Michael SeedsDana BackmanChapter 4Light and Telesc.docx

Michael Seeds
Dana Backman
Chapter 4
Light and Telescopes
*
He burned his house down
for the fire insurance and
spent the proceeds on a telescope.
ROBERT FROST
The Star-Splitter
*
Light is a treasure that links us to the sky. An astronomer’s
quest is to gather as much light as possible from the moon, sun,
planets, stars, and galaxies—in order to extract information
about their natures.

*
Telescopes, which gather and focus light for analysis, can help
us do that. Nearly all the interesting objects in the sky are very
faint sources of light.So, large telescopes are built to collect the
greatest amount of light possible.
*
This chapter’s discussion of astronomical research concentrates
on large telescopes and the special instruments and techniques
used to analyze light.
*
To gather light that is visible to your unaided eye, a normal
telescope would work fine.However, visible light is only one
type of radiation arriving here from distant objects.
*
Astronomers can extract information from other forms of
radiation by using other types of telescopes. Radio telescopes,
for example, give an entirely different view of the sky. Some of

these specialized telescopes can be used from Earth’s
surface.Others, though, must go high in Earth’s atmosphere or
even above it.
*
Radiation: Information from SpaceAstronomers no longer study
the sky by mapping constellations or charting the phases of the
moon.
*
Modern astronomers analyze light using sophisticated
instruments and techniques to investigate the compositions,
motions, internal processes, and evolution of celestial objects.
To understand this, you must learn about the nature of light.
Radiation: Information from Space
*
If you have noticed the colors in a soap bubble, then you have
seen one effect of light behaving as a wave.When that same
light enters the light meter on a camera, it behaves as a
particle.How light behaves depends on how you treat it.Light
has both wavelike and particlelike properties.
Light as a Wave and as a Particle

*
Sound is another type of wave that you have already
experienced.Sound waves are an air-pressure disturbance that
travels through the air from source to ear.
*
Sound requires a solid, liquid, or gas medium to carry it.So, for
example, in space outside a spacecraft, there can be no sound.
*
In contrast, light is composed of a combination of electric and
magnetic waves that can travel through empty space.Unlike
sound, light waves do not require a medium and thus can travel
through a vacuum.
*

As light is made up of both electric and magnetic fields, it is
referred to as electromagnetic radiation. Visible light is only
one form of electromagnetic radiation.
*
Electromagnetic radiation is a wave phenomenon.That is, it is
associated with a periodically repeating disturbance (a wave)
that carries energy.
*
Imagine waves in water.If you disturb a pool of water, waves
spread across the surface.Now, imagine placing a ruler parallel
to the travel direction of the wave. The distance between peaks
is the wavelength.
*
The changing electric and magnetic fields of electromagnetic
waves travel through space at about 300,000 kilometers per
second (186,000 miles per second). That is commonly referred
to as the speed of light.It is, however, the speed of all
electromagnetic radiation.

*
It may seem odd to use the word radiation when discussing
light.Radiation really refers to anything that spreads outward
from a source. Light radiates from a source, so you can
correctly refer to light as a form of radiation.
*
The electromagnetic spectrum is simply the types of
electromagnetic radiation arranged in order of increasing
wavelength. Rainbows are spectra of visible light.
*
The colors of visible light have different wavelengths.Red has
the longest wavelength.Violet has the shortest.
The Electromagnetic Spectrum
*

The average wavelength of visible light is about 0.0005 mm.50
light waves would fit end-to-end across the thickness of a sheet
of paper.
*
It is too awkward to measure such short distances in
millimeters.So, physicists and astronomers describe the
wavelength of light using either of two units:Nanometer (nm),
one billionth of a meter (10-9 m)Ångstrom (Å), named after the
Swedish astronomer Anders Ångström, equal to 10-10 m or 0.1
nm
*
The wavelength of visible light ranges between about 400 nm
and 700 nm, or, equivalently, 4,000 Å and 7,000 Å.Infrared
astronomers often refer to wavelengths using units of microns
(10-6 m).Radio astronomers use millimeters, centimeters, or
meters.
*

