Mercury,
May/June 1998 Table of Contents
Anthony
G. A. Brown
Universidad Nacional Autónoma de México
Outlining
the head of Taurus the Bull, stars in the Hyades cluster are important
to us. Oh, they give us pleasure to behold, but they also enable
us to measure the universe.
A
careful examination of the proper motions of all fixed stars in
the catalogues published by Messr. Main and Stone...has led me to
a somewhat interesting result. I find that in parts of the heavens
the stars exhibit a well-marked tendency to drift in a definite
direction. In the catalogues of proper motions, owing to the way
in which stars are arranged, this tendency is masked; but when the
proper motions are indicated in maps, by affixing to each star a
small arrow whose length and direction indicate the magnitude and
direction of the star's proper motion, the star-drift (as the phenomenon
may be termed) becomes very evident.
The
most famous of these "star-drifts" in the heavens discussed by Richard
A. Proctor in his entry in the 1869 Proceedings of the Royal Society
of London is the Hyades star cluster. It is the nearest moderately
rich star cluster and can easily be seen with the naked eye: All
the stars around Taurus's "V"-shaped head, with the exception of
red Aldebaran, are cluster members. Because of its proximity the
Hyades cluster has always played a central role in astronomy. It
forms the first rung on the ladder of the distance scale of the
universe and provides astronomers with an excellent laboratory for
testing their theories of stellar structure and evolution.
Astrometry
and the Measurement of Stellar Distances
The
subject of astrometry, the measurement of stellar positions in the
sky, is the oldest discipline in astronomy. It was the Greek astronomer
Hipparchus who around 150 BCE produced the first systematic catalogue
of stellar positions. Edmund Halley compared Hipparchus's measurements
with those of his 18th-century contemporaries and discovered in
1718 that three stars were not at their expected positions; from
this he deduced they have their own proper motion. Gradual improvements
in measurement precision showed that many more stars have proper
motions, and the realization by William Herschel that these motions
could be partly explained by the motion of the Sun itself, suggested
that some stars may be close enough to the Sun to measure their
distances using their trigonometric parallaxes.
The
parallax of a nearby star is its apparent, annual, angular movement
on the sky due to Earth's motion around the Sun-the star appears
to move back and forth relative to more distant, background stars.
The size of this displacement, together with the known Earth-Sun
distance, gives the distance to the star. This technique provides
a direct measure of stars' distances independent of any assumptions
about, for instance, the brightnesses of the stars. The measurement
of parallaxes is, thus, of fundamental importance to astronomy.
Notwithstanding the efforts made since Herschel's time, however,
the measurement of stellar parallaxes is very difficult in practice
and can only be done accurately for relatively few stars close to
the Sun (within roughly 120 lightyears). For stars outside this
small solar neighborhood, the uncertainties associated with their
measured parallaxes are simply too large.
This
poses serious problems because not all types of stars are located
near the Sun. Notably, there are no nearby Cepheid variables. For
these late-life stars there exists a very well defined relation
between the regular timing of their brightness variations and their
average true brightnesses. Therefore, by simply measuring the time
for the periodic variations in a Cepheid's apparent brightness,
one immediately knows the star's distance. We do need at least one
independent direct measurement of a Cepheid's distance, however,
to calibrate this technique.
This
is where the Hyades cluster comes in. Using the fact that the Hyades
stars move parallel in space, one can deduce the distance to the
cluster using the ingenious convergent-point method (see Hyades
SideBar). Like parallax measurement, this is also a geometrical
method for measuring distances that is independent of any properties
of the individual stars. And once the distance to the Hyades is
known, one can use the technique of main-sequence fitting to derive
distances to other clusters located much further away (see box).
Some of these far-away clusters contain Cepheids, which we find
in other galaxies outside our own. Thus, the Hyades star cluster
is a fundamental stepping stone in our path to deriving distances
to stars and galaxies, and ultimately, to determining the size and
age of the universe.
Doing
All That Measuring From Space
Although
the distance to the Hyades has been derived many times, the uncertainties
in this distance have always remained relatively large. This is
because the convergent-point method, which is in principle very
simple, is in practice fraught with many uncertainties: There may
be errors in the measured proper motions of the stars; some of the
stars under study may not be actual members of the Hyades cluster;
and motions of the cluster members are not perfectly parallel. All
these effects lead to errors in the derived cluster distance.
