Category Archives: X Ray

30th Anniversary of SN 1987A

The supernova SN 1987A, that exploded in the outskirts of the Tarantula Nebula (1) in the Large Magellanic Cloud (LMC) on February 23rd, 1987, was an unique a rare event that still puzzles astronomers and theoretical physicists. SN 1987A originated by the core-collapse of a very massive star, hence it was classified as a “type II” supernova (2).

SN 1987A provided astrophysicists the very first opportunity to study the behavior and evolution of a supernova in detail. Among other things, SN 1987A was the first supernova for which the progenitor star was identified on archival photographic plates, permitted the first direct observation of supernova nucleosynthesis (including accurate masses of 56Ni, 57Ni and 44Ti), the first observation of the dust in a supernova, the first detection of circumstellar and interstellar material around a supernova, and, importantly, the first detection of neutrinos coming from the core collapse of a massive stars.

Figure 1. Supernova 1987A after exploding in February 1987 (left), and an image taken before the explosion (right). Credit: David Malin / Australian Astronomical Observatory.

Figure 1. Supernova 1987A after exploding in February 1987 (left), and an image taken before the explosion (right). Credit: David Malin / Australian Astronomical Observatory.

Commemorating the 30th anniversary of this very rare event, we at the Australian Astronomical Observatory (AAO) have published today a new media release about it. In this post I extend this media release providing a kind of review of what it has been the study of the SN 1987A.

The discovery of SN 1987A

Although the light from SN 1987A reached Earth on February 23rd, 1987, it was discovered at ~23:00 UTC February 24th, 1987 (~10am Feb 25th, 1987 in Sydney time) by astronomers Ian Shelton and Oscar Duhalde using the 10-inch astrograph (also seen by the naked eye) at Las Campanas Observatory in Chile. Within few hours it was also independently identified by amateur astronomer Albert Jones (who has one of the highest records of observing variable stars, more than half a million measurements during his life) from New Zealand. Figure 1 compares the view of this region of the Tarantula Nebula using the Anglo-Australian Telescope (AAT) after (left) and before (right) the star exploded.

SN 1987A is located at approximately 168,000 light years from Earth. It was the closest observed supernova since famous Kepler’s Supernova (SN 1604), which occurred in the Milky Way itself in 1604. Although SN 1987A could be seen only from the Southern Hemisphere because its location in the famous Tarantula Nebula (Figure 2) within the Large Magellanic Cloud, and it was visible to the naked eye during months. Its brightness peaked in May, when it reaches an apparent magnitude of about 3, meaning it was within the 300 brightest stars in the sky at that moment.

Figure 2: The Tarantula Nebula loom to the upper left of where the star Sanduleak -69° 202 exploded as supernova 1987A. Three-colour image made from BGR plates taken at the Anglo-Australian Telescope (AAT) prime focus Credit: David Malin / Australian Astronomical Observatory.

Figure 2: The Tarantula Nebula loom to the upper left of where the star Sanduleak -69° 202 exploded as supernova 1987A. Three-colour image made from BGR plates taken at the Anglo-Australian Telescope (AAT) prime focus Credit: David Malin / Australian Astronomical Observatory.

The first confirmation of the position of the SN 1987A came from Robert McNaught from Siding Spring Observatory (SSO, Australia) using the University of Aston Hewitt Satellite Schmidt camera. Within four days after the discovery the progenitor star was tentatively identified as the blue supergiant Sanduleak -69° 202. This was later confirmed when the supernova faded, as Sanduleak -69° 202 having disappeared. The progenitor star of SN 1987A had around 20 solar masses, a diameter around 40 times larger that our Sun, and had a spectral and luminosity type B3 I. However, the chemical composition of the progenitor star was very unusual (particularly, the abundance of helium in the outer layers of the star was more than twice larger than expected, as if part of the material of the core, where helium was produced, was somehow mixed into the outer layers).

First observations of SN 1987A

Once alerted to news of the supernova in February 1987, astronomers and engineers working at the Australian Astronomical Observatory (AAO) immediately devised plans for how to make the best observations with the Anglo-Australian Telescope (AAT).  Observing the supernova became a top priority for the next three weeks, the assumed time that it would remain bright.

But just in case the supernova continued to be visible, AAO’s Peter Gillingham rapidly assembled a very high resolution “Wooden Spectrograph”, since no telescope in the southern hemisphere at the time had this type of technology available to take advantage of observing such a rare, bright supernova.  With luck, Supernova 1987A remained observable for several months after it exploded.

