is no doubt that the Hubble Space Telescope in its first decade
of operation has had a profound impact on astronomical research.
But Hubble did much more than that. It literally brought a
glimpse of the wonders of the universe into millions of homes
worldwide, thereby inspiring an unprecedented public curiosity
and interest in science.
has seen farther and sharper than any optical/ultraviolet/infrared
telescope before it. Unlike astronomical experiments that
were dedicated to a single, very specific goal (like the Cosmic
Background Explorer), Hubble's achievements are generally
not of the type of singular discoveries. More often, Hubble
has taken what had been existing hints and suspicions from
ground-based observations and has turned them into certainty.
other cases, Hubble's level of detail forced theorists to
re-think previous broad-brush models and construct new ones
that would be consistent with the superior emerging data.
In a few instances, the availability of Hubble's razor-sharp
vision at critical events provided unique insights into individual
total, by observing more than 14,000 astronomical targets,
Hubble has contributed significantly to essentially all the
topics of current astronomical research, covering objects
from our own solar system to the most distant galaxies.
the solar system, Hubble allowed for a front seat view of
the impact of comet Shoemaker-Levy 9 on Jupiter. The high-resolution
images provided exquisite details on the plumes' geometry,
on the growth and dispersion of the impact features, and on
atmospheric waves expanding around the impact sites. The precise
nature of these waves is still the subject of some debate,
thus this is a case where by unveiling more details the "simple"
can become complex. The comet impact is a relatively rare
phenomenon, where a thousand years may pass before a similar
event is observed again.
CYCLE OF STARS
from planets to stars, Hubble has documented in unprecedented
detail the births and deaths of stars. It has visually demonstrated
that protoplanetary dust disks around young stars are common,
suggesting that at least the raw materials for planet formation
are in place.
has shown for the first time that jets in young stellar objects
emanate from the centers of accretion disks (in objects such
as Herbig Haro 30), thus turning what were previously merely
theoretical expectations into an observed reality. Hubble
has provided many spectacularly detailed images of stellar
deaths, in the form of morphologies of planetary nebulae,
a mysterious three-ring structure around Supernova 1987A,
and corrugated bipolar lobes in the Luminous Blue Variable
Eta Carinae. While some of the basic physics developed for
these objects from earlier ground-based observations has not
changed significantly, the dramatic realization that almost
none of these objects is spherically symmetrical, but rather
that bi-polarity and point-symmetry are extremely common,
has stimulated a flurry of theoretical work on nebular shaping.
Thus, again, broad-brush models proved insufficient given
the level of detail of the Hubble data.
the observations of Supernova 1987A, the closest supernova
in four centuries, have already provided (for the first time)
and will continue to provide for the next decade, details
on the interaction of a blast wave from a supernova with its
surrounding environment. Not only has the three-ring structure
been an unexpected feature, but the fireworks expected when
the supernova ejecta will hit the central ring (an event which
has already started) during the next decade will illuminate
the surrounding material and thereby literally throw light
on the exploding star's history.
STARS IN DENSE REGIONS
superb resolution is one of its major assets when observing
dense stellar environments. Hence, it is no wonder that Hubble
has produced a plethora of results on resolved stellar populations
in globular clusters (galactic and in the Local Group), in
field populations of nearby galaxies, and in stellar aggregates
in the Magellanic clouds. Exciting results in this area include:
The spread of ages among galactic globulars is relatively
narrow, implying a short time scale for the formation of spheroidal
components of galaxies.
The horizontal branch morphology has been determined in globulars
as far as in M31 and M33, providing clues concerning the formation
age of globulars in the Local Group.
Hubble has revealed for the first time the sequence of cooling
white dwarfs in several nearby globulars and explored the
bottom of the main sequence.
Hubble has shown that the star formation histories of resolved
dwarf galaxies exhibit a wide variety of behaviors.
Hubble has provided valuable information on star formation
and the Initial Mass Function (IMF) in the Magellanic clouds.
These data may have important implications for star formation
in the (a universe deficient in the heavier elements) early
the dense environments of galactic centers Hubble has confirmed
previous suspicions and provided decisive evidence showing
that supermassive black holes reside in the centers of many
(not necessarily active) galaxies. High-resolution images
revealed the presence of dusty gas tori around the central
object. The ability to spectroscopically determine velocities
at multiple locations led to reliable determinations of the
black hole masses.
the violent environment of colliding galaxies, Hubble showed
that young, massive, compact star clusters are formed when
two galaxies collide or interact strongly. The formation time
is of the order of 10 million years or less, and these clusters
may be the progenitors of globular clusters.
HOMES OF QUASARS
from ground-based observations suggested that quasars reside
in host galaxies, but Hubble unambiguously confirmed it. Furthermore,
using its superb resolution, Hubble has demonstrated that
a very high fraction of the hosts are interacting galaxies.
