It’s probably fair to say that the Hubble Space Telescope, which recently celebrated its 25th birthday, has become the world’s most famous telescope in large part due to the breathtaking astronomical images it has captured.
Hubble’s images reveal the complex, three-dimensional structure of galaxies, nebulae and star-forming regions with incredible acuity, chiefly because the telescope is in space. For ground-based telescopes, the Earth’s atmosphere has a blurring effect, limiting the sharpness of the images they produce. Hubble’s images are limited only by the telescope’s engineering and the properties of light itself.
In 1990 I was privileged to be present at the space shuttle launch which carried Hubble into orbit. The combination of the launch’s powerful demonstration of the defiance of gravity, coupled with the promise of what Hubble would do for astronomy was overwhelming. Curmudgeonly male scientists wept.
Perhaps the affection directed towards Hubble is also partly due to the telescope’s troubled start: the primary mirror was very precisely manufactured, but to the wrong shape. For the first three years of operation, Hubble’s ability to produce sharp images was compromised, to the point that “Hubble Telescope” was a joke appearing in cartoons and punch-lines.
Astronauts Hoffman and Musgrave working on the Hubble Space Telescope in 1993. NASA
So engineers produced COSTAR, a component that would correct the optical problems with the primary mirror. Installed during the first space shuttle visit to Hubble, it worked a treat. Like a flawed hero or a prodigal child, Hubble’s triumph over adversity has universal appeal – the best stories, like the best images, have contrasts between darkness and light.
More than just light
Fortunately, not all of Hubble’s science had to wait three years for the first servicing mission. Scientific astronomy is carried out in other regions of the electromagnetic spectrum than just visible light, for example ultraviolet (UV) light. UV is invisible to our eyes, but forms the continuation of the visible spectrum beyond the violet.
Hubble had always been intended to serve as an ultraviolet telescope – from space, UV light that would otherwise be absorbed by Earth’s atmosphere can be collected. Light in this part of the spectrum is more energetic than visible light and is emitted by most stars, including our own, and many other astrophysical objects. Studies in ultraviolet radiation reveal things that can’t be learnt from telescopes on the ground.
A ‘hot Jupiter’ exoplanet’s atmosphere is stripped away by the heat of its star. ESA/Alfred Vidal-Madjar/NASA
Hubble has produced many, many UV science results. My favourite is the spectacular discovery in 2003 that the exoplanet HD209458b is surrounded by a huge cloud of hydrogen gas. This type of exoplanet, known as a “hot Jupiter”, orbits its star so closely – only a 20th of the Earth’s distance from the sun – that the star’s heat boils off the planet’s atmosphere.
Insight into the future
This sort of discovery offers a great opportunity to learn what exoplanets are made of. Spectroscopy is the key: each chemical substance has its own spectroscopic fingerprint that allows astronomers to measure chemical compositions – and the UV region of the spectrum is particularly sensitive and useful for this purpose. Hubble has used these strong UV features to reveal the presence of hydrogen, magnesium, iron, silicon and other chemicals in the atmospheres of several hot Jupiter-style exoplanets.
Our ultimate fate: a white dwarf star collapsed from a giant red, surrounded by remnants of its inner planets. G Bacon/NASA/ESA
The loss of the atmosphere of these exoplanets is a preview of the ultimate fate of the Earth, when the sun becomes a red giant star in about four billion years time. As the sun begins to exhaust hydrogen at its core and begins to burn helium, it will swell and become hotter and brighter, engulfing Earth and the inner planets. Once it has exhausted its nuclear fuel, it will collapse into a white dwarf star – about the size of the Earth, and surrounded by the remnants of our solar system.
Hubble UV spectroscopy of white dwarf stars has revealed that many of them are being continually bombarded by asteroids feeding the stars with rocky material. These observations allow us to learn the types of rocks present in extinct planetary systems which were perhaps once very similar to our own solar system.
Appearance in UV of aurorae on Ganymede. NASA/ESA
Most recently and closer to home, UV images reveal the aurorae around Ganymede, Jupiter’s largest moon. Just as with Earth’s aurora borealis and australis (northern and southern lights), Ganymede’s aurorae change continuously with the influence of Jupiter’s magnetic field. Hubble captured changes in the aurorae caused by the presence of an underground salt-water ocean on Ganymede – an ocean that probably has more water than all of Earth’s oceans combined and may provide a habitat for life.
Hubble has continued its mission well beyond its original planned lifetime. It has made over a million observations and generates about ten terabytes of new data each year. The current plan is for it to operate beyond 2020, to allow some overlap with its replacement, the NASA/ESA/Canadian Space Agency joint project, the James Webb Space Telescope (JWST).
Sadly for UV astronomy, the JWST will work predominantly in infrared and has no UV instruments. This leaves many astronomers keen to see a successor to Hubble that will continue it’s unique work in UV, which has added so much to human understanding by working beyond what we can see.
During its impressive 25 years the Hubble Space Telescope has captured numerous remarkable views of the universe, providing astronomers with a wealth of data for making astounding discoveries. Of all the beautiful astronomical objects observed by Hubble one of the most awe-inspiring is the massive, dying star V838 Moncerotis.
