The Role Of Stellar Population Types In The Discussion Of Stellar Evolution

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The Role Of
Stellar Population Types In The Discussion Of Stellar Evolution





CRSQ Volume 30(1) June 1993


Danny R. Faulkner


Abstract

Stars can be grouped into
two general types called population I and population II. The criteria
for classification include space velocity, location in the galaxy, composition,
differences in distribution on the Hertzsprung-Russell diagram, integrated
color, and the presence of nearby dust and gas. The current evolutionary
theory of stellar evolution and galaxy formation succeeds in giving
a qualitative explanation for the population types. In establishing
a creation model of stellar (and galactic) astronomy, it is important
to keep in mind the two different populations. If an alternate model
is to be taken seriously, then the observed population types should
be explained in a very plausible fashion.



Introduction

In a recent paper Faulkner
and DeYoung (1991) briefly surveyed the state of creationist astronomy.
It was noted that most work to date has been primarily concerned with
the ages of solar system objects or with criticism of the current standard
(Big Bang) model of the universe. This trend has overlooked the middle
scale of stellar astronomy between these two extremes. For several decades
astronomy has been dominated by the concept of stellar evolution which
has achieved some success in giving a natural and totally physical explanation
for a great number of observed properties of stars. Not much of this
has been challenged by creationists and it was the purpose of the paper
of Faulkner and De Young to call attention to this deficiency and spark
discussion on these matters. To this end that paper presented a very
brief discussion of stellar structure and its relation to the development
of stellar evolution. The Hertzsprung-Russell (H-R) diagram was described,
as well as its importance in interpretation of stellar evolution. Several
predictions of theoretical stellar evolution and purported observation-
al evidences were presented without comment in that paper as well. This
included the coincidence in location and age of planetary nebulae with
white dwarfs and the coincidence in location and age of supernova remnants
with neutron stars.

In addition the previous
paper briefly discussed the differences in observed H-R diagrams of
globular and open star clusters. These differences qualitatively agree
with the. predictions of stellar evolution for young clusters (open
clusters) and for old clusters (globular clusters). Armed with the results
of stellar evolution it is generally argued that certain features of
the H-R diagram, such as the turn off point can be used to deter- mine
the age of-a- particular cluster.

This paper will develop
the differences between the two types of clusters further and expand
those differences to all stars. The parlance for this is stellar populations,
and the two populations will be defined and examined. Very little creationist
criticism or commentary will be provided here: the purpose of this paper
is to inform readers interested in developing a creationist astronomy
of some of the stellar features that should be kept in mind with the
goal of explaining them from the creationist perspective.



Figure 1. Schematic
side and top views of the Milky Way galaxy. The diameter of the luminous
disk is about 100,000 light years across and a few thousand light years
thick. The nuclear bulge is about 10,000 light years across. The sun
is located in the disk about half or two thirds the distance out from
the center. The halo is a spherical distribution of stars concentric
with the disk (DeYoung, 1989, p. 84).


Stellar
Populations

The Milky Way galaxy is
believed to be a disk with a fainter, but massive, roughly spherical
halo that is concentric with the disk (see Figure 1). The galaxy has
a total mass of at least 100 billion, and perhaps as much as 250 billion,
times that of the sun. The luminous disk appears to contain most of
the brightest stars and is about 100,000 light years across, while it
is only a few thousand light years in thickness. There is a thickening
of up to perhaps 10,000 light years at the center of the disk, a feature
called the nucleus. Most of the hotter and brighter stars in the disk
are found in spiral arms that extend from the nucleus, and the Milky
Way, along with other similar appearing galaxies, are thus called spiral
galaxies. The halo is a fainter, roughly spherical distribution of stars
that is concentric with the disk as well. Despite being fainter, the
halo does contain a substantial portion of the galaxy’s mass. This is
particularly indicated from velocity profile studies of the Milky Way
and other spiral galaxies. These show that there must be a large amount
of mass located beyond where we see most of the stars and other visible
mass. Since this inferred additional matter is not visible, it has been
termed “dark matter,” and though a number of theories have
been put forth about its source, its identity is still a mystery.

