4. Large-Scale Universe (Chps. 23-26)

8/10/2019 4. Large-Scale Universe (Chps. 23-26)

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Milky Way:

Galaxycollection of stellar and interstellar matter, isolated

in space and held together by own gravity

Galactic diskcircular, flattened region containing galaxysluminous stars and interstellar matter

Galactic bulgefattened part

at center

Galactic halospherical

region of faint old stars within

which disk and bulge are

embedded

These are components ofa spiral galaxy.

Galactic centerhub of vast collection of matter where most

of the globular clusters are located8 kpc from Sun in

Milky Way

New stars form in interstellar clouds close to the disk

plane, but as they grow older they drift out towards the

galactic halo. Stars in the bulge and halo (Population I) are

redder, while those in the disk (Population II) are more blue-

white/yellow in color. The disk and bulge are heavy in

interstellar matter, while the halo contains almost none.


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Individual stars and clouds seem to move randomly, but

the Galactic disk as a whole is actually rotating differentially

(closer objects take less time than farther objects to orbit)

about the Galactic center. Out of the four Galactic quadrantsaround the Sun, stars in the upper right and lower left are

usually blueshifted and moving towards us, while stars in the

upper left and lower right are usually redshifted and moving

away from us.

The spiral arms are pinwheel-shaped structures

originating close to the Galactic center where star formation

takes place. Due to differential rotation, spiral arms are not

likely to be tied to rotating material in the disk. Instead,

they are probably accounted for by spiral density waves,

which are coiled, compressed gas that move through the

Galactic disk and trigger star formation. The wave pattern is

predicted to rotate more slowly than the stars and gas, so

material is temporarily slowed down and compressed as it

passes through. Another possibility is that formation of stars

drives the waves through self-propagating star formation, but

on a smaller scale. Spiral arms may have originated from (1)


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gravitational effects of satellite galaxies, (2) instabilities in

the gas near the Galactic bulge, or (3) a possible asymmetry

in the bulge that influenced the disk.

For galaxy mass, Keplers second law can be applied: ( )

()

()

This determines the mass of the portion of the galaxys mass

that lies within the orbit of the star in question.

Rotation curveplot of galactic rotation speed versus

distance from the center

Unlike planetary orbital speeds around a star, which decreasewith increasing distance, the rotation curve of galactic mass

keeps increasing. This suggests that the Milky Way is in

reality much larger than its luminous portion. The inner

Galactic halo may be surrounded by a gigantic, invisible

dark halo. This means that most of the mass exists as dark

matter, which is undetectable at all wavelengths.

Possibilities for their identity include stellar-mass blackholes, brown/white/red dwarfs (also called MAssive

Compact Halo Objects, or MACHOs), or unidentified

subatomic particles (Weakly Interacting Massive Particles,

or WIMPs) that were produced early after the universe was

created. As scale increases, so does the rate of the presence


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of dark matter, which may add up to upwards of 90% of the

entire universes mass.

Gravitational lensinginstance in which a distant star is

brightened by a faint foreground object (called thegravitational lens and is usually a MACHO) that passes in

front of it; apparent brightness of background star increases

by factor of 2-5 for several weeks

The Galactic center is invisible from Earth because the

interstellar medium in the disk shrouds it from view, but an

energetic ring of molecular gas exists there. Doppler

broadening shows that the gas is moving very rapidly, so an

object of more than 1 million solar masses, probably a blackhole, must exist at the center.

The accretion disk surrounding the black hole may

generate strong magnetic fields, which create cosmic rays

extremely fast, high-energy particles that frequently reach

the Earth and from all directions.

Normal Galaxies:The American astronomer Edwin Hubble

comprehensively categorized galaxies into spirals, barred

spirals, ellipticals, and irregulars.

Spirals are denoted by the letter S and classified as type

a, b, or c depending on the size of its central bulge, from

largest to smallest. The larger the bulge, the more tightly

wrapped the spiral arms are, and the smaller the bulge, the

more open the spiral pattern becomes. Spiral galaxies are

rich enough in interstellar gas to provide for continued stellar

birth. A variation called barred-spiral galaxies have an

elongated bar of matter passing through the center and


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extending past the bulge into the disk. The Milky Way is

most likely a barred spiral of type SBb or SBc.

