Astronomy Final

central bulge

(or nuclear bulge) the central (round) part of the Milky Way or a similar galaxy

4. What is a biomarker? Give some possible examples of biomarkers we might look for beyond the solar system.

A biomarker is a feature—a chemical substance, a structure, or a signal—that could only have been formed by life. Beyond our solar system, we can only detect planet-scale biosignatures—biological impacts so great that they affect the way a planet appears in reflected or emitted electromagnetic radiation. An example of such an exoplanet biomarker would be unusual atmospheric composition, such as the mutual presence of methane and oxygen. While this would be a strong indication of life, it would not be unequivocal because methane and oxygen can be produced in the absence of life under special circumstances. Another possible example might be very short, very energetic pulses of visible light or infrared radiation or radio waves that are not just natural static, but are coded with information given off by huge structures in space built near or around stars. Both would be biomarkers of technologically advanced civilizations.

9. What is a habitable zone?

A habitable zone is the range of distances from a star where, if water existed on the surface of a planet, that water would likely be liquid.

4. Which formed first: hydrogen nuclei or hydrogen atoms? Explain the sequence of events that led to each.

A standard hydrogen nucleus consists of just a proton. Protons were formed by quark condensation at around 10-6 seconds after the Big Bang. Nucleosynthesis of other isotopes of hydrogen, such as deuterium (one proton and one neutron) and tritium (one proton and two neutrons), could happen when the universe was cool enough for more complex nuclei to form, at around three to four minutes. Hydrogen atoms (which also include an electron) did not form until the universe was about 380,000 years old, when its temperature dropped below about 3000 K. This was when the random motion of electrons became slow enough for them to be electromagnetically captured by protons to form hydrogen atoms.

3. Give a short history of the atoms that are now in your little finger, going back to the beginning of the universe.

All the hydrogen atoms in your little finger have been around since the universe first cooled enough for protons and electrons to get together into atoms. Elements heavier than hydrogen in your finger were fused in stars through a process called nucleosynthesis. Fusion of lighter elements inside the "furnace" of stars creates new heavier elements, which are then dispersed when stars explode or lose material more peacefully. The newly made elements eventually join the clouds of gas and dust between the stars. Out of these, new stars form and further heavier elements are created inside their furnace. Elements up to iron can be formed during normal fusion inside stars, and elements heavier than that are formed during the violence of supernova explosions. The atoms in your little finger were made available in this way to the cloud from which the solar system formed. They then became part of the planetesimals that collided with each other to form the proto-Earth. Then through twists and turns of planetary evolution and life's evolution on our own planet, the atoms found their way to your little finger. But they likely won't be there for long because a human lifetime is but a blink of the eye compared to the age of the universe.

1. What are the basic observations about the universe that any theory of cosmology must explain?

Any cosmological theory must explain the expansion of the universe and the various stages the expansion has gone through (inflation, radiation-dominated, matter-dominated, and the present dark energy-dominated). Another key element of any good model is the cosmological principle: that on the large scale, the universe in any given time is the same everywhere (homogeneous and isotropic). More specific observations that require a cosmological explanation include why there is the background radiation (CMB) filling space all around us and why there is more helium in the universe than stars could have made since the beginning.

14. When comparing two isolated spiral galaxies that have the same apparent brightness, but rotate at different rates, what can you say about their relative luminosity?

Applying the Tully-Fisher method, the faster spinning galaxy should be more massive and thus more luminous.

2. Why did it take so long for the existence of other galaxies to be established?

Astronomers had no method of determining distance to objects so far away. Geometric methods work only for the nearest stars and for a long time (until the cepheid period-luminosity relationship was discovered) there were no reliable methods for finding distances farther away. In order to find individual cepheids in other galaxies, however, we needed a larger telescope, and so distance measurement to other galaxies had to await the building of the 100-in. telescope in the 1910s. Without distances, astronomers were not sure if the fuzzy objects we now call galaxies were inside the Milky Way or not.

10. What is the typical structure we observe in a quasar at radio frequencies?

At radio frequencies, quasars have a very small but extremely luminous center that coincides with the center of its host galaxy. There are often two very narrow but bright jets that extend in opposite directions from the center of the host galaxy and which terminate by spreading out into two large amorphous bright radio lobes.

8. What is the "cosmic haystack problem"? List as many of its components as you can think of.

Because so many factors go into detecting a signal from extraterrestrial intelligence, some astronomers have compared the effort to searching for a needle in a haystack. Some of the problem's components include the origin and direction of the signal containing the message from among all the possible directions one could "listen," the frequency chosen for that signal from among the vast range of potential frequencies in the electromagnetic spectrum, the frequency width of that signal, the strength of that signal compared with background noise, the continuity of that signal (whether it's on all the time, or only sweeps over us periodically), the frequency drift of that signal (caused by the relative motion of the sources to Earth), the system used for encoding any message in that signal, and whether we would recognize the nature of the message, when it was coded by alien minds.

