The Milky Way – Our Galaxy

Exam II

• A: 25 or better

• B: 20 or better

• C: 17 or better

• D: 15 or better

• F: 13 or below

• Average: 20.7

Solutions to Exam III (updated)

• 1a,2b,3b,4d,5e,6a,7b,8c,9a,10c,11d,12a, 13c,14c,15c,16c,17b,18d,19c,20a,21b,22c, 23c,24e,25b,26d,27b,28c,29e,30a,31d,32b

Toughest Questions

• Hydrostatic Equilibrium

• Temperature of Sun’s surface

• Star of absolute mag 4.6, apparent magnitude 3.2

Galaxies – Island Universes

• A historic tour of the discovery of the dwindling significance of humans in the universe:

• From the center of the universe towards the edge of an average galaxy amongst 100 billion others

How do we know where we are?

• “Obviously” we are living on a flat Earth at the center of the universe, as a quick look tells us:– The stars, Sun, Moon and planets rotate us– There is no apparent curvature of the ground– The Milky Way is a band that surrounds us – There are no signs for any movement of the

Earth (like wind, or forces throwing us off)

Logic to the Rescue

• How do we avoid these wrong conclusions?– Sound data – Flawed interpretation/reasoningFurther observations are necessary to decide!

• Do we have to question everything?– Yes, in principle.– The signature of genius is to ask the right

question, not necessarily to answer them.

Exploring our own Island Universe: The Milky Way

• A galaxy is a huge collection of stars, gas, dust, neutron stars, and black holes, isolated from others and held together by gravity

Our view of the Milky Way

• Appears as a milky band of light across the sky

• A small telescope reveals that it is composed of many stars (Galileo again!)

• Our knowledge of the Milky Way comes from a combination of observation and comparison to other galaxies

How do we know?

• Question: How can we say anything about our Milky Way, if we cannot see it from outside?

Obviously a bogus picture of our milky way!

Enter: the Genius

• William Herschel (XVIII century)• Simple model:

– Assumed all stars have the same absolute brightness

– Counts stars as a function of apparent magnitude

– Brighter stars closer to us; fainter stars further away

– Cut off in brightness corresponds to a cut off at a certain distance.

• Conclusion: there are no stars beyond a certain distance

Herschel’s Findings• Stars thinned out very fast at right angles to Milky Way• In the plane of the Milky Way the thinning was slower

and depended upon the direction in which he looked • Flaws:

– Observations made only in visible spectrum– Did not take into account absorption by interstellar gas and dust

Discovering other Island Universes

• Data: Lots of nebulous spots known in the nightsky

• Questions: What are they? All the same? Different things?

• Need more observations!

Build bigger telescopes

Famous Telescopes - Herschel

Herschel detected Uranus (1781)(Uranus is visible with the unaided eye)

Famous Telescopes – Lord Ross

• 72 inch Reflector

• built during potato famine in Ireland

• Largest Telescope until Mt Wilson (1917)

The first nebula discovered to have spiral structure: M51

Lord Rosse (1845): M51

Hubble Space Telescope (2007): M51

M99 is a spiral, too!

• Q: do we live in a spiral?

• Q: Are we in the center of the spiral?

• Most probable answer: No!

Enter: next genius

• Harlow Shapley used variable stars, e.g. RR Lyrae stars, to map the distribution of globular clusters in the galaxy

• Found a spherical distribution about 30 kpc (30,000 pc) across– This is the true size of the

galaxy

• Sun is (naturally!) not at the center – it’s about 26,000 ly out

Standing on the shoulders of Giants

• Shapley used methods developed by others to measure the distance to globulars

• Cepheid variables show luminosity-period correlations discovered by Henrietta Leavitt

• Shapley single-handedly increase the size of the universe tenfold!

Structure of the Galaxy

An observer far outside our galaxy would best describe our galaxy and the

Sun's position in it as a …

a) disk of stars with our Solar System 2/3 towards the edge.

b) disk of stars with a bulge containing our Solar System.

c) sphere of stars centered on our Solar System.

d) sphere of stars with our Solar System near the edge.

Intra-galactic Dynamics

• Three main parts of a galaxy:– Bulge (center of

galaxy)– Disk (rotating

around center)– Halo (orbiting

around bulge with randomly inclined orbits)

Properties of Bulge, Disk and Halo

Disk Halo Bulge

Highly flattened spherical football-shaped

young and old stars only old stars young and old stars

has Gas and dust none lots in center

Star formation none since 10 billion yrs in inner regions

White colored, reddish yellow-white blue spiral arms

An up-to-date “Reconstruction”

Activity: Milky Way Scales

• Form groups of 3-5

• Work on the questions on the handout

• Hold on the the sheets until we talked about your findings

• Turn them in with your names on (one sheet per group)

Other Galaxies: Hubble supersedes Shapley

• Edwin Hubble identified single stars in the Andromeda nebula (“turning” it into a galaxy)

• Measured the distance to Andromeda to be 1 million Ly (modern value: 2.2 mill. Ly)

• Conclusion: it is 20 times more distant than the milky way’s radius Extragalacticity!

Shapley’s theory falsified!

Q: How many galaxies are there?

• Hubble Deep Field Project– 100 hour exposures

over 10 days – Covered an area of

the sky about 1/100 the size of the full moon

• Probably about 100 billion galaxies visible to us!

• About 1,500 galaxies in this patch alone

• Angular size ~ 2 minutes of arc

Other Galaxies

• there are ~ 100 billion galaxies in the observable Universe

• measure distances to other galaxies using the period-luminosity relationship for Cepheid variables

• Type I supernovae also used to measure distances– Predictable luminosity – a standard candle

• Other galaxies are quite distant– Andromeda (M31), a nearby (spiral) galaxy, is 2 million

light-years away and comparable in size to Milky Way

• “Island universes” in their own right

Q: How does our galaxy look like from the outside?