The
only way really to obtain an accurate calibration of the distance
scale of our universe then, is to measure accurate parallaxes for
as many types of stars as possible (including the Hyades cluster
stars) in order to obtain direct distance measurements. For this
reason, a proposal was made in France in the 1960s to carry out
astrometrical measurements from outer space. The advantages of such
a spaceborne operation are that one can perform measurements for
thousands of stars in only a few years, which is impractical from
the ground, and that the measurements can be made in a very precise,
uniform way all over the sky, which avoids the systematic differences
that exist among ground-based astrometric measurements for different
parts of the sky (differences that exist because such measurements
were made by different observatories).
The
proposal ultimately led to the 1989 launch of the European Space
Agency's HIgh Precision PARallax COllecting Satellite, a spacecraft
named in memory of Hipparchus that measured positions, parallaxes,
and proper motions for over 100,000 stars. The precision with which
these measurements were carried out was one milli-arcsecond (an
arcsecond is 1/3600th of a degree, and a milli-arcsecond is one
thousandth of that). To put this degree of precision in perspective,
consider that the human Hipparchus was able to measure stellar position
to about one degree, which corresponds to the angular height of
a man seen from 100 meters. Modern ground-based astrometric measurements
can discern the height of a man from 4000 kilometers. The satellite
HIPPARCOS, however, can see a man standing on the Moon at a distance
of 380,000 kilometers!
These
precision figures translate into uncertainties of only 5% in the
distances to individual stars in the Hyades cluster. This means
that averaging about one hundred of these individual distances will
lead to a cluster distance known to better than 1%. This kind of
accuracy for distance measurements is unprecedented in astronomy.
A
Three-Dimensional View of the Hyades
The
HIPPARCOS Catalogue, containing positions, parallaxes, and proper
motions for about 118,000 stars, became available in August 1996
and represents 37 months of measurements by the satellite. Immediately
following its release, astronomers from the European Space Agency,
Leiden Observatory, Observatoire de Paris-Meudon, University of
Lausanne, and Observatoire de la Côte d'Azur started work
on the analysis of the HIPPARCOS data for the Hyades cluster. The
first thing they set out to do was to isolate members of the Hyades
cluster from all the other stars surrounding the cluster. Remember
that although Aldebaran is part of the "V"-shape in Taurus, it is
not a member of the Hyades-it is actually much closer to the Sun.
These non-members had to be weeded out first; the weeding was accomplished
by again making use of the fact that the cluster stars have the
same velocity through space. And to search for Hyades stars, it
was possible for the first time to use directly measured, three-dimensional
velocities of the stars by combining the stars' parallaxes, proper
motions, and radial velocities (measured from the ground, see
Hyades SideBar). This led to a very secure list of true Hyades
cluster members that could subsequently be studied in detail.
Based
on this list, the distance to the Hyades cluster was determined
to be 46.34 parsecs, or 151 lightyears, with an uncertainty of less
than 0.27 parsec (one lightyear). Figure 1 shows
a number of recent distance determinations for the Hyades cluster
together with the HIPPARCOS distance (rightmost point). The other
distance determinations are based on a variety of different techniques,
many of them invoking some form of the convergent-point method.
The vertical lines through the points indicate the uncertainty of
each distance determination; note the considerable spread in the
Hyades distances prior to the HIPPARCOS result, indicative of that
uncertainty. The HIPPARCOS measurements firmly tie down the distance
to the cluster and also lead to an explanation of the discrepancies
among the previous results.
Recent distance determinations for the Hyades cluster. The vertical
lines indicate the uncertainty associated with each distance. Note
the considerable spread among previous distance determinations and
the very small uncertainty on the HIPPARCOS distance (the rightmost
point).
Figure
3 shows the structure of the cluster in more detail: three projections
of the cluster on the three principal coordinate planes of the Milky
Way Galaxy. In the plots, the Earth is located at the origin (0,
0) of the coordinate system, the X-axis points towards the center
of our Galaxy, the Y-axis points in the direction in which the Sun
orbits the Galactic center, and the Z-axis points upwards, perpendicular
to the plane of the Milky Way. The distances along the axes are
given in parsecs. From the images and plots we can clearly see the
Hyades consists of a concentrated central part surrounded by a more
diffuse halo of stars. That central part (within a diameter of roughly
65 lightyears) is bound together by the mutual gravitational attraction
of the stars there. The "halo" stars originated with the rest of
the Hyades stars when the cluster was born but have gradually moved
away from the central regions and are no longer bound to the cluster.
However, they (and their remains after their deaths) will remain
in its vicinity for some time before being completely dispersed
throughout the Galaxy.