2. Where in the solar system (and beyond) have scientists found evidence of organic molecules?

Beyond our solar system, organic molecules have been found in giant clouds of dust and gas between stars (the "interstellar medium") and in star-forming regions. In our solar system, besides Earth, organic molecules have been discovered on comets, in meteorites, on Saturn's moon Titan, in the plumes of water expelled from Saturn's moon Enceladus, and on Neptune's moon Triton.

5. Describe at least two characteristics of the universe that are explained by the standard Big Bang model.

Characteristics of the universe explained by the standard Big Bang model include its expansion, observation of a cosmic microwave background, and the relative abundances of the very light elements (such as the existence of more helium than stars could have made since the Big Bang).

12. What causes the largest mass-to-light ratio: gas and dust, dark matter, or stars that have burnt out?

Dark matter can create the largest mass-to-light ratios, since it dominates the mass of large galaxies and galaxy clusters without adding to the total luminosity.

1. How are distant (young) galaxies different from the galaxies that we see in the universe today?

Distant galaxies look very different from nearby galaxies such as the Milky Way or Andromeda. Young galaxies do not fit in Hubble's galaxy classification scheme. These distant systems lack identifiable bulges or spiral arms. In addition to being visually different, the distant galaxies are also smaller, fainter, and gas-rich when compared to galaxies in the nearby universe.

5. Why are Mars and Europa the top targets for the study of astrobiology?

Five decades of observation of our neighbor world, Mars, strongly suggest that in the distant past it had an environment (thicker atmosphere, running surface water, perhaps even lakes) that could have sustained life on its surface. Even if such life no longer survives on Mars, its "fossils" might still be found on the red planet. Life could also exist on modern Mars just below the surface, where liquid water is thought to exist. Europa is a top target because of the high likelihood of an extensive salty ocean under the thick ice shell that covers this moon of Jupiter. This ocean, substantially deeper than Earth's ocean, is probably in contact with a rocky seabed and may be warmed by internal heat; thus the interaction of water and rocks could provide a chemical energy source for life.

Can you name five environmental conditions that, in their extremes, microbial life been challenged by and has learned to survive on Earth?

Five environmental conditions that microbial life has overcome are extreme temperature, pressure, salinity, acidity, and radiation.

11. What evidence do we have that the luminous central region of a quasar is small and compact?

Fluctuations in the energy output of a quasar can change over relatively short time periods (a few months to a few years at most). This means that the region from which the changing energy is coming cannot be larger than the distance that light can travel over a few month or a few years. In other words, the region that is fluctuating must be no more than a few light months to a few light years wide. Typical galaxies are tens of thousands to hundreds of thousands of light-years across. The short timescale energy fluctuations suggest that the region of greatest luminosity in a quasar must be much smalcold dark matter slow-moving massive particles, not yet identified, that don't absorb, emit, or reflect light or other electromagnetic radiation, and that make up most of the mass of galaxies and galaxy clusters

7. Why do astronomers believe there must be dark matter that is not in the form of atoms with protons and neutrons?

Galaxies could not have formed as early as they did without dark matter gravitationally attracting ordinary matter and inducing galactic formation. The existence of dark matter is also necessary to explain the long-term stability of both spiral galaxies and galactic clusters. In both cases, we see material in their outer regions moving around their centers too fast for the gravity we deduce from ordinary matter to hold. There must be some other form of material there with gravity. Yet searches for electromagnetic radiation from this additional matter have been fruitless, leading scientists to believe that this "dark matter" does not consist of ordinary particles, such as protons and neutrons.

2. Explain where in a spiral galaxy you would expect to find globular clusters, molecular clouds, and atomic hydrogen.

Globular clusters consist of older population II stars and are typically found in the spherical haloes of spiral galaxies. Clouds of molecular gas and atomic hydrogen are the sites of current or future star formation and are typically found in the disk; they are especially concentrated along the spiral arms.

4. Explain what we mean when we call the universe homogeneous and isotropic. Would you say that the distribution of elephants on Earth is homogeneous and isotropic? Why?

Homogeneous means uniform, and isotropic means it looks the same in all directions. On the largest scales (larger than galaxy superclusters) the universe does appear that way: we see the same sort of pattern no matter where we look. Since elephants are strongly clustered in a few parts of Africa and Asia but mostly absent elsewhere, their distribution on Earth is neither homogeneous nor isotropic.

7. What will be the long-term future of our Galaxy?

In a few billion years, the Milky Way Galaxy and Andromeda galaxy (M31) will merge. During the merger process, great tidal streams of stars will be drawn out of both galaxies. Ultimately, the two spiral galaxies will blend into a single elliptical galaxy surrounded by tidal streams. Students might also mention that as time goes on, and more cycles of star birth and death occur in the Galaxy, there will be a greater and greater enrichment of the heavier elements.

11. Why does the disk of a spiral galaxy appear dark when viewed edge on?

In a spiral galaxy, dust is concentrated in the plane of the disk. When seen edge on, light from the stars must reach us by traveling through the disk, and the dust in the plane of the galaxy absorbs the starlight within the galaxy, making it appear darker.

