Chapter 6 home work 11 Telescopes Flashcards Preview

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Flashcards in Chapter 6 home work 11 Telescopes Deck (65):
1

Which of the following best describes what we mean by the focal plane of a telescope?

It is the lower surface of the telescope's primary lens or mirror.
It is the surface of the lens on the eyepiece, through which you would look to see objects in the telescope's field of view.
It is the upper surface of the telescope's primary lens or mirror.
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.

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.

2

When your eye forms an image, the _____ plays a role analogous to the detector in a camera.

pupil
retina
optic nerve
lens

retina

3

What does angular resolution measure?

The size of an image.
The angular size of the smallest features that the telescope can see.
The number of electromagnetic waves captured by an image.
The brightness of an image.

The angular size of the smallest features that the telescope can see.

4

What is the angular resolution of the human eye?

about 1 degree
about 1 arcsecond (1/3600 of a degree)
about 1 arcminute, or 1/60 of a degree
about 1 milliarcsecond

about 1 arcminute, or 1/60 of a degree

5

Which of the following statements best describes the two principle advantages of telescopes over eyes?

Telescopes can collect far more light with far greater magnification.
Telescopes collect more light and are unaffected by twinkling.
Telescopes have much more magnification and better angular resolution.
Telescopes can collect far more light with far better angular resolution

Telescopes can collect far more light with far better angular resolution.

6

Which of the following statements best describes the difference between a refracting telescope and a reflecting telescope?

A refracting telescope produces refracted images while a reflecting telescope produces reflected images.
It is much easier to make a large refracting telescope than a large reflecting telescope.
Reflecting telescopes make much clearer images than can refracting telescopes of the same size.
A refracting telescope uses a transparent glass lens to focus light while a reflecting telescope uses a mirror to focus light.

A refracting telescope uses a transparent glass lens to focus light while a reflecting telescope uses a mirror to focus light.

7

What do we mean by the diffraction limit of a telescope?

It describes the maximum exposure time for images captured with the telescope.
It is the angular resolution the telescope could achieve if nothing besides the size of its light-collecting area affected the quality of its images.
It is the maximum size to which any telescope can be built.
It describes the farthest distance to which the telescope can see.

It is the angular resolution the telescope could achieve if nothing besides the size of its light-collecting area affected the quality of its images.

8

Which of the following is not one of the three main categories of observation generally used by astronomers?

Spectroscopy to spread an object's light into a spectrum.
Filtering to look at just a single color from an object.
Imaging to get a picture of an astronomical objects.
Time monitoring to track how an object's brightness varies with time.

Filtering to look at just a single color from an object.

9

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 angular resolution.
High spectral resolution.
High turbulence.
A radio telescope.

High spectral resolution.

10

Which of the following is always true about images captured with X-ray telescopes?

They are always very pretty.
They show us light with extremely long wavelengths compared to the wavelengths of visible light.
They always have very high angular resolution.
They always are made with adaptive optics.
They are always shown with colors that are not the true colors of the objects that were photographed.

They are always shown with colors that are not the true colors of the objects that were photographed.

11

What do astronomers mean by light pollution?

Light pollution means contamination of light caused by chemicals in the Earth's atmosphere.
Light pollution is a type of air pollution created by lightweight gases such as hydrogen and helium.
Light pollution is a term used to describe the appearance of the sky in regions that are crowded with stars.
Light pollution is light from human sources that makes it difficult to see the stars at night.

Light pollution is light from human sources that makes it difficult to see the stars at night.

12

Which of the following effects is caused by atmospheric turbulence?

light pollution
magnification of images
twinkling of stars
diffraction of light

twinkling of stars

13

What is the purpose of adaptive optics?

It is a special technology that allows the Hubble Space Telescope to adapt to study many different types of astronomical objects.
It allows ground-based telescopes to observe ultraviolet light that normally does not penetrate the atmosphere.
It allows several small telescopes to work together like a single larger telescope.
It reduces blurring caused by atmospheric turbulence for telescopes on the ground.