Elliptical galaxies have no spiral arms and little internal

structure. They are denoted by the letter E and classifiedfrom types 0-7, from most circular to most elliptical. They

range greatly in size and number of stars, so they are often

divided into giant ellipticals and dwarf ellipticals with a ratio

of occurrence of about 1:10. Stellar orbits show little

rotation and follow random paths. Some giant ellipticals

may contain disks, which make them intermediates between

E7 ellipticals and Sa spirals. These are classified as SB0

galaxies if a bar is present and S0 if there is none.Irregular galaxies are usually rich in interstellar matter

and young blue stars, and are divided into Irr I and Irr II

galaxies. The former resemble spirals but are smaller, with

the smallest called dwarf irregulars. Dwarf ellipticals and

dwarf irregulars are equally common, are found close to

larger parent galaxies, and make up the vast majority of all

galaxies that exist. Irr II galaxies are much rarer, and areoften explosive or filamentary in appearance. Some of them

may have resulted from a collision between two ordinary

galaxies.

A famous pair of Irr I galaxies that orbit the Milky Way

are known as the Magellanic Clouds, which are

approximately 50 kpc from our Galaxy center. Both contain

plenty of matter and blue stars, as well as older globular

clusters.

Hubble also arranged the galaxy types into a tuning

fork diagram, which seems to suggest an evolutionary

sequence. While isolated normal galaxies do not evolve

from one type to another, collisions and tidal interactions


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between interactions may be a main driving force in galaxy

transformations.

Standard candlebright, easily recognizable astronomical

object whose luminosity is confidently known and narrowly


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defined; distance can be measured by comparing luminosity

with apparent magnitude

Planetary nebulae and Type I supernovae are reliable

standard candles, and the latter all have consistent peakluminosities and similar properties.

Tully-Fisher relationused to determine absolute luminosity

of spiral galaxies; rotational velocity can be measured from

broadening of spectral lines, and then compared to total mass

to yield total luminosity

Faber-Jackson lawused to correlate luminosity with

velocity dispersion of an elliptical galaxy

Masses of spiral galaxies can be calculated bydetermining rotation curves. Binary and multiple systems of

galaxies may also provide more information. Finally, a

general estimate of a clusters total mass may come from

calculating how much mass is required to gravitationally

bind the galaxies. In general, the order of galaxy types from

most to least massive is spiral, elliptical, irregular, and dwarf

elliptical/irregular.Galaxies grow by repeated merging of smaller objects.

Collisions and interactions are common, because the

distances between galaxies in clusters are not much greater

than the sizes of the galaxies themselves.

Starburst galaxyviolent event has rearranged galaxys

internal structure and triggered sudden, intense burst of star

formation

Active Galaxies and Quasars:

Active galaxyhighly luminous

galaxy that emits hundreds to

thousands of times more energy per


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second than Milky Way in the form of long-wavelength,

nonthermal radiation

Seyfert galaxiesintermediate between normal galaxies and

more violent active galaxies; emits almost same amount ofvisible radiation as normal galaxies, but much more radio

and infrared; has small central region of less than 1 light year

across (known from sharp intensity variations) called

galactic nucleus

Radio galaxiesemit most of energy at longer (radio portion

of spectrum) wavelengths than those of Seyferts; long thin

jet of matter travels outward at very high speeds

Lobe-radio galaxyenergy is released from two extendedregions called radio lobes, which span 1 megaparsec across

and are aligned with center

Head-tail radio galaxylobes trail behind main galaxy

Core-halo radio galaxyenergy is emitted from extremely

small nucleus (core) less than 1 pc across, with additional

emissions coming from extended

halo 50 kpc acrossJets point to the possibility that

lobe-radio and core-halo galaxies

may actually refer to the same type

of galaxy but simply viewed from

different perspectives.