High-spatial resolution observations using cameras at the Anglo-Australian Telescope and the Cerro Tololo International Observatory independently found a “mystery spot” close to the supernova. This was another indication of the broken symmetry of the SN 1987A. Furthermore, the spectroscopic evolution of SN 1987A provided further evidence of the asymmetries in the explosion.

Figure 3. Image of the peculiar remnant of the SN 1987A as seen using the ACS camera of the Hubble Space Telescope. Two glowing loops of stellar material and a very bright ring surrounding the dying star at the centre of the frame are clearly identified. All together form an structure named “Hourglass” that still is not fully understood. The field of view of this image is 25x25 arcseconds. Credit: ESA/Hubble & NASA.

Figure 3. Image of the peculiar remnant of the SN 1987A as seen using the ACS camera of the Hubble Space Telescope. Two glowing loops of stellar material and a very bright ring surrounding the dying star at the centre of the frame are clearly identified. All together form an structure named “Hourglass” that still is not fully understood. The field of view of this image is 25×25 arcseconds. Credit: ESA/Hubble & NASA.

In 1994 images obtained with the very new Hubble Space Telescope (NASA/ESA) revealed the unusual remnant of SN 1987A (Figure 3). It consists in three rings aligned along an axis of symmetry, giving it the shape of an hourglass. The rings are dense regions in the stellar wind that were ionized by ultraviolet radiation from the supernova. The asymmetry of the SN 1987A remnant implies that the progenitor star was either spinning rapidly or was orbiting a companion star.

However, it is important to clarify that these rings are material ejected by Sanduleak -69° 202 tens of years before it exploded as supernova, but later ionized by the explosion. Interestingly, as the rings weren’t seen till the light of the explosion reached them and the size of the inner ring (that has a radius of 0.808 arcsec) was known, astronomers were able to use trigonometry to accurately calculate the distance to the supernova, that turned to be 168 000 light years.

Figure 4: (Left) Intense X-ray emission detected in 2005 as result of the collision of the expanding supernova ejecta with the inner ring released by the progenitor of the SN 1987A. (Right) Optical image using the Hubble Space Telescope. Credit: X-ray: NASA/CXC/PSU/S.Park & D.Burrows.; Optical: NASA/STScI/CfA/P.Challis

Figure 4: (Left) Intense X-ray emission detected in 2005 as result of the collision of the expanding supernova ejecta with the inner ring released by the progenitor of the SN 1987A. (Right) Optical image using the Hubble Space Telescope. Credit: X-ray: NASA/CXC/PSU/S.Park & D.Burrows.; Optical: NASA/STScI/CfA/P.Challis

Figure 4 shows the intense generation of X-ray emission when the expanding supernova ejecta, that was moving at more than 7000 km/s, collided with the inner ring of the SN 1987A remnant between 2001 and 2009. This collision also induced an increasing in the optical light emitted by the supernova remnant during this time. A timelapse movie of the images obtained with the HST between 1994 and 2009 (Figure 5) shows the collision of the expanding material with the inner ring. It is predicted the inner ring will disappear between 2020 and 2030 after the shock wave destroys the clumps of material within it.

Figure 5: A time sequence of Hubble Space Telescope images, taken in the 15 years from 1994 to 2009, showing the collision of the expanding supernova remnant with a ring of dense material ejected by the progenitor star 20,000 years before the supernova. Credit: Larson et al. 2011, Nature, 474, 484.

Figure 5: A time sequence of Hubble Space Telescope images, taken in the 15 years from 1994 to 2009, showing the collision of the expanding supernova remnant with a ring of dense material ejected by the progenitor star 20,000 years before the supernova. Credit: Larson et al. 2011, Nature, 474, 484.

Peculiarities of SN 1987A

SN 1987A was an unusual supernova in many aspects. The optical light curve of SN 1987A (Figure 6) was rather different from the one previously observed core-collapse supernovae. Astronomers expected that the progenitor stars were red supergiants with extended envelope, but Sanduleak -69° 202 was a blue supergiant. Furthermore, the way the ejecta of the supernova mixed induced changes in the expected light curve of SN 1987A. As a consequence, the old models of spherical explosions had to be revisited to include density inhomogeneities in the stellar structure.

Figure 6: Light curve of SN 1987A over the first 12 years. The figure marks some of the most important events in the history of the supernova. Credit: ESO, figure extracted from Leibundgut and Suntzeff 2003.

