HUBBLE

    15 YEARS OF DISCOVERY

    BY LARS LINDBERG CHRISTENSEN BOB FOSBURY

    ILLUSTRATIONS AND LAYOUT BY MARTIN KORNMESSER

    This book is dedicated to all the hard working people in the USA and Europe who have made the Hubble

    Space Telescope an incredible scientific successLars Lindberg Christensen ESAST-ECF

    NASAESA Hubble Space Telescope

    Garching 85748, Munich, Germany

    lars@eso.org

    Robert A. Fosbury ESAST-ECF

    NASAESA Hubble Space Telescope

    Garching 85748, Munich, Germany

    rfosbury@eso.org

    Library of Congress Control Number: 2005935456

    ISBN-10: 0-387-28599-7

    ISBN-13: 978-0387-28599-3

    Printed on acid-free paper.

    006 Springer Science+Business Media, Inc.

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    ·2HUBBLE

    15 YEARS OF DISCOVERYThe Sombrero Galaxy

    The Sombrero galaxy is one of the

    Universe's most stately and photogenic

    galaxies. The galaxy's hallmark is a

    brilliant white, bulbous core encircled by

    thick dust lanes comprising the spiral

    structure of the galaxy.

    NASAESA and The Hubble Heritage Team (STScIAURA)TABLE OF CONTENTS

    FOREWORD BY MALCOLM LONGAIR 5

    PREFACE 7

    INTRODUCTION 9

    1THE HUBBLE STORY 11

    2HUBBLE UP CLOSE 21

    3PLANETARY TALES 29

    4THE LIVES OF STARS 39

    5COSMIC COLLISIONS 59

    6MONSTERS IN SPACE 71

    7GRAVITATIONAL ILLUSIONS 81

    8BIRTH AND DEATH OF THE UNIVERSE 89

    9LOOKING TO THE END OF TIME 97

    10 HUBBLE GALLERY 105The Hubble Space Telescope has undoubtedly had a greater public impact than any

    other space astronomy mission ever. The images included in this beautiful volume

    are quite staggering in what they reveal about the Universe we live in and have already

    become part of our common scientific and cultural heritage.

    But what about the science impact? It is no exaggeration to say that the scientific

    output of the mission has far exceeded the most optimistic expectations of all those

    involved in the planning and execution of the project.When I joined the project in 1977,I had to describe the astronomy programme I would carry out with the Hubble once it

    was in operation in orbit. Seventeen years later when I receivedmy first data, I was quite

    staggered by the quality of the images and also by the totally new science which they

    revealed about the ways in which relativistic jets can illuminate the environments of

    active galaxies. This is a repeated theme in essentially all areas explored by the

    Telescope. The images are not only beautiful, but are full of spectacular new science,much of it undreamed of by the astronomers involved. A good example is the discovery

    of protostellar discs seen in silhouette against the bright background of the Orion

    Nebula. Another is the ability to discover distant star forming galaxies by imaging in a

    number of wavebands. The observation of distant supernovae has enabled the present

    acceleration of the Universe as a whole to be measured – an undoubted triumph. And

    then there are the spectacular images of the Hubble Deep and Ultra-Deep Fields which

    have revealed what are almost certainly young galaxies in the process of forming the

    galaxies and larger scale structures we observe about us today. But these are only a few

    random samples of the wealth of scientific knowledge which has accrued from the

    mission. Every picture tells a wonderful story which has already been built into our

    picture of the evolving Universe.

    What are the lessons to be learned from this spectacular success? The route to new

    understanding is through the ability to observe the Universe in new ways with tech-

    niques, that extend observational capability by a factor of 10 or more. In the case of the

    Hubble Space Telescope, the gains in angular resolution, or sharpness, and correspond-

    ing sensitivity, as well as the remarkable stability of the instruments in the remote

    environment of space, have given it unprecedented power to uncover new astrophysics.

    The results are a wonderful tribute to the dedicated efforts of many scientists, astrono-

    mers, engineers, managers and administrators, as well as to the vision of NASA and ESA

    in enabling the Hubble Space Telescope to come about. Long may this vision and the

    ability to inspire the public imagination continue as an essential means of deepening

    our understanding of the Universe.

    MalcolmLongair

    4 April 2005

    FOREWORD

    5

    NGC 346

    Hubble's exquisite sharpness has

    plucked out an underlying

    population of infant stars

    embedded in the nebula NGC 346

    that are still forming from

    gravitationally collapsing gas

    clouds.

    NASA, ESA and A. Nota (STScIESA)The long-term wellbeing and cultural development of humanity depend on scientific

    research and technological development. The communication of scientific discoveries

    and information about scientists and their work to the public are vital components of the

    scientific process. However, the competition for attention in today’s mass-media market

    is fierce.

    This book takes a closer look at whatmay be the world’smost successful scientific project.

    The fifteenth anniversary of Hubble’s launch, which took place on the 24th April 1990,presented the ideal opportunity for a spectacular project to seize the attention of the

    public. The story of a journey through space and time revealed by the telescope is told in a

    way that we hope will appeal especially to the younger generation. It will be their enthu-

    siasmthat powers the future of the scientific endeavour.

    We should like to thank Stefania Varano, Stuart Clark and Anne Rhodes who all worked on

    the film manuscript that laid the foundation for important parts of this book. Unless

    otherwise noted, the images in this book were taken by the NASAESA Hubble Space

    Telescope and should be credited to NASA, ESA and the individual scientists (see

    www.spacetelescope.org for the exact details).

    Lars Lindberg Christensen and Bob Fosbury

    Munich, 23 November 2005

    PREFACE

    The Cone Nebula

    Radiation from hot, young stars

    (located beyond the top of the

    image) has slowly eroded the

    nebula over millions of years.

    Ultraviolet light heats the edges of

    the dark cloud, releasing gas into

    the relatively empty region of

    surrounding space.

    7

    NASA, Holland Ford (JHU), the ACS Science Team and ESAINTRODUCTION

    On 24 April 2005 the NASAESA Hubble Space Telescope will exceed its original

    estimated lifetime of 15 years in orbit around the Earth. Hubble has been hugely

    successful in many different areas of astronomy. How does it differ from other famous

    telescopes?

    Hubble orbits 600 km above the Earth's surface, placing it well above our image-

    distorting atmosphere. It can be upgraded to take advantage of the latest developments

    in instrumentation and software. The telescope is designed to take high-resolution

    images and accurate spectra by concentrating light to form sharper images than are

    possible from the ground, where the atmospheric 'twinkling' of the stars limits the

    clarity. Therefore, despite its relatively modest aperture of 2.4 metres, Hubble is more

    than able to compete with ground-based telescopes that have light-collecting (i.e.

    mirror) areas 10 or even 20 times larger.

