How do astronomers find quasars using blue-light surveys?

Astronomers identify quasars by searching for objects that appear bluer than normal stars. This process is highly efficient and involves photographing large sections of the sky through two or more different-colored filters. By comparing these photographs, researchers can isolate unusually blue candidates, which are then verified through spectroscopy.1

While this method originally relied on film, modern surveys utilize electronic charge-coupled devices (CCDs). The technique has also evolved to include:1

• The use of multiple filter combinations to isolate quasars at varying redshifts.1
• The extension of surveys into infrared wavelengths.1

Although blue-light surveys remain a primary discovery tool, astronomers also identify these luminous objects by searching for starlike sources with irregular brightness variations or by conducting X-ray surveys, as high X-ray emission is a strong indicator of an accreting black-hole system.1