Research in the early part of the last century

showed that the city of stars that we live in is not the only thing in the universe.

The Milky Way is just one of billions of galaxies spread across billions of

light years in space.

In this section of astronomy state of the art we'll look at galaxies.

Their properties and behaviors and how they're distributed to a vast cosmic Most.

We'll start with the Milky Way, our system of stars, 400 billion strong.

We'll look at the evidence for dark matter initially coming from, what are called,

rotation curves of galaxies.

Or maps of the ways the stars rotating in a disk

as you move further out from the center.

In the center of our galaxy, infrared and

radio techniques have revealed the dense star cluster that hides another huge

surprise, a supermassive black hole several million times the mass of the sun.

Exquisitely detailed observations have now revealed individual

stars in orbit around this black hole showing its mass.

With high degree of precision, and

venturing close to the event horizon of this extraordinary object.

We'll talk about quasars and other active galaxies, and

what kind of extraordinary energetic phenomenon occur in their nuclei.

We'll look at the properties of galaxies in general.

The different morphological types, spirals like our own Milky Way,

giant ellipticals, And smaller, irregular galaxies.

And how we measure the distances to these galaxies and

map out their positions in three dimensional space.

Also discovered early last century, the galaxies are all receding from us.

A cosmic ray shift that's an indication of the expanding universe.

As surveys with large telescopes have to grown to span hundreds of thousands,

or millions, of galaxies.

An exquisite filigree of large-scale structure is seen,

on the scale of hundreds of millions of light-years.

Galaxies are not randomly or uniformly distributed through space.

They cluster into filaments and huge aggregations and

super clusters with voids in between, almost bereft of luminous matter.

Tracing back the cosmic expansion in time revealed a surprise 15 years ago.

The detection of an accelerating universe in the last five billions years.

Not part of the normal Heuvel expansion, and

in fact working in the opposite sense to gravity.

Which should slow down the expansion with cosmic time.

This single observation has implicated dark energy.

A component of the universe larger than dark matter, and

many times larger than the normal matter that we are made of.

Galaxies introduction video