Figure 6: Light curve of SN 1987A over the first 12 years. The figure marks some of the most important events in the history of the supernova. Credit: ESO, figure extracted from Leibundgut and Suntzeff 2003.

Of special importance, SN1987A provided the first chance to confirm by direct observation the radioactive source of the energy for visible light emissions by detection of predicted gamma-ray line radiation from two of its abundant radioactive nuclei, 56Co and 57Co. This proved the radioactive nature of the long-duration post-explosion glow of supernovae.

In 2007 astronomers Thomas Morris and Philipp Podsiadlowski presented simulations which support the hypothesis that the merger of to stars generated the triple-ring system around SN 1987A around 20,000 years before the explosion itself. The two stars were a 15-20 solar mass red giant star and an 5 solar mass star. This would also explain why Sanduleak -69° 202 was a blue supergiant and other peculiarities such as its very strange chemical composition.

Interestingly, several “light echoes” (the light emitted by the supernova at its brightness peak reflected of interstellar sheets between the supernova and us) have been observed around the supernova over the years. Figure 7 shows another David Malin’s (AAO) image obtained at the AAT where two “light echoes” are seen. These observations have been used to map the diffuse interstellar medium within the LMC nearby the supernova.

Figure 7. The light echo of supernova 1987A. When supernova 1987A was seen to explode in the Large Magellanic Cloud, the Milky Way's nearest companion galaxy, the brilliant flash of light from the self-destructing star had taken about 170,000 years to arrive at the telescope. Some light was deflected by two sheets of dust near the supernova, and is seen after the star has faded away because the reflected light covers a longer path to reach us. The dust responsible for the rings seen here lies in two distinct sheets, about 470 and 1300 light years from the supernova, close to our line of sight to it. The colour picture was made by photographically subtracting negative and positive images of plates of the region taken before and after the supernova appeared. The only major difference between them is the light echo itself. However, the bright stars do not cancel perfectly and appear black, while in other, bright parts of the image, the photographic noise does not cancel either. Despite this the image is an accurate reproduction of the colour of the extremely faint light echo, which in turn reflects the yellow colour of the supernova when it was at its brightest, in May, 1987. Photo and text credit: David Malin (AAO).

Figure 7. The light echo of supernova 1987A. When supernova 1987A was seen to explode in the Large Magellanic Cloud, the Milky Way’s nearest companion galaxy, the brilliant flash of light from the self-destructing star had taken about 170,000 years to arrive at the telescope. Some light was deflected by two sheets of dust near the supernova, and is seen after the star has faded away because the reflected light covers a longer path to reach us. The dust responsible for the rings seen here lies in two distinct sheets, about 470 and 1300 light years from the supernova, close to our line of sight to it. The colour picture was made by photographically subtracting negative and positive images of plates of the region taken before and after the supernova appeared. The only major difference between them is the light echo itself. However, the bright stars do not cancel perfectly and appear black, while in other, bright parts of the image, the photographic noise does not cancel either. Despite this the image is an accurate reproduction of the colour of the extremely faint light echo, which in turn reflects the yellow colour of the supernova when it was at its brightest, in May, 1987. Photo and text credit: David Malin (AAO).

In 2011, observations using the infrared Herschel Space Observatory (European Space Agency) indicated that SN 1987A released between 160,000 and 230,000 Earth masses (~0.4 to 0.7 solar masses) of fresh dust into the interstellar medium. Contrary to previously thought, this suggests that supernovae may have produced much of the dust in the very early Universe, as old red giant stars (that are thought are the main producers of dust in the local Universe) did not exist then.

Recent observations using the Atacama Large Millimeter/submillimeter Array (ALMA) telescope confirmed that SN 1987A freshly formed dust. Figure 8 shows a colour image of the remnant combining ALMA data (red) showing the newly formed dust, optical HST image (green) and X-ray Chandra data (blue) showing where the expanding shock wave is colliding with a ring of material around the supernova. The ALMA observations suggest that the SN 1987A remnant now contains about 25 percent the mass of the Sun in newly formed dust, including significant amounts of carbon monoxide and silicon monoxide.

