What is X in the following figure?

Stars die in a variety of ways, depending on their size: Sun-sized stars: Die slowly, releasing gas and dust in the form of planetary nebulas. The nebula fades from view as the star's light diminishes. Medium-sized stars: Shed their outer layers and form a planetary nebula. Massive stars: Die suddenly in a supernova, blasting gas, dust, and energy as they collapse into black holes or neutron stars. Extremely massive stars: Explode in a supernova, blasting their chemical creations into space.  Here's some more information about how stars die: Red giants: When hydrogen runs out, stars... Show more

Stars die in a variety of ways, depending on their size:
Sun-sized stars: Die slowly, releasing gas and dust in the form of planetary nebulas. The nebula fades from view as the star's light diminishes.
Medium-sized stars: Shed their outer layers and form a planetary nebula.
Massive stars: Die suddenly in a supernova, blasting gas, dust, and energy as they collapse into black holes or neutron stars.
Extremely massive stars: Explode in a supernova, blasting their chemical creations into space. 

Here's some more information about how stars die:
Red giants: When hydrogen runs out, stars about the size of the sun expand into red giants, which can be up to 100 times their original diameter.
Core shrinkage: As a red giant loses heat, its core loses mass, blowing off outer layers and shrinking to become a white dwarf star.
Supernovas: Massive stars collapse in seconds when gravity takes over after the pressure drops low enough. The explosion is extremely bright and has been seen by the naked eye throughout human history.

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