A Decade of Discovery
All About Hubble

There 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.

Hubble 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.

In 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 phenomena.

In 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.


In 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.


Moving 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.

Hubble 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.


Furthermore, 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.


Hubble's 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:

(i) The spread of ages among galactic globulars is relatively narrow, implying a short time scale for the formation of spheroidal components of galaxies.

(ii) 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.

(iii) 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.

(iv) Hubble has shown that the star formation histories of resolved dwarf galaxies exhibit a wide variety of behaviors.

(v) 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 universe.


In 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.


In 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.


Findings 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.

Using 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 observations.

As 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.

The 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 the past.

The 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 knots).

Both 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.

Determinations 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.


Ever 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.

By 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 its resolution.


Hubble 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.


One 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 Hubble Law.

An 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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