1	variable stars		_____	Milky Way and Andromeda are examples
2	period-luminosity relationship		_____	no arms, smooth, called "E" or"E-type"
3	binary stars		_____	many star clusters found here, dark nebulae limit vision here
4	the Sun		_____	spiral local group galaxy in which the Sun resides
5	Milky Way Galaxy		_____	our nearest galactic neighbor, called M31 and NGC224
6	spiral galaxy		_____	correlation between luminosity and pulsation
7	galactic bulge		_____	a bar of material passes through the galactic center
8	irregular galaxy		_____	luminosity changes with time, some erratically others regularly
9	Andromeda Galaxy		_____	without rotational symmetry, may be young and ejecting bubbles of gas
10	quasar		_____	English siblings who discovered & studied galaxies
11	nebulae		_____	located in our galaxy's suburbs, 30,000 ly from center
12	barred - spiral galaxy		_____	two stars orbiting around their center of mass held by gravitational attraction
13	Local Group		_____	a cluster of galaxies in which our solar system belongs
14	galactic disk		_____	huge cloud of gas and dust in intergalactic space
15	W. & C. Herschel		_____	region of galaxy far above and below center
16	galactic halo		_____	American astronomer who classified the galaxies
17	elliptical galaxy		_____	known as a QSO, star-sized energy source with excess UV radiation
18	E. Hubble		_____	visible light cannot penetrate clouds here