The figure shows how the visible spectrum makes up only a
small part of the electro-magnetic spectrum.
*
Beyond the red end of the visible range lies infrared (IR)
radiation—with wavelengths ranging from 700 nm to about 1
mm.
*
Your eyes are not sensitive to this radiation.Your skin, though,
senses it as heat. A heat lamp is nothing more than a bulb that
gives off large amounts of infrared radiation.
*
The figure is an artist’s conception of the English astronomer
William Herschel measuring infrared radiation—and, thus,
discovering that there is such a thing as invisible light.
*

Radio waves have even longer wavelengths than IR
radiation.The radio radiation used for AM radio transmissions
has wavelengths of a few hundred meters.FM, television, and
also military, governmental, and amateur radio transmissions
have wavelengths from a few tens of centimeters to a few tens
of meters.
*
Microwave transmissions, used for radar and long-distance
telephone communications, have wavelengths from about 1
millimeter to a few centimeters.
*
Electromagnetic waves with wavelengths shorter than violet
light are called ultraviolet (UV).
*
Shorter-wavelength electromagnetic waves than UV are called X

rays.The shortest are gamma rays.
*
The distinction between these wavelength ranges is mostly
arbitrary—they are simply convenient human-invented
labels.For example, the longest-wavelength infrared radiation
and the shortest-wavelength microwaves are the same.
Similarly, very short-wavelength ultraviolet light can be
considered to be X rays.
*
Nonetheless, it is all electromagnetic radiation, and you could
say we are “making light” of it all.All these types of radiation
are the same phenomenon as light.Some types your eyes can
see, some types your eyes can’t see.
*
Although light behaves as a wave, under certain conditions, it
also behaves as a particle. A particle of light is called a
photon.You can think of a photon as a minimum-sized bundle of
electromagnetic waves.

*
The amount of energy a photon carries depends on its
wavelength.Shorter-wavelength photons carry more
energy.Longer-wavelength photons carry less energy. A photon
of visible light carries a small amount of energy.An X-ray
photon carries much more energy.A radio photon carries much
less.
*
Astronomers are interested in electromagnetic radiation because
it carries almost all available clues to the nature of planets,
stars, and other celestial objects.
*
Earth’s atmosphere is opaque to most electromagnetic radiation.
Gamma rays, X rays, and some radio waves are absorbed high in
Earth’s atmosphere.

*
A layer of ozone (O3) at an altitude of about 30 km absorbs
almost all UV radiation. Water vapor in the lower atmosphere
absorbs long-wavelength IR radiation.
*
Only visible light, some short-wavelength infrared radiation,
and some radio waves reach Earth’s surface—through what are
called atmospheric windows.
*
To study the sky from Earth’s surface, you must look out
through one of these ‘windows’ in the electromagnetic
spectrum.
*
Astronomers build optical telescopes to gather light and focus it
into sharp images. This requires careful optical and mechanical

designs.It leads astronomers to build very large telescopes. To
understand that, you need to learn the terminology of
telescopes—starting with the different types of telescopes and
why some are better than others.
Telescopes
*
Astronomical telescopes focus light into an image in one of two
ways. A lens bends (refracts) the light as it passes through the
glass and brings it to a focus to form an image.A mirror—a
curved piece of glass with a reflective surface—forms an image
by bouncing light.
Two Kinds of Telescopes
*
Thus, there are two types of astronomical telescopes. Refracting
telescopes use
a lens to gather and focus
the light.Reflecting telescopes use
a mirror.
*

