Columbia subject headings:
Galaxies--Evolution--Congresses.
Gases--Congresses.
Interstellar matter--Congresses.
Harvard subject headings:
Galaxies--Evolution--Congresses
Gases--Congresses
Interstellar hydrogen--Congresses.
Interstellar matter--Congresses.
Princeton subject headings:
Galaxies--Evolution--Congresses.
Gases--Congresses.
Interstellar matter--Congresses.