Oct. 18th - Doppler Effect, Telescopes Flashcards
The Doppler Effect - Train Example
When the train is moving toward you, each pulse of a sound wave is emitted a little closer to you.
The result is that waves are bunched up between you and the train, giving them a shorter wavelength and higher frequency (pitch).
After the train passes you by, each pulse comes from farther away, stretching out the wavelengths and giving the sound a lower frequency
The Doppler Effect
The Doppler effect causes similar shifts in the wavelengths of light
If an object is moving toward us, the light waves bunch up between us and the object, so its entire spectrum is shifted to shorter wavelengths.
Because shorter wavelengths of visible light are bluer, the Doppler shift of an object coming toward us is called a blueshift
If an object is moving away from us, its light is shifted to longer wavelengths. We call this a redshift because longer wavelengths of visible light are redder.
The Doppler Effect - Spectral Lines & Rest Wavelengths
Provide the reference points we use to identify and measure Doppler shifts
* For example, suppose we recognize the pattern of hydrogen lines in the spectrum of a distant object
* We know the rest wavelengths of the hydrogen lines—that is, their wavelengths in stationary clouds of hydrogen gas — from laboratory experiments in which a tube of hydrogen gas is heated so that the wavelengths of the spectral lines can be measured.
* If the hydrogen lines from the object appear at longer wavelengths, then we know they are redshifted and the object is moving away from us
* The larger the shift, the faster the object is moving; can apply to either redshift moving away from us (right), or blueshift moving towards us (left)
* If the lines appear at shorter wavelengths, then we know they are blueshifted and the object is moving toward us.
Doppler shifts do not give us any information about..
It’s important to note that a Doppler shift tells us only the part of an object’s full motion that is directed toward or away from us
NOT how fast an object is moving across our line of sight (and can’t give any information about objects that aren’t coming towards/away from us)
To measure how fast an object is moving across our line of sight, we must observe it long enough to notice how its position gradually shifts across our sky
Rotation Rates: The Doppler effect not only tells us how fast a distant object is moving toward or away from us but also can reveal information about motion….
- WITHIN the object
- Suppose we look at spectral lines of a rotating planet or star: as the object rotates, light from the part of the object rotating toward us will be blueshifted, light from the part rotating away from us will be redshifted, and light from the center of the object won’t be shifted at all.
How can we determine rotation rate from spectral lines?
- The net effect, if we look at the whole object at once, is to make each spectral line appear wider than it would if the object were not rotating.
- The faster the object is rotating, the broader in wavelength the spectral lines become. We can therefore determine the rotation rate of a distant object by measuring the width of its spectral lines.
Most observations fall into one of three basic categories:
- Imaging
- Spectroscopy
- Time Monitoring
Observation Categories
Imaging
- Yields photographs (images) of astronomical objects
- At its most basic, an imaging instrument is simply a camera. Astronomers often place filters to allow only particular colors or wavelengths of light to pass through
- Images made with invisible light cannot have any natural color, because “color” is a property only of visible light. However, we can use color-coding to help us interpret them
- Images may be color-coded according to the wavelength of the light, the intensity of the light, or the physical properties of the objects in the image.
Observation Categories
Spectroscopy
- In which astronomers obtain and study spectra
- Instruments called spectrographs use diffraction gratings (or other devices) to separate the various colors of light into spectra, which are then recorded with a detector
- A spectrum can reveal a wealth of information about an object, including its chemical composition, temperature, and motion
- However, just as the amount of information we can glean from an image depends on the angular resolution, the information we can glean from a spectrum depends on the spectral resolution: The higher the spectral resolution, the more detail we can see
(However, higher spectral resolution comes at a price. A telescope collects only so much light in a given amount of time, and the spectral resolution depends on how widely the spectrograph spreads out this light.)
Observation Categories
Time monitoring
Tracks how an object changes with time
* Many astronomical objects vary with time
* Some objects vary periodically; for example, small, periodic changes in a star’s brightness can reveal the presence of an orbiting planet
* For a slowly varying object, time monitoring may be as simple as comparing images or spectra obtained at different times. For more rapidly varying sources, time monitoring may require instruments that make rapid multiple exposures
* The results of time monitoring are often shown as light curves: graphs that show how an object’s intensity varies with time
Working with astronomical data
You can think of the job of astronomers today as consisting of three major roles:
- Planning the observations to be obtained with telescopes
- Analyzing the data received from telescopes
- Developing theoretical models to explain the astronomical observations.
Telescopes: Portals of Discovery
How does earth observe atmosphere affect-ground-based observations:
- Our daytime sky is bright because the atmosphere scatters sunlight, and this brightness drowns out the dim light of most astronomical objects
- The constraints of daylight and weather affect the timing of observations, but by themselves do not hinder observations on clear nights
How does earth observe atmosphere affect-ground-based observations
Our atmosphere creates three other problems that inevitably affect astronomical observations:
- The light pollution from city lights and orbiting satellites
- The blurring of images by atmospheric motion
- The fact that most forms of light cannot reach the ground at all.
Our atmosphere creates 3 problems that affect astronomical observations
The light pollution from city lights and orbiting satellites
- Just as our atmosphere scatters sunlight in the daytime, it also scatters the bright lights of cities at night, creating what astronomers call light pollution
- Light pollution has become an increasing problem as cities have grown
- A new type of light pollution has begun to cause problems for astronomical observations: light reflected by artificial satellites in low-Earth orbit
- You can recognize a satellite because of its steady motion with respect to the stars. As you might guess, moving satellites can create streaks or other distortions in long-exposure astronomical images.
- The recent advent of satellite “constellations,” launched to provide Internet access around the world, has dramatically increased
Our atmosphere creates 3 problems that affect astronomical observations
The blurring of images by atmospheric motion
The ever-changing motion, or turbulence, of air in the atmosphere bends light in continually shifting patterns
As a result, our view of things outside Earth’s atmosphere appears to jiggle around (like water from the bottom of a swimming pool)
In most cases, the blurring of images by turbulence limits the angular resolution of ground-based telescopes to no better than about 0.5 arcsecond, even if a telescope’s diffraction limit is much smaller than that.