The main lens in a refracting telescope is called the primary
lens.The main mirror in
a reflecting telescope
is called the primary
mirror.
*
Both kinds of telescopes form a small, inverted image that is
difficult to observe directly.So, a lens called
the eyepiece is used
to magnify the image
and make it convenient
to view.
*
The focal length is the distance from a lens or mirror to the
image it forms of a distant light source such as

a star.
*
Creating the proper optical shape to produce a good focus is an
expensive process. The surfaces of lenses and mirrors must be
shaped and polished to have no irregularities larger than the
wavelength of light. Creating the optics for a large telescope
can take months or years; involve huge, precision machinery;
and employ several expert optical engineers and scientists.
*
Refracting telescopes have serious disadvantages.Most
importantly, they suffer from an optical distortion that limits
their usefulness.When light is refracted through glass, shorter
wavelengths bend more than longer wavelengths. As a result,
you see a color blur around every image. This color separation
is called chromatic aberration and it can be only partially
corrected.
*

Another disadvantage is that the glass in primary lenses must be
pure and flawless because the light passes all the
way through it. For that same reason, the weight of
the lens can be supported only
around its outer edge.
*
In contrast, light reflects from the front surface of a reflecting
telescope’s primary mirror but does not pass through it.So,
reflecting telescopes have
no chromatic aberration.
*
Also, mirrors are less expensive to make than similarly sized
lenses and the weight of telescope mirrors can be supported
easily.For these reasons, every large astronomical telescope
built since 1900 has been a reflecting telescope.
*

Astronomers also build radio telescopes to gather radio
radiation. Radio waves from celestial objects—like visible light
waves—penetrate Earth’s atmosphere and reach the ground.
*
You can see how the dish reflector of a typical radio telescope
focuses the radio waves so their intensity can be measured. As
radio wavelengths are so long, the disk reflector does not have
to be as perfectly smooth as the mirror of a reflecting optical
telescope.
*
Astronomers struggle to build large telescopes because a
telescope can help human eyes in three important ways.These
are called the three powers of a telescope.The two most
important of these three powers depend on the diameter of the
telescope.
The Powers of a Telescope
*

Most celestial objects of interest to astronomers are faint.So,
you need a telescope that can gather large amounts of light to
produce a bright image.
*
Light-gathering power refers to the ability of a telescope to
collect light. Catching light in a telescope is like catching rain
in a bucket—the bigger the bucket, the more rain it catches.
*
The light-gathering power is proportional to the area of the
primary mirror—that is, proportional to the square of the
primary’s diameter. A telescope with a diameter of 2 meters has
four times (4X) the light-gathering power of a 1-meter
telescope. That is why astronomers use large telescopes and
why telescopes are ranked by their diameters.
*
One reason radio astronomers build big radio dishes is to collect
enough radio photons—which have low energies—and

concentrate them for measurement.
*
Resolving power refers to the ability of the telescope to reveal
fine detail.
*
One consequence of the wavelike nature of light is that there is
an inevitable small blurring called a diffraction fringe around
every point of light in the image.You cannot see any detail
smaller than the fringe.
*
Astronomers can’t eliminate diffraction fringes.However, the
fringes are smaller in larger telescopes.That means they have
better resolving power and can reveal finer detail.For example,
a 2-meter telescope has diffraction fringes ½ as large, and thus
2X better resolving power, than a 1-meter telescope.

*
The size of the diffraction fringes also depends on
wavelength.At the long wavelengths of radio waves, the fringes
are large and the resolving power is poor. That’s another reason
radio telescopes need to be larger than optical telescopes.
*
One way to improve resolving power is to connect two or more
telescopes in an interferometer.This has a resolving power
equal to that of a telescope
as large as the maximum
separation between
the individual telescopes.
*
The first interferometers were built by radio astronomers
connecting radio dishes kilometers apart. Modern technology
has allowed astronomers to connect optical telescopes to form
interferometers with very high resolution.