It reduces blurring caused by atmospheric turbulence for telescopes on the ground.

14

Which of the following wavelength regions can be studied with telescopes on the ground?

all light with wavelengths shorter than infrared wavelengths
infrared, visible, and ultraviolet light.
all light with wavelengths longer than ultraviolet wavelengths
radio, visible, and very limited portions of the infrared and ultraviolet regions

radio, visible, and very limited portions of the infrared and ultraviolet regions

15

What is the purpose of interferometry?

It allows two or more small telescopes to achieve the angular resolution of a much larger telescope.
It is designed to prevent light pollution from interfering with astronomical observations.
It allows two or more small telescopes to achieve a larger light-collecting area than they would have independently.
It reduces the twinkling of stars caused by atmospheric turbulence.

It allows two or more small telescopes to achieve the angular resolution of a much larger telescope.

16

Which of the following forms of light can be observed with telescopes at sea level?
Select all that apply.

X rays
ultraviolet light
visible light
infrared light
radio waves

visible light
radio waves

17

If our eyes were sensitive only to X rays, the world would appear __________.

gray, black, and white like a medical X ray
brighter than normal because X rays carry more energy than visible light photons
dark because X-ray light does not reach Earth’s surface
green, yellow, and orange, because those are the colors of X rays

dark because X-ray light does not reach Earth’s surface

18

If you had only one telescope and wanted to take both visible-light and ultraviolet pictures of stars, where should you locate your telescope?

on Earth’s surface
on a tall mountain
in an airplane
in space

in space

19

Shown following are six different types of light that travel to Earth from the Sun. 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).

x-rays
most ultra violet light
most infrared light
green visble light
most radio waves

x-rays
most ultra violet light
most infrared light

Same: green visble light
most radio waves

20

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 lare is the Andromeda Galaxy?
Are stars in the Orion Nebula surrounded by dusty disks of gas?
What are the major surface features of Mars?
What is the temperature of Jupiter's atmosphere?
Is the star Vega moving toward us or away from us?
What is the chemical composition of the Crab Nebula?
Does the star Mira vary in brightness?
Is the X-ray emission from the galactic center steady or changing?

How lare is the Andromeda Galaxy?
Are stars in the Orion Nebula surrounded by dusty disks of gas?
What are the major surface features of Mars?

21

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. Spectroscopy

How lare is the Andromeda Galaxy?
Are stars in the Orion Nebula surrounded by dusty disks of gas?
What are the major surface features of Mars?
What is the temperature of Jupiter's atmosphere?
Is the star Vega moving toward us or away from us?
What is the chemical composition of the Crab Nebula?
Does the star Mira vary in brightness?
Is the X-ray emission from the galactic center steady or changing?

What is the temperature of Jupiter's atmosphere?
Is the star Vega moving toward us or away from us?
What is the chemical composition of the Crab Nebula?

22

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. Timing

How lare is the Andromeda Galaxy?
Are stars in the Orion Nebula surrounded by dusty disks of gas?
What are the major surface features of Mars?
What is the temperature of Jupiter's atmosphere?
Is the star Vega moving toward us or away from us?
What is the chemical composition of the Crab Nebula?
Does the star Mira vary in brightness?
Is the X-ray emission from the galactic center steady or changing?

Does the star Mira vary in brightness?
Is the X-ray emission from the galactic center steady or changing?

23

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
Measure the brightness of a star that is similar to our sun
Obtain a spectrum of the sunlight reflected by Mars.
Observe the hot (1-million K) gas in the Sun's corona.
Look for high-energy radiation from a supernova.

