Deck 18: Quasars and Other Active Galaxies

Question 1

Astronomers initially had difficulty identifying the emission lines in quasar spectra at optical wavelengths because

A) the emission lines were smeared out by the extremely high speed of the quasars, making them hard to measure.
B) the emission lines were from ionized atoms that had not been seen before.
C) the emission lines were found to be created from elements that do not exist on Earth.
D) no one expected to see the pattern of spectral lines characteristic of the ultraviolet hydrogen spectra in the visible region.

Answer 1

no one expected to see the pattern of spectral lines characteristic of the ultraviolet hydrogen spectra in the visible region.

Question 2

The energy output per second of a typical quasar is equal to that emitted by the Sun in

A) 1 year.
B) its whole lifetime.
C) 200 years.
D) 1 million years.

Answer 2

200 years.

Question 3

The energy output per second of a typical quasar is equal to that emitted by the Sun in 200 years. So, if the luminosity of the Sun is L, what is the luminosity of a typical quasar?

A) 3.2 * 10⁷ L
B) 6.3 * 10⁹ L
C) 1.9 *10¹¹ L
D) 5.3*10¹² L

Answer 3

6.3 * 10⁹ L

Question 4

Cygnus A, a strong radio source, is also designated 3C 405. What does the "3C" stand for?

A) "3C" stands for the Third Cambridge Catalog, an important list of radio sources.
B) This radio source is a Type 3C radio source as opposed to a Type 1A or a Type 2B.
C) Cygnus A was discovered through a joint program run by Cambridge, Connecticut, and Calgary universities.
D) A chopper is a device needed to cut off and delineate a signal from a source, and three such choppers are required to isolate an intelligible signal from Cygnus A.

Question 5

What is the MOST outstanding feature of a quasar compared with other objects in deep space?

A) small size
B) great distance from Earth
C) short lifetime
D) prodigious output of energy

Question 6

Astronomy with a radio telescope was initiated by

A) the British Broadcasting Corporation in England.
B) Marconi in Europe.
C) the National Radio Astronomical Observatories of the United States, with the support of the National Science Foundation and the American Astronomical Society.
D) an amateur astronomer, Grote Reber, after Jansky had detected radio energy from the Galaxy.

Question 7

Quasars typically appear to be

A) extremely massive objects in the Milky Way Galaxy, their spectra showing very high gravitational redshift.
B) moving away from Earth at very high speeds, up to about 90% of the speed of light.
C) moving across Earth's line of sight at very high speeds, as seen in time-lapse photographs.
D) moving toward Earth at high speeds, as high as 90% of the speed of light.

Question 8

One of the first three astronomical objects detected at radio wavelengths by Grote Reber in the late 1930s was

A) Jupiter.
B) the Sun.
C) the galactic center.
D) the Moon.

Question 9

The emission lines in quasar spectra were difficult to identify initially because

A) no one expected violet and ultraviolet spectral lines to be shifted so far toward the red.
B) the observed emission lines were so broad because of internal motions in the quasar that they were difficult to identify.
C) they appeared to be created by elements that did not exist on Earth.
D) they were very faint and could not be measured accurately.

Question 10

Redshifts revealed the radio source Cygnus A to be moving at 17,000 km/s. At this speed it could not have been a part of the Milky Way Galaxy. At this speed how long would it have taken to cross the whole of the visible Galaxy?

A) a few years
B) a few million years
C) a few hundred million years
D) 10 billion years, almost the age of the Galaxy

Question 11

Most quasi-stellar objects (QSOs) produce their strongest emissions in which region of the electromagnetic spectrum?

A) X- ray
B) visible
C) infrared
D) radio

Question 12

The discovery of the peculiar galaxy Cygnus A was a surprise to astronomers because

A) the redshift of its radio wavelength signal was as high as anything measured up to that time.
B) it was first discovered at X-ray wavelengths and detected at optical and radio wavelengths only some time later.
C) it was so bright at optical wavelengths that no one expected it to be a galaxy.
D) it was very faint at visible wavelengths but extremely bright at radio wavelengths.

Question 13

Quasars were originally thought to be stars in the Milky Way Galaxy. Which of these properties of quasars was LEAST unusual in comparison to normal stars?

A) They were dim in the visible part of the spectrum.
B) They were very energetic in the radio part of the spectrum.
C) Their spectra were dominated by emission lines rather than absorption lines.
D) Their spectra showed patterns that were, at first, unrecognizable.

