Light pollution significantly impacts the visibility of stars, thus affecting their apparent magnitude as observed from Earth. In urban areas with high levels of artificial lighting, the night sky becomes illuminated with scattered light, reducing the contrast between the stars and the background sky. This effect makes fainter stars, which would be visible under darker conditions, undetectable to the naked eye, effectively increasing their apparent magnitude number. For instance, in a heavily polluted area, a star with a natural apparent magnitude of +6 might become invisible, suggesting a higher apparent magnitude. Additionally, light pollution alters the color and intensity perception of stars, which can further complicate visual magnitude estimations. Astronomers often use photometric methods to accurately measure magnitudes in such conditions, but the fundamental issue of reduced stellar visibility remains a significant challenge in observational astronomy.

Apparent magnitude and absolute magnitude are two fundamental concepts in astronomy that describe the brightness of celestial objects, but from different perspectives. Apparent magnitude measures how bright a star or celestial object appears from Earth, which varies depending on the object's distance and intervening material. In contrast, absolute magnitude is a measure of the intrinsic brightness of a celestial object as it would appear at a standard distance of 10 parsecs (about 32.6 light-years) from the observer. The relationship between the two is described by the distance modulus formula, which incorporates the distance to the star. This formula shows that if we know the apparent magnitude and the distance to a star, we can calculate its absolute magnitude, providing a true comparison of stellar brightness regardless of their distances from Earth. This distinction is crucial for astronomers to understand the real luminosity of stars and other celestial bodies, making it a fundamental tool in the study of the universe.

Some stars with low apparent magnitudes may not be visible to the naked eye due to several factors that affect their observability. One primary factor is their location in the sky relative to the observer's position. Stars that are close to the horizon often suffer from atmospheric extinction, where Earth's atmosphere absorbs and scatters light, reducing the star's apparent brightness. Another factor is the star's light wavelength. Human eyes are more sensitive to certain wavelengths, and if a star emits light predominantly in wavelengths outside this range, it may appear fainter. Moreover, environmental conditions like light pollution, atmospheric turbulence, and weather conditions (e.g., cloud cover, humidity) can significantly diminish the visibility of stars, even those with low apparent magnitudes. Additionally, individual differences in human eyesight, including visual acuity and sensitivity, also play a role in the detection of faint stars.

The apparent magnitude of a star can indeed change over time, and these changes are attributable to several astronomical phenomena. Variable stars, which are stars that experience fluctuations in luminosity due to intrinsic processes such as pulsation or eruptions, can show significant changes in their apparent magnitude. Cepheid variables and long-period variables are classic examples, where periodic changes in their size and temperature lead to variations in brightness. Additionally, eclipsing binary stars, where two stars orbit each other in such a way that they periodically eclipse one another from our viewpoint, can also display changes in apparent magnitude. External factors like interstellar dust can also affect a star's apparent magnitude; as a star moves behind a dust cloud, the dust can absorb and scatter its light, making it appear dimmer. Furthermore, apparent magnitude changes can be caused by relative motion between the star and the observer, particularly in high-velocity stars.

Technological advancements have significantly enhanced the precision and accuracy of measuring stellar magnitudes. One of the major developments is the advent of charge-coupled devices (CCDs), which replaced photographic plates in telescopes. CCDs are more sensitive, have a wider dynamic range, and are capable of detecting fainter objects with greater accuracy. Space telescopes like the Hubble Space Telescope and the Gaia observatory have also played a crucial role, as they operate above Earth's atmosphere, eliminating atmospheric interference and providing clearer, more consistent observations. Photometric systems have been refined, with multiple filters allowing for precise measurements across different wavelengths. Automated surveys and telescopes have enabled the continuous monitoring of large sections of the sky, increasing the chances of detecting variability in stellar magnitudes. Additionally, advancements in data processing and analysis techniques have improved the ability to extract accurate magnitude information from observational data, making significant contributions to the field of astrometry and the study of celestial objects.