Determine the surface temperature of Venus
Study a dense cloud of cold gas in space

24

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. Visible light telescope

Determine the surface temperature of Venus
Study a dense cloud of cold gas in space
Measure the brightness of a star that is similar to our sun
Obtain a spectrum of the sunlight reflected by Mars.
Observe the hot (1-million K) gas in the Sun's corona.
Look for high-energy radiation from a supernova.

Measure the brightness of a star that is similar to our sun
Obtain a spectrum of the sunlight reflected by Mars.

25

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. X-ray telescope

Determine the surface temperature of Venus
Study a dense cloud of cold gas in space
Measure the brightness of a star that is similar to our sun
Obtain a spectrum of the sunlight reflected by Mars.
Observe the hot (1-million K) gas in the Sun's corona.
Look for high-energy radiation from a supernova.

Observe the hot (1-million K) gas in the Sun's corona.
Look for high-energy radiation from a supernova.

26

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.
The distortion is an artifact of the photographic process that your eyes would not actually see.
Air scatters different colors of light by different amounts.
The distortion is an illusion that you notice by eye but does not show up in photographs.

The distortion arises because of the way air affects the paths of light.

27

Suppose you have a camera attached to a telescope, and you want to record an image of a very faint galaxy. Which of the following will help the most?

a lot of pixels and a long exposure time
a small number of pixels and a long exposure time
a small number of pixels and a short exposure time
a lot of pixels and a short exposure time

a lot of pixels and a long exposure time

28

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 while the other one becomes fuzzy (grainy) looking. Which of the following statements is true?

Both photographs have the same angular resolution, because they are both photographs of the same object.
Both photographs have the same angular resolution, because they were both printed at the same sizes in each case.
The one that looks fuzzy at large size has better angular resolution (smaller) than the one that looks sharp.
The one that still looks sharp at large size has better (smaller) angular resolution than the one that looks fuzzy.

The one that still looks sharp at large size has better (smaller) angular resolution than the one that looks fuzzy.

29

The angular separation of two stars is 0.1 arcseconds and you photograph them with a telescope that has an angular resolution of 1 arcsecond. What will you see?

You will see two distinct stars in your photograph.
The stars will not show up at all in your photograph.
The two stars will appear to be touching, looking rather like a small dumbbell.
The photo will seem to show only one star rather than two.

The photo will seem to show only one star rather than two.

30

How does the light-collecting area of an 8-meter telescope compare to that of a 2-meter telescope?

The 8-meter telescope has 16 times the light-collecting area of the 2-meter telescope.
The 8-meter telescope has 8 times the light-collecting area of the 2-meter telescope.
The answer cannot be determined from the information given in the question.
The 8-meter telescope has 4 times the light-collecting area of the 2-meter telescope.

The 8-meter telescope has 16 times the light-collecting area of the 2-meter telescope.

31

Which of the following best describes the development of astronomical telescopes over the past 65 years?

The world's most powerful telescope remained the same for most of this period, but in the past 20 years many new and more powerful telescopes have been built.
Over the 65-year period, telescopes have gradually gotten bigger and more powerful.
Although there have been advances in cameras and computing power, telescopes themselves have not changed much in the last 65 years.
The only major change in telescope power has occurred because of our ability to launch telescopes into space rather than operating them only from the ground.

The world's most powerful telescope remained the same for most of this period, but in the past 20 years many new and more powerful telescopes have been built.

32

Which of the following best describes why radio telescopes are generally much larger in size than telescopes designed to collect visible light?

It is because radio telescopes are used in the daytime and visible light telescopes are used at night.
Radio telescopes are designed to collect sound rather than light.
Getting an image of the same angular resolution requires a much larger telescope for radio waves than for visible light.
Objects that emit radio waves are always much larger than objects that emit visible light, and therefore require larger telescopes.

Getting an image of the same angular resolution requires a much larger telescope for radio waves than for visible light.

33

Which of the following studies is best suited to astronomical observations that fall into the category called time monitoring?

Measuring the rotation rate of a distant star.
Studying how different planets differ in their surface compositions.
Determining the age of the solar system.
Studying how a star's brightness varies over a period of 3 years.

