Walter Baade was a German national working in the United States during World War II. Baade was prevented from working in the war effort, at the same time other scientists were required or volunteered to do so, leaving telescope time at the California Institute of Technology's great 100-inch refracting telescope on Mount Wilson to Baade.

In 1944, Baade categorized groups of stars within the Milky Way into two stellar populations though he said Jan Oort had in 1926 suggested a similar distinction (based on color alone?).

Baade observed that bluer stars were strongly associated with the spiral arms, and yellow stars dominated near the central galactic bulge and within globular star clusters well outside the spiral arms. Two main divisions he called population I and population II stars, . The spectrographic properties of the two different stellar populations suggest different chemical compositions.

A newer, hypothetical division called population III was added in 1978. By definition, each population group shows the trend where lower metal content indicates higher age of stars. Hence, the first stars in the universe (very low metal content) are called population III, old stars (low metallicity) are population II, and recent stars (high metallicity) are population I. The Sun is considered population I, a recent star with a relatively high 1.4% metallicity.

[A personal note: During Baade's 1959 lecture at Harvard, my classmates pointed out that I had suggested the Sun might actually be older population II disguised by the accretion of dust and gas from the spiral arms of the Milky Way.

In Chuck Whitney's class on stellar interiors, I had calculated the solar interior might be pure hydrogen (with helium in the thermonuclear core) and still be the same radius. A thin outer layer might contain accreted heavy elements and the convection mixing length might be very short, keeping metallic elements in just the thin solar surface.]

Recently formed stars are often seen surrounded with the gas and dust clouds (nebulae) from which they were born (cf. the Orion Nebula). Other nebulae are surrounded by the detritus from a star dying - an exploding supernova (e.g. the Crab Nebula). The Crab, in the constellation Cassiopeia, contains the first supernova ever observed, by Danish astronomer Tycho Brahe in 1572.

Wartime added a great advantage to Baade’s efforts. Light pollution in the Los Angeles area was greatly lessened by grayouts (partial blackouts) to hide targets from enemy submarines. As a result, Baade was able to resolve individual stars in the Andromeda galaxy. He saw two different types of Cepheid variable stars, corresponding to his populations I and II, The absolute luminosities had been discovered to be a function of their period of variation by Harvard astronomer Henrietta Swan Leavitt in 1908.

The Cepheid variables became "standard candles" for astronomers, used to calculate the distances to many objects out beyond the "local group" of galaxies

In 1929, Edwin Hubble had used Leavitt’s work on Cepheids in the Large Magellanic Cloud to calculate the distance to Andromeda and to dozens of other nearby galaxies. Red-shift measurements by Hubble’s colleague Vesto Slipher established the linear velocity-distance relationship that proved the universe is expanding, now known as the Hubble law.

Comparing the Hubble expansion rate with the known overall matter density led astronomers to claim that the universe would not expand indefinitely. The favored cosmological model until the middle 1950’s was an unbounded but finite space, inside which all paths ultimately curved back on themselves. Space was thought to be curved positively according to Albert Einstein's theory of general relativity. The universe was thought to be unbounded but a finite volume.

Baade recognized that the Cepheids Leavitt had studied were what we now call Population I stars, and that his second type are a part of older Population II. The second type are much brighter intrinsically, putting the Andromeda galaxy much farther away and the universe much larger. Baade first reported his results in Rome in 1952, but it was not until 1956 that the fact of the bigger universe was accepted. Since that time the universe has not been thought to contain enough matter to close it. It cannot be held back from an infinite expansion.

Baade's Cal Tech colleague Fritz Zwicky had questioned the interpretation of red shifts as velocities because they suggested too high velocities and he proposed another source of wavelength shifting. Zwicky's analysis of velocities in the great Coma cluster of galaxies showed there is not enough observable matter to satisfy the virial theorem. Zwicky was thus the first to infer the existence of unseen dark matter that he called "dunkle Materie."

In 1934, working with Zwicky, Baade proposed exploding stars could throw off most of their matter and crush their cores to form tiny stars made entirely of neutrons. He and Baade called such an exploding star a "supernova." Exploding supernovae, especially type Ia, have become the farthest seen "standard candles," their absolute luminosity determined by the shape of their "light curves," the increase in brightness as a function of time.

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