Post-main-sequence stars lose up to 50 percent of their mass because a. jets from the poles release material at an increasing rate. b. the mass of the star drops because of mass loss from fusion. c. the magnetic field causes increasing numbers of coronal mass ejections. d. the star swells until the surface gravity is too weak to hold material

Stellar evolution describes the life cycle of stars, from their formation in clouds of gas and dust to their ultimate fate. After a star spends most of its life in the main sequence, where it fuses hydrogen into helium, it eventually exhausts its nuclear fuel. At this point, the star undergoes significant changes and moves into the post-main-sequence phase.
During this phase, stars like the Sun may expand into a red giant, dramatically increasing in size and changing their internal structure. Understanding stellar evolution helps us track these transformations and the processes that follow, such as planetary nebulae or supernovae, depending on the star's initial mass.
Key stages of stellar evolution:

• Formation: A star is born from a collapsing cloud of gas and dust called a protostar.
• Main Sequence: Hydrogen fusion occurs in the core, providing stability for billions of years.
• Post-Main-Sequence: The star exhausts its core hydrogen, expands, and undergoes various changes.

Mass loss is a critical factor during the later stages of a star's lifecycle, especially in the post-main-sequence phase. A significant portion of a star's mass can be lost as its surface gravity weakens, allowing material to escape more easily.
One major contributor to mass loss is the expansion of the star, reducing the gravitational pull on its outer layers. As the star expands, the outer layers are more prone to being shed into space, often forming beautiful and complex structures like planetary nebulae.
Factors affecting mass loss:

• Expansion and Cooling: As stars swell into red giants or supergiants, the surface cools and the grasp of gravity weakens.
• Stellar Winds: Powerful winds can blow material off the star's surface.
• Mass Ejections: Events like coronal mass ejections can occasionally cause mass to be lost, but not typically in large quantities.

Surface gravity is the gravitational pull experienced at the outer layers, or surface, of a star. This force plays a crucial role in a star's ability to hold onto its mass. In the post-main-sequence stage, as the star expands, its surface gravity decreases.
When surface gravity diminishes, the star cannot effectively retain its outer layers, leading to significant mass loss. This is a natural part of the star's lifecycle and contributes to the dispersal of elements into space, enriching the interstellar environment.
Understanding surface gravity:

• Impact of size: As a star swells during its post-main-sequence phase, surface gravity becomes significantly weaker.
• Escape velocity: Lower surface gravity means the escape velocity decreases, allowing material to drift away.