Background Science
The Deep Fields
One of the main scientific justifications for building Hubble was to measure the size and age of the Universe and test theories about its origin. Images of faint galaxies give 'fossil' clues as to how the Universe looked in the remote past and how it may have evolved with time. The Deep Fields gave astronomers the first really clear look back to the time when galaxies were forming.
The idea for the Hubble Deep Fields originated in results from the first deep images taken after the repair in 1993. These images showed many galaxies, which were often quite unlike those we see in the local Universe and could not otherwise be studied using conventional ground-based telescopes.
The first Deep Field, the Hubble Deep Field North (HDF-N), was observed over 10 consecutive days during Christmas 1995. The resulting image consisted of 342 separate exposures, with a total exposure time of more than 100 hours, compared with typical Hubble exposures of a few hours. The observed region of sky in Ursa Major was carefully selected to be as empty as possible so that Hubble would look far beyond the stars of our own Milky Way and out past nearby galaxies.
The results were astonishing! Almost 3000 galaxies were seen in the image. Scientists analysed the image statistically and found that the HDF had seen back to the very young Universe where the bulk of the galaxies had not, as yet, had time to form stars. Or, as the popular press dramatically reported, 'Hubble sees back to Big Bang'...
These very remote galaxies also seemed to be smaller and more irregular than those nearby. This was taken as a clear indication that galaxies form by gravitational coalescence of smaller parts.
In 1996 it was decided to observe a second Deep Field, the Hubble Deep Field South (HDF-S), to assess whether the HDF-N was indeed a special area and thus not representative of the Universe as a whole. This time the field also contained a quasar, which was used as a cosmological lighthouse and provided valuable information about the matter between the quasar and the Earth.
After the Hubble observations of HDF-N and -S, other ground- and space-based instruments targeted the same patches of sky for long periods. Some of the most interesting results seem to emerge from these fruitful synergies between instruments of different sizes, in different environments and with sensitivity to different wavelengths.
Stefano Cristiani
Space Telescope-European Coordinating Facility (ST-ECF)
In my view the Hubble Deep Fields are some of the images that have made the greatest impact on observational cosmology so far. These impressive dips into the depths of space and time have allowed astronomers to glimpse the first steps of galaxy formation more than 10 billion years ago and are without doubt some of the great legacies of the Hubble Space Telescope.
The Age and Size of the Universe
Cepheids
The top-ranked scientific justification for building Hubble was to determine the size and age of the Universe through observations of Cepheid variables in distant galaxies. This scientific goal was so important that it put constraints on the lower limit of the size of Hubble's primary mirror.
Cepheids are a special type of variable star with very stable and predictable brightness variations. The period of these variations depends on physical properties of the stars such as their mass and true brightness. This means that astronomers, just by looking at the variability of their light, can find out about the Cepheids' physical nature, which then can be used very effectively to determine their distance. For this reason cosmologists call Cepheids 'standard candles'.
Several groups of astronomers have used Hubble to observe Cepheids with extraordinary results. The Cepheids have then been used as stepping-stones to make distance measurements for supernovae, which have, in turn, given a measure for the scale of the Universe. Today we know the age of the Universe to a much higher precision than before Hubble: around 15 billion years.
Gustav A. Tammann
Astronomer, University of Basle
We certainly live in exciting times. Hubble has made enormous progress possible within cosmology. Today we have a much more unified cosmological picture than was possible even five years ago when people were talking of 'the cosmology in crisis'. We have seen a dramatic change from misery to glory!
Bruno Leibundgut
Astronomer, European Southern Observatory (ESO)
Hubble gave us the distance measurements of the first four supernovae that made us realise something was wrong with our present understanding of the Universe. Even though the definite proof that the Universe is accelerating came later, we could not reconcile our Hubble observations with a Universe where the expansion is slowing down.
Supernovae
Hubble's sharp vision means that it can see exploding stars, supernovf that are billions of light years away and difficult for other telescopes to study.
Most scientists today believe that the expansion of the Universe is accelerating. This result came from combined measurements of remote supernovf with most of the world's top-class telescopes, including Hubble, and it was a very surprising one. For many years cosmologists have discussed whether the expansion of the Universe would stop in some distant future or continue ever more slowly. From the new results it seems clear that the expansion is nowhere near slowing down. In fact, due to some mysterious property of space itself (called vacuum energy), the expansion is accelerating and will continue forever.
Image above: Missing link found
between supernovae and black
holes
Hubble has given these supernovae measurements an added precision, mostly due to its high resolution. From the ground an image of the supernova usually blends in with the image of its host galaxy. Hubble can distinguish the light from the two sources and thus measure the supernova directly.
