G.K. Miley Prof.dr. UL UU UL UL Galaxies contain billions of stars, as well as interstellar gas and dust, and are embedded in dark halos of unknown constitution. Astronomers are able to look back in time, by observing galaxies at ever greater distances. Because light travels at finite velocity, distant objects are seen at a time when the Universe was young. The expansion of the Universe causes light to be redshifted, so that the most distant galaxies are those with the highest redshift.
New stars continue to be born deep inside molecular clouds in galaxies.
The birth process leads to a circumstellar disk of gas and dust from which planets and comets may subsequently form. During the late stages of their life, stars inject nucleosynthetically enriched material into the interstellar medium. E.P.J. van den Heuvel Prof.dr. A. Achterberg Prof.dr. At the end of its life, a massive star explodes and ejects its outer layers.