6. Stars Transformed
In the preceding chapter we examined the ways in which an aging
Sun might be rejuvenated and how the timescale over which life
might be supported within the Solar System can be prolonged.
In this chapter the focus will shift toward the broader question,
which asks whether any galactic civilizations might have managed
to alter their parent stars into a long-lived state.
Revisiting Carter
Physicist Brandon Carter has argued (recall Chapter 1) that the
most likely time for an intelligent species to emerge is that corresponding
to TMS, the main-sequence lifetime of the planetary
system’s parent star. In this sense most intelligent species are
likely to appear, if indeed they appear at all, at a time close
to that in which the habitability zone is about to dramatically
shift outward in response to the parent star becoming a red-giant.
Stars with planets that have ages close to their main-sequence
lifetime limits, therefore, are perhaps the most likely candidates
for which the effects of rejuvenation might be evident. Indeed, in
these systems the imperative to rejuvenate the parent star is at its
highest for any late-emerging intelligent civilization.
An Exoplanet Review
As of this writing, astronomers have detected 252 extrasolar
planets in orbit around 204 stars,1 and the numbers keep growing
every month! Age estimates have been made for a good number of
the parent stars12 to exoplanetary systems and the age, expressed
as a fraction of the main-sequence lifetime, for 123 of these stars
is shown in Figure 6.1. The number of stars in each bin is not
181
182 Rejuvenating the Sun and Avoiding Other Global Catastrophes
0
2
4
6
8
10
12
14
16
18
20
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5
Age / Main sequence lifetime
Orbit reorganization
Dyson sphere
construction
Star-engineering
Interstellar
migration
Systems
‘sterilized’
Sun
Figure 6.1. Age distribution of stars (expressed as a fraction of the mainsequence
lifetime) known to support planetary systems. Superimposed on
the diagram are the approximate time windows when planetary, as well as
star-engineering and interstellar migration, might be initiated.
of specific relevance in this study, other than it being non-zero,
but it can be seen that a complete main-sequence age spread has
been sampled from 0.0 < Age/TMS < 1.6. About 56 percent of the
planetary systems with published age estimates are associated with
stars that are more than halfway through their main-sequence
lifetime. There are 17 planetary systems in orbit around stars that
have completed between 75 and 90 percent of their main-sequence
lifetimes, and six systems that orbit stars that are within 1 percent
of completing their main-sequence lifetime limit. The properties
of the latter six systems are shown in Table 6.1.
Superimposed on Figure 6.1 are a set of suggested time
windows when a civilization might wish to engage in the redesign
of its planetary system. Clearly, we are many generations away
from the initiation of any truly large-scale space engineering
projects in our Solar System, and perhaps some 5 million years
away from the initiation of any such endeavors. At this advanced
time our Sun will be halfway through its main sequence. On
this basis, the first possible appearance of engineering-related
phenomena (i.e., Dyson spheres, or partial stellar wraps) in relation
to planet-supporting systems might be seen when Age/TMS ∼0.5.
As outlined in Chapter 4, if orbit manipulation or star-engineering
Stars Transformed 183
Table 6.1. Properties of the planetary systems with parent stars within
1 percent of their main-sequence lifetime limit. The last column refers
to the study by Brian Jones and co-workers who looked at the possibility
of stable orbits existing within each system’s habitability zone (HZ). A
‘Yes’ indicates that stable orbits are possible; ‘No’ indicates that no stable
planetary orbits are possible.
Star
Mass
(M
Age/
Tms
MPlanet/
MJupiter
(a, e)
of planet’s
orbit Habitability
HD4308 0.83 0.98 0.05 (0.1, 0.0) Yes
HD99109 0.93 0.98 0.5 (1.1, 0.09) ?