    As well as being able to take sharper wide-field images, the other huge advantage

    Hubble has over ground-based telescopes is its ability to observe the near-infrared and

    ultraviolet light that is otherwise filtered away or masked by the atmosphere before it

    can reach the ground.

    In many areas of astronomical investigation, Hubble has pushed the limit of our knowl-

    edge far, far beyond anything possible before its launch.

    NGC 1300

    NGC 1300 is considered to be

    prototypical of barred spiral

    galaxies. Barred spirals differ from

    normal spiral galaxies in that the

    arms of the galaxy do not spiral all

    the way into the centre, but are

    connected to the two ends of a

    straight bar of stars containing the

    nucleus at its centre.

    9

    NASA, ESA, and The Hubble Heritage Team (STScIAURA)THE HUBBLE STORY

    Hubble finally allowed astronomers to realise their dream of

    escaping the distorting effects of the Earth’s atmosphere to

    make their observations. Achieving an operational observatory in space

    was no small task: it took decades of planning and construction in a

    project of such scale and cost that it demanded international collabora-

    tion and the work of many dedicated engineers and scientists. The con-

    cept of a telescope that could be upgraded and serviced regularly by

    astronauts has resulted in capabilities and scientific discoveries far

    beyond the expectations of the designers.

    Hubble in Dock

    The Hubble Space Telescope in the

    Shuttle's payload bay during

    Servicing Mission 3A.

    11

    1

    NASA12

    Hubble has vastly improved our view of the skies, sharpened our

    perception of the Universe, and allowed us to penetrate ever deeper

    toward the furthest edges of time and space.

    Looking at the night sky we see the familiar twinkle of starlight; light that

    has travelled enormous distances to reach us. But the stars themselves do

    not flicker. The Universe is gloriously transparent, allowing light from

    distant stars and galaxies to travel unchanged across space for thousands,millions, even billions of years. Then, in the last few microseconds before

    the light reaches our eyes, the fine details in the view of those stars and

    galaxies are snatched away. This is because, as light passes through our

    atmosphere, the ever changing blankets of air, water vapour and dust, blur

    the image that finally reaches us.

    To solve this problem, astronomers around the world longed for an

    observatory in space for many years. As early as 1923, the famed German

    rocket scientist Hermann Oberth suggested a space-based telescope.

    However, it was decades before technology caught up with the dream. The

    American astronomer Lyman Spitzer proposed a more realistic plan for a

    space telescope in 1946.

    From a position in space above the Earth's atmosphere, a telescope could

    detect the pristine light fromstars, galaxies, and other objects before their

    images become distorted by the air we breathe. The result: much sharper

    images than even the largest telescopes on the ground could achieve;

    images limited in sharpness only by the quality of the optics.

    In the 1970s, NASA – the National Aeronautics and Space Administration

    and ESA the European Space Agency began working together to

    design and build what would become the Hubble Space Telescope. The

    name is a tribute to Edwin Powell Hubble, the founder of modern cosmol-

    ogy, who, in the 1920s, first showed that not all we see in the sky lies

    within the Milky Way. Instead, the cosmos extends far, far beyond.

    Hubble's work changed our perception of mankind's place in the Universe

    forever and the choice of naming th most magnificent telescope after

    Edwin Hubble could not have beenmore appropriate.

    –– –

    is

    For many years astronomers

    longed for an observatory in space

    ESAAbove the Ocean of Air

    A ground-based telescope similar in size to Hubble

    produced the image of the barred galaxy NGC 1300 on the

    left. From its position in space above the Earth's

    atmosphere, Hubble obtained the picture on the right of

    the same galaxy. A technique called 'Adaptive Optics' can

    be used to sharpen ground-based images and is extremely

    effective when used with telescopes much larger than

    Hubble (see box below).

    Hubble vs. Adaptive Optics

    The technique of Adaptive Optics can sharpen images from large ground-based telescopes to attain a

    higher resolution and is being vigorously developed by astronomers around the World. Such

    observations are highly complementary to those made by Hubble since they can exploit the collecting

    power of much larger telescopes and can be used very effectively to feed such light-hungry

    instruments as spectrographs. This higher resolution is achieved over a small patch of the sky and and

    the technique works better at infrared wavelengths than it does in the visible spectrum used by

    Hubble. Hubble remains supreme for mapping parts of the sky in exquisite detail in ultraviolet, visible

    and near-infrared light.

    Why is Hubble in orbit around the Earth?

    The Earth’s atmosphere both absorbs and emits light. Beyond the blue end of the visible spectrum,the presence of ozone ensures that little ultraviolet light reaches the ground. Towards the red end and

    beyond, into what astronomers call the near-infrared spectrum, there is considerable absorption by

    water vapour and molecular oxygen but also, the sky is brightened by intense emission from the OH

    radical (an unstable molecule consisting of an oxygen and a hydrogen atom). The visible spectrum

    alone remains reasonably free from these effects. The entire spectral range from ultraviolet through

    the near-infrared remains cleanly accessible to Hubble.

    13

    NASA, ESA, and The Hubble Heritage Team (STScIAURA)

    Hillary MathisNOAOAURANSFIt took two decades of dedicated collaboration between scientists, engineers and

    contractors from many countries before Hubble was finally finished. On April 24, 1990,five astronauts aboard the space shuttle Discovery left on a journey that changed our

    vision of the Universe for ever! They deployed the eagerly anticipated Space Telescope in

    an orbit roughly 600 kmabove the Earth’s surface.

    On Earth, astronomers waited impatiently for the first results. After extensive technical

    verification and testing, it soon became obvious that Hubble’s vision was anything but

    sharp. The mirror had a serious flaw. A defect in the shape of the mirror prevented

    Hubble from taking clear images. The mirror’s edge was too flat by only a mere fiftieth

    of the width of a human hair. But to accomplish itsmission, Hubble had to be perfect in

    every tiny detail. The disappointment was almost too great to bear. Not only amongst

    astronomers, but also for American and European taxpayers.

    Nevertheless, over the following two years, scientists and engineers fromNASA and ESA

    worked together to design and build a corrective optics package, named COSTAR, for

    Corrective Optics Space Telescope Axial Replacement. They were also able to build in a

    perfect correction to the replacement camera that was already planned for installation.

    Hubble’s masters now faced another tough decision: which science instrument should

    they remove so that COSTAR could be fitted to Hubble? They eventually chose the High

    Speed Photometer.

    Hubble’s First Servicing Mission, performed in 1993, has gone down in history as one of

    the supreme highlights of human spaceflight. It captured the attention of both astrono-

    mers and the public at large to a degree that no Space Shuttle mission has since

    achieved. Meticulously planned and brilliantly executed, the mission succeeded on all

    counts. COSTAR and the new Wide Field and Planetary Camera 2 (WFPC2) corrected

    Hubble’s eyesightmore perfectly than anyone had dared to hope.