Question 14

The major surprise about Cygnus A, one of the first three sources of strong radio emission detected by Grote Reber in the late 1930s, when examined with large optical telescopes, was that it

A) had no corresponding optical counterpart at all.
B) was at the center of the Milky Way Galaxy.
C) was a very distant object that was relatively faint at optical wavelengths yet extremely luminous at radio wavelengths.
D) was a very bright supernova remnant.

Question 15

The puzzle about quasar spectra, which was finally solved by Maarten Schmidt, was

A) that the characteristic pattern of hydrogen spectral lines was seen but at much greater blueshifts than is usual from hydrogen in stellar sources.
B) that the brightnesses of hydrogen lines in the familiar spectral sequence were seen to fluctuate wildly over times of seconds from such a bright and therefore presumably large object.
C) that the characteristic pattern of hydrogen spectral lines was seen but at much greater redshifts than is usual from hydrogen in stellar sources.
D) that a familiar spectral line sequence of hydrogen lines was detected from these starlike objects, but they had intensity ratios between lines radically different from those seen in spectra from nearby stars.

Question 16

The specific characteristics that identify MOST quasars are

A) that they look like elliptical galaxies but with high spectral redshifts.
B) luminous, starlike appearance, and very high spectral blueshift, indicating that they are approaching the Sun very fast, and rapid intensity fluctuations, indicating small intrinsic size.
C) luminous, starlike appearance with very high redshifts and hence very large distances, indicating very energetic sources.
D) spiral galaxy appearance and very high spectral blueshift, indicating that they are coming toward the Sun at high speed.

Question 17

The energy output per second of a typical quasar is equal to that emitted by the Sun in 200 years. How much energy is this? The Sun's luminosity is 3.8 * 10²⁶ watts.

A) 6.8 * 10²⁸ Joules
B) 1.2 * 10³⁴ Joules
C) 2.4 *10³⁶ Joules
D) 5.5 *10³⁹ Joules

Question 18

The first three strong radio sources, discovered by Reber in the 1930s and 1940s, included all of these EXCEPT

A) galactic nucleus.
B) supernova remnant.
C) globular cluster.
D) distant galaxy.

Question 19

When, by whom, and by what means was the first quasar detected?

A) in 1944 by Reber with a small radio telescope
B) in 1951 by Baade and Minkowski with the 200-inch Hale telescope on Mount Palomar
C) in 1992 by Zeeman with the Hubble Space Telescope
D) in 1955 by a British team using the Cambridge radio telescope

Question 20

The observed characteristics of a quasar are a

A) diffuse circular image, no redshift of the spectrum, and often a very bright radio source.
B) starlike image, with a highly variable Doppler spectrum that shifts from red to blue, and often a very bright radio source.
C) starlike image, a highly redshifted spectrum, and sometimes an intense radio emitter.
D) starlike image, an extremely blueshifted spectrum, and often an intense radio emitter.

Question 21

The distance to the bright quasar 3C 273 is estimated to be

A) 2 billion ly.
B) 20,000 ly.
C) just beyond the Milky Way.
D) 3 million ly.

Question 22

Quasars are

A) sources of great energy, very large in actual size, and shaped like galaxies.
B) sources of intense radio energy only, not visible at other wavelengths, and relatively large but very distant.
C) prolific sources of energy, starlike in appearance, and intrinsically small.
D) starlike sources of great energy located in the Milky Way Galaxy and intrinsically very small.

Question 23

The highest recession velocities that have recently been detected for quasars are

A) 3000 km/s, or about 1% the speed of light.
B) more than 99% the speed of light.
C) about 10% the speed of light.
D) almost half the speed of light.

Question 24

Quasars appear to be

A) very distant and intrinsically bright objects moving in random directions at high speeds.
B) very distant, intrinsically faint objects with very high blueshifts in their spectra.
C) relatively close with very high redshifts.
D) very distant, intrinsically bright objects with very high redshifts.

Question 25

Which of these statements MOST accurately characterizes the abundance of quasars over the universe's history?

A) Quasars were most common very early in the universe's history, about 100 million years after the Big Bang, and their abundance has been declining ever since.
B) Quasars have been increasing in abundance over time, so that they are most common today.
C) Quasars peaked in their abundance about 11-12 billion years ago.
D) Quasars go through cycles in which their abundance increases dramatically then decreases. At the present day, they are at a low point in the cycle.

