fuel is exhausted, its final fate depends on its mass: the smallest stars slowly fade away, while the more massive ones expand into red giants. Eventually, the red giants explode as a supernova, spreading various elements into space. Depending on the mass of the exploded star, for example, the remnant of an explosion may remain a neutron star or a black hole.  

By observing the motion, luminosity, and spectra of a star, it is possible to determine its other properties such as mass, age, and metal content. There are several different types of stars. The most common tool for classifying them is the Hertzsprung-Russell diagram.

Gravitationally bound star systems of two or more stars are quite common. On a large scale, stars are an important part of galaxy structure, together with interstellar gas and dark matter.

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