Hubble’s longevity has provided astronomers with a series of detailed images of V838 Mon captured between May 2002 and September 2006: the result is a fascinating “time-lapse” that uniquely illuminates the evolution of this massive, super-giant star. Hubble’s exceptionally sharp focus of V838 Mon offered a ring-side seat at the slow death of the star and excited astrophysicists with the chance to study the physics of the light, matter and microscopic dust of the interstellar medium.
V838 Monocerotis in April 2002, as the first flash of bright blue light emanates from the star. NASA\ESA\H E Bond
V838 Mon is about 20,000 light-years away from Earth, in the direction of the constellation of Monoceros (the unicorn). This enormous distance places the star at the outskirts of our galaxy, the Milky Way. In 2002 the star underwent an enigmatic and spectacular brightening, briefly becoming one of the most luminous stars in the Galaxy.
This stellar flash blasted out radiation at a rate 600,000 times the output of our sun. The astounding Hubble images taken over the following years reveal the flash of light illuminating the shells of dust and gas that surround the star.
At first glace, the ring of colour suggests we are witnessing the gradual expansion of the ejected shell of the exploding star. But in fact what are seeing is an extraordinary “light echo”.
As light from the powerful flash propagates outward at 300,000km per second, it travels through successive rings of dust that surround the star. Some light is scattered by micron-sized dust particles in the dust clouds, reflected back towards the telescope and Earth. The scattered light has travelled a greater distance than the light that arrived directly from the original flash, and so arrives later. This light echo is the optical analogue of the more familiar sound echo, generated when for example thunder bounces of surrounding mountain-sides.
The route the ‘light echo’ took, from star to telescope. NASA\ESA\A Feild
The light echo discovered by Hubble reveals successive layers of matter and dust, spanning light-years across. The images betray earlier phases in the life of a now ageing, red super-giant star. The latest images in this time-lapse from 2005 and 2006 show an object around about six light-years in diameter, and reveal the turbulent and intricate structure of the interstellar medium, a region where it seems magnetic fields thread the space between stars.
All of these sharp images were taken using the Advanced Camera for Surveys (ACS), which had only just been installed on Hubble during a service mission in March 2002. The images approach true colours by combining data filtered to isolate blue, green and infrared light.
After the initial flash, the light reflected from successively more distant dust clouds reaches Earth. NASA\ESA\Z Levay
The cause of the powerful light flash remains mysterious and debated. One suggestion is that as V838 Mon evolves, its outer layers expand to engulf one or more unseen gas giant planets (or “hot Jupiters”) that may have orbited the star. The subsequent release of the planets’ gravitational energy could result in a brightening of the star, though it’s not certain whether this would be sufficient to explain the magnitude of the light flash. Another scenario is that it may have been the result on an unusual nova-like eruption, where matter from a companion star is pulled by gravity onto the surface of a collapsed white dwarf star, triggering a thermonuclear explosion.
The current leading theory is that a low-mass star, perhaps one-third of the mass of the sun, collided and merged with V838 Mon. The space between stars in enormous compared to the diameter of any star, and therefore stellar collisions are very rare. However, if a pair of stars is born together they will orbit each other, and with loss of orbital energy the low-mass companion may eventually be drawn into the more massive star. The result would be a huge blast of energy.
Transforming from an unremarkable speck in the sky to one of the most extraordinary and beautiful objects, the echoes of V838 Mon will be monitored by astronomers from space telescopes and those on the ground. V838 Mon continues to reveal more to us about the structure of stars, the nature of dust shells spreading into the interstellar medium, and the life – and death – of the most massive stars.
The nature of dark energy is one of the most important unsolved problems in all of science. But what, exactly, is dark energy, and why do we even believe that it exists?
Step back a minute and consider a more familiar experience: what happens when you toss a ball straight up into the air? It gradually slows down as gravity tugs on it, finally stopping in mid-air and falling back to the ground. Of course, if you threw the ball hard enough (about 25,000 miles per hour) it would actually escape from the Earth entirely and shoot into space, never to return. But even in that case, gravity would continue to pull feebly on the ball, slowing its speed as it escaped the clutches of the Earth.
But now imagine something completely different. Suppose that you tossed a ball into the air, and instead of being attracted back to the ground, the ball was repelled by the Earth and blasted faster and faster into the sky. This would be an astonishing event, but it’s exactly what astronomers have observed happening to the entire universe!
This illustration shows abstracted ‘slices’ of space at different points in time as the universe expands. Ævar Arnfjörð Bjarmason, CC BY-SA
Scientists have known for almost a century that the universe is expanding, with all of the galaxies flying apart from each other. And until recently, scientists believed that there were only two possible options for the universe in the future. It could expand forever (like the ball that you tossed upward at 25,000 miles an hour), but with the expansion slowing down as gravity pulled all of the galaxies toward each other. Or gravity might win out in the end and bring the expansion of the universe to a halt, finally collapsing it back down in a “big crunch,” just like your ball plunging back to the ground.
