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The Milky Way - DOC


									There are dust in the ISM.

                                     The Shape of the Galaxy

Difficult to determine, due to us being located within the Galaxy.

Look in all directions, see stars.

However, in some directions (e.g. in the „Milky Way‟), we see more stars.

Led to the „star count‟ method of mapping out the shape of the Galaxy

Difficulty: _____________ (the absorption of light by the ISM)
                             Modern Structure of Galaxy.

How to determine our location in the Galaxy?

       -Globular Clusters

Globular clusters are gravitationally-bound systems of several hundred thousand stars in

orbit around the center of the Galaxy.

Mapping distances to these shows that they are centered on a point ~8.5 kpc from the


We are located within the ____________ of the Galaxy.
                                 21 cm radio emission

Due to _________________________________.

Due to extremely long wavelength as compared to optical or infrared light, can penetrate

dust and gas very easily.

Detected first in 1951 (Harold Ewen and Edward Purcell)

Used to map the gas distribution of the Milky Way, also seen in many other galaxies.
                                      The Milky Way

                                     Our home galaxy


        Disk of stars and gas/dust, surrounded by an extended spherical halo of widely-

distributed stars and thin gas.

Major components:

        Nucleus: very dense core of galaxy

        Bulge: central ~spherical (somewhat flattened) volume of dense stars, gas, dust.

Radius ______________________

        Disk: _________________ diameter, _____________________ thick disk of

stars, gas, dust.

        Halo: spherical, radius _______________________________, filled with globular

clusters, scattered stars, hot gas, and some (thin) dust.
                               Components of the Galaxy

Halo: old stars (11 billion years or so), very metal-poor

Disk: young stars (0-5 billion years), metal rich

Why difference?

       Halo stars formed early, before Galaxy flattened. Gas mostly hydrogen and

       helium, little metal enrichment by SN had occurred.

       Disk stars: formed later, after many SN had exploded. Gas of Galaxy had

       flattened into disk by this time.

Present-day star formation:

       Only in the disk, in a very thin (200 pc think) layer of gas & dust.
                            Open Star Cluster

Found in ____________of Galaxy
size: 1-10pc
Number of stars: _____________________________

Example: Pleiades

Stars are relatively young, most stars near main sequence
                             Globular Star Cluster

Found in _______________ of Galaxy
contain old stars
Number of stars: 104 – 107
No stars present on upper main sequence
                         The Sun and the Galaxy

Orbital distance: _______________

Orbital period: 240 million years

*Note: the Sun has only orbited the center of the Galaxy ~20 times since its


      (Earth around Sun: ~5 billion times)

Mass interior to the Sun: 1011 solar masses
                       Rotation Curve of the Galaxy

A rotation curve is a plot of orbital radius versus velocity.


       ___________________________: all radii „orbit‟ in the same period,

so velocity increases as the radius does.

       ___________________________: velocity drops with radius (non-


Galaxy: from 1-20 kpc, the rotation speed is constant!

       Since orbital velocity set by gravity, this means that there is much

more material inside 20 kpc than 1 kpc.
                     Galactic Rotation and Dark Matter

For the Sun‟s distance, one needs 1011 solar masses to explain the velocity.

This is ~ what we see visibly.

For 2 times the Sun‟s distance, we need 2*1011 solar masses of stuff, but

there is little visible matter (i.e. stars) beyond the Sun in the Galaxy,

certainly not another 1011 solar masses.

Therefore, there must be matter in a form not visible to us either by emission

of light directly or by absorption of light from more distant objects (like

other galaxies)!

This matter is called ______________________.
                                   What is dark matter?

Short answer: unknown.


        MACHOs: Massive Compact Halo Objects.

________________________________. (Unlikely, these has been looked for and not

found in sufficient quantities to fully explain the dark matter)

        WIMPs: ________________________________________. Unknown at this

time, is a matter of physics to find or disprove these.
                        Spiral Structure of the Milky Way

The Galaxy (disk) is a spiral of material separated by less-dense regions.

Spiral structure derived from 21-cm maps of gas, along with distances to star-forming

regions in the nearby region of the Galaxy.

Most star formation seems to take place in the spiral arms of the Galaxy.

Spiral arms near the Sun:
                                     The Galactic Center

Visible light: can‟t be seen from the Galactic center.

However, infrared and radio gets to us from there.

What we see:

       100 pc from the center: 100 stars/cubic pc.