Question 26

Today, the number density of quasars in the universe is quite small. For how long in the past has this been true?

A) 7 billion years
B) 9 billion years
C) 11 billion years
D) 13 billion years

Question 27

The redshift of the quasar Cygnus A is about 0.046. In the spectrum of this quasar what is the wavelength of the H_α line in the hydrogen spectrum (in nanometers)? The rest wavelength of the H_α line is 656.3 nanometers.

A) 30.2
B) 626.1
C) 656.3
D) 686.5

Question 28

The extreme redshifts of quasar spectra are caused by

A) very high recession speeds of the sources away from the Milky Way Galaxy.
B) absorption of all but the red parts of the quasar spectrum by intergalactic matter.
C) Zeeman effects from the very intense magnetic fields in the vicinity of the source.
D) high gravitational fields at the surfaces of the quasars (gravitational redshift).

Question 29

All quasars appear to be

A) moving toward Earth at very high speeds.
B) moving away from Earth at very high speeds.
C) very distant, intrinsically faint objects.
D) relatively close, very bright objects.

Question 30

Quasars emit significant amounts of radiation from the Lyman-alpha transition. When the spectrum is observed on Earth, it is found that the Lyman-alpha line is accompanied by many absorption lines, called the Lyman-alpha forest. What is the origin of these lines? See Figure 18-5 in the text.

A) The absorption lines are caused by the rotation of the quasar. Different parts of the quasar thus give rise to Lyman-alpha lines with different Doppler shifts.
B) The absorption lines are the result of gravitational lensing by objects between the quasar and Earth.
C) The emitted Lyman-alpha radiation is absorbed by many gas clouds between the quasar and Earth. The lines are receding at various velocities and thus are absorbed at different Doppler-shifted wavelengths.
D) Because the quasar's jets are aimed at various directions, the jet plasma has Doppler shifts that are different from those of the quasar itself. The result is a variety of Lyman-alpha wavelengths in the spectrum received on Earth.

Question 31

Which observation of quasars convinced astronomers that quasars were very distant objects?

A) extremely red spectrum, reddened by extreme interstellar absorption of the blue part of the spectrum, meaning that the source must be a very long way away
B) extreme redshift of visible Balmer and UV Lyman hydrogen emission lines, indicating high recessional velocities and hence, by the Hubble law, very large distances
C) extreme faintness at visible wavelengths; the inverse square law for visible light showed that they must be very distant
D) appearance as pointlike star images under the highest magnification, meaning that they must be very far away

Question 32

An intense radio source is found to coincide with a starlike object whose spectrum contains a pattern of intense emission lines in the visible range that matches that of the Lyman UV hydrogen spectral lines but is very redshifted. What is this object?

A) exploding shell of a supernova
B) black hole
C) quasar
D) pulsar

Question 33

The closest quasar is Cygnus A, 635 million light-years away. What does the Hubble law give for its redshift?

A) 0.0326
B) 0.046
C) 0.091
D) 0.15

Question 34

Quasars

A) are more common in nearby clusters of galaxies and less common in distant clusters of galaxies.
B) are rare in the Local Group, with only one or two examples.
C) increase in number as redshift increases, a relationship that persists to the highest redshifts astronomers can measure.
D) peaked in number about 2 billion years after the Big Bang.

Question 35

Quasars have now been detected at redshifts so large that, if translated to "recession velocities," indicate motion away from the Milky Way Galaxy of

A) about 90% the speed of light.
B) speeds as large as 3 * 10³ km/s, or 1% the speed of light.
C) about 1/10 the speed of light.
D) speeds greater than the speed of light, indicating that the interpretation of redshifts in terms of velocity is faulty.

Question 36

What are the typical characteristics of a quasar in terms of energy generation?

A) output of 100 galaxies from a volume with diameter of 1 light-day
B) output of 10⁶ galaxies from a volume with diameter of 1 light-day
C) output of 100 galaxies from a volume with diameter of 1 light-year
D) output of single galaxy from a volume with diameter of 1 light-year

Question 37

Faint starlike objects producing intense radio energy that eventually were called quasars were found to have an unusual feature in their optical spectra. This feature was

A) an extreme redshift of emission lines that indicated high recessional velocities and hence great distances, requiring extremely high energy output to be detected.
B) a periodic variation of the Doppler shift from red to blue, indicating a light source oscillating back and forth over a few weeks.
C) two sets of spectral lines that indicated simultaneous motion of sources toward and away from the Sun, possibly from a rapidly expanding shell of material around the radio source.
D) an extreme blueshift, meaning that these stars in the Milky Way Galaxy were coming toward Earth at very high velocities.

