Archive of Astronomy Questions and Answers

How do some stars make dust?

Among the more interesting red supergiant stars are those whose distended atmospheres ( extending beyond the orbit of Earth) are so cool that carbon atoms can collide and get stuck together to form graphite dust grains at temperatures below 1600 Kelvin. Stars that are 'oxygen-rich' rather than 'carbon rich' prefer to form grains of silicon monoxide (SiO) instead. As more and more of these dust grains begin to form and condense out of the star's atmosphere, the light from the star begins to fade until in some cases the star becomes invisible. A bright infrared star called IRC+10216 seems to be one of these supergiant stars which has permanently enshrouded itself in dust so that we can now only see it by its infrared light. Some variable stars also make dust grains. As their outer layers expand and cool, dust grains form and further attenuate the light reaching us. The variation in the infrared light from these stars lags the optical light variation slightly because the dust grains do not form instantly as the stars outer layers expand. The above image is of the dust shell around Lambda Orionis as sen by the IRAS satellite.