What are Magellanic Clouds?

Since ancient times, human beings have been staring at the night sky and been amazed by the celestial objects looking back at them. Whereas these objects were once thought to be divine in nature, and later mistaken for comets or other astrological phenomena, ongoing observation and improvements in instrumentation have led to these objects being identified for what they are.

For example, there are the Small and Large Magellanic Clouds, two large clouds of stars and gas that can be seen with the naked eye in the southern hemisphere. Located at a distance of 200,000 and 160,000 light years from the Milky Way Galaxy (respectively), the true nature of these objects has only been understand for about a century. And yet, these objects still have some mysteries that have yet to be solved.

Characteristics:

The Large Magellanic Cloud (LMC) and the neighboring the Small Magellanic Cloud (SMC) are starry regions that orbit our galaxy, and look conspicuously like detached pieces of the Milky Way. Though they are separated by 21 degrees in the  night sky – about 42 times the width of the full moon – their true distance is about 75,000 light-years from each other.

An ultraviolet view of the Large Magellanic Cloud from Swift's Ultraviolet/Optical Telescope. Almost 1 million ultraviolet sources are visible in the image, which took 5.4 days of cumulative exposure to do. The wavelengths of UV shown in this picture are mostly blocked on Earth's surface. Credit: NASA/Swift/S. Immler (Goddard) and M. Siegel (Penn State)
Ultraviolet view of the Large Magellanic Cloud from Swift’s Ultraviolet/Optical Telescope. Credit: NASA/Swift/S. Immler (Goddard) and M. Siegel (Penn State)

The Large Magellanic Cloud is located about 160,000 light-years from the Milky Way, in the constellation Dorado. This makes it the 3rd closest galaxy to us, behind the Sagittarius Dwarf and Canis Major Dwarf galaxies. Meanwhile, the Small Magellanic Cloud is located in the constellation of Tucana, about 200,000 light-years away.

The LMC is roughly twice the diameter of the SMC, measuring some 14,000 light-years across vs. 7,000 light years (compared to 100,000 light years for the Milky Way). This makes it the 4th largest galaxy in our Local Group of galaxies, after the Milky Way, Andromeda and the Triangulum Galaxy. The LMC is about 10 billion times as massive as our Sun (about a tenth the mass of the Milky Way), while the SMC is equivalent to about 7 billion Solar Masses.

In terms of structure, astronomers have classified the LMC as an irregular type galaxy, but it does have a very prominent bar in its center. Ergo, it’s possible that it was a barred spiral before its gravitational interactions with the Milky Way. The SMC also contains a central bar structure and it is speculated that it too was once a barred spiral galaxy that was disrupted by the Milky Way to become somewhat irregular.

Aside from their different structure and lower mass, they differ from our galaxy in two major ways. First, they are gas-rich – meaning that a higher fraction of their mass is hydrogen and helium – and they have poor metallicity, (meaning their stars are less metal-rich than the Milky Way’s). Both possess nebulae and young stellar populations, but are made up of stars that range from very young to the very old.

The Small Magellanic Cloud as seen by Swift's Ultraviolet/Optical Telescope. This composite of 656 separate pictures has a cumulative exposure time of 1.8 days. Credit: NASA/Swift/S. Immler (Goddard) and M. Siegel (Penn State)
The Small Magellanic Cloud as seen by Swift’s Ultraviolet/Optical Telescope. This composite of 656 separate pictures has a cumulative exposure time of 1.8 days. Credit: NASA/Swift/S. Immler (Goddard) and M. Siegel (Penn State)

In fact, this abundance in gas is what ensures that the Magellanic Clouds are able to create new stars, with some being only a few hundred million years in age. This is especially true of the LMC, which produces new stars in large quantities. A good example of this is it’s glowing-red Tarantula Nebula, a gigantic star-forming region that lies 160,000 light-years from Earth.

Astronomers estimate that the Magellanic Clouds were formed approximately 13 billion years ago, around the same time as the Milky Way Galaxy. It has also been believed for some time that the Magellanic Clouds have been orbiting the Milky Way at close to their current distances. However, observational and theoretical evidence suggests that the clouds have been greatly distorted by tidal interactions with the Milky Way as they travel close to it.