       10 pc from the center: several thousand stars/pc3

       Much dust and gas also collects in the center

       2-8 pc from the center: a ring of gas, rotating at 110 km/s.

                  Implies that there is 10 million solar masses of material within the central

2 pc of the nucleus!

       Most likely this mass is all contained within a black hole at the center of the


                  Other evidence for this: radio emission from likely near-motionless

accretion disk.
                               Galaxies in General (Ch. 23)

What is a galaxy?

       Initially, it was unclear.

Appearance of galaxies (early on):

       Fuzzy patches in sky

The major debate in astronomy was if these objects were located within or outside the

Milky Way.

                                     Types of Galaxies
Two major types

       Spiral (SB/S)

               (Barred or un-barred)

       Elliptical (E)

Classification system:

       Hubble „tuning-fork‟ diagram


               S/SB (based on bar)

               Sa,Sb,Sc (based on bulge size and tightness of arm wrap)

       Ellipticals (E)

               0-7 (based on apparent flatness)
                                    Elliptical Galaxies

Nearly featureless

Round balls of stars, have very little gas/dust (as compared to spirals

Consist of only old stars, typically metal-poor.

No (or nearly no) star formation taking place!

Appearance (0-7) not due to intrinsic property of galaxy, rather due to viewing angle.

Range in sizes:

       Smallest: _____________________ galaxies:

               few kpc across

               few 100,000 solar luminosities

       Largest: __________ galaxies

               Up to 2000 kpc across

               Few 100 billion solar luminosities
                                       Spiral Galaxies

(Around 80% of all galaxies)

Have flattened disks of stars as primary feature

Most have spiral arms within the disk.

Major types:

        Barred spiral

        Un-barred spiral

Split based on presence of a central bar-like structure in place of a simple round nucleus.

If the bar is present, the spiral arms (typically) will begin at the ends of the bar.
                               Spiral Galaxy Classification

Classed as (for most galaxies) type „a‟, „b‟, or „c‟.

Type ___ galaxies have (as compared to type ___)

       More tightly-wound arms

       Larger bulges

All spirals have a large component of gas and dust, leading to active star formation

(typically within the spiral arms).

Consist of stars with a wide range of ages (and metallicity).
                                    Irregular Galaxies

Catch-all category for galaxies which are neither spiral nor elliptical.

Typically patchy in appearance, with no major organized structure.

Many have even more gas/dust than spirals.
                                      Galaxy formation

Similar to stars, in that they form from the collapse of huge clouds of gas and dust.

Spiral galaxies retained the initial rotation of the cloud, elliptical galaxies got rid of most

of the rotational energy.

How to get rid of rotational energy? (Remember, rotational energy can‟t be created or


        Transfer of energy to the dark matter of the galaxy.

Requires a lumpy initial distribution of matter for formation of ellipticals, and a smooth

distribution for spirals.
                                    Galaxy collisions

Compared to stars, galaxies are rather closely-packed.

Average star-star distance: 30 million stellar diameters

Average galaxy-galaxy distance (in clusters): under 1 diameter.

Therefore: Collisions between galaxies are common, while collisions between stars are

                             Collision between two galaxies

The stars don‟t collide, they simply pass on by.

Gas clouds, however, (as they are much larger), do collide, and get left behind.

High-speed collision:
       Stars keep going (structure of galaxies are disrupted, however)

Low-speed collision:
       Can result in eventual merger of two galaxies.

Typical result of collision between two spirals: an elliptical
Result of this (seems to): more ellipticals in dense regions of galaxies, and
more spirals in less-dense regions of galaxies.

In very dense regions, large galaxies can become galactic cannibals as they
absorb multiple smaller galaxies. This leads to the giant cD galaxies.
                                   Rotation of Galaxies

Rotation curves of galaxies like the Milky Way also show evidence for dark-matter halos.

In fact, spiral galaxies must be made mostly of dark matter.

Rotation curves of various galaxies also show extreme orbital speeds in the nucleus,

probably due to massive black holes in the centers of many galaxies. However, this is a

very difficult observation to do, due to the small angular scale of the nucleus in the

distant galaxies.
                                     Cosmic Distances

So how do we measure distances to galaxies?

We use a sequence of measuring „rods‟, each calibrated by the previous one.

