The Milky Way

Just as the Sun is "our" star, the Milky Way is "our" galaxy.
Unfortunately we see it from the inside, so first a look at a galaxy which is reasonably similar: M100. Large spiral galaxy, Note:
  • ∼ 100 billion stars
  • Large spiral arms outlined by hot stars
  • Large dust lanes
  • Bright centre
  • Size: ∼ 50000 pc (50 kpc) across
  • Bright patches are clumps of young stars

M100: A Grand Design Credit: NASA

Looked at from the side, the Milky way looks a bit like NGC 4565 (ignore the foreground stars): note
  • bulge
  • dust
  • Thickness of arms ∼ 1 kpc

Credit: Bruce Hugo and Leslie Gaul, Adam Block (KPNO Visitor Program), NOAO, AURA, NSF
So the Milky Way probably looks like this

Illustration Credit & Copyright: Mark Garlick (Space-art
This is an old picture that shows the whole Milky Way

7,000 Stars And The Milky Way Credit: Knut Lundmark (Copyright: Lund Observatory)
and we can pick out the same general structure in radio waves, but note very intense source at centre

Credit: C. Haslam et al., MPIfR, SkyView
and γ-rays from the EGRET satellite

Credit: EGRET Team, Compton Observatory, NASA

Galactic Centre Not visible directly (too much dust)

Credit: W. Keel (U. Alabama, Tuscaloosa), Cerro Tololo, Chile

but strong radio source

We can see through the dust (partially) with infra-red: note how dense the star field is

Credit: 2MASS Project, UMass, IPAC/Caltech, NSF, NASA
and X-rays Credit: Fred Baganoff (MIT), Mark Morris (UCLA), et al., CXC, NASA

Centre is very intense (and confused) source of radio waves (note the old supernova remnant)

Credit: N. E. Kassim, D. S. Briggs, T. J. W. Lazio, T. N. LaRosa, J. Imamura (NRL/RSD)
Close to centre a lot of rapidly moving (300 km/s) hot (i.e. ionised) gas

(Gravitational field at centre of galaxy should be very small, so would expect velocities to be small.)

and hot stars
Could be very dense cluster of stars..........but note M31 (Andromeda), M100 and many others show a star-like central nucleus

and very rapidly moving stars Credit: A. Eckart (U. Koeln) & R. Genzel (MPE-Garching), SHARP I, NTT, La Silla Obs., ESO
Now there seems to be evidence for multiple black holes: very intense small X-ray sources close to the centre

Credit: M. P. Muno (UCLA) et al., CXC, NASA

Size of centre < 1pc
Mass of object ∼ 3000000 M₀

Just recently have tracked star as it came within 17 light hours (3x distance to Pluto) of centre

Credit: Rainer Schdel (MPE) et al., NAOS-CONICA, ESO

Whole picture is consistent with a very large black hole at centre, but not nearly as active as we see in other galaxies: e.g. this shows gas at the centre of NGC 1365

Credit: Ground-based image: Allan Sandage (Carnegie Observatories), John Bedke (CSC, STScI) WFPC2 image:John Trauger (JPL), NASA NICMOS image: C. Marcella Carollo (JHU, Columbia U.), NASA, ESA
and if you heard "As it Happens" on Tuesday... Star SDSS J090745.0+24507 is escaping from galaxy at ∼ 500 km/s. velocity points back towards galactic centre. Probably was one part of a binary system, companion was absorbed into black hole and speed acquired from black hole

Credit: SDSS Collaboration (www.sdss.org)

Suspect that there are black holes (1 million to 100 million Mo at the centre of all galaxies: these are very different from BH left over from supernovae.

So are all galaxies the same?