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Punctuation: Commas
Punctuation is a good example of this effort to use clearly defined rules in technical writing. In journalistic punctuation style, you punctuate according to what you feel are the needs for clarity. But this is likely to be viewed differently by different people. Therefore, punctuation style in technical writing is based on the structure of the sentence.
Use a comma after all introductory elements. Any element, regardless of the length, coming before the main clause should be punctuated with a comma. (The main clause is that core part of a sentence that makes it a complete sentence; that is, it expresses a complete thought.) Here are some examples:
When an atom acquires enough energy to leave its orbit, the
atom is positively charged.
As for the energy required to produce plastic automobile parts,
the auto makers view the additional cost as justified by the savings in
petroleum by a lighter car during its lifetime.
Because the high-pressure turbopumps rotate at speeds of 30,000
rpm, the weight distribution on the turbine blades must be balanced
with great accuracy.
Because there is no belt of doldrums in the Atlantic south of the
equator, hurricanes do not usually occur there.
Between 40 and 50 degrees west and just south of 10 degrees north
in the western end of the doldrums belt, calms do occur with
frequency, and hurricanes originate there with great frequency.
In 1831, Michael Faraday discovered that if a magnet was moved
in the vicinity of a coil, a current could be induced in the coil.
(Punctuate even short introductory phrases like this and the next
two sentences.)
Using this concept, Faraday arrived at a relation between the
changing flux and the induced electromagnetic field.
Today, the computer consortium of IBM, Mototrola, and Apple is
announcing its new PowerPC chip.
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Doublecheck commas between the parts of a sentence. A single comma should never break the flow of the main subject, verb, and object or complement of a sentence. Instead, commas should occur in pairs. Here are some examples (the bracketed commas indicated where commas are typically but mistakenly placed):
The discovery that moving a magnet within a coil could produce
current[,] was a major breakthrough in the history of
electronics.
(Yes, it's a long way from the subject "discovery" to the
verb "was" but there should be no comma.)
Decreasing the radar operating frequency [,] increases the
effective velocity coverage for the same sampling rate.
(The whole phrase "Decreasing the radar operating
frequency" is the subject of the verb "increases.")
It can be assumed that [,] precipitation particles move with the
air in their environment and are therefore good tracers for air
motion.
(Don't know why people would put a comma here—does it feel like a
pause?)
The separator between black mix and the zinc electrode [,]
consists of a paper barrier coated with cereal or methyl cellulose.
That European refuse incineration costs are substantially lower
than U.S. costs [,] is particularly evident when income from
by-product recovery and salvage operations is included.
(The whole clause, "That European refuse incineration costs
are substantially lower than U.S. costs," is the subject for the verb
"is.")
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Use a comma between all independent clauses. Whenever you have a compound sentence (those are the ones joined by and, but, or, nor, for, whereas), put a comma before the conjunction (the words I just listed in italics). Length of the compound sentence does not matter. Here are some examples (conjunctions are italicized):
The tank is made of aluminum, but the outer surface is protected by a
spray-on foam.
By the mid-1970s, the free-spending ways of the Apollo Program were
gone, and NASA now had to grapple with large technical challenges on a
limited budget.
It first appeared that Hurricane Betsy would reach the eastern U.S.,
but a looping path took her around the tip of Florida and into the Gulf
instead.
Gamma rays produce few pairs, but they travel farther.
(These next three examples are short, but to keep life as simple as
possible we punctuate them.)
One grate turns at 50 mph, but the others turn at 15 mph.
Type your name, and then press the Enter key.
(These are two imperative sentences—this qualifies as a compound
sentence. But check out the next example.)
You should type your name and then press the Enter key.
(In this case "you" is the subject for the compound
verb—it's the subject for both "should type" and
"press." This is not a compound sentence, and therefore
there is no comma before "and.")
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Do not use a comma between two compound verb phrase. Watch out about what you think are compound sentences. A complete sentence has to be on both sides of the conjunction (that means subject, verb, object, or complement—the works). Compare the following examples (subjects are italicized, and verbs are bold):
Offspring exposed to significant amounts of alcohol in utero
are much more active than controls and sometimes seem to fly around the
room.
(This is a compound verb phrase, not a compound sentence:
"Offspring" is subject for both verbs.)
Plastic parts are not weldable and must be repaired by other
methods.