Studying how a star's brightness varies over a period of 3 years.

34

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?

Dry regions mean less rain and clouds, and mountaintops in dry regions may even allow some infrared observations.
Mountaintops far from cities are generally subject to less light pollution than locations nearer to cities.
Being on a high mountain top means being relatively high in the atmosphere, which tends to limit turbulence.
The thin air on mountaintops makes the glass in telescope mirrors less susceptible to warping.

The thin air on mountaintops makes the glass in telescope mirrors less susceptible to warping.

35

The stars in our sky twinkle in brightness and color because of ______.

light pollution
turbulence in the Earth's atmosphere
rapid changes in the brightnesses and colors of stars caused by changes in their spectra
the bubbling and boiling of gases on the surfaces of stars

turbulence in the Earth's atmosphere

36

Which of the following is not an advantage of the Hubble Space Telescope over ground-based telescopes?

Stars do not twinkle when observed from space.
It is closer to the stars.
It never has to close because of cloudy skies.
It can observe infrared and ultraviolet light, as well as visible light.

It is closer to the stars.

37

The Chandra X-ray Observatory must operate in space because:

X rays are too dangerous to be allowed on the ground.
It was built by NASA.
X rays do not penetrate Earth's atmosphere.
X-ray telescopes require the use of grazing incidence mirrors.

X rays do not penetrate Earth's atmosphere.

38

Which of the following telescopes benefits most from adaptive optics?

The Arecibo radio telescope in Puerto Rico
The Chandra X-ray Observatory
The Keck I telescope on Mauna Kea
The Hubble Space telescope

The Keck I telescope on Mauna Kea

39

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.
Observatory Y can detect dimmer stars and Observatory X reveals more detail in images.
Observatory Y both detects dimmer stars and reveals more detail in images.
Observatory X both detects dimmer stars and reveals more detail in images.
Both observatories have the same capabilities, but Observatory Y would be cheaper to build.

Observatory X can detect dimmer stars and Observatory Y reveals more detail in images.

40

Why can't X-ray and gamma-ray telescopes use the same designs as visible-light telescopes?

X rays and gamma rays do not penetrate Earth's atmosphere.
X rays and gamma rays have so much energy that they cannot be reflected in the same way as visible light.
X rays and gamma rays are so intense that they would melt the glass in a visible-light telescope.
Visible-light telescopes can only reflect light that we see, not any other kind of light.

X rays and gamma rays have so much energy that they cannot be reflected in the same way as visible light.

41

How much greater is the light-collecting area of one of the 10-meter Keck telescopes than that of the 5-meter Hale telescope?

2
4
8
16

4

42

Suppose astronomers built a 60-meter telescope. How much greater would its light-collecting area be than that of the 10-meter Keck telescope?

36

43

What is the angular separation of the two stars? Give your answer in degrees.

θ = 2.4×10^−6 ∘

44

What is the angular separation of the two stars? Give your answer in arcseconds.

θ = 8.7×10^−3 ′′

45

Can the Hubble Space Telescope resolve the two stars?

yes
no

no

46

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.

Large binocular telescope with 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 2 one 6.5 m mirorr

Large binocular telescope with 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 2 one 6.5 m mirorr

47

Listed below are the names, spectral types (in parentheses), and approximate masses of several nearby main-sequence stars. Rank the stars based on the distances to their habitable zones (from the central star), from shortest to longest.

