Mapping The Milky Way (Old School)

With a telescope, Galileo was first to resolve the Milky Way into stars. I. Kant (1755) deduced that we occupy a disk of stars. Later astronomers counted stars in various directions to deduce its structure. This would help deduce the shape if: - All stars are the same magnitude - The view to the edge is not obscured

W.&C. Herschel (c.1800) counted stars in 683 regions. Herschel’s Map Mapping The Milky Way (Old School)

J. Kapteyn, also using star counts, launched a massive project (1906-1922) to survey 200 regions. With data from 40 observatories, he built a detailed model: -similar shape to Herschel’s model; Sun near center of a disk -included a distances: disk radius ~ 8000 pc. -But....

These models failed to include the effects of “extinction”, the decrease in starlight due to intervening dust. Jacobus Kapteyn: “to no other astronomer was the Galaxy so cruel.” Finding the Center of the Milky Way

Harlow Shapley realized interstellar dust could confound a map of the MW. So he studied globular clusters which are mostly out of the plane of the Galaxy, and thus unaffected by dust. Most globulars congregate near Sagittarius & Scorpius.

The distance to a globular cluster can Herschel’s Map of the Milky Way. be measured if a standard candle can be found in it. Several G.C.’s have RR Lyrae stars: globular star cluster “47 Tucanae”

optical

Several G.C.’s have RR Lyrae stars: xray 20th Century Copernicus

Several G.C.’s have RR Lyrae stars, which are standard candles similar to Cephieds. Shapely measured the distances to these clusters first, then... Assumed the brightest star in each cluster had the same magnitude. ... a (cruder) use of standard candles to get distance:

He deduced that the Sun was NOT at center of the Milky Way. (it is 8 kpc away) His MW model was about 10x larger than Kapteyn’s. Shapley’s Globular Cluster Data The Milky Way’s Size and Shape 21 cm (neutral hydrogen) map of the Milky Way reveals spiral structure Mapping The Milky Way

It is difficult to map the structure of the MW because dust and gas obscure visible light.

However, longer light wavelengths (IR, radio waves) are less blocked.

Our best maps of the plane of the Milky Way come from radio waves emitted by Hydrogen with λ = 21 cm.

But why do H atoms emit radio waves? 21 cm Line (a.k.a “hyperfine transition”)

The lowest energy state of Hydrogen (n=1, “ground state”) is actually 2 states:

If the spin of the electron & proton are anti-aligned, the total energy is slightly (6 μeV) lower.

If the atom flips to a lower energy state, it emits a photon.

ΔE = h ν = h c / λ, so, λ = 21 cm. Predicted in 1945, this 21 cm line was first observed in 1951. It has since been used to map the Milky Way Galaxy. Milky Way’s spiral structure was first revealed by a 21 cm map (made by pioneering radio astronomer Jan Oort.) A more recent map 21 cm of the MW