So imagine scientists’ surprise when two different teams of astronomers discovered, back in 1998, that neither of these behaviors was correct. These astronomers were measuring how fast the universe was expanding when it was much younger than today. But how could they do this without building a time machine?
Luckily, a telescope is a time machine. When you look up at the stars at night, you aren’t seeing what they look like today – you’re seeing light that left the stars a long time ago – often many hundreds of years. By looking at distant supernovae, which are tremendously bright exploding stars, astronomers can look back hundreds of millions of years. They can then measure the expansion rate back then by comparing the distance to these far-off supernovae with the speed at which they are flying away from us. And by comparing how fast the universe was expanding hundreds of millions of years ago to its rate of expansion today, these astronomers discovered that the expansion is actually speeding up instead of slowing down as everyone had expected.
Instead of pulling the galaxies in the universe together, gravity seems to be driving them apart. But how can gravity be repulsive, when our everyday experience shows that it’s attractive? Einstein’s theory of gravity in fact predicts that gravity can repel as well as attract, but only under very special circumstances.
Repulsive gravity requires a new form of energy, dubbed “dark energy,” with very weird properties. Unlike ordinary matter, dark energy has negative pressure, and it’s this negative pressure that makes gravity repulsive. (For ordinary matter, gravity is always attractive). Dark energy appears to be smoothly smeared out through the entire universe, and it interacts with ordinary matter only through the action of gravity, making it nearly impossible to test in the laboratory.
Scientists used to think that the expansion of the universe was described by the yellow, green, or blue curves. But surprise, it’s actually the red curve instead.
The simplest form of dark energy goes by two different names: a cosmological constant or vacuum energy. Vacuum energy has another strange property. Imagine a box that expands as the universe expands. The amount of matter in the box stays the same as the box expands, but the volume of the box goes up, so the density of matter in the box goes down. In fact, the density of everything goes down as the universe expands. Except for vacuum energy – its density stays exactly the same. (Yes, that’s as bizarre as it sounds. It’s like stretching a string of taffy and discovering that it never gets any thinner).
Astronomers continue to probe the skies, looking for finer details that can build on what we suspect about dark energy. Reidar Hahn, CC BY
Since dark energy can’t be isolated or probed in the laboratory, how can we hope to understand exactly what it’s made of? Different theories for dark energy predict small differences in the way that the expansion of the universe changes with time, so our best hope of probing dark energy seems to come from ever more accurate measurements of the acceleration of the universe, building on that first discovery 17 years ago. Different groups of scientists are currently undertaking a wide range of these measurements. For example, the Dark Energy Survey is mapping out the distribution of galaxies in the universe to help resolve this puzzle.
There is one other possibility: maybe scientists have been barking up the wrong tree. Maybe there is no dark energy, and our measurements actually mean that Einstein’s theory of gravity is wrong and needs to be fixed. This would be a daunting undertaking, since Einstein’s theory works exceptionally well when we test it in the solar system. (Let’s face it, Einstein really knew what he was doing). So far, no one has produced a convincing improvement on Einstein’s theory that predicts the correct expansion for the universe and yet agrees with Einstein’s theory inside the solar system. I’ll leave that as a homework problem for the reader.
In this special feature, we have invited top astronomers to handpick the Hubble Space Telescope image that has the most scientific relevance to them. The images they’ve chosen aren’t always the colourful glory shots that populate the countless “best of” galleries around the internet, but rather their impact comes in the scientific insights they reveal.
Tanya Hill, Museum Victoria
NASA,ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team
My all-time favourite astronomical object is the Orion Nebula – a beautiful and nearby cloud of gas that is actively forming stars. I was a high school student when I first saw the nebula through a small telescope and it gave me such a sense of achievement to manually point the telescope in the right direction and, after a fair bit of hunting, to finally track it down in the sky (there was no automatic ‘go-to’ button on that telescope).
Of course, what I saw on that long ago night was an amazingly delicate and wispy cloud of gas in black and white. One of the wonderful things that Hubble does is to reveal the colours of the universe. And this image of the Orion Nebula, is our best chance to imagine what it would look like if we could possibly go there and see it up-close.
So many of Hubble’s images have become iconic, and for me the joy is seeing its beautiful images bring science and art together in a way that engages the public. The entrance to my office, features an enormous copy of this image wallpapered on a wall 4m wide and 2.5m tall. I can tell you, it’s a lovely way to start each working day.
The impact of the fragments of Comet Shoemaker Levy 9 with Jupiter in July 1994 was the first time astronomers had advance warning of a planetary collision. Many of the world’s telescopes, including the recently repaired Hubble, turned their gaze onto the giant planet.
The comet crash was also my first professional experience of observational astronomy. From a frigid dome on Mount Stromlo, we hoped to see Jupiter’s moons reflect light from comet fragments crashing into the far side of Jupiter. Unfortunately we saw no flashes of light from Jupiter’s moons.
However, Hubble got an amazing and unexpected view. The impacts on the far side of Jupiter produced plumes that rose so far above Jupiter’s clouds that they briefly came into view from Earth.