HD190360 0.96 1.07 1.5 (3.92, 0.36) Yes
0.06 (0.13, 0.01)
HD213240 1.22 0.93 4.5 (2.03, 0.45) No
HD216435 1.25 1.03 1.2 (2.6, 0.14) No
70 Virginis 1.1 0.94 7.4 (0.48, 0.4) Yes
options are going to be initiated then, in the case of our Solar
System, these must start within the next 1 to 1.5 billion years
(i.e., when the Sun has completed about 60 percent of its canonical
main-sequence lifetime). If no large-scale engineering takes place
within a planetary system that has reached the limit Age/TMS ∼1,
interstellar migration might be initiated. And, again, if no system
‘redesign’ is initiated within a life-supporting planetary system,
then once Age/TMS > 1 for the parent star, the previously nurturing
habitability zone will be sterilized and all life (if it evolved) will
be killed off.
The fact that there are some stars with Age/TMS > 1 in
Figure 6.1 underscores the point made in Chapter 1 and Chapter 4,
that parent star aging is a problem that must have already been
faced by some galactic civilizations (although it must be emphasized
that we do not know for certain if any of the systems listed in
table 6.1 host planets capable of supporting advanced life forms).
Brian Jones and co-workers3 at the Open University in the UK have
numerically investigated the possibility that Earth-like planets
might exist in stable orbits within the habitability zones of most of
the known planetary systems. In many cases they find that stable
orbits can exist and, for the six systems listed in Table 6.1 (see
last column), three might potentially support habitable planets.
In addition, 26 of the 152 exoplanetary systems studied by Jones
and co-workers with Age/TMS > 0.65 allow for stable orbits to exist
184 Rejuvenating the Sun and Avoiding Other Global Catastrophes
Table 6.2. Exoplanetary systems that are known to have circumstellar dust
disks. The disk extent data (fifth column) is taken from Trilling and coworkers.4
The last column refers to the possibility of stable planetary orbits
within the system habitability zone, as deduced by Jones and co-workers.3
Star Mass (M Age/Tms N(planets) Disk extent (AU) Habitability
55 Cancri 1.03 0.6 4 ∼20 to 70 Yes
CrB 0.95 0.6 1 ∼30 to 80 Yes
HD210277 0.99 0.7 1 ∼40 to 100 No
within their habitability zones. Time and improved ground-based
observations will tell, of course, if there are any Earth-like planets
within the habitability zones of the systems identified, but the
possibility is certainly intriguing.
A survey of the properties of known exoplanetary systems
having parent stars with Age/TMS > 0.6 reveals that 55 Cancri,
Corona Borealis, and HD 210277 have associated circumstellar
disks.4 Table 6.2 presents a summary of these three systems.
The presence of disks around these stars is certainly interesting
for a number of reasons, not least from the point of view that
they might be an indication of systems undergoing ‘redesign’ or
star-engineering. It is not our intent to say that the exoplanetary
systems with disks must support advanced civilizations in
the process of large-scale engineering, but that the available data
supports the possibility of them hosting ancient civilizations.
The fifth column in Table 6.2 indicates that the observed dust
disks appear to begin at distances of between 20 to 40 AU from the
parent stars. Within our Solar System the inner edge of the Kuiper
Belt is located at about 40 to 45 AU from the Sun, so by analogy
the exoplanets sporting dust disks apparently have Kuiper Belt-like
regions composed of large ice/silicate objects of their own.
The Case of the Blue Stragglers
Sounding something like an adventure from the archives of
Sherlock Holmes, the blue stragglers have long been a mystery
to astronomers. They are oddities in that they have apparently
evolved in a different way to their similar-aged and similar-mass
companions.
Stars Transformed 185
The blue stragglers stand out from their neighbors as a consequence
of the gregarious nature of stars. Rather than producing
isolated stars – one at a time, here and there – nature shows
a distinct preference for producing stars in relatively compact
clusters. Indeed, dotted throughout the plane of our Milky Way
galaxy, there are numerous open clusters5 containing between
a few hundred to several thousand stars. With characteristic
dimensions of just a few parsecs across, galactic clusters provide
astronomers with a very useful test bed for stellar evolution
models. Since all the stars in a given cluster formed at the same
time out of material with the same composition, and because they
are all at essentially the same distance away from us,6 then any
observed variations in luminosity and temperature, from one star
to the next, must be solely due to differences in their mass.