Stellar Evolution
Most of the light and radiation we can observe in the Universe originates in stars - individual stars, clusters of stars, nebulae lit by stars and galaxies composed of billions of stars. Stars are spheres of glowing hydrogen and other chemical elements which produce their prodigious energy output by converting lighter elements to heavier ones through nuclear processes similar to those in hydrogen bombs. Like human beings they are born, mature and eventually die, but their lifetimes are vastly longer than our own.
Hubble has gone beyond what can be achieved by other observatories by linking together studies of the births, lives and deaths of individual stars with theories of stellar evolution. In particular Hubble's ability to probe stars in other galaxies enables scientists to investigate the influence of different environments on the lives of stars. This is crucial in order to be able to complement our understanding of the Milky Way galaxy with that of other galaxies.
Hubble was the first telescope to directly observe white dwarfs in globular star clusters. White dwarfs are stellar remnants and provide a 'fossil' record of their progenitor stars which shone so brightly that they long ago exhausted their nuclear fuel. Through these measurements it is possible to determine the ages of these ancient clusters which is an important cosmological tool.
Another area where Hubble's work has been widely acknowledged is the linking of star formation (also see pages 28-29) with stellar evolution. Hubble's infrared instrument, NICMOS, is capable of looking through the dust surrounding newly born stars. Some of the most surprising discoveries so far have come about by peering through the clouds of dust surrounding the centre of our Milky Way. Astronomers found that this centre, which was thought to be a calm and almost 'dead' region, is in fact populated with massive infant stars gathered into clusters.
Gerard Gilmore
Astronomer, University of Cambridge
Hubble has in my view revolutionised the study of globular clusters - especially those in other galaxies. These objects are so dense and the stars so tightly packed together that it is almost impossible to separate the stars from each other with ground-based telescopes. We have been able to measure what kind of stars they are composed of, how they evolve and how gravity works in these complex systems.
The last phases of solar-like stars have been investigated through observations of planetary nebulae and proto-planetary nebulae. These are colourful shells of gas expelled into space by dying stars. The varying shapes and colours of these intricate structures with different colours tracing different, often newly created, chemical elements, have shown that the final stages of the lives of stars are more complex than once thought.
Our Solar System
Rudi Albrecht
Space Telescope-European Coordinating Facility (ST-ECF)
We conducted an intensive series of observations of Pluto with Hubble followed by advanced data processing on the ground. We saw surface features emerge for the first time in history on our screens. For me, personally, it was a memorable experience to be able to show this image to the original discoverer of Pluto, Clyde Tombaugh, and in this way let Hubble pay a tribute to his great discovery.
Hubble's high-resolution images of the planets and moons in our Solar System can only be surpassed by pictures taken from spacecraft that actually visit them. Furthermore Hubble can return to look at these objects periodically and so observe them over much longer periods (years) than any passing probe.
Regular monitoring of planetary surfaces is vital in the study of planetary atmospheres and geology, where evolving weather patterns such as dust storms can reveal much about the underlying processes. Hubble can also observe geological phenomena such as volcanic eruptions directly. The asteroid Vesta is only 500 km in diameter and was surveyed by Hubble from a distance of 250 million km. The resulting map of the surface shows a strange world with many lava flows, dominated by a gigantic impact crater.
Hubble is also able to react quickly to sudden dramatic events occurring in the Solar System. Most of the world kept an eye on Comet Shoemaker-Levy 9 when it made its fiery plunge into the atmosphere of the giant planet Jupiter during the period 16-22 July 1994. Hubble followed the comet fragments on their last journey and delivered stunning high-resolution images of the impact scars, from which important new information on conditions in the Jovian atmosphere was obtained.
On their fly-bys past Jupiter and Saturn the Voyager probes showed that these gas giants had aurorf similar to the northern lights here on Earth. However, Hubble's images of the aurorf were the first to reveal the delicate structure that so impressed many scientists. Hubble carries cameras that are sensitive to ultraviolet light, which is absorbed by the atmosphere and hence not seen by ground-based observatories.
Pluto is the only planet not yet visited by space probes, but in 1994 Hubble made the first clear images showing Pluto and its moon Charon as separate objects from a distance of 4.4 billion kilometres.
Black Holes, Quasars, and Active Galaxies
A black hole
In the 1950s and 1960s astronomers had found objects, such as quasars and radio sources, whose energy output was so immense that it could not be explained by traditional sources of energy such as that produced by normal stars. It was suggested that their vast energy output could best be explained if massive black holes were at the centres of these objects.