The observation and measurement of such small frequency shifts
require excellent radar frequency-stability characteristics that
are not usually found in conventional radar but can be added without a
drastic increase in equipment costs.
Pulse Doppler radar effectively samples the backscattered
signal at the radar repetition rate and therefore can provide
unambiguous Doppler frequency observations only in the frequency range
allowed by the sampling rate.
The manganese dioxide used in batteries is usually obtained
from natural ore (mainly from Gabon, Greece, and Mexico) but can be a
synthetic product prepared by chemical precipitation or by
electrolytic methods.
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These last three sentences probably seem incredibly long to you and needy of commas at and and but. Rather than break our rule (and remember it's not breaking the rule that matters; it's creating more and more exceptions that will drive us all crazy), why not split these into two sentences each?
The observation and measurement of such small frequency shifts require
excellent radar frequency-stability characteristics that are not
usually found in conventional radar. However, this same observation
and measurement can be added without a drastic increase in equipment
costs.
Pulse Doppler radar effectively samples the backscattered
signal at the radar repetition rate. This type of radar therefore can
provide unambiguous Doppler frequency observations only in the
frequency range allowed by the sampling rate.
The manganese dioxide used in batteries is usually obtained
from natural ore (mainly from Gabon, Greece, and Mexico). It can also
be a synthetic product prepared by chemical precipitation or by
electrolytic methods.
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Use commas around all nonrestrictive elements. Nonrestrictive elements are phrases and clauses that a sentence literally does not need to say what it wants to say. These elements can be taken out of the sentence without hurting its basic message. Use commas around these nonrestrictive elements. Here are some examples:
Eighty percent of the work done by the heart is carried out by the
left ventricle, which pumps blood into the arteries serving the organs
and the tissues.
(Nice of the writer to remind us what the left ventricle does, but
the sentence could live without it; it would still make sense.)
The test produced a speed in the high-pressure hydrogen turbopump of
7000 rom, which is 19 percent of design speed.
(This is additional detail, not essential to the sense of the
sentence.)
The Coriolis force, caused by the rotation of the earth, always acts
at right angles to the pressure gradient in the northern hemisphere.
(This is a helpful definition but again is not essential to the
sentence.)
The bulky equipment, although placed on a rolling cart, must always
remain within 6 feet of the heart transplant patient.
(Nonessential stuff—put commas around it!)
The formation of hurricane, a type of atmospheric vortex, involves the
combined effect of pressure and circular wind.
Researchers also found that heavy drinkers — women drinking at least
1.6 ounces of absolute alcohol during pregnancy — have infants
averaging 59 grams less than the infants of lighter drinkers.
(Nonessential stuff—put commas around it, or in this case dashes,
which are commas by another name.)
When added to liquids, detergent materials decrease the contact angle,
thereby decreasing the wettability.
(Nonessential stuff—put commas around it!)
When waterproofing material is added to a fabric, it increases the
contact angle, making the fabric water-repellent.
(Nonessential stuff—put commas around it!)
Molecules may also have some degree of ordered as well as disordered
motion, in which case the total energy is the sum of the mechanical and
thermal energies.
(Nonessential stuff—put commas around it!)
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Do not use comma around restrictive elements. Restrictive elements are phrases and clauses that a sentence desperately needs to make sense, to say what it means to say. If you take restrictive elements out of a sentence, you wreck the sentence!
Problem: You can use the system, when the login prompt appears.
(The way this sentence is punctuated implies that you can
use the system any old time! The comma indicates that the clause
beginning with "when" can be lifted from the
sentence.)
Revision: You can use the system when the login prompt appears.
(The clause beginning with "when" is restrictive—
it can't be omitted from the sentence and therefore should not be
punctuated. Now the sentence means that you can use the system
only when the prompt appears.)
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Here are some additional examples of this rather tricky rule:
A turbopump is a pump that is turned by the action of a turbine that
shares a common shaft with the pump.
(It's not any old pump; it's one that does what the latter part of
this sentence says it does. Imagine this sentence ending at
"essentially a pump.")
Eighty percent of the work done by the heart is carried out by the
left ventricle.
(Imagine this sentence without "done by the heart," which is the
restrictive element in this sentence. No commas here!)
A drop of water almost flattens out when it is placed on a glass
plate.
(Imagine this sentence without "when it is placed on a glass
plate," which is the restrictive element here. No commas need
apply!)
