Chapter 6
Why are astronomers interested in building observatories capable of detecting neutrinos, cosmic rays, and gravitational waves?
These things are NOT forms of light, and therefore can provide different types of information than light about objects that emit them.
The stars in our sky twinkle in brightness and color because of ______.
turbulence in the Earth's atmosphere
What is the angular resolution of the human eye?
about 1 arcminute, or 1/60 of a degree
If you had only one telescope and wanted to take both visible-light and ultraviolet pictures of stars, where should you locate your telescope?
in space
The James Webb Space Telescope is designed primarily to observe __________.
infrared light
Which of the following wavelength regions can be studied with telescopes on the ground?
radio, visible, and very limited portions of the infrared and ultraviolet regions
When your eye forms an image, the _____ plays a role analogous to the detector in a camera.
retina
What is the purpose of adaptive optics?
to eliminate the distorting effects of atmospheric turbulence for telescopes on the ground
Which of the following studies is best suited to astronomical observations that fall into the category called time monitoring?
Studying how a star's brightness varies over a period of 3 years.
Which of the following statements best describes the two principal advantages of telescopes over eyes?
Telescopes can collect far more light with far better angular resolution
Which of the following statements best describes the two principle advantages of telescopes over eyes?
Telescopes can collect far more light with far better angular resolution.
What are the two most important properties of a telescope?
- angular resolution - light-collecting area
Which of the following forms of light can be observed with telescopes at sea level?
- visible light - radio waves
How much more light does an 8-meter telescope gather than a 2-meter telescope?
16 times as much
Match the words at the left to the correct blanks in the sentences at right.
A camera is an example of an instrument used for IMAGING observations. SPECTROSCOPY refers to telescopic observations in which we separate an object's light so we can measure its intensity at different wavelengths. If we want to confirm that a star's brightness alternately dims and brightens, we need TIME MONITORING observations of the star. The familiar twinkling of the stars is caused by ATMOSPHERIC TURBULENCE, which also blurs telescopic images. Human civilization is responsible for what astronomers call LIGHT POLLUTION.
Which of the following statements best describes the difference between a refracting telescope and a reflecting telescope?
A refracting telescope uses a transparent glass lens to focus light, whereas a reflecting telescope uses a mirror to focus light.
Which of the following is a principal advantage of CCDs over photographic film?
CCDs capture a much higher percentage of the incoming photons than does film.
Which of the following is NOT one of the three main categories of observation generally used by astronomers?
Filtering to look at just a single color from an object.
Which of the following best describes why radio telescopes are generally much larger in size than telescopes designed to collect visible light?
Getting an image of the same angular resolution requires a much larger telescope for radio waves than for visible light.
Suppose you want to determine the chemical composition of a distant planet or star. Which of the following will be most useful to have?
High spectral resolution.
Shown following are six different types of light that travel to Earth from sources in space. Rank these types of light from left to right based on the altitude in the atmosphere where they are completely absorbed, from highest to lowest (Earth's surface). If two (or more) of the choices reach the same altitude or the surface, rank them as equal by dragging one on top of the other(s).
Highest altitude: - X-ray - most ultraviolet light - most infrared light - green visible light and most radio waves
Sort each of the astronomical questions below into the appropriate bin based on the type of observation you would need to perform to answer it.
Imaging: - How large is the Andromeda Galaxy? - What are the major surface features of Mars? - Are stars in the Orion Nebula surrounded by dusty disks of gas? Spectroscopy: -What is the chemical composition of the Crab Nebula? - What is the temperature of Jupiter's atmosphere? - Is the star Vega moving toward us or away from us? Time Monitoring: - Is the X-ray emission from the galactic center steady or changing? - Does the star Mira vary in brightness?
Each of the following statements describes an astronomical measurement. Place each measurement into the appropriate bin based on the type of telescope you would use to make it.
Infrared telescope: - Determine the surface temperature of Venus. - Study a dense cloud of cold gas in space. Visible light telescope: - Obtain a spectrum of the sunlight reflected by Mars. - Measure the brightness of a star that is similar to our Sun. X-ray telescope: - Look for high-energy radiation from a supernova. - Observe the hot (1-million K) gas in the Sun's corona.
What is the purpose of interferometry?
It allows two or more small telescopes to achieve the angular resolution of a much larger telescope.
What does the technique of interferometry allow?
It allows two or more telescopes to obtain the angular resolution of a single telescope much larger than any of the individual telescopes.
What is meant by spectral resolution?
It is a measure of how close two spectral lines can be distinguished.
Which of the following is NOT an advantage of the Hubble Space Telescope over ground-based telescopes?
It is closer to the stars.
What do we mean by the diffraction limit of a telescope?
It is the best angular resolution the telescope could achieve with perfect optical quality and in the absence of atmospheric distortion.
Which of the following best describes what we mean by the focal plane of a telescope?
It is the place where, if we mounted film or an electronic detector, we could get a clear (not blurry) image of an object viewed through the telescope.
Listed following are the names and mirror diameters for six of the world's greatest reflecting telescopes used to gather visible light. Rank the telescopes from left to right based on their light-collecting area from largest to smallest. For telescopes with more than one mirror, rank based on the combined light-collecting area of the mirrors.