When a cluster Hertzsprung-Russell (HR) diagram is
constructed, the lower mass stars will be located on the mainsequence,
since the main-sequence lifetime of a star decreases with
increasing stellar mass, while the more massive stars will have
evolved to become red-giants. As the age of the cluster increases,
so the mass of a star that can remain on the main-sequence (that
is, not having evolved into a red-giant) decreases. Given the rapid
evolution toward lower surface temperatures with the exhaustion
of hydrogen within the core of a star, the cluster main-sequence
develops a distinct turn-off point beyond which no stars will fall
on the main-sequence. As the cluster ages, the luminosity and
temperature of the turn-off point in the HR diagram decrease. The
oddity of the blue stragglers is that they are cluster members found
in the vicinity of the main-sequence, but beyond the turn-off point
determined by their companion stars. Figure 6.2 shows the blue
straggler region in a schematic cluster HR diagram.
With reference to Figure 6.2, the blue part of blue straggler
refers to the fact that the stars are typically hotter than the mainsequence
turn-off point, while the straggler part refers to the fact
that other stars of the same mass, formed at the same time, have
evolved into red-giants. Therefore, the question that follows is,
why? What is it about blue stragglers that have made them evolve
differently?
186 Rejuvenating the Sun and Avoiding Other Global Catastrophes
Main
sequence
MS Turn-off
Giant
branch
⇐ Temperature
⇑
Luminosity
MS continuation
Blue stragglers
Figure 6.2. Schematic HR diagram for an old cluster of stars containing
blue stragglers. The normally evolved stars delineate the main-sequence
and the giant branch. The region where the blue stragglers are found is a
continuation of the main-sequence, but beyond the main-sequence turnoff
for the cluster. Some blue stragglers are also found to have higher
temperatures than similar-mass stars located on the main-sequence. The
observed location of blue stragglers in the HR diagram compares favorably
with the predicted temperatures and luminosities expected for fully mixed
stellar models as shown in Figure 5.2
Over the last 50 years a number of distinct formation
scenarios have been developed for the blue stragglers. Among the
suggested origins are the following:
1. They are stars that formed much later than their companions in the
cluster.
2. They are young stars captured by the cluster.
3. They are stars that have accreted additional material from the
surrounding interstellar medium, thereby increasing their mass,
surface temperature, and luminosity, and have accordingly moved up
the main-sequence.
4. They are coalesced stars produced by two stars actually colliding.
5. They are the result of a mass exchange between the two stars in a
binary system.
6. They are stars that have undergone quasi-homogenous evolution.
7. They are stars containing a centrally accreting black hole.
Stars Transformed 187
Of the scenarios listed above the first three and last two seem
to be the least popular among modern astronomers, at least as
a general explanation, although there are no physical reasons to
suppose that these scenarios could never apply. Scenarios 4 and 5
have been studied extensively in recent years and certainly appear
capable of producing stars that qualify as blue stragglers. Scenarios
6 and 7 might, in some sense, be viewed as nature’s version of the
engineered stars that our descendants and advanced extraterrestrial
civilizations might try to produce.7 Craig Wheeler (University of
Texas, Austin) introduced the term quasi-homogeneous evolution8
to describe the partial extra mixing that was being invoked to
produce the straggling effect. Full mixing of a star and red-giant
phase elimination was not something that Wheeler was specifically
trying to produce in his blue straggler models. In addition, as
illustrated in Figure 5.1, a fully mixed star will eventually evolve
‘blueward’ to surface temperatures much hotter than that of the
main-sequence. Most, but not all, blue stragglers have surface
temperatures consistent with those expected for ordinary mainsequence
stars.