    Hubble'smirror problem

    The cause of the problem was a defect in the 2.4 metre diameter primary mirror caused by the incorrect assembly of

    the optical system used to test the mirror during manufacture. This resulted in what is called ‘spherical aberration’.

    Fortunately, the test system remained untouched in the lab and it was possible for engineers to go back and use it to

    reconstruct the nature of the error with great precision. This is why the Servicing Missions were so successful in

    correcting Hubble’s optics to near-perfection.

    14When the first images after the servicing came up on the computer screens it was

    instantly clear that the ‘glasses’ taken up by the astronauts were completely correcting

    Hubble's vision. Hubblewas finally in business!

    That was only the first time the Space Shuttle visited Hubble. The telescope was

    designed to be upgraded, enabling it to take advantage of new technologies and

    software. When more advanced instruments, electrical or mechanical components

    became available, they could be installed by the astronauts. So, just as a car needs

    servicing so Hubble needs tuning-up from time to time. Engineers and scientists

    periodically send the Shuttle to Hubble so that astronauts can upgrade it, using

    wrenches, screwdrivers and power tools, just as amechanicmight with a car.

    There have been four Servicing Missions so far: in 1993, 1997, 1999 and 2002. All were

    undertaken by astronauts transported into space by NASA’s Space Shuttle. The next one

    was supposed to occur in 2005, but was unfortunately cancelled in the aftermath of the

    tragic Columbia crash.

    Hubble’s future is uncertain. It was originally designed to operate for 15 years, but it is

    now expected that its life could be extended to 20 years. Hubble is still producing the

    most astonishing results that astronomers have ever known.

    Changing instruments on Hubble

    An astronaut exchanging cameras

    on Hubble during the first Servicing

    Mission in 1993.

    15

    Hubble was finally in business!

    The centre of M100

    The central regions of this grand-

    design spiral galaxy taken before

    and after Hubble's first servicing

    mission. Left: A picture taken with

    theWFPC1 camera in wide field

    mode, on November 27, 1993, just

    a few days prior to the STS-61

    servicing mission. The effects of

    optical aberration in HST's 2.4-

    metre primary mirror blur starlight,smear out fine detail and limit the

    telescope's ability to see faint

    structure. Right: The same field

    imaged withWFPC2 in its high

    resolution channel. TheWFPC2

    contains modified optics that

    correct for Hubble's previously

    blurry vision. For the first time the

    telescope was able to resolve

    cleanly faint structure as small as

    30 light-years across in a galaxy

    that is tens of millions of light years

    away. The image was taken on

    December 31, 1993.Fine Guidance Sensor

    Astronaut Gregory J. Harbaugh

    on the robot-arm manoeuvring a

    Fine Guidance Sensor (FGS)

    during the second Servicing

    Mission.

    16

    NASAEventually, however, Hubble’s active life will end and the telescope will have to be guided

    to a safe resting place in the ocean. It is too massive a spacecraft to burn up completely

    in the atmosphere on re-entry and an uncontrolled plunge into the atmosphere is a

    potential danger to residents of regions covering a broad swathe of our planet. The plan

    is for an unmanned probe to link up with Hubble in orbit and dock with it. The probe will

    leave behind a rocket-module so that, after some more years of fruitful observing,engineers on the ground can activate these rockets to control Hubble’s final descent into

    the atmosphere.

    However, the retirement of the Hubble Space Telescope will not signal the end of our

    unrivalled view of the Universe. Rather, it will mark a new beginning, an era of even

    more amazing discoveries and images fromspace. For Hubble has a successor.

    The James Webb Space Telescope is currently being designed and may be launched as

    early as 2011. When that day comes, scientists using the James Webb Space Telescope

    hope to discover and understand evenmore about our fascinating Universe.

    Space power tools

    Astronaut Claude Nicollier, mission

    specialist from the European Space

    Agency (ESA), works at a storage

    enclosure using one of the Hubble

    power tools during the second of

    three extravehicular activities (EVA)

    of the third Servicing Mission.

    17

    Just as a car needs servicing so Hubble

    needs tuning-up from time to time

    NASAJamesWebb Space Telescope

    Artist's impression of the JamesWebb

    Space Telescope. Shaded behind a huge

    sunscreen, the telescope and its

    instruments will remain cool enough to

    make ultra-sensitive infrared observations

    of the most distant objects in the

    Universe.

    ESAESA21

    HUBBLE UP CLOSE

    The NASAESA Hubble Space

    Telescope

    Hubble is a large satellite; about 16

    metres long or the size of a small

    bus. It is also one of the most

    complicated pieces of technology

    ever built.

    Hubble is a large satellite, about the size of a school bus. As well as

    the 2.4 metre aperture telescope, it carries six scientific instru-

    ments that can be regularly replacedwithmoremodern and capable ones

    by space suited astronauts in orbit 600 km above the Earth. The systems

    that allow it to be pointed and stabilised are very sophisticated and are

    working extraordinarily well. Far from being an isolated resource for

    astronomers, Hubble has worked in close harmony with other satellites

    and ground-based observatories to lead, during its 15 years of operation,a huge leap in our understanding of the Universe.

    222 ESA23

    Hubble is a space-based telescope that is designed to be upgraded and to adapt to

    changing needs and technologies. It orbits at almost 600 km above the Earth’s

    surface, placing it well above most of our image-distorting atmosphere and takes about

    96minutes to complete each orbit.

    It is designed to take high-resolution images and accurate spectra by concentrating

    starlight to form sharper images than are possible from the ground, where the atmo-

    spheric ‘twinkling’of the stars limits the clarity.

    To gather asmuch light as possible fromthe faint objects it studies, any telescope needs

    the largest mirror it can get. Despite Hubble’s relatively modest mirror diameter of 2.4

    metres, it is more than able to compete with ground-based telescopes that have mirrors

    10 or 20 times larger in collecting area.

    Hubble is a large satellite; about 16metres long or the size of a small bus. It is also one of

    the most complicated pieces of technology ever built. It contains more than 3000

    sensors that continually read out the status of the hardware so that technicians on the

    ground can keep an eye on everything.

    Time on Hubble is a precious commodity. Astronomers across the world regularly ask for

    much more time than is available. Keeping Hubble working 247 is no small task. Not a

    single secondmust be lost and all tasks either observations or so-called ‘housekeeping’

    tasks, such as repositioning of the telescope, or uploading new observing schedules —

    aremeticulously planned.

    –

    Hubble's orbit

    Hubble orbits the Earth every 96

    minutes at an altitude of nearly

    600 km. The ever-changing aspect

    of its orbit makes the process of

    scheduling observations rather

    complicated.

    Is there competition between different observatories?

    A large and ambitious astronomical research project today would use large amounts of time on a whole range of

    different telescopes on the ground and in space. These telescopes, far from competing with one another, provide

    different and complementary views of astronomical sources that greatly increase our ability to understand the

    physical processes that create them. It is usually a case of the whole being greater than the sum of the parts.