As Jupiter rotated on its axis, enormous dark scars came into view. Each scar was the result of the impact of a comet fragment, and some of the scars were larger in diameter than our moon. For astronomers around the globe, it was a jaw dropping sight.
William Kurth, University of Iowa
NASA, ESA and Jonathan Nichols (University of Leicester), CC BY
This pair of images shows a spectacular ultraviolet aurora light show occurring near Saturn’s north pole in 2013. The two images were taken just 18 hours apart, but show changes in the brightness and shape of the auroras. We used these images to better understand how much of an impact the solar wind has on the auroras.
We used Hubble photographs like these acquired by my astronomer colleagues to monitor the auroras while using the Cassini spacecraft, in orbit around Saturn, to observe radio emissions associated with the lights. We were able to determine that the brightness of the auroras is correlated with higher radio intensities.
Therefore, I can use Cassini’s continuous radio observations to tell me whether or not the auroras are active, even if we don’t always have images to look at. This was a large effort including many Cassini investigators and Earth-based astronomers.
This far-ultraviolet image of Jupiter’s northern aurora shows the steady improvement in capability of Hubble’s scientific instruments. The Space Telescope Imaging Spectrograph (STIS) images showed, for the first time, the full range of auroral emissions that we were just beginning to understand.
The earlier Wide Field Planetary Camera 2 (WFPC2) camera had shown that Jupiter’s auroral emissions rotated with the planet, rather than being fixed with the direction to the sun, thus Jupiter did not behave like the Earth.
We knew that there were aurora from the mega-ampere currents flowing from Io along the magnetic field down to Jupiter, but we were not certain this would occur with the other satellites. While there were many ultraviolet images of Jupiter taken with STIS, I like this one because it clearly shows the auroral emissions from the magnetic footprints of Jupiter’s moons Io, Europa, and Ganymede, and Io’s emission clearly shows the height of the auroral curtain. To me it looks three-dimensional.
Fred Watson, Australian Astronomical Observatory
Take a good look at these images of the dwarf planet, Pluto, which show detail at the extreme limit of Hubble’s capabilities. A few days from now, they will be old hat, and no-one will bother looking at them again.
Why? Because in early May, the New Horizons spacecraft will be close enough to Pluto for its cameras to reveal better detail, as the craft nears its 14 July rendezvous.
Yet this sequence of images – dating from the early 2000s – has given planetary scientists their best insights to date, the variegated colours revealing subtle variations in Pluto’s surface chemistry. That yellowish region prominent in the centre image, for example, has an excess of frozen carbon monoxide. Why that should be is unknown.
The Hubble images are all the more remarkable given that Pluto is only 2/3 the diameter of our own moon, but nearly 13,000 times farther away.
Chris Tinney, University of New South Wales
HST / Adam Schneider (University of Toledo)/Chris Tinney (UNSW)
I once dragged my wife into my office to proudly show her the results of some imaging observations made at the Anglo-Australian Telescope with a (then) new and (then) state-of-the-art 8,192 x 8,192 pixel imager. The images were so large, they had to be printed out on multiple A4 pages, and then stuck together to create a huge black-and-white map of a cluster of galaxies that covered a whole wall.
I was crushed when she took one look and said: “Looks like mould”.
Which just goes to show the best science is not always the prettiest.
My choice of the greatest image from HST is another black-and-white image from 2012 that also “looks like mould”. But buried in the heart of the image is an apparently unremarkable faint dot. However it represents the confirmed detection of the coldest example of a brown dwarf then discovered. An object lurking less than 10 parsecs (32.6 light years) away from the sun with a temperature of about 350 Kelvin (77 degrees Celsius) –- colder than a cup of tea!
And to this day it remains one of the coldest compact objects we’ve detected outside out solar system.
Lucas Macri, Texas A&M University
NASA/ESA/STScI, processing by Lucas Macri (Texas A&M University). Observations carried out as part of HST Guest Observer program 9810.
In 2004, I was part of a team that used the recently-installed Advanced Camera for Surveys (ACS) on Hubble to observe a small region of the disk of a nearby spiral galaxy (Messier 106) on 12 separate occasions within 45 days. These observations allowed us to discover over 200 Cepheid variables, which are very useful to measure distances to galaxies and ultimately determine the expansion rate of the universe (appropriately named the Hubble constant).
This method requires a proper calibration of Cepheid luminosities, which can be done in Messier 106 thanks to a very precise and accurate estimate of the distance to this galaxy (24.8 million light-years, give or take 3%) obtained via radio observations of water clouds orbiting the massive black hole at its center (not included in the image).
A few years later, I was involved in another project that used these observations as the first step in a robust cosmic distance ladder and determined the value of the Hubble constant with a total uncertainty of 3%.
One of the images that excited me most – even though it never became famous – was our first one of the light echo around the strange explosive star V838 Monocerotis. Its eruption was discovered in January 2002, and its light echo was discovered about a month later, both from small ground-based telescopes.
Although light from the explosion travels straight to the Earth, it also goes out to the side, reflects off nearby dust, and arrives at Earth later, producing the “echo.”