Question 38

A starlike object seen on deep sky photographs coincides with an intense radio source and has a spectrum in which the characteristic Lyman pattern of hydrogen spectral lines has been shifted from the ultraviolet to the visible spectral range. What is this object?

A) quasar
B) pulsar
C) supernova explosion
D) black hole

Question 39

A quasar is observed to have a redshift of about 0.06, with a recessional velocity about 6% of the speed of light. Approximately what distance, in light-years, does the Hubble law give for this object? The Hubble constant is about 70 kilometers per second per megaparsec.

A) 100 million
B) 800 million
C) 3.2 billion
D) 12.1 billion

Question 40

The nearest quasar is

A) in the Milky Way Galaxy.
B) in the Local Group of galaxies.
C) about 600 million light-years from the Milky Way.
D) barely visible at the remote edge of the observable universe, 13 billion light-years away.

Question 41

The energy output of a bright quasar is equivalent to

A) that of the Milky Way Galaxy.
B) 10⁶ spiral galaxies.
C) 1000 Suns.
D) 100 Milky Way galaxies.

Question 42

An astronomer is observing the radiation from a distant active galaxy and notices that the amplitude of the signal varies in strength regularly over a certain period. The maximum possible size for the source of this radiation can now be calculated from the

A) period and the amplitude of the signal.
B) period but not the amplitude of the signal.
C) amplitude but not the period of the signal.
D) frequency of the radiation composing the signal.

Question 43

The luminosity of the Milky Way Galaxy is about 10 billion times the luminosity of the Sun. During peak periods of brightness, the quasar 3C 279 has a luminosity about ten thousand times the luminosity of the Galaxy. Suppose the flux (energy per second per emitting area) from the Sun remains the same, but it grows in radius. How big would it have to be to produce the same luminosity as 3C 273 at its peak?

A) about the size of Pluto's orbit
B) about 100 times the diameter of Pluto's orbit
C) about a light-year across
D) about a tenth the diameter of the Galaxy

Question 44

"It is starlike in appearance, showing very high redshift and an energy output of at least 100 galaxies from a small region about 1 light-day across." This statement describes which of these astronomical objects?

A) red supergiant star
B) center of the Milky Way Galaxy
C) quasar
D) supernova explosion in a neighboring galaxy

Question 45

A quasar is now thought to be a

A) long-lived supernova explosion.
B) very luminous object at a very large distance from the Sun.
C) nearby star, ejected with great violence and velocity from the center of a galaxy.
D) distant but very luminous and active star in the Milky Way Galaxy.

Question 46

Which of these is NOT a characteristic of active galaxies?

A) jets
B) strong emission lines in the spectrum
C) Doppler shifts indicating a large rate of rotation
D) bright starlike nuclei

Question 47

The fluctuation time for the brightnesses of quasars can be as short as a

A) few seconds.
B) month.
C) day.
D) year.

Question 48

An active galaxy is measured to be 100 Mpc away. How many million years ago did the light observers see now leave this galaxy?

A) 30.7
B) 100
C) 326
D) 652

Question 49

Which observations of the radiation from quasars indicate that quasars are physically very small objects compared with galaxies?

A) their emission, which is mostly in the infrared and radio range
B) their starlike appearance on photographs, showing no structure
C) rapid fluctuations in output, often in less than 1 day
D) very high redshift of their light

Question 50

What observations convince astronomers that the energy source of a quasar is physically very small?

A) rapid fluctuations in brightness since variations over 1 day mean that the source must be less than 1 light-day across
B) instant disappearance of the quasar when occulted by the Moon's edge as the Moon moves in front of a quasar, indicating a very small source size
C) sharpness of the emission lines in their optical spectra since motions within a large source would smear out the line shapes
D) extremely small size of the image of a quasar, even from Hubble Space Telescope images and radio interferometry measurements

Question 51

Compared with the total output of energy from the Milky Way Galaxy, what is the equivalent output of a typical quasar?

