ISNS 3371 - Phenomena of Nature 1. Imaging –use a camera to take pictures (images) –Photometry measure total amount of light from an object 2.Spectroscopy.

ISNS 3371 - Phenomena of Nature

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)

Uses of Telescopes


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.


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 NebulaVisible Infrared

Radio Waves X-rays


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.

Atmospheric Effects


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!


Space Astronomy


Space Based Telescopes

Chandra X-ray Obs. Hubble Space Telescope

Compton Gamma Ray Obs. Spitzer Space Telescope (IR)

FUSE (Far UV)


Radio Telescopes

305-meter radio telescope at Arecibo, Puerto Rico

• The wavelengths of radio waves are long.• So the dishes which reflect them must be very large to achieve any

reasonable angular resolution.


Spectrum of a Low Density Cloud Animation



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.

.


So what astronomical body has this spectrum?


The Doppler Effect


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.

2. Light emitted from an object moving away from you will have its wavelength lengthened.

3. Light emitted from an object moving perpendicular to your line-of-sight will not change its wavelength.

BLUESHIFT

REDSHIFTREDSHIFT


Doppler Shift of Emission Lines Animation


v c

=

The amount of spectral shift tells us the velocity of the object:


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.


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.


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.


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.


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


Doppler shift allows detection of slight motion of star caused by orbiting planet


Determining Star’s Velocity Animation


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.


Determining Planet Mass and Orbit Animation


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.


• 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

Planetary Transit Animation


Orbital distances and approximate masses of first 77 planets discovered

ISNS 3371 - Phenomena of Nature 1. Imaging –use a camera to take pictures (images) –Photometry measure total amount of light from an object 2.Spectroscopy.

Documents