Astronomical Imaging: Overview
• When you think of a clear, dark night
sky, what do you think of?
– The human visual system is fine-tuned to focus,
detect, & process (= image) the particular
wavelengths where the Sun emits most of its
energy
– As a result, when we look at the night sky, what
we see is dominated by starlight
» We think stars and planets when we think of
astronomy
The Night Sky: Orion
This is approximately what you can see with your
unaided eye when you look at Orion on a clear night
Magnitudes
• Magnitudes: a “backwards,” logarithmic scale to
measure the brightnesses of stars
• For each increase of 1 magnitude, an object is
fainter by a factor of 2.5
– an increase of 5 magnitudes is a factor 100 decrease in brightness
– an increase of 2.5 magnitudes is a factor 10 decrease in brightness
• magnitude = -2.5*log(F/F0)
– F and F0 represent the number of photons/second received
from an object and reference
Magnitudes and Human Vision
• Sensitivity of human vision is limited (in large part)
by the length of time your brain is willing to wait to
receive and interpret the signals from the eye
– The brightest stars have magnitudes of about 0
» (well, OK: the magnitude of Sirius is -1)
» Venus gets as bright as -4!
– The faintest stars you can see have magnitudes of about 5
• What if you could have your retina store up the
signals it detects, then report them to the brain?
You might see this when you look at Orion!
Note that not all stars are the same color
Betelgeuse
(a red supergiant)
Rigel
(a blue supergiant)
Furthermore:
visible light is a small part of the whole story
Multiwavelength astronomy
• All-sky views at various
wavelengths
• Note the dominance of the
Milky Way (the galaxy
where our solar system is
located)
Gamma Ray
X-ray
Visible
Also note that stars are only
one ingredient in a galaxy!
Infrared
Radio Waves
Images from NASA
The Orion Nebula:
Stellar Nursery
The constellation of Orion
(wide-field optical)
The Orion Nebula
(Hubble Space Telescope optical)
Three views of the young stars in Orion
Orion Nebula region
left: optical (HST); center: infrared
(2MASS); right: Xray (Chandra)