Short (astronomical) distances:

       Primary distance indicators


              Cepheid variable stars

       These are defined by objects in the Milky Way
                                Other Distance Indicators

Moderate distances:

       Secondary distances:

       (Defined by objects in nearby galaxies)

              Largest HII region

                      (scales with galaxy brightness)

              Average brightness of globular clusters

              Tully-Fisher relation

              Type Ia SN

Far distances (beyond 25 Mpc)

       Tertiary distance indicators

       (Based on entire galaxies)

              Brightest galaxy in cluster

              Hubble‟s Law
                                    Type Ia Supernova

A binary system with ____________________________.

Mass transfer in this case is very rapid

        Result: layer of accreted material on WD is kept so hot it doesn‟t go degenerate.

Instead, get a steady shell of hydrogen fusion at the bottom of the accretion layer.

Eventually, the core temperature reaches 10 billion K, and carbon fusion begins.

        The core is degenerate, so it doesn‟t expand, just gets hotter, fusion takes place

faster, etc…

        The WD ends up running through all the fusion reactions, even generating

elements heavier than iron (by other processes). The energy released blows the star apart


1) Extremely bright, so ___________________________________________.

2) The exploding object is _____________________________________________


        Therefore, the explosion will always be the same!

This last makes SNIa exceedingly useful as standard candles.

Distinguishing them from Type II SN done by examination of the lightcurve (Type Ia

doesn‟t have the plateau in the decay) or by spectral analysis (Type Ia supernovae have

no hydrogen lines).
                                  Tully-Fisher Relation

For spiral galaxies, there is a relation between the rotation rate of the HI gas (measured in

21cm emission) and the luminosity of the galaxy.

Astronomers can measure the Doppler width of the 21cm emission line for the galaxy,

then derive the luminosity. Having measured the apparent brightness, they can derive the


Method calibrated using local galaxies and local clusters of galaxies (i.e. Virgo cluster, at

15 Mpc).

       (Revised calibration of this relation: M. Pierce & A. Rogel, in publication.)

A somewhat similar relation exists for elliptical galaxies.
                                     Hubble’s Law

This relation is based on the observation that (almost) all galaxies are moving away from


First measurement of radial velocity: V.M. Slipher (1913) of Andromeda. It is moving

towards us at ~300 km/s. (15 times the average radial velocity of stars)

By 1917, 25 spirals had radial motions measured, and 21 were moving away.

Edwin Hubble continued this process, and also was able to estimate distances to the

galaxies by other means. He discovered that




H0 is what is called Hubble’s constant.
                               Hubble’s Law: the math

Hubble‟s constant (today‟s value) is

        71+/- 7 km/s/Mpc

This means that

        A galaxy at 1 Mpc from us is moving away at ____ km/s.

        A galaxy at 2 Mpc from us is moving away at ____ km/s.

        A galaxy at 10 Mpc from us is moving away at ____ km/s.


        A galaxy moving away from us at 284 km/s is _____ Mpc away.

                      Hubble’s Law and the Expanding Universe

Clearly, if everything is moving away from us, the Universe must be expanding!

But are we the center of the expansion?


Think of a spring being stretched on the table.

ANY point on the spring will see all other points getting further away, and (per time), the

more distant points will move away more. Therefore (extending the analogy to an

infinite spring), every point on the spring sees the same thing, that is: Every point on the

spring is moving away from it. Extend this to 3 dimensions (e.g. expanding raisin bread),

and this is what is happening to the Universe.

       Astronomers in every galaxy will see the same effect, that is everything moving

away from them, and the more distant galaxies moving away faster.
                           Meaning of the Redshift of a galaxy

1st: the redshift observed is not actually (totally) due to a true motion of the galaxy.

Rather, it is due (mostly) to the expansion of space between us and that galaxy.

Therefore, the redshift is properly termed an expansion redshift

2nd: As we look farther away, the light that we are seeing has taken longer and longer to

get to us. So, knowing a galaxy‟s redshift (and the evolution of the Hubble constant over

time), we can compute the lookback time.

Lookback time: _________________________________________________________
                       Questions about the expanding Universe

1) So does the space between objects here on the Earth expand? What about within the

Solar System? The Galaxy?

2) How is it that the Andromeda Galaxy (at ~0.7 Mpc) is moving towards us at 300 km/s,

instead of away from us at ~50 km/s?

3) What does this say about the history of the Universe?

4) Is the expansion rate constant?

5) Will the Universe keep expanding forever?
                                 Does local space expand?