Figure 8: This image shows the remnant of Supernova 1987A seen in light of very different wavelengths. ALMA data (in red) shows newly formed dust in the centre of the remnant. Hubble (in green) and Chandra (in blue) data show where the expanding shock wave is colliding with a ring of material around the supernova. This ring was initially lit up by the ultraviolet flash from the original explosion, but over the past few years the ring material has brightened considerably as it collides with the expanding shockwave. Credit: ALMA (ESO/NAOJ/NRAO)/A. Angelich. Visible light image: the NASA/ESA Hubble Space Telescope. X-Ray image: The NASA Chandra X-Ray Observatory.

Figure 8: This image shows the remnant of Supernova 1987A seen in light of very different wavelengths. ALMA data (in red) shows newly formed dust in the centre of the remnant. Hubble (in green) and Chandra (in blue) data show where the expanding shock wave is colliding with a ring of material around the supernova. This ring was initially lit up by the ultraviolet flash from the original explosion, but over the past few years the ring material has brightened considerably as it collides with the expanding shockwave. Credit: ALMA (ESO/NAOJ/NRAO)/A. Angelich. Visible light image: the NASA/ESA Hubble Space Telescope. X-Ray image: The NASA Chandra X-Ray Observatory.

The birth of neutrinos astronomy

Approximately two to three hours before the visible light from SN 1987A reached Earth, a burst of neutrinos was observed at three separate neutrino observatories. In total 25 neutrinos were detected during this event: 12 recorded in Kamiokande II observatory (Japan), 8 found with the IMB (Irvine–Michigan–Brookhaven) detector (U.S.), and 5 detected by Russian Baksan Neutrino Observatory. Detecting 25 neutrinos in a very short lapse of time was a significant increase from the previously observed background level. Indeed, soon it was clear that the neutrinos were actually emitted from SN 1987A. The neutrino emission occurs simultaneously with core collapse, but preceding the emission of visible light. Transmission of visible light is a slower process that occurs only after the shock wave reaches the stellar surface.

This was the first time neutrinos from a supernova were observed directly. The most important implication of this observation was the confirmation of the hydrodynamic core collapse of the massive star. Indeed, these observations were consistent with theoretical supernova models in which 99% of the energy of the collapse is radiated away in the form of neutrinos. The analysis also suggested that the star collapsed into a neutron star but not further collapse to a black hole occurred. However, till date, the predicted neutron stars has not been detected in SN 1987A.

The detection of neutrinos from the explosion of SN 1987A marked the beginning of neutrino astronomy. As all the neutrino observatories were located in the northern hemisphere, this meant that the detected neutrinos were found after passage through the Earth. Masatoshi Koshiba was awarded the Nobel Prize in 2002 in recognition for the first detection of neutrinos from a celestial object other than the Sun. The 2002 Nobel Prize in Physics was shared with Riccardo Giacconi for X-ray astronomy and Raymond Davis Jr. for solar neutrinos.

SN 1987A as seen today

30 years after the explosion, the remnants of the SN 1987A are still monitored in all wavelengths. An optical spectrum of the object will reveal a very prominent emission coming from ionized hydrogen (H-alpha). This feature is also seen using narrow-band images, as SN 1987A appears as a small “pink” blob. Figure 9 shows as example the optical spectrum of SN 1987A obtained in March 2016 by the Global Jet Watch project.

Figure 9. Optical spectrum of the remnant of the SN 1987A obtained using a 0.5m telescope of the Global Jet Watch project. It is a single 3000 seconds exposure obtained in March 2016 as part of a sequence that have been taken to monitor changes in this object. Credit: Global Jet Watch project, http://www.globaljetwatch.net, Acknowledgment: Steve Lee (AAO).

Figure 9. Optical spectrum of the remnant of the SN 1987A obtained using a 0.5m telescope of the Global Jet Watch project. It is a single 3000 seconds exposure obtained in March 2016 as part of a sequence that have been taken to monitor changes in this object. Credit: Global Jet Watch project, http://www.globaljetwatch.net, Acknowledgment: Katherine Blundell (Oxford Un.) and Steve Lee (AAO).

What does the neighborhood around Supernova 1987A look like today?  The surroundings of the SN 1987A remnant is filled by clouds of diffuse gas and dust. There are also some nearby bright blue stars that have an age of around 12 million years. These stars are from the same generation of stars that created Sanduleak -69° 202, the star that exploded as SN 1987A. The light of many of these massive stars makes shine the diffuse surrounding gas, that glows with green and red colors as seen in Figure 10.