Barnard's Star (m4) 0.2 Msun
61 Cygni A (K5) 0.2 Msun
Alpha Centauri A (G2) 1 Msun
Siruis (A1) 2 Msun
Spica (B1) 11Msun

Barnard's Star (m4) 0.2 Msun
61 Cygni A (K5) 0.2 Msun
Alpha Centauri A (G2) 1 Msun
Siruis (A1) 2 Msun
Spica (B1) 11Msun

48

Consider again the same set of five stars. This time, rank the stars based on the size (width) of their habitable zones, from smallest to largest.
Barnard's Star (m4) 0.2 Msun
61 Cygni A (K5) 0.2 Msun
Alpha Centauri A (G2) 1 Msun
Siruis (A1) 2 Msun
Spica (B1) 11Msun

Barnard's Star (m4) 0.2 Msun
61 Cygni A (K5) 0.2 Msun
Alpha Centauri A (G2) 1 Msun
Siruis (A1) 2 Msun
Spica (B1) 11Msun

49

Imagine that each of the five stars is orbited by a terrestrial planet at a distance of 1 AU (Earth’s distance from the Sun). Rank the stars based on the planet’s expected surface temperature (not including any greenhouse effect), from lowest to highest.

Barnard's Star (m4) 0.2 Msun
61 Cygni A (K5) 0.2 Msun
Alpha Centauri A (G2) 1 Msun
Siruis (A1) 2 Msun
Spica (B1) 11Msun

Barnard's Star (m4) 0.2 Msun
61 Cygni A (K5) 0.2 Msun
Alpha Centauri A (G2) 1 Msun
Siruis (A1) 2 Msun
Spica (B1) 11Msun

50

The items below describe worlds or selected localities on worlds. Based on our current scientific understanding, match these items to the appropriate category below. - Likely to be habitable

Moon with atmosphere orbiting jovian planet 1 au from 1 msun Star
Underground on Mars
Subsurface ocean on Europa
Surface of mars
Surface of terrestrial plante 10 au from 0.5 Msun star
volcanoes on Io


Moon with atmosphere orbiting jovian planet 1 au from 1 msun Star
Underground on Mars
Subsurface ocean on Europa

51

The items below describe worlds or selected localities on worlds. Based on our current scientific understanding, match these items to the appropriate category below. - unlikely to be habitable

Moon with atmosphere orbiting jovian planet 1 au from 1 msun Star
Underground on Mars
Subsurface ocean on Europa
Surface of mars
Surface of terrestrial plante 10 au from 0.5 Msun star
volcanoes on Io

Surface of mars
Surface of terrestrial plante 10 au from 0.5 Msun star
volcanoes on Io

52

Which statement explains the observations that make it seem possible that Mars could have life underground?

We have detected water ice on Mars, and Mars still has some volcanic heat.
We have detected subsurface wells of liquid water in equatorial regions of Mars.
We have found surface liquid water on Mars, so it should also have water underground.
Mars is located within the Sun's habitable zone.

We have detected water ice on Mars, and Mars still has some volcanic heat.

53

In Part A you found that the terrestrial planet 10 AU from a 0.5MSun star is unlikely to be habitable. Could this planet be habitable if it were in a different orbit around its star?

Yes, if it were 1 AU from its star.
Yes, but it would have to be less than 0.5 AU from its star.
Yes, if it had an eccentric orbit that sometimes brought it within 0.01 AU of its star.
No, because its star is too small to have a habitable planet.

Yes, but it would have to be less than 0.5 AU from its star.

54

As the mass of the central star increases, the distance to the habitable zone __________ and the size (width) of the habitable zone __________.
Select from the choices in the format first blank / second blank.

decreases / increases
decreases / decreases
increases / decreases
increases / increases

increases / increases

55

Suppose that our Sun was cool enough to include Mercury in its habitable zone. Which of the following would be true in that case?

Only Mercury would be in the Sun's habitable zone.
Mercury and Venus would be in the Sun’s habitable zone, but Earth and Mars would not.
Mercury, Venus, and Earth would be in the Sun’s habitable zone, but Mars would not.
All the terrestrial planets would be in the Sun's habitable zone.

Only Mercury would be in the Sun's habitable zone.

56

Scientists think it is very unlikely that complex and large forms of life could evolve on planets that orbit stars that are much more massive than the Sun. Why?