There
are several reasons why a particular star may escape the central,
gravitationally bound regions of the cluster. As it orbits the center
of the cluster, the gravitational pull of all the other stars will
lead to slight changes in the particular star's velocity and may
ultimately lead to its gradual wandering away from cluster center.
And if the star is far enough away from the center, the gravitational
forces of the Galaxy will come to dominate those due to the cluster,
and the star will find itself no longer bound to the cluster, but
rather to the Galaxy. Another mechanism that can remove stars from
a cluster's center are star-star encounters. In the dense central
regions, where stars pass closer to each other than in the diffuse,
outer halo, encounters between stars or, more importantly, between
single stars and double stars, can lead to the rapid ejection from
the cluster's center of one of the stars taking part in the encounter.
Just consider the analogous gravitational slingshot effect that
we use to accelerate interplanetary spacecraft: The spacecraft falls
in its trajectory just close enough to a big planet to get a velocity
kick. And finally, a star cluster may also lose stars during encounters
with other massive objects such as giant interstellar clouds of
gas and dust. All these effects will together ultimately lead to
the total dissolution of the Hyades cluster at some point in the
future.
Apart
from the velocities of the Hyades members, the stars' distances
are now known well enough for us to construct for the first time
a three-dimensional picture of the arrangement of stars in the cluster.
The image on the left shows the constellation Taurus, easily identified
by the "V"-shape of the Bull's head (except for the Aldeberan,the
large sphere in the middle, which is much closer to the Sun than
the Hyades), and the image on the right is Hyades as seen from Earth
if we could discern the cluster's three-dimensional structure with
the naked eye. Each sphere in the picture represents one star, and
all spheres are the same size. Hence, stars closer to us appear
as larger spheres. Note, however, that relative sphere sizes do
not reflect brightness differences among the cluster stars. This
image of the Hyades shows that the cluster members are strongly
concentrated towards the center.
A
movie of the 3D Hyades cluster has been created by the author and
can be viewed at http://astro.estec.esa.nl/Hipparcos.
Taurus
and the Hyades.
This image shows the constellation Taurus as seen from Earth.
This
image shows all the stars that are part of the Hyades cluster as
seen from Earth. Note the strong concentration of cluster members
towards the center.
The
images below show the structure of the cluster in more detail: three
projections of the cluster on the three principal coordinate planes
of the Milky Way Galaxy. In the plots, the Earth is located at the
origin (0, 0) of the coordinate system, the X-axis points towards
the center of our Galaxy, the Y-axis points in the direction in
which the Sun orbits the Galactic center, and the Z-axis points
upwards, perpendicular to the plane of the Milky Way. The distances
along the axes are given in parsecs. From the images and plots we
can clearly see the Hyades consists of a concentrated central part
surrounded by a more diffuse halo of stars. That central part (within
a diameter of roughly 65 lightyears) is bound together by the mutual
gravitational attraction of the stars there. The "halo" stars originated
with the rest of the Hyades stars when the cluster was born but
have gradually moved away from the central regions and are no longer
bound to the cluster. However, they (and their remains after their
deaths) will remain in its vicinity for some time before being completely
dispersed throughout the Galaxy.
There
are several reasons why a particular star may escape the central,
gravitationally bound regions of the cluster. As it orbits the center
of the cluster, the gravitational pull of all the other stars will
lead to slight changes in the particular star's velocity and may
ultimately lead to its gradual wandering away from cluster center.
And if the star is far enough away from the center, the gravitational
forces of the Galaxy will come to dominate those due to the cluster,
and the star will find itself no longer bound to the cluster, but
rather to the Galaxy. Another mechanism that can remove stars from
a cluster's center are star-star encounters. In the dense central
regions, where stars pass closer to each other than in the diffuse,
outer halo, encounters between stars or, more importantly, between
single stars and double stars, can lead to the rapid ejection from
the cluster's center of one of the stars taking part in the encounter.
Just consider the analogous gravitational slingshot effect that
we use to accelerate interplanetary spacecraft: The spacecraft falls
in its trajectory just close enough to a big planet to get a velocity
kick. And finally, a star cluster may also lose stars during encounters
with other massive objects such as giant interstellar clouds of
gas and dust. All these effects will together ultimately lead to
the total dissolution of the Hyades cluster at some point in the
future.
How
long has the Hyades cluster survived all these destructive forces?
This
question brings us to discuss the Hyades main-sequence in the Hertzsprung-Russell
diagram, a topic of central importance in astronomy. Because the
distances to the Hyades stars can be obtained from a geometrical
method, the stars can be placed in the theoretical H-R diagram (with
their true luminosities as opposed to their apparent luminosities).