Figure 10: New CACTI AAT image of the neighborhood of SN1987A Diffuse gas and dust in the outskirts of the Tarantula Nebula within the Large Magellanic Cloud. The remnant of SN 1987A appears as a bright red blob near the centre of the image. Data taken on 16th February 2017 using the CACTI camera in 2dF at the 3.9m Anglo-Australian Telescope. Color image using B (6 x 40s, blue) + V (4 x 30 s, yellow) + [O III] (4 x 180 s, green) + Ha (4 x 180 s, red) filters. A Hubble Space Telescope (HST) image is included as luminosity at the position where SN 1987A is located. Credit: Ángel R. López-Sánchez (AAO/MQU), Steve Lee, Robert Patterson, Robert Dean and Jennifer Riding (AAO) & Sarah Martel (UNSW / AAO).

Figure 10: New CACTI AAT image of the neighborhood of SN1987A Diffuse gas and dust in the outskirts of the Tarantula Nebula within the Large Magellanic Cloud. The remnant of SN 1987A appears as a bright red blob near the centre of the image. Data taken on 16th February 2017 using the CACTI camera in 2dF at the 3.9m Anglo-Australian Telescope. Color image using B (6 x 40s, blue) + V (4 x 30 s, yellow) + [O III] (4 x 180 s, green) + Ha (4 x 180 s, red) filters. A Hubble Space Telescope (HST) image is included as luminosity at the position where SN 1987A is located. Credit: Ángel R. López-Sánchez (AAO/MQU), Steve Lee, Robert Patterson, Robert Dean and Jennifer Riding (AAO) & Sarah Martel (UNSW / AAO).

The image, obtained on 16th February 2017 using the CACTI auxiliary camera of the Anglo-Australian Telescope (AAT), shows the remnant of Supernova 1987A, with the pink glow of its hydrogen gas, and filaments of gas and dust that stretch over 300 light years to either side. A comparison of this AAT image with those obtained by the Hubble Space Telescope is included in Figure 11.  An animation showing the zooming into the remnants of the SN 1987A and also including the famous AAT view of the Tarantula Nebula obtained by David Malin (AAO)  before the explosion is also available.

Figure 11: The neighborhood and the remnant of SN 1987A. Left: New image around the remnant of SN 1987A in the Large Magellanic Cloud taken with the 3.9m Anglo-Australian Telescope. Credit: Ángel R. López-Sánchez (AAO/MQU), Steve Lee, Robert Patterson, Robert Dean and Jennifer Riding (AAO) & Sarah Martel (UNSW / AAO).Top right: Wide Hubble Space Telescope image of the central area, data collected between 1994 and 1997. Credit: Hubble Heritage Team (AURA/STScI/NASA/ESA). Bottom right: Deep Hubble Space Telescope image obtained in 2011 showing the asymmetric structure of the SN 1987A remnant. Credit: ESA/Hubble & NASA.

Figure 11: The neighborhood and the remnant of SN 1987A. Left: New image around the remnant of SN 1987A in the Large Magellanic Cloud taken with the 3.9m Anglo-Australian Telescope. Credit: Ángel R. López-Sánchez (AAO/MQU), Steve Lee, Robert Patterson, Robert Dean and Jennifer Riding (AAO) & Sarah Martel (UNSW / AAO).Top right: Wide Hubble Space Telescope image of the central area, data collected between 1994 and 1997. Credit: Hubble Heritage Team (AURA/STScI/NASA/ESA). Bottom right: Deep Hubble Space Telescope image obtained in 2011 showing the asymmetric structure of the SN 1987A remnant. Credit: ESA/Hubble & NASA.

The peculiar shapes of the gas and the dust also indicate that, previously to SN 1987A, many other stars have ended their lives as supernovae. In any case, there are also indications of new star forming now within these clouds. The new image also reveals a group of pearl-like bubbles, 110 light years away from the explosion site. These bubbles are a sign of youth, indicating previous supernova explosions in this fertile nursery, that still is forming stars. Who knows, perhaps this region gives us more celestial fireworks soon.

Animation: Zooming into the SN 1987A remnant. This 40 seconds animation shows a zooming into the SN1987A remnant in the Large Magellanic Cloud. It compiles 4 images: the full view of the Tarantula Nebula, as seen by the AAT years before the explosion on 23 February 1987, a new image of the neighborhood of the supernova obtained with the new CACTI camera at the AAT, and wide and deep images obtained with the Hubble Space Telescope showing the asymmetry of the SN 1987A remnant.

