Types of Galaxies

Elliptical:

Spiral:

Irregular:

Milky Way Galaxy

Supermassive Black Hole at center

~ 1 Million Solar Masses

Sun - 2/3 of the way out on a spiral arm

Normal and Active Galaxies How do the luminosities of normal and active

galaxies compare?

What about their spectra?

What resides at the center of most galaxies, including our own? Where is the Sun?

Active Galactic Nuclei (AGN)

Several hundreds of trillions of solar luminosities!

Spectrum is not black body => luminosity not due to starlight alone!

What is the power source?

Evolution of Galaxies:

Caused by merging and interactions between Galaxies.

AGN Energy Source

Rapidly varying intensity => small size

High luminosity and small size

=> supermassive black hole

Billions of solar masses!

Consumes entire stars!

Cosmic Distance Scale

“Standard Candles” Extremely bright events

with well-known luminosities

Ex. Type Ia (carbon-detonation) supernovae Works out to ~ 3 billion light

years!

Recall: Apparent Brightness = Luminosity/Distance^2

Structures of Galaxies

On intermediate scales, galaxies form identifiable structures.

Groups

Clusters

Superclusters

The “Local” Supercluster

Cosmological Structure?

How is matter distributed in the universe on the very largest scales?

No Structure on Largest Scales (Galaxies distributed fairly uniformly)

Surprising given observations of groups, clusters, and superclusters on smaller scales

Cosmological Principle:Universe is homogeneous and isotropic

Homogeneity => ?

Isotropy => ?

Olbers` Paradox If universe is homogeneous, isotropic, infinite in extent, and unchanging, then any line of sight from Earth shouldeventually run into a star.

What observable effect would this have on the night sky?

Why isn't this what we see?

The universe is expanding

=> Assumption that universe is unchanging is incorrect

=> Doesn’t matter whether universe is finite or infinite, we only see a finite part

=> Light from objects greater than 14 billion light years away has not had time to reach us!

Hubble Expansion Analogy

1 Mpc 6 Mpc 12 Mpc

2 Mpc 12 Mpc 24 Mpc

Accuracy increases with distance. The farther out , the faster the expansion. Works for any chosen point of reference. Hubble's Law: v = H

0 x distance

1 pc ~ 3 Light Years

The Fate of the Universe

What property determines the ultimate fate of the universe?

What does the best available data tell us about how the expansion rate of the universe is changing?

Affect of Density on the Universe

As best we can tell, the expansion rate of the universe is increasing!

Question

What is it that is causing the expansion of the universe to accelerate?

What is the biggest single contributor to the overall density of the universe?

DARK ENERGY

The Big Bang Primordial explosion thought to be the creation of

our universe – all matter and space, “beginning” of time.

Hot and dense conditions => early universe was opaque to radiation!

Primordial nucleosynthesis

Source of Cosmic Microwave Background Radiation

Primordial Nucleosynthesis Creation of base level of helium (~25%) in

the early universe

Stopped because temperature and density became too low (due to expansion) for fusion to continue

Also, Hubble's Law and CMBR

Evidence for the Big Bang

The Horizon Problem

CMBR - Leftover radiation from the Big Bang

How is the CMBR distributed across the sky?

Why is this a problem?

The Flatness Problem

The cosmic density appears to be fairly close to the critical value

No obvious reason why universe should have exactly the critical density!

What theory solves both the horizon and flatness problems?

The Theory of Inflation

The universe swelled in size by a factor of about 1050 in only 10-32 seconds!

Solves both horizon and flatness problems! How?

Extraterrestrial Life (as We Know It)

“Life as we know it” is generally taken to mean carbon-based life that originated in a liquid-water environment.

Requirements?Liquid water, protection from radiation, temperate climate

What are some important characteristics of living organisms?

Characteristics of a Living Organism

Responsiveness to environment

Reproduction/replication

Capacity to evolve

Ability to take in nourishment, grow, and heal

The Miller-Urey experiment attempts to recreate the chemical conditions of the primitive Earth in the laboratory, and synthesize some of the building blocks of life.

Supports the assumptions of mediocrity!

The Drake Equation

What is the Drake equation designed to estimate?

Which terms have astronomers measured most accurately?

The Drake Equation

number of technological, intelligent civilizations in the Milky Way

rate at which new stars are formed (avg. over galactic lifetime)

= x

fraction of stars having planetary systems

x

average number of habitable planets within those planetary systems

xfraction of those habitable planets on which life arises

xfraction of those life-bearing planets on which intelligence evolves

fraction of those planets with intelligent life that develop technological society

average lifetime of a technological civilization

xx

Each term is less certain than the preceding one! Only in last ten years have we addressed the second term.

SETI

Search for ExtraTerrestrial Intelligence- Listens for radio signals from alien worlds

Microwave Window - in radio part of EM spectrum - Minimal noise

- Referred to as the “Water Hole”

Good luck and have a great Summer!