There are a number of ways in which nature can apparently
produce blue stragglers, with the cluster environment typically
dictating which scenario operates most efficiently. Stars are more
likely to collide and coalesce to produce blue stragglers in more
densely populated clusters, for example, than sparsely populated
ones. This being said, a star that has been engineered into a
homogeneous, long-lived state (as described in Chapter 5) would
also take on many of the characteristics of a blue straggler. In
particular, a homogeneous star evolving with mass-loss such that
the ratio L/d2 remains constant (Figure 5.3) will evolve upward
along the main-sequence, mimicking the blue straggler effect. In
other words, an engineered star might, at least initially, be indistinguishable
from a naturally produced blue straggler. Of course,
not all blue stragglers have been engineered, but these stars are
probably good candidates to study for additional signs that might
betray an artificially engineered origin.
What characteristics might an engineered star have that
would make it distinguishable from a naturally formed blue
straggler? First, the environment in which the blue straggler is
found could be a good indicator. It is highly unlikely, for example,
188 Rejuvenating the Sun and Avoiding Other Global Catastrophes
that an isolated blue straggler, or one in a low-density cluster,
will have been formed via a collision or through the capture
and accretion of additional interstellar medium (in other words,
Scenarios 1 to 4 listed above are unlikely to apply). Further, if
no companion star is detected, then Scenario 5 can also be ruled
out, leaving just the mixing and accreting black hole scenarios.
The two mechanisms that are usually invoked to produce extra
mixing are rapid rotation and strong internal magnetic fields.
Both of these phenomena provide non-thermal pressure support
within the interior of a star, and this enables the star to run
at a slower evolutionary rate than a similar-mass star without
additional pressure support. Certainly some blue stragglers show
clear signs of supporting strong surface magnetic fields and some
also show signs of rapid rotation, but the observations are by no
means complete (or easy to obtain). Some blue stragglers also show
clear signs of having undergone extensive, additional mixing.9
All in all, therefore, it is not entirely clear how an engineered
star might look significantly different from a blue straggler
produced through the natural mixing scenario, at least to begin
with. Perhaps the best blue straggler candidates for further study
with respect to the possibility of having been engineered are
those with the highest surface temperatures (resulting from their
being fully, rather than partially, mixed stars). If such stars also
show indications of having strong magnetic fields (related to the
magnetic mass-loss engines described earlier), and if they also
show indications of mass-loss, then these stars might just be
the ones undergoing rejuvenation by an advanced extraterrestrial
race.
Recently, Orsola De Marco (American Museum of Natural
History) and co-workers10 have discovered four blue stragglers that
have low-mass circumstellar disks with estimated radii of order
0.1 AU. Such disks might be the result of rapid rotation, or they
might possibly be composed of the material being ejected from a
star by a polar mass-loss engine (Figure 5.5). All the blue stragglers
with disks are located within ancient globular clusters and are,
therefore, already old stars. Should planets exist around such earlyformed,
relatively metal-poor stars, then their ancient heritage
should favor the appearance of intelligent life.
Stars Transformed 189
The Time of Their Lives
In the last chapter it was argued that there was little advantage in
seeding the Sun with a low-mass black hole in order to prolong
its lifetime. Indeed, it is shown in Appendix B that the black hole
mass increases exponentially with time, and the time for a lowmass
black hole to consume the entire Sun is of order Tconsume
∼1 billion years. For humanity this timescale is not of great use,
but for an intelligent civilization emerging in a planetary system
in orbit around a star more massive than the Sun, 1 billion years
might be of great importance with respect to survival. Equation
(B.6) reveals, for example, that Tconsume > TMS once the mass of the
parent star is greater than about 2 M. The black hole consumption
time is nearly 2½ times greater than the canonical main-sequence
lifetime for a 3 M star. For civilizations that emerge in planetary
systems with parent stars more massive than the Sun, the black
hole seeding scenario might, therefore, buy a useful amount of time.
