The Milky Way Galaxy

A galaxy is a collection of stellar and interstellar matter –stars, gas, dust, neutron stars, black holes – held togetherby gravityby gravity.

This is what we see if we look towards the Galactic center from our vantage point on Earth.g p

We live in the Milky Way Galaxy.

Looking at other spiral galaxies gives us an idea of how our galaxy looks.

Andromeda Galaxy – 3 million lyr

3 main parts of a spiral galaxy:

•Galactic disk

G l i b l•Galactic bulge

•Galactic halo

In the 18th century William Herschel estimated the size and h f l b i l ti t i ll di tishape of our galaxy by simply counting stars in all directions

(assumed all stars are the same luminosity).

SunSun

What didn’t Herschel know about? DUST!

(+ all stars are NOT the same luminosity)More dust along the disk causes the distribution of stars to drop-off artificially – objects more than a few kpc away are hidden by all of the dust.

Variable Stars

-stars whose luminosity change with time because y gthey are physically pulsating

There are 2 types of Variable Stars:

RR LyraeRR Lyrae CepheidCepheid

•All pulsate in the same way•Periods 0.5 to 1 day

•Each pulsate somewhat differently•Periods range from 1 to 100•Periods range from 1 to 100 days

• Variable stars areVariable stars are located on the “instability strip” in the HR diagram

• The star is internally unstable

T t d di• Temperature and radius vary in a regular way causing pulsations

Cepheid variables are tl hi h tmostly high-mass stars

evolving across the top of the HR diagram

RR Lyraes are lower mass horizontal branch stars

There is a relationship between the pulsation periodof a variable and its luminosity.luminosity.

We can see Cepheids in nearby galaxies!

What does this mean?

They can be used to determine distances!!determine distances!!

Flux Luminosity∝

distance2

Edwin Hubble Discovered Cepheids in the spiral nebula in the constellation Andromeda in the 1920s

His notes about a variable star

Hubble then derived the distance to Andromeda, and showed that it was external to our Galaxy (~3 million light years away) – another galaxy in( 3 million light years away) another galaxy in its own right!

This had a profound impact on our Note the date: 6 Oct 1923 understanding of our place in the universe, similar to the Copernican revolution.

RR Lyraes are too faint to be seenRR Lyraes are too faint to be seen in distant galaxies.

BUTBUT

They can be seen in Globular Clusters tightly bound groups ofClusters - tightly bound groups of stars in our galaxy.

In the early 20th century HarlowIn the early 20th century, Harlow Shapley observed RR Lyraes in GCs and determined 2 things:

•Most GCs are at great distances (1000s of pc) from the Sun•Their 3-d distribution is a roughly spherical volume of space b 30 kabout 30 kpc across

Globular Clusters in our Galaxy

Shapley realized that the GCs map out the true extentout the true extent of our galaxy!

Galactic Halo

The hub of the l i thgalaxy is the

Galactic Center -about 8 kpc from the Sun

The Structure of our Galaxy

Disk is about 300pc thick at the Sun

Globular clusters reside in a nearly spherical “halo.”

Globular clusters (and halo stars) are 10 – 15 billion years old (red in color).

Clusters (and stars) in the bulge are similarly old.

The disk contains a mix of stellar ages – from intermediate ages to still forming…

•young O and B starsy g

•young open clusters

•nebulae from which stars form

The disk also contains considerable amounts of gas and dust, while the halo does not…

Wh i th littl t f ti i th h l ?Why is there so little star formation in the halo?Why are halo stars and GCs deficient in heavy elements?

Components of the Milky WayComponents of the Milky Way

1. Disk (approximately Solar metallicity)1. Disk (approximately Solar metallicity)a. Stars a. Stars -- mostly young stars many in open clustersmostly young stars many in open clusters

-- all have ages less than 7 billion yearsall have ages less than 7 billion yearsb. Gasb. Gas -- HI (neutral Hydrogen) and HII (singly ionized Hydrogen)HI (neutral Hydrogen) and HII (singly ionized Hydrogen)b. Gas b. Gas HI (neutral Hydrogen) and HII (singly ionized Hydrogen)HI (neutral Hydrogen) and HII (singly ionized Hydrogen)c. Dustc. Dust

2 Halo (metal2 Halo (metal poor)poor)2. Halo (metal2. Halo (metal--poor)poor)a. Stars a. Stars -- mostly old stars some located in globular clustersmostly old stars some located in globular clustersb. Gas b. Gas -- a small amount of HI, no HIIa small amount of HI, no HIIc. Dust c. Dust -- little or nonelittle or none

3. Bulge (metal3. Bulge (metal--rich)rich)3. Bulge (metal3. Bulge (metal rich)rich)a. Stars a. Stars -- mostly intermediate to old stars some in globular clustersmostly intermediate to old stars some in globular clustersb. Gas b. Gas -- little or nonelittle or nonec Dustc Dust little or nonelittle or nonec. Dust c. Dust -- little or nonelittle or none

How are things moving in the galaxy?

