Majority of visible light coming to us from space originated in stars, which shine because their surfaces are hot; some glow red or orange like a hot iron, some yellow or nearly white like a filament in a classical light bulb, and others shine blue. Due to different temperatures of stars their prevailing color varies, but, in general, starlight covers the whole visible spectrum. When astronomers capture objects composed mainly by stars (galaxies, star clusters, …), they use RGB filters to capture all the colors and later create a so-called true-color image.

Still, not all objects shine because they are hot. For example, the light from nebulae originates in electron shells of their atoms. Such light, emitted by electrons going from one quantum state to another, has only a very specific energy and thus also the wavelength (color). Astronomers use this fact by employing narrow-band filters to exclude everything but a small part of the incoming light and thus can greatly increase signal-to-noise ratio (we get all signal we care about, but background noise is strongly suppressed). Since both the Hα and SII filters land in seemingly similar very dark-hued reds, astronomers often assign false colors to the narrow-band channels.

The NGC300, captured by CielAustral team, is a galaxy and as such shines mainly in continuous spectrum. But every galaxy contains also many nebulae emitting much narrower spectrum. So, the CielAustral team used both RGB and narrow-band filter sets to acquire enormous 132 hours of total exposure time. The result is a stunning image of the NGC300 galaxy with stars in real colors and, at the same time, with highly enhanced nebulae, belonging to the NGC300, shown here in false colors associated with OIII, Hα, and SII spectral lines.