Martyn Fogg has developed a detailed model describing
accretion-powered blue stragglers, and suggests that they might
represent stars seeded with the primordial mini black holes
postulated by Stephen Hawking (University of Cambridge, UK).11
Although astronomers have not been able to confirm the presence
of very low-mass black holes in interstellar space, some recent
speculations concerning the formation of ball lightning on Earth
may indicate that such objects really do exist. Although it
is certainly a controversial proposal, plasma physicist Pace
VanDevender has argued that long-lasting ball lightning events
might be explained in terms of very low-mass, 1 (?) million kg,12
black holes. VanDevender suggests that such minuscule black
holes might form GEAs, the gravitational equivalent of an atom,
in which the black hole acts as the nucleus around which atoms
(rather than electrons) move within bound and stable orbits. All
the above being said, the actual existence of low-mass black holes
still remains highly uncertain at the present time, but should they
exist – and provided a civilization can locate, transport, and then
use one to prolong the main-sequence lifetime of its parent star – it
is presumably among the more massive blue stragglers where such
seeding might be evident. These are the stars with the shorter
canonical main-sequence lifetimes and for which Tconsume > TMS.
190 Rejuvenating the Sun and Avoiding Other Global Catastrophes
This situation is probably not common, however, since the time
for life to first appear and then evolve into an intelligent form
around, say, a 2 M star would have to be compressed into an
uncomfortably short 1 billion years or less.
Under Construction
One of the problems inevitably encountered when trying to
identify engineered stars is that many of the controlling processes
that an asteroengineer might try to employ have natural counterparts.
At some level, virtually all stars lose mass and have magnetic
fields. Perhaps, therefore, the signs of ongoing stellar rejuvenation
might be more readily identified by the debris and system reorganization
being directed by the engineers. Among the various possibilities
one can list are:
• Extensive Kuiper Belt-like and asteroid-belt-like debris clouds in
older stellar systems – such debris resulting from asteroid mining
and the deliberate rearrangement of terrestrial planet orbits.
• Bright optical flashes (periodic and non-periodic) due to nearspecula
reflection from large solar sails involved in the
dynamical reorganization of system orbits (Figure 4.5).
• Periodic transits of a star by objects with distinctive, non-natural
shapes (i.e., squares, louvers, and triangles), as suggested by Luc
Arnold.
• Terrestrial planets with orbits that should be unstable given
the age of the host star and the orbital radii of its companion
planets. Such orbits would have to be continuously andactively
maintained.
• Terrestrial planets with atmospheres betraying odd or even biotic
chemistry, yet not situated in the habitability zone. Such observations
would suggest terraforming in action.
• Rapid (milliseconds) brightness variations due to Dyson sphere
islands transiting the disk of a parent star.
• Periodic X-ray emission from a Sun-like star due to the accretion
of material onto a black hole undergoing oscillatory motion
within its interior (Figure 5.3). The X-ray emission will be
detected only when the black hole is close to the surface of the
Stars Transformed 191
host star, and the emission variation will have a period of about
three hours.
• Stars with distinctive asymmetric mass-loss (Figure 5.6). Such
an effect might be associated with a dwarf star close to or
approaching a Sun-like star. The star-lifting of the dwarf would
have been initiated to head off a close encounter with the Sunlike
star’s surrounding cometary cloud.
• Stars with unusually high abundances of helium relative to
hydrogen. As a fully mixed star ages and the hydrogen is
consumed throughout its interior, the He/H ratio will increase.
• Repeated brightness variations of a star associated with the
passage of a giant ramscoop through its outer atmosphere (recall
Figure 5.4).
• Stars with close-in (less than 1 AU) low-mass circumstellar disks
produced by rapid spin induction or by a polar mass-loss engine
(see Figure 5.5).
• Multiple dwarf stars occupying the same orbit (Figure 5.7) around
a central star that is undergoing extensive mass-loss. The central
star might also betray an unusually strong magnetic field due to
the mass-loss driving engines.