3. Describe several characteristics that distinguish population I stars from population II stars.

In our Galaxy, population I stars are restricted to the disk. As such, they have circular co-planar orbits of relatively high velocity. Compared to population II stars, they are richer in elements heavier than hydrogen and helium—although population I stars in the outer disk are significantly depleted in these elements. They have a range of ages—from the most recently born to more than 10 billion years. Population II stars occupy the spheroidal component of the Galaxy—and the spheroids of other spiral galaxies. They have orbits that are highly elliptical and with little sense of a common plane. Their overall rotational velocity is very low. Their abundances of elements heavier than hydrogen and helium are much lower than in population I stars—with the exception of the population II stars occupying the galactic bulge. Almost all population II stars are ancient, with ages of 10 to 13 billion years.

9. Thinking about the ideas of space and time in Einstein's general theory of relativity, how do we explain the fact that all galaxies outside our Local Group show a redshift?

In the general relativistic view, the expansion of the universe is a "stretching" of space. As the scale of space increases, waves of light from a distant galaxy, moving through that space, will also be stretched. That is, their wavelengths increase, which is a redshift. The farther away a galaxy is, the more space it travels through, and the greater the increase in its wavelength by the time it arrives.

6. Why is traveling between the stars (by creatures like us) difficult?

Interstellar travel is difficult for many reasons. The first is certainly the vast distances between the stars. Even at speeds very close to the speed of light, the maximum theoretical speed achievable, it would require four years or more to travel between stars. At more realistic speeds, trips would take far longer than a human lifetime. And the faster you go, the more expensive (in fuel costs) the trip would be. Since we can't depend on fuel being available at our destination, such travel would require carrying all the fuel necessary for both the trip there and the return trip and require accelerating all that fuel to tremendous speeds—a truly gargantuan effort, and an extraordinarily expensive one. To be sure, those issues only come up if creatures like us are along on the trip. Travel by machines (such as robots, computers, or smartphones) could proceed much more slowly and less expensively. As this book went to press, a billionaire in Silicon Valley gave $100 million to a project to find technology that could get a very tiny probe to the nearest star using laser propulsion. See: Project Breakthrough Star-shot: https://breakthroughinitiatives.org/News/4

8. What does it mean to say that the universe is expanding? What is expanding? For example, is your astronomy classroom expanding? Is the solar system? Why or why not?

It is space (or more properly space-time) that is expanding. The matter inside the universe is not expanding—gravity holds things together at that local level. The empty space between groups and clusters of galaxy is where expansion can be seen.

11. What are two characteristic properties of life that distinguish it from nonliving things?

Life extracts energy from its environment and has a means of encoding and replicating information in order to make faithful copies of itself.

3. In what ways are active galaxies like quasars but different from normal galaxies?

Like quasars, active galaxies have very luminous central regions that emit prodigious amounts of energy, particularly in the radio region of the spectrum. Both quasars and active galaxies are powered by infalling material that forms an accretion disk around a central supermassive black hole. Both quasars and active galaxies emit jets of mass and energy perpendicular to the plane of the host galaxy's disk.

6. Describe the observations that convinced astronomers that M87 is an active galaxy.

M87 possesses jets that stream from a very luminous and compact center perpendicular to the plane of the galactic disk. In addition, Hubble Space Telescope observations show a disk of hot material circling the central black holes in M87, and Doppler shift measurements indicate that this black hole is 3.5 billion solar masses, about 1000 times more massive than the Milky Way's central supermassive black hole, and more than sufficient to produce the tremendous energy we observe radiating from the galaxy in its radio jets.

3. Explain what the mass-to-light ratio is and why it is smaller in spiral galaxies with regions of star formation than in elliptical galaxies.

Mass-to-light ratio is a comparison of mass (usually in solar masses) to luminosity (usually in solar luminosities). The Sun by definition would be a 1 in mass-to-light ratio. Regions of recent star formation have many new massive stars in addition to the large number of low-mass stars that all galaxies have; thus, their mass to light ratio tends to be lower (in the range of 1-10). Elliptical galaxies tend to be devoid of gas and dust, and are not doing a lot of new star formation. Thus, many high-mass stars have consumed all of their fuel and "burnt out." The result is that the high mass stars no longer contribute to the overall luminosity, so the mass-to-light ratio needs to increase to show this additional mass.

6. What are the two best ways to measure the distance to a distant, isolated spiral galaxy, and how would it be measured?

Method 1: Type Ia supernovae can be used as a standard bulb. First, look for a supernova explosion, and determine what kind of supernova it was. If it is a type Ia, it will reach the same peak luminosity as other type Ia's. Compare that peak luminosity with the apparent brightness of the supernova at maximum to determine the distance. Method 2: The rotation rate of the spiral galaxy can be used to determine the distance using the Tully-Fisher relation. Take a spectrum of the galaxy. The line widths of the 21-cm line can then be used to determine the rotation rate of the galaxy.

