Earth Blog

Space Shuttle Discovery, Orbit of the Earth

March 25, 2009

An international team of astronomers, lead by researchers at the Max-Planck Institute for Extraterrestrial Physics (MPE) , has directly observed an otherwise normal star orbiting the supermassive black hole at the center of the Milky Way Galaxy. Ten years of painstaking measurements have been crowned by a series of unique images obtained by the Adaptive Optics (AO) NAOS-CONICA (NACO) instrument on the 8.2-m VLT YEPUN telescope at the ESO Paranal Observatory.

It turns out that earlier this year the star approached the central Black Hole to within 17 light-hours - only three times the distance between the Sun and planet Pluto - while travelling at no less than 5000 km/sec. Previous measurements of the velocities of stars near the center of the Milky Way and variable X-ray emission from this area have provided the strongest evidence so far of the existence of a central Black Hole in our home galaxy and, implicitly, that the dark mass concentrations seen in many nuclei of other galaxies probably are also supermassive black holes.

However, it has not yet been possible to exclude several alternative configurations. In a break-through paper appearing in the research journal Nature on October 17th, 2002, the present team reports their exciting results, including high-resolution images that allow tracing two-thirds of the orbit of a star designated "S2" . It is currently the closest observable star to the compact radio source and massive black hole candidate "SgrA*" ("Sagittarius A") at the very center of the Milky Way. The orbital period is just over 15 years. The new measurements exclude with high confidence that the central dark mass consists of a cluster of unusual stars or elementary particles, and leave little doubt of the presence of a supermassive black hole at the centre of the galaxy in which we live.

Quasars and Black Holes

Ever since the discovery of the quasars (quasi-stellar radio sources) in 1963, astrophysicists have searched for an explanation of the energy production in these most luminous objects in the Universe. Quasars reside at the centres of galaxies, and it is believed that the enormous energy emitted by these objects is due to matter falling onto a supermassive Black Hole, releasing gravitational energy through intense radiation before that material disappears forever into the hole (in physics terminology: "passes beyond the event horizon").

To explain the prodigious energy production of quasars and other active galaxies, one needs to conjecture the presence of black holes with masses of one million to several billion times the mass of the Sun. Much evidence has been accumulating during the past years in support of the above "accreting black hole" model for quasars and other galaxies, including the detection of dark mass concentrations in their central regions.

However, an unambiguous proof requires excluding all possible other, non-black hole configurations of the central mass concentration. For this, it is imperative to determine the shape of the gravitational field very close to the central object - and this is not possible for the distant quasars due to technological limitations of the currently available telescopes.

The centre of the Milky Way

The centre of our Milky Way galaxy is located in the southern constallation Sagittarius (The Archer) and is "only" 26,000 light-years away. On high-resolution images, it is possible to discern thousands of individual stars within the central, one light-year wide region (this corresponds to about one-quarter of the distance to "Proxima Centauri", the star nearest to the solar system).

Using the motions of these stars to probe the gravitational field, observations with the 3.5-m New Technology Telescope (NTT) at the ESO La Silla Observatory (Chile) (and subsequently at the 10-m Keck telescope, Hawaii, USA) over the last decade have shown that a mass of about 3 million times that of the Sun is concentrated within a radius of only 10 light-days of the compact radio and X-ray source SgrA* ("Sagittarius A") at the center of the star cluster.

This means that SgrA* is the most likely counterpart of the putative black hole and, at the same time, it makes the Galactic Center the best piece of evidence for the existence of such supermassive black holes . However, those earlier investigations could not exclude several other, non-black hole configurations.

"We then needed even sharper images to settle the issue of whether any configuration other than a black hole is possible and we counted on the ESO VLT telescope to provide those", explains Reinhard Genzel, Director at the Max-Planck Institute for Extraterrestrial Physics (MPE) in Garching near Munich (Germany) and member of the present team. "The new NAOS-CONICA (NACO) instrument, built in a close collaboration between our institute, the Max-Planck Institute for Astronomy (MPIA: Heidelberg, Germany), ESO and the Paris-Meudon and Grenoble Observatories (France), was just what we needed to take this decisive step forward".

The NACO observations of the Milky Way centre

The new NACO instrument was installed in late 2001 at the VLT 8.2-m YEPUN telescope. Already during the initial tests, it produced many impressive images, some of which have been the subject of earlier ESO press releases.

"The first observations this year with NACO gave us right away the sharpest and 'deepest' images of the Milky Way Centre ever taken, showing a large number of stars in that area in great detail", says Andreas Eckart of the University of Cologne, another member of the international team that is headed by Rainer Schödel, Thomas Ott and Reinhard Genzel from MPE. "But we were still to be overwhelmed by the wonderful outcome of those data!"