Largest area: - Large Binocular Telescope: Two 8.4-m mirrors - Keck 1: One 10-m mirror - Hobby-Ebberly: One 9.2-m mirror - Subaru: One 8.3-m mirror - Gemini North: One 8-m mirror - Magellan 11: One 6.5-m mirror
What do astronomers mean by light pollution?
Light pollution is light from human sources that makes it difficult to see the stars at night
Consider two future observatories in space. Observatory X consists of a single 50-meter telescope. Observatory Y is an interferometer consisting of five 10-meter telescopes, spread out over a region 100 meters across. Which observatory can detect dimmer stars, and which one can see more detail in its images? (Assume all else is equal, such as quality of optics, types of instruments, and so on.)
Observatory X can detect dimmer stars, and Observatory Y reveals more detail in images.
Match the examples of invisible wavelengths with the appropriate description
RADIO astronomy uses dishes like satellite dishes to observe objects. In some cases, the dishes work together to form images at a much higher resolution than any one of them could achieve alone. An example is Very Large Array in New Mexico. INFRARED telescopes are very similar to optical telescopes, except that they use different detectors to sense the light. An example is the Spitzer INFRARED Space Telescope. Most of the ULTRAVIOLET wavelengths act enough like the visible wavelengths that we can use the same sorts of telescopes, with different detectors. An example is the Hubble Space Telescope. X-RAYtelescopes have to gently redirect the light because it tends to go straight through things in its path. Mirrors are arranged so that the light just grazes them. An example is the Chandra Space Telescope. GAMMA-RAY telescopes must be massive in order to intercept this high-energy light. An example is the Fermi GAMMA-RAY Telescope.
Listed following are distinguishing characteristics and examples of reflecting and refracting telescopes. Match these to the appropriate category.
Reflecting: - Most commonly used by professional astronomers today - The Hubble Space Telescope - world's largest telescope Refracting: - very large telescopes become "top-heavy" - incoming light passes through glass - Galileo's telescopes - The world's largest is 1-meter in diameter
Which of the following telescopes benefits most from adaptive optics?
The Keck I telescope on Mauna Kea
What does angular resolution measure?
The angular size of the smallest features that the telescope can see.
Why does the Sun's image look distorted in shape at sunset?
The distortion arises because of the way air affects the paths of light.
Which of the following statements about light focusing is NOT true?
The focal plane of a reflecting telescope is always located within a few inches of the primary mirror.
Suppose that two stars are separated in the sky by 0.01 arcsecond, and you observe them with a telescope that has an angular resolution of 1 arcsecond. What will you see?
The light from the two stars will be blended together so that they look like one star
Suppose you have two small photographs of the Moon. Although both look the same at small size, when you blow them up to poster size one of them still looks sharp, whereas the other one becomes fuzzy (grainy) looking. Which of the following statements is true?
The one that still looks sharp at large size has better (smaller) angular resolution than the one that looks fuzzy.
Which of the following is NOT a reason why telescopes tend to be built on mountaintops that are relatively far from cities and are in regions with dry climates?
The thin air on mountaintops makes the glass in telescope mirrors less susceptible to warping.
Which of the following is always true about images captured with X-ray telescopes?
They are always shown with colors that are NOT the true colors of the objects that were photographed.
Why can't X-ray and gamma-ray telescopes use the same designs as visible-light telescopes?
X rays and gamma rays have so much energy that they cannot be reflected in the same way as visible light.
The Chandra X-ray Observatory must operate in space because __________________________.
X rays do not penetrate Earth's atmosphere
What is the primary reason that we launch X-ray telescopes into space rather than building them on the ground?
X-rays from space do not reach the ground.
Suppose you have a camera attached to a telescope, and you want to record an image of a faint galaxy. Which of the following will help the most?
a lot of pixels and a long exposure time
Shown following are five different colors of visible light that travel to Earth from the Sun. Rank these colors of visible light from left to right based on the altitude in the atmosphere where they are completely absorbed, from highest to lowest (Earth's surface). If two (or more) of the choices reach the surface, rank them as equal by dragging one on top of the other(s).
all are equal As your correct answer indicates, all wavelengths of visible light reach Earth's surface, which is why we can see all colors and why visible-light telescopes work fine on the ground.
Shown following are the primary mirror arrangements and total light-collecting area of five different telescopes. Each mirror uses a different segmented arrangement, but assume that they are all equivalent in quality and in their ability to focus light. Also assume that the telescopes use identical detectors and have the same observing conditions. Rank the telescopes from left to right based on their ability to detect very dim objects, from greatest to least. To rank two (or more) telescopes as equal, drag one on top of the other(s).
all equal If all else is equal (such as mirror quality, detector, and observing conditions), the ability to detect dim objects depends only on light-collecting area. Because all the telescopes shown have the same light-collecting area, they all can detect dim objects equally well. The arrangement of the mirrors does not matter, as long as they are arranged and shaped so they bring light to a perfect focus.
If our eyes were sensitive only to X rays, the world would appear __________.
dark because X-ray light does not reach Earth's surface
Order the following in order of increasing efficiency of detecting photons of visible light.
eye, photographic film, CCD