Chapter 31 Galaxies & the

Universe

Review& Recap

It does this by precisely measuring the speed of gas and stars around a black hole.

This provides clues for the existence of a black hole.

Astronomers measure the motion of stars swirling around the black hole to determine the mass.

The Hubble Space Telescope measures black holes that cannot be seen.

A Black Hole

A black hole is an object with a gravitational pull so intense that not even light can escape. It pulls in other stars and gas with its gravitational force.

31.1 The Milky Way Galaxy

Objectives:Determine the size and shape of the

Milky Way, as well as Earth’s Location within it.

Describe how the Milky Way was formed.

By measuring a star's period of pulsation, astronomers can determine its luminosity and calculate how far away a variable star must be to appear as dim or as bright as it does.

Variable Stars

Variable stars comparing and contrastingBoth are variable stars that have a relationship between its luminosity and its pulsation period.

RR Lyrae variables • have periods of

pulsations between 1.5 hours and 1 day and, on average, have the same luminosity

Cepheid variables • have pulsation periods

between 1 day and more than 100 days.

• the longer the pulsation period the greater the luminosity

The Shape of the Milky Way Astronomers have determined the shape of the Milky Way by using radio waves because they penetrate the interstellar gas and dust without being scattered or absorbed.

Discovering Other Discovering Other GalaxiesGalaxies

Other galaxies were first Other galaxies were first believed to be nebulae or believed to be nebulae or star clusters within the star clusters within the Milky Way.Milky Way.

Discovering Other Discovering Other Galaxies – cont.Galaxies – cont.

Edwin Hubble's discovery of Edwin Hubble's discovery of CepheidCepheid variable stars in the variable stars in the Great Nebula in the Andromeda Great Nebula in the Andromeda constellation proved the constellation proved the existence of galaxies outside existence of galaxies outside the Milky Way. the Milky Way.

Mass of the Milky Way

Studies provide evidence that there is a great amount of unseen matter called dark matter composed of dim stellar remnants that have mass.

Estimates vary from 200 billion x , up to 1,000 billion x the Mass of our Sun.

Dark Matter

The strongest evidence that the universe contains a larger amount of dark matter than visible matter is that by analyzing the motion of member galaxies, the mass of galaxy clusters can be determined. This mass of whole galaxy clusters is always much larger than the sum of only the visible masses of the galaxies.

The halo and bulge of the Milky Way are made of old stars which suggests that the halo and bulge formed first.

Formation and Evolution of the Milky Way

Quiz – 31.1 The Shape of the Milky Way

31.2 Other Galaxies in the

Universe

Objectives:Describe how astronomers classify

galaxies.Identify how galaxies are organized into

clusters and super clusters.Describe the expansion of the universe.

Superclusters

Gigantic formation of clusters of galaxies hundreds of millions of light-years in size are called superclusters.

Active Galaxies

Core of a galaxy in which highly energetic objects or activities are located. (also known as AGNs!)

Quasars

The redshift in many quasars is very large, which means that those quasars are far away.

The Expanding Universe – Hubble’s Law

The farther away from Earth a galaxy is, the faster it is moving.

Hubble Constant - A value of approximately 70 kilometers per second per megaparsec

Elliptical Galaxy

This is an elliptical galaxy. Galaxies that are not flattened into disks and do not have spiral arms. Their shapes range from round ellipticals to very elongated ellipticals. Possible example: Virgo A

Please copy illustration

Elliptical Galaxy – cont.

Elliptical galaxies are divided into subclasses based on the ratio of their major and minor axes.

Spiral Galaxy

This is a spiral galaxy. Disklike galaxy with spiral arms consisting of interstellar matter and young star clusters. A bulge consists of an old stellar population. There are normal and barred spirals. Possible example: Milky Way.

Please copy illustration

Irregular Galaxy

This is an irregular galaxy. These galaxies do not fit the elliptical or spiral classification. They typically have no distinct shape. Possible example: Large and Small Magellanic Clouds.

Please copy illustration

Galaxy ClusterThis is a galaxy cluster. These groups of galaxies may have from a few to hundreds of member galaxies and may range in sizes up to 30 million ly. In a cluster, most of the inner region galaxies are ellipticals. Galaxies in the outer portions are a mix of ellipticals and spirals. The galaxies often merge to form strangely shaped galaxies with more than one nucleus. Possible example: Local Group.

Please copy illustration

Galaxy Clusters – cont.

Most galaxies in the inner region of a large cluster are ellipticals.

Galaxy Shapes An Sc galaxy is a normal spiral with loosely wound arms and a small, dim nucleus.

M74 Spiral Galaxy

Galaxy Shapes – Cont.Galaxy shapes

a Tightly wound arm and large, bright nucleus

c Loosely wound arms and a small, dim nucleus

E7 Very elongated elliptical

EO Round elliptical

Irr Irregular galaxy

S Normal spiral

SB Barred spiral

SO Flat disks that do not have spiral arms

Quiz – 31.2 Other Galaxies in the Universe (10pts)

31.3 Cosmology Objectives:Explain the different theories about the

formation of the universe.Describe the possible outcomes of

universal expansion.

Models of the Universe

Steady-state theory - Proposes that the universe looks the same on large scales to all observers and that it has always looked that way

Cosmic background radiation

Has a wavelength of approximately 1 mm, which makes it microwave radiation.

Cosmic background radiation

Provides information about conditions very early in the expansion of the universe.

The Hubble Constant

Recent observations show that the rate of expansion of the universe is speeding up.

Models of the Universe – cont.

Big Bang Theory - States that the universe began as a point and has been expanding ever since

The Big Bang – cont.

The theory does not suggest an explosion into space, but instead that there is an expansion of space with matter going along for the ride.

Models of the Universe – cont.

Inflationary Universe Theory - Model says the universe began as a fluctuation in a vacuum and expanded very rapidly for a fraction of a second before settling into a more orderly expansion

Inflationary Universe Theory – cont.

A flat universe was first predicted in the 1980’s by the inflationary universe model.

The Critical Density

Average density and Critical Density are factors in the expansion of the universe.

The Critical Density – cont.

The average density is the total matter of the universe.

The Critical Density – cont.

Critical density is the dividing point between a closed or an open universe.

The Critical Density – cont.

If the average density is higher than the critical density, the universe is closed.

The Critical Density – cont.

If the average density is lower than the critical density, the universe is open.

The Critical Density – cont.

If the average density equals the critical density, the universe is flat.

Quiz – 31.2 Other Galaxies in the Universe (10pts) Quiz – 31.3 Cosmology (10pts)