Combining their infrared images with high-resolution radio data, the team was able to determine - during a ten-year period - very accurate positions of about one thousand stars in the central area with respect to the compact radio source SgrA*.

"When we included the latest NACO data in our analysis in May 2002, we could not believe our eyes. The star S2, which is the one currently closest to SgrA*, had just performed a rapid swing-by near the radio source. We suddenly realised that we were actually witnessing the motion of a star in orbit around the central black hole, taking it incredibly close to that mysterious object", says a very happy Thomas Ott, who is now working in the MPE team on his PhD thesis.

In orbit around the central black hole

No event like this one has ever been recorded . These unique data show unambiguously that S2 is moving along an elliptical orbit with SgrA* at one focus, i.e. S2 orbits SgrA* like the Earth orbits the Sun.

The superb data also allow a precise determination of the orbital parameters (shape, size, etc.). It turns out that S2 reached its closest distance to SgrA* in the spring of 2002, at which moment it was only 17 light-hours away from the radio source, or just 3 times the Sun-Pluto distance. It was then moving at more than 5000 km/s, or nearly two hundred times the speed of the Earth in its orbit around the Sun. The orbital period is 15.2 years. The orbit is rather elongated - the eccentricity is 0.87 - indicating that S2 is about 10 light-days away from the central mass at the most distant orbital point.

"We are now able to demonstrate with certainty that SgrA* is indeed the location of the central dark mass we knew existed. Even more important, our new data have "shrunk" by a factor of several thousand the volume within which those several million solar masses are contained", says Rainer Schödel, PhD student at MPE and also first author of the resulting paper.

In fact, model calculations now indicate that the best estimate of the mass of the Black Hole at the centre of the Milky Way is 2.6 ± 0.2 million times the mass of the Sun .

No other possibilities

According to the detailed analysis presented in the Nature article, other previously possible configurations, such as very compact clusters of neutron stars, stellar size black holes or low mass stars, or even a ball of putative heavy neutrinos, can now be definitively excluded.

The only still viable non-black hole configuration is a hypothetical star of heavy elementary particles called bosons, which would look very similar to a black hole. "However", says Reinhard Genzel, "even if such a boson star is in principle possible, it would rapidly collapse into a supermassive black hole anyhow, so I think we have pretty much clinched the case!"

Next observations

"Most astrophysicists would accept that the new data provide compelling evidence that a supermassive black hole exists in the center of the Milky Way. This makes even more likely the supermassive black hole interpretation for the enormous concentration of dark mass detected at the center of many other galaxies", says Alvio Renzini, VLT Programme Scientist at ESO.

So what remains to be done? The next big quest now is to understand when and how these supermassive black holes formed and why almost every massive galaxy appears to contain one. The formation of central black holes and that of their host galaxies themselves increasingly appear to be just one problem and the same. Indeed, one of the outstanding challenges for the VLT to solve in the next few years.

There is also little doubt that coming interferometric observations with instruments at the VLT Interferometer (VLTI) and the Large Binocular Telescope (LBT) will also result in another giant leap within this exciting field of research.

Andreas Eckart is optimistic: "Perhaps it will even be possible with X-ray and radio observations in the next few years to directly demonstrate the existence of the event horizon."

In the search for rogue planets and failed stars astronomers using the NASA/ESA Hubble Space Telescope have created a new mosaic image of the Orion Nebula. During their survey of the famous star formation region, they found what may be the missing piece of a cosmic puzzle; the third, long-lost member of a star system that had broken apart.

The Orion Nebula is the closest star formation region to Earth, only 1400 light-years away. It is a turbulent place — stars are being born, planetary systems are forming and the radiation unleashed by young massive stars is carving cavities in the nebula and disrupting the growth of smaller, nearby stars.

Because of this ongoing turmoil, Hubble has observed the nebula many times to study the various intriguing processes going on there. This large composite image of the nebula’s central region, combining visual and near-infrared data, is the latest addition to this collection.

Astronomers used these new infrared data to hunt for rogue planets — free-floating in space without a parent star — and brown dwarfs in the Orion Nebula. The infrared capabilities of Hubble also allow it to peer through the swirling clouds of dust and gas and make the stars hidden within clearly visible; the unveiled stars appear with bright red colours in the final image. Among these, astronomers stumbled across a star moving at an unusually high speed — about 200 000 kilometres per hour. This star could be the missing piece of the puzzle of a star system that had been broken apart 540 years ago.

Astronomers already knew about two other runaway stars in the Orion Nebula which were most likely once part of a now-defunct multiple-star system. For years it was suspected that the original system contained more than just these two stars. Now, by virtue of accident and curiosity, Hubble may have found the missing third piece of this cosmic puzzle.

