|
As an amateur
astronomer I used to wonder how it was that professional astronomers
could tell so much about what was happening thousands of lightyears
away. Just as many of you now do, I remember reading all of
the popular journals to keep up to date on recent discoveries.
But reasdin those articles was like watching the crankshaft
in a car turning, but not being able to tell just how the
gears were spinning around in the transmission to make the
whole thing work.
As a professional astronomer, I now realize that there is
nothing especially mysterious about this process, after all,
trained artists and musicians view DaVinci's paintings and
Bach's cantatas in a much more abstract way than the novice.
When astronomers contemplate the universe and study Nature's
handiworks, they take the simple perceptions provided by their
instruments and refine them to the point where the subtilities
of how things work can be made more easily recognizable. Astronomers
try to 'see' the hidden relationships that underly seemingly
unrelated characteristics of the world; relationships that
lie at the core of Nature.
You have already
been on part of this journey and have seen how infrared astronomers
gather their data ( Astronomical Ballooning)
and how they analyze it to make maps of particular objects
in the sky ( A Quickie Guide to Analyzing
Balloon Data). What I would now like to do is to show
you some of the 'behind-the-scenes' thinking that goes along
with transforming a calibrated, infrared map into a quantitative
statement about a particular astronomical object. To focus
this excercise, let's have a look at the center of our own
galaxy; a region of considerable mystery and excitement.
A
PORTRATE OF THE GALACTIC CENTER
In the last
50 years, over 550 papers have been written by hundreds of
astronomers on the galactic center. Like painters working
on a canvas, some astronomers have used this data to paint
bold, agressive swatches of color, others have filled-in details
here and there. From these activities, astronomers have built-up
a tantalyzing picture of what lurks in the depths of the Milky
Way's core. This undertaking is even more impressive when
you consider that the galactic center is completly hidden
from view by ten thousand parsecs of intervening dust and
gas from the disc of the Milky Way. Only radio, infrared and
some X-ray observations have been successful in penetrating
this murky veal.
Within 800 parsecs
of the galactic core, radio astronomers have detected the
weak signals from molecular hydrogen and carbon monoxide,
suggesting the presence of a rapidly rotating disc of hydrogen
gas that is inclined to the galactic plane by about 24 degrees!
No one knows why this structure is tilted like this, but it
might have something to do with tidal torques between this
region of the galaxy and the rest of the Milky Way's disc,
or with gravitational perturbations due to the Magellanic
Clouds.
Within 250 parsecs
of the core, molecular clouds can be detected that look, superficially,
like those which astronomers have observed in the spiral arms
of the Milky Way. The total amount of molecular hydrogen in
these clouds has been estimated by Thomas Bania of Boston
University, as 350 million suns. Early mathematical models
by Nick Scoville at Columbia University in 1972 suggested
that the bulk of the molecular clouds form a giant ring 250
parsecs in radius that is expanding at about 50 km/sec. The
discovery of outwardly expanding clouds from the galactic
core soon led many astronomers to speculate that the 'molecular
ring' may have been ejected in a titanic explosion in the
Milky Way's core millions of years ago.
To compliment
the molecular picture, radio astronomers have been able to
show over the years that the entire region, extending hundreds
of parsecs from the core, is suffused by a hot, ionized hydrogen
gas, some of which is in the form of clumps that look suspiciously
like ordinary HII regions. HII regions are normally associated
with star-forming regions in the spiral arms of the Milky
Way, containing massive, young O or B-type stars.
The galactic
center also contains a number of objects that remain enigmatic
for example: in 1983, F. Yusef-Zadeh, Mark Morris and Don
Chance at Columbia and UCLA, used the VLA to map the galactic
center. They discovered enormous filiments extending nearly
100 parsecs in length above and below the molecular ring.
These filiments suggest the galactic center contains an enormous
magnetic field that is well-organized over at least this same
scale, but where does it come from?
Tantalyzing
glimpses of the galactic core have also been provided by Robert
Brown, Ken Johnston and Ronald Ekers in 1983. These astronomers
seem to have observed spiral-shaped streamers of ionized matter
falling into ... what? A blackhole?
The inner few
parsecs of the galactic core are surrounded by a dusty ring
according to a 1982 study by the infrared astronomers Eric
Becklin, Mike Werner and Ian Gatley. Inside this dust ring,
dozens of clouds were discovered by John Lacy and his co-workers.