Prior to the launch of Hubble a handful of black-hole candidates had been studied, but the limitations of ground-based astronomy were such that irrefutable evidence for their existence could not be obtained. Black holes themselves, by definition, cannot be observed, since no light can escape from them. However, astronomers can study the effects of black holes on their surroundings. These include powerful jets of electrons that travel huge distances, many thousands of light years from the centres of the galaxies. Matter falling towards the black hole can also be seen emitting bright light and, if the speed of this falling matter can be measured, it is possible to determine the mass of the black hole itself. This is not an easy task and it requires the extraordinary capabilities of Hubble to carry out these sophisticated measurements.
Hubble observations have been fundamental in the study of the jets and discs of matter around a number of black holes. Accurate measurements of the masses have been possible for the first time. Hubble has found black holes 3 billion times as massive as our Sun at the centre of some galaxies.
While this might have been expected, Hubble has surprised everyone by providing strong evidence that black holes exist at the centres of all galaxies. Furthermore, as it appears that larger galaxies are the hosts of larger black holes, there must be some mechanism that links the formation of the galaxy to that of its black hole and vice versa. This has profound implications for theories of galaxy formation and evolution and will certainly be the subject of considerable additional research with Hubble during the next decade.
Quasars
In the 1980s observations made with different ground-based telescopes showed that some quasars were surrounded by fuzzy light. It was suspected that the quasars reside in galaxies and that the fuzzy patches of light could be those host galaxies.
Hubble's high-resolution Faint Object Camera images showed with clarity that this is indeed the case. More importantly the hosts of quasars appear to be galaxies of all types, contrary to earlier predictions that favoured the idea that quasars were to be found only in elliptical galaxies. This is important since the light from quasars is believed to be produced by black holes at the centres of their host galaxies. Astronomers can now show that this is indeed the case and that quasar host galaxies are the same types of galaxies found in our neighbourhood.
This realisation also leads to the question of why most of the nearby galaxies, including our own Milky Way have 'dormant' black holes, namely black holes which are inactive at this time. This will be the subject of new studies with Hubble.
Unified model
Today most astronomers believe that quasars, radio galaxies and the centres of so-called active galaxies are just different views of more or less the same phenomenon: a black hole with energetic jets beaming out from two sides. When the beam is directed towards us we see the bright lighthouse of a quasar. When the orientation of the system is different we observe it as an active galaxy or a radio galaxy. This 'unified model' has gained considerable support through a number of Hubble observational programmes. The simplistic early ideas have, however, been replaced by a more complex view of this phenomenon - a view that will continue to evolve in the years to come.
Duccio Macchetto
ESA astronomer, Head of the Science Policies Division, STScI
Hubble provided strong evidence that all galaxies contain black holes millions or billions of times heavier than our Sun. This has quite dramatically changed our view of galaxies. I am convinced that over the next 10 years Hubble will find that black holes play a much more important role in the formation and evolution of galaxies than we believe today. Who knows, it may even influence our picture of the whole structure of the Universe...?
Formation of Stars
To astronomers and laymen alike the topic of star formation has always been a particularly appealing one. Important clues about our genesis lie hidden behind the veil of the dusty, and often very beautiful, star- forming molecular clouds. Our Earth and the Solar System were born 4.6 billion years ago and our knowledge of the event is sparse. Astronomers turn their eyes to the birth of other stars and stellar systems in neighbouring stellar 'maternity wards' and use these as a time machine to see a replay of the events that created our own Solar System.
Hubble looks through
cosmic zoom lense
Hans Zinnecker
Astrophysicist, Astrophysical Institute Potsdam
Hubble has had a major impact in two areas in the field of star formation. Firstly it has studied the formation of stars like our Sun and has literally seen dusty discs which may end up as planetary systems around those stars. Secondly Hubble has made an impact in the area one could call 'cosmological star formation', that is, the formation of stars all over the Universe. The Hubble Deep Field North for instance opened up the box and allowed us to follow the history of star formation through the entire Universe and in this way enabled us to study the 'cosmic evolution' of the stars.
The large mosaic of 15 Hubble images showing the central part of the Orion complex is one of the most detailed images of a star-forming region ever made. It shows a very young star cluster blowing a 'bubble' in its remnant parent cloud of glowing gas so that the stars start to be seen in visible light - like the smoke in a forest fire being driven away by the heat.