A) only about 1/10 of the Galaxy's output but from a small volume of space
B) 100 times brighter
C) about a million times brighter
D) about the same energy output

Question 52

The surprising observational fact about quasars is that they appear to

A) produce the energy output of greater than 100 galaxies in a volume similar to that of the solar system.
B) be the largest known structures in the universe, although they produce only modest amounts of energy.
C) be associated with ancient supernova explosions.
D) be moving rapidly toward Earth while emitting large amounts of energy.

Question 53

Suppose the X-ray signal from a quasar fluctuates with a period of 3 hours. What is the maximum size of the source?

A) 150,000 km, about the diameter of Jupiter
B) 10.8 au, about the diameter of Jupiter's orbit
C) 4.5 ly, about the distance between the Sun and the nearest star
D) 10,800 ly, a tenth the size of the Milky Way Galaxy

Question 54

What observational fact convinces astronomers that the source of energy in a typical quasar is physically very small?

A) rapid variation in the energy output of the source
B) extreme distance of all quasars
C) starlike appearance of quasars in Earth's sky
D) narrowness of the emission lines in the spectra of quasars

Question 55

Variations in the luminosity of quasars over the span of a few hours are an indication

A) of the rapid rotation of the sources.
B) that these objects are actually binary systems in which one sees mutual eclipses.
C) of the relatively small size of the emitting regions.
D) of obscuring gas and dust clouds moving in front of them from Earth's point of view.

Question 56

What is the relationship between the size of an object and the rate at which its radiant output can vary?

A) Small objects can vary more rapidly.
B) Large objects can vary more rapidly.
C) The relationship depends on the wavelength of the radiant output. Large objects can vary more rapidly at long wavelengths, and smaller objects can vary more rapidly at shorter wavelengths.
D) There is no relationship between size and variation rate.

Question 57

Which of these would NOT necessarily be categorized as "active galaxies"?

A) radio galaxies
B) double-radio sources
C) BL Lacertae objects
D) galaxies with supermassive black holes

Question 58

How can astronomers determine the size of an emission region in a very distant and unresolvable source?

A) by measuring the object's mass and using a reasonable value for the average density for matter to calculate its volume and hence its diameter
B) by using radio interferometry because this technique can resolve far greater detail than optical imaging
C) by measuring brightness variability because an object cannot vary more rapidly than the time taken for light to cross the source
D) by measuring the redshift of its spectrum because the redshift will depend on the source size

Question 59

The fact that quasars can be detected from distances from which even the biggest and most luminous galaxies cannot be seen means that

A) they must be in directions where gravitational focusing by the masses of nearer galaxies makes them visible from Earth.
B) they must be far more luminous than the brightest galaxies.
C) they must be in directions where intergalactic absorption by dark matter is minimum, allowing observers to see them.
D) their spectra have not been as redshifted by their motion as those of galaxies and hence they can still be seen.

Question 60

Why is it that astronomers will NOT see fluctuations in light output in times shorter than about 1 day when they observe an extragalactic source whose diameter is about 1 light-day?

A) Absorption of light by intergalactic matter will smooth out rapid fluctuations within the beam.
B) The light from different parts of the source will be Doppler-shifted by different amounts, allowing observers to see only an average shift.
C) It is inconceivable that a source of this size could vary on such short time scales.
D) Arrival times will be different from different parts of the source, which will smooth out short-term fluctuations.

Question 61

The spectrum of a BL Lacertae object shows

A) strong, highly redshifted emission lines.
B) doubled emission lines split by the Doppler shift from oppositely directed jets of material.
C) absorption lines of highly ionized atoms.
D) A fairly smooth continuum with exceptionally weak spectral lines.

Question 62

A moving electron in a magnetic field in space follows a spiral pattern, emitting what type of radiation as it does so?

A) synchrotron radiation
B) no radiation at all, since it is moving smoothly without acceleration
C) Cherenkov radiation
D) Lyman radiation

Question 63

Which of these designations does NOT refer to the shape of a galaxy?

A) peculiar
B) barred spiral
C) elliptical
D) spiral

Question 64

Which of these objects is NOT classified as an active galaxy?

A) Seyfert galaxy
B) BL Lacertae object
C) quasar
D) barred spiral

Question 65

Which one of these properties do Seyfert galaxies and the Milky Way Galaxy share?