Astronauts had serviced Hubble in March 2002, installing the new Advanced Camera for Surveys (ACS). In April, we were one of the first to use ACS for science observations.
I always liked to think that NASA somehow knew that the light from V838 was on its way to us from 20,000 light-years away, and got ACS installed just in time! The image, even in only one color, was amazing. We obtained many more Hubble observations of the echo over the ensuing decade, and they are some of the most spectacular of all, and VERY famous, but I still remember being awed when I saw this first one.
Galaxies form stars. Some of those stars end their “normal” lives by collapsing into black holes, but then begin new lives as powerful X-ray emitters powered by gas sucked off a companion star.
I obtained this Hubble image (in red) of the Medusa galaxy to better understand the relation between black hole X-ray binaries and star formation. The striking appearance of the Medusa arises because it’s a collision between two galaxies – the “hair” is remnants of one galaxy torn apart by the gravity of the other. The blue in the image shows X-rays, imaged with the Chandra X-ray Observatory. The blue dots are black hole binaries.
Earlier work had suggested that the number of X-ray binaries is simply proportional to the rate at which the host galaxy forms stars. These images of the Medusa allowed us to show that the same relation holds, even in the midst of galactic collisions.
Some of the Hubble Space Telescope images that appeal to me a great deal show interacting and merging galaxies, such as the Antennae (NGC 4038 and NGC 4039), the Mice (NGC 4676), the Cartwheel galaxy (ESO 350-40), and many others without nicknames.
These are spectacular examples of violent events that are common in the evolution of galaxies. The images provide us with exquisite detail about what goes on during these interactions: the distortion of the galaxies, the channeling of gas towards their centers, and the formation of stars.
I find these images very useful when I explain to the general public the context of my own research, the accretion of gas by the supermassive black holes at the centers of such galaxies. Particularly neat and useful is a video put together by Frank Summers at the Space Telescope Science Institute (STScI), illustrating what we learn by comparing such images with models of galaxy collisions.
Michael Drinkwater, University of Queensland
NASA, Holland Ford (JHU), the ACS Science Team and ESA
Our best computer simulations tell us galaxies grow by colliding and merging with each other. Similarly our theories tell us that when two spiral galaxies collide, they should form a large elliptical galaxy. But actually seeing it happen is another story entirely!
This beautiful Hubble image has captured a galaxy collision in action. This doesn’t just tell us that our predictions are good, but it lets us start working out the details because we can now see what actually happens.
There are fireworks of new star formation triggered as the gas clouds collide and huge distortions going on as the spiral arms break up. We have a long way to go before we’ll completely understand how big galaxies form, but images like this are pointing the way.
Roberto Soria, Curtin University
NASA and The Hubble Heritage Team (STScI/AURA)
This is the highest-resolution view of a collimated jet powered by a supermassive black hole in the nucleus of the galaxy M87 (the biggest galaxy in the Virgo Cluster, 55 million light years from us).
The jet shoots out of the hot plasma region surrounding the black hole (top left) and we can see it streaming down across the galaxy, over a distance of 6,000 light-years. The white/purple light of the jet in this stunning image is produced by the stream of electrons spiralling around magnetic field lines at a speed of approximately 98% of the speed of light.
Understanding the energy budget of black holes is a challenging and fascinating problem in astrophysics. When gas falls into a black hole, a huge amount of energy is released in the form of visible light, X-rays and jets of electrons and positrons travelling almost at the speed of light. With Hubble, we can measure the size of the black hole (a thousand times bigger than the central black hole of our galaxy), the energy and speed of its jet, and the structure of the magnetic field that collimates it.
When my Hubble Space Telescope proposal was accepted in 1998 it was one of the biggest thrills of my life. To imagine that, for me, the telescope would capture Stephan’s Quintet, a stunning compact group of galaxies!
Over the next billion years Stephan’s Quintet galaxies will continue in their majestic dance, guided by each other’s gravitational attraction. Eventually they will merge, change their forms, and ultimately become one.
We have since observed several other compact groups of galaxies with Hubble, but Stephan’s Quintet will always be special because its gas has been released from its galaxies and lights up in dramatic bursts of intergalactic star formation. What a fine thing to be alive at a time when we can build the Hubble and push our minds to glimpse the meaning of these signals from our universe. Thanks to all the heroes who made and maintained Hubble.
Geraint Lewis, University of Sydney
NASA, Andrew Fruchter and the ERO Team [Sylvia Baggett (STScI), Richard Hook (ST-ECF), Zoltan Levay (STScI)] (STScI)
When Hubble was launched in 1990, I was beginning my PhD studies into gravitational lensing, the action of mass bending the paths of light rays as they travel across the universe.
Hubble’s image of the massive galaxy cluster, Abell 2218, brings this gravitational lensing into sharp focus, revealing how the massive quantity of dark matter present in the cluster – matter that binds the many hundreds of galaxies together – magnifies the light from sources many times more distant.
As you stare deeply into the image, these highly magnified images are apparent as long thin streaks, the distorted views of baby galaxies that would normally be impossible to detect.
