The Milky Way

The giant communities in which stars live…

How did our Galaxy form and evolve?

What are the spiral arms?

What lies at the center?

Henrietta Leavitt and Cepheid variables:

Delta Cephei – the first variable star observed

The apparent magnitude changes from 3.5 to 4.4 over a period of 5.366 days

Typical periods for other “Cepheids” range from 1 to 60 days

1912 Leavitt photographed stars in the SMC

She noticed stars that pulsate

She assumed they were all at about the same distance from us (…on the cosmological scale, that’s not a bad assumption)

Same distance => she could directly compare the luminosity of the stars…

She noticed a period-magnitude relation!

Q: How might this be of use…?

What makes a Cepheid a “Cepheid”:

These are post main-sequence stars

Helium in the atmosphere absorbs and releases energy like a spring

When the He is absorbing E dimmer

releasing E brighter

The “instability strip” exists because if the star is too hot He atmosphere too high

too cool He atmosphere too low

Too hot or cool and the star is stable

Higher mass variable stars pulsate slower

Lower mass pulsate faster

Harlow Shapley and his clusters:

Noticed;

Open clusters were concentrated along the Milky way…

Globular clusters scattered all over sky

Globular clusters also strongly concentrated toward Sagittarius…

He assumed that this pattern was governed by the mutual gravitational force between all the stars in the entire system…

He set out to study the size and extent of the “system” by studying the clusters

But there was one problem…

He didn’t know the distances to the clusters

Shapley + Leavitt = distance:

Shapley knew that M d

He also noticed that closer stars had larger proper motions than did the farther stars…

He found 11 variable stars in a globular cluster…

Using the proper motions and statistical techniques, average distance to these stars

Knowing m and d, he could then calculate M (Leavitt couldn’t because dSMC wasn’t known at the time)

Q: Combining Shapley’s M with Leavitt’s P-m work… what does all this do for us?

P-m relation P-M relation

Upshot: Measure P find M find d!

“Period-luminosity relation”

( 5) /510 m Md 5log( ) 5m M d

The results of Shapley’s study:

Shapley had two unknown handicaps though…

1. ISM dims light

2. Used two types of Cepheids, those in his clusters were Type II, and those used for calibration (the 11 stars) were Type I

His estimate for the size of the Milky Way was too large (300,000 ly in dia.)

Shapley – Curtis Debate:

Astronomers began to suspect some of the “nebulae” were other galaxies…

In 1920, they held a debate

In 1923, Edwin Hubble photographed individual stars in the Andromeda Nebula

and in 1924 found the distance via Cepheids

Andromeda Nebula Andromeda Galaxy

Shapley argued that Andromeda was part of

our star system

Curtis argued that Andromeda was far

away and itself a galaxy

The anatomy of the Milky Way:

Disk: 1000 – 3000 ly thick (300 – 900 pc) 75000 ly (25 kpc) diameter location of the spiral arms contains the most gas & dust sun ~ 8.5 kpc from center

Halo: ~ same diameter as disk very little gas & dust 2% - 10% as many stars as disk stars are typically older than in disk

Bulge: ~ 3000 ly radius (2 kpc) little gas & dust little star formation old, cool stars

Corona (extended halo): 300000 ly (100 kpc) diameter little gas & dust

“Aitoff projection”

visual

visual

Infrared Blue = 12 microns green = 60 microns red = 100 microns

Infrared 3.5 microns false color

Infrared 240 microns false color

Infrared

Blue = cool Galactic stars

Yellow-green = galaxies

Red = extremely cold Galactic material

Radio 75.5 cm

Gas clouds via UV studies

X-ray point sources

Gamma ray

WMAP CMBR ~ 2.74K

A = visible

B = radio

C = infrared

D = x ray

E = gamma

Mass of the Milky Way:

Q: To find the masses of stars, what do we need?

A: A binary system (study orbits of…)

Mass of galaxy => orbits of stars around galaxy

Doppler radial, measure proper combine to get orbital velocity

Take the orbit of the sun; r ~ 8.5 kpc = 8.5*1016 km v ~ 220 km/s toward Cygnus 2*pi*r / v P ~ 240 million years

Kepler # 3; M = r3 / P2 100 billion Msun

Q: Why might this be inaccurate? a. values subject to uncertainty b. only accounts for the mass inside the sun’s orbit (Gauss’ law)

100 billion Msun a lower limit

Mass of the Milky Way II (rotation curves):

Q: Would you expect orbital velocities of stars to increase or decrease the farther away from the center they are?