• Probably like others, so observe them!

Hubble Classification Scheme

• Edwin Hubble (~1924) grouped galaxies into four basic types:– Spiral– Barred spiral– Elliptical– Irregular

• There are sub-categories as well

Spirals (S)

• All have disks, bulges, and halos• Type Sa: large bulge, tightly wrapped, almost circular

spiral arms• Type Sb: smaller bulge, more open spiral arms• Type Sc: smallest bulge, loose, poorly defined spiral arms

Barred Spirals (SB)

• Possess an elongated “bar” of stars and interstellar mater passing through the center

Elliptical (E)• No spiral arms or clear internal structure

• Essentially all halo

• Vary in size from “giant” to “dwarf”

• Further classified according to how circular they are (E0–E7)

S0/SB0• Intermediate between E7 and Sa

• Ellipticals with a bulge and thin disk, but no spiral arms

Test: What type is this galaxy?

• Spiral

• Barred Spiral

• Irregular

• Elliptical

And this one?

• Spiral

• Barred Spiral

• Irregular

• Elliptical

Type?

• Spiral

• Barred Spiral

• Irregular

• Elliptical

Here’s a weird one!

• Spiral Elliptical

• Barred Spiral Irregular

Solutions

• Sb (Andromeda Galaxy M31)

• E2 (Elliptic Galaxy)

• SBb (Barred spiral galaxy)

• Ir II (Irregular galaxy M82)

Q: How do we know we live in a Spiral Galaxy?

• After correcting for absorption by dust, it is possible to plot location of O- and B- (hot young stars) which tend to be concentrated in the spiral arms

• Radio frequency observations reveal the distribution of hydrogen (atomic) and molecular clouds

• Evidence for– galactic bulge– spiral arms

Rotation of the Galaxy• Stars near the center

rotate faster; those near the edges rotate slower (Kepler)

• The Sun revolves at about 250 km/sec around the center

• Takes 200-250 million years to orbit the galaxy – a “galactic year”

How do spiral arms persist?

Why don’t the “curl up”?

“Spiral Density Waves”

• A spiral compression wave (a shock wave) moves through the Galaxy

• Triggers star formation in the spiral arms

• Explains why we see many young hot stars in the spiral arms

Density (Shock) Waves

The Mass of the Galaxy

• Can be determined using Kepler’s 3rd Law– Solar System: the orbital velocities of planets determined by

mass of Sun– Galaxy: orbital velocities of stars are determined by total

mass of the galaxy contained within that star’s orbit

• Two key results:– large mass contained in a very small volume at center of our

Galaxy– Much of the mass of the Galaxy is not observed

• consists neither of stars, nor of gas or dust • extends far beyond visible part of our galaxy (“dark

halo”)

Galaxy Masses

• Rotation curves of spiral galaxies comparable to milky way

• Masses vary greatly

The Missing Mass Problem

• Dark Matter is dark at all wavelengths, not just visible light

• The Universe as a whole consists of up to 25% of Dark Matter! Strange!

• What is it?– Brown dwarfs?– Black dwarfs?– Black holes?– Neutrinos?– Other exotic subatomic particles?

• Actually: Most of the universe (70%) consists of Dark Energy Even stranger!

Missing Mass Problem

• Keplerian Motion: more distance from center less gravitational pull slower rotational speed

Actual data

Hypothetical Keplerian motion

Galaxy Formation

• Not very well understood– More complicated than stellar formation, and

harder to observe

• Formation of galaxies begins after Big Bang

• Different than star formation because galaxies may collide and merge

Galaxy Formation

• Galaxies are probably built up by mergers– Contrast to break

up of clouds in star formation

• Our own Milky Way is eating up the neighboring Sagittarius Dwarf Galaxy

Galaxy Mergers• Start with high density of small proto-galaxies

• Galaxies merge and turn into bigger galaxies

Actual photo (HST): lots of small galaxies

Galaxy Interaction

Galaxy Collision: NGC2207 vs. IC2163

Collision between NGC 4038 and NGC 4039

The Tully-Fisher Relation• A relation between the rotation speed of a spiral galaxy

and its luminosity• The more mass a galaxy has the brighter it is the

faster it rotates the wider the spectral lines are• Measuring rotation speed allows us to estimate

luminosity; comparing to observed (apparent) brightness then tells us the distance

Active Galaxies

Types of Active Galaxies

• Radio galaxies: radiate a long radio frequencies

• Seyfert Galaxies: between normal and active, compact core

• Quasars: quasi stellar objects, very far away, maybe early stage of galaxy

• Note: most active galaxies look ‘normal’ in visible frequencies

Seyfert Galaxies• Look like normal spiral galaxies• Energy output mostly in IR and radio frequencies• Emitted from small region: nucleus of galaxy• Nucleus of Seyfert galaxy 10,000 times brighter than of

normal galaxy

NGC 5728

Energy Output

• Active galaxies emit most of their energy in radio frequencies

Quasars

• Quasi-stellar objects

• Appear like stars on photographs

• Very distant objects

• Very high luminosity

• Essentially a quasar is a galaxy with exceptionally bright core

• Might be young galaxies

A typical Quasar

• Very distant

Very faint

Appears star-like

Beyond the Galactic Scale – Clusters of Galaxies

The Local Group The Virgo Cluster

Superclusters

Beyond Superclusters• Strings, filaments,

voids• Reflect structure

of the universe close to the Big Bang

• Largest known structure: the Great Wall (70 Mpc 200 Mpc!)