The composition of a star significantly affects its luminosity, primarily through its impact on the star’s temperature and energy generation processes. Stars are primarily composed of hydrogen and helium, with heavier elements present in smaller amounts. The proportion of these heavier elements, often referred to as 'metals' in astronomical parlance, influences a star's opacity - how easily energy can pass through its outer layers. Higher metallicity increases a star’s opacity, causing it to retain heat more efficiently and potentially increasing its luminosity. Furthermore, the nuclear fusion processes at a star’s core, which are the primary source of its energy, are sensitive to its composition. Stars with higher proportions of hydrogen fuel can maintain high-energy fusion reactions for longer periods, affecting their luminosity over their lifetime. Additionally, as stars evolve, changes in their core composition (like the depletion of hydrogen and the fusion of heavier elements) lead to changes in luminosity, as seen in stages like the red giant phase.

A star's luminosity varies over its lifetime due to changes in its internal structure and energy generation processes. In the initial stages, a star's luminosity is primarily governed by the gravitational contraction of its mass, heating up the core until nuclear fusion begins. Once the star enters the main sequence phase, its luminosity is relatively stable, driven by the stable fusion of hydrogen into helium in its core. However, as the star exhausts its hydrogen fuel, the core contracts and heats up, leading to the expansion of the outer layers and an increase in luminosity. This is observed when stars evolve into red giants. In later stages, changes in the fusion processes (e.g., helium burning) and further structural changes cause significant variations in luminosity. These changes are observed through telescopic measurements of brightness and spectral analysis, allowing astronomers to track a star's evolutionary path on the Hertzsprung-Russell Diagram. The variability in luminosity is a key indicator of a star’s age and stage in its lifecycle, offering insights into the processes occurring within and the eventual fate of the star.

Luminosity and absolute magnitude are both measures of a star’s intrinsic brightness, but they are quantified differently. Luminosity is the total amount of energy a star emits per second, measured in watts or solar luminosities. It reflects the actual power output of a star. Absolute magnitude, on the other hand, is a logarithmic scale that measures the apparent brightness a star would have if it were placed at a standard distance of 10 parsecs (32.6 light-years) from Earth. Lower absolute magnitude values correspond to higher luminosities. While luminosity is a direct measurement of a star’s energy output, absolute magnitude is a comparative measure that allows astronomers to rank stars based on how bright they would appear at a fixed distance. This makes absolute magnitude a useful tool for comparing the intrinsic brightness of stars without the variable of differing distances.

The luminosity of a star is intrinsically linked to its colour and temperature, a relationship governed by the laws of blackbody radiation. Stars emit a spectrum of light that approximates a blackbody, a perfect radiator. The colour of a star, which ranges from red to blue, corresponds to its surface temperature. Cooler stars emit more red light and are less luminous, whereas hotter stars emit more blue light and are more luminous. This is because the energy output (and hence luminosity) increases steeply with temperature, following the Stefan-Boltzmann law, which states that the total energy radiated per unit surface area of a black body across all wavelengths per unit time is directly proportional to the fourth power of the black body's temperature. Therefore, a slight increase in a star’s temperature can cause a significant increase in its luminosity. This relationship is crucial for understanding why stars of different temperatures and colours appear at different positions on the Hertzsprung-Russell Diagram.

Luminosity can indeed be used to infer the size of a star, although this process requires additional information about the star, such as its temperature. The luminosity of a star is determined by two primary factors: its surface area and its effective temperature. According to the Stefan-Boltzmann law, the luminosity (L) of a star is proportional to its surface area (A) and the fourth power of its temperature (T), given as L = A × σ × T⁴, where σ is the Stefan-Boltzmann constant. Therefore, if the temperature of the star is known, its luminosity can be used to estimate its surface area and, hence, its radius. For instance, if a star has the same temperature as the Sun but a luminosity 100 times greater, it must have a surface area 100 times that of the Sun. By calculating the surface area, the radius of the star can be inferred. This method is particularly useful for distant stars whose direct size measurements are not feasible.