Short answer: ___________

The forces between materials (nuclear forces, electro-magnetic forces, gravity) all act to

counteract this expansion. So on scales smaller than galaxies or so, the other forces act to

keep the expansion from spreading out the local material.

        Think of the raisin-bread example (in text). The bread expands (this is space),

but the raisins do not (these are galaxies, etc.).
                                  Motion of Andromeda

The expansion of space as described by Hubble‟s Law is a universal phenomenon.

However, galaxies can also have „peculiar‟ motions, motions with respect to space itself.

In the case of Andromeda, the galaxy, under the influence of the gravity of the Milky

Way, is moving towards us faster than the expansion of space is carrying Andromeda

away from us. (Think of walking backwards on a slidewalk; one can make progress in

the direction opposite that of the slidewalk if one walks fast enough.)

Any given galaxy‟s measured motion will consist of the combination of its Hubble‟s Law

expansion and its own peculiar velocity.

Locally, the peculiar velocities can be larger than the Hubble expansion speed, so one can

get galaxies moving towards us.
                                     Other Questions

History of the Universe

Does Hubble‟s constant change with time

Fate of the Universe

All these questions belong to the subject of Cosmology, which will be covered later on.

But first, a look at some of the other strange objects in the Universe.
                                     Active Galaxies


In the mid 1900‟s, some very strange objects were discovered. They looked like stars,

but their spectral lines did not match any known elements. They were called „quasi-

stellar objects‟, or quasars.

In 1963, it was finally realized that the quasar spectra were normal elemental spectra, but

highly redshifted! (Redshifts up to a factor of 6 have been measured for quasars to date,

which means that a spectral line normally located at (say) 91.2 nm (UV) would be located

at 638.4 nm (red visible).

However, these objects can appear to be point-like (stellar) sources, so they must be

extremely bright! What are they?

General (Observational) Properties of Quasars

1) Luminosity: up to ________________________

2) Spectrum:

       Continuous radiation (from X-ray and above down to radio) of near-uniform


       Emission lines:

               Broad lines of various elements

               Narrow lines of various elements
                            Examples of Spectra of Quasars

Look at page 557 in your text for a „typical‟ quasar spectrum (note: this spectrum is not

redshifted. Optical emission is from about 400 nm to ~700 nm).

       *Note: 100 nm = 1000 angstroms

Compare that spectrum to these recently acquired at WIYN:
                                   Clusters of Galaxies

Most galaxies are found in groups of one kind or another.

Range from a few galaxies to thousands

Richness of a cluster:

       Measures how many galaxies the cluster contains.

       Measured by counting bright galaxies only.

       *Note: not all clusters will contain the same ratio of bright/dim galaxies, so this is

not a true measure of galaxy number…

Shape of a cluster

       Regular: round

       Irregular: no overall symmetry.

               (possibly several groups)
                                   The Local Group

Small cluster of ~35 galaxies

Major members:

       Milky Way


Rest are small, dim galaxies of various types

Size of cluster: ~1 Mpc.
                                       Virgo Cluster

*Note: text is wrong as to distance

~2000 galaxies

~15 Mpc away

Classed as a poor (in richness) irregular cluster.

Brightest members:

       Both spirals and ellipticals

Notable members:

       M100 (spiral)

       M87 (giant elliptical): has spectroscopic evidence for a 2-3 billion solar mass

black hole at the center
                                    Other Clusters:

Many other clusters exist, spacing ~10s of Mpc.

Some examples:

       Fornax Cluster (~20 Mpc)

              Also used for distance scale calibrations

       Coma Cluster (~70 Mpc)

              Extremely rich, regular cluster

              Several hundred bright galaxies

              No spirals in core.

two clusters in Ursa Major ( ~ 270Mpc and 680Mpc)
                                     Galaxy clusters:

                                         Other matter

Clusters are made up not only of galaxies, but also other things:

Inter-galactic medium

       Seems to be very hot (X-ray producing) this gas filling the cluster

       Mass similar to the galaxies in the cluster

Dark matter

       Inferred by galactic velocities

       90% of total mass of the cluster is dark matter.

       This is NOT counting the dark matter in the galaxies…

As galaxies are arranged in clusters, clusters are arranged in superclusters


       Local (or Virgo) Supercluster

               Consists of local group, Virgo cluster, etc.

               Spread over ~40 Mpc

       Great Wall

               Great sheet of clusters spanning ~100 Mpc, at a distance of ~60 Mpc

Between the superclusters are voids, with very very few galaxies

Why this arrangement?