*
Aside from diffraction fringes, two other factors limit resolving
power:Optical qualityAtmospheric conditions
*
A telescope must contain high-quality optics to achieve its full
potential resolving power. Even a large telescope shows little
detail if its optical surfaces have imperfections.
*
Also, when you look through a telescope, you look through
miles of turbulence in Earth’s atmosphere, which makes images
dance and blur—a condition astronomers call seeing.
*

A related phenomenon is the twinkling of a star. The twinkles
are caused by turbulence in Earth’s atmosphere.A star near the
horizon—where you look through more air—will twinkle more
than a star overhead. On a night when the atmosphere is
unsteady, the stars twinkle, the images are blurred, and the
seeing is bad.
*
Even with good seeing, the detail visible through a large
telescope is limited.This is not just by its diffraction fringes but
by the steadiness of the air through which the observer must
look.
*
A telescope performs best on a high mountaintop—where the air
is thin and steady.
*
However, even at good sites, atmospheric turbulence spreads
star images into blobs 0.5 to 1 arc seconds in diameter.That
situation can be improved by a difficult and expensive

technique called adaptive optics.By this technique, rapid
computer calculations adjust the telescope optics and partly
compensate for seeing distortions.
*
This limitation on the amount of information in an image is
related to the limitation on the accuracy of a measurement. All
measurements have some built-in uncertainty, and scientists
must learn to work within those limitations. a focal length of 14
mm has a magnifying power of 503.
*
Higher magnifying power does not necessarily show you more
detail. The amount of detail you can see in practice is limited by
a combination of the seeing conditions and the telescope’s
resolving power and optical quality.
*
A telescope’s primary function is to gather light and thus make
faint things appear brighter, so the light-gathering power is the
most important power and the diameter of the telescope is its

most important characteristic. Light-gathering power and
resolving power are fundamental properties of a telescope that
cannot be altered, whereas magnifying power can be changed
simply by changing the eyepiece.
*
Most major observatories are located far from big cities and
usually on high mountains.
Observatories on Earth—Optical and Radio
*
Optical astronomers avoid cities because light pollution—the
brightening of the night sky by light scattered from artificial
outdoor lighting—can make it impossible to see faint objects. In
fact, many residents of cities are unfamiliar with the beauty of
the night sky because they can see only the brightest stars.
*
Radio astronomers face a problem of radio interference
analogous to light pollution. Weak radio signals from the
cosmos are easily drowned out by human radio interference—
everything from automobiles with faulty ignition systems to

poorly designed transmitters in communication.
*
To avoid that, radio astronomers locate their telescopes as far
from civilization as possible. Hidden deep in mountain valleys,
they are able to listen to the sky protected from human-made
radio noise.
*
As you learned previously, astronomers prefer to place optical
telescopes on mountains because the air there is thin and more
transparent.Most important, though, they carefully select
mountains where the airflow is usually not turbulent—so the
seeing is good.
*
Building an observatory on top of a high mountain far from
civilization is difficult and expensive.However, the dark sky
and good seeing make it worth the effort.

*
There are two important points to notice about modern
astronomical telescopes.
*
One, research telescopes must focus their light to positions at
which cameras and other instruments can be placed.
*
Two, small telescopes can use other focal arrangements that
would be inconvenient in larger telescopes.
*
Telescopes located on the surface of Earth, whether optical or
radio, must move continuously to stay pointed at a celestial
object as Earth turns on its axis.This is called sidereal tracking
(‘sidereal’ refers to the stars).

*
The days when astronomers worked beside their telescopes
through long, dark, cold nights are nearly gone. The complexity
and sophistication of telescopes require a battery of computers,
and almost all research telescopes are run from warm control
rooms.
*
High-speed computers have allowed astronomers to build new,
giant telescopes with unique designs. The European Southern
Observatory has built the Very Large Telescope (VLT) high in
the remote Andes Mountains of northern Chile.
*
The VLT actually consists of four telescopes, each with a
computer-controlled mirror 8.2 m in diameter and only 17.5 cm
(6.9 in.) thick. The four telescopes can work singly or can
combine their light to work as one large telescope.