Each cloud has about the mass of the sun and seems to be expanding
at 100 km/sec. The clouds don't seem to have the basic properties
we normally associate with planetary nebulae or HII regions.
What are they?
Gamma ray astronomers
over the last 10 years have also discovered a powerful source
of radiation in the core, of the kind produced when electrons
and positrons annihilate one another. Theoretical models suggest
that the source must be less than about 1,300 miles across.
Could this be the fingerprint of a 1000 solar mass blackhole?
The galactic
center is certainly a dynamic, energetic place, unlike any
similar-sized local to be found anywhere else in our galaxy.
Is there any new piece of information that we can contribute,
using our infrared map, that will provide us with a better
understanding of this strange region in the Milky Way? Let's
see if we can use our map to work-out an answer to this basic
question, 'What is it that makes the galactic center such
a powerful source of infrared radiation?'.
THROUGH
A LOOKING GLASS, DARKLEY
Looking at our
map, we notice two kinds of emission at far-infrared wavelengths,
a smooth component that extends to a radius of about 200 parsecs
along the plane and 30 parsecs above and below it; and a clumpy
component occupying the same region along the plane but more
narrowly spread above and below it. If we treat the individual
clumps as independent sources and assume that they are all
located in the galactic center, we can use the amount of detected
infrared emission to calculate the luminosity of each source.
In several cases, we see that some infrared sources are adjacent
to optical nebulae. Since we know that the dust in the galactic
plane makes it impossible to see optical emission beyond a
few kiloparsecs, these sources are probably nearby, and not
related to the galactic center at all. So, let's ignore these
foreground sources. They are irrelevant to our present analysis
but may be useful, later-on, in another avenue of research.
The estimates
we get for the luminosities of each of the sources, contains
several kinds of uncertainty. First of all, from studies by
other astronomers, we only know the approximate distance to
the galactic center to 10,000 +/- 500 parsecs. Also, our measurements
only give infrared brightness to an accuracy of 20_% or so.
Keeping these, and other, sources of uncertainty in mind ,
we plug our best estimates into the proper equation and turn
the crank. We discover that the far-infrared sources shine
with a range of luminosities between 140,000 and 81 million
times that of the sun! The WEAKEST source has about the same
output as the Great Nebula in Orion!
Comparing our
far-infrared map with the radio map made in 1978 by Dennis
Downes and his co-workers at Bonn, also reveals another important
fact, there is a striking similarity between the radio and
infrared emissions. The far-infrared and radio emissions seem
to be coming from the same parts of the sky with about the
same relative brightness and angular distributions. This suggests
that, whatever these sources might be, they seem to be producing
both kinds of radiation at the same time. There are many physical
processes that can produce both radio and far-infrared emission.
Astronomers have observed a number of these at work in other
parts of the Milky Way. The most common mechanism seems to
involve very hot and luminous stars between spectral classes
O and B. These stars are capable of producing large quantities
of radiation, powerful enough to ionize the surrounding hydrogen
gas and to heat the nearby dust.
Now that we
suspect that O or B-type stars along with dust and gas are
involved in each of these sources in the galactic center,
we use these ingredients in a detailed, mathematical model,
to predict just how much radio and far-infrared emission we
should expect to see if a young, luminous star were embedded
in a dense cloud of dust and gas. This model, like all others
astronomers use, is based on a specific set of physical principles
or laws which we think are relevant to the particular data
we are trying to understand. Mathematical models of dusty,
HII regions, for example, show that the energy from the star
can be converted into radio and far-infrared emission as the
radiation makes its way from the stars surface into the surrounding
cloud of gas and dust. For the objects we see in the galactic
center, these models predict that each source could be powered
by small clusters containing 1 to 5 stars between spectral
classes O4 and O8, although the precise numbers are somewhat
ambiguous. The models also predict that each source contains
about as much gas and dust as HII regions found elsewhere
in the Milky way. At least in their gross physical properties,
the objects that appear in the maps do not represent some
revolutionary new species of astronomical object. We can also
conclude from this analysis, that the formation of massive
O-type stars has occurred in the heart of our galaxy during
the last few million years. This is how long other mathematical
models tell us it takes for such stars to be born. The BIG
QUESTION that astronomers have yet to answer is whether the
star-forming mechanism for O stars is the same in the galactic
center as in the spiral arms of the Milky Way. We just don't
have enough information to answer this tantalyzing question.
