What is the Milky Way?

• Some history: Galileo and the stars in the Milky Way; In the 1750s it was proposed that it may be similar to other "island universes"; The issue whether some "nebulae" are inside the Milky Way or separate galaxies was resolved only in the 1920s using Cepheid stars in nearby ones.
• The view from here: Like being inside a forest, and seeing only the trees; Because our view is obscured, we see about as many stars in all directions; Need to use radio waves or other forms of radiation to see through interstellar matter.
• What we now know: The Milky Way is a spiral galaxy containing about 100 billion stars; So are the "spiral nebulae" like M31, each one a distant "island".

Our location: About 28 kly or 8.6 kpc from the center; we can only see out to 1/10 of that distance; We happen to be in a spiral arm, but not the one we were in when the Solar System formed.

  Size and Structure of the Milky Way

• Disk: Diameter about 100,000 ly, thickness varies (about 1000 ly here), slightly warped; We get the shape from 21-cm wavelength radio waves emitted by interstellar matter, or from open clusters, bright stars and nebulae if we see them; It contains stars with more heavy elements [population I].
• Spiral arms: We can't yet map the spiral arms as well as those of other galaxies, but there seem to be two main arms (Scutum-Centaurus and Perseus) and other small, partial arms.
• Inner halo: 60,000 ly or so radius; We get the shape from globular clusters; Contains redder stars, with fewer heavy elements [population II], but most of its composition is unknown.
• Outer halo: Extends out to at least 200,000, possibly 500,000 ly.
• Central bulge: Football shaped, dense, 6 by 4 kpc.
• Core: Towards Sgr A, near M8; 10 ly across, difficulty to see, look with IR, radio and X-rays; A 4-million-solar-mass black hole (from the motion of nearby stars), possibly surrounded by many thousands of smaller ones.

Evolution and Motion, Mass

• Formation: The Milky Way seems to be about 13.6 Gyr old and one of the original galaxies, but grew as a result of mergers with smaller galaxies and fragments; The flattening developed from rotation, details depend on how the mergers happened.
• Spiral arms: Thicker where there is more interstellar matter, not just carried around by the rotation but also affected by moving density waves and self-propagating star formation, with events like star explosions affecting future star generations.
• Nearby stars: Apparently random directions, as if our environment had been stirred, at about 40,000 mph with respect to us.
• More distant stars: On average, disk stars revolve around the center at 220 km/s (about 600,000 mph!), with a period P = 225 Myr, and bob up and down across the disk; Halo stars move along huge ellipses in random directions.
• Mass: Using Kepler's laws and rotation curves; inside the Sun's orbit, there are 10¹¹ solar masses, and for the whole galaxy the value may be more than 10¹² solar masses!
• Future evolution: The galaxy seems destined to collide and merge with the Andromeda galaxy, and in 50-80 Gyr may plunge into the Virgo cluster and be shredded.

• Rotation curve: The Milky Way has a differential rotation, but the rotation curve showing speeds at different distances from the center cannot be explained by the matter we see.

  Interesting and Puzzling Aspects

• Anomalous motions: Some packs of stars and outlying high-velocity clouds move on paths unrelated to that of the rest of the galaxy's stars; They seem to be remnants of small galaxies absorbed by the Milky Way (the Solar System sits in the Sagittarius stream...).
• Immediate surroundings: Our galaxy has at least 11 small satellite galaxies, and a giant ring of several hundred million stars outside the galactic disk, which could also be the remnant of a dwarf galaxy.