Gamma ray bursts (GRBs) are some of the brightest, most dramatic events in the Universe. These cosmic tempests are characterized by a spectacular explosion of photons with energies 1,000,000 times greater than the most energetic light our eyes can detect. Due to their explosive power, long-lasting GRBs are predicted to have catastrophic consequences for life on any nearby planet. But could this type of event occur in our own stellar neighborhood? In a new paper published in Physical Review Letters, two astrophysicists examine the probability of a deadly GRB occurring in galaxies like the Milky Way, potentially shedding light on the risk for organisms on Earth, both now and in our distant past and future.
There are two main kinds of GRBs: short, and long. Short GRBs last less than two seconds and are thought to result from the merger of two compact stars, such as neutron stars or black holes. Conversely, long GRBs last more than two seconds and seem to occur in conjunction with certain kinds of Type I supernovae, specifically those that result when a massive star throws off all of its hydrogen and helium during collapse.
Perhaps unsurprisingly, long GRBs are much more threatening to planetary systems than short GRBs. Since dangerous long GRBs appear to be relatively rare in large, metal-rich galaxies like our own, it has long been thought that planets in the Milky Way would be immune to their fallout. But take into account the inconceivably old age of the Universe, and “relatively rare” no longer seems to cut it.
In fact, according to the authors of the new paper, there is a 90% chance that a GRB powerful enough to destroy Earth’s ozone layer occurred in our stellar neighborhood some time in the last 5 billion years, and a 50% chance that such an event occurred within the last half billion years. These odds indicate a possible trigger for the second worst mass extinction in Earth’s history: the Ordovician Extinction. This great decimation occurred 440-450 million years ago and led to the death of more than 80% of all species.
Today, however, Earth appears to be relatively safe. Galaxies that produce GRBs at a far higher rate than our own, such as the Large Magellanic Cloud, are currently too far from Earth to be any cause for alarm. Additionally, our Solar System’s home address in the sleepy outskirts of the Milky Way places us far away from our own galaxy’s more active, star-forming regions, areas that would be more likely to produce GRBs. Interestingly, the fact that such quiet outer regions exist within spiral galaxies like our own is entirely due to the precise value of the cosmological constant – the factor that describes our Universe’s expansion rate – that we observe. If the Universe had expanded any faster, such galaxies would not exist; any slower, and spirals would be far more compact and thus, far more energetically active.
In a future paper, the authors promise to look into the role long GRBs may play in Fermi’s paradox, the open question of why advanced lifeforms appear to be so rare in our Universe. A preprint of their current work can be accessed on the ArXiv.
The Milky Way Galaxy is an immense and very interesting place. Not only does it measure some 120,000–180,000 light-years in diameter, it is home to planet Earth, the birthplace of humanity. Our Solar System resides roughly 27,000 light-years away from the Galactic Center, on the inner edge of one of the spiral-shaped concentrations of gas and dust particles called the Orion Arm.
But within these facts about the Milky Way lie some additional tidbits of information, all of which are sure to impress and inspire. Here are ten such facts, listed in no particular order:
1. It’s Warped:
For starters, the Milky Way is a disk about 120,000 light years across with a central bulge that has a diameter of 12,000 light years (see the Guide to Space article for more information). The disk is far from perfectly flat though, as can be seen in the picture below. In fact, it is warped in shape, a fact which astronomers attribute to the our galaxy’s two neighbors -the Large and Small Magellanic clouds.
These two dwarf galaxies — which are part of our “Local Group” of galaxies and may be orbiting the Milky Way — are believed to have been pulling on the dark matter in our galaxy like in a game of galactic tug-of-war. The tugging creates a sort of oscillating frequency that pulls on the galaxy’s hydrogen gas, of which the Milky Way has lots of (for more information, check out How the Milky Way got its Warp).
2. It Has a Halo, but You Can’t Directly See It:
Scientists believe that 90% of our galaxy’s mass consists of dark matter, which gives it a mysterious halo. That means that all of the “luminous matter” – i.e. that which we can see with the naked eye or a telescopes – makes up less than 10% of the mass of the Milky Way. Its halo is not the conventional glowing sort we tend to think of when picturing angels or observing comets.