Doppler shifts reveal orbital motions about the Galactic Center:pp

The Sun, at 8 kpc radius, orbits at 220 km/s.

With one orbit taking g225 million years.

These data suggest a model for the formation of our Galaxy:

Stars formed early keep their randomkeep their random orbits.

The rotation causes the dust to flatten in a disk

When gas + dust clouds collide they experience “friction,”experience friction, which tends to organize their orbits.

Many stars form later, in the disk.

How did our Galaxy get its spiral arm structure???

Many galaxies look like this.

Some look like this.

No galaxies look like this.

Galaxies are old enough for many orbits of the stars.

Stars nearer the Galactic center orbit faster, ,so why doesn’t the spiral structure “wind up?”

A leading theory for galactic spiral i i l d itarms is spiral density waves.

Density waves are something that everyone over 16 years of y g y yage are familiar with……

Galactic Spiral Arm Structure: Spiral Density Waves

The spiral wave pattern is nearly stationary (it moves around as well but not as much), while the gas, dust and stars move through it.

An alternative theory is that of Self-propagating Star Formation

The formation of stars drives the waves – shock waves from the later evolution of stars creates denser regions where new stars are created.

H d thHow do we measure the mass of the Galaxy?

• From Kepler’s 3rd Law (as modified by Newton):• From Kepler s 3rd Law (as modified by Newton):

P2 (in Earth years) = a3 (in AU) / Mtotal (in solar mass units)

• Parameters of the Sun’s orbit around the Galactic Center:

radius (= a) = 8 kpc = 1 65 × 109 AUradius (= a) = 8 kpc = 1.65 × 109 AU

period (=P) = 225 million years

→ Mtotal ≈ 9 × 1010 Msun ≈ 100 billion Suns!

Mass of the Galaxy: Rotation Curve

I th f 2 b di biti h th ( E th biti th S )• In the case of 2 bodies orbiting each other (e.g. Earth orbiting the Sun) Mtotal is just the Sun + Earth mass ≈ Msun.

• In the case of the Sun orbiting around the Galaxy, what is Mtotal?

A di t N t M i th S ’ l th f th• According to Newton, Mtotal is the Sun’s mass plus the mass of the Galaxy interior to the Sun’s orbit.

• The orbital speed is: v = sqrt(G Minterior / radius)

• The orbital speeds of the planets orbiting the sun follow v = sqrt(G MSun / radius)

• Inside the Galaxy M increases with radius so velocity

• Outside the Galaxy as in the Solar System M = M

• Inside the Galaxy, Minterior increases with radius, so velocity may stay constant or even increase with radius.

• Outside the Galaxy, as in the Solar System, Minterior = Mtotaland again v = sqrt(G Mtotal / radius).

Mass of the Galaxy: Rotation CurveEdge of visible Galaxy at 15 kpc -rotation speed yields 2 × 1011 Msun

At 40 Kpc, rotation speed yields 6 × 1011 Msunv ∝ sqrt(Minterior / radius)

“Dark Matter” Problem

Within Galactic bulge -spherical distribution → v ∝ r

If the mass ended at 15 kpc, we If the mass ended at 15 kpc, we should find v should find v ∝∝ sqrt(1/radius). sqrt(1/radius).

“Dark Matter” Problem – 2/3 of the Galaxy’s mass is invisible!?!– mostly beyond the visible light radius!

And here…

And here…

And here

And here…

here…

And here… And

here…

← There is lots of “stuff” out here →And here…

And here…

Dark Matter candidates:

White Dwarf StarsWhite Dwarf Stars

Very Low Mass Stars – Red Dwarfs (0.2 Msun)

Brown Dwarfs (<0.08 Msun)

Neutron StarsShould be far too few

Black Holes

Exotic sub-atomic particles

Should be far too few.

p“Weakly Interacting Massive Particles” – WIMPs

The Search for Stellar Dark Matter (brown or white dwarfs): Massive Compact Halo Objects – MACHOsMassive Compact Halo Objects MACHOs

•The faint foreground bj (b hiobject (brown or white

dwarf) bends the light of the background star gbecause of its gravitational field

•The light from the background star is f d “l d” bfocused or “lensed” by this effect and the star appears brighter.pp g

What’s at the center of the Milky Way Galaxy?

Visible light images show many stars but dust hides many more.

In IR and radio wavelengths,we see MANY stars (a million times more dense thanmillion times more dense than the solar neighborhood).

Sagittarius A• bright radio source at the center of theat the center of the Galaxy

Sagittarius (Sgr) A*• object at the very center of the Galaxyy• million times more luminous than the Sun (IR radio XSun (IR, radio, X-ray, and gamma ray source)Massive Black Hole!!

Black Hole at Sgr A*

• entire region less than 10 AU across• Stars and gas near Sgr A* are moving fast!• Stars and gas near Sgr A* are moving fast!• Mass of the black hole - 2 to 3 million solar masses!• Radiation arises from the accretion disk

Black Hole here

100 kpc 1 parsec(over 200000 AU)(over 200000 AU)