ELECTRO MAGNETIC SPECTRUM
• The electromagnetic spectrum extends from
wavelengths of many meters to wavelengths
of submicroscopic size.
• Visible light has wavelengths of less than
one millionth of a meter.
• Chemical elements in a star’s atmosphere
emit and absorb light at different
wavelengths.
• We can tell what elements are on a star by
the light we see coming from it.
• A spectrograph breaks the light down for us
a simple spectrograph
top: continuous spectrum
bottom: emission spectrum
Emission spectra of some of the
elements
Absorption spectra are produced when a substance such as an element
or a colored liquid is placed in front of a full spectrum light source; the
element will absorb some of the light and let some of it pass through.
Which colors does hydrogen absorb?
The next slide shows emission
spectra on top and absorption
spectra underneath. Compare the
emission spectrum of helium
(top), with its absorption
spectrum (bottom)
absorption spectra
Emission
spectra
TOOLS OF MODERN ASTRONOMY
How do we find what stars are made of?
• First we need to improve our eyesight. We do
this by using telescopes.
• There are telescopes that detect visible light
and those that detect light outside the visible
range.
• There are two type: Refracting and
Reflecting.
TELESCOPES THAT USE VISIBLE LIGHT
• The reflecting telescope bounces the light
from the object off a concave mirror and
focuses it in the eye or camera .
• The refracting telescope uses a convex lens
to bend the light from the object and focus it
in the eye or camera .
• Most visible light telescopes are reflecting
telescopes.
• Other telescopes use wavelengths of the
spectrum that are not visible.
• Radio telescopes use radio waves which are very long.
Don’t forget that stars produce all kinds of radiation,
including radio waves. The larger the radio telescope, the
more waves it can detect and the better the image will be.
• There are telescopes that detect short wavelengths like xrays and gamma rays. So some images are the result of xrays that were emitted by the star and captured by an x-ray
telescope.
• Earth based telescopes use visible, ultra-violet, infra-red, or
radio waves because the atmosphere does not block them,
unlike the shorter wavelengths such as x-rays.
• Telescopes in space have opened up more possibilities
because they can use shorter wavelengths.
The Hubble
telescope
(top)
and the VLT
(bottom)
Yerkes: the
largest refracting
telescope (40
inch diameter)
(a) Mona Koa is 4-km above sea level. The air is so thin that astronomers
wear oxygen masks. (b) the 10-m mirror of Keck1.
Photo (a) was taken with a telescope
half the diameter of (b). Which has
more detail?
Arecibo has a dish with a diameter
of 305 m to detect radio waves
Focal
point
Radio (top)
compared
with optical
image of
spiral
galaxy M51
Crab nebula:
visible light
Crab nebula:
radio waves