The universe isn't what it used to be.

Just a few billion years ago, according to a Yale University astronomer, some 90 percent of its mass was in the form of giant stars, some more than 100 times the mass of the sun.

Nowadays, says Richard Larsen, most of the giants are gone, collapsed into invisible black holes, and the stars are more predominantly of middling-to-midget size, like the sun.

Larsen's views on changes during the history of the universe challenge a long-held assumption that large and small stars have been forming and living out their lives at about the same rates since the beginning of time, about 15 billion years ago.

Stars are thought to form when vast clouds of interstellar dust and gas begin condensing into a mass with growing gravitational strength. When the mass reaches a critical size, its internal pressures ignite nuclear fires, forming a star.

Using observations of nearby stars in the Milky Way galaxy, Larsen speculates that high-mass stars formed mainly during the earliest phases of the universe, when it was dominated by hotter, larger clouds of primordial matter.

Low-mass stars, by contrast, seem to have formed more often in the smaller, colder clouds of matter typical of the galaxy today.

Larsen, who published his views in the February Monthly Notices of Britain's Royal Astronomical Society, says that as the universe cooled following the Big Bang, fewer big stars formed, but more small stars were born.

Larsen suggests that most of the ancient giants have disappeared, having lived out their lives, exploded in a supernova and then collapsed to form black holes from their residual mass. Because the mass is so densely packed, its gravitational pull is strong enough to prevent even light from escaping.

Nearly 90 percent of the mass in the universe, Larsen believes, exists in the form of these extinct giants.