In one study, 11 percent of the offspring whose mothers consumed 2 to
4 drinks per day showed partial features of fetal alcohol syndrome (FAS),
while 19 percent of those whose mothers consumed 4 or more drinks per
day showed FAS features.
(Imagine this sentence without "whose mothers consumed 2 to 4
drinks per day" or without "whose mothers consumed 4 or more
drinks per day." The sentence simply wouldn't make any sense. No
commas!)
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Use a comma before the "and" in a series of three or more. In series of three or more words or phrases, go ahead and put the comma before the and that occurs before the final element. You may have heard that this series-and comma rule is optional. However, there are situations where the lack of the series-and comma can cause confusion. And when you consider that using the series-and comma cannot hurt the sense of the sentence, it makes sense to use it in all cases. Here are some examples:
Instrument panels, bumper components, door liners, seat covers,
and grille panels are the most common parts produced directly by
automakers.
A 12-ounce can of beer, a 5-ounce glass of wine, and a mixed
drink with 1.5 ounces of 80-proof liquor all contain approximately the same
amount of alcohol.
The development years involved designing the components for the Space
Shuttle's engines, testing the original designs, and retesting the
redesigned components.
In humans, the period of rapid brain development begins at
mid-pregnancy, peaks in the third trimester, and ends by the
postnatal year.
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Do not use a comma between a series of only two. Be careful not to apply the series-and comma rule to a series of only two elements. Watch out also for those situations where it looks like you have a series of three elements but it is actually a series of two noun phrases and a compound verb phrase (if this is meaningless—see the example).
We brought bread and cheese and read poetry.
(Sorry for the Dick-and-Jane sentence, but notice that
"bread," "cheese," and "poetry" are not
really in a series. No commas for either "and" here.)
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Punctuate series adjectives carefully. It gets tricky knowing how to punctuate when two or more adjectives pile up in front of a noun. One fairly reliable technique is this: if you can switch the order of the adjectives or if you can insert and between them without making the phrase sound weird, then you should use commas. (Remember that in no case is there a comma between the final series adjective and the noun it modifies.)
He's having his third mid-life crisis. Now he wants a new red sports
car.
(You couldn't say "mid-life third crisis" nor could you
say "sports red new car"—so no commas in or amongst these
adjectives.)
Each door is held shut with an adjustable, spring-loaded door
latch.
(You probably could switch "adjustable" and
"spring-loaded"—use a comma here.)
As each rack passes through the wash chamber, the dishes get a
thorough soil-stripping wash and a final, automatic hot-water rinse.
(You probably could switch "final" and
"automatic"—use a comma here.)
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These last two examples may have felt a bit "iffy" to you—the technique is only "fairly" reliable.
Note: This doesn't cover all commas rules; see a standard handbook like one of the ones mentioned at the beginning of this appendix for the full set. (Incidentally, you'll notice a lot more flexibility in the rules in those standard reference books—they weren't written for the technical-documentation context.)
Punctuation: Colons
Although the colon has other uses in writing, its most important function is to act as a signal to the reader—it says something like "Okay, reader! Here it comes!" For example:
To make a kite, you need the following items: string, paper, thin
sticks, glue, and scissors.
Notice the words before the colon make a complete statement—at least
grammatically. Here are some additional examples:
The main engines of the Space Shuttle consist of six main components:
the external tank, the low-pressure turbopump, the high-pressure
turbopumps, the preburners, the combustion chamber, and the nozzle.
Hurricane size is expressed in three ways: the strength of the maximum
winds, the diameter of the hurricane-force winds, the diameter of the
gale-force winds, and the overall size the cyclone circulation.
To make a metal dashboard, three steps are required: (1) the metal
must be stamped; (2) the texture must be stamped into the metal; and (3)
the part must be painted.
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Notice in the last example that the first sentence introduces a series of complete sentences. In fact, you can use the colon to connect two complete sentences—as long as the first sentence introduces or prepares for the second. Here are some examples of this possibility:
The grades of the students in the caffeine research project told a
dramatic story: the higher the caffeine intake, the lower the grades, both
for semester and overall grade point average.
In general, shelf-life increases as the cell size of the battery
becomes smaller: with well-constructed cells, shelf-lives of three years
with a No. 6 telephone cell and ten years with a penlight cell are
possible.
The line-of-sight in a communication satellite can be a problem:
communication satellites can see the earth's surface only between about 83
degrees north latitude and 83 degrees south latitude.
