The cosmic distance ladder for measuring galactic distances. Credit: NASA,ESA, A. Feild (STScI), and A. Riess (STScI/JHU)
The universe is expanding. When we look in all directions, we see distant galaxies speeding away from us, their light redshifted due to cosmic expansion. This has been known since 1929 when Edwin Hubble calcuated the relation between a galaxy’s distance and its redshift. Then in the late 1990s, two studies of distant supernovae found that the expansion of the universe is accelerating. Something, some dark energy, must be driving cosmic expansion.
A composite image of SN 1987A from Hubble, Chandra, and ALMA. Image Credit: By ALMA (ESO/NAOJ/NRAO)/A. Angelich. Visible light image: the NASA/ESA Hubble Space Telescope. X-Ray image: The NASA Chandra X-Ray Observatory - http://www.eso.org/public/images/eso1401a/, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=30512379
Astronomers at Cardiff University have done something nobody else has been able to do. A team, led by Dr. Phil Cigan from Cardiff University’s School of Physics and Astronomy, has found the neutron star remnant from the famous supernova SN 1987A. Their evidence ends a 30 year search for the object.
Japanese astronomers have captured images of an astonishing 1800 supernovae. 58 of these supernovae are the scientifically-important Type 1a supernovae located 8 billion light years away. Type 1a supernovae are known as ‘standard candles’ in astronomy.
Two of the largest galaxies in the Fornax galaxy cluster. Image Credit: NASA/Chandra
A rogue star is one that has escaped the gravitational pull of its home galaxy. These stars drift through intergalactic space, and so are sometimes called intergalactic stars. Sometimes, when a rogue star is ejected from its galaxy, it drags its binary pair along for the ride.
A look at The Cow (approximately 80 days after explosion) from the W.M. Keck Observatory in Maunakea, Hawaii. The Cow is nestled in the CGCG 137-068 galaxy, 200 million light years from Earth. Image Credit:Raffaella Margutti/Northwestern University
On June 17th 2018, the ATLAS (Asteroid Terrestrial-impact Last Alert System) survey’s twin telescopes spotted something extraordinarily bright in the sky. The source was 200 million light years away in the constellation Hercules. The object was given the name AT2018cow or “The Cow.” The Cow flared up quickly, and then just as quickly it was gone.
Artist's impression of a Type II supernova. Credit: ESO
For many years, scientists have been studying how supernovae could affect life on Earth. Supernovae are extremely powerful events, and depending on how close they are to Earth, they could have consequences ranging from the cataclysmic to the inconsequential. But now, the scientists behind a new paper say they have specific evidence linking one or more supernova to an extinction event 2.6 million years ago.
About 2.6 million years ago, one or more supernovae exploded about 50 parsecs, or about 160 light years, away from Earth. At that same time, there was also an extinction event on Earth, called the Pliocene marine megafauna extinction. Up to a third of the large marine species on Earth were wiped out at the time, most of them living in shallow coastal waters.
“This time, it’s different. We have evidence of nearby events at a specific time.” – Dr. Adrian Melott, University of Kansas.
Artist's impression of the blue supergiant that underwent a Type Ic supernova in 2017. Credits: NASA, ESA, and J. Olmsted (STScI)
As astronomical phenomena go, supernovae are among the most fascinating and spectacular. This process occurs when certain types of stars reach the end of their lifespan, where they explode and throw off their outer layers. Thanks to generations of study, astronomers have been able to classify most observed supernovae into one of two categories (Type I and Type II) and determine which kinds of stars are the progenitors for each.
However, to date, astronomers have been unable to determine which type of star eventually leads to a Type Ic supernova – a special of class where a star undergoes core collapse after being stripped of its hydrogen and helium. But thanks to the efforts of two teams of astronomers that pored over archival data from the Hubble Space Telescope, scientists have now found the long sought-after star that causes this type of supernova.
The planetary Nebula M3-1, obtained by Hubble Space Telescope. The central star is actually a binary system with one of the shortest orbital periods known. Credit: David Jones/Daniel López/IAC
Planetary nebulae are a fascinating astronomical phenomena, even if the name is a bit misleading. Rather than being associated with planets, these glowing shells of gas and dust are formed when stars enter the final phases of their lifespan and throw off their outer layers. In many cases, this process and the subsequent structure of the nebula is the result of the star interacting with a nearby companion star.
Recently, while examining the planetary nebula M3-1, an international team of astronomers noted something rather interesting. After observing the nebula’s central star, which is actually a binary system, they noticed that the pair had an incredibly short orbital period – i.e. the stars orbit each other once every 3 hours and 5 minutes. Based on this behavior, the pair are likely to merge and trigger a nova explosion.
