A planetary nebula glows because a. it is hot. b. fusion is happening in the nebula. c. it is heating up the interstellar medium around it. d. light from the central star causes emission lines.

Understanding the glow of a planetary nebula starts with grasping stellar evolution. Stars evolve through a series of stages over millions or even billions of years. When a medium-sized star near the end of its life exhausts its nuclear fuel, it sheds its outer layers. This process forms a planetary nebula.
At this stage, the star’s core becomes a hot, dense object called a white dwarf, which is surrounded by the expelled gas. These outer layers dispersed into space are what we observe as a beautiful, glowing nebula. Summarizing the broad steps in stellar evolution:

• Formation from a protostar
• Main sequence phase where nuclear fusion occurs
• Red giant phase, expanding and shedding outer layers
• Formation of planetary nebula, leaving behind a white dwarf

Thus, the origin of the glowing nebula is deeply tied to the lifecycle of its central star. This leads us to the next key concept involving the central star in more detail.

A striking feature of planetary nebulae is their emission lines. These lines are specific wavelengths of light emitted by the ionized gas in the nebula. To put it simply, emission lines are like the nebula’s 'fingerprints'.
Here's how it works: the intense ultraviolet radiation from the central star excites the atoms in the nebula. When these atoms return to their lower energy states, they release light at specific wavelengths, visible as colorful lines in the spectrum.
Some common emission lines observed in planetary nebulae include:

• Hydrogen-alpha (Hα) - gives off a red hue
• Oxygen-III (O[III]) - often appears turquoise or greenish blue
• Nitrogen-II (N[II]) - can show as red or orange

These lines not only add to the visual appeal but also help astronomers determine the composition and physical conditions of the nebula.

Central to the glowing of a planetary nebula is ionization. This process begins when the central star emits high-energy ultraviolet photons. These photons collide with atoms in the surrounding gas, knocking electrons free. This state, where atoms lose electrons and become charged, is called ionization.
For example, consider hydrogen, the most abundant element in a nebula:

• An ultraviolet photon strikes a hydrogen atom.
• The hydrogen atom loses its electron, becoming ionized (H+).
• When the electron recombines with the ionized atom, energy is released in the form of light, typically in the visible spectrum.

The glow we see from planetary nebulae comes from numerous such ionization and recombination events among various elements, painting a colorful and dynamic picture of the interstellar medium.

The true hero behind the glow of a planetary nebula is its central star. After a star has shed its outer layers to become a nebula, the remaining core is the central star. This stellar remnant is often a white dwarf, incredibly hot and dense.
Key traits of the central star include:

• High temperature: It emits strong ultraviolet radiation.
• Small size but very dense: Its gravity is extremely strong due to compact mass.
• Central role in ionization: Its powerful radiation ionizes the gases in the nebula, leading to the emission lines we observe.

Without the central star's intense energy output, the surrounding gas would not ionize and no glowing nebula would form. This star not only marks the end of the life cycle of the original star but also illuminates the final stage of its existence in the universe.