One of the most important
considerations in deducing the structure of the galaxy is the study
of stellar kinematics, or the motions of stars. A star’s space velocity
can be expressed in terms of two component velocities with respect to
the solar system: the radial velocity in the line of sight to the star,
and the tangential velocity perpendicular to the radial velocity. The
radial velocity is easily measured by using the Doppler shift observed
to occur in the spectral lines of stars, and can be determined at any
distance, as long as the star is bright enough. Unfortunately the tangential
velocity is more difficult to determine. Over time the tangential velocity
will cause a gradual change in a star’s position in the sky, and generally
two photographs made many years apart are carefully measured to determine
a star’s change in position. The change in position is divided by the
time interval to obtain an annual change, which is called the proper
motion and is measured in arc seconds per year. Obviously, the proper
motion and the tangential velocity are directly
related, but the distance is also involved. The three quantities are
related to each other by the equation


vT = 4.74md

where vT is
the tangential velocity in km/sec,
m is the
proper motion and d is the distance in parsecs. The distance can be
directly measured by the method of trigonometric parallax, which entails
the determination of the annual slight change in the apparent positions
of stars that occurs because of the earth’s orbital motion around the
sun. If
p is the
parallax, in seconds of arc, then the distance is given by


d = 1/p,

where the distance is
measured in parsecs (1 pc = 3.26 light years). It is often stated that
this method of distance determination works accurately up to a distance
of 100 parsecs (roughly 300 light years). Actually, at 100 parsecs the
error in the measurement is equal to the measurement itself, and this
method is only reliable (errors within 10 percent) to about 20 parsecs
(65 light years) (Mihalas and Routly, 1968; Smart and Green, 1977).
Beyond this distance other, indirect, methods must be used.

As would be expected,
the nearest stars have the largest proper motions, while distant stars
have small proper motions. Since parallax measurements are very difficult
and tedious, it is only profitable to attempt measurements of stars
which we guess are nearby. Proper motion studies have been conducted
for the entire sky, and parallax studies have generally used proper
motion surveys to identify candidates for measurement by searching for
stars having large proper motions. It is probable that a few faint,
near- by stars have been missed this way because they happen to have
small proper motions or are very faint, but the sample of nearby stars
is otherwise nearly complete.

The space velocity of
a star can be determined by knowing its two components, the tangential
and radial velocities. It was discovered several decades ago that stellar
kinematics naturally divide stars into two classes: those with large
space velocities (high velocity stars) and those with small space velocities
(low velocity stars). In the original and classic paper on the subject
Baade (1944) proposed that there are two types, or populations of stars,
population I being the low velocity stars and population II being the
high velocity stars. Additional discussion of stellar populations may
be found in Mihalas and Routly (1968); Mould (1982); Sandage (1986);
Binney and Tremaine (1987). From a creationist perspective Steidl (1979)
has briefly described populations as well.

The kinematic differences
between the two populations are caused by their different orbits about
the galactic center. Space velocities are actually velocities of stars
relative to that of the sun. The low velocity or population I stars
must then have orbits similar to that of the sun. This suggests that
the sun is a population I star, and that population I stars orbit the
galactic center in roughly circular orbits confined to the galactic
plane. On the other hand, high velocity, or population II, stars must
have elliptical orbits that are highly inclined to the galactic plane,
while population II stars are found throughout the halo.

There are other differences
that Baade noted between the two stellar types. One was the amount of
heavier elements present. Chemistry is very simple to astronomers: There
is hydrogen, helium, and then there is everything else. Astronomers
do, of course, note the complex chemical makeup of interstellar gases
and molecules. For stars, however, a gross view may be taken using X,
Y, and Z, where X stands for hydrogen abundance, Y for helium abundance,
and Z for the abundance of elements above helium. Since many of the
other elements are metals, all of these other elements are collectively
called “metals,” even though some, such as carbon, nitrogen,
and oxygen, are not actually metals. The metalicity, Z, is the fraction
of mass that is comprised of the metals. Most of the universe and the
stars in it are primarily made of hydrogen and helium, with only a few
percent of metals. Generally the abundances of the heavier elements
increase in about the same proportions to one another, so the measurement
of a few elements is sufficient to estimate the abundances of all.

