Hydrogen is the most abundant element in the universe and constitutes about 75% of the interstellar medium.

Helium is the second most abundant element in the universe and constitutes about 25% of the interstellar medium.

Star formation is the process by which interstellar gas and dust are converted into stars.

Molecular clouds are characterized by high density and low temperature and are the primary sites of star formation.

H II regions are regions of ionized hydrogen surrounding hot, young stars.

Supernovae are the primary source of heavy elements in the interstellar medium.

Hydrogen cyanide (HCN) is responsible for the characteristic blue-green color of reflection nebulae.

Interstellar emission is the process by which interstellar dust grains are heated by starlight and emit infrared radiation.

Water vapor (H₂O) is responsible for the characteristic maser emission observed in some star-forming regions.

Interstellar absorption is the process by which interstellar dust grains absorb and scatter starlight.

Atomic hydrogen (H I) is responsible for the characteristic absorption feature observed at 21 cm.

Interstellar polarization is the process by which interstellar dust grains align with the magnetic field.

Ammonia (NH₃) is responsible for the characteristic emission feature observed at 1.3 cm.

Interstellar erosion is the process by which interstellar dust grains are destroyed by collisions with other particles.