5. What are the two best ways to measure the distance to a nearby spiral galaxy, and how would it be measured?

Method 1: Use the period-luminosity relationship for cepheid variable stars. First, look for a star that varies at a rate consistent with cepheids, then use the period to determine the luminosity of the star. Finally, compare the luminosity with the apparent brightness to determine the distance. Method 2: Type Ia supernovae can be used as a standard bulb. First, look for a supernova explosion and determine what kind of supernova it was. If it is a type Ia, it will reach the same peak luminosity as other type Ia's. Compare that peak luminosity with the apparent brightness of the supernova at maximum to determine the distance.

9. Was Hubble's original estimate of the distance to the Andromeda galaxy correct? Explain.

No, although this was not known in the early 1920s, there are really two kinds of cepheid-like variable stars, and Hubble was using the other kind to estimate the distance to Andromeda. The distance estimate increased by more than a factor of two once it was corrected for this.

16. Is the Hubble constant actually constant?

No, astronomers understand that if the universe is decelerating (because of gravity) or accelerating, then the Hubble "constant" actually changes with time. Observations suggest that Hubble's constant has increased over time, meaning that the universe's expansion is accelerating.

10. Does an elliptical galaxy rotate like a spiral galaxy? Explain.

No, in an elliptical, the stellar motions are randomized and do not travel systemically or in any predominant direction. We can only measure the variations of the motion of the stars within an elliptical galaxy.

15. If all distant galaxies are expanding away from us, does this mean we're at the center of the universe?

No, you can show that if the expansion follows a simple proportional relationship (Hubble's law), then all points in space within the expanding universe could make the same observation and claim to be the center.

9. What do we now understand to be the primary difference between normal galaxies and active galaxies?

Normal galaxies have very little material feeding their central supermassive black holes and thus very little activity near or from their centers. Active galaxies have gas and dust falling into an accretion disk around their central supermassive black holes. These days, we understand that the material to make the core of a galaxy active comes from a galaxy that collided with the one that became active.

1. Describe some differences between quasars and normal galaxies.

Normal galaxies host anywhere from thousands of stars in the smallest galaxies to trillions of stars in the largest ones. Quasars appear to be single, solitary energy sources. Normal galaxies are observed throughout the universe, while quasars are generally found only at very large distances with very high redshifts. Luminous quasars are typically 100 times brighter than a large galaxy. Large galaxies can be hundreds of thousands of light-years in diameter, while quasars are typically much less than 1 light-year in diameter. Normal galaxies do not emit much energy at radio frequencies, while many quasars are extremely luminous at radio frequencies.

8. How does the presence of an active galactic nucleus in a starburst galaxy affect the starburst process?

Once an active galactic nucleus forms, its great jets and outward flowing particles disrupt the denser clouds of gas and dust where star formation happens. This slows down or stops the great burst of star formation and inhibits the formation of many new stars in the galaxy.

4. Briefly describe the main parts of our Galaxy.

Our Milky Way Galaxy contains a barred bulge; a thin disk of stars, gas, and dust with concentrations in spiral arms; a much less substantial thick disk of stars; and a spheroidal halo of ancient stars and globular star clusters. Deep within the central bulge dwells a supermassive black hole. In addition, there appear to be considerable amounts of unknown dark matter surrounding the Galaxy.

10. Why is the simultaneous detection of methane and oxygen in an atmosphere a good indication of the existence of a biosphere on that planet?

Oxygen and methane chemically react with each other, so we would not see them together unless there are active sources for both. At least on Earth, biology is responsible for essentially all the oxygen and the majority of the methane in our atmosphere.

8. Why were quasars and active galaxies not initially recognized as being "special" in some way?

Prior to the advent of radio telescopes, quasars were generally thought to be stars (after all, they looked like points of light on a photograph). Most active galaxies did not look significantly different from normal galaxies at visible wavelengths. When radio telescopes came onto the scene, astronomers detected much more activity in quasars than they did from stars, and much more activity from active galaxies than normal galaxies. The surprising "radio loud" objects invited more intense scrutiny, including analyses of their spectra and observations in many other regions of the electromagnetic spectrum. These observations then began to show astronomers how different these objects actually were.

7. Why do astronomers believe that quasars represent an early stage in the evolution of galaxies?

Quasars and their host galaxies all have very large redshifts, indicating that they are at least 1 billion light-years away, and often much farther. When we recall that light travels at a finite speed, we see that looking out in space is the same thing as looking back in time. Because all the quasars we see are so far away, the light we see from them today must have left the quasar billions of years ago, back when the universe was much younger than it is today. Thus, we are seeing galaxies in an early stage of their formation. The number of quasars increases as we look further back in time and was greatest when the universe was about 20% of its current age.