Many of the new applications of microcomputers are
"interactive": there is frequent interaction between the computer
and one or more users.
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However, don't use a colon inside a complete sentence. It should connect only complete sentences to complete sentences or complete sentences to lists.
Problem: The typical Doppler velocity sensor consists of: a
transistor, an antenna, and a receiver.
Revision: The typical Doppler velocity sensor consists of a
transistor, an antenna, and a receiver.
Problem: Three significant types of generating plants are:
hydroelectric, fossil-fuel-electric, and nuclear-electric.
Revision: Three significant types of generating plants are
hydroelectric, fossil-fuel-electric, and nuclear-electric.
Problem: You will need the following items: string, paper, thin
sticks, glue, and scissors, to make a kite.
Revision: You will need the following items—string, paper, thin
sticks, glue, and scissors—to make a kite.
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Look at this last example closely: the grammatical core of the sentence is "You will need the following items . . . to make a kite." And don't forget the important role of the colon in introducing a vertical list. A colon should punctuate the lead-in that sets up the list items.
The semicolon could be called a strong comma. Its two main uses are to connect two (or more) sentences that seem very closely related and to clarify the punctuation of a series of items that have their own internal commas.
You may have had some unhappy encounters with run-ons and comma splices (discussed beginning on page D-12) in the past. These two "comma faults" usually result from the writer's sense that the sentences involved in the problem are very closely related—the full stop signaled by the period seems like too full of a stop. (It's almost like music; makes you wonder why we don't have the equivalent of whole, half, quarter, and eighth rests in punctuation.) Often, these run-on sentences and comma splices can be fixed by substituting a semicolon for the offending comma.
But not always. Some writers go way overboard in sensing close relations between sentences. Well, yes, every sentence in a document is related to every other—they ought to be! But they need to be reeeaaally closely related. Here are some examples:
"Plaque-fissuring" refers to the formation of an opening
from the lumen to the intima; it leads to an intra-intimal thrombus
containing not just red cells but mainly fibrin and platelets.
In 1940, philanthropy accounted for 24 per cent of the total operating
budget of nonprofit hospitals in New York City; in 1948, it had dropped to
17 per cent.
Gray mold is one of the most important fungal diseases in Italian
viticulture; its growth causes serious production losses and adversely
affects wine quality.
The other use of the semicolon worth noting here is how it can clarify
items in a series that have commas within them already:
Injury caused by pollutants can easily be mistaken for injury caused
by other stresses; or, just the opposite, injury symptoms from adverse
temperature or moisture relations may resemble, and can be incorrectly
attributed to, air pollutants.
Possible research areas announced recently have included genetics,
fermentation microbiology, and immobilized biocatalysts; but environmental
biotechnology, such as metal recovery and waste recycling, is also
included.
A typical membrane potential of about one-tenth of a volt sounds
relatively small; but, because it occurs across a membrane that is only
about 10 nanometers thick, it represents an enormous voltage gradient
of about 10 million volts per meter.
The heart undergoes two cardiac cycle periods: diastole, when blood
enters the ventricles; and systole, when the ventricles contract and blood
is pumped out.
An organization may be functional, with responsibility assigned on the
basis of buying, selling, promotion, distribution, and other tasks;
production-oriented, with production managers for each product
category and brand managers for each individual brand in addition to
functional categories; or market-oriented, with managers assigned on
the basis of geographical markets and customer types in addition to
functional categories.
Electric power substations are used for some or all of the following
purposes: connection of generators, transmission or distribution lines, and
loads to each other; transformation of power from one voltage level to
another; interconnection of alternate sources of power; and detection of
faults, monitoring and recording of information, power measurement,
and remote communication.
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A common misuse of the semicolon is to plunk it down between what appear to be two complete sentences:
Problem: The slide rule was an important device for scientists and
engineers for many years; although its use has all but vanished since the
advent of the pocket calculator.
Revision: The slide rule was an important device for scientists and
engineers for many years, although its use has all but vanished since the
advent of the pocket calculator.(The "although" clause is not
complete; it can't stand on its own.)
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Apostrophes
Pity the poor apostrophe—it's practically an endangered species. The problem with the apostrophe is that it has some conflicting tasks: it is used primarily to show possession and, minimally, to show plurals. But people have gotten it all mixed up. For example, the likes of "John love's Mary" is becoming pretty common in telephone booths. A scant two to three hundred years ago, people didn't even use apostrophes (yes—a world without apostrophes!). But the thing does add precision to writing; it does prevent confusion. The rules are stupidly simple; here they are:
- To show the plural of numbers or letters when they are discussed as
such, add 's (again usage varies on this, but this is a safe choice):
Do you know how many c's and s's are in the word ne-e-ry?