    Hubble plays an absolutely pivotal role inmany of these programmes.

    Hubble is a space-based telescope

    that is designed to be upgradedHubble exposed

    This cutaway view of Hubble shows the

    configuration of the telescope, the

    instruments and the many other essential

    systems that allow it to point, operate and

    communicate.

    24

    FGS

    Hubble has three Fine Guidance Sensors on board. Two of

    them are needed to point and lock the telescope on the target

    and the third can be used for position measurements, also

    known as astrometry.

    STIS

    The Space Telescope Imaging Spectrograph (STIS) is currently

    not operating, but is a versatile multi-purpose instrument

    taking full advantage of modern technology. It combines a

    camera with a spectrograph and covers a wide range of

    wavelengths from the near-infrared region into the ultravio-

    let.

    NICMOS

    The Near Infrared Camera and Multi-Object Spectrometer

    (NICMOS) is an instrument for near-infrared imaging and

    spectroscopic observations of astronomical targets. NICMOS

    detects light with wavelengths between 800 and 2500

    nanometres.

    ACS

    ACS is a so-called third generation Hubble instrument. Its

    wide field of view is nearly twice that of Hubble’s previous

    workhorse camera,WFPC2. The name, Advanced Camera for

    Surveys, comes from its particular ability to map relatively

    large areas of the sky in great detail.

    Primary mirror

    Hubble’s primary mirror is made of a special glass coated with

    aluminium and a special compound that reflects ultraviolet

    light. It is 2.4 metres in diameter and collects the light from

    stars and galaxies and reflects it to the secondary mirror.

    COSTAR

    COSTAR is not really a science instrument: it is the

    corrective optics package that replaced the High

    Speed Photometer (HSP) during the first servicing

    mission. COSTAR was designed to correct the effects

    of the primary mirror’s aberration.

    HUBBLE'S INSTRUMENTS SYSTEMS

    ESASupport systems

    Containing essential support systems such as

    computers, batteries, gyroscopes, reaction wheels

    and electronics.

    Communication antennae

    Once Hubble observes a celestial object, its onboard

    computers convert the image or spectrum into long

    strings of numbers that, via one of Hubble's two

    antennae, are sent to one of the two satellites that form

    the Tracking and Data Relay Satellite System (TDRSS).

    Secondary mirror

    Like the primary mirror, Hubble’s secondary mirror is made of

    special glass coated with aluminium and a special compound

    to reflect ultraviolet light. It is 13 metre in diameter and

    reflects the light back through a hole in the primary mirror

    and into the instruments.

    Solar panels

    Hubble’s third set of solar arrays produce enough

    power to enable all the science instruments to

    operate simultaneously, thereby making Hubble

    even more efficient. The panels are rigid and unlike

    earlier versions, do not vibrate, making it possible

    to perform stable, pinpoint sharp observations.

    WFPC2

    WFPC2 was Hubble’s workhorse camera until the installation

    of ACS. It records excellent quality images through a selection

    of 48 colour filters covering a spectral range from far-

    ultraviolet to visible and near-infrared wavelengths.WFPC2

    has produced most of the stunning pictures that have been

    released as public outreach images over the years.

    Aperture door

    Hubble’s aperture door can be closed if

    Hubble is in danger of letting light

    from the Sun, Earth or Moon into the

    telescope.

    2526

    For astronomers, the most important components of Hubble are its scientific instru-

    ments. There are two groups of instruments in Hubble, known as ‘radial’ mounted

    around Hubble’s waist; and ‘axial’ fitted at the back end of the spacecraft. The

    different instruments serve different purposes: some are for making images and some

    are designed to dissect the light from the stars and galaxies by spreading it out to form

    a rainbow-like spectrum.

    Hubble’s unique vantage point in space makes it capable of observing over a broader

    band of wavelengths than ground-based (optical) telescopes. It can observe ultraviolet

    light that is completely absorbed by Earth’s atmosphere. It can also see much more

    clearly in the near-infrared part of the spectrum where the Earth’s sky is very bright and

    not very transparent. These forms of light reveal properties of celestial objects that are

    otherwise hidden fromus.

    Some instruments, like ACS the Advanced Camera for Surveys are better for visible

    and ultraviolet observations, some, like NICMOS the Near Infrared Camera and Multi-

    object Spectrograph are designed for infrared observations.

    Different mechanical and electrical components keep Hubble functioning. The power

    for Hubble comes from solar panels on the side that convert sunlight into electricity.

    Gyroscopes, star trackers and reaction wheels keep Hubble steady and pointing in the

    right direction for hours or days at a time: not too close to the Sun, Moon or Earth as

    they would destroy the light-sensitive instruments; and accurately towards the objects

    being studied. The Hubble pointing and tracking systemis a triumph of engineering and

    relies on a complex hierarchy of systems that keep the entire spacecraft stable in space

    to an almost incredible precision. It can point to the same spot on the sky for weeks at a

    timewithout deviating bymore than a fewmillionths of theMoon’s diameter.

    Hubble has several communications antennae on its side that are used for sending

    observations and other data down to Earth. Hubble sends its data first to a satellite in

    the Tracking and Data Relay Satellite System, which then downlinks the signal toWhite

    Sands, New Mexico, USA. The observations are sent from NASA in the United States to

    Europewhere they are stored in a huge data archive inMunich, Germany.

    No single nation could undertake such an enormous project. Hubble has been a major

    collaboration between NASA and ESA, the European Space Agency, from an early stage

    in its life. ESA has contributed an instrument, two sets of Solar Arrays, various electronic

    systems and a substantial group of people to the project.

    –

    –

    ––

    –

    –

    No single nation could undertake

    such an enormous project27

    Hubble facts

    A few of the lesser known facts about Hubble are: it has orbited the Earth more than 80 000 times

    and travelled nearly 4 billion kilometres – more than 25 times the distance to the Sun. It has made

    700 000 exposures of 22 000 different astronomical targets, producing 20 Terabytes of data that

    have resulted in about 6 000 scientific papers–avery high number even given the considerable

    outlay on the project.

    Orbital altitude: 568 km

    Orbital time: 96minutes

    Mission lifetime: 20 years

    Exposures: approx. 700 000

    Different objects observed: approx. 22 000

    Data:more than 20 TB downloaded to Earth

    Distance travelled: 80 000 times around the Earth (nearly 4 billion kilometres)

    Number of scientific papers: approx. 6 000

    Angular resolution: 0.05 arc-seconds

    Wavelength range: 110 – 2400 nm(fromultraviolet to near-infrared)

    Mirror diameter: 2.4m

    Pointing stability: Hubblemoves less than 0.007 arc-seconds in 24 hours

    Costs: ESA's financial contribution over 20 years is 593million Euros

    Dimensions: 15.9metres long, diameter 4.2metres

    Launch Date: 24 April, 1990, 12:33:51 UT

    Weight: 11 110 kg!!!!!!!!!!!!!!!!