The technique of gravitational lensing relies on the presence of a large cluster of matter between the observer and the object to magnify light coming from that object. Credit: NASA
For the sake of studying the most distant objects in the Universe, astronomers often rely on a technique known as Gravitational Lensing. Based on the principles of Einstein’s Theory of General Relativity, this technique involves relying on a large distribution of matter (such as a galaxy cluster or star) to magnify the light coming from a distant object, thereby making it appear brighter and larger.
However, in recent years, astronomers have found other uses for this technique as well. For instance, a team of scientists from the Harvard-Smithsonian Center for Astrophysics (CfA) recently determined that Gravitational Lensing could also be used to determine the mass of white dwarf stars. This discovery could lead to a new era in astronomy where the mass of fainter objects can be determined.
The study which details their findings, titled “Predicting gravitational lensing by stellar remnants” appeared in the Monthly Noticed of the Royal Astronomical Society. The study was led by Alexander J. Harding of the CfA and included Rosanne Di Stefano, and Claire Baker (also from the CfA), as well as members from the University of Southampton, Georgia State University, the University of Nigeria, and Cornell University.
A Hubble image of the white dwarf star PM I12506+4110E (the bright object, seen in black in this negative print) and its field which includes two distant stars PM12-MLC1&2. Credit: Harding et al./NASA/HST
To put it simply, determining the mass of an astronomical object is one the greatest challenges for astronomers. Until now, the most successful method relied on binary systems because the orbital parameters of these systems depend on the masses of the two objects. Unfortunately, objects that are at the end states of stellar evolution – like black holes, neutron stars or white dwarfs – are often too faint or isolated to be detectable.
This is unfortunate, since these objects are responsible for a lot of dramatic astronomical events. These include the accretion of material, the emission of energetic radiation, gravitational waves, gamma-ray bursts, or supernovae. Many of these events are still a mystery to astronomers or the study of them is still in its infancy – i.e. gravitational waves. As they state in their study:
“Gravitational lensing provides an alternative approach to mass measurement. It has the advantage of only relying on the light from a background source, and can therefore be employed even for dark lenses. In fact, since light from the lens can interfere with the detection of lensing effects, compact objects are ideal lenses.”
As they go on to state, of the 18,000 lensing events that have been detected to date, roughly 10 to 15% are believed to have been caused by compact objects. However, scientists are unable to tell which of the detected events were due to compact lenses. For the sake of their study then, the team sought to circumvent this problem by identifying local compact objects and predicting when they might produce a lensing event so they could be studied.
Animation showing the white dwarf star Stein 2051B as it passes in front of a distant background star. Credit: NASA
“By focusing on pre-selected compact objects in the near vicinity of the Sun, we ensure that the lensing event will be caused by a white dwarf, neutron star, or black hole,” they state. “Furthermore, the distance and proper motion of the lens can be accurately measured prior to the event, or else afterwards. Armed with this information, the lensing light curve allows one to accurately measure the mass of the lens.”
In the end, the team determined that lensing events could be predicted from thousands of local objects. These include 250 neutron stars, 5 black holes, and roughly 35,000 white dwarfs. Neutron stars and black holes present a challenge since the known populations are too small and their proper motions and/or distances are not generally known.
But in the case of white dwarfs, the authors anticipate that they will provide for many lensing opportunities in the future. Based on the general motions of the white dwarfs across the sky, they obtained a statistical estimate that about 30-50 lensing events will take place per decade that could be spotted by the Hubble Space Telescope, the ESA’s Gaia mission, or NASA’s James Webb Space Telescope (JWST). As they state in their conclusions:
“We find that the detection of lensing events due to white dwarfs can certainly be observed during the next decade by both Gaia and HST. Photometric events will occur, but to detect them will require observations of the positions of hundreds to thousands of far-flung white dwarfs. As we learn the positions, distances to, and proper motions of larger numbers of white dwarfs through the completion of surveys such as Gaia and through ongoing and new wide-field surveys, the situation will continue to improve.”
The future of astronomy does indeed seem bright. Between improvements in technology, methodology, and the deployment of next-generation space and ground-based telescopes, there is no shortage of opportunities to see and learn more.
This image shows the incredibly distant and ancient supernova DES16C2nm. The supernova was discovered by the Dark Energy Survey. Image: Mat Smith and DES collaboration.
Astronomers have discovered the most distant supernova yet, at a distance of 10.5 billion light years from Earth. The supernova, named DES16C2nm, is a cataclysmic explosion that signaled the end of a massive star some 10.5 billion years ago. Only now is the light reaching us. The team of astronomers behind the discovery have published their results in a new paper available at arXiv.