There must be many more ways than those listed above by which
the active and intelligent manipulation of a planetary system
might be deduced. All we need to do is spend the time and effort
to look, and then we need to look with a non-jaundiced eye.
The Search for Extraterrestrial Intelligence (SETI) community has
battled long and hard to achieve the levels of its present funding,
and this is the situation in spite of SETI being one of the few
scientific endeavors that has great public support. If we fail to look
(and hope) for the signs of extraterrestrial civilizations within our
galaxy, then we may fail ourselves and our imagination.
On the Threshold
Michel Mayor and Didier Queloz discovered the first exoplanet
orbiting a Sun-like star in 1995. Surprisingly, however, the 0.47
Jupiter-mass planet was found to be orbiting its parent star – 51
Pegasus – at a distance of a mere 0.05 AU. Many such hot Jupiterlike
planets have now been found, and these worlds are not likely
192 Rejuvenating the Sun and Avoiding Other Global Catastrophes
to support intelligent life. To at least be consistent with what
we know has occurred within our own Solar System, the goal of
future studies is to find an Earth-like planet orbiting a Sun-like
star at a distance of about 1 AU. There is no reason why such
worlds shouldn’t exist, but detecting them is far from easy.13 We
are, however, on the very threshold of being able to identify such
planets around nearby stars.
In December 2006 the European Space Agency successfully
launched its COROT satellite with the aim (at least in part) of
detecting Earth-like planets in orbit around nearby stars. The
mission name is derived from the title COnvection ROtation and
planet Transits mission, and the satellite carries a 27-cm telescope
specifically designed to detect planet-sized objects as they transit
their parent star. When a transit occurs for a few short hours
(depending on the orbit), it blocks a small fraction of its parent
star’s light from reaching the spacecraft detector. The satellite’s
instrumentation is able to record exceptionally small changes in
the brightness of the star being monitored, and this should enable
planets larger than about twice the size of Earth to be detected.14
The NASA Kepler mission is due for launch in late 2008 and,
like COROT, it will search for sub-Earth-sized planets via transit
observations. Named in honor of Johannes Kepler (1570–1630), the
satellite will carry a 0.95-m diameter telescope and will be able to
measure relative changes as small as 10−4 in brightness. During the
spacecraft’s four-year mission its telescope will stare at the same
star field and continuously monitor 100,000 stars for the small,
telltale brightness variations due to planetary transits.
The various satellite components that constitute NASA’s
Terrestrial Planet Finder (TPF) mission are currently scheduled for
launch between 2014 and 2020. The suite of TPF satellites will
be able to study all aspects of the formation and development of
Earth-sized planets around nearby stars. Not only will the size,
temperature, and location of any planets be determined, but their
atmospheric chemistry will also be analyzed, thereby determining
whether the planet might presently or someday support life. The
detection of ozone or methane or both, for example, would indicate
an active biotic system.
The mass range of terrestrial planets that might eventually be
detected by future spacecraft mission will vary from perhaps a 10th
Stars Transformed 193
that of Earth (equivalent to a Mars-sized planet) to a maximum
of about 10 times the mass of Earth.15 Terrestrial planets that
are smaller than Earth (i.e., Mars-sized planets) will have a low
surface gravity and a more rigid crust, upon which high mountain
ranges and deep valleys might form.16 The large surface area to
volume ratio for smaller planets will also result in their relatively
rapidly cooling, leading to a decline in volcanic activity and active
crustal recycling (i.e., as required by the carbon cycle; see Figure
2.24).17 Such planetary characteristics are probably not conducive
to the emergence and evolution of advanced life forms. Planets
more massive than Earth, however, will have hotter interiors,
higher surface gravity, and less rigid crusts. Under these circumstances
the surface topology is likely to be muted, and maybe
such planets will support deep (possibly global) oceans. These
latter planetary characteristics are potentially more conducive to
the appearance and evolution of advanced life forms. Intriguingly,
William Dietrich and J. Taylor Perron (University of California,
Berkeley) have recently argued that the existence of life might be
betrayed on Earth-like planets by the kinds of topological features
that they reveal.18 Specifically, they point out chemical reactions
precipitated by biotic activity will have potentially measurable,
short-term effects on such processes as rock weathering, soil
formation, upland slope stability, and river system dynamics.