Credit: Australian Astronomical Observatory. Credit of the composition: Ángel R. López-Sánchez (AAO/MQU). Credit of the individual images: Tarantula Nebula with the AAT: David Malin (AAO), CACTI image with the AAT:  Credit: Ángel R. López-Sánchez (AAO/MQU), Steve Lee, Robert Patterson, Robert Dean and Jennifer Riding (AAO) & Sarah Martel (UNSW / AAO), Wide Hubble Space Telescope image: WFPC2, Hubble Heritage Team (AURA/STScI/NASA/ESA), Deep Hubble Space Telescope image: ACS, ESA/Hubble & NASA.
Extra info:

(1) The Tarantula nebula (also known as 30 Doradus and NGC 2070) is a massive star-forming region within the LMC that sprawls across more than 2000 light years. This nebula hosts the open clusters R136 and Hodge 301. This clusters contain some of the largest, brightest and most massive stars known, including some that are very likely to explode soon as supernova.

(2) Stars that are more massive than 8 solar masses end their lives as Type II supernova. The star first burned hydrogen (H) into helium (He) in its core. Later the He itself is burned, producing a smaller core of oxygen (O) and carbon (C). As the core contracts and heats to high temperatures, carbon and oxygen are also ignited. Their fusion produces neon (Ne), magnesium (Mg), silicon (Si), and sulfur (S). Finally a core of silicon and sulfur produces iron (Fe) and nickel (Ni). The internal structure of such stars resembles an onion, with deeper shells burning heavier elements, until a central core of iron is created. However it is impossible to produce nuclear fusions with iron. As the core colds down, the gravitational pressure of the rest of the star make the material collapse towards the core. This implosion is so violent that is generates an enormous burst of energy that rebounds as neutrinos back outward and blows the star apart as a supernova.

Light and Astrophysics: My post for the IYL15 blog

DP ENGLISH: This story belongs to the series “Double Post” which indicates posts that have been written both in English in The Lined Wolf and in Spanish in El Lobo Rayado.

DP ESPAÑOL: Esta historia entra en la categoría “Doble Post” donde indico artículos que han sido escritos tanto en español en El Lobo Rayado como en inglés en The Lined Wolf.

Post originally published on 17th March 2015 in the International Year of Astronomy 2015 (IYL15) blog with the title Light and Astrophysics. The Spanish version of this article was published in Naukas.com.

Unlike the rest of sciences, Astrophysics is not based on carefully experiments designed in a laboratory but in the direct observation of the Universe. Astrophysicists get their data via the analysis of the light we receive from the Cosmos. For achieving this we use extremely sensitive instruments that collect the light emitted by planets, stars, nebulae and galaxies. Certainly, there are some alternative ways to study the Universe besides using the light, as analyzing meteorites or moon rocks, detecting energetic particles such as cosmic rays and neutrinos, or perhaps even using gravitational waves if they actually exist. But the main tool astrophysicists have today to investigate the Cosmos is the study of the radiation we receive from the outer space. Light is the key piece of the Astrophysics we make today.

As the aim is to observe the very faint light coming from objects located even billions of light years away, astronomical observatories are built in relatively isolated places, which are typically located high over the sea level. To observe the Universe, we astrophysicists need dark skies that are not affected by the nasty light pollution created by our society. The inadequate use of the artificial light emitted by streetlight of the cities induces an increasing of the brightness of the night sky. This happens as a consequence of the reflection and diffusion of the artificial light in the gases and particles of dust of the atmosphere. Besides the huge economic waste that it means, light pollution also has a very negative impact on the ecosystem, increases the amount of greenhouse gases in the atmosphere, and drastically diminishes the visibility of the celestial bodies. Unfortunately the light pollution is the reason that a large part of the mankind cannot enjoy a dark starry sky. How is the firmament when we observe it from a dark place? This time-lapse video shows as an example the sky over Siding Spring Observatory (Australia), where the Anglo-Australian Telescope (AAT), managed by the Australian Astronomical Observatory (AAO) and where I work, is located. The darkness of the sky in this observatory allows us to clearly see with our own eyes the Milky Way (the diffuse band of stars that crosses the sky) and many other celestial bodies such as the Magellanic Clouds, the Orion and Carina nebulae, or the Pleiades and Hyades star clusters.


Movie: Time-lapse video “The Sky over the Siding Spring Observatory”. More information about this video in this post in the blog. Credit: Ángel R. López-Sánchez (AAO/MQ).

