In this lecture we'll learn how the lives
of galaxies are connected with the history
of the universe, and the three major types
of galaxies, and how galaxies are grouped
together in the cosmos.
The images in the background is called the
Hubble Deep Field.
The size of the piece of sky is about the
size of a grain of sand at arm's length.
The Hubble Space Telescope looked at this
tiny region of sky for ten days.
Every spot you see is an individual galaxy.
Imagine, if there are this many galaxies in
a random spot of sky the size of a grain of
sand, just how many galaxies must there be
in our universe?
We actually estimate there are 100 billion
galaxies in the observable universe.
The lives of galaxies are long.
To understand the life story of a typical
galaxy, we have to look for different galaxies
at different life stages.
This is no easy task.
Most of the galaxies near us are about the
same age.
To study young galaxies, we have to look back
into the universe's past, which means looking
to great distances.
Remember the farther away we look in space,
the further back we look in time.
So when we observe galaxies that are very
far away, we see them as they were when they
and the universe were young.
The relationship between a galaxy's age, its
distance, and the age of the universe makes
it impossible to consider the histories of
galaxies without at the same time considering
the evolution of the universe as a whole.
The study of galaxies is therefore closely
connected with cosmology? the study of the
structure and evolution of the universe.
Galaxies are so beautiful.
If you have the time, look at the Hubble website
gallery, you can enjoy the wide variety of
gorgeousness our universe has to offer.
If you look at galaxies long enough, you're
likely to agree with astronomers that galaxies
fall into three major types.
The three major types of galaxies that we
see are: spiral, elliptical, and irregular
First, let's look at spiral galaxies.
Spiral galaxies have a thin disk that extends
outward from a central bulge.
The bulge merges smoothly into a halo that
can extend to a radius of more than 100,000
light-years.
Spiral galaxies have two distinct populations,
the disk population, which includes stars
of all ages and masses that orbit in the disk
of the galaxy, and the spheroidal population,
which consists of the halo and bulge stars.
Lenticular galaxies have a disk like a spiral
galaxy but have much less dusty gas.
They are intermediate between spiral and elliptical
galaxy types.
Elliptical galaxies differ from spiral galaxies
primarily in that they have only a spheroidal
component, and lack a significant disk component.
Elliptical galaxies come in a wide range of
sizes, from the rare giant ellipticals, to
the dwarf ellipticals which often have fewer
than a billion stars.
Elliptical galaxies usually contain very little
dust or cool gas.
Therefore, there is little or no star formation
within ellipticals.
Ellipticals often look reddish in color because
all the massive blue stars have died away.
Some galaxies are neither clearly spiral nor
elliptical.
These irregular galaxies are a miscellaneous
class, encompassing small galaxies
As well as larger "peculiar galaxies" that
appear to be in disarray.
The astronomer Edwin Hubble invented a system
for classifying galaxies that organizes the
galaxy types into a diagram shaped like a
tuning fork.
Elliptical galaxies appear on the "handle"
at the left, designated by the letter ?E'
and a number.
The larger the number, the flatter the elliptical
galaxy.
An E- galaxy is a sphere, and an E7 is highly
elongated.
The two forks show spiral galaxies, designated
by the letter S for ordinary spirals and SB
for barred spirals, followed by a lowercase
a, b, or c.
The bulge size decreases from 'a' to 'c',
while the amount of dusty gas increases.
Lenticular galaxies are designated S0.
Astronomers had once thought the Hubble sequence
was an evolutionary sequence in which galaxies
flattened and spread out as they aged, but
we now know that is not the case.
The evolution of galaxies is much more complex.
Edwin Hubble knew this from the beginning.
So, how are galaxies grouped together?
Well, most of the galaxies in the universe
are gravitationally bound together with neighboring
galaxies.
Spiral galaxies are often found in loose collections
of up to a few dozen galaxies called groups.
Our Milky Way is part of the Local Group,
for example.
Elliptical galaxies are particularly common
in clusters of galaxies, which can contain
hundreds and sometimes thousands of galaxies
extending over more than 10 million light-years.
I will leave you with this Hubble image of
the Peculiar Galaxies of Arp 273.
Their distorted appearance is due to gravitational
tides as the pair engages in a close encounters
From our perspective they look like a lovely
flower.
In fact, if you are looking for a creative
thing to make your sweetie, you can put this
image on a card.
It's a unique way to give someone you love
some an amazing cosmic flower.
Take care, I will talk to you soon.