On a computer, O's are represented by O's and 0's with 0's.
His speech was filled with annoying uh's, okay's, and you know's.
Now, there are others rules involving apostrophes such as for contractions or for quotes within quotes, but we'll leave those for the reference books to handle.
Someone once said, "Take hyphens seriously and you will surely go mad." They weren't lying! (By the way, this previous sentence has a pronoun-reference problem.)
Hyphens are supposed to keep us from misreading things and show us how words in complex phrases relate to each other. The problem is that the rules for hyphens simply cannot be applied absolutely consistently—you end up hyphenating everything including the kitchen sink. Professional editors end up keeping long lists of exactly which word pairs they will hyphenate in a specific document (so that they don't end up in therapy).
Hyphens do matter, however (save the hyphen!). Our language culture seems to be very "into" piling up ambitious noun phrases. These sentences verge on having a problem called "noun stacks," as described in Appendix E. But to read this kind of stuff, we need hyphens—they show us what goes with what. Hyphens show that a pair of words is acting as a unit and must be read that way. The common types of unit modifiers—which are two or more words acting as a unit—are discussed in the following (but it's by no means exhaustive):
Once you get a partial feel for hyphens, watch out! You might start acting like Lucy in that show where she has been on the assembly line too long and starts going after everything and everybody with her wrenches. Everything will seem like it needs a hyphen! When that happens, back off, and ask yourself—could someone misread this sentence without a hyphen, even if they were just being mean? If it positively cannot be misread, then maybe you can give your hyphen key a break.
Comma Splices and Runons
The comma-splice and run-on sentence (and the fused sentence, as a variant is called) are all examples of the problem in which two or more sentences are improperly joined. In the typical comma-splice sentence, two sentences are joined by a comma without an intervening coordinating conjunction (and, or, nor, but, yet). Technically, the run-on sentence is a sentence that goes on and on and needs to be broken up; it's likely to be a comma splice as well. A fused sentence is two complete sentence just jammed together without any punctuation and without any conjunction.
We write comma-splice and run-on sentences because we sense that the sentences involved are closely related—a full-stop period just doesn't seem right. Actually, the semicolon is the right choice in these situations (although it's easy to go semicolon crazy when you first start using them). Here are some examples of this type of problem and their revisions:
Problem: Sometimes, books do not have the most complete information,
it is a good idea then to look for articles in specialized periodicals.
Revision: Sometimes, books do not have the most complete
information; it is a good idea then to look for articles in specialized
periodicals.
Problem: Most of the hours I've earned toward my associate's
degree do not transfer, however, I do have at least some
hours the University will accept.
Revision: Most of the hours I've earned toward my associate's degree
do not transfer. However, I do have at least some hours the University
will accept.
Problem: The opposite is true of stronger types of stainless steel,
they tend to be more susceptible to rust.
Revision: The opposite is true of stronger types of stainless steel:
they tend to be more susceptible to rust.
Problem: Some people were highly educated professionals, others were
from small villages in underdeveloped countries.
Revision: Some people were highly educated professionals, while
others were from small villages in underdeveloped countries.
Problem: This report presents the data we found concerning the cost
of the water treatment project, then it presents comparative data from
other similar projects.
Revision: This report first presents the data we found concerning
the cost of the water treatment project and then comparative data from
other similar projects.
Problem: Most of this firm's contracts have been with major
metropolitan hospitals, included among them is Memorial East in
Luckenbach.
Revision: Most of this firm's contracts have been with major
metropolitan hospitals, included among which is Memorial East in
Luckenbach.
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See comma splices to get some additional practice recognizing and correcting comma splices.
Fragments
Fragments are simply incomplete sentences—grammatically incomplete. They usually come about because the sentence may already seem too long. Also, in conversation, we typically speak in fragments. Here are some examples and their revisions:
Problem: Mary appeared at the committee meeting last week. And made
a convincing presentation of her ideas about the new product.
Revision: Mary appeared at the committee meeting last week and made
a convincing presentation of her ideas about the new product.