On the other hand, after traveling during hundreds, millions, or billions of years throughout the deep space, the information codified in the light that reaches us is disrupted by the atmosphere of the Earth in the last millionth of a second of its trip. Hence professional telescopes are built on the top of the mountains, where the atmosphere is more stable than a sea level. Even though, many times this is not enough: our atmosphere distorts the light coming from space and prevents the identification of objects located very close in the sky. New techniques have been developed for compensating the effect of the atmosphere in the quality of the light we receive from the Cosmos. In particular, the adaptive optics technique induces in real time slight modifications to the shape of the primary mirror of the telescope, and therefore they counteract the distortion created by the atmosphere. In any case, astrophysicists need to direct the light received by the telescope to a detector, which transforms light energy into electric energy. This has been the purpose of the CCD (Charge-Couple Device) chips, firstly used by astronomers, and later popularized in smartphones and digital cameras. Very sophisticated optical systems are built to direct the light from the telescope to the detectors. Some of the systems created to manipulate our collection and processing of light are based on optical fibres. This new technology has created the branch of Astrophotonic. Indeed, the AAO, together with the University of Sydney and Macquarie University (Australia), are pioneers in the field of Astrophotonic. The next video shows how the light from the Cosmos is studied at the AAT. First it is collected using the primary mirror of the telescope, which has a diameter of 4 meters, and then it is sent using optical fibres to a dark room where the AAOmega spectrograph is located. This spectrograph, which is a series of special optics, separates the light into its rainbow spectrum, in a similar way a prism separates white light into a rainbow. The separated light is later focussed onto the CCD detector.


Movie: Rainbow Fingerprints, showing how the light of distant galaxies in collected by the Anglo-Australian Telescope and directed to the AAOmega spectrograph using optical fibres. More information: at the AAO webpages. Credit: Australian Astronomical Observatory (AAO), Movie produced by Amanda Bauer (AAO).

Specifically, this video shows how astrophysicists analyse the light coming from distant galaxies to understand their nature and properties. In particular, the video reveals the final science quality spectra for two different types of galaxies, one spiral (top panel) and one elliptical (bottom panel), using actual data obtained with the AAT and the AAOmega spectrograph. The information codified in the rainbow fingerprint identifies each galaxy unambiguously: distance, star formation history, chemical composition, age, physical properties as the temperature or the density of the diffuse gas, and many more. All this information has been captured within a single ray of light that has travelled hundred of millions of years to reach us. Similarly, the properties of stars (luminosity, mass, temperature, chemical composition, kinematics, …), nebulae, and any other celestial body (planets, comets, asteroids, quasars, …) are analyzed through its light. And studying tiny changes in the amount of light we receive from nearby stars we are now finding thousands of exoplanets in the Milky Way.

The “rainbow fingerprints” video shown before includes only the observations of two galaxies, but actually the AAT is able to observe around 350 objects at the same time. This is achieved using the 2dF robot, which can configure 400 optical fibres within a circular field of view with a diameter of 4 full moons. The majority of the optical fibres are allocated to observe galaxies (or stars), but some few optical fibres are used to get an accurate guiding of the telescope or to obtain important calibration data. With this technology the AAT is a survey machine, and indeed it is a pioneer of galaxy surveys. Around 1/3 of all the galaxy distances known today have been obtained using the AAT. The most recent galaxy survey completed at the AAT is the “Galaxy And Mass Assembly” (GAMA) survey, that has collected the light of more than 300 thousand galaxies located in some particular areas of the sky. The next movie shows the 3D distribution of galaxies in one of the sky areas observed by GAMA. This simulated fly through shows the real positions and images of the galaxies that have been mapped by GAMA. Distances are to scale, but the galaxy images have been enlarged for a viewing pleasure.


Movie: “Fly through of the GAMA Galaxy Catalogue”, showing a detailed map of the Universe where galaxies are in 3D. More information in the Vimeo webpage of the video. Crédito: Made by Will Parr, Dr. Mark Swinbank and Dr. Peder Norberg (Durham University) using data from the SDSS (Sloan Digital Sky Survey) and the GAMA (Galaxy And Mass Assembly) surveys.

