The Milky Way

• The phrase "Milky Way" is used in two ways:

  1) The name of the star system otherwise known as "our galaxy", or the "local galaxy" and

  2) The name of the "way" (in the sense of "path" or "street") or milky looking, blurry region that extends completely around the sky.

• The word "galaxy" has the same root as the Greek for milk (as in lactate, lactose, etc.).

• All the naked eye objects, except for one, in our sky are in the (star system) Milky Way. The exception is the great galaxy in the constellation Andromeda.
• The scale of the Milky Way is determined by several means. One is the locations of the globular clusters. They are distributed roughly in a sphere 100, 000 light years across. The majority of the stars are distributed in a thin disc, with the same center and diameter as the sphere outlined by the globular clusters. The Milky Way in the sky is the plane of this disc.

• The stars dominate the appearance of the Milky Way, but there are other things which make up the Milky Way. The interstellar space contains matter which constitutes the "other things". This matter consists of isolated atoms, isolated molecules, and small dust-like pieces of matter.

  The atoms make their presence known as absorption lines in stellar spectra, as well as by producing their own (emission) spectra.

  The dust can be seen to obscure objects behind it, and more subtly, to "redden" the light of stars seen through it. This reddening arises from the same phenomenon which causes the reddening of the sun as it sets, and which is responsible for the blue of the sky.

  Cold hydrogen gas (isolated atoms) produces light of wavelength 21 cm. The atomic transition responsible for this is the flipping of the orientation of the nucleus (which is a magnet) in the magnetic field of the orbiting electron. Because this flipping requires such a small amount of energy compared to that needed to produce visible light, it can happen at the very low temperature of interstellar space; and because so little energy is involved, the wavelength is very long compared to visible light. The Doppler shift of this 21 cm radiation allows the identification of spiral arms of the galaxy.

  We suspect we are in a spiral because so many other galaxies are spirals. But the evidence that we are actually a spiral galaxy is indirect; we get our evidence from this 21 cm data, and from corresponding visible light (from stars) Doppler shift data.

  The spectral evidence for interstellar molecules ("chemistry") is surprising and intriguing. Surprising because the interstellar medium is so tenuous that the collisions between atoms (needed before the atoms can join together into molecules) are so rare. Intriguing because chemistry is the precursor to biology, and if there is interstellar chemistry, perhaps there could be interstellar biology.

• The mass of the Milky Way is estimated by counting the stars and multiplying that number by the mass of an average star. This is obviously not very precise, but one does what one can. Another way is to use Kepler's law: measure the orbital speed (from its radial and tangential velocities), and distance from the galactic center, of a star (for as many stars as one can); Kepler's law gives the mass of the object (which in this case is the galaxy) being orbited. This second method gives a much bigger number than the first method. More precisely, the orbital speeds of the stars do not decrease, with distance from the galactic center, as fast as they would have to if the only mass inside the orbit were the stars. This discrepancy appears in other measurements as well: There is mass "missing" in the sense that the gravitational effect is more than can be accounted for by visible matter; there is "dark matter," of unknown nature, in the galaxy. (Corresponding measurements for the motions of entire galaxies within clusters of galaxies imply missing mass there, also.)