As of this writing no nearby Sun-like star has been found
to harbor a terrestrial planet, nor has any data been obtained
to demonstrate the clear-cut existence of life having evolved
anywhere in the universe other than on Earth. What is remarkable,
however, is that we are literally on the threshold of possibly
making such discoveries. Within the next 25 years humanity might
actually know the answer to the age-old questions concerning the
existence of other Earths and the evolution of ecosystems other
than our own.
Notes and References
1. The most comprehensive website on exoplanetary discoveries and
related publications is located at http://exoplanet.eu.
194 Rejuvenating the Sun and Avoiding Other Global Catastrophes
2. A set of age estimates for planet-supporting stars has recently been
published by Carlos Saffe and co-workers [On the ages of exoplanet
host stars, Astronomy and Astrophysics, 443, 609–625, (2005)]. To
convert the system ages into main-sequence lifetime fractions, recall
from Chapter 2 [Equation (2.1)] that the main-sequence lifetime
depends upon both the mass and luminosity of a star. Also recall,
however, from Chapter 3 that main-sequence stars satisfy a massluminosity
relationship. In this manner, the main-sequence lifetime
can be written in terms of the mass alone. The observations, nicely
summarized by Dr. R. C. Smith [An empirical stellar mass luminosity
relationship. The Observatory, 103, 29–31 (1983)], indicate that for
stars more massive than 0.4 , the luminosity varies as: L/ L =
(M/M 4. Accordingly, for Sun-like stars the main-sequence life time
will vary according to the relationship: TMS (yr) = 1010 (M/M) −3.
3. Brian Jones and co-workers, Habitability of known exoplanetary
systems based on measured stellar properties, Astrophysical Journal,
649, 1010–1019, (2006).
4. D. E. Trilling et al., Circumstellar dust discs around stars with known
planetary companions. Astrophysical Journal, 529, 499–505 (2000).
5. In addition to the galactic (or open) clusters, the galaxy also hosts
an extensive halo of globular clusters. The globular clusters are
composed of the oldest known stars, with ages ∼12 billion years, and
they contain ∼500,000 stars in a region that is ∼20 pc across.
6. This condition holds provided that the size of the cluster D is small
compared to its distance d from us. The closest galactic cluster to the
Sun is that of the Hyades (visible to the naked eye in the constellation
of Taurus), which is about 46 pc away. For this cluster, estimated to
be about 800 million years old, D/d ∼0.25.
7. These points were first raised in the article, Blue stragglers as
indicators of extraterrestrial civilizations? Earth, Moon and Planets,
49, 177–186 (1990).
8. The seminal theoretical paper on the blue straggler phenomena is that
by J. Craig Wheeler, Blue stragglers as long-lived stars, Astrophysical
Journal, 234, 569–578 (1979). The idea of quasi-homogeneous mixing
was further studied in the paper by Hideyuki Saio and Craig Wheeler,
The evolution of mixed long-lived stars, Astrophysical Journal, 242,
1176–1182 (1980).
9. A key indicator of extensive mixing is the lithium abundance of blue
stragglers. The fusion reaction 7Li + p ==> 4He + 4He + energy can
proceed efficiently once the temperature is above just a few million
degrees. The relatively low fusion temperature for 7Li dictates that
it is rapidly destroyed once it is mixed into the interior of a star.
Stars Transformed 195
In a number of observational studies it has been found that the
lithium abundances of the blue stragglers within a cluster are significantly
lower that that of the main-sequence stars near the turnoff
point. These observations suggest that the blue stragglers have
indeed undergone some form of additional mixing. A comprehensive
review of the observations relating to blue stragglers is provided by
L. L. Stryker in Blue stragglers, Publications of the Astronomical
Society of the Pacific, 105, 1081–1100 (1993).