       Totally unclear at this time.
                                  Fate of the Universe:


From Hubble‟s Law, the Universe is expanding.

Looking back in time, we can calculate (for a constant Hubble parameter) how long the

Universe has been expanding to reach its present size. This is called the Hubble time.

       For H0=70 km/s/Mpc, tH= 13 billion years.

So…according to this, 13 billion years ago, all of space itself and everything in it was at

the same place. It then started to expand.

       This is the Big Bang theory
                                 Notes on the Hubble time

Space is expanding, but gravity is an attractive force. Therefore, gravity is acting to slow

the expansion of the Universe.

Therefore, the Universe should have been expanding faster in the past than it is now, so

the true age of the Universe is less than a Hubble time.

*NOTE: Certain globular clusters have ages of ~15 billion years…

       Something is wrong

Solving this is one of the primary mission of the Hubble space telescope.

(If the Hubble constant is lower, the Hubble time is longer. Therefore, HST set out to

measure H0 as accurately as possible, and (if possible) the change in the Hubble

parameter over time due to gravity.)
                                      The Big Bang

                                 (or ‘Let there be light’)

All of space itself was wrapped up in a singularity. Then, for whatever reason, it


Initially (prior to 10-43 seconds after the explosion), physics doesn’t work, we can make

no statements about this time.

After that, we can try to unravel what happened.
                                    Significant events

       At first: pure energy. Temperature >1013 K Particles of all kinds are created, then

destroyed immediately in the radiation field.

       10-6 seconds: protons and neutrons „freeze out‟, no longer being created, as the

photons no longer carry enough energy to create them.

       1 second: electrons freeze out.

               (neutrinos „decouple‟ from other matter, leaving them free to travel

forever. a lot of them pass through you each second.)

       100 seconds: Helium nuclei form.

       Over the next 200 seconds, other reactions form lithium, beryllium, boron.

       Few 100 thousand years: Universe becomes transparent to radiation. Origin of

the Cosmic Microwave Background, which is extremely redshifted photons from the

3000K transparent Universe.

       1 billion years: matter in the Universe had gathered into concentrations that

would become the galaxy clusters
                 Cosmic Background Radiation (CBR)

- created in the __________________Epoch (after few 100 thousand years)

Discovered accidentally in mid-1960s

Spectrum: corresponds to a blackbody of _____________K

(originally:          K , redshift: 1000)
                                  Fate of the Universe

The other great question of cosmology is the ultimate fate of the Universe:

Will gravity finally reverse the expansion, bringing everything back together in a Big


Or will the expansion slow down over time, but always keep expanding?

These are the only two options consistent with physics as we understand it.

                                Perlmutter, et al, 1999 ApJ

                     The Expansion of the Universe is Accelerating!

By careful examination of distant SNIa, Perlmutter, et. al. determined that the expansion

of the Universe is not slowing down, as was thought it had to be. Instead, just recently

(last billion years or so), the expansion has begun to accelerate, increasing the Hubble


This violated just about everything we thought we knew about how the Universe works,

in particular:

1) Gravity is the only long-range force

2) Gravity is only attractive

Therefore: the expansion must be slowing down!

But actually…
                            So what is driving the expansion?

        5th force


        vacuum pressure

        dark energy

        Cosmological constant


We really don‟t know…this result is still so recent and so fundamental (and it has been

fairly well confirmed by completely independent techniques) that it will take some time

to sort it all out.
Assumptions of modern science revisited

the Perfect Cosmological Principle

      -“We are not at a special place or time in the universe.”

      (Scientists do not like to think there is anything special about us or our

location in time and space.)


      The acceleration just started! So we are in a unique (relatively) time

in the Universe!
                         A NEW theory -- the cyclic universe

                          by Paul Steinhardt & Neil Turok

Big bang  expansion (as the stand picture)

Finally, everything are "diluted away,"

leaving the universe smooth, empty, and flat.

Then everything contracts in “big crunch”,

and a fresh cycle begins.

two universes: the other one - dark matter

 - still controversial
                      Fate of the Universe (general thought)

Expansion forever, with rate first slowed by gravity, then (once the Universe reaches a

critical size) an acceleration of the expansion rate. Nothing (as we understand it

currently) will halt the expansion, so eventually matter will be very very thinly

distributed in a cold, dark Universe.

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