Our location: About 28,000 ly from the center [we can only see
out to 1/10 of that distance], moving along with the Sun at 220
km/s, almost 500,000 mph!
Size and Structure
of the Milky Way
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; We can't yet map the
spiral arms as well as those of other galaxies; Contains stars
with more heavy elements [population I].
Radius of inner part 60,000 ly, outer part 200,000 ly or more;
We get the shape from globular clusters; Contains redder stars,
with fewer heavy elements [population II], but 90% or more of
its composition is unknown (and may have some clumpy gas clouds).
- Central bulge: Football shaped, dense, 6 by 4 kpc.
Towards Sgr A, near M8; 10 ly across, difficulty to see, look
with IR, radio and X-rays; A 3-million-solar-mass black hole,
possibly surrounded by many thousands of smaller ones.