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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
Orion image at
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
Image at
Spiral Galaxies
have flat disk, spiral arms, central bulge, and a
surrounding halo
some have a “barred” bulge
are fairly large (no dwarf spirals)
have lots of gas and dust and younger stars in their
arms, but older stars and little gas or dust in their
halos and central bulges
Spiral galaxy--Andromeda
NOAO/AURA/NSF Images at http://www.noao.edu/image_gallery/html/im0606.html
The Milky Way
Image at
Our location in the Galaxy
Elliptical galaxies
range from spherical to football shaped
range from very small to giant
have very little gas or dust
mostly old stars
similar to the central bulge of a spiral galaxy
Elliptical Galaxies
Images at
http://hubblesite.org/newscenter/archive/releases/galaxy/elliptical/2007/08/image/a/format
Elliptical Galaxies
Images at
http://hubblesite.org/newscenter/archive/releases/galaxy/elliptical/2007/08/image/a/format
Irregular Galaxies
any galaxy that isn’t a Spiral, Elliptical, or
Lenticular
usually have lots of gas and dust and young
stars
may have a distorted shape from interaction
with another galaxy
Irregular Galaxies
NASA and NOAO/AURA/NSF Images at
http://hubblesite.org/newscenter/archive/releases/galaxy/irregular/2005/09/results/50/ ,
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 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
Radiation
There are lots of types of light (radiation),
including visible and invisible
Electromagnetic spectrum
http://coolcosmos.ipac.caltech.edu/cosmic_classroom/ir_tutorial/what_is_ir.html
.
The Electromagnetic Spectrum
 The EM spectrum is the ENTIRE range of EM waves in
order of increasing frequency and decreasing wavelength
As you go from left  right, the wavelengths get smaller and the frequencies get higher.
This is an inverse relationship between wave size and frequency. (As one goes up, the other goes
down.) This is because the speed of ALL EM waves is the speed of light (300,000 km/s).