Whether the new star is indeed the missing — and the last — piece of the puzzle will require further observations. So will the answer to the question of why the original star system broke apart in the first place. While there are several theories — interactions with other, nearby stellar groups, or two of the stars getting too close to each other — none can be ruled out or confirmed yet.

And while the astronomers are looking for the answers to these questions, who knows what mystery they will find next?

2009

Astronomers using NASA's Spitzer Space Telescope have detected what they believe is an alien world just two-thirds the size of Earth – one of the smallest on record. The exoplanet candidate, known as UCF-1.01, orbits a star called GJ 436, which is located a mere 33 light-years away. UCF-1.01 might be the nearest world to our solar system that is smaller than our home planet.

Although probably rocky in composition like Earth, UCF-1.01 would be a terrible place for life. The world orbits scorchingly close to its star, so in all likelihood this planet lacks an atmosphere and might even have a molten surface, as shown in this artist's impression.

Evidence for UCF-1.01 turned up when astronomers were studying a known, Neptune-sized exoplanet, called GJ 436b, seen in the background in this image. The identification of nearby small planets may lead to their characterization using future instruments. In this way, worlds like UCF-1.01 might serve as stepping stones to one day finding a habitable, Earth-like exoplanet.

Because of GJ 436's proximity to our solar system, the star field around it shares many of our culture's famous cosmic landmarks. To the far left, the constellation of Orion gleams, though in a distorted shape compared to our vantage point on Earth. The red giant Betelgeuse (Orion's right shoulder) and blue Rigel (Orion's left foot) stand out, as well as the three belt stars. From GJ 436's perspective, however, the stars do not align as they do in our sky. The Pleiades star cluster is located to the upper left of UCF-1.01.

At the centre of this amazing image is the elliptical galaxy NGC 3610. Surrounding the galaxy are a wealth of other galaxies of all shapes. There are spiral galaxies, galaxies with a bar in their central regions, distorted galaxies and elliptical galaxies, all visible in the background. In fact, almost every bright dot in this image is a galaxy — the few foreground stars are clearly distinguishable due to the diffraction spikes that overlay their images.

NGC 3610 is of course the most prominent object in this image — and a very interesting one at that! Discovered in 1793 by William Herschel, it was later found that this elliptical galaxy contains a disc. This is very unusual, as discs are one of the main distinguishing features of a spiral galaxy. And NGC 3610 even hosts a memarkable bright disc.

The reason for the peculiar shape of NGC 3610 stems from its formation history. When galaxies form, they usually resemble our galaxy, the Milky Way, with flat discs and spiral arms where star formation rates are high and which are therefore very bright. An elliptical galaxy is a much more disordered object which results from the merging of two or more disc galaxies. During these violent mergers most of the internal structure of the original galaxies is destroyed. The fact that NGC 3610 still shows some structure in the form of a bright disc implies that it formed only a short time ago. The galaxy’s age has been put at around four billion years and it is an important object for studying the early stages of evolution in elliptical galaxies.

West End of Grand Bahama Island, Bahamas

August 23, 2009

The magnificent reflection nebula NGC 2023 lies nearly 1500 light-years from Earth. It is located within the constellation of Orion (The Hunter), in a prestigious area of the sky close to the well-known Flame and Horsehead Nebulae. The entire structure of NGC 2023 is vast, at four light-years across. This NASA/ESA Hubble Space Telescope picture just takes in the southern part, with the subtle shades of colour closely resembling those of a sunset on Earth.

NGC 2023 surrounds a massive young B-type star. These stars are large, bright and blue-white in colour, and have a high surface temperature, being several times hotter than the Sun. The energy emitted from NGC2023’s B-type star illuminates the nebula, resulting in its high surface brightness: good news for astronomers who wish to study it. The star itself lies outside the field of view, at the upper left, and its brilliant light is scattered by Hubble’s optical system, creating the bright flare across the left side of the picture, which is not a real feature of the nebula.

Stars are forming from the material comprising NGC 2023. This Hubble image captures the billowing waves of gas, 5000 times denser than the interstellar medium. The unusual greenish clumps are thought to be Herbig–Haro objects. These peculiar features of star-forming regions are created when gas ejected at hundreds of kilometres per second from newly formed stars impacts the surrounding material. These shockwaves cause the gas to glow and result in the strange shapes seen here. Herbig–Haro objects typically only last for a few thousand years, which is the blink of eye in astronomical terms.