Quasarsquasi-stellar objects/radio sources with large

redshifts that indicate extremely far distance from Earth;

most luminous objects knownoutshine brightest galaxies

by factor of 100-1000; vary irregularly in brightness in many

parts of electromagnetic spectrum, which signifies its small

energy-generating region


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Active galaxies and quasars show at least some of these

properties: 1) high luminosities, 2) nonstellar energy

emission, 3) variable energy output, 4) jets and other signs of

explosive activity, and 5) spectra with broad emission lines,which indicate rapid internal motion within the energy-

producing region. Because they contain so much material

packed into a small space, their central engine probably takes

the form of a supermassive black hole whose accretion disk,

which spins perpendicularly to the jet, efficiently converts

infalling mass (gas) into energy (electromagnetic radiation).

The fuel supply may be reignited by companion galaxies.

Jets are accompanied by strong magnetic fields, which arepossibly generated by gas swirling within the disk. Charged

particles, mostly low-mass electrons, accelerate and spiral

around the field lines producing synchrotron radiation, which

depends on both their speed of travel and the strength of the

magnetic field.

Burned-out quasars in the form of black holes dont

exist, so quasars probably spend only a short time in their

highly luminous phase. A theory for the reason suggests that

black holes eat out cavities at the centers of their hot

galaxies, prematurely cutting off their fuel supply.


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Quasar spectra allow astronomers to probe previously

undetected parts of the universe by exhibiting Lyman-alpha

absorption features, or Lyman-alpha forests, produced by gas

clouds in foreground structures along the line of sight.Absorption lines of atomic hydrogen, the most common

element, are especially important. In the picture, the peak

shows the Lyman-alpha emission line from a quasar, which

was emitted at 122 nm but redshifted to 564 nm. The

Lyman-alpha forest represents the emissions from hundreds

of clouds of hydrogen gas in the foreground.

Gravitational lensing from foreground objects maygreatly deflect light from a quasar and cause it to appear as a

double image. In addition, the light rays follow paths of

different lengths, so there is often a time delay, ranging from

several days to several years, between the two images. The

time delay lets astronomers determine the distance to the

lensing galaxy. Microlensing from additional stars can also

cause fluctuations in the quasars brightness. Maps of dark

matter have been created by measuring the gravitational

distortions produced in the images of bright background

objects.

The redshift of light is the fractional increase in its

wavelength resulting from the sources motion away from


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us, so for example, a redshift of 1 corresponds to a 100%

increase in wavelength. However, in terms of relativity the

redshift is infinite at speeds equal to light. Redshifts greater

than 1 are not moving faster than the speed of light, butinstead indicate relativistic (comparable to the speed of light)

velocities; in these cases the general formula for redshift is

not applicable. The distance to

faraway objects is not well-defined,

because it differs now from when the

light was emitted, due to the fact that

the universe is expanding. Look-

back time refers to how long ago anobject emitted the radiation detected

now.

Quasars were once more

common than they are today, and

were originated from small black

holes and abundant fuel. The theory

states as follows: the black holessank to the centers of their parent galaxies and merged to

form supermassive black holes. The intense fuel

consumption and emission of quasars lasted a short time, and

they became active galaxies. As central activity continued to

decline, some transformed into normal galaxies. A possible

evolutionary sequence is shown below.


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BL Lac objects, which are a type of blazar (compact

quasar at the center of a giant, active elliptical galaxy), are

active galaxies with much more synchrotron than thermal

radiation. They may represent a transition phase betweenradio galaxies and quasars.

Cosmology:

Galaxy clustergroup of galaxies held together by mutual

gravitational attraction; Milky

Way is part of Local Group,

which consists of 45 galaxies

Superclustercluster ofgalaxy clusters

Galaxies and galaxy

clusters move in very ordered

patterns. They are all also

moving away from each other

in a universal recession

sometimes called the Hubbleflow.

