Galaxies

Astronomy 115

First, which of the following is a galaxy?

• Open cluster• Globular cluster• Nebula• Interstellar medium (gas and dust)• Supernova remnant• None of these

First, which of the following is a galaxy?

• Open cluster• Globular cluster• Nebula• Interstellar medium (gas and dust)• Supernova remnant• None of these

What is a “star cluster”?What is a “star cluster”?

• stars formed together at same time

• stars are at least weakly gravitationally bound together

• two types: open (galactic) and globular (shown to right)

Image at http://hubblesite.org/newscenter/archive/releases/star%20cluster/globular/2007/18/image/a/format/web/results/50/

Open ClustersOpen Clusters• dozens to

thousands of stars• young stars! only

a few million years old

• may still be surrounded by nebula from which they formed

• located in the spiral arms of a galaxy

• example: Pleiades• Fate: generally,

the stars drift apart (not enough gravity)

Image at http://hubblesite.org/newscenter/archive/releases/star%20cluster/open/2004/20/image/a/results/50/

More open star clusters

Globular ClustersGlobular Clusters• millions to hundreds of millions of

stars• old! 6 to 13 billion years• mostly red giants and dwarfs• stars are clumped closely together,

especially near the center of the cluster (densely); stars don’t drift apart

• surround our disk as a halo

Image at http://hubblesite.org/newscenter/archive/releases/star%20cluster/globular/1999/26/image/a/results/50/

What is a “nebula”?What is a “nebula”?

• A cloud in space• Made of gas and dust

– Can have stars inside

• Most of the ones we see are inside our Milky Way Galaxy

• Different types

Large, massive, bright nebulae

Emission NebulaEmission Nebula•The hot gas is emitting lightThe hot gas is emitting light

Colder, darker nebulaeDark dust blocking Dark dust blocking the hot gas behind the hot gas behind itit

Leftovers from an Explosion

Supernova remnantSupernova remnant (smaller, less gas) (smaller, less gas)

So, what is a “galaxy”?So, what is a “galaxy”?

• A large group of stars outside of our own Milky Way• Made of billions to trillions of stars, held together by

its own gravity, with all different ages of stars– Also may have gas and dust

• Spiral, or elliptical, or irregular shaped

Galaxy ClassificationSa

Sb

Sc

E0 = Spherical

Small nucleus; loosely

wound arms

E1

E6

E0, …, E7 Large nucleus;

tightly wound arms

E7 = Highly elliptical

Spiral galaxy--Andromeda

NOAO/AURA/NSF Images at http://www.noao.edu/image_gallery/html/im0606.html and http://www.noao.edu/image_gallery/html/im0685.html

Elliptical Galaxies

Irregular Galaxies

NASA and NOAO/AURA/NSF Images at http://hubblesite.org/newscenter/archive/releases/galaxy/irregular/2005/09/results/50/ , http://www.noao.edu/image_gallery/html/im0560.html , and http://www.noao.edu/image_gallery/html/im0993.html

Irregular GalaxiesOften: result of galaxy collisions / mergers

Often: Very active star formation (“Starburst galaxies”)

Some: Small (“dwarf galaxies”) satellites of larger galaxies

(e.g., Magellanic Clouds)

Large Magellanic

Cloud

NGC 4038/4039

The Cocoon Galaxy

Galaxy Diversity

The Hubble Deep Field:

10-day exposure on an apparently empty field in the sky

Even seemingly empty regions

of the sky contain

thousands of very faint, very distant galaxies

Large variety of galaxy

morphologies:

Spirals

Ellipticals

Irregular

(some interacting)

Gas and Dust in GalaxiesSpirals are rich in

gas and dustEllipticals are almost

devoid of gas and dust

Galaxies with disk and bulge, but no dust are termed S0

Barred Spirals

• Some spirals show a pronounced bar structure in the center

• They are termed barred spiral galaxies

• Sequence:

SBa, …, SBc,

analogous to regular spirals

Our Galaxy: the Milky WayOur Galaxy: the Milky Way

• has about 200 billion stars, and lots of gas and dust

• is a barred-spiral (we think)• about 100,000 light-years wide• our Sun is halfway to the edge, revolving at

half a million miles per hour around the center of the Galaxy

• takes our Solar System about 200 million years to revolve once around our galaxy

The Milky WayThe Milky Way

Mapping the Milky Way

We can see: • Stars and star clusters – microwaves generated by water from H II

regions (called the MASER technique) traces the Milky Way’s spiral arms

• Nebulae – infrared light (detected by the Spitzer Space Telescope) shows the outline of the heat generated by the bar

• Other galaxies (analogous structure as our galaxy)

How do we know what our Galaxy looks like?How do we know what our Galaxy looks like?

Do galaxies evolve over time?

Edwin Hubble (whom we’ll hear more about next lecture) in 1926 classified known galaxies according to shape, and suggested an “evolution” of galaxies from elliptical to spiral as they aged. The diagram was called a “tuning fork” due to its shape

No one evolutionary path for galaxies

As detection methods grew more sophisticated, using the infrared (Spitzer telescope), radio (Very Long Baseline Array) and gamma ray (Compton telescope) portions of the EM spectrum, the tuning fork is no longer regarded as containing an evolutionary sequence – it’s simply a way of classifying galaxies.

It is true that irregular galaxies seem to form from galactic collisions, and that some spiral galaxies lose their arms to become elliptical (Milky Way + Andromeda fate), there is no good model to describe galactic evolution.