5. Describe the process by which the action of a black hole can explain the energy radiated by quasars.

Quasars are galaxies that contain a supermassive black hole, surrounded by an accretion disk in which material is heated to extreme temperatures. Because the gravity of the black hole is very strong near its event horizon, material in the accretion disk is falling in and releasing a huge amount of gravitational energy as it falls. It is this energy that powers the glowing accretion disk and explains the luminosity of quasars. Quasars represent an early stage in the development of galaxies,when vastly more material, mostly interstellar gas and dust, fell into the central supermassive black hole during the galaxy's accretion stage. Quasars can also be "reborn" when another galaxy collides with the quasar host, and its material provides new fuel for the quasar accretion disk.

2. Describe the arguments supporting the idea that quasars are at the distances indicated by their redshifts.

Quasars have extremely large redshifts, indicating that they are receding from us at large fractions of the speed of light. Any objects moving this rapidly from a nearby galaxy would easily achieve escape speeds from even the largest host galaxies. Hubble Space Telescope observations have shown that quasars sit in the middle of host galaxies, and the host galaxies have the same redshifts as their quasars, confirming that quasars obey Hubble's law and their high redshifts are due to their distance.

7. What are the advantages to using radio waves for communication between civilizations that live around different stars? List as many as you can.

Radio waves travel at the speed of light, are cheap to produce (they are the lowest-energy electromagnetic waves), are not significantly absorbed by interstellar clouds, go right through planetary atmospheres, and, most importantly, can be modulated in a way that carries information.

3. Describe the evolution of an elliptical galaxy. How does the evolution of a spiral galaxy differ from that of an elliptical?

Some giant ellipticals probably formed in a dramatic gravitational collapse of huge isolated clouds of gas and dark matter. But most ellipticals seem to have formed from the "bottom up": the assembly by gravity of small pieces of galaxies that had already formed their stars. Star formation in most ellipticals essentially stopped when the universe was less than half its current age. The bulges of spirals may have formed very similarly, but the disks formed over a more protracted period, and their growth included fresh gas from small galaxies and streams of gas flowing under the influence of gravity. The gas was able to lose energy and settle into a disk and also to fuel star formation that continues today in spiral galaxies.

​1. Describe the main distinguishing features of spiral, elliptical, and irregular galaxies.

Spiral galaxies have a disk, spiral arms, and a central bulge. Elliptical galaxies appear as only a bulge—they do not have any disk or spiral arm structure. Irregular galaxies do not fit into either of the other categories and don't have well-defined or clear structure.

11. Describe the anthropic principle. What are some properties of the universe that make it "ready" to have life forms like you in it?

The "anthropic principle" is the idea that the physical laws that we observe must be what they are precisely because these are the only physical laws that allow for the existence of humans. Properties of the universe that make it "ready" to have life forms like us include: (a) a 1-part-in-105 mass-energy fluctuation in the early universe that allowed formation of galaxies like ours (which include regions for solar systems with sufficiently low intensity of X-rays and gamma rays), (b) a balance between the forces of expansion and contraction for the universe (so it didn't expand or collapse too fast), resulting from a mass-energy density very close (or equal) to critical density, (c) a very slight initial excess of matter that survived matter-antimatter annihilation, (d) nuclear fusion reactions at rates that produce long-lasting stars, (e) the strength of gravity not being much stronger (so that stars form with smaller masses and live too short a time), and (f) the structure of atomic nuclei providing sufficient production of carbon nuclei in stars via fusion of three helium nuclei.

10. Astronomers have found that there is more helium in the universe than stars could have made in the 13.8 billion years that the universe has been in existence. How does the Big Bang scenario solve this problem?

The Big Bang scenario begins with a universe that is very hot and full of energy. At the temperatures and densities that existed between 3 and 4 minutes after the beginning, conditions (temperature and density) in the entire universe were right for protons and deuterium nuclei to fuse into helium. In essence, the entire universe was acting the way centers of main-sequence stars do now. After 4 minutes, the universe had expanded so much that temperatures were not sufficient for the fusion of helium. This explains the extra helium we see in the universe today and why there is neither less nor more of it.

1. What is the Copernican principle? Make a list of scientific discoveries that confirm it.

The Copernican principle is the idea that Earth and the Sun are in no way specially favored bodies in the universe. Several discoveries confirm this, including (in order of discovery) the following: Earth orbits the Sun and is not the center of our solar system, our Sun is one among billions of other stars in the Milky Way Galaxy and is not in any central position within the Galaxy, our Galaxy is one among billions of other galaxies in the universe, and planets are commonly found orbiting other stars. You could also discuss that the elements that make up most of Earth and the Sun are commonly found in other stars and other planets.

3. What does the term Hubble time mean in cosmology, and what is the current best calculation for the Hubble time?

The Hubble time is the age of the universe, estimated from calculating the Hubble constant and then taking its reciprocal (1/H). The current estimate for the Hubble time is about 14 to 15 billion years.

​1. Explain why we see the Milky Way as a faint band of light stretching across the sky.

