Galaxies and More Galaxies!

It is now believed that there are over 100 billion galaxies, each with

an average of 100 billion stars… 1020 stars altogether!

Types of Galaxies

Photographs of distant galaxies indicate that they fall into four basic categories: spiral, bar spiral, elliptical, and irregularirregular.

The Milky Way is a spiral galaxy.

Spiral Galaxy• A typical spiral galaxy

has a tight center, with long trailing arms extending outwards.

• As seen from the side, they are flattened disks.

• Many have halos surrounding them, with old Population II stars.

The “Full” Milky Way

Magellanic Clouds

Barred Spiral Galaxies

• The central region forms a straight “bar”, surrounded by two arms.

• Like normal spirals, they are flattened as seen from the side.

• One arm shows a blue shift, the other arm a red shift. Why?

Density Waves

The formation of new stars in spiral type galaxies.

Elliptical Galaxies• These galaxies show no

signs of arm formation.• They can be circular or

very oval in shape.• The center of the galaxy

is the most dense, gradually thinning out toward the edges.

Irregular Galaxies

• They appear to be galaxies still unformed.

• However, they have old stars in them, so it is not believed that they are changing to another type.

How Far Away are the Galaxies?

• For relatively close galaxies, we can observe Cepheid variables.

• Let’s say that we find out that the variable has an absolute magnitude of -4 and an apparent magnitude of +21.

• Use the formula: D = 10(Mapp - Mabs +5)/5

D = 10(21– -4 +5)/5 = 106 pc D = 1 megaparsec (Mpc)

Hubble’s Law• Most galaxies are too

far away to use this method.

• Edwin Hubble discovered that most galaxies are receding. The faster they recede, the farther away they are.

• He produced a graph similar to the one at the right.

Using Hubble’s Law to Estimate Distances

Usually Hubble’s Law is written as a formula:

Vr = H • d (Vr is the recessional

velocity, H is Hubble’s constant, d is distance in

megaparsecs).

We can use Doppler shifts to measure the velocity, so the formula is often rewritten as

d =Vr (H = 70 approximately)

H

Putting the formula to work…

• Let’s say that the Sombrero galaxy shows a red shift of 2.5 nm from a rest wavelength of 750.

Vradial = shift x c

rest

Vradial = 2.5 nm x 300,000

750 nm

Vradial = 1000 km/sec

Why Hubble’s Law Works…

• Nearly all galaxies show a red shift, which means they are moving away.

• The universe itself is expanding, like a rising loaf of bread.

• The farther the galaxies are from us, the faster they are moving.

Plug into the formula:

• Hubble’s Law:

d = Vr

H

d = 1000 = 14.3 Mpc

70

1000

14.3

Measuring the Hubble Deep FieldMeasuring the Hubble Deep Field

Mapping the Galaxies

The Milky Way is part of a cluster of galaxies known as the Local Group.

Most other galaxies are clustered as well.

Looking even further into deep space…

Old Theory of Galactic Evolution

The Facts Don’t Fit!• If this evolutionary path was correct, the elliptical

galaxies would only have young stars. They don’t!• It is now believed that galaxies change by interacting

with other galaxies, by gravitational pull, or by cannibalizing them.

Colliding Galaxies• Since most galaxies are

in clusters, they are attracted to each other.

• Sometimes they pass nearby and pull material out of the other.

• In some cases they merge, especially if one is much larger than the other.

Just passing through…

• The Cartwheel Galaxy shows evidence that a smaller galaxy passed through it.

• The ring around the galaxy is one of the remnants of this interaction.

Changes to Our Own Milky Way Galaxy

• On the other side of the Milky Way, a small galaxy known as the Sagittarius Dwarf is being cannibalized.

• Eventually the two Magellanic Clouds will also be absorbed.

Animation

Possible Collision with the Andromeda Galaxy

A Super Elliptical Galaxy?

• Andromeda and the Milky Way are traveling between 100-300 kilometers a second on a collision course for each other… or they could miss completely, it could go either way.

• If the two giant galaxies do collide much of the structure of each will be destroyed. They will no longer be two spiral Galaxies but one super elliptical galaxy.

• However, we have 3 billion years to wait for that to happen!

Animation

Gas Cloud Accretion

• Recent evidence has shown that clouds of gas are being pulled into the galaxy.

• They are the raw materials for new star production.

Black HolesBlack Holes in the in the Galactic CentersGalactic Centers

• As galaxies develop, black holes coexist with them, near the center of the galaxy.

• In our galaxy, a black hole has been detected with X-ray imagery known as SgrA*.

The “Seyfert” Galaxy

• Galaxies with massive black holes in the center have high activity.

• Often we observe massive explosions in the centers of these Seyfert galaxies.

Different Views of a Seyfert Galaxy

Journey to Andromeda’s Center

• Andromeda Galaxy is around 2.5 million light years from us.

• It has a large black hole in its center.

• The animation here takes us to the center of this galaxy:

• ANIMATION

White Holes, Wormholes

• Black holes “suck in” just about anything that gets too close.

• Where does all this matter end up?

• Some theorists say that it emerges out a “white hole.”

• Perhaps they are both connected by worm holes of science fiction fame.