A) bright starlike nucleus
B) spiral galaxy classification
C) significantly varying energy output over a few weeks
D) spectrum containing strong emission lines from elements such as highly ionized iron

Question 66

Seyfert galaxies are a distinct class of galaxies because they

A) are very close to the Milky Way and appear to be gravitationally bound to it.
B) have very bright, very hot starlike central cores with variable energy output.
C) are completely devoid of structure, appearing to be amorphous spheres of gas and dust.
D) are in the constellation of Seyfert in Earth's southern hemisphere sky.

Question 67

Which of these is a characteristic of a Seyfert galaxy?

A) A Seyfert galaxy has no spiral arms.
B) The nucleus of a Seyfert galaxy is unusually bright compared with the spiral arms.
C) The nucleus of a Seyfert galaxy is unusually faint compared with the spiral arms.
D) The spectrum of a Seyfert galaxy shows particularly narrow absorption lines.

Question 68

Seyfert galaxies are

A) elliptical galaxies with extremely bright nuclei.
B) supergiant elliptical galaxies that are periodically disturbed by supernova explosions within them.
C) spiral galaxies with extremely active cores.
D) active galaxies that shine mainly by radiation from two relatively widely spaced radio lobes.

Question 69

BL Lacertae objects appear to be

A) giant irregular galaxies with neither spiral arms nor the smooth shape of elliptical galaxies.
B) elliptical galaxies with bright, starlike nuclei.
C) spiral galaxies with bright, starlike nuclei.
D) active galaxies, most of whose energy is emitted by two widely spaced radio lobes.

Question 70

What is a "peculiar" galaxy?

A) a galaxy with a supermassive black hole at the center
B) an older name for an active galaxy
C) a galaxy that looks like it is exploding
D) an active galaxy with large radiation output at many different wavelengths across the spectrum

Question 71

Seyfert galaxies are

A) spiral galaxies with bright, starlike nuclei and strong emission lines.
B) irregular galaxies with no shape or structure.
C) very small elliptical galaxies.
D) the largest galaxies in the universe.

Question 72

Which one of these objects typically has the greatest luminosity?

A) giant elliptical galaxy
B) Seyfert galaxy
C) radio galaxy
D) quasar

Question 73

The typical timescale for variability of a blazar (a parameter related to true physical size of these objects) is

A) about 1 week.
B) less than 1 day.
C) a few seconds.
D) about 1 year.

Question 74

What mechanism appears to produce the double-radio sources seen in intergalactic space?

A) two oppositely directed jets of matter, ejected from a small source
B) two radio galaxies orbiting each other much like two binary stars
C) radio-bright galaxy with a dark absorbing disk edge-on to Earth, splitting the source into two as seen from Earth
D) two black holes orbiting around a small but massive galactic nucleus

Question 75

An electron moving in a magnetic field in space is forced to move in a spiral pattern. As it does so, it emits

A) characteristic X-rays as the electron is slowed down by this motion.
B) visible light, mostly blue in color because the electron is forced to move in a circular path.
C) synchrotron radiation, mostly radio waves, since the electrons are accelerating.
D) nothing since such electrons are moving with a constant speed and are not therefore accelerating.

Question 76

Observationally, the biggest difference between quasars and other active galaxies such as Seyfert and radio galaxies appears to be that

A) quasars appear to be located inside elliptical galaxies, whereas Seyfert and radio galaxies are spirals.
B) Seyfert and radio galaxies do not have the bright, starlike nuclei of quasars.
C) the brightness of Seyfert and radio galaxies does not vary with time.
D) Seyfert and radio galaxies are less powerful energy emitters than quasars.

Question 77

Synchrotron radiation is produced whenever

A) electrons jump from level to level in an atom.
B) electrons spiral around a magnetic field.
C) atoms in a molecule vibrate back and forth.
D) electrons move in a transparent medium at a speed faster than the speed of light in the medium.

Question 78

Which of these phenomena will produce synchrotron radiation?

A) spiraling high-speed electrons in a magnetic field
B) heating of matter by compression as it spirals into a black hole
C) radioactive decay of an atomic nucleus
D) slowing down of charged particles as they enter a dense medium such as the atmosphere of a star or of Earth

Question 79

A BL Lacertae object is a(n)

A) emission nebula containing a young T Tauri star.
B) rapidly spinning neutron star.
C) eclipsing binary star with a black hole as one component.
D) active galactic nucleus.

Question 80

A spiral galaxy with a bright, starlike nucleus showing strong emission lines is called a

A) quasar.
B) gravitational lens.
C) BL Lacertae object.
D) Seyfert galaxy.