It gives you pause to think that such gravitational lenses, acting as natural telescopes, use the gravitational pull from invisible matter to reveal amazing detail of the universe we cannot normally see!
Gravitational lensing is an extraordinary manifestation of the effect of mass on the shape of space-time in our universe. Essentially, where there is mass the space is curved, and so objects viewed in the distance, beyond these mass structures, have their images distorted.
It’s somewhat like a mirage; indeed this is the term the French use for this effect. In the early days of the Hubble Space Telescope, an image appeared of the lensing effects of a massive cluster of galaxies: the tiny background galaxies were stretched and distorted but embraced the cluster, almost like a pair of hands.
I was stunned. This was a tribute to the extraordinary resolution of the telescope, operating far above the Earth’s atmosphere. Viewed from the ground, these extraordinary thin wisps of galactic light would have been smeared out and not distinguishable from the background noise.
My third year astrophysics class explored the 100 Top Shots of Hubble, and they were most impressed by the extraordinary, but true colours of the clouds of gas. However I cannot go past an image displaying the effect of mass on the very fabric of our universe.
With General Relativity, Einstein postulated that matter changes space-time and can bend light. A fascinating consequence is that very massive objects in the universe will magnify light from distant galaxies, in essence becoming cosmic telescopes.
With the Hubble Space Telescope, we have now harnessed this powerful ability to peer back in time to search for the first galaxies.
This Hubble image shows a hive of galaxies that have enough mass to bend light from very distant galaxies into bright arcs. My first project as a graduate student was to study these remarkable objects, and I still use the Hubble today to explore the nature of galaxies across cosmic time.
Alan Duffy, Swinburne University of Technology
NASA, ESA, H. Teplitz, M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (Arizona State University), and Z. Levay (STScI)
To the human eye, the night sky in this image is completely empty. A tiny region no thicker than a grain of rice held at arms length. The Hubble Space Telescope was pointed at this region for 12 full days, letting light hit the detectors and slowly, one by one, the galaxies appeared, until the entire image was filled with 10,000 galaxies stretching all the way across the universe.
The most distant are tiny red dots tens of billions of light years away, dating back to a time just a few hundred million years after the Big Bang. The scientific value of this single image is enormous. It revolutionised our theories both of how early galaxies could form and how rapidly they could grow. The history of our universe, as well as the rich variety of galaxy shapes and sizes, is contained in a single image.
To me, what truly makes this picture extraordinary is that it gives a glimpse into the scale of our visible universe. So many galaxies in so small an area implies that there are 100 thousand million galaxies across the entire night sky. One entire galaxy for every star in our Milky Way!
This is what Hubble is all about. A single, awe-inspiring view can unmask so much about our Universe: its distant past, its ongoing assembly, and even the fundamental physical laws that tie it all together.
We’re peering through the heart of a swarming cluster of galaxies. Those glowing white balls are giant galaxies that dominated the cluster center. Look closely and you’ll see diffuse shreds of white light being ripped off of them! The cluster is acting like a gravitational blender, churning many individual galaxies into a single cloud of stars.
But the cluster itself is just the first chapter in the cosmic story being revealed here. See those faint blue rings and arcs? Those are the distorted images of other galaxies that sit far in the distance.
The immense gravity of the cluster causes the space-time around it to warp. As light from distant galaxies passes by, it’s forced to bend into weird shapes, like a warped magnifying glass would distort and brighten our view of a faint candle. Leveraging our understanding of Einstein’s General Relativity, Hubble is using the cluster as a gravitational telescope, allowing us to see farther and fainter than ever before possible. We are looking far back in time to see galaxies as they were more than 13 billion years ago!
As a theorist, I want to understand the full life cycle of galaxies – how they are born (small, blue, bursting with new stars), how they grow, and eventually how they die (big, red, fading with the light of ancient stars). Hubble allows us to connect these stages. Some of the faintest, most distant galaxies in this image are destined to become monster galaxies like those glowing white in the foreground. We’re seeing the distant past and the present in a single glorious picture.
Astronomers and planetary scientists have been waiting with bated breath for the first detailed close-up images of Ceres, the solar system’s largest asteroid. Now, with NASA’s Dawn spacecraft approaching closer each day, tantalising new colour imagery has revealed new details of the geological processes that formed Ceres.
Orbiting the sun between Mars and Jupiter, Ceres is also given the somewhat controversial classification “dwarf planet”. The Dawn spacecraft eased into orbit around Ceres a few weeks ago, and has since then been slowly circling across the dark, night-side of Ceres away from the sun. As the spacecraft begins to creep within sight of the day-side of Ceres, we wait for more detailed images of the “white spots” and other intriguing features seen during the spacecraft’s initial approach.