    Hubble has been of paramount importance to European astronomy. European

    astronomers regularly win more than 15 percent of the observing time with

    Hubble, resulting in several thousand scientific publications over the years. Much

    of the work done by astronomers with Hubble is complemented by observations

    madewith ground-based and other space telescopes.

    Two groups of European specialists work with Hubble. There are 15 people from

    ESA currently working at the Space Telescope Science Institute in the USA, and 20

    others make up the Space Telescope–European Coordinating Facility in Munich,Germany.PLANETARY TALES

    Planetary systems are made up of material leftover from the forma-

    tion of their parent star. Astronomers expect the formation of

    ‘debris disks’ to be a common result of star formation and so expect there

    to be many ‘Solar Systems’ awaiting discovery: indeed, over the last few

    years, the first hundred or so of these have already been found around

    nearby stars. Hubble performs long-termstudies ofmembers of our Solar

    Systemand hasmade unique observations of planets in others.

    A 'terrestrial' planet orbiting a sun-

    like star (artist's impression)

    In contrast to the successful

    searches for massive gas giants like

    Jupiter, finding small, Earthlike,rocky planets around other stars

    will be a very difficult task. None

    have been found yet although

    experiments are being planned to

    search for, and eventually to study,them. The goal is, of course, to

    search for signs of habitation.

    29

    3

    ESA30

    There are no boundaries in space. In this vast Universe, our closest relatives are the

    objectswithin the Solar System:we share the same origin and the same destiny.

    Our Solar System was formed about four and a half billion years ago from a huge gas

    cloud. Ironically, it could have been the deadly force of a thermonuclear blast from an

    exploding star in the vicinity that triggered our creation. The devastating force of the

    blast may have disturbed the precarious equilibrium of the original gas cloud, causing

    some of the matter to collapse inwards and creating a new star, our Sun. A minute

    percentage of the collapsing matter became the multifaceted assembly of planets that

    we have around us today.

    We are, in other words, just the leftovers of our Sun’s birth. The planets were born in the

    rotating disk of dust and gas left behind as our mother star was formed. The rocky

    planets formed in the inner Solar System while the enig

    sizable planets

    could maintain their gaseous surroundings and the last wisps of the tenuous cloud

    between the planets was whipped away. So, in our Solar System’s zoo of celestial bodies

    there are both rockyworlds and giant gaseous planets.

    Even now, there is no exact estimate of how much matter or even how many planets

    exist within our Solar System. Since Pluto’s discovery in the 1930s, and its satellite

    Charon’s in the 1970s, astronomers have tried to figure out if there is anything else out

    there beyond the ninth planet.

    In 2003, Hubble spotted something moving fast enough across the background of

    faraway stars to be an object within the Solar System. Estimates show that it could be

    about the size of a planet and it has been named Sedna, after an Inuit goddess. Sedna

    may be 1500 km in diameter about three quarters the size of Pluto, but it is so far

    away that it appears as just a small cluster of pixels even to Hubble. Nevertheless, it is

    the largest object discovered in the Solar System since Pluto. The Sun is about 15 billion

    km from Sedna 100 times further than Earth’s distance from the Sun and barely

    gives out as much light and heat as the full moon. So Sedna is engulfed in an eternal

    bleakwinter.

    Sedna is not the only mysterious object out there. Debris from the formation of the

    planets is still floating everywhere in the form of asteroids and comets of various

    shapes and sizes. Sometimes their orbits can lead themon catastrophic courses.

    matic gas giants were formed

    further out. And then, when a fierce wind of smashed atoms began to blow from the

    Sun – or perhaps from hot nearby stars or a nearby supernova – only

    –

    ––

    We are just the leftovers

    of our Sun’s birth31

    Comet impact

    This true colour image of the giant

    planet Jupiter, taken with Hubble's

    WFPC2 camera, reveals the impact

    sites of fragments 'D' and 'G' from

    Comet Shoemaker-Levy 9.

    H. Hammel, MIT and NASAESA32

    NASAESA, J. Bell (Cornell U.), and M.Wolff (Space Science Inst.)33

    Hubble is able to react quickly to dramatic events occurring within the Solar System. This

    has allowed it to witness the dramatic plunge of comet Shoemaker-Levy 9 into Jupiter’s

    atmosphere. The comet was torn into numerous pieces by Jupiter's gravitational pull

    when it passed the massive planet in the summer of 1992. Two years later, these

    fragments returned and drove straight into the heart of Jupiter’s atmosphere.

    Hubble followed the comet fragments on their last journey and delivered stunning high-

    resolution images of the impact scars. Our Earth could easily fit into any of these black

    bruises. The consequences of the impact could be seen for days afterwards and, by

    studying the Hubble data, astronomers were able to assemble fundamental information

    about the composition and density of the giant planet’s atmosphere.

    Space probes with sophisticated instruments are frequently sent to the planets of our

    Solar System. They provide close-up investigations of these distant places. While a few

    go into orbit around their destination planets and so can monitor them for long periods,most fly by quickly and gather some snapshots on the way. Although Hubble’s high

    resolution images can be surpassed by close-up pictures taken by planetary space

    probes, Hubble has the advantage of being able to carry out long-term monitoring. This

    is crucial for the study of planetary atmospheres and geology. Weather systems can

    revealmuch about underlying atmospheric processes.

    Hubble provides its own unique service, by opening a window on our Solar System that

    is never closed. It can be used tomonitor almost any planet in the Solar System(Mercury

    is too close to the Sun) regularly and to provide a long-term view of changes that is

    impossible to achieve in any other way. This is how we see developing storms on other

    planets; their changing seasons; and unprecedented views of other atmospheric events,such as aurorae, known on Earth as the northern and southern lights.

    Hubble’s extremely high resolution and sensitivity have resulted in unique observations

    of objects within the Solar System, providing amazing images and rich streams of data

    about the nature of these bodies. Hubble has seen unprecedented detail in Jupiter’s

    aurorae: while similar to those seen above the Earth's polar regions, they are almost

    1000 times more energetic and much more complex. Jupiter’s aurorae can only be seen

    in ultraviolet light and, so they can never be studiedwith ground-based telescopes.

    Astonishing images of Saturn’s aurorae have also been taken and reveal that the

    glowing curtains of ultraviolet light rise more than a thousand kilometres above the

    cloud tops of the planet’s north and south poles.

    Mars up close

    This view of Mars, the sharpest

    photo of it ever taken from the

    vicinity of Earth, reveals small

    craters and other surface markings

    only a few tens of kilometers

    across. The Advanced Camera for

    Surveys (ACS) aboard Hubble took

    this image on the 24th August

    2003, just a few days before the red

    planet's historic 'close encounter'

    with Earth.

