Galaxies

Objectives

• Identify three types of galaxies.

• Describe the contents and characteristics of galaxies.

• Explain why looking at distant galaxies reveals what young galaxies looked like..

Solar System

What is a Galaxy ?Distance from Earth to Sun = 93,000,000 miles = 8 light-minutes

Size of Solar System = 5.5 light-hours

What is a Galaxy?Stellar Region

30light-years

Sun(solar system

too small to beseen on this scale)

What is a Galaxy?

A Galaxy is:

100,000 light-years

a massivecollection ofstars, gas, and dust kept togetherby gravity

Sun’s Stellar Region

What is a Galaxy?

If our solar system was the size

of a cell inthe human body,

our galaxywould still

measure overone mile across.

Contents of Galaxies

• Gas Clouds A large clouds of gas and dust in interstellar space is called a nebula.

• Star Clusters A globular cluster is a tight group of stars that looks like a ball and contains up to 1 million stars.

• An open cluster is a group of stars that are close together relative to surrounding stars.

• Contents of Galaxies• Quasars

There are two main classes of galaxies organized by Hubble

ELIPTICAL• Spherical galaxies

classified on a scale of 0 to 7• The number indicates

how egg-shaped the ellipse is - 0 means a ball shape, and 7 looks like a dish

SPIRAL• Central nucleus

surrounded by arms of stars

• Spiral or barred spiral• The bar is a dense

area of stars that extends along the diameter of the galaxy

Types of GalaxiesSpiral disk-like appearance with arms of stars and dust forming a spiral pattern

Barred Spiral similar to spirals but with a bright bar of stars and gas through the center

Elliptical elliptically-shaped, with less gas and dust than spirals; no disk or “arms”

Irregular neither elliptical nor spiral in shape; gas and dust as in spirals but no defined “arms”

Peculiar distorted form of one of the above types, often due to collision with another galaxy or similar catastrophic event

Where did they come from?

• Galaxies first formed in the densest spots in the Universe.

• By the time the Universe was a few hundred million years old, the densest regions had ceased expanding and began to collapse

• Matter in these areas did not expand rapidly with the big bang

• These areas formed dense groups of rotating stars which became galaxies

Where did they come from?

• A gigantic star formed at the center of the star cluster from the inflowing matter.

• Stars containing more than about 100 times the mass of the Sun are unstable. If slightly compressed, they collapse faster and faster until a black hole forms.

• In every observed galaxy, supermassive black holes have been found.

Classification of Galaxies

Hubble Tuning Fork

The Hubble Tuning Fork• After he discovered what galaxies really were, Edwin

Hubble became the first person to classify galaxies. • Astronomers use his system, called the "Hubble

Tuning Fork," even today. – First, Hubble divided the galaxies into two general

categories: elliptical and spiral galaxies. • Elliptical galaxies are shaped like ellipses, and

spiral galaxies are shaped like spirals, with arms winding in to a bright center.

The Hubble Tuning Fork

• Elliptical galaxies are classified by how round or flat they look.

• An E0 galaxy is very round and an E7 galaxy is very flat.

• In detail, the number after the "E" is determined by the galaxy's ellipticity - the ratio of the ellipse's major axis to minor axis.

• Galaxies with higher ellipticities have higher numbers.

The Hubble Tuning Fork

• Hubble noticed that some spiral galaxies have a bright line, or bar, running through them.

• He called these galaxies "barred spiral galaxies."

• Galaxies with spiral arms, but without the bar, are just called "spiral galaxies."

The Hubble Tuning Fork

• Spiral galaxies are further classified by how tightly their arms are wound.

• Type a galaxies have their arms wound very tightly and have large central bulges.

• Type c galaxies have very their arms would loosely and have small central bulges.

The Hubble Tuning Fork

• Some galaxies are a transition type between the elliptical and spiral galaxies, labeled S0 on the tuning fork.

• These are called "lenticular galaxies." • Lenticular galaxies have a central bulge and a

disk but no spiral arms.

The Hubble Tuning Fork

• The third class of galaxies is irregular galaxies. • Irregular galaxies are neither spiral nor

elliptical, and can have any number of shapes. • They are frequently the product of two

galaxies colliding with each other, or at least affecting each other through the force of gravity.

• For each slide, write the type of galaxy as classified by the Hubble Tuning Fork diagram.

• 1• 2• 3• 4• 5• 6• 7• 8• 9• 10• 11• 12• 13• 14• 15

Spiral

Spiral

Lenticular Spiral

Barred Spiral

Elliptical

SpiralMilky Way galaxy - Spiral

Barred Spiral

Barred Spiral

Elliptical

Irregular

Elliptical

Spiral

Irregular

Lenticular Spiral

Spiral

Milky Way Galaxy = Our Home•Our solar system is located in a spiral galaxy called the Milky Way. •From the side, the Milky Way appears to be a narrow disk with a bulge in the middle. •The galaxy’s spiral structure is visible only from above or below.

The Milky Way Galaxy• The Milky Way might contain one trillion

stars.

• The visible disk of stars is about 100,000 light-years across.

• The Sun is located about 26,000 light-years from the galaxy's center in one of the spiral arms.

The Milky Way Galaxy

• In the galaxy, all stars orbit around a central region, or core.

• It takes about 225 million years for Sun to orbit the center of the Milky Way.

• The last time our solar system was in its current position was during the Triassic Period, when dinosaurs first appeared on Earth!

The Milky Way Galaxy

• The Milky Way often is classified as a normal spiral galaxy.

• However, recent evidence suggests that it might be a barred spiral.

The Milky Way Galaxy• You can, see the Milky Way stretching across

the sky as a misty band of faint light. • You can see the brightest part of the Milky

Way if you look low in the southern sky on a moonless summer night.

• Like many other galaxies, the Milky Way has a supermassive black hole at its center.

• This black hole might be more than 2.5 millions times as massive as the Sun.

The Milky Way Galaxy

• Evidence for the existence of the black hole comes from observing the orbit of a star near the galaxy's center.

• Additional evidence includes X-ray emissions detected by the Chandra X-ray Observatory.