Ceres, seen from 21,000km. Source: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
Gravity, ice and the white spots
Ceres measures nearly 1,000km across its equator, massive enough for it to have been pulled into a rough sphere by the force of its own gravity. This is made somewhat easier by roughly a quarter of its outer portion comprising of ice, whereas its interior is rocky. Is Ceres’ icy shell solid all the way down to the rock, or have lower layers of the ice melted to produce the sort of internal ocean known to exist within some of the icy satellites of Jupiter (Europa) and Saturn (Enceladus)? If there is an internal ocean, this could account for plumes of water vapour seen venting from Ceres last year by the Herschel space telescope – not to mention those mysterious white spots seen on the Ceres’ surface.
A Ceres of images taken by Dawn during its approach. White spots, possibly water vapour plumes, can be glimpsed in the 2nd and 3rd images. NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
Dawn has also taken colour images, now released, of Ceres as it approached. The colour images are formed by combining data recorded through three different colour filters in Dawn’s camera: blue, green, and near-infrared. These three channels, displayed as blue, green and red and amplified a little, look much like what human eyes would see if we were able to see a little beyond the red end of the spectrum. A colour map made of Ceres has been created by stitching together a series of images taken as Ceres rotated underneath the spacecraft, as seen in the main image above. The colours are different to the image released by NASA as the channels have been rearranged to match natural RGB more closely.
The detail and colour from the images reveal many variations across the surface of Ceres. What is probably relatively pure or clean ice appears blue, whereas areas contaminated by rocky or carbonaceous material appear relatively red, as in this image of Europa.
Jupiter’s moon Europa, showing relatively pure ice in blue and rocky or contaminated ice in red. NASA/JPL/University of Arizona
Within weeks Dawn will reach its final orbit distance of around 250km from the surface, and we’ll start seeing Ceres, bathed in the light, in much greater detail.
NASA released the first-ever color images of Pluto on Tuesday, captured by its New Horizons probe as the spacecraft continues its marathon journey to the remote dwarf planet. The picture, which was taken by the probe’s Ralph color imager last week, shows the rust-colored Pluto and its biggest moon, Charon, as little more than blurry specks.… Continue reading →
Twenty-five years ago a set of images were taken that provided a unique view of Earth and helped highlight the fragility of our existence, and the importance of our stewardship.
As the Voyager 1 planetary spacecraft went beyond the orbit of Pluto on February 14, 1990, it took one last look at Earth. Three exposures, each one in a different filter, contained a very small and faint Earth.
The images were then stored on-board, on a tape recorder but because of competing planetary missions the data took until May 1990 to arrive back on Earth.
Despite Voyager 1 being more than 6 billion kilometres from Earth, the three exposures ranged only between 0.48 and 0.72 seconds in duration. But the data took five and a half hours, travelling at the speed of light, to span the distance between the spacecraft and Earth.
A ‘selfie’ of Earth
Three images (separately in blue, green and violet light) were combined to produce the now famous Pale Blue Dot image, Voyager 1’s “selfie” of Earth.
It is an image that contains all of Earth and yet NASA says Earth was a crescent at the time and only 0.12 of one pixel in size.
Earth has a blue appearance due to reflected light scattering off oceans, clouds and land. The faint band of light in which it is seemingly suspended is not some celestial filament but an artifact of scattered sunlight.
Pale Blue Dot was part of a remarkable larger “family” portrait of the solar system, an idea the famous American astronomer Carl Sagan, a member of the Voyager imaging team, came up with many years before 1990.
Despite our planet being so small, the image has a strangely magical quality in which for the first time we can begin to appreciate our place, not only in the much larger solar system, but in the galaxy we reside, that is part of our universe.
These six narrow-angle colour images were made from the first ever
‘portrait’ of the solar system taken by Voyager 1. NASA, Voyager 1
Our Earth dot is not distinguishable from the other dots in the larger solar system portrait.
Yet, it is of course special. For one thing, in 1977 we launched a spacecraft called Voyager 1 from that dot.
Our posturings, our imagined self-importance, the delusion that we have some privileged position in the universe, are challenged by this point of pale light.
Other ‘selfies’ of Earth
The Pale Blue Dot was not the first image of Earth taken from space. On Christmas Eve 1968, Apollo 8 astronauts William Anders, James Lovell and Frank Borman were in lunar orbit and took several images of an Earth rising above the moon’s horizon.
One image in particular – known as Earthrise – has resonated like the Pale Blue Dot.
The image of a hemispherically illuminated blue sea, white cloud (with traces of brown land) Earth seemingly floating above the lunar horizon as the astronauts orbited the moon, is iconic.
The Earth was too small to easily identify known features. Craters and other features of the moon’s surface clearly show that the photographers had left their home.
You can watch a recreation of the time in the mission when the images were taken in this video (below).
Apollo astronauts, the first to journey to the moon, were also the first humans to take images of the whole Earth. At such distances, at least in the daylight part, cities and evidence of humans are invisible.
Apollo 8 command module pilot Jim Lovell, picturing himself as a first-time Earth visitor, commented to his mission commander Frank Borman:
Frank, what I keep imagining is if I am some lonely traveller from another planet what I would think about the Earth at this altitude, whether I think it would be inhabited or not […] I was just curious if I would land on the blue or brown part of the Earth.
