Galaxies

Background

• Billions of them in the universe

• Only largest telescopes can gather the light that travels from them to Earth

• Hubble Deep Field– Relatively nearby galaxies– 10 billion LY away

Discovery

• William Parsons– 1845 built a telescope– 72 in. diameter– “spiral nebulae” great spirals of stars – Foggy swirls of dust and gas

December 30, 1924

• Mt. Wilson observatory

• Edwin Hubble

• Discovery of cepheid variables

• Supergiants are faint, very distant

• Hence, they must exist outside of the Milky Way

Spiral

Types of Galaxies

• Spiral

• Elliptical

• Irregular

Barred Spiral

Elliptical

Irregular

Dark Matter

• We estimate the mass of stars and galaxies from luminosity

• Measured masses are often too large…something must be missing

• The mass that is not visible is called “dark matter”

• What we can see is only the central, luminous parts of much larger structures.

• X-ray images reveal more evidence

X-ray images

• Galaxy clusters filled with very hot, low-density gas (not the dark matter)

• Something must hold the gas there (gravity)

• What is creating the gravity is the dark matter.

• 90-99% of all matter in the universe is dark

• Could be low-luminosity white dwarfs and brown dwarfs

• We have found them, but not enough to account for all the missing mass!

• Could be a yet discovered type of matter.

The Life of a Galaxy

• How do they form?

• There is NO answer to this question!

• How do they evolve?– Interactions, collisions, and mergers with

other galaxies

Colliding Galaxies

• Should happen often (they are large and in motion)

• There are hundreds that do this

“The mice”

• They don’t need to collide for their gravitational fields to affect one another

• Can last hundreds to millions of years

• The results are ring galaxies and galactic cannibalism (a small galaxy is pulled apart by a larger one)

• The Milky Way is cannibalizing the Magellanic clouds!

Galaxy Clusters

• A few to a few thousand members

• We (the Milky Way) are a member of a cluster containing 36+ galaxies

• Rich clusters- over 1000 (Virgo cluster)

• Poor clusters- less than 1000 (the Local Group, of which we are a member)

Virgo Cluster

The Local Group

Superclusters

• Clusters of galaxies that are associated with each other

• Local group is part of the local supercluster– Includes the local group, the Virgo cluster,

and others– So faint and far away, they are difficult to see,

even with the best telescopes

Evolution

• Elliptical galaxies formed from the merger of spirals

• Spirals have not experienced collisions with large galaxies since their formation