Hubbles lawrate at which galaxy recedes is directly

proportional to distance

Hubbles constantdenoted by ; about 65 km/s/MpcCosmological redshiftredshift resulting from Hubble flow

On very large scales, distribution of galaxies is

nonrandom. They are arranged in a network of

strings/filaments surrounding large, empty regions of space

called voids, which resemble giant bubbles. There appear to

be no structures in the universe on scales greater than 200

Mpc.


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Cosmologystudy of structure and evolution of the entire

universe

The universe is most likely homogeneous, which means

that its contents appear the same in any equally-sizedsection, and isotropic, which means that they appear the

same in all directions. These two assumptions make up the

cosmological principle. Implications include the universe

having no center and no edges.

Olberss paradoxThe sky is dark at night, not entirely

bright, despite the assumption that any line of sight from

Earth should eventually run into stars whose apparent

luminosities add up to an equal amount.

Age of the universe:

If Hubbles constant is 65 km/s/Mpc, then this time

equals about 15 billion years. All matter and radiation was

once confined into a single point, and then it expanded in a

gargantuan explosion known as the Big Bang, the beginning

of the universe. The elements of hydrogen and most of the

helium in the universe are primordial.

As a photon moves through space, its wavelength is

stretched by the expansion of the universe. This

cosmological redshift is unrelated to velocity.

The universe may either expand forever as an unbound

universe if it is low-density, or stop and begin to contract as

a bound universe if it is high-density. The dividing line is

called the critical density, and in this case would correspondto a marginally bound universe in which gravity can halt, but

not reverse, the present expansion. The critical density

equals about km/m3for km/s/Mpc, and

the critical density parameter, denoted by (omega


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nought), is the ratio of

the universes actual

density to the critical

value. This ratio islikely to be about 1:3,

which means that the

universe will expand

forever, but the

unknown distribution

of dark matter on larger

scales and high-density accumulations of mass such as the

Great Attractor in the Local Supercluster have not beenaccounted for.

Standard candles help determine the rate of cosmic

expansion in the distant past. If the universe was

decelerating, then objects that emitted their radiation long

ago should be receding faster than the current Hubble flow,

and their velocities should lie

above the curve that representsconstant universal expansion. In

the picture to the right, the red and

purple curves represent redshifts

of objects in a decelerating

universe, and the black curve

represents the redshift of constant

expansion. The actual

observations suggest that the

universes expansion is actually accelerating.

Cosmological constantvacuum pressure force associated

with empty space on large scales; major factor in controlling

cosmic expansion


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Dark energycosmic field that causes the universe toaccelerate; repulsive force that opposes attractive force of

gravity; estimated to account for 65% of energy in universe

Theory and observation sometimes conflict in terms of

the age of the universe, in which it appears to be younger

than the estimated ages of the oldest globular clusters. The

best compromise is that the Big Bang happened 14 billion

years ago, the first quasars appeared 13 billion years ago,and the oldest-known stars formed during the following 3

billion years.

Space is curved, and the degree of curvature is

determined by the total density of the cosmos, including

matter, radiation, and dark energy. If the average density of

the universe is above the critical value, the curvature is high

enough that the universe bends back on itself as a closed

universe, sort of like the sphere of the Earth. If the average

density is below the critical value, the negative curvature

makes the surface of the open universe curve up like a

saddle. A critical universe, whose density is equal to the

critical density, is flat and follows Euclidean geometry.


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Cosmic microwave backgroundelectromagnetic remnant

of the Big Bang that takes the form of an almost isotropic

radio signal; discovered by scientists Arno Penzias and

Robert Wilson in New Jersey in 1964; has a temperature ofabout 2.735 K

Since the Earth is moving at about 380 km/s in the

direction of the constellation Leo, the microwave

background appears a little hotter (blueshifted) in that

direction, and a little cooler (redshifted) behind. For a static

observer, the cosmic microwave background is isotropic.

Pictures below: theoretically derived blackbody curve

of the universe; redshifted microwave background withrespect to Earth; COBEmap of microwave sky; accurate

spectrum of microwave background


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Cosmic Distance Ladder:

4. Large-Scale Universe (Chps. 23-26)

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