This information could be an important clue for how the central
black hole is fed.
its unique capability to collect ultraviolet light, Hubble
discovered low-redshift counterparts to the quasar absorption
systems discovered initially at high redshifts in ground-based
data. The Hubble discovery further allowed the low-redshift
absorbers to be directly identified as galaxies and confirmed
the theoretical expectation that the density of Lyman Alpha
forest lines is higher at low-redshift than might have been
suggested by a simple extrapolation from the high-redshift
expected, Hubble really shined in observations of the high-redshift
universe. Using deep imaging, as in the Medium Deep Survey,
and in the two Hubble Deep Fields, Hubble showed that the
angular sizes of faint galaxies are small. Since beyond a
redshift of 1 a small angular size corresponds to a physically
small source, this observation, when coupled to the one below,
can have important implications for galaxy formation.
high resolution obtained in the Hubble Deep Fields allowed
for a determination of galaxy morphologies at high redshift
and demonstrated that high-redshift galaxies have less well-defined,
more disturbed appearances. Generally, there is an increasing
fraction of irregular and multiple-component systems into
Near Infrared and Multi-Object Spectrometer observations showed
that this conclusion remains true, even when one takes into
account that optical images really give the ultraviolet rest
frame appearance of galaxies (and thus show mostly star formation
of the above findings generally support the hierarchical model
for structure formation, in that high redshift galaxies are
often only the building blocks from which present day galaxies
formed, via interactions and mergers.
of the ultraviolet luminosity density (using the Hubble Deep
Fields) have helped to sketch out the cosmic star formation
history back into the distant past. Much of the lower-redshift
data came from ground-based observations, but the Hubble Deep
Fields provided and inspired much of the follow-up, high-redshift
work in this area. It now appears that the cosmic star formation
rate was higher in the past, with a peak value around redshift
1.25. In the still more distant past, the star formation rate
was about constant up to about redshift 5.
since Edwin Hubble's discovery of the expansion of the universe
in the late 1920s, measuring the value of the Hubble constant
(the reciprocal of which indicates the age of the universe)
has been a prime target for observational cosmology. In May
1999 a Hubble key project team announced the completion of
a program aimed at measuring the distances to 18 galaxies,
some as far as 20 megaparsecs away (e.g. the Virgo cluster
galaxies).By calibrating a variety of methods with Cepheid
Variable distances, the team arrived at a value of 70 km/s/Mpc
for the Hubble constant, with an uncertainty of about 10 percent.
This project would have been absolutely impossible without
Hubble's resolution and depth.
calibrating the absolute magnitudes at maximum of a sample
of Type Ia supernovae, another team determined the distances
to galaxies in the Hubble flow, finding a value of 60 km/s/Mpc
(with a 10 percent uncertainty) for the Hubble constant. Thus,
the decades-long discrepancy among the values determined by
different groups (and different methods) is finally reaching
teamed up with X-ray and gamma-ray satellites, as well as
with ground-based optical telescopes in a quest for understanding
gamma-ray burst sources. Gamma-ray bursts may represent the
most powerful explosions in the universe since the Big Bang.
Before 1997 astronomers were frustrated: although more than
2,000 "bursts" had been observed, it was impossible even to
determine whether these fireballs occurred in our own galaxy's
halo or at cosmological distances. The discovery of X-ray
afterglows by the BeppoSax satellite, followed by the discovery
of optical transients (from the ground), led eventually to
a clear confirmation of the cosmological nature of at least
a subclass of bursts. Hubble provided images that showed unambiguously
that the gamma-ray bursts actually reside in galaxies that
are forming stars at high rates. Furthermore, by pinpointing
a burst's precise location, Hubble showed that at least in
one case the gamma-ray burst is probably not associated with
an active galactic nucleus.
of the most dramatic astronomical discoveries of this century
came in 1998, when two teams found (independently) strong
evidence that the cosmic expansion is accelerating. This conclusion,
based on distance measurements to Type Ia supernovae (if confirmed),
also implies the existence of a cosmological constant, which
contributes about 70 percent of the cosmic energy density.
While many of the observations were made with the Keck telescope,
Hubble provided the resolution needed for the high-redshift
(z>~0.5) supernovae, for their light to be correctly distinguished
from that of the host galaxies. Hubble's contribution was
crucial in establishing that the more distant Type Ia supernovae
are dimmer (by about 0.25 magnitude) than expected from the
examination of the above list of accomplishments reveals that
Hubble has enormously improved our understanding of the cosmos,
from the universe's size, age, and fate, to the meteorology
of planets, from stellar births to their deaths. But perhaps
even more importantly, Hubble has not only established itself
as a premier observatory that makes discoveries that are at
the forefront of astronomy, it has become the public's premier
gateway to science.
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