In December 1972, only four years after Apollo 8, the Apollo 17 crew took one of the most famous and widely used whole-Earth images from a distance of 45,000km on its outbound leg, dubbed the “The Blue Marble” for obvious reasons.
Similar whole-Earth images had been taken as early as 1967 by satellites but the Blue Marble combines human photographer, a unique alignment of sun, spacecraft and Earth and the most artistic (even abstract!) mix of land, ocean and cloud.
This combination has elevated this image above many similar ones. It is an Earth we know.
The human ‘selfie’ in space
The first human selfie in space – which also includes Earth – was seemingly taken by Edwin “Buzz” Aldrin in 1966 during the Gemini 12 mission.
Edwin ‘Buzz’ Aldrin during the Gemini 12 mission in 1966. NASA
Note that this was taken while in a bulky spacesuit, wearing thick gloves, pre-smartphone and many years before the common use of the selfie-stick.
Selfies of International Space Station (ISS) residents and even ISS spacewalkers appear regularly in social media and these images now seem to be somewhat expected and slightly mundane.
Not to be outdone, machines have joined the quest for self reflection. As far back as 1976 the Viking 2 lander on Mars took partial self-portraits, containing part of the lander with the Martian horizon in the background.
In 2013, also on Mars, NASA’s Curiosity rover took 66 high-resolution images, which together made this wonderful self-portrait.
But selfies of robots do not strongly resonate with me. What strikes me the most are images of Earth taken by us or our machines. Whether it is from high orbit, en route to the moon, from another part of the solar system or even from outside the solar system, our ability to take images of our planet has changed our perspective forever. It is us we are looking back upon.
Most of the Apollo astronauts commented that their original mission objective was the moon, yet their biggest impact came from viewing the (their) Earth.
Some 40 years after his mission to the moon, Apollo 8’s William (Bill) Anders told a television documentary that:
It’s tiny out there […] it’s inconsequential. It’s ironic that we had come to study the moon and it was really discovering the Earth.
In the near future the images taken during Apollo will be joined by images of Earth taken from human travellers to, and residents of Mars.
In the short term though the Pale Blue Dot will not be matched in the impact and reaction it creates.
In that one dot is our 5 billion year old planet with its unique mix of favourable position from the sun, liquid water, tectonic activity, thin atmosphere, life and unique flora and fauna.
It represents all human history, our discoveries, our evolved intellect, our social achievements, our destructive wars, our families and loved ones, all those before us and the current seven billion humans and rising, in lockstep with increasing environmental and resources impact.
Beyond the Pale Blue Dot
Voyager 1 is now more than 19 billion kilometres from Earth. It has travelled 13 billion kilometres in the 25 years since since taking the original Pale Blue Dot. Even if the camera that took the image could be brought back to life, it is unlikely a similar set of images would detect Earth.
From the more distant Voyager 1 perspective Earth is both fainter by a factor of ten and closer to a still bright sun.
Voyager 1 is now regarded as being in interstellar space, as it is outside the influence of our sun and is travelling toward the constellation Ophiuchus. In the year 40,272 AD, Voyager 1’s next encounter will be to come within 1.7 light years of a star in the constellation Ursa Minor called AC+79 3888.
What will Earth look like in 40,272 AD? More importantly, what will Earth look like in 4027 AD, only slightly more than 2,000 years from now? Will future spacecraft over the next 2,000 years take as potent images as Pale Blue Dot, Earthrise or the Blue Marble?
Apollo showed a recognisable but seemingly fragile home planet. Voyager 1 showed an unremarkable dot much like several others in our star’s planetary system. Has the evolving Earth “selfie” changed our behaviour?
Human selfies are often perceived as having a negative narcissistic component. The Earth “selfie” has only positive attributes.
A century ago, the famous naturalist John Muir seemed to anticipate the Earth “selfie” when he wrote in his book Travels in Alaska:
[…] when we contemplate the whole globe as one great dewdrop, striped and dotted with continents and islands, flying through space with other stars all singing and shining together as one, the whole universe appears as an infinite storm of beauty.
Such self-portraits of Earth should make us continually ask are we worthy stewards to help navigate our home through John Muir’s stormy yet beautiful universe?
NASA’s Hubble space telescope has captured images of a set of thin, green objects, which astronomers have identified as the short-lived ghosts of quasars — extremely bright masses of energy and light — that flickered to life and then faded away. Scientists expect the new findings to help them better understand the baffling behavior of galaxies… Continue reading →
WASHINGTON, April 1, 2015 /PRNewswire-USNewswire/ — NASA Television will air an event from 1 – 2 p.m. EDT on Tuesday, April 7, featuring leading science and engineering experts discussing the recent discoveries of water and organics in our solar system, the role our sun plays in water-loss in neighboring planets, and our search for habitable worlds… Continue reading →
Mercury, the planet closest to the Sun, is apparently also the dirtiest. According to a study published in the journal Nature Geoscience, Mercury’s surface is constantly bombarded by a steady stream of carbon from passing comets, a phenomenon that answers a longstanding question — why does the planet’s surface reflect so little sunlight? “It appears that… Continue reading →
