Knowledge of our celestial surroundings began with the early studies of the sky by the ancient Greeks and their careful observations of the stars and planets Accurate star catalog - Hipparchus

Models of the “universe” were simple and included the Earth, Sun, Moon, planets and stars Geocentric – Aristotle, Ptolemy Heliocentric – Asistothenes Milky Way of stars - Democritus

Next significant advance in understanding of our surroundings came with the discovery of the telescope and its use by Galileo and others in the 16th century Planets and moons Heliocentric solar system Patches of dense stars too numerous to count

Careful observations of the diffuse “clouds” of stars first studied in the 10th century by Arab astronomers were made by Charles Messier in the late 18th century 109 brightest nebula were cataloged (Messier Objects) Included the Andromeda Galaxy (M31) – Our Sister

Galaxy

Over 5,000 nebula were later cataloged by William Herschel

“Island Universes” was coined by Immanuel Kant in the 1750s to describe the distant diffuse star clouds that appeared to be independent of the Milky way galaxy

The “Great Debate” in 1920 between Harlow Shapley and Heber Curtis was a discussion on nature of the many star-rich nebula that could be either: Small objects within our Milky Way Galaxy (our Milky

represented the entire universe) Distant star clouds were far beyond the boundaries of

our Milky Way Galaxy (the universe would be much larger than the Milky Way)

The debate was settled later in the 1920s when spectroscopic measurements of the diffuse nebulae showed them to be much farther than the stars in the Milky Way The universe was discovered to be a collection of

many galaxies, and the Milky Way was just one of many

Precise measurements of many of the galaxies by Ernst Opik and Edwin Hubble in the 1920s led to distance determination methods that included Doppler shift measurements based on relative velocities between distant galaxies and an observer on Earth

Following Hubble’s classification of galaxy shapes and types, the distance to many galaxies showed that the universe was in constant expansion Distant galaxies were receding faster than nearby galaxies Velocity of recession was proportional to distance

Hubble law provided an accurate measurement of Universe expansion Distance to galaxies and clusters of galaxies (easier to

measure at very large distances)

Variations of and improvements in the Hubble classification of galaxies are still used today

Hubble’s data of receding galaxy clusters (velocity vs. distance)

Inverse of the Hubble relation (velocity/distance) gives the age of the universe

Receding galaxy clusters Hubble law states that velocity is proportional to

distance

Interpretation (hypothesis) As we look back in time, the universe expands

faster Highest expansion rate is at the beginning of the

formation of the universe

Big Bang

Initially called the “primeval atom”, George Lemaitre hypothesized that the universe began as a small entity that expanded into the present-day universe according to the Hubble relation

Refinements over several decades transformed the primeval atom hypothesis of Lemaitre into the Big Bang theory that is accepted today as the broad collection of theories that best describes the formation of the observable universe

Early mythology reflected the primitive efforts to interpret or understand the external world represented in the night sky

Our efforts today to understand the universe contain many theories that tie in the smallest and largest components of the physical world Smallest – quantum physics Largest – Einstein’s theory of general relativity Still missing – gravity, dark matter, dark energy

Research into the origin and evolution of the universe include both observation and theory

Theory must fit observation to be valid, and must be validated by scientists in the field to become a robust model (theory)

Basic observations that must be fit to the theory

Dark night sky (Obler’s paradox)

Receding galaxy clusters (expanding universe) Good correlation between distance and recessional velocity

Cosmic background radiation left over from beginning of universe

3:1 H to He ratio

Oldest stars are 13 billion years old

Age of galaxies Oldest are 13 billion years Some dwarf galaxies are still forming

Evolution of galaxies Earliest galaxies are small

Dwarf galaxies often with irregular shapes Later galaxies are larger, better formed, and more complex

Scales used to measure the universe (small to large)

Solar system - near Distance to planets – measured in months and years in spacecraft

travel time for a spacecraft traveling at 10-20 km/s = 22,500-45,000 mph

Light minutes to light hours to the nearby planets Light minute is the distance traveled by light in one minute = 18,000,000

km Light second is the distance traveled by light in one second = 299,800 km

Solar system – far Pluto – 6 light hours (45 years at spacecraft speeds in a Hohmann

transfer orbit)

Nearest stars 4-5 light years Distance of one light year is the distance traveled by light in one

year = 9.461×1012 km = 6.324×104 AU Approximately 100,000 years at spacecraft speeds

Distance to Galaxy center – 30,000 light years (ly)

Diameter of Milky Way Galaxy – 100,000 ly

Distance to Andromeda galaxy (nearest large galaxy) – 2,000,000 ly

Distance to nearby Virgo cluster of galaxies – 60,000,000 ly (6x107 ly)