*
Italian and American astronomers have built the Large
Binocular Telescope, which carries a pair of 8.4-m mirrors on a
single mounting.
*
The Gran Telescopio Canarias, located atop a volcanic peak in
the Canary Islands, carries a segmented mirror 10.4 meters in
diameter.It holds, for the moment, the record as the largest
single telescope in the world. Other giant telescopes are being
planned with innovative designs.
*
The largest fully steerable radio telescope in the world is at the
National Radio Astronomy Observatory in West Virginia. The
telescope has a reflecting surface 100 meters in diameter made
of 2,004 computer-controlled panels that adjust to maintain the
shape of the reflecting surface.
*

The largest radio dish in the world is 300 m (1,000 ft) in
diameter, and is built into a mountain valley in Arecibo, Puerto
Rico. The antenna hangs on cables above the dish, and, by
moving the antenna, astronomers can point the telescope at any
object that passes within 20 degrees of the zenith as Earth
rotates.
*
The Very Large Array (VLA) consists of 27 dishes spread in a
Y-pattern across the New Mexico desert. Operated as an
interferometer, the VLA has the resolving power of a radio
telescope up to 36 km (22 mi) in diameter.
*
Such arrays are very powerful, and radio astronomers are now
planning the Square Kilometer ArrayIt will consist of radio
dishes spanning 6,000 km (almost 4,000 mi) and having a total
collecting area of one square kilometer.
*

Just looking through a telescope doesn’t tell you much.To learn
about planets, stars, and galaxies, you must be able to analyze
the light the telescope gathers. Special instruments attached to
the telescope make that possible.
Astronomical Instruments and Techniques
*
The photographic plate was the first image-recording device
used with telescopes. Brightness of objects imaged on a
photographic plate can be measured with a lot of hard work—
yielding only moderate precision.
Imaging Systems and Photometers
*
Astronomers also build photometers.These are sensitive light
meters that can be used to measure the brightness of individual
objects very precisely.
*
Most modern astronomers use charge-coupled devices (CCDs)
as both image-recording devices and photometers. A CCD is a

specialized computer chip containing as many as a million or
more microscopic light detectors arranged in an array about the
size of a postage stamp. These array detectors can be used like a
small photographic plate.
*
CCDs have dramatic advantages over both photometers and
photographic plates. They can detect both bright and faint
objects in a single exposure and are much more sensitive than a
photographic plate. CCD images are digitized—converted to
numerical data—and can be read directly into a computer
memory for later analysis.
*
Although CCDs for astronomy are extremely sensitive and thus
expensive, less sophisticated CCDs are now used in commercial
video and digital cameras. Infrared astronomers use array
detectors similar in operation to optical CCDs. At other
wavelengths, photometers are still used for measuring
brightness of celestial objects.
*

The digital data representing an image from a CCD or other
array detector are easy to manipulate—to bring out details that
would not otherwise be visible.
*
For example, astronomical images are often reproduced as
negatives—with the sky white and the stars dark. This makes
the faint parts of the image easier to see.
*
Astronomers also manipulate images to produce false-color
images.The colors represent different levels of intensity and are
not related to the true colors of the object.
*
For example, humans can’t see radio waves.So, astronomers
must convert them into something perceptible.

*
One way is to measure the strength of the radio signal at various
places in the sky and draw a map in which contours mark areas
of uniform radio intensity.
*
Compare such a map to a seating diagram for a baseball stadium
in which the contours mark areas in which the seats have the
same price.
*
Contour maps are very common in radio astronomy and are
often reproduced using false colors.
*
To analyze light in detail, you need to spread the light out
according to wavelength into a spectrum—a task performed by a
spectrograph. You can understand how this works by

reproducing an experiment performed by Isaac Newton in 1666.
Spectrographs
*
Boring a hole in his window shutter, Newton admitted a thin
beam of sunlight into his darkened bedroom.When he placed a
prism in the beam, the sunlight spread into a beautiful spectrum
on the far wall. From this, Newton concluded that white light
was made of a mixture of all the colors.
Spectrographs
*
Newton didn’t think in terms of wavelength.You, however, can
use that modern concept to see that the light passing through the
prism is bent at an angle that depends on
the wavelength.Violet (short-
wavelength) light
bends most,
and red (long
wavelength) light
least.
Spectrographs