The Milky Way in the sky is our particular view of the inward part of the Milky Way Galaxy as seen from our location within the Galaxy's disk. Since we are part of the disk, we see a band of diffuse light that completely encircles us.

5. Describe the organization of galaxies into groupings, from the Local Group to superclusters.

The Milky Way is one of three spiral galaxies (with the Andromeda galaxy and M33) in the Local Group. The Local Group is part of the Virgo supercluster, which is centered on the massive Virgo cluster of galaxies. On even larger scales, clusters and superclusters of galaxies are distributed on sheets and filaments like beads on a string spanning hundreds of millions of light-years.

6. What is the evidence that a large fraction of the matter in the universe is invisible?

The existence of dark matter is supported by three main pieces of evidence. (1) Stars and clusters orbit the centers of their host galaxies faster than they would if only visible matter (stars, gas, dust, planets) made up most of the mass. (2) Galaxies in clusters likewise move much faster than can be explained by the gravity of only luminous matter. (3) Galaxy clusters emit copious X-rays best explained by fast motion of gas particles under the influence of gravity much stronger than just the luminous matter can supply.

6. Explain why the abundances of heavy elements in stars correlate with their positions in the Galaxy.

The lowest abundances of elements heavier than hydrogen and helium are found in stars belonging to the galactic halo. These stars are also known to be very ancient—formed before a lot of supernovae had a chance to explode and enrich the interstellar medium with heavier elements. Much higher abundances are found in the younger disc component of the Galaxy, which is explained by the disk having been formed after the halo. There prior generations of stars have had time to seed the disk's interstellar medium with heavy elements. Stars and gas and dust were most concentrated in the inner bulge of the Galaxy. There more star formation occurred and enriched the interstellar medium there to a greater extent.

5. Describe the evidence indicating that a black hole may be at the center of our Galaxy.

The most compelling evidence consists of recorded stellar tracks within 1 arcsecond (0.13 light-years) of the galactic center, whose orbital periods and radii indicate the presence of a central source of gravity having a mass equivalent to more than 4 million Suns, yet being concentrated within a radius less than 17 light-hours. Other evidence includes unique radio and X-ray emissions from the galactic center.

8. What is dark energy and what evidence do astronomers have that it is an important component of the universe?

The notion of dark energy was suggested to help explain measurements, using Type Ia supernovae as distance indicators, that the expansion of the universe is speeding up. Such acceleration requires a source of energy. Scientists don't yet fully understand what dark energy is. It may be a new form of energy for which there is not yet a theoretical explanation. Alternately, it may be the vacuum energy associated with "empty" space itself, as predicted by quantum mechanics.

12. Describe the evidence that the expansion of the universe is accelerating.

The only direct evidence of acceleration comes from supernovae (as described in the chapter), although other evidence fits the standard model we have described in the book that includes dark energy. In order to determine whether the expansion is accelerating, it is necessary to measure the rate of expansion at different distances, which is equivalent to making measurements at different times in the history of the universe. We have only one "standard bulb" that allows us to measure large enough distances to perform this experiment—the supernovae produced when white dwarfs in binary systems acquire too much mass and explode. The observations show that distant supernovae are fainter than would be expected if the universe were expanding at a constant rate. This means that when we detect the light from supernovae, we are farther away from them than we would be if the expansion rate were constant. The only way that can happen is if the rate at which we are moving away from the supernovae has sped up since the time the light left them.

2. Describe some possible futures for the universe that scientists have come up with. What property of the universe determines which of these possibilities is the correct one?

The possible futures for the universe will be determined by the fate of its expansion. In two models for the universe, the rate of expansion slows down. In one of these, expansion comes to a stop and reverses, with the universe ending in a "big crunch"—the implosion of matter, energy, space, and time. For this model, the mass-energy density r is greater than the critical density (which is presently rcrit = 9.6 × 10-27 kg/m3). In the second model, the universe continues to expand forever, but ever more slowly, coming to a stop only after infinite time. For this model, the mass-energy density is exactly equal to the critical density. In a third model, the universe expands forever at a constant rate (given by a Hubble constant that does not vary with time). This would occur in an empty universe. In a fourth model, the universe accelerates at a faster and faster rate forever. For this model, the mass-energy density is less than the critical energy density for a given amount of dark energy. Current mass-energy density measurements indicate that the universe will expand forever.

What are the three requirements that scientists believe an environment needs to supply life with in order to be considered habitable?

The requirements are a solvent (water may be the best example), the biogenic elements (CHNOPS) in biologically accessible form, and energy.

6. Describe two properties of the universe that are not explained by the standard Big Bang model (without inflation). How does inflation explain these two properties?

The standard Big Bang model without inflation does not explain why the mass-energy density of the universe would be equal to the critical density, nor does it explain the amazing uniformity of the universe. However, both of these features can be explained when an inflationary stage is added to the standard Big Bang model. A Big Bang model with a rapid, early expansion stage (inflation) is identical to the standard Big Bang model after 10-30 s, but it is significantly different prior. In an inflationary model, within the first 10-30 s, the universe expanded by a factor of about 1050 times that predicted by standard Big Bang. Such an expansion over a very short time drives any initial mass-energy density to the critical density and also produces the scale of uniformity we observe.

