Transcript What is a light-year and how is it used?

How do we
know our
Universe is
expanding?
• Throughout our universe, light is bursting
from stars, bouncing off planets, diving into
black holes, wandering into nebulae, and
generally going every which way.
• Meanwhile, a little bit of it actually shows up
here on earth.
• The light that does arrive here all seems to
bear the same message: the universe is
expanding.
Each type of atom gives off a unique set of
colors. The colored lines (or Spectral Lines)
are a kind of "signature" for the atoms.
Red Shift
• How can light from the night sky tell us
that the universe is growing in size?
The main clue comes from something called
redshift.
• Redshift is light's version of a
phenomenon we experience all the time
with sound.
•
Have you ever noticed how the pitch of a police siren seems to drop
suddenly as the car zooms by you?
As the siren approaches
you, the waves of sound are
squeezed together, and you
hear them as being higherpitched. After the car
passes by, sound waves
from the receding siren are
stretched apart. You hear
these stretched waves as
being lower-pitched.
• Just like a cosmic police car, a star
zooming toward you has its light waves
squeezed together.
• You see these light waves as having a
higher frequency than normal.
• Since blue is at the high-frequency end
of the visible spectrum, we say the light
from an approaching star is shifted
toward blue, or blueshifted.
• Likewise, if a star is zooming away from you,
any light it emits gets stretched.
• You see these stretched-out light waves as
having a lower frequency.
• Since red is at the low-frequency end of the
visible spectrum, we say that light from a
receding star is shifted toward red, or
redshifted.
Red Shift
• Observers looking
at an object that is
moving away from
them see light that
has a longer
wavelength than it
had when it was
emitted-- a
redshift.
Blue Shift
• Observers looking
at an approaching
source see light
that is shifted to
shorter
wavelength-- a
blueshift.
• The amount of the shift depends on the speed
of the star, relative to you.
• For a moving object to create an appreciable
redshift or blueshift requires some pretty
serious speeds.
• To get just a 1% change in the frequency of
light, a star has to be moving 1,864 miles per
second.
• For a blue lightbulb to look red, it would have
to be flying away from you at 3/4 of the speed
of light.
• Studying light from galaxies throughout
our universe, astronomers have noticed
something surprising: almost all of it is
redshifted.
• In fact, not only is it redshifted, galaxies
that are farther away are more redshifted
than closer ones.
• So it seems that not only are all the
galaxies in the universe moving away from
us, the farther ones are moving away from
us the fastest.
Red Shift
• Observers looking
at an object that is
moving away from
them see light that
has a longer
wavelength than it
had when it was
emitted-- a
redshift.
Blue Shift
• Observers looking
at an approaching
source see light
that is shifted to
shorter
wavelength-- a
blueshift.
Birth of the Big Bang
• Edwin Hubble noticed:
– the further away a galaxy is, the faster it is
moving away from us.
– galaxies are all moving, and are all moving
away from each other
• Interpretation: Hubble said that the Universe
was expanding.
Birth of the Big Bang
• This implied that the galaxies were once
much closer together.
• This must mean that the whole Universe was
once a single point, which exploded into
existence!
• The idea of the Big Bang was formed.
Big Bang and Universe Expanding
• Big Bang- The universe was
formed billions of years ago in
an enormous explosion. The
universe has been expanding
ever since.
• We see evidence for this big
bang because astronomers
looking at stars and galaxies in
the sky see that over time,
they are gradually moving
apart.
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