© Sierra College Astronomy Department1 The Milky Way Galaxy

© Sierra College Astronomy Department 1

The Milky Way GalaxyThe Milky Way Galaxy


By Claudius Ptolemy:Now the Milky Way is not strictly speaking a circle, but rather a belt of a sort of milky color overall (whence it got its name); moreover this belt is neither uniform nor regular, but varies in width, color, density and situation, and in one section is bifurcated. All that is very apparent even to the casual eye…

The Milky Way Galaxy

Overview


The Modern Eye of the Milky Way: • Photographic Image in the visible: The glow of stars in the

interstellar medium• Images in the Infrared: Thinner extent and a dust component• 21-cm Radio Emissions: Location of hydrogen gas• Other radio Emissions: Synchrotron radiation and fast moving

electrons• X-rays: Very hot gas and the galactic polesAn Overall Picture• The Milky Way Galaxy is a nearly self-contained system that

cycles stellar remnant materials into the interstellar medium and then back into stars.

• The Milky Way galaxy is rich in structure and physical processes.


Overview


The Milky Way Galaxy• The Milky Way Galaxy is the spiral galaxy of

which the Sun is a part. From Earth, it appears as a band of light around the sky.

• Most stars in the Galaxy are arranged in a wheel-shaped disk with a diameter of 30,000 parsecs (100,000 light-years) that circles around a bulging center.

• About 100 billion stars occupy the Milky Way.• The Sun and solar system is a little more than

halfway out from the galactic center.


Our Galaxy – A Quick View


Showing the Way• Initial arguments that the shape of the Milky Way

was that of a disc were made by Thomas Wright and Immanuel Kant (18th century).

• Sir William Herschel (late 1700s) and Jacobus Kapteyn (early 1900s) used star counts to determine the size of the disc distribution.

• Generally speaking, by the early 1900s it was believed that:– The Solar System was at the center of the Milky Way.– The Milky Way was only a few thousand parsecs across.– The only galaxy in the Universe was the Milky Way.


Early History


Towards a Modern View of the Galaxy• Globular clusters in 1915:

– Did not appear to be uniformly distributed across the sky.– Appear to be concentrated in the direction of Sagittarius.

• Harlow Shapley used the existing period-luminosity relation for the RR Lyrae variables to determine the distance to the globular clusters.– Found center of globular cluster distribution was 15 kpc from the Sun in

the direction of Sagittarius (1kpc = 1000 pc).– Claimed that the center of the globular cluster distribution was also the

center of the Milky Way: therefore, the Sun was no longer at the center.• Shapley had not taken into account the extinction of light due to

interstellar dust.– This did not alter his center of the galaxy claim.– With dust accounted for, the center is about 8.5 kpc away from the Sun.– The Milky Way is still bigger than previously thought.


Early History


The Three Components of the Galaxy• Galactic Disk

– Contains most of the stars (including the Sun) and the interstellar gas and dust in a 100,000 light-year diameter disk with a 1,000 light-year thickness.

– Star density is greater in the central plane (where most of the O stars are) and falls off above and below this plane.

– Stars generally rotate in the same direction with a “bobbing up and down” motion that gives the disk its thickness.

• Galactic Bulge– Somewhat flattened collection of stars about 20,000 ly across with the galactic

nucleus at its center.– Elongated to form a bar 2-3 times longer than wide, pointing towards the Sun– Star orbits are more or less elliptical in shape, but randomly oriented.– Bulge stars have a lower metallicity than those in the disc (in astronomy, a metal

is any element other than hydrogen and helium).• Galactic Halo

– Consists of globular clusters (and a few small galaxies) and very low metallicity stars (lower than bulge).

– Spans disc diameter and star orbits are like those of the bulge.


Components of the Galaxy


Motion of the Sun and Mass of the Inner Galaxy• The Sun is traveling in a nearly circular path around the

galactic center at a speed of about 220 km/s.• With Sun’s orbit radius equal to 8,500 pc, the Sun takes

about 230 million years to complete one revolution around the center of the Galaxy.

• 230 million years is about 5% of the Sun’s age (which is about 20 galactic rotations) and about 2% of the Galaxy’s age (about 50 galactic rotations).

• From the Sun’s period of rotation and its distance from the center, Kepler’s Third Law, as modified by Newton, gives the mass of the Milky Way interior to the Sun’s orbit as 100 billion M⊙.


Galactic Motion and Mass


The Rotation Curve of the Galaxy• The galactic rotation curve of objects in the Milky Way’s

disk shows that the galaxy undergoes differential rotation.• Between 2 kpc and 20 kpc, and perhaps even out to 50 kpc,

the galactic rotation curve remains relatively flat.• Kepler’s modified 3rd law, v = (GM/r)1/2 or M = v2r/G:

– Constant v implies M linearly increases with r– At 50 kpc, M will be 6x greater than mass found interior to Sun’s

orbit

• Very little of the mass beyond the Sun’s orbit is luminous enough to be detected: dark matter.