    Hubble has opened a window on our

    Solar System that is never closed34

    Glowing curtains of ultraviolet light

    that rise more than a thousand

    kilometres above the cloud tops

    NASA, ESA, J. Clarke (Boston University, USA), and Z. Levay (STScI)35

    Even though the solar systemclearly hasmanymore surprises in store for us, Hubble has

    also turned its eye out towards other stars, looking for planetary systems. Astronomers

    are beginning their search for life elsewhere in the Universe. The primary objective is to

    find earth-like planets. These are verymuch harder to detect thanmassive ‘Jupiters’ and,as yet, none have been found.

    Hubble had been in orbit for five years when the first planet around a Sun-like star was

    discovered. Although it was not designed to study these objects, Hubble’s versatility has

    allowed it to make significant contributions to this intensely interesting area of study.

    For example, Hubble’s high resolution has been indispensable in the investigation of the

    gas and dust disks, dubbed ‘proplyds’, around the newly born stars in the Orion Nebula.

    The proplyds may very well be young planetary systems in the early stages of creation.

    The details revealed by Hubble are superior to anything seen to date with ground-based

    instruments and, thanks to Hubble’s capability, we now have visual proof that dusty

    disks around young stars are common.

    Hubble has also measured the mass of a planet – only the second time such a calcula-

    tion has been performed with any accuracy – by detecting the way in which the planet

    causes its star to wobble. Hubble found the oldest planet so far known: it orbits a tiny

    stellar husk, which was once a blazing star like the Sun, and is located 5,600 light years

    away. The planet was once like Jupiter and is around 13 billion years old, almost three

    times older than our own planetary system.

    Saturn's aurora

    Astronomers combined ultraviolet

    images of Saturn's southern polar

    region with visible-light images of

    the planet and its rings to make

    this picture. The auroral display

    appears blue because of the glow of

    ultraviolet light. In reality, the

    aurora would appear red to an

    observer at Saturn because of the

    presence of glowing hydrogen in

    the atmosphere. The ultraviolet

    image was taken on 28 January

    2004 by Hubble's Imaging

    Spectrograph (STIS). The ACS was

    used on 22 March 2004 to take the

    visible-light image.

    Io's shadow cast on Jupiter

    Jupiter's volcanic moon Io zips

    around Jupiter every 1.8 days. Here,Hubble'sWFPC2 captures the 3,640

    km diameter moon casting its black

    shadow on the giant planet.

    J. Spencer (Lowell Observatory) and NASAESA36

    With ground-based telescopes, the gas giant planet HD 209458b, 150 light-years from

    Earth, was discovered in 1999 through its slight gravitational tug on its ‘mother-star’. In

    2001 Hubblemade highly accuratemeasurements of the dip in the star’s light when the

    planet passed in front. The first detection of an atmosphere around an extrasolar planet

    was also made in this object. The presence of sodium as well as evaporating hydrogen,oxygen and carbon was detected in light filtered through the planet's atmosphere as it

    passed in front of the star.

    Measuring the chemical makeup of extra-solar planetary atmospheres will one day

    allow us to search for the markers of life beyond Earth. All living things breathe and this

    changes the composition of the atmosphere in readily detectable ways. Light-

    harvesting plants will impose their own colourful ‘biomarkers’ on the light reflected

    fromplanetary surfaces.

    Astronomers believe there are many planetary systems similar to ours orbiting other

    stars throughout the Galaxy. The birth, life, death and rebirth of stars continues in an

    unending cycle in which stars, born of gas and dust, will shine for millions or billions of

    years, die and return as gas and dust to form new stars. The by-products of this contin-

    ual process include planets and the chemical elements thatmake life possible.

    And so, through the entire vastness of space, the eternal ebb and flowof life continues.

    One day we will search for the

    markers of life beyond Earth10

    Transiting exoplanet

    This artist's impression shows the

    planet HD 209458b transiting its

    parent star. Hubble's spectrometer

    STIS has been used to detect – for

    the first time – the signature of the

    giant planet's atmosphere

    evaporating off into space.

    Astronomers call HD 209458b a

    'hot Jupiter' because it orbits much

    closer to its star than our own

    planet of that name.

    ESACaption caption caption caption caption

    caption. Caption caption caption caption

    caption caption. Caption caption caption

    caption caption caption. Caption caption

    caption caption caption caption. Caption

    caption caption caption caption caption.

    T

    he Sun is a typical star amongst the 100 billion or so in our Milky

    Way galaxy. Some are more massive – living relatively brief and

    spectacularly brilliant lives; some are less so and can live longer than the

    present age of the Universe. Stars are chemical factories, constructing the

    elements from which we and the Earth are made: most of the atoms in

    the newly-formed Universe were hydrogen and helium and the stars had

    to convert this raw material into what we need for life. Some short-lived

    phases of a star’s evolution have produced themost remarkably beautiful

    structures that Hubble has ever imaged.

    THE LIVES OF STARS

    39

    M17 in Sagittarius

    ThisWFPC2 image, taken in the

    light of glowing hydrogen (green),oxygen (blue) and sulphur (red),shows a small region within the

    star-forming Omega or Swan

    nebula. The wave-like patterns of

    gas have been sculpted and

    illuminated by a torrent of

    ultraviolet radiation from young,massive stars that lie outside the

    picture to the upper right.

    4

    European Space Agency, NASA, and J. Hester (Arizona State University)40

    Jeff Hester and Paul Scowen (Arizona State University), and NASAESAOur Sun, that vital source of energy for life on Earth, is a star. A totally unexceptional

    star, just like billions of others that we can find throughout the Galaxy.

    A star is a sphere of glowing gas. It forms out of a cloud of gas compressed by gravity and

    releases energy steadily, throughout its life, because a chain of nuclear reactions is

    continuously taking place in its core. Most stars combine hydrogen atoms to form

    helium through the process called nuclear fusion; the same process that powers a

    devastating hydrogen bomb. In fact, stars are nuclear factories that convert lighter

    elements into heavier elements in a series of fusion reactions. They will keep glowing

    until they run out of ‘fuel’. And that’s it; a star’s life; a quiet beginning and a steady

    progress to a sometimes violent end. But how can we be certain of this picture when an

    individual star like the Sun outlives humans by a factor of a fewhundredmillion?

    To investigate the lifecycle of a particular organism on Earth, we don’t have to track an

    individual specimen’s entire life. Instead, we can observemany of the organisms at once.

    This will show us all the different phases of its life cycle. For example, each stage of a

    person’s life is a snapshot of the human experience. And so it iswith stars.

    Stars live and die overmillions, or even billions, of years. Even themost reckless stars live

    for at least one million years; longer than the entire history of mankind! And this is why

    it is extremely unusual to be able to track age-related changes in individual stars.

    To learnmore about stars, wemust sample different stars at every stage of life and piece

    together the whole cycle from birth to death. Hubble’s vivid images have documented

    the tumultuous birth of stars and delivered many astonishing pictures documenting

    their evolution. The birth of stars in neighbouring stellar ‘maternity wards’ can be used

    as a timemachine to replay the events that created our Solar System.