Yet.
I mentioned
that there seemed to be two components to the galactic center's
far-infrared and radio emission. If the clumpy component represents
young, star- forming regions, what does the extended component
consist of? Only about 15_% of the galactic centers total
infrared luminosity can be accounted for by the clumpy component
we have discovered, where does the remaining 85_% come from?
A simple model consistent with the radio and infrared luminosity
of the extended emission, suggests that 30 to 50 thousand
B0-type stars embedded in a uniform, dusty interstellar medium
could give us something that looks superficially like the
extended component. These stars would be too numerous and
faint for us to see individually with our balloon-borne instruments
so all we would observe is their combined, unresolved emission.
A search through some of the 550 papers published on the galactic
center reveals that large populations of B stars in the galactic
center (though not quite so large as the one we are proposing)
was already proposed by Dr. Luis Rodriguez and Prof. Eric
Chaisson at Harvard University back in 1978. This is encouraging
since it means that other astronomers, using entirely different
kinds of data, have already come to a similar conclusion as
our data seems to be sugesting to us.
If our inference
for the sizes of the two populations were correct, even by
a factor of 10, we would be presented with an interesting
problem; How do we explain the mutual existence of several
hundred O-type stars in the discrete sources, and the several
tens of thousands of B-type stars implied by the energy requirements
of the extended emission? If O and B-type stars were born
in the galactic center with the same relative frequency as
in the rest of the galaxy, 30,000 B0 stars would imply about
160,000 O-type stars. We only infer 1 percent of this number
of O-type stars in our radio and infrared data: Where are
all the others? There are several possibilities, among which
are:
1) In the last
10 million years, most of the O-type stars have all evolved
into red supergiants, neutron stars or even blackholes, leaving
only a few stragglers left which are the ones we are now observing.
2) For some
unknown reason, the frequency of forming O-type stars in the
galactic center is lower than elsewhere in the Galaxy. Only
one O star is formed in the galactic center for every 200
B0 stars, rather than one in five as in the rest of the Galaxy.
3) Our estimates
for the number of B stars are too high by a factor of 40;
the extended far-infrared emission is due to the indigeneous
Pop-II stars. The radio emission we are seeing comes from
supernovae remnants not from B-type stars.
It is at this
point that we begin to see a glimpse of perhaps part of the
explaination. Our mathematical model for the Sagittarius-A
source, the core of the Milky Way, predicts that a cluster
of about 10 O4 stars are required to give the combined radio
and infrared emission. Such a source would have an effective
temperature equivalent to that of a typical O4 star which
is about 50,000 K. Instead, astronomers who have studied the
abundances of certain ionic species of atoms, tell us that
the radiation field in the core cannot have a temperature
greater than about 37,000 K. Only stars with spectral classes
later than O8 satisfy this constraint. This means that our
mathematical model is not valid but that if we used about
150 O8 stars, our predictions would more nearly match the
available data on this particular source. This also means
that there are no stars in classes O4 through O7.5 so there
must be some kind of bottle-neck in the mechanism that produces
them. This could substantiate the second proposal we made
in the previous list. Future studies, or a more critical evaluation
of the existing body of data, may help us to decide between
these, and other, possibilities.
In the last
50 years, hundreds of astronomers all over the world have
turned the ir instruments towards the galactic center. Although
we may never visit this mysterious enigma in the sky, in admiring
it from a distance we have learned a great deal about the
stellar furnaces that illuminate it, the molecular clouds
that inhabit it, and the strange objects that lurk in its
murky depths.
There is no
real end to this process. As the quality of our data increases,
we can begin to ask more specific questions; their answers
help refine our physical models and light the way for new
insights. We will never be able to visit the galactic center
so our models provide us with our only physical means of exploring
this region 'first-hand'.
In our little
exercise, we have reduced the mysterious heart of the Milky
Way into an odd collection of numbers. Anyone who sees this
process as the dissection of Beauty into hard, cold, Facts
has sadly missed the point altogether. To paraphrase Oliver
Wendell Holmes, Astronomy '...is painting a picture, not doing
a sum'. Today we only draw crude, child-like sketches of the
galactic center, perhaps tomorrow we may acheive a DaVincian
mastery of its detail.
|
