1

•Number of Stars•Mass•Shape•Size•Age•Sun’s location•First ideas about MW structure

The Milky Way

2

• Hershel could not determine the actual size of the galaxy.• In this model, the Sun was located very close to the center of the

galaxy.

6400 Light Years

1300

Lig

ht Y

ears

The shape and size of MWStar Counts: William Herschel: 1785“The further the Milky Way extends in some direction, the more stars we should see.”

3

Globular Clusters

1900’s: Harlow Shapley

4

M3: Inconstant Star Cluster

Credit & Copyright: J. Hartman & K. Stanek (Harvard CfA)

2

Shapely’s Map of 93 Globular Clusters

Estimation of distances:

1.Variable stars in clusters

2. Angular size of clusters

• Shapely (~1900): “The system of globular clusters is centered on a point some 26,000 light years away. If these clusters are evenly distributed, that should be where the center of the Milky Way is located.”

6

The revolution of the Sun The revolution of the Sun around the Galactic centeraround the Galactic center

2 32 4

( )dP

G m Mπ

=+

2 32 4 dP

G Mπ

=

2 dPvπ

=

2v dMG

=

Mass of material inside Sun’sorbit:100 billion Solar masses

• The predicted and observed rotation curve of a typical spiral galaxy

The discovery of stellar populations

• Stellar groups with different distribution, composition, age

• Walter Baade, in 1944, based on blue and red photographs of Andromeda

3

Globular clusters and open (galactic) clusters

10

Location of the interstellar dust•30-300 µm : 10-90 K : distribution of warm dust•Disk (40 kpc (50 kpc ?) : 0.6 kcp); bulge (diameter 2 kpc); bar

The MW structure

11

A Map of Neutral Hydrogen in the Milky Way

12

Radio-astronomy:detailed structure of

the MW

Doppler shift of 21-cm line

4

13

Spiral Structure of the Milky Way

Spiral arms look strung out along the line of sight Different parts overlapped Need distances to bright spiral structure tracersOur understanding entirely depends on observational data

14

Norma-Centaurusor

Norma-Scutum

Carina-Sagittarius

The spiral structure of the Milky Way

Galactic Spiral Arms are due to Density Waves

• Basic facts about spiral arms• Density of material is higher in the spiral arms• Within the spiral arms additional gravitational

influence is exerted on stars and ISM• In spiral arms matter moves slow and piles up• Increase in density, change of direction of motion• Not “material” arms • Entire spiral pattern rotates with the same orbital

period (500 mil yr)

Star-formation within the spiral arms

5

The age and evolution of the MW• Oldest globular clusters: turnoff point of MS, ~ 13 billion yrs

• As stars form the amount of ISM decreases– The rate of star-formation is not fixed

– Current rate of star-formation

• Chemical composition of ISM changes– More “metal rich” stellar generations

• Fate of MW– End of star-formation

– The last low-mass stars

• Merger events with nearby galaxies– LMC, SMC

– Andromeda18

Galaxies

Collection of stars, gas and dust bound together by their common gravitational pull.

Galaxies range from 10,000 to 200,000 light-years in size.

19

1920’sClassification based on appearance

6

The Local Group

• Members and Size

•Dominant galaxies (M31, MW, M33)

• Dwarf elliptical galaxies

• 50 similar groups in a sphere with a radius 20 Mpc

•Virgo Cluster at 20 Mpc: moving away from the Local group at 1000 km/s (should be moving at 1300 km/s)

Clusters of galaxies

The closest clusters – Vigro(20 Mpc) and Coma (90 Mpc)

Virgo

Coma

23

Gravitational vs. Visible Mass

Fritz Zwicky(1933)

The Coma Cluster

Gravitational / Visible Mass = 10/124

Cosmology

The known part of the Universe - to a distance of ~ 4000 Mpc

• Study of the structure and origin of the universe

• Observational science– The large-scale distribution of galaxies– Looking out to extremely large distances – The motions of galaxies