This picture was created from multiple images taken with the Wide Field Camera of Hubble’s Advanced Camera for Surveys. Exposures through a blue filter (F475W) are coloured blue, exposures through a yellow filter (F625W) are coloured green and images through a near-infrared filter (F850LP) are shown as red. The total exposure times per filter are 800 s, 800 s and 1200 s, respectively, and the field of view spans 3.2 arcminutes.

Yoho National Park is located in the Canadian Rocky Mountains along the western slope of the Continental Divide of the Americas in southeastern British Columbia. Yoho NP is bordered by Kootenay National Park on the southern side and Banff National Park on the eastern side in Alberta. The name Yoho comes from the Cree word for awe and wonder.

Yoho covers 1,313 km2 (507 mi2) and it is the smallest of the four contiguous national parks. Yoho, together with Jasper, Kootenay and Banff National Parks, along with three British Columbia provincial parks—Hamber Provincial Park, Mount Assiniboine Provincial Park, and Mount Robson Provincial Park—form the Canadian Rocky Mountain Parks World Heritage Site. The park's administrative and visitor centre are located in the town of Field, British Columbia, beside the Trans-Canada Highway.

Portrayed in this image is the spiral galaxy NGC 4945, a close neighbour of the Milky Way. Belonging to the Centaurus A group of galaxies, it is located at a distance of almost 13 million light-years. Showing a remarkable resemblance to our own galaxy, NGC 4945 also hides a supermassive black hole behind the thick, ring-shaped structure of dust visible in the picture. But, unlike the black hole at the centre of our Milky Way, the million-solar-mass black hole inside NGC 4945 is an Active Galactic Nucleus that is frantically consuming any surrounding matter, and so releasing tremendous amounts of energy.

This image combines observations performed through three different filters (B, V, R) with the 1.5-metre Danish telescope at the ESO La Silla Observatory in Chile.

This dazzling image shows the globular cluster Messier 69, or M 69 for short, as viewed through the NASA/ESA Hubble Space Telescope. Globular clusters are dense collections of old stars. In this picture, foreground stars look big and golden when set against the backdrop of the thousands of white, silvery stars that make up M 69.

Another aspect of M 69 lends itself to the bejewelled metaphor: As globular clusters go, M 69 is one of the most metal-rich on record. In astronomy, the term “metal” has a specialised meaning: it refers to any element heavier than the two most common elements in our Universe, hydrogen and helium. The nuclear fusion that powers stars created all of the metallic elements in nature, from the calcium in our bones to the carbon in diamonds. Successive generations of stars have built up the metallic abundances we see today.

Because the stars in globular clusters are ancient, their metallic abundances are much lower than more recently formed stars, such as the Sun. Studying the makeup of stars in globular clusters like M 69 has helped astronomers trace back the evolution of the cosmos.

M 69 is located 29 700 light-years away in the constellation Sagittarius (the Archer). The famed French comet hunter Charles Messier added M 69 to his catalogue in 1780. It is also known as NGC 6637.

The image is a combination of exposures taken in visible and near-infrared light by Hubble’s Advanced Camera for Surveys, and covers a field of view of approximately 3.4 by 3.4 arcminutes.

The MPG/ESO 2.2-metre telescope at La Silla in Chile is a powerful instrument that can capture distant celestial objects, but it has been used here to image a heavenly body that is much closer to home: the Moon.

This image of the crescent Moon shows sunlight skimming across the heavily pocked surface, filling its craters with shadows. This is a fairly flat region of the Moon, but elsewhere, high mountains can be found, with some peaks reaching about 5000 metres. When backlit by the Sun, these mountains cast long shadows on the lunar surface. In the 1600s, Galileo Galilei used these long shadows to determine the height of the peaks.

At the Moon’s poles (not seen in this picture), some craters are permanently shadowed and the floors of some may have not seen sunlight for billions of years. Scientists had long suspected that these dark and constantly cold regions of the Moon could harbour water ice, but it wasn’t until late 2009 that evidence for this was found.

In a NASA mission called LCROSS (Lunar Crater Observation and Sensing Satellite), a spent rocket booster was sent on a one-way collision course to the Moon’s south pole, while the remaining part of the spacecraft hunted for evidence of water in the ejected debris. The mission was a success and its findings confirmed the presence of water ice within these dark craters. The finding has important implications for the future of human exploration of the Moon and elsewhere in the Solar System.

This view of the Moon was taken through a narrowband red filter (H-alpha). The height of the image is about 30 arcminutes.

With their spectacular glowing arms, grand spiral galaxies seem to get all the attention — but NGC 6822, a barred irregular dwarf galaxy, proves that regular spirals do not have a monopoly on galactic beauty. Also called Barnard’s galaxy, NGC 6822 is located in the constellation of Sagittarius just 1.6 million light-years away and is brimming with rich star formation regions.

This new image is a composite of older observations made with the Wide Field Imager attached to the 2.2-metre MPG/ESO telescope at ESO’s La Silla Observatory and new data collected by the Atacama Large Millimeter/submillimeter Array (ALMA).

The observations by ALMA reveal the structure of star-forming gas clouds in unprecedented resolution. Observations in our own galaxy have shown that stars form in the dense cores of giant clouds of molecular hydrogen gas, the only places where gas can become cold enough to collapse under its own gravity. These conditions also foster the formation of other molecules, such as carbon monoxide, which are an indispensable tool in helping astronomers to detect galactic molecular hydrogen gas.

Until recently, astronomers have only been able to resolve star formation regions within the Milky Way — but now ALMA’s sharp sight provides a window into star formation in other galaxies. The analysis of the data revealed that, unlike in our own galaxy, the observed molecules are concentrated into small, dense cores of gas. This explains why it has been so hard to observe extragalactic star formation regions so far, especially in low mass, low metallicity galaxies. ALMA also found that the cores in NGC 6822 behave remarkably similarly to stellar nurseries in the Milky Way, indicating that the physics of star formation in these low-mass galaxies resemble that which we see in our own galaxy.

New observations indicate that massive, star-forming galaxies during the peak epoch of galaxy formation, 10 billion years ago, were dominated by baryonic or “normal” matter. This is in stark contrast to present-day galaxies, where the effects of mysterious dark matter seem to be much greater. This surprising result was obtained using ESO’s Very Large Telescope and suggests that dark matter was less influential in the early Universe than it is today. The research is presented in four papers, one of which will be published in the journal Nature this week.

We see normal matter as brightly shining stars, glowing gas and clouds of dust. But the more elusive dark matter does not emit, absorb or reflect light and can only be observed via its gravitational effects. The presence of dark matter can explain why the outer parts of nearby spiral galaxies rotate more quickly than would be expected if only the normal matter that we can see directly were present.

Now, an international team of astronomers led by Reinhard Genzel at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany have used the KMOS and SINFONI instruments at ESO’s Very Large Telescope in Chile to measure the rotation of six massive, star-forming galaxies in the distant Universe, at the peak of galaxy formation 10 billion years ago.

What they found was intriguing: unlike spiral galaxies in the modern Universe, the outer regions of these distant galaxies seem to be rotating more slowly than regions closer to the core — suggesting there is less dark matter present than expected.

“Surprisingly, the rotation velocities are not constant, but decrease further out in the galaxies,” comments Reinhard Genzel, lead author of the Nature paper. “There are probably two causes for this. Firstly, most of these early massive galaxies are strongly dominated by normal matter, with dark matter playing a much smaller role than in the Local Universe. Secondly, these early discs were much more turbulent than the spiral galaxies we see in our cosmic neighbourhood.”

Both effects seem to become more marked as astronomers look further and further back in time, into the early Universe. This suggests that 3 to 4 billion years after the Big Bang, the gas in galaxies had already efficiently condensed into flat, rotating discs, while the dark matter halos surrounding them were much larger and more spread out. Apparently it took billions of years longer for dark matter to condense as well, so its dominating effect is only seen on the rotation velocities of galaxy discs today

This explanation is consistent with observations showing that early galaxies were much more gas-rich and compact than today’s galaxies.

The six galaxies mapped in this study were among a larger sample of a hundred distant, star-forming discs imaged with the KMOS and SINFONI instruments at ESO’s Very Large Telescope at the Paranal Observatory in Chile. In addition to the individual galaxy measurements described above, an average rotation curve was created by combining the weaker signals from the other galaxies. This composite curve also showed the same decreasing velocity trend away from the centres of the galaxies. In addition, two further studies of 240 star forming discs also support these findings.

Detailed modelling shows that while normal matter typically accounts for about half of the total mass of all galaxies on average, it completely dominates the dynamics of galaxies at the highest redshifts.

Florence, Montana, USA

December 10, 2011

Star clusters such as the Pleiades are often considered some of the most beautiful objects in the sky. Yet in this image taken by NASA's Wide-field Infrared Survey Explorer, or WISE, the star cluster NGC 2259 is overshadowed by the surrounding stars and dust which glow brightly in infrared light.

Star clusters are groups of stars held together by their own gravity. Larger star clusters, with hundreds of thousands or more stars, tend to form into spheres and are called globular clusters. Open star clusters with up to a few thousand stars are more irregularly shaped. NGC 2259 is an example of an open cluster where the stars are more loosely organized.

As seen in visible light through a telescope, NGC 2259 appears as a smattering of stars loosely clustered in an irregular group. In the infrared light captured by WISE's detectors, nearby gas and dust clouds which are dark in visible light, along with stars whose visible light the clouds block, emerge into view. Seeing more stars as well as glowing gas and dust in the infrared makes the star cluster less obvious. In the annotated version of this image, NGC 2259 is located within the white circle.

One of the more prominent objects in this image is the bright blue star seen on the left side. This star is HR 258. Although it appears to be one star, it is actually a binary, or double, star system. The two stars are also known as CCDM J00549+2337 A & B. At a distance of 127 light-years, this star system is much closer than the NGC 2259 star cluster, which is roughly 12,000 light-years from Earth.

The NGC 2259 star cluster is hundreds of millions (300,000,000 to 700,000,000) years old. While this sounds old, it is much younger than the age of the sun, which is 5.5 billion (5,500,000,000) years old. However, NGC 2259 is much older than the red-colored young stellar objects seen in this image, which are typically only a few million years old. Seen scattered around the image, these young stars are forming within the clouds of gas and dust.

The colors used in this image represent specific wavelengths of infrared light. Blue and cyan represent 3.4- and 4.6-micron light, mainly emitted by hot stars. Green and Red represent 12- and 22-micron wavelengths, primarily light emitted from warm dust.

Enceladus is a world divided. To the north, we see copious amounts of craters and evidence of the many impacts the moon has suffered in its history. However, to the south we see a smoother body with wrinkles due to geologic activity.

Most solar system bodies lacking an atmosphere are heavily cratered like Enceladus' (313 miles or 504 kilometers across) northern region. However, the geologic activity in the south, including the famous plume above the moon's south pole, can erase craters and leave a younger, smoother-looking surface.

This view looks toward the anti-Saturn hemisphere of Enceladus. North on Enceladus is up and rotated 4 degrees to the right. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Nov. 27, 2016.

The view was obtained at a distance of approximately 41,000 miles (66,000 kilometers) from Enceladus. Image scale is 1,310 feet (398 meters) per pixel.

An outburst of Perseid meteors lights up the sky in August 2009 in this time-lapse image.

This image from Hubble’s Wide Field Camera 3 (WFC3) shows NGC 1448, a spiral galaxy located about 50 million light-years from Earth in the little-known constellation of Horologium (The Pendulum Clock). We tend to think of spiral galaxies as massive and roughly circular celestial bodies, so this glittering oval does not immediately appear to fit the visual bill. What’s going on?

Imagine a spiral galaxy as a circular frisbee spinning gently in space. When we see it face on, our observations reveal a spectacular amount of detail and structure — a great example from Hubble is the telescope’s view of Messier 51, otherwise known as the Whirlpool Galaxy. However, the NGC 1448 frisbee is very nearly edge-on with respect to Earth, giving it an appearance that is more oval than circular. The spiral arms, which curve out from NGC 1448’s dense core, can just about be seen.

Although spiral galaxies might appear static with their picturesque shapes frozen in space, this is very far from the truth. The stars in these dramatic spiral configurations are constantly moving and spinning around the galaxy’s core, with those on the inside whirling around faster than those sitting further out. This makes the formation and continued existence of a spiral galaxy’s arms something of a cosmic puzzle, because the arms wrapped around the spinning core should become wound tighter and tighter as time goes on — but this is not what we see. This is known as the winding problem.

This graphic features an artist's impression of a star found in the closest orbit known around a black hole, as reported in our latest press release. This discovery was made using data from NASA's Chandra X-ray Observatory (shown in the inset where low, medium, and high-energy X-rays are colored red, green, and blue respectively), plus NASA's NuSTAR telescope and the Australia Telescope Compact Array.

Astronomers found this extraordinarily close stellar pairing in the globular cluster named 47 Tucanae, a dense collection of stars located on the outskirts of the Milky Way galaxy, about 14,800 light years from Earth.

This particular source, known as X9, has been of interest to scientists for many years. Until a couple of years ago, astronomers thought X9 contained a white dwarf pulling material from a companion star like the Sun. (Astronomers call a pair of objects orbiting one another a 'binary' system.) However, a team of scientists in 2015 used radio data to show that X9 likely consisted instead of a black hole pulling gas from a white dwarf companion. These researchers predicted that the white dwarf would take only about 25 minutes to orbit the black hole.

New Chandra data likely verify this hypothesis and reveal that the X-rays change periodically over about 28 minutes. Additionally, Chandra data show evidence for large amounts of oxygen in the system, a characteristic for the presence of a white dwarf. Therefore, a strong case can be made that that the companion star is a white dwarf, which would then be orbiting the black hole at only about 2.5 times the separation between the Earth and the Moon.

As seen in the artist's illustration, the white dwarf is so close to the black hole that much of its material is being pulled away. If it continues to lose mass, this white dwarf may evolve into some exotic sort of planet or completely evaporate.

In order to make such a close pairing, one possibility is that the black hole smashed into a red giant star, and then gas from the outer regions of the star was ejected from the binary. The remaining core of the red giant would form into a white dwarf, which becomes a binary companion to the black hole. The orbit of the binary would then have shrunk as gravitational waves were emitted, until the black hole started pulling material from the white dwarf. The gravitational waves currently being produced by X9 have a frequency that is too low to be detected with Laser Interferometer Gravitational-Wave Observatory (LIGO). It could potentially be detected with future gravitational wave observatories in space.

An alternative explanation for the observations is that the binary contains a neutron star, rather than a black hole, that is spinning faster as it pulls material from a white dwarf companion via a disk. This process can lead to the neutron star spinning around its axis thousands of times every second. A few such objects, called transitional millisecond pulsars, have been observed near the end of this spinning up phase. The authors do not favor this possibility as transitional millisecond pulsars have properties not seen in X9, such as extreme variability at X-ray and radio wavelengths. However, they cannot disprove this explanation.

In addition to Chandra, NASA's NuSTAR telescope, which observes higher-energy X-rays, and the radio telescope Australia Telescope Compact Array were used to make this discovery.

A setting Sun and Earth's horizon are featured in this image photographed by an Expedition 13 crewmember on the International Space Station.

ISS, Orbit of the Earth

September 5, 2006

In this image the NASA/ESA Hubble Space Telescope takes a close look at the spiral galaxy NGC 4217, 60 million light-years away. The galaxy is seen almost perfectly edge on and is a perfect candidate for studying the nature of extraplanar dust structures — the patterns of gas and dust above and below the plane on the galaxy, seen here as brown wisps coming off NGC 4217.

These tentacle-like filaments are visible in the Hubble image only because the contrast with their surroundings is so high. This implies that the structures are denser than their surroundings. The image shows dozens of dust structures some of which reach as far as 7000 light-years away from the central plane. Typically the structures have a length of about 1000 light-years and are about 400 light-years in width.

Some of the dust filaments are round or irregular clouds, others are vertical columns, looplike structures or vertical cones. These structures can help astronomers to identify the mechanisms responsible for the ejection of gas and dust from the galactic plane of spiral galaxies and reveal information on the transport of the interstellar medium to large distances away from galactic discs.

The properties of the observed dust structures in NGC 4217 suggest that the gas and dust was driven out of the midplane of the galaxy by powerful stellar winds resulting from supernovae — explosions that mark the deaths of massive stars.

The NASA/ESA Hubble Space Telescope has captured a crowd of stars that looks rather like a stadium darkened before a show, lit only by the flashbulbs of the audience’s cameras. Yet the many stars of this object, known as Messier 107, are not a fleeting phenomenon, at least by human reckoning of time — these ancient stars have gleamed for many billions of years.

Messier 107 is one of more than 150 globular star clusters found around the disc of the Milky Way galaxy. These spherical collections each contain hundreds of thousands of extremely old stars and are among the oldest objects in the Milky Way. The origin of globular clusters and their impact on galactic evolution remains somewhat unclear, so astronomers continue to study them through pictures such as this one obtained by Hubble.

As globular clusters go, Messier 107 is not particularly dense. Visually comparing its appearance to other globular clusters, such as Messier 53 or Messier 54 reveals that the stars within Messier 107 are not packed as tightly, thereby making its members more distinct like individual fans in a stadium's stands.

Messier 107 can be found in the constellation of Ophiuchus (The Serpent Bearer) and is located about 20 000 light-years from the Solar System.

French astronomer Pierre Méchain first noted the object in 1782, and British astronomer William Herschel documented it independently a year later. A Canadian astronomer, Helen Sawyer Hogg, added Messier 107 to Charles Messier's famous astronomical catalogue in 1947.

This picture was obtained with the Wide Field Camera of Hubble’s Advanced Camera for Surveys. The field of view is approximately 3.4 by 3.4 arcminutes.

Puyallup, Washington, USA

November 20, 2004

This Voyager 2 image shows the region of Jupiter extending from the equator to the southern polar latitudes in the neighborhood of the Great Red Spot. A white oval, different from the one observed in a similar position at the time of the Voyager 1 encounter, is situated south of the Great Red Spot. The region of white clouds now extends from east of the red spot and around its northern boundary, preventing small cloud vortices from circling the feature. The disturbed region west of the red spot has also changed since the equivalent Voyager 1 image. It shows more small scale structure and cloud vortices being formed out of the wave structures. The picture was taken on July 3 from 6 million kilometers (3.72 million miles).

The longest set of HARPS measurements ever made has firmly established the nature of the smallest and fastest-orbiting exoplanet known, CoRoT-7b, revealing its mass as five times that of Earth's. Combined with CoRoT-7b's known radius, which is less than twice that of our terrestrial home, this tells us that the exoplanet's density is quite similar to the Earth's, suggesting a solid, rocky world. The extensive dataset also reveals the presence of another so-called super-Earth in this alien solar system.

"This is science at its thrilling and amazing best," says Didier Queloz, leader of the team that made the observations. "We did everything we could to learn what the object discovered by the CoRoT satellite looks like and we found a unique system."

In February 2009, the discovery by the CoRoT satellite of a small exoplanet around a rather unremarkable star named TYC 4799-1733-1 was announced one year after its detection and after several months of painstaking measurements with many telescopes on the ground, including several from ESO. The star, now known as CoRoT-7, is located towards the constellation of Monoceros (the Unicorn) at a distance of about 500 light-years. Slightly smaller and cooler than our Sun, CoRoT-7 is also thought to be younger, with an age of about 1.5 billion years.

Every 20.4 hours, the planet eclipses a small fraction of the light of the star for a little over one hour by one part in 3000. This planet, designated CoRoT-7b, is only 2.5 million kilometres away from its host star, or 23 times closer than Mercury is to the Sun. It has a radius that is about 80% greater than the Earth's.

The initial set of measurements, however, could not provide the mass of the exoplanet. Such a result requires extremely precise measurements of the velocity of the star, which is pulled a tiny amount by the gravitational tug of the orbiting exoplanet. The problem with CoRoT-7b is that these tiny signals are blurred by stellar activity in the form of "starspots" (just like sunspots on our Sun), which are cooler regions on the surface of the star. Therefore, the main signal is linked to the rotation of the star, with makes one complete revolution in about 23 days.

To get an answer, astronomers had to call upon the best exoplanet-hunting device in the world, the High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph attached to the ESO 3.6-metre telescope at the La Silla Observatory in Chile.

"Even though HARPS is certainly unbeaten when it comes to detecting small exoplanets, the measurements of CoRoT-7b proved to be so demanding that we had to gather 70 hours of observations on the star," says co-author François Bouchy.

HARPS delivered, allowing the astronomers to tease out the 20.4-hour signal in the data. This figure led them to infer that CoRoT-7b has a mass of about five Earth masses, placing it in rare company as one of the lightest exoplanets yet found.

"Since the planet's orbit is aligned so that we see it crossing the face of its parent star — it is said to be transiting — we can actually measure, and not simply infer, the mass of the exoplanet, which is the smallest that has been precisely measured for an exoplanet," says team member Claire Moutou. "Moreover, as we have both the radius and the mass, we can determine the density and get a better idea of the internal structure of this planet."

With a mass much closer to that of Earth than, for example, ice giant Neptune's 17 Earth masses, CoRoT-7b belongs to the category of "super-Earth" exoplanets. About a dozen of these bodies have been detected, though in the case of CoRoT-7b, this is the first time that the density has been measured for such a small exoplanet. The calculated density is close to Earth's, suggesting that the planet's composition is similarly rocky.

"CoRoT-7b resulted in a 'tour de force' of astronomical measurements. The superb light curves of the space telescope CoRoT gave us the best radius measurement, and HARPS the best mass measurement for an exoplanet. Both were needed to discover a rocky planet with the same density as the Earth," says co-author Artie Hatzes.

CoRoT-7b earns another distinction as the closest known exoplanet to its host star, which also makes it the fastest — it orbits its star at a speed of more than 750 000 kilometres per hour, more than seven times faster than the Earth's motion around the Sun. "In fact, CoRoT-7b is so close that the place may well look like Dante's Inferno, with a probable temperature on its 'day-face' above 2000 degrees and minus 200 degrees on its night face. Theoretical models suggest that the planet may have lava or boiling oceans on its surface. With such extreme conditions this planet is definitively not a place for life to develop," says Queloz.

As a further testament to HARPS' sublime precision, the astronomers found from their dataset that CoRoT-7 hosts another exoplanet slightly further away than CoRoT-7b. Designated CoRoT-7c, it circles its host star in 3 days and 17 hours and has a mass about eight times that of Earth, so it too is classified as a super-Earth. Unlike CoRoT-7b, this sister world does not pass in front of its star as seen from Earth, so astronomers cannot measure its radius and thus its density.

Given these findings, CoRoT-7 stands as the first star known to have a planetary system made of two short period super-Earths with one that transits its host.