Distance to the Great Attractor – 2.5x108 ly

Distance to the early surface of the expanding universe – 45x109 ly (45 billion light years) This distance is 3.3 times the age of the universe in light travel

time, but remember the universe is expanding and that mass curves space

Largest structures

The largest structures in the universe are the superclusters that string together and form web-like features composed of galaxies

These filamentary structures can extend billions of light years in length with large collections of superclusters making up the dense regions and little visible mass in the dark voids between them

Galaxies are large collections of stars, gas, dust, and dark matter

For the typical galaxy, the gas, dust and dark matter are an order of magnitude more massive than the stars

The atomic composition of a galaxy is predominantly hydrogen and helium, with a fractional amount of other material

The arrangement of stars and gas within a galaxy produces a characteristic shape to each galaxy, with several common features found among the various galaxy shapes

The original classification of the galaxies according to their basic shape was devised by Edwin Hubble in the 1920s Elliptical Spiral Barred spiral

Galaxies range in size from roughly 107 to 1014 solar masses (Mo)

There are approximately 100 billion galaxies making up the baryonic (atomic) universe

Formation process is hierarchical Galaxy and galaxy cluster formation are in high-

density regions of H-He gas (top-down) Galaxies form as small objects first, then evolve

to form larger galaxies (bottom-up) Early galaxies are mostly dwarfs Galaxies evolve to giants by collisions and

cannibalism

Galaxies, like solar systems, are gas-dominated, rotating systems that have a large central mass

The rotational patterns in the disk-shaped (spiral) galaxies follow the same orbital relation as the planets in our solar system

Rotation speed slows and orbital period increases with increasing distance from the center

Keplerian rotation is not uniform in most spiral galaxies

Rather than decrease as 1/r1/2 as the planets do, the orbital velocities often exhibit constant rotational velocity with increasing distance, suggesting significant other mass not visible in the outer regions of the visible galaxy

This extra mass is also not visible in any of the electromagnetic bands, and hence is called dark mass, or dark matter

A diagram of the flat rotation curve due to hypothesized dark matter is shown below for spiral galaxy NGC 3198

Dark matter is also found surrounding groups, clusters and superclusters of galaxies

Dark matter is unusual because it does not form in the same central concentration as the galaxies or groups (its not cold) , yet it does not disperse over time (its not hot)

Both "cold" and "hot" dark matter models for dark matter are problematic since neither accurately portrays the actual distribution based on its gravitational influence

Spiral galaxies are easily identified by their armed spiral structures that extend from the center of the galaxy outward to the edge of the equatorial disk

Spiral arms that typify spiral galaxies outline elongated sites of ongoing star formation and are brighter than the surrounding disk because of the young, hot OB star

M101 is shown on the right

Barred spiral galaxies include a straight bar feature in the central region of the otherwise spiral galaxy

Roughly half of all spirals are observed to have a bar structure that extends from the galaxy center to the inner reach of the spiral arms

NGC 1300 is shown on the right

Elliptical galaxies are generally egg-shaped with little if any disk feature and can be spherical

These elliptical galaxies are generally the most massive, and also represent the end-point in collisions between large spiral galaxies

M87 is shown on the right

Irregular galaxies have few of the dominant features of the larger spirals and elliptical galaxies

Irregular shapes generally lack an equatorial disk of the spiral galaxies and the symmetrical shape of the ellipsiodal galaxies

The irregular galaxy morphology (shape) is generally associated with smaller dwarf galaxies, analogous to the irregularly-shaped planetesimals compared to the spherical planets and large moons

NGC 1569 is shown on the right

Our own Milky Way Galaxy is barred spiral type classified as an SBc type SB = barred spiral C = diffuse outer

arms

Diameter = 100,000 ly Mass = 100 billion

solar masses Sun’s orbit period =

250 My

The Milky Way Galaxy is but one of nearly 50 galaxies located in a gravitationally-bound collection of large and small galaxies called the Local Group

The two primary galaxies of the Local Group are the Milky Way and the Andromeda galaxies Each has more than a dozen

dwarf satellite galaxies orbiting their center

Andromeda and the Milky Way are in orbit around the Local Group’s center of mass and on a collision course in about 5 By

Diameter of the Local Group of galaxies is approximately 10 Million light years (10 Mly)

The Milky Way Galaxy and the Local Group of neighboring galaxies are also members of the huge Virgo supercluster of galaxies

The Virgo supercluster contains more than 100 groups and clusters (including the Virgo cluster) and spans more than 100 Mly in diameter

The Virgo supercluster is thought to be just one of millions of superclusters in the observable universe