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)

So are all galaxies the same?

Structure of the Galaxy II

MOTION OF STARS recap:

If we think of the sun as fixed, then stars can move at random (space velocity). Taking photo's at widely separated times (∼ 100 years) gives proper motion. Barnard's star moves ∼ 10"/year i.e. diameter of moon in 180 years. Distance ∼ 1.8 p.c.

Space velocity v = 4.74 μ/π km s^-1

Proper motion is cumulative (i.e. just wait longer) so can be measured very accurately radial motion can be measured via Doppler.
=>space velocity
v² = v²_|| + v² _⊥
From proper motion
a) can deduce binaries
b) could detect Jupiter sized planets round nearby dwarfs
c) can measure distances to clusters
d) can deduce how galaxy rotates

Galactic rotation

Obvious (almost) that spiral structure occurs because the outer arms are being dragged behind

Stars will rotate about centre of galaxy in independent orbits.
Assume they are ∼ circular. Local system of rest (LSR) is fixed so that it moves round galaxy at the same rate as average of stars in stellar neighbourhood.

Stars will move partly at random Some of this motion will be due to sun's motion: consider sun to be at rest (=> local standard of rest). But presumably relative to the galaxy, the sun is moving....
The local system of rest is defined as moving at this average orbital velocity: w.r.t. the L.S.R., sun moves at 20 km/sec. towards Hercules. Peculiar motions are defined w.r.t. L.S.R.

Measuring distance to clusters: If we see a group of stars, all apparently moving together at ∼ the same velocity, then when motions are projected on celestial sphere, they appear to converge at a point.
Radial velocity vr measured by Doppler Proper motion μ", so tangential component v = 4.74 μ"/π" (but we don't know π" or d)
But vt/vr = tan(θ), so if we knew θ we could find d......... But if we have a cluster, we do know θ! It is given by angular distance in sky of vanishing point. Hence can get distance d = vr tan(θ)/ 4.74μ"

So if we can measure the proper motion, μ", and the radial velocity, we can deduce distance
For Hyades v = 39.1 km/s, θ ∼ 28⁰, μ = .36"

⇒ 44.3 (±2%!) pc.
Beehive cluster (Prasepe) at 158 pc
χ Perseus at 2290 pc

Gives us scale well beyond limits of parallax

Rotation of Galaxy:

Assume (simplest) that the galaxy rotates as if there is a single large mass at the centre then mv²/r = GMm/r² so v = √(GM/r)Stars closer to centre rotate faster
This will give an illusion of motion relative to the sun
Stars at l = 0 and 1800 will have no Doppler shift Stars at 90 and 270 will have max.
To do this quantitatively: we want to find relative velocity of a star - relative to sun

Angular vel of sun = ω₀, so R₀ω₀ = V₀

Angular vel of star = ω, so R ω = V

SinƖ/R = sin (90+α)/R₀ = cos α/R₀ Also from geometry: Using these gives V = R₀(ω-ω₀) sin Ɩ

Now if star is fairly close to sun

ω = ω₀  +  (R-R₀) dω/dr +............... 

A = -1/2 R₀ dω|
                   dR|R=R₀

R₀ - R = d cosƖ

V = A d sin (2Ɩ)

By a similar argument

Vt = d (A cos(2Ɩ) + A-ω₀)

i.e. If we measure velocities of more distant stars relative to LSR, they will appear to have different motions depending on GALACTIC LONGITUDE Ɩ Want stars of known brightness and distance i.e. Cepheids. Observed velocities of cepheids between 1 & 2 kpc distant

Current numbers:

A = 15 km s-1 kpc-1, A-ω₀ = -10 km s-1 kpc-1 so  ω₀ = 25 km s-1 kpc-1

= 25x10³/3x1019 = 8.3x10-16 s-1

If we can measure v₀, we can then find

R₀ = v₀/ω₀, v₀ ∼ 220 km/s 

Stellar distribution in galaxy

Clear difference between external galaxies in red & blue light: spiral structure only visible in blue, Implies spiral is made up of young stars:

Since the spiral arms do not represent stars gravitationally orbiting in separate orbits, what are they? Look at Populations

Globular cluster stars are extreme population II; (metal deficient) means they must have been formed early & have had very little interaction with dust in galactic plane. "halo" stars are also pop. II, but less extreme Disc stars lying within 500 pc of galactic plane contain more metal, but also very old (i.e. highly evolved on H.R. diagram). Density of stars large in this region< Pop. I Stars: more metal (1-2%) found in open clusters, associated with spiral arms. Bright (O & B) must have been formed recently

Density wave theory

Sound wave in a gas is a localised pressure/density fluctuation which travels at a definite speed v, (almost) unrelated to speeds of molecules In galaxy, density wave will trigger the formation of both large and small stars By time wave has moved on, hot stars will have burnt through their life-cycle, but cool ones will be left

Note: velocity of density wave is not related to velocity of stars: - probably 1/2 V Note: time for density wave to go round galaxy ∼ 5x108yr. Lifetime of O & B stars ∼ 107 yr.
If this is true, then we ought to be able to see the spiral structure outlined by blue stars & H clouds in our own galaxy