The habitable zone of a massive star is too far from the star to allow for the evolution of complex life
The expected lifetime of a massive star is too short to allow for the evolution of complex life
The habitable zone of a massive star covers too wide a range of distances from the star to allow for the evolution of complex life

The expected lifetime of a massive star is too short to allow for the evolution of complex life

57

Which of the following best describes what we mean by a habitable world?

a planet or moon that could support life, if any life happened to be on it
a planet or moon on which humans could survive if we happened to go there
a planet or moon with life
a planet or moon that lies within its star's habitable zone

a planet or moon that could support life, if any life happened to be on it

58

Which of the following places is not generally considered a potential home for life in our solar system?

Titan
Europa
Jupiter's atmosphere
Mars

Jupiter's atmosphere

59

The Sun's habitable zone _________.

consists only of Earth, since Earth is the only planet known to be inhabited
extends from the orbit of Earth to the orbit of Jupiter
extends from some place a little beyond the orbit of Venus to some place near the orbit of Mars
extends from just beyond the orbit of Mercury to just beyond Earth's orbit

extends from some place a little beyond the orbit of Venus to some place near the orbit of Mars

60

Why don't we expect to find life on planets orbiting high-mass stars?

The lifetime of a high-mass star is too short.
The high-mass stars emit too much ultraviolet radiation.
The stars are too hot to allow for life.
Planets cannot have stable orbits around high-mass stars.

The lifetime of a high-mass star is too short.

61

The "rare Earth hypothesis" holds that Earth-like planets will prove to be quite rare. Which of the following statements best sums up the current status of the debate over this hypothesis?

The debate raged for a while, but is now settled. We are now quite certain that Earth-like planets are common.
The debate raged for a while, but is now settled. We are now quite certain that Earth-like planets are rare.
It is no longer discussed, because as part of its broad cover-up of UFOs, the United States government has classified all the material relating to this debate as Top Secret.
We do not have enough data to settle the debate, because counterarguments can be made for each argument suggesting Earth-like planets may be rare.

We do not have enough data to settle the debate, because counterarguments can be made for each argument suggesting Earth-like planets may be rare

62

At present, what is the primary way that the search for extraterrestrial intelligence (SETI) is carried out?

by using X-ray telescopes to search for exhaust from interstellar spacecraft
by searching for planets around distant stars
by using radio telescopes to search for signals from extraterrestrial civilizations
by seeking access to the secret records and alien corpses kept at the military's Area 51 in Nevada
by analyzing high-resolution images of nearby stars in search of evidence for structures that could not have developed naturally

by using radio telescopes to search for signals from extraterrestrial civilizations

63

In the Drake equation (Number of Civilizations = NHP × flife × fciv × fnow), what do we mean by fnow?

the fraction of planets in the galaxy on which a civilization could theoretically develop right now
the fraction of all species ever to exist that we currently are aware of
the fraction of civilizations in the universe that currently are sending messages to us
the fraction of planets with civilizations at the present time (as opposed to only in the past or future).

the fraction of planets with civilizations at the present time (as opposed to only in the past or future).

64

If there are other civilizations at present in the Milky Way Galaxy, which statement is almost undoubtedly true?

They have social structures that are completely different from our own; for example, different types of "family" units, and so on.
They are anatomically much like us, with two arms, two legs, two eyes, and two ears.
They are far more technologically advanced than we are.
For fun, they enjoy "buzzing" to Earth and temporarily abducting people, showing a clear preference for people located in less-developed rural areas.

They are far more technologically advanced than we are.

65

In the Drake equation, suppose that the term flife=12. What would this mean?
Half the stars in the Milky Way Galaxy have a planet with life.
Half of all life-forms in the universe are intelligent.
Half of the habitable worlds in the galaxy actually have life, while the other half don't.

Half of the habitable worlds in the galaxy actually have life, while the other half don't.