7

Redshift in spectra of galaxies

1929 1931

Distance and Velocity (away) are related!v = H x D

Implications of the Hubble’s Law

• Space between galaxies is expanding uniformly - the universe itself is expanding- the further away a galaxy is from us the higher the recession

velocity

• Even though the galaxies appear to be moving away from us, we are not at the centre of the universe

– an observer in a distant galaxy would see the same effect

• Cosmological principle: the universe is homogeneous and isotropic“no preferred places and directions”

• What exactly is expanding?– Ordinary things are not expanding – The expansion is noticeable only at fairly large separations 28

The Big Bang and the expansion age of the Universe

• Everything was located very close together in the past, Universe was very dense and hot about 14 billion years ago

• The expansion began with the Big Bang – expansion of space – the universe is expanding everywhere we look: there is no special location in the universe where the Big Bang originated – the Universe has no centre

• How long a galaxy has been traveling: t=d / v

• t=d/(Hd) t=1/H Hubble time

• “one Hubble time” ago all of the matter of the Universe was located together - estimation of the expansion age of the Universe

8

29

• Tremendous explosion (filling all of space with all of the particles )

• We can’t see radiation produced during the first 300 000 yrs

• Enormous increase of scale during a very short time in the early universe (inflation model)

• 300 000 yrs after BB universe had cooled to about 3000 billion degrees Kelvin

• The universe became transparent for the radiation – isotropic cosmic background radiation

The Big Bang

30

Galaxy Cluster in the Early Universe

The cosmic microwave background (CMB) radiation

Satellite missions detected fluctuations in the microwave background:

COBE (1989) and WMAP (2001)

2001, NASA’s -- Wilkinson Microwave Anisotropy Probe

9

33

Expansion forever or collapse?

A sketch of how gravity and the energy of expansion determine the behavior of the Universe. 34

The fate of the Universe• Not enough information about the parameters of

the Universe• Cosmological models: the fate depends on

average density• Critical density 3H2 / 8πG = 8.3 x 10 -30 g/cm3

• Ω = actual density / critical density – Ω = 0 , the universe is empty, will expand forever – Ω > 1 , the universe will recollapse, bound universe– Ω < 1 , the universe will expand forever, unbound

universe– Ω = 1, still unbound universe

35

In which model (theory) of the universe will all the galaxies eventually show a blue shift in their spectrum instead of a redshift? (open, closed or flat)

36

Geometry of the Universe Density of matter connected to geometry of space

Geometry:Positively curved

Closed space; finite Universe; Ω > 1 (Big Crunch)

Negatively curvedOpen space; infinite Universe; Ω < 1 (expands forever)

FlatOpen space, infinite Universe; Ω = 1

10

37

Geometry of Universe

2001, NASA’s -- Wilkinson Microwave Anisotropy Probe

Flat, open, infinite Universe; Ω = 1

38

The makeup of the universe

• Deduced from observations of the brightness variations in the CMB

39

The fate of the Universe

40

Primordial ripples in density created the large-scale structure that we observe now

What What created created the the galactic galactic clusters?clusters?

11

Fig. 25.26

Other SuperclustersSuperclustersVirgoComaCentaurus…

Large Sheets of GalaxiesGreat WallPerseus-Pisces Great Wall…

Voids

“Great Wall”Large single structure Dimensions are about 600x250x30 million light yearsGiant quilt of galaxies across the sky

Fig. 25.27

Mapping the UniverseA slice of the Universe

Redshift surveys are ways of mapping the distribution of galaxies around us

Redshift - radial velocity –distance

Distance + coordinates = location within Universe

Spherical coordinate system centered on the Milky Way

Location of galaxies in slices of the Universe centered on the Earth

43The known part of the Universe - to a distance of ~ 4000 Mpc

Flat, open, infinite Universe; average density = critical density