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.
Last modified: Wed Oct 4 22:24:42 CDT 2000