We would expect them to decrease (move slower), however…

There must be more mass than we can see 3 3 2

2 2 2(2 / ) (2 )

2

tot

tot

r r rM

P r

M

r

Appears that ~ 2 x 1012 Msun

WDs & low-luminosity stars

Also “dark matter”

Origin of the Milky Way I (Age):

Age: Clusters turnoff point

Suspect to uncertainties though…

• Older clusters change more slowly

• Turnoff point depends on chemical composition

• Open clusters not bound by gravity may have lost older stars which migrated away

But… MW > 9 billion years old

Globular clusters 13 billion years (still uncertain though…)

2.5M

Origin of the Milky Way II (Population I & II stars):

Two types of stars: Population I & Population II:

Pop I: young, disk metal-rich

Pop II: older, halo, bulge metal-poor

Type I Cepheids Pop I

Type II Cepheids Pop II

(Metallicity affects the easy with which radiation flows through the gas (the star)

Origin of the Milky Way III (formation):

Predictions from this model:

• Halo stars formed first should all have ~ same age

• Halo stars formed when the MW was very young should have an extremely low metallicity (or metal free)

• Most stars along the disk should be younger

Problems with this model:

• Some stars in halo have a much lower metallicity than the globular clusters appear to be much older (should all have a uniform age…)

• There are young globular clusters in the outer halo

• Some of the oldest stars are in the bulge

• Oldest stars are metal-poor, but not metal-free

• Seems to be too few WDs in the disk (assuming it’s 10 – 13 billion years old

Classical picture:

Origin of the Milky Way IV (formation):

Ages are a key problem in the classical model

Modern picture:

• Gas cloud condensed to form the bulge first (a much larger bulge than present day). Oldest stars in bulge

• As stars began to form, slightly metal enriched gas formed the halo. Halo stars slightly older and not metal-free

• The disk formed later from the gas in the Galaxy and as the bulge flattened. Disk not al old as previously thought… the WD problem

• The Milky Way merged with other galaxies some of the stars & clusters in the halo could be the cannibalized leftovers. Explains why halo stars & clusters are not all the same age

…

M51 Whirlpool

M81 IR

NGC 1300

NGC 6946

Spiral arms of the Milky Way:

Two difficulties;

1. View obscured by gas and dust

2. Why doesn’t differential rotation destroy the arms?

NGC 6946

Spiral arms of the Milky Way:

Looking at other galaxies spiral arms contain hot-blue stars

Locate these stars in the Milky Way as they are likely in the arms

“Spiral tracers”:

1. OB associations (globular clusters of type O and B stars)

2. Young open clusters

3. HII regions (as they are ionized by hot stars)

4. High mass variable stars (bright, young, and give distances)

Hot-blue stars are massive:

Massive stars last for only a few million years young

Orbital velocity ~ 250 km/s, move less than 500 pc during their lifetime, 500 pc < width of spiral arm

Assuming these stars are in the arms, then these young stars must have formed there

Thus, spiral arms are regions of star formation

NGC 6946

Spiral arms of the Milky Way (what are spiral arms?):

- Not physically connected

- Common to disk-shaped galaxies

- Last a long time (seem to be a permanent feature)

Spiral arms of the Milky Way (density wave theory):

Arms are formed by waves of compression traveling through the Galaxy

These waves trigger star formation

Theory predicts they should last ~ billion years

Differential rotation doesn’t wind them up because they are not physically connected

O & B stars live short lives don’t have time to leave arms they reside in the arms

Two problems:

1. What stimulated the formation of the spiral pattern?

2. Doesn’t account for the branches or spurs

M51 Whirlpool

Spiral arms of the Milky Way (density wave theory – possible amendments):

Two problems (revisited):

1. What stimulated the formation of the spiral pattern?

Something starts it, but what maintains it?

Possibly the galaxy is unstable to certain disturbances which can generate a density wave; fluctuations in the disk, or the gravity of passing galaxies….

2. Doesn’t account for the branches or spurs

Computer models can only produce stable two-armed galaxies “the grand design”

Possibly the solution lies in the process that sustains star formation once it begins….

NGC 1300

M51 Whirlpool

Spiral arms of the Milky Way (star formation in the spiral arms):

Makes the arms visible

May itself shape the spiral pattern;

Arms can regenerate themselves if star formation can continue through many generations of stars

Radiation from hot stars can compress nearby gas and

trigger more star formation

Massive stars die quickly & the SN shock can generate more

formation

Upshot: Self-sustaining star formation is possible (and even observed – e.g., in Orion))

Self-sustaining star formation can produce branches and spurs as differential rotation occurs

The Galactic nucleus of the Milky Way:

This is the most mysterious region of our Galaxy;

- Hidden by gas and dust which dim the light by 30 magnitudes

If 1012 photons left the nucleus, only one would make it to Earth

For IR, 1/10 photons make it through

IR a good probe

Radio is also a good probe

The Galactic nucleus of the Milky Way:

Shapley center is near Sagittarius

Radio maps powerful radio source near same region

“Sgr A*”

Only a few AU in diameter

Q: What could be as small as Sgr A* and produce so much energy?

In Sgr A*, we see evidence for star formation, gas, and SNRs

Massive stars

Can’t be that old

The Galactic nucleus of the Milky Way:

The Galactic nucleus of the Milky Way (a supermassive black hole?):

Observations reveal a swirl of gas around the central object

Also know that it’s a powerful radio source

The Galactic nucleus of the Milky Way (a supermassive black hole):

Using Kepler’s 3rd law, we can calculate the mass of Sgr A*

The period of SO-2 is 15.2 years

The semi-major axis is 950 AU

2.6 million solar masses

Can’t be anything other that a supermassive black hole – only a single black hole can have so much mass in such a small area

A measly 0.0002 solar masses of gas flowing into this black hole per year could produce the observed energy

A star that falls into this BH would produce a sudden eruption

Our Galaxy:

Our Galaxy:

Our Galaxy:

Our Galaxy:

Our Galaxy: