Twenty-five years ago, one of the most famous and awe-inspiring pieces of technology – The Hubble Space Telescope – was launched.
Hitching a ride with the Space Shuttle Discovery in 1990, Hubble was placed in low earth orbit, where it has been continuously observing the night sky ever since. Observations have been carried out across all wavelengths of light, from ultraviolet to infrared, that have given the astronomers an unprecedented window to the Universe.
But what have they learned from its breathtaking pictures? Well list is…….
CAUSE OF GAMMA-RAY BURSTS
The fuzzy looking galaxy shown was home to one of the most energetic events in the universe: a gamma-ray burst (GRB). These flashes of gamma-ray radiation are an enigma because they are so rare – a typical galaxy produces only a few every million years. Yet they release so much energy in a few seconds as our Sun does in 10 billion years. On 3 June 2013, a GRB lasting one-tenth of a second occurred, and was spotted by NASA’s swift satellite. When Hubble looked 10 days later, it found an infrared glow where the burst had been. But by 3 July it had faded. This disappearing glow was the dying embers of another kind of cosmic explosion – a kilonova – believed to be the result of extremely dense stars called ‘neutron stars’ merging.
Since the kilonova was found in the same location as the GRB, it was the ‘smoking gun’ revealing that short GRBs should well be caused in the same way. The kilonova was investigated by Prof Nial Tanvir of Leicester University, who says Hubble played a vital role. “Although Swift discovered this particular short gamma-ray burst, and observations from ground based telescopes gave us its precise position and distance, Hubble was the only option for seeing the faint kilonova emission.
HOW PLANETARY COLLISIONS WORK
On 16 July 1994, telescopic eyes were turned on Jupiter as the first of 2 fragments of the broke-up comet, Shoemaker-Levy 9, crashed into the planet. Blotches scarred the atmosphere for a month before the fading.
Hubble’s observations provided a wealth of information about Jupiter’s atmosphere. “Obvious waves emanated from the largest impacts, like ripples in a pond. From this, we could make deductions about the deep atmosphere and water below the clouds,” explains Dr Amy Simon, senior scientist for planetary atmosphere research at NASA Goddard.
While ground-based observatories were so involved, Hubble was the only one that could look across an entire range of wavelengths, irrespective of the time of day or weather conditions. Ultraviolet was particularly important for imaging dust and aerosols whipped up by the impacts. “Hubble observed leftover debris and molecules high in the atmosphere for months, and even years, afterwards,” says Dr Simon.
Looking like little islands, these flat discs of cold dust and gas are left over from the formation of a new star in the Orion nebula. Although part of this material will be lost over time, some will eventually clump together in pebble-sized grains before potentially building up to form a baby planet. As such, they are known as protoplanetary discs, or ‘proplyds’. By learning about proplyds, astronomers hope to find out more about the formation of Earth and other planets. Ground-based telescopes had previously detected the objects, which were initially believed to be stars. The idea that they were discs of material surrounding the star goes back to the 1700s, but confirmation didn’t come until late 1980s, where astronomers managed to detect the disc through observation of its modules. Hubble provided the breakthrough – directly imaging numerous proplyds for the first time within the Orion nebula.
AGE OF UNIVERSE
The spiral galaxy, M81, was the first of many galaxies observed by Hubble to find the expansion rate, and therefore the age, of universe. “Before the launch of Hubble, there was a heated debate over whether the Universe was 10 or 20 billion years old,” says Prof Wendy Freedman, an astronomer at the University of Chicago. Freedman set out to measure Cepheid variable stars – pulsating stars, whose brightness increases and decreases over timescale of days to months. By determining the relationship between Cepheid’s brightness and its pulsation rate, it is possible to estimate its distance.
Cepheids are the most accurate way of measuring the distances of galaxies, and for setting the expansion rate of universe.
The Hubble measurements helped to determine that the age of Universe is 13.8 billion years.
SUPERMASSIVE BLACK HOLES
Black holes are difficult to find. Their intense gravitational is so strong that not even light can escape their pull, making them ‘invisible’. But by measuring the material that surrounds a black hole, it is possible to calculate its mass using the laws of gravity. If there is more mass than is accounted for by the stars we see, the rest could be due to a black hole. By the early 1990s, it was suspected that a supermassive black hole (SMBH) was at the center of a handful of galaxies. Soon after its launch, Hubble confirmed earlier SMBH detections by taking images five times sharper than those obtained from the ground,” explains Dr Marc Sarzi, an astronomer at the University of Hertfordshire.
Hubble became known as the ‘black hole hunter’, due to its ability to measure the speed of surrounding gas and stars. Results from its observations were surprising, says Dr Sarzi. “SMBHs have a radius comparable to our solar system, and yet can only directly affect the motion of stars and gas in the very central regions of their host galaxies,” he says. It suggests they evolved together, he explains. “It has turned SMBHs from being exotic curiosities to an integral part of our understanding of galaxy formation.
Some pictures from the telescope has revealed the presence of something we can’t see: ‘dark matter’. The galaxies, star and planets that we can see make up just 15 per cent of the Universe’s matter. The rest – the 85 per cent – is dark matter and it neither emits nor absorbs any known wavelength of light. “With this map we saw for the first time where dark matter is’: says Durham University physicist Dr Richard Massey. To construct it, half a million galaxies were observed by Hubble and ground-based telescopes. “When light travels across the universe, it passes through all the intervening dark matter on its way to us, leaving a telltale imprint of its journey. You can’t see such faraway, faint galaxies from the Earth because atmosphere blurs the details. This is why we needed Hubble,” explains Massey.
The dark matter bends light in a ‘gravitational lensing’ effect, making the galaxies appear distorted. By observing this, it’s possible to deduce where dark matter lies. Such a map is fundamental to understanding the Universe’s structure, as dark matter acts as ‘scaffoldings’, along which galaxies are assembled. “When the first explorer reached the American West, they sat on a ridge and tried to understand the lie of the land. We were doing the same thing on a new frontier,” says Massey.
GENERATIONS OF STARS
Globular cluster are compact crowds of hundreds of thousands of stars bound together by gravity. For many years it was believed that all the stars within must be very similar, having formed close together from the same dusty cloud. But in 2005, Hubble measured the brightness and colors of the stars inside the NGC 2808 globular cluster. Only one generation of stars was expected, but three were found.
Dr Giampaolo Piotto was the leader of the team that observed NGC 2808. “With an age up to 13.5 billion years – only 300 million years less than the age of the Universe – globular clusters are a benchmark for cosmology, and represent an ideal laboratory to understand star formation and chemical evolution in the Universe,” he explains. What defines the different generations of the stars, also known as ‘stellar populations’, are characteristics such as their chemical composition, age,and their location in the cluster. Hubble’s high resolution images allowed Dr Piotto and his team to look into the densely packed core of NCG 2808 and measure many stars – something that is difficult for ground-based telescopes to do. Hubble’s visible and ultraviolet light also made easier to spot multiple populations of stars and track their evolutionary paths.
“We have now used Hubble to observe more than 60 globular clusters – almost half of the known globular clusters in the Milky Way. Preliminary results show that all have multiple stellar populations,” says Dr Piotto.
As of February 2015, 1,890 planets had been detected orbiting stars other than our Sun. An impressive photo if one of these ‘ exoplanets’ is yet to be taken, but Hubble was the first to detect the atmosphere of one of these alien worlds.
HD 209458-b, also known as Osiris, is a planet 150 light-years from Earth. Temprature reaches a scorching 1,100 degree C as it orbits just 6.4 million kilometers from its parent star. As the orbiting planet moves in front of the star, some of the light passes through the planet’s atmosphere. This is analysed by a spectograph, which is an instrument that spills light into constituent wavelengths, explains Prof David Charbonneau, leader of the team behind the discovery. “The idea was to gather spectra when the planet was in front of the star and when it moved away. By comparing them, we could research the appearance of new features when the planet was in transit. This required an extremely stable platform that was free from the absorption effects of our atmosphere. Only Hubble could do it!” In 2001, the procedure revealed signs of sodium – the first atmospheric element detected on a planet outside of our Solar System.
“This same method has become the standard means to examine exoplanet atmospheres, and Hubble has now gathered similar data on dozens of worlds,” says Charbonneau.
ACCELERATING EXPANSION OF THE UNIVERSE
Many galaxies host energetic supernovae (exploding stars), contributed to one of the most talked-about discoveries in recent years. Not only is the expansion of the universe accelerating, it is being fueled by a phenomenon dubbed ‘dark energy’.
In 1998, astronomers released new data on how the brightness of supernovae changed over time. It showed that the light coming from the most distant exploding stars was fainter and more stretched (red-shifted) than predicted. It meant that they were further away than astronomers calculated – a result that didn’t fit with the existing idea that the tug of gravity was causing the expansion of the Universe to slow down. For the team leading the project, this could only mean one thing: the expansion rate is not slowing at all. It’s speeding up.
Hubble played a supporting role in this initial discovery by providing data of three of the supernovae that the team wanted to observe, with the rest coming from ground-based telescopes in Chile, Europe and the USA. “This result was so extraordinary that it required extraordinary evidence,” explains Dr Adam Riess, one of the three Noble prize-winning team members of the discovery. “This confirming evidence came from Hubble Space Telescope.”
By finding and precisely measuring another 16 supernovae at distances up to 10 million light-years away, Hubble was able to confirm not just the acceleration, but that the Universe had indeed been decelerating in earlier times, just as predicted.
But to overcome gravity, something must be giving an opposing, repulsive force as the universe expands and matter is spread out. This ‘something’ is dark energy, which makes up approximately 75 percent of the entire known Universe. Hubble observations showed that this cause the acceleration we see today to begin about five billion years ago.
HOW GALAXIES EVOLVE
Many awe-inspiring images, dappled with beautiful shapes and a whole array of colors, changed the way we think about the distant Universe forever. One of Hubble’s most famous images, the Hubble Deep Field (HDF) is a snapshot of tiny patch of sky in the constilation of Ursa Major. It covers an area of just one 24-millionth of the whole sky. And yet this minute window reveals around 3,000 galaxies crowded together, giving astronomers a vital window into the past.
There had been predictions that the light emitted from such distant objects would be stretched out so much that they would appear nothing more than faint smudges against the blackness. They could not have been more wrong. The image, made up of 342 separate exposures taken over more than 100 hours, showcased the power of Hubble. It revealed an incredible amount of detail and structure to galaxies that had never been seen before.
“A lot of astronomers were sceptical that we would learn a lot from simply pointing the telescope at a fairly arbitrary spot in the sky and taking long exposures,” says Dr Henry Ferguson, a member of the original HDF team. However, the plethora of information that appeared convinced most that this was a good technique. As the telescope’s capabilities were upgraded, projects such as the Hubble Ultra Deep Field continued where HDF left off.
Today, astronomers are finding galaxies from a time when the Universe was only 500 million years old. As a result, it has become possible to chart galaxy evolution directly, by measuring how properties such as size, shape and color change over time. “The HDF became one of the major ‘watering holes’ for studying galaxy evolution, with deep observations spanning X-ray to radio wavelengths,” continues Dr Ferguson. “It is one of the most important observations ever made with any telescope.