PHYS205 Galaxies

Where we live:

Milkyway Galaxy

Orion Arm

System of Sol

Third Planet

PHYS205 Herschel

Band of light stretching in the night sky was known to the ancients.

Methodical study of the sky (1785).

Divide into little squares and count the number of stars.

More stars on the diagonal.

Sun at the center.

PHYS205 Milkyway Galaxy

Spiral galaxy with 3 ½ arms.

Perseus, Cygnus, Carina, Orion

80,000 lightyears diameter

50,000 ly radius halo (non-luminous matter)

1,000 ly nuclear bulge

Average size galaxy

PHYS205

Center of our galaxy

• At the center of our galaxy there is a super massive blackhole.

• The size of this blackhole is estimated to be 4 million Solar masses.

PHYS205 Size of the galaxies

Copernicus Earth is not the center of the Solar system

Harlow Shapley Sun is not the center of the Milkyway galaxy.

Cepheids and RR Lyrae are at the end of their lives and they change their brightness in a well known way.

As their brightness is known they can be used as standard candles to determine distances.

Shapley used 93 variable stars to measure the distances in our galaxy.

And proved that we are not at the center of our galaxy.

PHYS205 Rotation Speed

Our galaxy rotates around its center.

The sun revolves around the galactic center every 200,000,000 years.

The density of stars increase when we go closer to the galactic center.

The mass of the stars in a galaxy can be calculated by the revolution speed of the stars around the galactic center.

The more mass, the higher should be the revolution speed.

However…

PHYS205 Dark Matter

Sun revolves around the galactic center much faster than the visible matter of the galaxy suggests

There is more mass in the galaxy than we can see

DARK MATTER

Not just a little to make the calculations correct, but around 80-90% of Milkyway’s mass in dark matter.

PHYS205 Types of Galaxies

Classification made by Edwin Hubble in the 1920s.

Three types of galaxies: Elliptical, Spiral and Irregular

Spirals are further divided into regular spirals and barred spirals.

These groups are further divided among themselves depending on how large the nucleus is (spirals) or how spherical they are (elliptical).

PHYS205 Other Galaxies

10th century astronomer Al-Sufi M31 – Andromeda galaxy, Little Cloud

Magellan Clouds in the Southern Sky Magellanic Clouds

Messier 1781 Catalog of nebulous objects (M31)

Herschel New General Catalog (NGC3034 = M82)

Big question: Where are they located??

Shapley (no galaxies) – Curtis (galaxies) debate (1920)

Hubble and Humason resolved in 1932.

discovered many Cepheid variables in the Andromeda galaxy

they are not in this galaxy, far far away.

PHYS205 Elliptical Galaxies

M87: Virgo cluster

M32: Satellite ofAndromeda

M110: Satellite ofAndromeda Leo I: Local group

PHYS205 Spiral Galaxies

Triangulum Galaxy (M33) in our local cluster

NGC 2997

NGC 1365

NGC 3351 = M95

PHYS205 Irregulars

Large Magellanic Cloud

Small Magellanic Cloud

PHYS205 Clusters

Galaxies love to cluster together.

Coma cluster: A rich cluster

Hercules cluster: A poor cluster

Milkway is part of a poor cluster called the Local Group.

PHYS205 Galaxy Collisions

If there is clustering because of gravitational interactions, there is bound to be some collisions.

BTW, we are going to collide with the Andromeda galaxy in a few hundred million years.

PHYS205 Hubble’s Law

In 1931, Hubble and Humason discovered that more distant galaxies are moving faster away from us.

PHYS205 Quasars

Very difficult to distinguish from regular stars in the sky.

However, they have a very large red-shift meaning that they are very far away (Hubble’s Law).

They are also changing their luminosity very rapidly, we can estimate their sizes.

Optical observations confirm that they are at the center of galaxies far away.

The combination of all this information tells us that:

The size of a quasar is less than a lightyear in diameter.

They are about 1000 times brighter than our galaxy.

They can be at distances about 10 billion lightyears away, but there are no quasar located farther away than that.