Quasars significantly differ from typical stars in their emission spectra. While stars primarily emit light due to nuclear fusion processes, quasars emit light and other electromagnetic radiation across a much broader spectrum, from radio waves to visible light, and even X-rays. This wide range of emissions is due to the various high-energy processes occurring around the supermassive black hole at the quasar's core. The material in the accretion disk around the black hole heats up to extreme temperatures as it spirals inward, emitting X-rays and ultraviolet light. In contrast, typical stars like our Sun have a more limited emission spectrum, primarily in the visible range, with some ultraviolet and infrared. Furthermore, the emission lines in a quasar's spectrum are often broadened due to the high velocities of the gas in the accretion disk, whereas stars typically exhibit narrower emission lines. This distinction in the emission spectrum is a fundamental characteristic that helps astronomers differentiate quasars from stars.

Yes, quasars can be used to map the large-scale structure of the universe. Quasars, being some of the brightest and most distant objects in the universe, serve as beacons that can illuminate the distribution of matter in the cosmos. By observing the distribution and redshifts of quasars, astronomers can infer the large-scale structure of the universe, including the distribution of galaxies and galaxy clusters. Additionally, the light from quasars can be affected by intervening matter, such as gas clouds and galaxies. This phenomenon, known as gravitational lensing, can reveal the presence and distribution of dark matter. As light from a quasar passes through these regions, its path is bent by gravity, providing clues about the mass and structure of the intervening matter. Thus, quasars are not only valuable for studying the properties of distant galaxies and supermassive black holes but also for understanding the overall structure and evolution of the universe.

Finding quasars in the early universe is significant for several reasons. Firstly, it provides information about the formation and growth of supermassive black holes shortly after the Big Bang. The existence of these massive objects, only a few hundred million years after the Big Bang, challenges our understanding of black hole formation and growth. It suggests that black holes can grow much more rapidly than previously thought or that they may have formed directly from large concentrations of mass in the early universe. Secondly, early quasars offer insights into the conditions of the young universe, including the formation of the first galaxies. Their light, travelling for billions of years, carries information about the intergalactic medium and the evolution of galaxies over time. Lastly, the study of these ancient quasars helps in understanding the reionization epoch, a key phase in the universe's history when the first light sources ionized the surrounding hydrogen gas, ending the cosmic dark ages and making the universe transparent to light.

Quasars contribute significantly to our understanding of galaxy evolution. They are thought to play a crucial role in the formation and development of galaxies, particularly in their early stages. The intense radiation and powerful winds emitted by quasars can influence the gas within their host galaxies, affecting star formation rates. This process, known as feedback, can either trigger or suppress the formation of new stars. Additionally, the study of quasars provides insights into the relationship between galaxies and their central supermassive black holes. Observations suggest a correlation between the mass of these black holes and the properties of their host galaxies, indicating a co-evolution. By studying quasars, astronomers gain insights into the mechanisms that drive this co-evolution, such as how material is accreted onto black holes and how energy is released back into the galaxy. Understanding these processes is crucial for a comprehensive picture of how galaxies form, evolve, and interact with their central black holes over cosmic time.

Observing quasars presents several challenges, primarily due to their extreme distances from Earth and the high-energy processes occurring within them. Firstly, their immense distances mean that their light is very faint when it reaches us, requiring highly sensitive telescopes equipped with advanced technology. Astronomers use large ground-based telescopes and space telescopes like the Hubble Space Telescope to observe quasars. These instruments are capable of detecting faint objects and have high-resolution capabilities to study distant quasars in detail. Secondly, the high-energy processes in quasars produce a wide range of emissions, from radio waves to X-rays. To fully understand these processes, a multi-wavelength approach is necessary, employing telescopes that can observe across the electromagnetic spectrum. This includes radio telescopes like the Very Large Array (VLA) and X-ray observatories like the Chandra X-ray Observatory. Finally, quasars are often surrounded by dust and gas, which can obscure observations. Infrared telescopes, which can see through dust, are used to overcome this challenge. Despite these challenges, the advancements in telescope technology and observational techniques have greatly enhanced our ability to study and understand quasars.