    Hubble has gone beyond what can be achieved with 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 individual stars in other galaxies

    enables scientists to investigate the influence of different environments on the lives of

    stars. These are crucial data that allow us to extend our understanding of the MilkyWay

    to other galaxies.

    41

    Pillars of creation

    This image, taken withWFPC2 in

    1995, has become a universally

    recognised icon. Part of M16, the

    Eagle nebula, these Evaporating

    Gaseous Globules (EGGs) are

    protrusions of cool, dusty,molecular gas into hotter, more

    tenuous material excited by young,hot stars in this nearby star-

    forming region.

    Cosmic recycling

    Lighter elements such as carbon, nitrogen, oxygen, silicon aremade as a result of fusion reactions taking place in stars.

    The heavier elements, however, are built during the cataclysmic stellar explosions we know as supernovae.When the

    Universe was very young—before any stars and galaxies had formed—hydrogen and helium were overwhelmingly

    its dominant atomic constituents.

    A star is a sphere of glowing gasColourful Tarantula

    The Tarantula nebula is situated 170,000

    light-years away in the Large Magellanic

    Cloud (LMC), visible to the naked eye in

    the Southern sky. Supernovae have

    already detonated in this huge star-

    forming region and the resulting blast

    waves have compressed the gas into

    filaments and sheets. This mosaic of

    images was created using Hubble

    archival data by 23 year old amateur

    astronomer, Danny LaCrue. It was

    constructed from 15 individual exposures

    taken through three narrow-band filters.

    ESANASA, ESO and Danny LaCrue44

    Hubble's view of Orion

    This spectacular colour panorama

    of the centre of the Orion Nebula is

    one of the largest pictures ever

    assembled from individual images

    taken with the Hubble telescope.

    The richly detailed tapestry

    revealed by Hubble shows a

    churning, turbulent star factory

    set within a maelstrom of flowing,luminescent gas. Though this 2.5-

    light-year-wide view is but a small

    portion of the entire nebula, it

    includes a star cluster and almost

    all of the light from the bright

    glowing clouds of gas that make

    up the nebula.

    C.R. O'Dell (Rice University), and NASAESAProto-Solar Systems?

    Disks around young stars (also known as

    circumstellar or protoplanetary disks) are

    thought to be made up of 99% gas and

    1% dust. Even that small amount of dust

    is enough to make the disks opaque and

    dark at visible wavelengths. These dark

    disks are seen here because they are

    silhouetted against the bright backdrop

    of the hot gas of the Orion nebula.

    Hubble has often had to work hard for this information because these important clues

    about our genesis lie hidden behind the veil of gently glowing, dust-laden molecular

    cloudswhere stars are formed.

    There are stars forming throughout the Universe. Enormous glowing pillars of dusty

    hydrogen gas stand sentinel over their cradles, basking in the light of nearby, newly-

    formed stars.

    Hubble’s ability to observe infrared light enables it to penetrate the dust and gas and

    reveal the newly born stars as never before.

    One of themost exciting of Hubble’smany discoveries was the observation of dust disks

    surrounding some newborn stars, buried deep inside the Orion Nebula. Here we are

    actually seeing the creation of new solar systems where planets will eventually form;

    just as they did in our own Solar Systemfour and a half billion years ago.

    In the first stages of their lives, stars can stock up on gas from their original birth cloud.

    Material falling into the star creates bubbles or even jets as it is heated and blasted

    along a path that follows the star's rotation axis, like an axle through awheel.

    Oftenmany stars are born fromthe same cloud of gas and dust. Somemay stay together

    through their whole lifetime, keeping step as they evolve, like the childhood friends that

    you keep for life.

    45

    Important clues about our genesis

    lie hidden behind the veil of gently

    glowing, dust-laden molecular clouds

    Mark McCaughrean (Max-Planck-Institute for Astronomy), C. Robert O'Dell (Rice University), and NASAESAThe Carina Nebula

    Previously unseen details of a mysterious,complex structure within the Carina Nebula

    (NGC 3372) are revealed by this image of the

    'Keyhole Nebula', obtained using four

    different pointings of theWFPC2 camera

    through six colour filters. The picture is

    dominated by a large, approximately circular

    feature, which is part of the Keyhole Nebula,named in the 19th century by Sir John

    Herschel. This region, about 8000 light-years

    from Earth, is located adjacent to the

    famous explosive variable star Eta Carinae,which lies just outside the field of view

    toward the upper right. The Carina Nebula

    also contains several other stars that are

    among the hottest and most massive

    known, each about 10 times as hot, and 100

    times as massive, as our Sun.

    NASAESA, The Hubble Heritage Team (AURASTScI)48

    The stars in a cluster will all have the same age, but will have a range of different

    masses. And thismeans that very different destinies await them.

    Human existence is the mere blink of an eye compared with the life of a star, so the

    direct observation of a transition between the different stages of a star’s life can only

    come about by lucky chance. Hubble uses its stability and exceptionally sharp focus to

    Young star's jet

    This view of a 5 trillion kilometre

    long jet called HH-47 reveals a

    very complicated jet pattern

    which indicates the star (hidden

    inside a dust cloud near the left

    edge of the image) might be

    wobbling, possibly caused by the

    gravitational pull of a companion

    star.

    Human existence is the mere blink of an eye

    compared with the life of a star

    J. MorseSTScI, and NASAESA49

    reveal changes on cosmic scales over periods of only a few years. From the ground it is

    usually not possible to see this kind of evolution taking place over such short periods. In

    the Universe, this sort of action normally takes place on timescales of many thousands

    or even millions of years, so being able to follow real time changes in astronomical

    objects is a considerable asset.

    At the other extreme of the stellar life cycle, Hubble has monitored Supernova 1987A

    since 1991, four years after it exploded. The result is a series of stunning observations

    that show the evolution following the violent explosion witnessed nearly two decades

    ago.

    The regular monitoring of an even older supernova remnant, the Crab Nebula, has

    enabled Hubble to capture the display ofmatter and antimatter particles propelled close

    to lightspeed by the Crab pulsar, a rapidly rotating neutron star. Thanks to Hubble,scientists can directly follow the motion of the gas remnant left behind by the super-

    nova explosionwitnessed by Chinese astronomers in 1054.

    Not all elderly stars end their lives as supernovae and Hubble has followed the final

    stages of their lives, with their very different outcomes. One such elderly star V838

    Monocerotis, located about 20,000 light-years from Earth put out a brief flash of energy

    that illuminated the surrounding dust. The progress of the light echo across the dust

    was captured by Hubble in a film-like sequence of unprecedented clarity.