In this case, the halo is actually invisible, but its existence has been demonstrated by running simulations of how the Milky Way would appear without this invisible mass, and how fast the stars inside our galaxy’s disk orbit the center.
The heavier the galaxy, the faster they should be orbiting. If one were to assume that the galaxy is made up only of matter that we can see, then the rotation rate would be significantly less than what we observe. Hence, the rest of that mass must be made up of an elusive, invisible mass – aka. “dark matter” – or matter that only interacts gravitationally with “normal matter”.
To see some images of the probable distribution and density of dark matter in our galaxy, check out The Via Lactea Project.
3. It has Over 200 Billion Stars:
As galaxies go, the Milky Way is a middleweight. The largest galaxy we know of, which is designated IC 1101, has over 100 trillion stars, and other large galaxies can have as many as a trillion. Dwarf galaxies such as the aforementioned Large Magellanic Cloud have about 10 billion stars. The Milky Way has between 100-400 billion stars; but when you look up into the night sky, the most you can see from any one point on the globe is about 2,500. This number is not fixed, however, because the Milky Way is constantly losing stars through supernovae, and producing new ones all the time (about seven per year).
4. It’s Really Dusty and Gassy:
Though it may not look like it to the casual observer, the Milky Way is full of dust and gas. This matter makes up a whopping 10-15% of the luminous/visible matter in our galaxy, with the remainder being the stars. Our galaxy is roughly 100,000 light years across, and we can only see about 6,000 light years into the disk in the visible spectrum. Still, when light pollution is not significant, the dusty ring of the Milky Way can be discerned in the night sky.
The thickness of the dust deflects visible light (as is explained here) but infrared light can pass through the dust, which makes infrared telescopes like the Spitzer Space Telescope extremely valuable tools in mapping and studying the galaxy. Spitzer can peer through the dust to give us extraordinarily clear views of what is going on at the heart of the galaxy and in star-forming regions.
5. It was Made From Other Galaxies:
The Milky Way wasn’t always as it is today – a beautiful, warped spiral. It became its current size and shape by eating up other galaxies, and is still doing so today. In fact, the Canis Major Dwarf Galaxy is the closest galaxy to the Milky Way because its stars are currently being added to the Milky Way’s disk. And our galaxy has consumed others in its long history, such as the Sagittarius Dwarf Galaxy.
6. Every Picture You’ve Seen of the Milky Way Isn’t It:
Currently, we can’t take a picture of the Milky Way from above. This is due to the fact that we are inside the galactic disk, about 26,000 light years from the galactic center. It would be like trying to take a picture of your own house from the inside. This means that any of the beautiful pictures you’ve ever seen of a spiral galaxy that is supposedly the Milky Way is either a picture of another spiral galaxy, or the rendering of a talented artist.
Imaging the Milky Way from above is a long, long way off. However, this doesn’t mean that we can’t take breathtaking images of the Milky Way from our vantage point!
7. There is a Black Hole at the Center:
Most larger galaxies have a supermassive black hole (SMBH) at the center, and the Milky Way is no exception. The center of our galaxy is called Sagittarius A*, a massive source of radio waves that is believed to be a black hole that measures 22,5 million kilometers (14 million miles) across – about the size of Mercury’s orbit. But this is just the black hole itself.
All of the mass trying to get into the black hole – called the accretion disk – forms a disk that has 4.6 million times the mass of our Sun and would fit inside the orbit of the Earth. Though like other black holes, Sgr A* tries to consume anything that happens to be nearby, star formation has been detected near this behemoth astronomical phenomenon.
8. It’s Almost as Old as the Universe Itself:
The most recent estimates place the age of the Universe at about 13.7 billion years. Our Milky Way has been around for about 13.6 billion of those years, give or take another 800 million. The oldest stars in our the Milky Way are found in globular clusters, and the age of our galaxy is determined by measuring the age of these stars, and then extrapolating the age of what preceded them.
Though some of the constituents of the Milky Way have been around for a long time, the disk and bulge themselves didn’t form until about 10-12 billion years ago. And that bulge may have formed earlier than the rest of the galaxy.