Galactic Motion and Mass


General Characteristics • Most of the Interstellar Medium or ISM is confined to

the Milky Way’s disk in a clumpy cloudlike distribution (with “clouds” a few light-years to a few hundred light-years across).

• 70% H, 28% He, 2% heavier elements.• Densities range from about 0.01 to 104 molecules per

cubic centimeter with temperatures ranging from 10 K to 1,000,000 K.

• Astronomers estimate that interstellar hydrogen contributes about a few percent of the Galaxy’s total mass.

• Dust is about 1% of the total mass of the ISM.


The Interstellar Medium – Some Data


Discovery• In 1930, Robert Trumpler showed cluster size tended to

increase with distance.– He also showed distant stars were redder than closer stars of the

same spectral type.– Astronomers soon concluded that dust grains exist throughout

space.

Important Processes and Consequences – In a Nutshell• Scattering of starlight – Diminishing of star’s brightness

(interstellar extinction and reddening) and the creation of galactic glow.

• Absorption of starlight – The infrared sky.


Interstellar Dust



Interstellar Dust

Scattering and extinction of light• Some numbers:

– On average, interstellar dust extinguishes light by one magnitude (2.512 times) for each 1000 pc.

– After 1000 pc, 40% of the light penetrates the dust; after 2000 pc, 16%; 3000 pc, 6.3%; 5000 pc, 1.0%; and 10,000 pc, 0.01 %.

– Extinction of blue is stronger than red: After 3000 pc, 2.5 % of blue light penetrates, whereas 6 % for red.

– Dust then reradiates the absorbed energy in the infrared.• An interstellar cloud with a significant dust component “seen”

silhouetted against a bright background of gas and stars is referred to as a dark nebula (e.g., Zone of Avoidance, Horsehead Nebula).

• Interstellar dust clouds may also become visible if the highly scattered blue light of a star is directed towards Earth. Such an observed dust cloud will look blue and is called a reflection nebula (e.g., Pleiades and the Trifid Nebula).

• About 75% of light in the night sky is due to stars, while the remaining galactic glow is due to starlight scattered by interstellar dust.


General Ideas• Fluorescence is the process of absorbing radiation of

one frequency and reemitting it at a lower frequency.• An emission nebula is a cloud of interstellar gas that

fluoresces due to ultraviolet light from a star near or within the nebula.

• The most conspicuous type of bright emission nebula is an HII region; elsewhere, interstellar gas is much harder to detect visually.

• Interstellar gas causes absorption lines in the stellar spectra.


Interstellar Gas


HII Regions• The bright nebulae around hot stars are referred to as HII

regions since most of the hydrogen in these regions is ionized (a region of predominantly atomic hydrogen would be called as HI region).

• The emission spectrum of an HII region is caused when protons capture electrons and these electrons subsequently jump to lower energy levels.

• The characteristic red color of an HII region is due to the bright hydrogen Balmer emission (level 3 to 2).

• O stars can produce HII regions 100 pc in diameter, while B stars can produce diameters of a few parsecs.


Interstellar Gas


The 21 cm Line of Hydrogen• Hydrogen can emit photons with a wavelength of 21 cm as

a result of its electron “flipping” its spin state relative to the spin state of the proton.

• 21 cm is in the radio band and in 1944, it was proposed as a means of detecting hydrogen away from stars.

Interstellar Molecules• Radio astronomers have discovered more than 100

different molecules through their radio emissions.• These molecules survive deep inside opaque interstellar

clouds.• Hydrogen is the most abundant molecule, followed by

CO.


Interstellar Gas


Overall Interstellar Gas Structure – Phases• Interstellar gas has several “phases” dependent on

densities, temperatures, and degree of ionization.• About half of the ISM gas is in relatively cool,

high-density regions called clouds (cool phase).– The densest clouds with temperatures near 10 K

are molecular clouds, whereas lower density regions are atomic clouds.

– Atomic clouds are more common in the outer galaxy.

– These atomic and molecular clouds are ripe for star formation.


Interstellar Gas


• Between clouds, temperatures are warmer (as high as 10,000 K) and less dense and may be ionized – this is the warm phase of the intercloud medium.

• Elsewhere temperatures can hit 1 million K (hot phase), emit X-rays, and appear to be associated with shock waves from supernovae and called bubbles.– Several supernovae bubbles can conspire to create hot

superbubbles.• A blowout may result wherein gas breaks out of the disc,

disperses over the disc, and rains back onto the disc via a proposed galactic fountain process.

– Amount of gas in the hot phase is not known.– The Sun may be in a hot superbubble (the Local Bubble)

created by Scorpius-Centaurus association.


Interstellar Gas


Observations of Spiral Structures• Objects found in other spiral galaxies that trace the spiral arms

– HII Regions (or ionized hydrogen nebulae)– Clusters of O and B stars– Clouds of interstellar hydrogen– Molecular clouds

• In early 1950s, mapping of HII regions and O and B stars indicated the existence of three spiral arms in the neighborhood of the Sun.