Problem: The committee considered her ideas for a new marketing
strategy quite powerful. The best ideas that they had heard in years.
Revision: The committee considered her ideas for a new marketing
strategy quite powerful, the best ideas that they had heard in years.
Problem: In a proposal, you must include a number of sections. For
example, a discussion of your personnel and their qualifications, your
expectations concerning the schedule of the project, and a cost breakdown.
Revision: In a proposal, you must include a number of sections: for
example, a discussion of your personnel and their qualifications, your
expectations concerning the schedule of the project, and a cost
breakdown.
Problem: The research team has completely reorganized the
workload. Making sure that members work in areas of their own expertise and
that no member is assigned proportionately too much work.
Revision: The research team has completely reorganized the
workload. They made sure that members work in areas of their own expertise
and that no member is assigned proportionately too much work.
Problem: She spent a full month evaluating his computer-based
instructional materials. Which she eventually sent to her supervisor with
the strongest of recommendations.
Revision: She spent a full month evaluating his computer-based
instructional materials. Eventually, she sent the evaluation to her
supervisor with the strongest of recommendations.
Problem: The corporation wants to begin a new marketing push in
educational software. Although the more conservative executives of the
firm are skeptical.
Revision: Although the more conservative executives of the firm are
skeptical, the corporation wants to begin a new marketing push in
educational software.
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See fragments to get some additional practice recognizing and correcting fragments.
Problem Modifiers
Modifier problems occur when the word or phrase that a modifier is supposed to modify is unclear or absent, or when the modifier is located in the wrong place within the sentence. A modifier is any element—a word, phrase, or clause--that adds information to a noun or pronoun in a sentence. Modifier problems are usually divided into two groups: misplaced modifiers and dangling modifiers:
Misplaced modifiers
They found out that the walkways had collapsed on the late
evening news. (Was that before or after the sports news?)
The committee nearly spent a hundred hours investigating the
accident. (Did they spend even a minute?)
The superviser said after the initial planning the in-depth study
would begin. (Just when did she say that, and when will the
study begin?)
Dangling modifiers
Having damaged the previous one, a new fuse was installed in the
car. (Who damaged that fuse?)
After receiving the new dumb waiter, household chores became so
much easier in the old mansion. (Who received the dumb waiter?)
Using a grant from the Urban Mass Transportation Administration,
a contraflow lane was designed for I-45 North. (Who used that
money?)
Pointing out the productivity and health problems plaguing US
workers, aerobic fitness programs may become much more common in
American industry, according to the spokeswoman. (Who pointed
that out?)
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To correct misplaced modifier problems, you can usually relocate the misplaced modifier (the word or phrase). To correct dangling modifiers, you can rephrase the dangling modifier, or rephrase the rest of the sentence that it modifies.
Revisions
On the late evening news, we heard that the walkways had
collapsed.
The committee spent nearly a hundred hours investigating the
accident.
The superviser said that the in-depth study would begin after the
initial planning.
Because the previous fuse had been damaged, a new one had to be
installed.
Having damaged the previous one, I had to install a new fuse in
my car.
After we received the dumb waiter, it was immediately installed.
After receiving the dumb waiter, we immediately installed it.
When the Urban Mass Transportation Administration granted funds
to the city, planners began designing a contraflow lane for
I-45 North.
Using a grant from the Urban Mass Transportation Administration,
city planners designed a contraflow lane for I-45 North.
Because of the productivity and health problems plaguing US
workers, aerobic fitness programs may become much more common
in American industry, according to the spokeswoman.
Pointing out the productivity and health problems plaguing US
workers, the spokeswoman said that aerobic fitness programs may
become much more common in American industry.
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One particularly effective way to correct dangling modifiers is to create a summary appositive, that is, a noun or pronoun summarizing what was just said followed by an adjective clause:
Dangling modifier problems Summary appositive revisions
Stars that were formed relatively Stars that were formed relatively
recently should have higher recently should have higher
concentrations of heavy elements concentrations of heavy elements
than do the older stars, which is than do the older stars, a
confirmed by observation. prediction that is confirmed by
observation.
Most astronomers now believe that Most astronomers now believe that
the energy of quasars comes from the energy of quasars comes from
giant black holes in the cores of giant black holes in the holes of
the quasars, which fits the quasars, a theory that fits the
growing belief that black holes growing belief that black holes
are present in the cores of many are present in the cores of many
galaxies, our own included. galaxies, our own included.
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