9. It’s Part of the Virgo Supercluster:
As big as it is, the Milky Way is part of an even larger galactic structures. Our closest neighbors include the Large and Small Magellanic Clouds, and the Andromeda Galaxy – the closest spiral galaxy to the Milky Way. Along with some 50 other galaxies, the Milky Way and its immediate surroundings make up a cluster known as the Local Group.
And yet, this is still just a small fraction of our stellar neighborhood. Farther out, we find that the Milky Way is part of an even larger grouping of galaxies known as the Virgo Supercluster. Superclusters are groupings of galaxies on very large scales that measure in the hundreds of millions of light years in diameter. In between these superclusters are large stretches of open space where intrepid explorers or space probes would encounter very little in the way of galaxies or matter.
In the case of the Virgo Supercluster, at least 100 galaxy groups and clusters are located within it massive 33 megaparsec (110 million light-year) diameter. And a 2014 study indicates that the Virgo Supercluster is only a lobe of a greater supercluster, Laniakea, which is centered on the Great Attractor.
10. It’s on the move:
The Milky Way, along with everything else in the Universe, is moving through space. The Earth moves around the Sun, the Sun around the Milky Way, and the Milky Way as part of the Local Group, which is moving relative to the Cosmic Microwave Background (CMB) radiation – the radiation left over from the Big Bang.
The CMB is a convenient reference point to use when determining the velocity of things in the universe. Relative to the CMB, the Local Group is calculated to be moving at a speed of about 600 km/s, which works out to about 2.2 million km/h. Such speeds stagger the mind and squash any notions of moving fast within our humble, terrestrial frame of reference!
A team of Australian astronomers has been busy utilizing some of the world’s leading radio telescopes located in both Australia and Chile to carve away at the layered remains of a relatively new supernova. Designated as SN1987A, the 28 year-old stellar cataclysm came to Southern Hemisphere observer’s attention when it sprang into action at the edge of the Large Magellanic Cloud some two and a half decades ago. Since then, it has provided researchers around the world with a ongoing source of information about one of the Universe’s “most extreme events”.
Representing the University of Western Australia node of the International Centre for Radio Astronomy Research, PhD Candidate Giovanna Zanardo led the team focusing on the supernova with the Australia Telescope Compact Array (ATCA) in New South Wales. Their observations took in the wavelengths spanning the radio to the far infrared.
“By combining observations from the two telescopes we’ve been able to distinguish radiation being emitted by the supernova’s expanding shock wave from the radiation caused by dust forming in the inner regions of the remnant,” said Giovanna Zanardo of the International Centre for Radio Astronomy Research (ICRAR) in Perth, Western Australia.
“This is important because it means we’re able to separate out the different types of emission we’re seeing and look for signs of a new object which may have formed when the star’s core collapsed. It’s like doing a forensic investigation into the death of a star.”
“Our observations with the ATCA and ALMA radio telescopes have shown signs of something never seen before, located at the centre or the remnant. It could be a pulsar wind nebula, driven by the spinning neutron star, or pulsar, which astronomers have been searching for since 1987. It’s amazing that only now, with large telescopes like ALMA and the upgraded ATCA, we can peek through the bulk of debris ejected when the star exploded and see what’s hiding underneath.”
A video compilation showing Supernova Remnant 1987A as seen by the Hubble Space Telescope in 2010, and by radio telescopes located in Australia and Chile in 2012. The piece ends with a computer generated visualization of the remnant showing the possible location of a Pulsar. Credit: Dr Toby Potter, ICRAR-UWA, Dr Rick Newton, ICRAR-UWA
But, there is more. Not long ago, researchers published another paper which appeared in the Astrophysical Journal. Here they made an effort to solve another unanswered riddle about SN1987A. Since 1992 the supernova appears to be “brighter” on one side than it does the other! Dr. Toby Potter, another researcher from ICRAR’s UWA node took on this curiosity by creating a three-dimensional simulation of the expanding supernova shockwave.
