The presence of dust is easily seen in the Horsehead Nebula and is an example of a Dark Nebula . The dust obscures the light from distant objects as well as from those inside it. Being able to see through the dust has now become possible thanks to advances radio and infrared astronomy which has revealed important new insights into the process of star formation. Interstellar dust can be detected due to four primary effects that it has on starlight. These are Extinction, Reddening, Polarisation and Infrared Emission .
Extinction is the dimming of starlight as it travels through the dust.
The dust particles can either absorb some of the light or scatter it so that less light emerges from the dust than was incident. Finally, the dust particles emit Infrared Radiation . The IRAS satellite showed that infrared emission was the strongest in regions where there is a high concentration of interstellar gas. Jeans realised that there must be a critical value for a mass of gas such that above a certain limit, gravity would overcome the thermal motion of the particles which would otherwise disperse the cloud. The critical mass of gas is called the Jeans Mass .