“…sometimes you just have to go out and look up to find something amazing.” – Dr. Bob Nichol, University of Portsmouth.
The supernova was discovered by astronomers involved with the Dark Energy Survey (DES), a collaboration of astronomers in different countries. The DES’s job is to map several hundred million galaxies, to help us find out more about dark energy. Dark Energy is the mysterious force that we think is causing the accelerated expansion of the Universe.
DES16C2nm was first detected in August 2016. Its distance and extreme brightness were confirmed in October that year with three of our most powerful telescopes – the Very Large Telescope and the Magellan Telescope in Chile, and the Keck Observatory, in Hawaii.
This image from 2015 shows the same area of sky before DES16C2nm exploded. Image: Mat Smith and DES collaboration.
DES16C2nm is what’s known as a superluminous supernova (SLSN), a type of supernova only discovered 10 years ago. SLSNs are the rarest—and the brightest—type of supernova that we know of. After the supernova exploded, it left behind a neutron star, which is the densest type of object in the universe. The extreme brightness of SLSNs, which can be 100 times brighter than other supernovae, are thought to be caused by material falling into the neutron star.
“It’s thrilling to be part of the survey that has discovered the oldest known supernova.” – Dr Mathew Smith, lead author, University of Southampton
Lead author of the study Dr Mathew Smith, of the University of Southampton, said: “It’s thrilling to be part of the survey that has discovered the oldest known supernova. DES16C2nm is extremely distant, extremely bright, and extremely rare – not the sort of thing you stumble across every day as an astronomer.”
Dr. Smith went on to say that not only is the discovery exciting just for being so distant, ancient, and rare. It’s also providing insights into the cause of SLSNs: “The ultraviolet light from SLSN informs us of the amount of metal produced in the explosion and the temperature of the explosion itself, both of which are key to understanding what causes and drives these cosmic explosions.”
“Now we know how to find these objects at even greater distances, we are actively looking for more of them as part of the Dark Energy Survey.” – Co-author Mark Sullivan, University of Southampton.
Now that the international team behind the Dark Energy Survey has found one of the SLSNs, they want to find more. Co-author Mark Sullivan, also of the University of Southampton, said: “Finding more distant events, to determine the variety and sheer number of these events, is the next step. Now we know how to find these objects at even greater distances, we are actively looking for more of them as part of the Dark Energy Survey.”
The instrument used by DES is the newly constructed Dark Energy Camera (DECam), which is mounted on the Victor M. Blanco 4-meter Telescope at the Cerro Tololo Inter-American Observatory (CTIO) in the Chilean Andes. DECam is an extremely sensitive 570-megapixel digital camera designed and built just for the Dark Energy Survey.
The DECam in operation at its home in the Chilean Andes. The extremely sensitive, 570 megapixel camera is mounted on the Victor M. Blanco 4-meter Telescope at the Cerro Tololo Inter-American Observatory. Image: DES/CTIO
The Dark Energy Survey involves more than 400 scientists from over 40 international institutions. It began in 2013, and will wrap up its five year mission sometime in 2018. The DES is using 525 nights of observation to carry out a deep, wide-area survey to record information from 300 million galaxies that are billions of light-years from Earth. DES is designed to help us answer a burning question.
According to Einstein’s General Relativity Theory, gravity should be causing the expansion of the universe to slow down. And we thought it was, until 1998 when astronomers studying distant supernovae found that the opposite is true. For some reason, the expansion is speeding up. There are really only two ways of explaining this. Either the theory of General Relativity needs to be replaced, or a large portion of the universe—about 70%—consists of something exotic that we’re calling Dark Energy. And this Dark Energy exerts a force opposite to the attractive force exerted by “normal” matter, causing the expansion of the universe to accelerate.
“…sometimes you just have to go out and look up to find something amazing.” – Dr. Bob Nichol, University of Portsmouth.
To help answer this question, the DES is imaging 5,000 square degrees of the southern sky in five optical filters to obtain detailed information about each of the 300 million galaxies. A small amount of the survey time is also used to observe smaller patches of sky once a week or so, to discover and study thousands of supernovae and other astrophysical transients. And this is how DES16C2nm was discovered.
Study co-author Bob Nichol, Professor of Astrophysics and Director of the Institute of Cosmology and Gravitation at the University of Portsmouth, commented: “Such supernovae were not thought of when we started DES over a decade ago. Such discoveries show the importance of empirical science; sometimes you just have to go out and look up to find something amazing.”