“By introducing asymmetry into the explosion and adjusting the gas properties of the surrounding environment, we were able to reproduce a number of observed features from the real supernova such as the persistent one-sidedness in the radio images”, said Dr. Toby Potter.
So what’s going on? By creating a model which spans over a length of time, researchers were able to emulate an expanding shock front along the eastern edge of the supernova remnant. This region moves away more quickly than its counterpart and generates more radio emissions. When it encounters the equatorial ring – as observed by the Hubble Space Telescope – the effect becomes even more pronounced.
A visualization showing how Supernova1987A evolves between May of 1989 and July of 2014. Credit: Dr Toby Potter, ICRAR-UWA, Dr Rick Newton, ICRAR-UWA
“Our simulation predicts that over time the faster shock will move beyond the ring first. When this happens, the lop-sidedness of radio asymmetry is expected to be reduced and may even swap sides.”
“The fact that the model matches the observations so well means that we now have a good handle on the physics of the expanding remnant and are beginning to understand the composition of the environment surrounding the supernova – which is a big piece of the puzzle solved in terms of how the remnant of SN1987A formed.”
It might be a bad idea to get close to dead stars. Like a White Walker from Game of Thrones, this “cosmic zombie” white dwarf star was dangerous even though it was just a corpse of a star like our own. The result from this violence is still visible in the Spitzer Space Telescope picture you see above.
Astronomers believe the giant star was shedding material (a common phenomenon in older stars), which fell on to the white dwarf star. As the gas built up on the white dwarf over time, the mass became unstable and the dwarf exploded. What’s left is still lying in a pool of gas about 160,000 light-years away from us.
“It’s kind of like being a detective,” stated Brian Williams of NASA’s Goddard Space Flight Center, who led the research. “We look for clues in the remains to try to figure out what happened, even though we weren’t there to see it.”
This explosion in the Large Magellanic Cloud — one of the closest satellite galaxies to Earth — is known as a Type 1a supernova, but it’s a rare breed of that kind. Type 1as are best known as “standard candles” because their explosions have a consistent luminosity. Knowing how luminous the supernova type is allows astronomers to estimate distance based on its apparent brightness; the fainter the supernova is, the further away it is.
Most Type 1as happen when two orbiting white dwarfs smash into each other, but this scenario is more akin to something that Earthlings saw in 1604. Informally called Kepler’s supernova, because it was discovered by astronomer Johannes Kepler, astronomers believe this arose from a red giant and white dwarf interaction. The evidence left for this conclusion showed the supernova leftovers embedded in dust and gas.
Investigators have submitted their results to the Astrophysical Journal.
Some 160,000 light years away towards the constellation of Dorado (the Swordfish), is an amazing area of starbirth and death. Located in our celestial neighbor, the Large Magellanic Cloud, this huge stellar forge sculpts vast clouds of gas and dust into hot, new stars and carves out ribbons and curls of nebulae. However, in this image taken by ESO’s Very Large Telescope, there’s more. Stellar annihilation also awaits and shows itself as bright fibers left over from a supernova event.
For southern hemisphere observers, one of our nearest galactic neighbors, the Large Magellanic Cloud, is a well-known sight and holds many cosmic wonders. While the image highlights just a very small region, try to grasp the sheer size of what you are looking at. The fiery forge you see is several hundred light years across, and the factory in which it is contained spans 14,000 light years. Enormous? Yes. But compared to the Milky Way, it’s ten times smaller.
Even at such a great distance, the human eye can see many bright regions where new stars are actively forming, such as the Tarantula Nebula. This new image, taken by ESO’s Very Large Telescope at the Paranal Observatory in Chile, explores an area cataloged as NGC 2035 (right), sometimes nicknamed the Dragon’s Head Nebula. But, just what are we looking at?
The Dragon’s Head is an HII region, more commonly referred to as an emission nebula. Here, young stars pour forth energetic radiation and illuminate the surrounding clouds. The radiation tears electrons away from the atoms contained within the gas. These atoms then gel again with other atoms and release light. Swirling in the mix is dark dust, which absorbs the light and creates deep shadows and create contrast in the nebula’s structure.