    The stars containing the most mass end their lives cataclysmically, destroying them-

    selves in titanic stellar explosions known as supernovae. For a few gloriousmonths, each

    becomes one of the brightest objects in the entire Universe, outshining all the other

    stars in its parent galaxy.

    Since its launch in 1990, Hubble has watched the drama unfold in Supernova 1987A, the

    nearest exploding star inmodern times. The telescope has beenmonitoring a ring of gas

    surrounding the supernova blast.

    Hubble has observed the appearance of bright spots along the ring, like gemstones on a

    necklace. These cosmic ‘pearls’ are now being lit by supersonic shocks unleashed during

    the explosion of the star.

    The ruins of an exploding star can hide a powerful engine. Hubble has probed the

    mysterious heart of the Crab Nebula, the tattered remains of an exploding star, vividly

    described by Chinese astronomers in 1054, and has revealed its dynamic centre. The

    innermost region of this nebula harbours a special type of star, a pulsar. This star rotates

    like a beacon, emitting light and energy in a beam. And it energizes and illuminates the

    vast nebula of dust and gas surrounding it.The Crab Nebula

    Much of the light emitted by an

    object like the Crab Nebula comes

    from what astronomers call a ‘non-

    thermal’ process. Electrons,travelling at speeds close to that of

    light, spiral around lines of

    magnetic field and so produce

    radiation covering the entire

    electromagnetic spectrum, fromX-

    rays to radio waves. This is why the

    pictures of the Crab taken in X-rays

    and optical light look so similar.

    This is a composite image of the

    Crab Nebula showing the X-rays

    (blue), and optical (red) radiation.

    Light echo movie

    In fifteen highly productive years, Hubble has allowed us to

    observe some stars ageing in real time. The telescope has

    produced startling ‘movies’ that allow us to witness how

    some of them do modify their appearance over this minute

    span of astronomical time.

    50

    NASAESAASUJ. Hester et al.V838 Monocerotis

    Some of the most impressive

    celestial ‘movies’ are created by

    pulses of light travelling out from a

    stellar explosion like a flash from a

    camera. As the ‘sphere of light’

    expands away from its origin, it can

    illuminate surrounding material to

    produce what we call ‘light-echoes’.

    These produce the illusion of

    material in rapid motion while, in

    reality, it is the pulse of light that is

    moving.

    NASA, the Hubble Heritage Team (AURASTScI) and ESA

    51Nordic Optical Telescope and Romano Corradi (Isaac Newton Group of Telescopes, Spain)

    5253

    Wide angle Cat's Eye

    Wide angle view of the enormous

    but extremely faint halo of gaseous

    material surrounding the Cat's Eye

    Nebula showing material ejected

    during earlier active episodes in the

    star's evolution. This probably

    happened some 50,000 to 90,000

    years ago.

    However, not all stars end their lives so violently. Sun-like stars cool down once they run

    out of hydrogen. The centre collapses in on itself and the heavier elements are burnt,causing the outer layers to expand and leak slowly into space. At this stage in a star’s life,it is called a red giant.

    Our Sun will become a red giant in a few billion years. At that time, it will expand so

    much that it will swallowMercury, Venus and our planet aswell.

    But these stars are not finished quite yet. They can still evolve into something extraordi-

    nary. Just before they breathe their last breath, stars like our Sun go out in a final blaze of

    glory.

    In its final stages of nuclear fusion, stellar winds blow from the star, causing the rem-

    nants of the red giant to swell to an enormous size. At the heart of this expansion, the

    exposed heart of the star, an intensely hot dwarf, floods the gaseous envelope with

    powerful ultraviolet light,making it glowin awhole range of beautiful colours.

    Hubble's close up view of the Cat's

    Eye

    Detailed view from Hubble

    focussing on central regions of the

    Cat's Eye Nebula seen on the

    previous page. Although this

    nebula was among the first

    planetary nebula ever to be

    discovered, it is one of the most

    complex planetary nebulae ever

    seen in space. A planetary nebula

    forms when Sun-like stars gently

    eject their outer gaseous layers to

    form bright nebulae with amazing

    twisted shapes.

    The Sun will swallow Mercury, Venus

    and our planet as well

    ESA, NASA, HEIC and The Hubble Heritage Team (STScIAURA)54

    Colours of Planetary Nebulae

    The intensely hot stars at the centres of planetary nebulae flood the surrounding volume of gas with

    ultraviolet light. This causes the atoms in the gas to lose one or more of their electrons. The resulting ‘ions’

    radiate their energy away in a series of discrete colours that astronomers can observe to measure gaseous

    temperatures, densities, chemical composition andmotions.

    Since these amazing constructions looked a bit like the newly discovered planet Uranus

    to early telescopic astronomers, they became known as planetary nebulae. From

    telescopes on Earth they look like round (planet-shaped) objects with fairly simple

    geometries. Hubble's keen perception shows that each planetary nebula is a distinct

    individual. Howa normal Sun-like star evolves froma relatively featureless gas sphere to

    a nebula with intricate glowing patterns is still one of the unsolved mysteries in

    astronomy. Each additional image of the glowing patterns of gas intrigues astronomers

    anew.

    From its unique position high above the distorting atmosphere Hubble is the only

    telescope that can observe the swollen outer envelope of these dying stars in full detail.

    Hubble has been able to observe the expansion of the nebula itself directly. The Cat’s

    Eye Nebula, for instance, has been observed with Hubble over a period of eight years

    and is amarvellous example of the resolving power of the telescope.

    A Collection of Planetary Nebulae

    Hubble’s dazzling collection of

    planetary nebulae show surpris-

    ingly intricate, glowing patterns:

    sprinkling jets, pinwheels, ghostly

    filaments, supersonic shocks,concentric rings and intricate

    tendrils of gas and fiery lobes.With

    their gauzy symmetrical wings of

    gas they resemble butterflies.

    ESAIC 4406, the Retina nebula

    Hubble reveals a rainbow of colours

    in this dying star, called IC 4406.

    Like many other so-called planetary

    nebulae, IC 4406 exhibits a high

    degree of symmetry. The nebula's

    left and right halves are nearly

    mirror images of one another. If we

    could fly around IC 4406 in a

    spaceship, we would see that the

    gas and dust form a vast doughnut

    of material streaming outward

    from the dying star.

    One of the greatest mysteries in modern

    astrophysics is how a simple, spherical gas

    ball can give rise to these intricate structures!

    One of the greatest mysteries in modern astrophysics is how a simple, spherical gas ball

    such as our Sun can give rise to these intricate structures!

    For some planetary nebulae it is as if a cosmic garden sprinkler created the jets that

    stream out in opposite directions; or could these amazing patterns possibly be sculpted

    by themagnetic field of a companion star that funnels the emitted gas into a jet?

    Whatever their cause, in only ten thousand years these fleeting cosmic flowers disperse

    in space. Just as real flowers fertilize their surroundings as they decompose, the chemi-

    cal elements produced inside the star during its life ......