The metalicity can be
measured from spectra, but it can be determined more easily and efficiently
by using Stromgren (intermediate band) photometry. A good discussion
of this technique can be found in Henden and Kaitchuck (1982). Photometry
is the precise measurement of star light, and is usually accomplished
by using a photosensitive detector with colored filters on a telescope.
Each filter has a certain wavelength interval, called the band pass,
through which light is transmitted to the detector. The band passes
are carefully selected to measure particular spectral features. For
example, in the spectra of most stars the near ultraviolet contains
numerous absorption lines due to metals. This causes the spectrum to
be depressed there, leading to the phenomenon called line blanketing.
One of the four filters of the Stromgren system is in this part of the
spectrum (the u filter), while a second nearby filter in the violet
part of the spectrum (the v filter) does not suffer from line blanketing.
The difference in the brightness in the u and v filters is therefore
a measurement of the amount of line blanketing present. This measurement
also depends upon the stellar temperature, but the temperature can be
independently measured by using the two other filters, which are in
the blue and yellow (the b and y filters). All of these measurements
can be combined in various ways to form several indices, one of which
is a metal index. The metal index has been well calibrated with the
amount of metals determined from detailed study of stellar spectra.
Such studies show that population II stars have a low metal abundance,
while population I stars have a high metal abundance. The difference
in metalicity between the most metal rich and the most metal poor stars
is on the order of about 100.

There is also a difference
in the H-R diagrams of typical population I and population II stars
as well. Population I stars have an H-R diagram similar to those for
open star clusters, while the H-R diagram of population II stars resembles
that of globular clusters. More specifically, upper main sequence stars
are not found among population II stars, while though they are rare
among population I stars, they are among the brightest population I
stars. Kinematic and chemical abundance studies of clusters show that
the other properties of the two stellar populations are shared with
the two types of star clusters. Globular clusters have low metalicity
and are found in the halo, and so are considered to be population II.
Open star clusters have high metal abundances and are found near the
galactic plane, and so are recognized as population I.

Population I stars generally
have clouds of dust and gas around and near them, while population II
stars generally are found in dust free and gas free environments. This
last characteristic is not independent of the others in that most of
the gas and dust in the galaxy is found near the galactic plane.

Baade (1944) was working
with the most extreme examples of stellar populations, and so it is
not surprising that many stars are found somewhere in between the population
classifications. It is now recognized that the stellar populations represent
a continuum in properties, rather than two distinct bins. Extreme, or
halo, population II stars are found high in the halo, possess high velocities,
and are very low in metals. Intermediate population II stars are found
closer to the galactic plane, have smaller space velocities, and are
even higher in metalicity. Old population I stars (in which the sun
is included) are found very close to the galactic plane and have high
metalicity. Extreme population I stars are the highest in metals and
are found in the galactic plane. Extreme population I stars have space
velocities that are slightly less than older population I stars. Generally
population I stars have a very patchy distribution, being found along
the spiral arms in the galactic plane. On the other hand population
II stars are found to have a very smooth distribution.


The Evolutionary
Explanations for the Stellar Populations

Most current cosmological
theories are predicated upon the assumption that the universe began
with only the elements hydrogen and helium. All other elements are assumed
to have been synthesized in the cores of stars. Certain isotopes up
to iron can be synthesized by successive alpha capture by nuclei that
results in an energy source for stars. Other isotopes, especially those
more massive than iron can be produced by the slow or rapid neutron
capture processes. The heavier elements particularly can be synthesized
in violent processes such as supernova explosions. Most nucleosynthesis
occurs in or around cores of stars, and since convection is usually
not present throughout stars, heavier elements that are synthesized
remain deep in stellar interiors. Therefore the composition determined
from spectral analysis or inferred from photometric measurements must
reflect the initial composition of stars.

The cosmology popular
today supposes that early in the universe large clouds of gas began
to form. These clouds were millions of light years across and slowly
condensed to form galaxies. It is recognized that a perfectly smooth
Big Bang cannot give rise to these structures, so it has been hypothesized
that the early universe contained small inhomogeneities that acted as
gravitational seeds to produce the structure in the universe that we
see today. The purpose of the COBE satellite has been to look for these
inhomogeneities as temperature variations in the background radiation.
However, the very subtle and questionable variations recently announced
from COBE measurements are far less than had been predicted. Let us
set this difficulty aside, and grant that somehow these large clouds,
usually termed proto-galaxies, did form. As the Milky Way protogalaxy
collapsed, it would have assumed a roughly spherical shape, and parts
of the cloud would have subfragmented, and in some locations the density
would have increased so that the very first stars would have formed.
The process of star formation would have continued as the galaxy collapsed,
with most of the leftover gas flattening into a plane. Today virtually
all of the remaining gas is confined to the plane. Early in the galaxy’s
history star formation would have occurred anywhere in the original
sphere of gas, but in later times star formation would have only occurred
near or in the disk. Since the collisional cross sections of stars are
so tremendously small compared to the size of the galaxy, stars would
generally continue to follow the orbit about the galactic center that
they possessed when they formed.