However, as we look deep into this image, there’s even more… a fiery finale. At the left of the photo you’ll see the results of one of the most violent events in the Universe – a supernova explosion. These troubled tendrils are all that’s left of what once was a star and its name is SNR 0536-67.6. Perhaps when it exploded, it was so bright that it was capable of outshining the Magellanic Cloud… fading away over the weeks or months that followed. However, it left a lasting impression!
Earth’s galactic next-door neighbors shine brighter than ever in new pictures taken by an orbiting telescope, focusing on ultraviolet light that is tricky to image from the surface.
The Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) — the two largest major galaxies near our own, the Milky Way — were imaged in 5.4 days and 1.8 days of cumulative exposure time, respectively. These produced two gorgeous, high-resolution photos in a spot of the light spectrum normally invisible to humans.
“Prior to these images, there were relatively few UV observations of these galaxies, and none at high resolution across such wide areas, so this project fills in a major missing piece of the scientific puzzle,” stated Michael Siegel, lead scientist for Swift’s Ultraviolet/Optical Telescope at the Swift Mission Operations Center at Pennsylvania State University.
Science isn’t interested in these pictures — taken in wavelengths ranging from 1,600 to 3,300 angstroms, mostly blocked in Earth’s atmosphere — because of their pretty face, however. Ultraviolet light pictures let the hottest stars and star-forming areas shine out, while in visible light those hotspots are suppressed.
“With these mosaics, we can study how stars are born and evolve across each galaxy in a single view, something that’s very difficult to accomplish for our own galaxy because of our location inside it,” stated Stefan Immler, an associate research scientist at NASA Goddard Space Flight Center and the lead of the SWIFT guest investigator program.
Although the galaxies are relatively small, they easily shine in our night sky because they’re so close to Earth — 163,000 light-years for the LMC, and 200,000 light years for the SMC.
The LMC is only about 1/10 of the Milky Way’s size, with 1% of the Milky Way’s mass. The punier SMC is half of LMC’s size with only two-thirds of that galaxy’s mass.
Immler revealed the large images — 160 megapixels for the LMC, and 57 megapixels for the SMC — at the American Astronomical Society meeting in Indianapolis on Monday (June 3.)
Precise observations of a rare class of binary stars have now allowed a team of astronomers to improve the measurement of the distance to one of our neighboring galaxies, the Large Magellanic Cloud, and in the process, refine the Hubble Constant, an astronomical calculation that helps measure the expansion of the Universe. The astronomers say this is a crucial step towards understanding the nature of the mysterious dark energy that is causing the expansion to accelerate.
The team used telescopes at ESO’s La Silla Observatory in Chile, the Las Campanas Observatory also in Chile and two from the University of Hawaii at Manoa, and the Las Campanas Observatoryas well as others around the globe. These results appear in the 7 March 2013 issue of the journal Nature.
The new distance to the LMC is 163,000 light-years. The LMC is not the closest galaxy to the Milky Way; Canis Major Dwarf Galaxy, discovered in 2003 is considered the actual nearest neighbor at 42,000 light-years from the Galactic Center, and the Sagittarius Dwarf Elliptical Galaxy is about 50,000 light-years from the core of the Milky Way.
Astronomers ascertain the scale of the universe by first measuring the distances to close-by objects and then using them as standard candles — objects of known brightness — to pin down distances farther and farther out in the universe.
Up to now, finding an accurate distance to the LMC has proved elusive. Stars in that galaxy are used to fix the distance scale for more remote galaxies, so it is crucially important.
“This is a true milestone in modern astronomy. Because we know the distance to our nearest neighbor galaxy so precisely, we can now determine the rate at which the universe is expanding — the Hubble constant — with much better accuracy. This will allow us to investigate the physical nature of the enigmatic dark energy, the cause of the accelerated expansion of the universe,” says Dr. Rolf-Peter Kudritzki, an astronomer at the University of Hawaii’s Institute for Astronomy.