*
Thus, the white light entering the prism is spread into what is
called a spectrum.
Spectrographs
*
A typical prism spectrograph contains more than one prism to
spread the spectrum wider.Also, it has lenses to guide the light
into the prism and to focus the light onto a photographic plate.
Spectrographs
*
Most modern spectrographs use a grating in place of a prism. A
grating is a piece of glass with thousands of microscopic
parallel lines scribed onto its surface. Different wavelengths of
light reflect from the grating at slightly different angles.So,
white light is spread into a spectrum and can be recorded—often
by a CCD camera.
Spectrographs
*

Recording the spectrum of a faint star or galaxy can require a
long time exposure.So, astronomers have developed multiobject
spectrographs that can record the spectra of as many as 100
objects simultaneously. Multiobject spectrographs automated by
computers have made large surveys of many thousands of stars
or galaxies possible.
Spectrographs
*
As astronomers understand how light interacts with matter, a
spectrum carries a tremendous amount of information.That
makes a spectrograph the astronomer’s most powerful
instrument.Astronomers are likely to remark, “We don’t know
anything about an object until we get a spectrum.”That is only a
slight exaggeration.
Spectrographs
*
You have learned about the observations that groundbased
telescopes can make through the two atmospheric windows in
the visible and radio parts of the electromagnetic spectrum.
Airborne and Space Observatories
*

Most of the rest of the spectrum—infrared, ultraviolet, X-ray,
and gamma-ray radiation—never reaches Earth’s surface. To
observe at these wavelengths, telescopes must fly above the
atmosphere in high-flying aircraft, rockets, balloons, and
satellites.
*
The only exceptions are observations that can be made in what
are called the near-infrared and the near-ultraviolet—almost the
same wavelengths as visible light.
*
Astronomers can observe from the ground in the near-infrared
just beyond the red end of the visible spectrum.This is because
some of this radiation leaks through the atmosphere in narrow,
partially open atmospheric windows ranging in wavelength from
1,200 nm to about 30,000 nm.
The Ends of the Visual Spectrum
*

Infrared astronomers usually describe wavelengths in
micrometers or microns (10-6 m).So, they refer to this
wavelength range as 1.2 to 30 microns.
*
In this range, most of the radiation is absorbed by water vapor,
carbon dioxide, or ozone molecules in Earth’s atmosphere.Thus,
it is an advantage to place telescopes on the highest mountains
where the air is especially thin and dry.
*
For example, a number of important infrared telescopes are
located on the summit of Mauna Kea in Hawaii—at an altitude
of 4,200 m (13,800 ft).
*
The far-infrared range, which includes wavelengths longer than
30 micrometers, can inform you about planets, comets, forming
stars, and other cool objects.

*
However, these wavelengths are absorbed high in the
atmosphere. To observe in the far-infrared, telescopes must
venture to high altitudes.
*
Remotely operated infrared telescopes suspended under balloons
have reached altitudes as high as 41 km (25 mi).
*
For many years, the NASA Kuiper Airborne Observatory (KAO)
carried a 91-cm infrared telescope and crews of astronomers to
altitudes of over 12 km (40,000 ft).This was in order to get
above 99 percent or more of the water vapor in Earth’s
atmosphere.
*

Now retired from service, the KAO will soon be replaced by the
Stratospheric Observatory for Infrared Astronomy (SOFIA).This
is a Boeing 747-P aircraft that will carry a 2.5-m (100-in.)
telescope to the fringes of the atmosphere.
*
If a telescope observes at far-infrared wavelengths, then it must
be cooled. Infrared radiation is emitted by heated objects.If the
telescope is warm, it will emit many times more infrared
radiation than that coming from a distant object. Imagine trying
to look at a dim, moonlit scene through binoculars that are
glowing brightly.
*
In a telescope observing near-infrared radiation, only the
detector—the element on which the infrared radiation is
focused—must be cooled.To observe in the far-infrared,
however, the entire telescope must be cooled.
*
At the short-wavelength end of the spectrum, astronomers can

