Astronomy: Pre-bomb Astronomy (1910-1950)

• The Hertzsprung-Russell diagram made its appearance, showing the main sequence, plus other odd star groups like red giants and white dwarfs. It was understood that the spectral type sequence was also a sequence in color and surface temperature.
• Cecilia Payne (in 1920, I think) combined what physicists knew about atoms and energy levels and spectral lines with what astronomers knew about the temperatures of stars to show that, even though hydrogen lines are not prominent in every star, hydrogen is very abundant (present in great quantities). She understood for the first time that cool stars are not able to excite very many electrons, so the hydrogen spectrum is weak. And she understood that very hot stars excite hydrogen to such high orbital levels that few electrons are left to participate in the 3-2, 4-2, 5-2, etc. orbital transitions of the Balmer series. This marks the start of abundance analysis, where the amount of an element present in a star is measured.
• Meanwhile, studies of binary stars yielded complete orbit information, which yielded the masses of the two stars in orbit around each other. These studies showed that the main sequence in the HR diagram is a sequence of mass. Massive (50 Msun) stars are hot (50000 K) and luminous (millions of Lsun). Low-mass stars (0.1 Msun) are cool (3000 K) and dim (1/10,000 Lsun).
• Henrietta Leavitt discovered that a particular kind of pulsating star, a Cepheid Variable, had a very predictable luminosity, if one measures the period of time for one pulsation. The hundred-day variables are about 100 times brighter than the 1-day Cepheids.
• The trouble with Cepheids is that their absolute magnitude (or luminosity) was not known. IF it were known, then the distance to these stars could be easily found via the inverse-square law (in magnitude form the inverse-square law is: m - M = 5logD - 5.)
• Enter Harlow Shapley. Shapley was able to show that the Cepheids were at least several hundred times more luminous than the sun. Using this tool, he was able to find the distances to many rich globular clusters of stars. He found that these clusters clustered around a spot toward Sagittarius, about 8000 parsecs (25,000 light years). This was very convincing evidence of the staggering size of the Milky Way Galaxy in which we live: 100,000 light years across. The date: 1917.
• Was this staggeringly large collection of stars and nebula the whole universe? The question hinged upon: what were the spiral nebulae? Were they within the Milky Way or outside the Milky Way?
• Edwin Hubble took photographs of one very large (looking) spiral nebula called M31, or the Andromeda Galaxy. Looking very carefully, he found that the "nebula" was really composed of innumerable, very faint stars. After a series of photographs, he discovered that some pulsated with the periods typical of Cepheid variables. But these Cepheid variables were so faint that the "spiral nebula" must be very far away. 2 million light years. Whoa.
• Hubble also classified the different types of galaxies according to how they look: elliptical, spiral, irregular.
• Hubble's most world-shaking discovery was that the most distant galaxies are receeding away from us. The farther away the galaxy, the faster it is fleeing from us. Hubble has discovered that the Universe is expanding. This needed two numbers for each galaxy: (1) a velocity obtained by taking a spectrum of the galaxy and measuring the doppler shift of its spectral lines, and (2) an estimate of the distance of the galaxy. Hubble used two methods. Initially, he simply noted that fainter galaxies must, on average, be farther away. By 1929, he was able to use the 'brightest star' method, where the brightest stars in each galaxy were assumed to have the same luminosity, and a distance was quickly obtained via the inverse-square law.