“For extragalactic astronomers,” said Dr. Fabio Bresolin, also from UH, “the distance to the Large Magellanic Cloud represents a fundamental yardstick with which the whole universe can be measured. Obtaining an accurate value for it has been a major challenge for generations of scientists. Our team has overcome the difficulties using an exquisitely accurate method, and is already working to cut the small remaining uncertainty by half in the next few years.”
The team worked out the distance to the LMC by observing rare close pairs of stars known as eclipsing binaries. As these stars orbit each other, they pass in front of each other. When this happens, as seen from Earth, the total brightness drops, both when one star passes in front of the other and, by a different amount, when it passes behind.
By tracking these changes in brightness very carefully, and also measuring the stars’ orbital speeds, it is possible to work out how big the stars are, what their masses are, and other information about their orbits. When this is combined with careful measurements of the total brightness and colors of the stars, remarkably accurate distances can be found.
“Now we have solved this problem by demonstrably having a result accurate to 2%,” states Wolfgang Gieren (Universidad de Concepción, Chile) and one of the leaders of the team.
As the Milky Way rises over the horizon at the European Southern Observatory, its companion galaxies also come into view. Credit: ESO/Y. Beletsky
A previously undetected heist of stars was uncovered by astronomers who were actually looking for why an unexpected amount of microlensing events were being seen around the outskirts of the Milky Way. Instead, they found the Large Magellanic Cloud (LMC) had been stealing stars from its neighbor, the Small Magellanic Cloud (SMC), leaving behind a trail of stars. Although the crime was likely committed hundreds of milllions of years ago during a collision between the two galaxies, the new information is helping astronomers to understand the history of these two galaxies that are in our neighborhood.
“You could say we discovered a crime of galactic proportions,” said Avi Loeb of the Harvard-Smithsonian Center for Astrophysics.
The Large Magellanic Cloud almost got away with it, if it wasn’t for those meddling astronomers….
Astronomers were originally monitoring the LMC to hunt for the reason for the unexpected microlensing events. Their initial hypothesis was that massive compact halo objects, or MACHOs were causing the effect, where a nearby object passes in front of a more distant star. The gravity of the closer object bends light from the star like a lens, magnifying it and causing it to brighten. The MACHOs were thought to be faint objects, roughly the mass of a star, but not much is known about them. Several surveys looked for MACHOs in order to find out if they could be a major component of dark matter – the unseen stuff that holds galaxies together.
In order for MACHOs to make up dark matter, they must be so faint that they can’t be directly detected. So, the team of astronomers hoped to see MACHOs within the Milky Way by lensing distant LMC stars.
“We originally set out to understand the evolution of the interacting LMC and SMC galaxies,” said lead author of a new paper on the results, Gurtina Besla of Columbia University. “We were surprised that, in addition, we could rule out the idea that dark matter is contained in MACHOs.”
“Instead of MACHOs, a trail of stars removed from the SMC is responsible for the microlensing events,” said Loeb.
Only a fast-moving population of stars could yield the observed rate and durations of the microlensing events. The best way to get such a stellar population is a galactic collision, which appears to have occurred in the LMC-SMC system.
“By reconstructing the scene, we found that the LMC and SMC collided violently hundreds of millions of years ago. That’s when the LMC stripped out the lensed stars,” said Loeb.
Their research also supports recent findings suggesting that both Magellanic Clouds are on their first pass by the Milky Way.
However, this isn’t a closed case. The evidence for the trail of lensed stars is persuasive, but they haven’t been directly observed yet. A number of teams are searching for the signatures of these stars within a bridge of gas that connects the Magellanic Clouds.
The simulation results will be published in the Monthly Notices of the Royal Astronomical Society.
Image credit: X-ray: NASA/CXC/U.Mich./S.Oey, IR: NASA/JPL, Optical: ESO/WFI/2.2-m. Zoom by John Williams/TerraZoom using Zoomify
When NASA combines images from different telescopes, they create dazzling scenes of celestial wonder and in the process we learn a few more things. Behold this wonder of combined light, known as LHA 120-N 44, or N 44 for short. Zoom into the scene using the toolbar at the bottom of the image. Click the farthest button on the right of the toolbar to see this wonder in full-screen. (Hint: press the “Esc” key to get back to work)