• Radio observations of hydrogen (at 21 cm) and CO have extended the map of the Milky Way spiral structure.

• The exact spiral structure of the Milky Way remains complicated and controversial.


The Spiral Structure of the Galaxy


Why Spiral Structure?• Puzzles

– Why are O and B stars forming only in the arms?– Why do the spiral arms not “wind up” and disappear?

• Answer: density wave theory – This theory proposes that the arms are the result of density waves

sweeping around the galaxy and through the interstellar medium at the same orbital period of about 500 million years (this solves the wind up problem).

– The theory also holds that the trigger for the collapse of gas clouds is the increased gravitational force in the high density portion of the wave traveling through space.

– The collapsing gas triggers star formation, including O and B stars which do not live long enough to escape the spiral arm in which they were born.


The Spiral Structure of the Galaxy


Observations of the Galaxy’s Center• Too much dust lies between Earth and the center for visible

observations to give reliable data.• Radio, infrared, and x-rays do reach us from the center• The wealth of data from the center has not yet yielded a

consensus model.The Stellar Picture• At the Sun’s orbit, there is 1 star per cubic pc (cpc).• At 100 pc from center, 100 stars per cpc• At 10 pc, several thousand stars per cpc• At a few pc, hundreds of thousands of stars per cpc (stars are

light-weeks apart, a few thousand AUs)


The Center of the Galaxy


The Gaseous Ring• A thin ring of gas and dust rotates about the center between 2

and 8 pc.• It rotates at 110 km/s meaning 10 million M⊙ resides within

its orbit.• The ring is 10,000 M⊙ and consists of atoms and molecules,

although the inner edge is ionized.• The high rate at which gas from the ring spirals into the

center and the low density inside the ring suggests that a violent explosion occurred in the center quite recently (within last 100,000 years).




A Supermassive Black Hole?• Inside the gaseous ring, streamers of hot gas form

a spiral-like pattern that converges to a point referred to as Sgr A* (a bright radio source).

• Gas and star speeds increase to very large values indicating that over a few million M⊙ exist within 0.1 pc from the center.

• Sgr A* is 1 AU or smaller and appears motionless.• A 1 AU object is about 10 times as large as the

Schwarzschild radius of a black hole containing a few million M⊙.




Stellar Populations in the Milky Way• A stellar population is a group of stars that resemble each other in

spatial distribution, age, and chemical composition.• A very popular grouping of star populations uses the Roman numerals

I, II, and III.– Population I stars are young and contain heavy elements. Found

primarily in the disk.– Population II stars mainly in the halo are older than those in the disk

and contain only about 1% of the heavy elements of population I .– Population III stars are pure H & He stars, the first stars born after the

Big Bang when very little metals existed.• No Pop III stars exist today since only massive Pop III stars can be

created and they have short lifetimes.• If Pop III stars do exist today (for some unknown reason), since none are

observed ,they must be masquerading as Pop II stars or are simply so rare they have not been found yet.


Stellar Populations


Stellar Populations in the Milky Way (continued)

• Another population grouping is: Thin Disk, Thick Disk (together referred to as the Disk Population and corresponds to Population I), and Halo and Bulge (also referred to as the Spheroidal Population and corresponds to Population II)– As one moves from the thin disk stars to the thick disk stars to the

halo stars (increasing distance from the galactic central plane), star age increases and metallicity decreases.

– It is proposed that stellar populations present a fundamental statement of the shape and chemical makeup of the gas in the galaxy at the time of stellar formation.


Stellar Populations


The Galaxy’s Evolution• Some 11 billion years ago, protogalactic clouds of H/He gas

begin to gravitationally contract and fragment into individual stars and globular clusters.

• Through mutual gravitation between these clouds, they merge into the Milky Way’s protogalactic cloud, a structure larger than the Galaxy’s current halo.

• Stars begin to form in the dense center.• Pop III stars, which had extremely short lives, begin the

“pollution” of the interstellar gas.• The bulge stars and halo globular clusters continue to orbit

within their respective regions, while the gas settles into a disc which establishes a distinct direction of rotation.


Evolution of the Milky Way


The Galaxy’s Evolution (continued)

• The death of Pop I and Pop II stars continuously adds to the “pollution”, increasing the metallicity of each succeeding generation of stars.

• The spiral arms of the disc begin to take shape.• As the Galaxy ages, less and less gas will be available to

make new stars as material becomes trapped in stellar remnants.

• Mergers with other galaxies may bring new sources of gas, but eventually the available gas will be depleted to the point where no new stars will form.

• The Milky Way will eventually fade into a slowly spinning, dark disk of stellar cinders.


Evolution of the Milky Way

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© Sierra College Astronomy Department1 The Milky Way Galaxy