The first stars to form
would consist entirely of hydrogen and helium, with heavier elements
being produced in their cores. The more massive stars among the first
generation would quickly end their life cycles and explode in violent
supernovae that would spew the heavier elements that they synthesized
into the gas then present in the galaxy. This would cause the next generation
of stars to have a higher metal content. The more massive stars of each
generation would repeat the process of synthesizing heavier elements
in their cores and then spreading their material into the interstellar
medium. Such a process is referred to as chemical enrichment and would
cause a gradual increase in the metal content as stars form progressively
later.

All together, this theory
suggests that the oldest stars generally should be found far from the
galactic plane, though a few will be found near the plane if they are
in the portion of their orbits where they cross the plane. Such stars
would also be expected to be low in metalicity. The youngest stars should
be in the galactic plane and have the highest metalicity. There should
be stars of intermediate age with intermediate metalicities and locations
in the galaxy. Thus the extreme population II stars are identified as
the oldest stars while extreme population I stars are the youngest.
The differences in the H-R diagram between the two populations discussed
earlier are also reflected in the differences of supposed ages as discussed
in the previous paper of Faulkner and De Young. The properties of the
two populations are reiterated in Table I.


 Property


 Population I


 Population II
 Space velocity


low 


 high
 Location


disk


halo
 Metalicity


high


low
 Color


blue


red
 Dust


yes


no
 Supposed age


 young


 old


Table I. Properties
of the Two Stellar Populations

Since current cosmological
theories demand that the universe began with a composition entirely
of hydrogen and helium, it is believed that the very first generation
of stars should have had no metals. Such a primordial generation has
been dubbed population III, and a vigorous but unsuccessful search for
these stars has been conducted. Even though the most extreme population
II stars have only one percent of the metal content of population I,
the fact that all stars have some metalicity is somewhat embarrassing
for the standard theory. There have been several suggested explanations.
One is that the Big Bang produced some of the heavier elements, so that
even the earliest stars contained some metals. Another is that there
was a brief intense period of star formation just before the collapse
of the galaxy. These stars are supposed to have been massive, which
would have caused them to have synthesized the elements and seeded the
interstellar medium very rapidly. Because of the short lifetimes of
massive stars, this primordial generation would no longer exist. How
or why such a primordial generation would have formed is not known.


Conclusion

The current theories of stellar and galactic evolution can qualitatively
explain the differences between the two population types. This agreement
has been put forth as evidence of the correctness of the theory. This
topic has not been discussed in the creationist literature until now,
and it is hoped that this paper will spark interest and discussion of
it. The observed differences between the two types is important information
that must be considered in developing a comprehensive stellar theory.
The evolutionary theory qualitatively explains the types in a plausible
and natural way. A creationist alternative must be able to do as good
a job in explaining the differences. To that end it is hoped that the
previous paper of Faulkner and DeYoung and this paper have provided
useful information and direction. The author encourages correspondence
with interested parties.


References

Baade, W. 1944. The resolution of Messier 32, NGC 205, and the central
region of the Andromeda Nebula. Astrophysical Journal. 100:137-146.

Binney, James J. and Scott Tremaine. 1987. Galactic dynamics. Princeton
University Press. Princeton, NJ.

DeYoung, Don. 1989. Astronomy and the Bible. Baker Book House. Grand
Rapids.

Faulkner, Danny R. and Donald B. DeYoung. 1991. Toward a creationist
astronomy. Creation Research Society Quarterly. 28:87-92.

Mihalas, Dimitri and Paul McRae Routly. 1968. Galactic astronomy. W.
H. Freeman. San Francisco.

Henden, Arne A. and Ronald H. Kaitchuck. 1982. Astronomical photometry.
Van Nostrand Reinhold. New York. (Now available from Willmann-Bell. Richmond,
VA).

Mould, J. R. 1982. Stellar populations in the galaxy. In Geoffrey Burbidge,
David Layzer, and John G. Phillips, editors. Annual reviews of astronomy
and astrophysics. Annual Reviews. Palo Alto, CA.

Sandage, Allan. 1986. The population concept, globular clusters, subgiants,
ages, and the collapse of the galaxy. In Geoffrey Burbidge, David Layzer,
and John G. Phillips, Editors. Annual Reviews. Palo Alto, CA.

Smart, W. M. and R. M. Green. 1977. Textbook on spherical astronomy.
Cambridge University Press. London.

Steidl, Paul. 1979. The earth, the stars, and the Bible. Presbyterian
and Reformed. Phillipsburg, NJ.

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