Galaxies

Astronomy 100

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

• stars formed together at same time

• stars may be gravitationally bound together

• two types: open (galactic) and globular

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

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)

• 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)

What is a “galaxy”?What is a “galaxy”?

• A large group of stars outside of our own Milky Way• Made of billions to trillions of stars– Also may have gas and dust

• Spiral, or elliptical, or irregular shaped

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

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• Nebulae• Galaxies

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