The location of the stars along the Hyades main-sequence can then
be used to test and even calibrate different models of stellar evolution.
And perhaps more importantly, the position of the Hyades main-sequence
in the H-R diagram is important if one wants to use the main-sequence
fitting technique (see Hyades SideBar) for
distance determinations.
With
the HIPPARCOS data, specifically stellar parallaxes, it is now possible
to circumvent the problems associated with the convergent-point
method and directly place each individual Hyades star in the H-R
diagram. The result is shown in the image below for the stars located
within 10 parsecs of the cluster's center. The closed symbols represent
the single stars; the open symbols, known double stars. The plot's
horizontal axis corresponds to the stars' (B-V) index, or color.
Low values of (B-V) correspond to blue stars, high values to red
stars. The blue stars are also the more massive ones, evolving more
quickly than the less-massive, red stars. The plot's vertical axis
indicates the absolute magnitude, or true brightness, of the stars.
Some cluster characteristics are immediately discernible from this
plot.
The
four isolated stars near the top are four red giants in the center
of the Hyades cluster. These are the most evolved stars in this
diagram; the other stars define the Hyades main-sequence. Note that
the double stars often lie above the main-sequence defined by the
single stars: They appear brighter when both components of the double
star are measured simultaneously. The red part of the main-sequence
(lower right) contains low-mass stars that have not moved from their
positions in the H-R diagram since they started their lives as main-sequence
stars. These stars define the so-called zero-age main-sequence of
the Hyades.
The
positions of the redder, single stars in the H-R diagram were used
to model this zero-age main-sequence. Theoretical calculations show
that the zero-age main-sequence of the Hyades is best modeled by
assuming that the cluster's stars contain the same amount of helium
as the Sun, even though it is known that Hyades stars are slightly
brighter and more "metal-rich," or heavy-element abundant, than
the Sun. Once the amount of metals and helium in the Hyades is known,
one can calculate how the stars will evolve in the H-R diagram.
And from these and intermediate calculations, we find the age of
the Hyades to be 625 million years, making it a middle-aged cluster.
(Compared to the oldest open clusters or globulars, the Hyades is
a relatively young cluster, but compared to the Pleiades it is quite
old.)
There's
More. . .There's Always More
So
can we now close the "Hyades case"? Not quite. A number of questions
still remain unanswered about this cluster.
Intracluster
motions.
One
of the things the HIPPARCOS measurements were not able to address
is the nature of the intracluster motions of the cluster stars.
Can we see the signs of past encounters with giant gas clouds in
the way the stars move inside the cluster? Does the cluster, as
a whole, rotate? From detailed analysis of the HIPPARCOS results,
we can barely estimate that the motions of the stars in the cluster
amount to no more than 300 meters per second on average. That sounds
pretty fast because we know it's about equal to the speed of sound
at sea level here on Earth, but it's actually a pretty tiny velocity;
the Hyades, like the Sun, zooms around the Galaxy with a velocity
of roughly 220 kilometers per second!
The
Hyades moving-group.
This group is made of stars that are spread out all over the sky,
but that appear to have velocities very similar to that of the Hyades
cluster. Are these stars that escaped from the cluster in the past?
Can we find more moving-group members by carefully searching the
HIPPARCOS Catalogue? Studies of the moving group will probably provide
more insight into the history and origins of the Hyades cluster,
and this in turn will help us better understand the evolution of
other open clusters in the Galaxy.
Mass
segregation.
Analysis of the cluster's three-dimensional structure shows that
there are relatively more massive stars in the center than in the
outer regions. This segregation in stellar mass is expected to occur
in any gravitationally bound cluster: Due to many distant encounters
between stars in the cluster, the heavy stars gradually sink to
the center while the lighter stars rise into the cluster's outer
regions. However, the Hyades cluster may be too young to have undergone
the amount of mass-segregation we observe today. Was the cluster
formed with the massive stars already preferentially located in
the center? This may well be. Observations of very young clusters
in star-forming regions, such as those of the Trapezium cluster
in Orion, show that heavy stars do indeed form in the centers of
clusters.
As
far as our hope of obtaining more accurate measurements are concerned,
the future looks promising. Several national and international space
agencies are already considering or planning future space-astrometry
missions that will carry out measurements 10 to 100 times more accurate
than those by HIPPARCOS!