4. If we now realize dwarf ellipticals are the most common type of galaxy, why did they escape our notice for so long?

They are extremely dim and small. This makes them difficult to observe and identify.

4. Why could the concentration of matter at the center of an active galaxy like M87 not be made of stars?

To explain the large masses in the centers of galaxies that we observe, there would have to be at least a million stars in a region about the size of the solar system. This requires that they would have be only two star-diameters apart, meaning there would be constant collisions between stars. Those stars would then merge, and soon, the resulting giant star or stars would collapse into a black hole.

7. When astronomers make maps of the structure of the universe on the largest scales, how do they find the superclusters of galaxies to be arranged?

To the surprise of astronomers, they found the superclusters to be arranged in filaments and sheets surrounding emptier regions that are now called voids. The layout reminds them of good Swiss cheese, where the walls of cheese surround large empty regions.

13. What is the most useful standard bulb method for determining distances to galaxies?

Type Ia supernovae; cepheid variable stars are limited by distance (since individual stars are hard to make out once a galaxy gets too far away). Type Ia supernovae, on the other hand, are very luminous, and can be seen at much greater distances.

2. What is the evidence that star formation began when the universe was only a few hundred million years old?

Using powerful telescopes, we observe galaxies of stars so far away that they must have formed more than 13 billion years ago. Even these early galaxies show evidence of heavier elements in their spectra, indicating that massive stars must have gone through their lives and exploded as supernovae. We also directly observe stars today, such as in the Milky Way's globular clusters, that apparently have ages of more than 13 billion years.

7. Why is Hubble's law considered one of the most important discoveries in the history of astronomy?

You could answer this several ways. Hubble's law allows us to estimate the distance for galaxies that are too far away to see individual cepheids. Hubble's law describes the expansion of the universe. It validates solutions to the equations of general relativity in which the universe is in motion (rather than static, as Einstein had fudged it to be).

merger

a collision between galaxies (of roughly comparable size) that combine to form a single new structure

organic molecule

a combination of carbon and other atoms—primarily hydrogen, oxygen, nitrogen, phosphorus, and sulfur—some of which serve as the basis for our biochemistry

photosynthesis

a complex sequence of chemical reactions through which some living things can use sunlight to manufacture products that store energy (such as carbohydrates), releasing oxygen as one by-product

organic compound

a compound containing carbon, especially a complex carbon compound; not necessarily produced by life

Hubble constant

a constant of proportionality in the law relating the velocities of remote galaxies to their distances

spiral galaxy

a flattened, rotating galaxy with pinwheel-like arms of interstellar material and young stars, winding out from its central bulge

deuterium

a form of hydrogen in which the nucleus of each atom consists of one proton and one neutron

Drake equation

a formula for estimating the number of intelligent, technological civilizations in our Galaxy, first suggested by Frank Drake

starburst

a galaxy or merger of multiple galaxies that turns gas into stars much faster than usual

irregular galaxy

a galaxy without any clear symmetry or pattern; neither a spiral nor an elliptical galaxy

protein

a key biological molecule that provides the structure and function of the body's tissues and organs, and essentially carries out the chemical work of the cell

supercluster

a large region of space (more than 100 million light-years across) where groups and clusters of galaxies are more concentrated; a cluster of clusters of galaxies

open universe

a model in which the density of the universe is not high enough to bring the expansion of the universe to a halt

closed universe

a model in which the universe expands from a Big Bang, stops, and then contracts to a big crunch

flat universe

a model of the universe that has a critical density and in which the geometry of the universe is flat, like a sheet of paper

RNA (ribonucleic acid)

a molecule that aids in the flow of genetic information from DNA to proteins

DNA (deoxyribonucleic acid)

a molecule that stores information about how to replicate a cell and its chemical and structural components

galactic cannibalism

a process by which a larger galaxy strips material from or completely swallows a smaller one

void

a region between clusters and superclusters of galaxies that appears relatively empty of galaxies

Hubble's law

a rule that the radial velocities of remove galaxies are proportional to their distances from us

