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ISNS 3371 - Phenomena of Nature
Uses of Telescopes
1.
Imaging
–
use a camera to take pictures (images)
–
Photometry  measure total amount of light from an object
2.
Spectroscopy
–
use a spectrograph to separate the light into its different
–
wavelengths (colors)
3.
Timing
–
measure how the amount of light changes with time
(sometimes in a fraction of a second)
ISNS 3371 - Phenomena of Nature
Imaging
• In astronomy, filters are
usually placed in front of
a camera to allow only
certain colors to be
imaged
• Single color images are
superimposed to form
true color images.
ISNS 3371 - Phenomena of Nature
Nonvisible Light
•
•
•
Most light is invisible to the human eye - gamma rays, x-rays, ultraviolet,
infrared, radio waves.
Special detectors/receivers can record such light - each type of light can
provide information not available from other types.
Digital images are reconstructed using false-color coding so that we can see
this light.
Chandra X-ray image of the Center of the Milky Way Galaxy
ISNS 3371 - Phenomena of Nature
The Crab Nebula
Visible
Radio Waves
Infrared
X-rays
ISNS 3371 - Phenomena of Nature
Atmospheric Effects
Earth’s atmosphere causes problems for astronomers on the ground:
• Bad weather makes it impossible to observe the night sky.
• Man-made light is reflected by the atmosphere, thus making the night
sky brighter.
– this is called light pollution
• The atmosphere absorbs light - dependent on wavelength
• Air turbulence in the atmosphere distorts light.
– That is why the stars appear to “twinkle”.
– Angular resolution is degraded.
ISNS 3371 - Phenomena of Nature
Atmospheric Absorption of Light
• Earth’s atmosphere absorbs most types of light.
– good thing it does, or we would be dead!
• Only visible, radio, and certain IR and UV light make it through to the
ground.
To observe the other wavelengths, we must put our telescopes in space!
ISNS 3371 - Phenomena of Nature
Space Astronomy
ISNS 3371 - Phenomena of Nature
Space Based Telescopes
Chandra X-ray Obs.
FUSE (Far UV)
Compton Gamma Ray Obs.
Hubble Space Telescope
Spitzer Space Telescope (IR)
ISNS 3371 - Phenomena of Nature
Radio Telescopes
• The wavelengths of radio waves are long.
• So the dishes which reflect them must be very large to achieve any
reasonable angular resolution.
305-meter radio telescope at Arecibo, Puerto Rico
ISNS 3371 - Phenomena of Nature
Spectrum of a Low Density Cloud Animation
ISNS 3371 - Phenomena of Nature
ISNS 3371 - Phenomena of Nature
Kirchhoff’s Laws of Radiation
First Law. A luminous solid, liquid or gas, such as a light bulb filament,
emits light of all wavelengths thus producing a continuous spectrum
of thermal radiation.
Second Law. If thermal radiation passes through a thin gas that is
cooler than the thermal emitter, dark absorption lines are superimposed
on the continuous spectrum. The gas absorbs certain wavelengths.
This is called an absorption spectrum or dark line spectrum.
Third Law. Viewed against a cold, dark background, the same gas
produces an emission line spectrum. It emits light of discrete
wavelengths. This is called an emission spectrum or bright line
spectrum.
.
ISNS 3371 - Phenomena of Nature
So what astronomical body has this spectrum?
ISNS 3371 - Phenomena of Nature
The Doppler Effect
ISNS 3371 - Phenomena of Nature
Doppler Effect for Light Animation
ISNS 3371 - Phenomena of Nature
The Doppler Effect
1. Light emitted from an object moving towards you will have its
wavelength shortened.
BLUESHIFT
2. Light emitted from an object moving away from you will have its
wavelength lengthened.
REDSHIFT
3. Light emitted from an object moving perpendicular to your line-ofsight will not change its wavelength.
ISNS 3371 - Phenomena of Nature
Doppler Shift of Emission Lines Animation
ISNS 3371 - Phenomena of Nature
The amount of spectral shift tells us the velocity of the object:
 = v

c
ISNS 3371 - Phenomena of Nature
The Doppler shift only tells us part of the object’s full motion - the radial
part or the part directed toward or away from us.
ISNS 3371 - Phenomena of Nature
Measuring Rotational Velocity
Doppler shift can be used to tell us how fast an object is rotating: As an
object rotates, light from side rotating toward us is blueshifted - light
from side rotating away from us is redshifted. Spectral lines appear
wider - the faster it rotates, the wider are the spectral lines.
ISNS 3371 - Phenomena of Nature
Extrasolar Planets
• Planets which orbit other stars are called extrasolar planets.
• Over the past century, we have assumed that extrasolar planets exist, as
evidenced from our science fiction.
• We finally obtained direct evidence of the existence of an extrasolar
planet in the year 1995.
– A planet was discovered in orbit around the star 51 Pegasi.
– Over 100 such extrasolar planets are now known to exist.
ISNS 3371 - Phenomena of Nature
Detecting Extrasolar Planets
• Can we actually make images of extrasolar planets?
– No, this is very difficult to do.
• The distances to the nearest stars are much greater than the distances
from a star to its planets.
– The angle between a star and its planets, as seen from Earth, is too
small to resolve with our biggest telescopes.
• A star like the Sun would be a billion times brighter than the light
reflected off its planets.
• As a matter of contrast, the planet would be lost in the glare of the star.
•
Improved techniques of interferometry may solve this problem
someday.
ISNS 3371 - Phenomena of Nature
Detecting Extrasolar Planets
•
We detect the planets indirectly by observing the star.
•
Planet gravitationally tugs the star, causing it to wobble.
•
This periodic wobble is measured from the Doppler Shift of the star’s spectrum.
Stellar Motion and Planets Animation
ISNS 3371 - Phenomena of Nature
Doppler shift allows
detection of slight motion
of star caused by orbiting
planet
ISNS 3371 - Phenomena of Nature
Determining Star’s Velocity Animation
ISNS 3371 - Phenomena of Nature
A plot of the radial
velocity shifts forms a
wave.
–Its wavelength
tells you the period
and size of the
planet’s orbit.
–Its amplitude tells
you the mass of the
planet.
Doppler shift in spectrum of star 51 Pegasi - shows presence of large planet
with orbital period of about 4 days.
ISNS 3371 - Phenomena of Nature
Determining Planet Mass and Orbit Animation
ISNS 3371 - Phenomena of Nature
Remember - Doppler shift only tells us radial motion. If plane of orbit
perpendicular to our line of sight - no shift seen. If we view it from edge on,
maximum Doppler shift seen. Orbit generally tilted at some angle - star’s full
speed not measured. So mass derived from Doppler technique is minimum
possible. If varying position in sky measured (as in one case) orbital tilt can be
determined and mass measured accurately.
ISNS 3371 - Phenomena of Nature
Planetary Transit
•
The Doppler technique yields only planet masses and orbits.
•
Planet must eclipse or transit the star in order to measure its radius.
•
Size of the planet is estimated from the amount of starlight it blocks.
•
We must view along the plane of the planet’s orbit for a transit to occur.
– transits are relatively rare
•
They allow us to calculate the density of the planet.
– extrasolar planets we have detected have Jovian-like densities.
Planetary Transit Animation
ISNS 3371 - Phenomena of Nature
Orbital distances and
approximate masses of
first 77 planets
discovered