Hubble's high resolution has been crucial in the investigation of the dust discs, dubbed proplyds, around the newly born stars in the Orion Nebula. The 'proplyds' may very well be young planetary systems in the early phases of their creation. The details that are revealed are far finer than than can be seen with ground-based instruments and, thanks to Hubble, we today have visual proof that dusty discs around young stars are common.
Since star birth always seems to take place in dusty environments, Hubble's infrared capabilities have been a very important factor. The infrared instrument NICMOS can peer through much of the dust and reveal the complex processes taking place in star-forming regions. Otherwise invisible close double and multiple stars have been discovered, as well as faint substellar brown dwarf companions. With NICMOS and its visual counterpart WFPC2, Hubble has observed giant jets of material spewing out from infant stars surrounded by large discs of dust, giving a fantastic view into the dramatic first steps in the lives of newly born stars.
The Composition of the Universe
The chemical composition of the Universe and the physical nature of its constituent matter are topics that have occupied scientists for centuries. From its privileged position above the Earth's atmosphere Hubble has been able to contribute significantly to this area of research.
All over the Universe stars work as giant reprocessing plants taking light chemical elements and transforming them into heavier ones. The original, so-called primordial, composition of the Universe is studied in such fine detail because it is one of the keys to our understanding of processes in the very early Universe.
Helium in the early Universe
Shortly after the First Servicing Mission successfully corrected the spherical aberration in Hubble's mirror a team led by European astronomer Peter Jakobsen investigated the nature of the gaseous matter that fills the vast volume of intergalactic space. By observing ultraviolet light from a distant quasar, which would otherwise have been absorbed by the Earth's atmosphere, they found the long-sought signature of helium in the early Universe. This was an important piece of supporting evidence for the Big Bang theory. It also confirmed scientists' expectation that, in the very early Universe, matter not yet locked up in stars and galaxies was nearly completely ionised (the atoms were stripped of their electrons). This was an important step forward for cosmology.
Quasar lighthouses
This investigation of helium in the early Universe is one of many ways that Hubble has used distant quasars as lighthouses. As light from the quasars passes through the intervening intergalactic matter, the light signal is changed in such a way as to reveal the composition of the gas. The results have filled in important pieces of the puzzle of the total composition of the Universe now and in the past.
Dark Matter
Today astronomers believe that close to 95% of the mass of the Universe consists of dark matter, a substance quite different from the normal matter that makes up atoms and the familiar world around us. Hubble has played an important part in work intended to establish the amount of dark matter in the Universe and to determine its composition. The riddle of the ghostly dark matter is still far from solved, but Hubble's incredibly sharp observations of, for instance, gravitational lenses (see pages 32-33) have provided stepping stones for future work in this area.
Peter Jakobsen
ESA astronomer, NGST Study Scientist
I believe that we now have a good understanding of the amount and composition of 'normal' matter of the Universe. By looking further and further back in time we are now beginning to piece together the history of this matter since it emerged from the Big Bang and eventually collapsed to form the stars and galaxies that we see in the present day Universe. Hubble has played a very important part in unravelling this history. With the Next Generation Space Telescope we hope to reach back to even earlier times and see the very first stars turn on.
Gravitational Lensing
Light does not always travel in straight lines. Einstein predicted in his Theory of General Relativity that massive objects will deform the fabric of space itself. When light passes one of these objects, such as a cluster of galaxies, its path is changed slightly. This effect, called gravitational lensing, is only visible in rare cases and only the best telescopes can observe the related phenomena.
Hubble's sensitivity and high resolution allow it to see faint and distant gravitational lenses that cannot be detected with ground-based telescopes whose images are blurred by the Earth's atmosphere. The gravitational lensing results in multiple images of the original galaxy each with a characteristically distorted banana-like shape.
Hubble was the first telescope to resolve details within these multiple banana-shaped arcs. Its sharp vision can reveal the shape and internal structure of the lensed background galaxies directly and in this way one can easily match the different arcs coming from the same background galaxy by eye.
Since the amount of lensing depends on the total mass of the cluster, gravitational lensing can be used to 'weigh' clusters. This has considerably improved our understanding of the distribution of the 'hidden' dark matter in galaxy clusters and hence in the Universe as a whole.
Richard Ellis
Astronomer, University of Cambridge and California Institute of Technology
When we first observed the galaxy cluster Abell 2218 with Hubble in 1995 we mainly aimed at studying the cluster and its galaxies. But we got a surprise. The images showed dozens and dozens of gravitationally lensed arcs. When we showed these ultrasharp images to our colleagues they could immediately see the importance of using gravitational lensing as a cosmological tool.