Galaxy Notes '23

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Galaxies

Open Ended

What is one thing you already know about galaxies? What's one question you have about galaxies?

What are Galaxies?

• A group of billions of stars and their planets, gas,

and dust that extends over many thousands of
light-years and forms a unit within the universe.

• Held together by gravitational forces.

• Estimated 50 billion galaxies in the universe.

How do Galaxies Form?

• Massive clouds of dust and gas collects by

gravitational pull and collapses under its
own weight, forming stars...maybe.

• OR, maybe primordial lumps of matter

from the early universe clumped together.

– Hubble Telescope

Galaxy Mergers

Galaxy evolution has not stopped!

The Milky Way is “digesting” two small galaxies right now.

Contents of Galaxies

Galaxies are made of
stars, planetary
systems, gas clouds,
and star clusters.
Nebulas are giant clouds
of gas and dust where
stars may be forming.

We're fairly certain stars are formed here, in the arms of galaxies because the arms of galaxies are full of nebulas!

​

​Star Formation

Multiple Choice

What holds Galaxies together?

• Space

• Time

• Gravitational force

• Light

Multiple Choice

Where do we believe stars are formed?

• Space

• Nebulas

• Black holes

• Prominences

Galaxy Classification

In 1924, Edwin Hubble

divided galaxies into

different “classes” based

on their appearance.

The Hubble Deep Field

The longest, deepest exposure ever taken.

Was an empty piece of sky!

From this image, we can estimate
the number of galaxies in the
universe! This is how:

1.

Count the number of
galaxies in this image

2.

Measure angular area on
the sky of this image

3.

Figure out how many
images of this size needed
to cover entire sky

4.

Multiply that number
(from 3.) by the number
of galaxies in this image
(from 1.)

Elliptical

Spiral

Irregular

Examples of Three Main Morphological Galaxy Types

The Hubble
Tuning Fork

© 2015 Pearson Education, Inc.

Spiral galaxy

Spiral Galaxies

Spirals are classified by their relative amount of
disk and bulge components.

We designate these Sa, Sb, Sc, in order of
decreasing bulge to disk ratio.

More bulge

More disk

Barred spirals are

called SBa, SBb, SBc

More disk means
more star formation!

The

“Milky Way”

galaxy is a

spiral galaxy.

The Sun is in a

spiral arm

half-way out

from the
center.

Map of Milky Way

Our Sun(yellow dot) is in a spiral arm (Orion’s Arm), ½-way out

Hotspot

Click where you think stars are formed!

Elliptical Galaxies

•Smooth structure and symmetric, elliptical contours
•Subtype E0 - E7 defined by flattening

•En where n = 10(a-b)/a

a and b are the projected major and minor axes

Elliptical Galaxies

Names of E galaxies give their shape.
E0 is round. E6 is elongated.

The way you name an E galaxy is to
measure its “major” and “minor” axis
and plug it into the formula above.

An Example of an E0 galaxy. The
bright objects surrounding it are its
own globular clusters.

Elliptical can be extremely large and massive. This galaxy is 2 million light years across.

The size of the Milky Way in comparison!

General trends within Hubble sequence E 🡪 Sc:

•Decreasing Bulge/Disk
•Decreasing stellar age
•Increasing fractional gas content
•Increasing ongoing star formation

Multiple Choice

What are the galaxies classified as E?

• Elliptical

• Spiral

• Irregular

• Milky Way

Match

Match the following

E0

E7

Barred Spiral

• E0

• E7

• Barred Spiral

Multiple Choice

Using the hubble tuning fork, what would this galaxy be classified as?

• E0

• E7

• Sa

• SBb

Multiple Choice

What type of galaxy is this?

• Spiral

• Barred Spiral

• Elliptical

• Irregular

Examples
of Galaxies

M31 - The

Great
Spiral

Galaxy in
Andromeda

This nearby galaxy in

the Local Group of

galaxies, of which the

Milky Way is a

member, is 2.5 million

light years away.

(NOAO/AURA Photos)

Dwarf

Irregular
Galaxy

in

Sagittarius

Hubble Space Telescope Image

Galaxies in Collision

In this close encounter between two spiral galaxies, their arms are dramatically warped and

massive star formation is triggered when the hydrogen gas clouds in the two collide.

Hubble Space Telescope Image

Draw

Draw 2 things you learned today!