observe in the near-ultraviolet. Human eyes do not detect this
radiation, but it can be recorded by photographic plates and
CCDs.
*
Wavelengths shorter than about 290 nm—the far-ultraviolet, X-
ray, and gamma-ray ranges—are completely absorbed by the
ozone layer extending from 20 km to about 40 km above Earth’s
surface.
*
No mountain is that high, and no balloon or airplane can fly that
high.So, astronomers cannot observe far-UV, X-ray, and
gamma-ray radiation—without going into space.
*
Telescopes that observe in the far-infrared must be protected
from heat and must get above Earth’s absorbing atmosphere.
They have limited lifetimes because they must carry coolant to
chill their optics.
Telescopes in Space

*
The most sophisticated of the infrared telescopes put in orbit,
the Spitzer Space Telescope was cooled to –269°C (–452°F).
Telescopes in Space
*
Launched in 2003, it observes from behind a sunscreen. In fact,
it could not observe from Earth’s orbit because Earth is such a
strong source of infrared radiation, so the telescope was sent
into an orbit around the sun that carried it slowly away from
Earth.
Telescopes in Space
*
Named after theoretical physicist Lyman Spitzer Jr., it has made
important discoveries concerning star formation, planets
orbiting other stars, distant galaxies, and more. Its coolant ran
out in 2009, but some of the instruments that can operate
without being chilled continue to collect data.
Telescopes in Space
*

High-energy astrophysics refers to the use of X-ray and gamma-
ray observations of the sky. Making such observations is
difficult but can reveal the secrets of processes such as the
collapse of massive stars and eruptions of supermassive black
holes.
Telescopes in Space
*
The largest X-ray telescope to date, the Chandra X-ray
Observatory, was launched in 1999 and orbits a third of the way
to the moon. Chandra is named for the late Indian-American
Nobel Laureate Subrahmanyan Chandrasekhar, who was a
pioneer in many branches of theoretical astronomy.
Telescopes in Space
*
Focusing X rays is difficult because they penetrate into most
mirrors, so astronomers devised cylindrical mirrors in which the
X rays reflect from the polished inside of the cylinders and form
images on special detectors.
Telescopes in Space
*

The telescope has made important discoveries about everything
from star formation to monster black holes in distant galaxies.
Telescopes in Space
*
One of the first gamma-ray observatories was the Compton
Gamma Ray Observatory, launched in 1991. It mapped the
entire sky at gamma-ray wavelengths.
Telescopes in Space
*
The European INTEGRAL satellite was launched in 2002 and
has been very productive in the study of violent eruptions of
stars and black holes.
Telescopes in Space
*
The GLAST (Gamma-Ray Large Area Space Telescope),
launched in 2008, is capable of mapping large areas of the sky
to high sensitivity.
Telescopes in Space

*
Modern astronomy has come to depend on observations that
cover the entire electromagnetic spectrum. More orbiting space
telescopes are planned that will be more versatile and more
sensitive.
Telescopes in Space
*
Professor’s Questions Set 4
Provide comprehensive answers to the following questions.
Remember to support your arguments where necessary by
websites and pictures.
Chapter 6 Readings
1. Why would you include the moon in a comparison of the
Terrestrial planets?
2. In what ways is Earth unique among the Terrestrial planets?

3. What has produced the oxygen in Earth’s atmosphere?
4. Why would a decrease in the density of the ozone layer cause
public health problems?
5. What characteristics must earth’s core have in order to
generate a magnetic field?
6. What evidence indicates that there has been liquid water on
Mars?

7. Why do astronomers conclude that the crust on Mars must be
thicker than Earth’s crust?
8. What evidence suggests that Venus has been resurfaced
within the last half billion years?

9. Discuss the hypotheses concerning the origin of the Earth’s
moon?
10. Why are atmospheres of Venus and Mars mostly carbon
dioxide? Why is the atmosphere of Venus very dense but
atmosphere of Mars very thin?

Michael SeedsDana BackmanChapter 4Light and Telesc.docx

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Michael SeedsDana BackmanChapter 4Light and Telesc.docx