180 PART 2^ |^ THE STARS
Visual-wavelength image■ Figure P-7
The beautiful symmetry of the spiral pattern is clear in this image of the galaxy NGC 1300. Young star clusters containing hot, bright stars and ionized hydro-
gen are located along the spiral arms, while dark lanes of dust mark the inner edges of the arms. (NASA, ESA, and The Hubble Heritage Team; STScI/AURA)
Furthermore, tidal forces twist and distort the colliding galaxies.
Images of such colliding galaxies show their twisted shapes, far-
fl ung streamers of stars and gas, and extensive regions of star
formation (■ Figure P-9). Such bursts of star formation salt the
galaxies with newly formed heavy atoms. Our own Milky Way
Galaxy has probably collided with more than one smaller galaxy,
and some of the atoms of which you are made may have been
formed because of those collisions.The Origin of the
UniverseA little less than a century ago, astronomers made an aston-
ishing discovery. Th e galaxies in the universe are moving away
from each other. Th e spectrum of a galaxy is the combined spec-
trum of billions of stars, and the spectral lines visible in galaxy
spectra are shifted slightly toward the red end of the spectrum.
Th is redshift is proportional to distance. Th at is, the farther away
a galaxy is, the larger is the redshift that is visible in its spectrum
(■ Figure P-10). Th is result, fi rst discovered by the American
astronomer Edwin Hubble in 1929, is clear evidence that the
universe is expanding.
If the galaxies are all rushing away from each other, then you
can imagine viewing a video running backward. You would seeP-5
■ Figure P-8
An apparently empty spot on the sky only 1/30 the diameter of the full moon
contains over 1500 galaxies in this extremely long time exposure known as the
Northern Hubble Deep Field. Presumably the entire sky is similarly fi lled with
galaxies. (R. Williams and the Hubble Deep Field Team, STScI/AURA, NASA, ESA)
Location
of Hubble
DipperBig Deep FieldVisual-wavelength image