<P> The properties of a galaxy can also be determined by analyzing the stars found within them . NGC 4550, a galaxy in the Virgo Cluster, has a large portion of its stars rotating in the opposite direction as the other portion . It is believed that the galaxy is the combination of two smaller galaxies that were rotating in opposite directions to each other . Bright stars in galaxies can also help determine the distance to a galaxy, which may be a more accurate method than parallax or standard candles . </P> <P> The interstellar medium is matter that occupies the space between star systems in a galaxy. 99% of this matter is gaseous - hydrogen, helium, and smaller quantities of other ionized elements such as oxygen . The other 1% is dust particles, thought to be mainly graphite, silicates, and ices . Clouds of the dust and gas are referred to as nebulae . </P> <P> There are three main types of nebula: absorption, reflection, and emission nebulae . Absorption (or dark) nebulae are made of dust and gas in such quantities that they obscure the starlight behind them, making photometry difficult . Reflection nebulae, as their name suggest, reflect the light of nearby stars . Their spectra are the same as the stars surrounding them, though the light is bluer; shorter wavelengths scatter better than longer wavelengths . Emission nebulae emit light at specific wavelengths depending on their chemical composition . </P> <P> In the early years of astronomical spectroscopy, scientists were puzzled by the spectrum of gaseous nebulae . In 1864 William Huggins noticed that many nebulae showed only emission lines rather than a full spectrum like stars . From the work of Kirchhoff, he concluded that nebulae must contain "enormous masses of luminous gas or vapour ." However, there were several emission lines that could not be linked to any terrestrial element, brightest among them lines at 495.9 nm and 500.7 nm . These lines were attributed to a new element, nebulium, until Ira Bowen determined in 1927 that the emission lines were from highly ionised oxygen (O). These emission lines could not be replicated in a laboratory because they are forbidden lines; the low density of a nebula (one atom per cubic centimetre) allows for metastable ions to decay via forbidden line emission rather than collisions with other atoms . </P>

Which astronomical bodies emit light in the visible spectrum