10. See Orsola De Marco et al., A spectroscopic analysis of blue stragglers,
horizontal branch stars, and turnoff stars in four globular clusters.
The Astrophysical Journal, 632, 894–919 (2005).
11. Martyn Fogg, Accretion-powered blue stragglers. Speculations in
Science and Technology, 13 (1), 20–25 (1990). Hawking initially
suggested that the solar neutrino problem might be solved by the Sun
containing a primordial mini black hole [Gravitationally collapsed
objects of very low mass, Monthly Notices of the Royal Astronomical
Society, 152, 75–78 (1971)], but this solution is no longer required
since the problem has been solved in terms of a new physical understanding
of neutrino characteristics. Hawking has further argued
that low-mass primordial black holes must eventually evaporate, and
produce what is known as Hawking radiation. The evaporation time
for a 5 x 1012 kg black hole (which would have a size similar to that
of a proton ∼5 x 10−15 m) is about the current age of the universe.
12. VanDevender’s ideas are discussed in Hazel Muir’s article, Black
holes in your backyard [New Scientist Magazine, 23/30 December
(2006)]. With respect to the non-detection of Hawking radiation,
VanDeveren argues that since there is currently no accepted theory of
quantum gravity there is, likewise, no compelling reason to suppose
that low-mass, primordial black holes must evaporate.
13. In a recent study published by Martyn Fogg and R. P. Nelson [On the
formation of terrestrial planets in hot-Jupiter systems, Astronomy
and Astrophysics, 461 (3), 1195–1208 (2007)] it is shown that material
capable of forming terrestrial planets is able to survive within planetforming
disks, even after a Jupiter-mass planet (which formed in the
outer part of the disk beyond the boundary where ice can form) has
migrated inward towards the parent star. Fogg and Nelson conclude,
“hot-Jupiter systems are likely to harbor water-abundant terrestrial
planets in their habitability zone.” This result is an important
negation of the ‘rare Earth’ hypothesis that has been advocated by
some scientists in recent years.
14. A number of planets have now been discovered through the transit
detection technique (see Note 1). The first transiting planet was
196 Rejuvenating the Sun and Avoiding Other Global Catastrophes
found to orbit the star HD209458 [R. A. Wittenmyer et al., System
parameters of the transiting extrasolar planet HD209458b. The
Astrophysical Journal, 632, 1157–1167 (2005)]. By studying the
brightness variations over repeated transits it is now known that the
radius of the 0.66 MJupiter mass planet is 1.35 RJupiter. The planet is
slightly inflated for its mass due to its close-in, 0.045 AU radius orbit
around the parent star.
15. A study by Diana Valencia and co-workers [Internal structure of
massive terrestrial planets, Icarus, 181, 545–554 (2006)] finds that the
size of a massive Earth-like planet scales as its mass M according
to the relationship M0 27. The radius increases, therefore, by about a
factor of 2 when the mass increases by a factor of 10. If a planetary
embryo grows to a mass greater than about 10 times that of Earth,
it will begin to accrete and retain an extensive envelope of hydrogen
and helium, resulting a gas giant (Jupiter-like) planet.
16. We know, for example, that Olympus Mons on Mars is nearly twice
as high as Mount Everest on Earth. Likewise Valles Marineris is much
larger and deeper than any similar such tectonically formed rift valley
found on Earth.
17. The expected properties of various-sized Earths are described by
J. J. Lissauer, How common are habitable planets? Nature, 404, C11–
C14 (1999). See also Note 11.
18. W. E. Dietrich and J. T. Perron. The search for a topological signature
of life. Nature, 439, 411–418 (2006). Climate change (as well as local
weather variations) is also dependent upon biotic activity. A climate
control example is that of the biotic sequestration of carbon to form
carbonate rock deposits, as observed on Earth (see Figure 2.18).
Comments
Post a Comment