However, to really understand what happens in the Universe, astrophysicists use not only the light that our eyes can see (the optical range) but all the other “lights” that make up the electromagnetic spectrum, from the very energetic gamma rays to the radio waves. The light codified in the radio waves is studied using radiotelescopes, many of them located in the surface of the Earth. The study of the light in radio frequencies allows us to detect the diffuse, cold gas existing in and around galaxies, the coldest regions of the interstellar medium and where the stars are formed, and energetic phenomena associated to galaxy nuclei hosting an active super-massive black hole in its centre. Many technological achievements, including the invention of the Wi-Fi, come from Radioastronomy. The study of the infrared, ultraviolet, X ray and gamma ray lights must be done using space telescopes, as the atmosphere of the Earth completely blocks these kinds of radiation. As an example, the next image shows how the nearby spiral galaxy M 101 is seen when we use all the lights of the electromagnetic spectrum. Light in X rays traces the most violent phenomena in the galaxy, which are regions associated to supernova remnants and black holes. The ultraviolet (UV) light marks where the youngest stars (those born less than 100 million years ago) are located. Optical (R band) and near-infrared (H band) lights indicate where the sun-like and the old stars are found. The emission coming from ionized hydrogen (H-alpha) reveals the star-forming regions, that is, the nebulae, in M 101. Mid-infrared (MIR) light comes from the thermal emission of the dust, which has been heated up by the young stars. Finally, the image in radio light (neutral atomic hydrogen, HI, at 21 cm) maps the diffuse, cold, gas in the galaxy.

Imagen: Mosaic showing six different views of the galaxy M 101, each one using a different wavelength. Images credit: X ray data (Chandra): NASA/CXC/JHU/K.Kuntz et al,; UV data(GALEX): Gil de Paz et al. 2007, ApJS, 173, 185; R and Hα data (KPNO): Hoopes et al. 2001, ApJ, 559, 878; Near-Infrared data (2MASS): Jarrett et al. 2003, AJ, 125, 525, 8 microns data (Spitzer): Dale et al. 2009, ApJ, 703, 517; 21cm HI data (VLA): Walter et al. 2008, AJ, 136, 2563, ”The H I Nearby Galaxy Survey”. Credit of the composition: Ángel R. López-Sánchez (AAO/MQ).

In any case, today Astrophysics does not only use observations of the light we collect from the Cosmos, but also includes a prominent theoretical framework. “Experiments” in Astrophysics are somewhat performed using computer simulations, where the laws of Physics, together with some initial conditions, are taken into account. When the computer runs, the simulated system evolves and from there general or particular trends are obtained. These predictions must be later compared with the real data obtained using telescopes. Just to name some few cases, stellar interiors, supernova explosions, and galaxy evolution are modeled through careful and sometimes expensive computer simulations. As an example, the next movie shows a cosmological simulation that follows the development of a spiral galaxy similar to the Milky Way from shortly after the Big Bang to the present time. This computer simulation, that required about 1 million CPU hours to be completed, assumes that the Universe is dominated by dark energy and dark matter. The simulation distinguishes old stars (red colour), young stars (blue colour) and the diffuse gas available to form new stars (pale blue), which is the gas we observe using radiotelescopes. This kind of cosmological simulations are later compared with observations obtained using professional telescopes to progress in our understanding of the Cosmos.

Movie: Computer simulation showing the evolution of a spiral galaxy over about 13.5 billion years, from shortly after the Big Bang to the present time. Colors indicate old stars (red), young stars (white and bright blue) and the distribution of gas density (pale blue); the view is 300,000 light-years across. The simulation ran on the Pleiades supercomputer at NASA’s Ames Research Center in Moffett Field, Calif., and required about 1 million CPU hours. It assumes a universe dominated by dark energy and dark matter. More information about this animation in this NASA website. Credit: F. Governato and T. Quinn (Univ. of Washington), A. Brooks (Univ. of Wisconsin, Madison), and J. Wadsley (McMaster Univ.).

In summary, thanks to the analysis of the light we know where stars, galaxies, and all the other celestial bodies are, what are they made of, how do the move, and more. Actually, much of the research that we astrophysicists do today combines observing and analyzing light coming from very different spectral ranges, X rays, ultraviolet, optical, infrared and radio waves. In many cases, we are using techniques that have been known for only few decades and that are still waiting to be fully exploited. The detailed study of the light coming from the Cosmos will provide new important astronomical discoveries in the nearby future and, at the same time, will impulse new technologies; many of them will be applied in medicine and communications. The light techniques we are developing for Astrophysics will have a direct application to our everyday life and will improve the welfare state of our society, besides deepens the understanding of the vast Universe we all live in.