One
of these possible missions is the Global Astrometric Interferometer
for Astrophysics, currently under consideration by the European
Space Agency. Early plans are for the instrument to carry out astrometric
measurements to a precision of 10 millionths of an arcsecond. And
these measurements would be performed not for a hundred thousand
stars, but for 50 million stars, with less precise measurements
(up to milli-arcseconds) for up to a billion stars to apparent magnitudes
as faint as 20! This would not only greatly advance our knowledge
of the Hyades star cluster, but would also enable us to construct
a detailed, three-dimensional map of a large portion of our Galaxy
for the first time.
ANTHONY
G. A. BROWN is
a postdoctoral fellow at the Instituto de Astronomía of the
Universidad Nacional Autónoma de México at Ensenada
in Baja California. His research interests are the formation and
evolution of star clusters, and he is currently involved in the
concept and technology study for the ESA's GAIA mission. While not
pursuing his astronomical research, he is occupied by the excellent
wines and food in Ensenada. His email address is
brown@bufadora.astrosen.unam.mx.
The
Convergent-Point Method and Main-Sequence Fitting
Before
the HIPPARCOS mission, it was very difficult to get reliable distances
to individual stars in the Hyades cluster by measuring their parallaxes
because the cluster is located just beyond the distance for which
one can more or less easily get good parallaxes from the ground.
Hence, other geometrical methods were devised to measure stellar
distances. The most widely used method is the so-called convergent-point
method. This technique makes use of the fact that stars in a cluster
follow roughly parallel trajectories through space, as shown in
Figure 5a: The parallel motions will result in the stars' apparent
motions on the sky converging toward a single point, the convergent
point. This is analogous to railroad tracks apparently meeting each
other in the distance as you look along them. That the proper motions
of the Hyades stars do indeed point to a particular spot on the
sky is illustrated in Figure 5b.
Figure 5a
Figure 5b
One
can use this perspective effect to measure the distances to the
stars in the cluster in the following way. Consider the motion of
a cluster star as seen from Earth in Figure 5c. This space-velocity
points in the direction of the convergent point and is perceived
by us as the sum of two components-the velocity along our line of
sight, the star's radial velocity, and the velocity perpendicular
to our line of sight, the star's proper motion. Now, a star's radial
velocity can be measured from its spectrum, and its proper motion
is determined by measuring its angular speed through space (i.e.,
arcseconds per year).
Figure 5c
Thus,
knowing the proper motions of stars in a cluster can enable one
to find the directions of the stars' space velocities. This direction
is identified in the diagram as the angle q. From a star's radial
velocity and the angle q, one can then calculate the space velocity
and also the velocity of the star perpendicular to the line of sight.
The latter is measured in kilometers per second, but we also know
the proper motion in arcseconds per year. A comparison of these
two numbers will yield the distance to the star. And this technique
can be applied to every star in the cluster.
Though
conceptually very simple, the method relies heavily on a couple
of assumptions. First, we assume the stars move strictly parallel
through space. This assumption is not valid in reality, however.
The stars move through the cluster with small velocities, and the
cluster itself may be rotating. These effects lead to errors in
the convergent-point determinations and in the derived distances.
Second, we assume that all the stars under study are actually members
of the same cluster (so we can assume they move parallel!). The
unintentional inclusion of non-cluster stars in the analysis leads
to errors.
In
spite of these error-inducing assumptions, the convergent-point
method applied to data from ground-based observations has yielded
a distance for the Hyades with an accuracy that could not be achieved
otherwise from the ground. And once the Hyades distance is known,
one can measure distances to other clusters. The way this works
is illustrated in Figure 6: By observing stars in a cluster, we
determine their apparent brightnesses and colors, which we in turn
use to place them in an observational Hertzsprung-Russell diagram.
The Hyades main-sequence is the middle line. We know how far away
the Hyades are, and by comparing the brightnesses of stars in other
clusters to those of the Hyades stars, we can deduce distances to
stars in those other clusters.
Two
main sequences for other clusters are also shown in Figure 6, one
from stars in a cluster closer than the Hyades (appearing brighter)
and one from stars in a cluster further away (and thus fainter).
This main-sequence comparison is best done using the whole main-sequence
rather than individual stars. But the method has some uncertainty.
We must assume that the main sequences of different clusters can
actually be compared. Other clusters' stars may have different compositions
from those in the Hyades, making them intrinsically brighter or
fainter (that is, not due to their distance).
Figure 6
Additionally,
the shapes of the main-sequences may differ, and this makes the
measurement of brightness differences even more complicated. Nevertheless,
this is still a very powerful method for extending distance measurements
out to far-away clusters, and it is one of the fundamental building
blocks for building the distance scale of the universe.
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