Local Group

a small cluster of galaxies to which our Galaxy belongs

spiral arm

a spiral-shaped region, characterized by relatively dense interstellar material and young stars, that is observed in the disks of spiral galaxies

population I star

a star containing heavy elements; typically young and found in the disk

population II star

a star with very low abundance of heavy elements; found throughout the Galaxy

type Ia supernova

a supernova formed by the explosion of a white dwarf in a binary system and reach a luminosity of about 4.5 × 109LSun; can be used to determine distances to galaxies on a large scale

inflationary universe

a theory of cosmology in which the universe is assumed to have undergone a phase of very rapid expansion when the universe was about 10-35 second old; after this period of rapid expansion, the standard Big Bang and inflationary models are identical

dark energy

an energy that is causing the expansion of the universe to accelerate; the source of this energy is not yet understood

habitable environment

an environment capable of hosting life

quasar

an object of very high redshift that looks like a star but is extragalactic and highly luminous; also called a quasi-stellar object, or QSO

extremophile

an organism (usually a microbe) that tolerates or even thrives under conditions that most of the life around us would consider hostile, such as very high or low temperature or acidity

thermophile

an organism that can tolerate high temperatures

weakly interacting massive particles (WIMPs)

are one of the candidates for the composition of dark matter

evolution (of galaxies)

changes in individual galaxies over cosmic time, inferred by observing snapshots of many different galaxies at different times in their lives changes in individual galaxies over cosmic time, inferred by observing snapshots of many different galaxies at different times in their lives

biomarker

evidence of the presence of life, especially a global indication of life on a planet that could be detected remotely (such as an unusual atmospheric composition)

​active galactic nuclei (AGN)

galaxies that are almost as luminous as quasars and share many of their properties, although to a less spectacular degree; abnormal amounts of energy are produced in their centers

active galaxies

galaxies that house active galactic nuclei

homogeneous

having a consistent and even distribution of matter that is the same everywhere

critical density

in cosmology, the density that is just sufficient to bring the expansion of the universe to a stop after infinite time

hot dark matter

massive particles, not yet identified, that don't absorb, emit, or reflect light or other electromagnetic radiation, and that make up most of the mass of galaxies and galaxy clusters; hot dark matter is faster-moving material than cold dark matter

cosmic microwave background (CMB)

microwave radiation coming from all directions that is the redshifted afterglow of the Big Bang

dark matter

nonluminous mass, whose presence can be inferred only because of its gravitational influence on luminous matter; the composition of the dark matter is not known

dark matter

nonluminous material, whose nature we don't yet understand, but whose presence can be inferred because of its gravitational influence on luminous matter

amino acids

organic compounds that are the molecular building blocks of proteins

grand unified theories (GUTs)

physical theories that attempt to describe the four forces of nature as different manifestations of a single force

stromatolite

solid, layered rock formations that are thought to be the fossils of oxygen-producing photosynthetic bacteria in rocks that are 3.5 billion years old

cosmological principle

the assumption that, on the large scale, the universe at any given time is the same everywhere—isotropic and homogeneous

Milky Way Galaxy

the band of light encircling the sky, which is due to the many stars and diffuse nebulae lying near the plane of the Milky Way Galaxy

gene

the basic functional unit that carries the genetic (hereditary) material contained in a cell

fusion

the building of heavier atomic nuclei from lighter ones

dark energy

the energy that is causing the expansion of the universe to accelerate; its existence is inferred from observations of distant supernovae

differential galactic rotation

the idea that different parts of the Galaxy turn at different rates, since the parts of the Galaxy follow Kepler's third law: more distant objects take longer to complete one full orbit around the center of the Galaxy

dark matter halo

the mass in the Milky Way that extends well beyond the boundary of the luminous stars to a distance of at least 200,000 light-years from the center of the Galaxy; although we deduce its existence from its gravity, the composition of this matter remains a mystery

astrobiology

the multidisciplinary study of life in the universe: its origin, evolution, distribution, and fate; similar terms are exobiology and bioastronomy

supermassive black hole

the object in the center of most large galaxies that is so massive and compact that light cannot escape from it; the Milky Way's supermassive black hole contains 4.6 millions of Suns' worth of mass

halo

the outermost extent of our Galaxy (or another galaxy), containing a sparse distribution of stars and globular clusters in a more or less spherical distribution

mass-to-light ratio

the ratio of the total mass of a galaxy to its total luminosity, usually expressed in units of solar mass and solar luminosity; the mass-to-light ratio gives a rough indication of the types of stars contained within a galaxy and whether or not substantial quantities of dark matter are present

habitable zone

the region around a star in which liquid water could exist on the surface of terrestrial-sized planets, hence the most probable place to look for life in a star's planetary system

isotropic

the same in all directions

SETI

the search for extraterrestrial intelligence; usually applied to searches for radio signals from other civilizations

multiverse

the speculative idea that our universe is just one of many universes, each with its own set of physical laws

cosmology

the study of the organization and evolution of the universe

cosmological constant

the term in the equations of general relativity that represents a repulsive force in the universe

Big Bang

the theory of cosmology in which the expansion of the universe began with a primeval explosion (of space, time, matter, and energy)

lithium

the third element in the periodic table; lithium nuclei with three protons and four neutrons were manufactured during the first few minutes of the expansion of the universe

redshift

when lines in the spectra are displaced toward longer wavelengths (toward the red end of the visible spectrum)

photon decoupling time

when radiation began to stream freely through the universe without interacting with matter

elliptical galaxy

​a galaxy whose shape is an ellipse and that contains no conspicuous interstellar material

anthropic principle

​idea that physical laws must be the way they are because otherwise we could not be here to measure them