This indicates that they are not likely to have frequently got as close to the Milky Way as they are now. For instance, measurements conducted with the Hubble Space Telescope in 2006 suggested that the Magellanic Clouds may be moving too fast to be long terms companions of the Milky Way. In fact, their eccentric orbits around the Milky Way would seem to indicate that they came close to our galaxy only once since the universe began.

Small and Large Magellanic Clouds over Paranal Observatory Credit: ESO/J. Colosimo
The Small and Large Magellanic Clouds visible over the Paranal Observatory in Chile. Credit: ESO/J. Colosimo

This was followed in 2010 by a study that indicated that the Magellanic Clouds may be passing clouds that were likely expelled from the Andromeda Galaxy in the past. The interactions between the Magellanic Clouds and the Milky Way is evidenced by their structure and the streams of neutral hydrogen that connects them. Their gravity has affected the Milky Way as well, distorting the outer parts of the galactic disk.

History of Observation:

In the southern hemisphere, the Magellanic clouds were a part of the lore and mythology of the native inhabitants, including the Australian Aborigines, the Maori of New Zealand, and the Polynesian people of the South Pacific. For the latter, they served as important navigational markers, while the Maori used them as predictors of the winds.

While the study Magellanic Clouds dates back to the 1st millennium BCE, the earliest surviving record comes from the 10th century Persian astronomer Al Sufi. In his 964 treatise, Book of Fixed Stars, he called the LMC al-Bakr (“the Sheep”) “of the southern Arabs”. He also noted that the Cloud is not visible from northern Arabia or Baghdad, but could be seen at the southernmost tip of Arabian Peninsula.

By the late 15th century, Europeans are believed to have become acquainted with the Magellanic Clouds thanks to exploration and trade missions that took them south of the equator. For instance, Portuguese and Dutch sailors came to know them as the Cape Clouds, since they could only be viewed when sailing around Cape Horn (South America) and the Cape of Good Hope (South Africa).

Panoramic Large and Small Magellanic Clouds as seen from ESO's VLT observation site. The galaxies are on the left side of the image. Credit: ESO/Y. Beletsky
Panoramic view of the Large and Small Magellanic Clouds above the ESO’s VLT observation site in Chile. Credit: ESO/Y. Beletsky

During the circumnavigation of the Earth by Ferdinand Magellan (1519–22), the Magellanic Clouds were described by Venetian Antonio Pigafetta (Magellan’s chronicler) as dim clusters of stars. In 1603, German celestial cartographer Johann Bayer published his celestial atlas Uranometria, where he named the smaller cloud “Nebecula Minor” (Latin for “Little Cloud”).

Between 1834 and 1838, English astronomer John Herschel conducted surveys of the southern skies from the Royal Observatory at the Cape of Good Hope. While observing the SMC, he described it as a cloudy mass of light with an oval shape and a bright center, and catalogued a concentration of 37 nebulae and clusters within it.

In 1891, the Harvard College Observatory opened an observing station in southern Peru. From 1893-1906, astronomers used the observatory’s 61 cm (24 inch) telescope to survey and photograph the LMC and SMC. One such astronomers was Henriette Swan Leavitt, who used the observatory to discover Cephied Variable stars in the SMC.

Her findings were published in 1908 a study titled “1777 variables in the Magellanic Clouds“, in which she showed the relationship between these star’s variability period and luminosity – which became a very reliable means of determining distance. This allowed the SMCs distance to be determined, and became the standard method of measuring the distance to other galaxies in the coming decades.

Hubble image of variable star RS Puppis (NASA, ESA, and the Hubble Heritage Team)
Hubble image of variable star RS Puppis, a Cepheid Variable located in the Milky Way Galaxy. Credit: NASA/ESA/Hubble Heritage Team

As noted already, in 2006, measurements made suing the Hubble Space Telescope were announced that suggested the Large and Small Magellanic Clouds may be moving too fast to be orbiting the Milky Way. This has given rise to the theory that they originated in another galaxy, most likely Andromeda, and were kicked out during a galactic merger.

Given their composition, these clouds – especially the LMC – will continue making new stars for some time to come. And eventually, millions of years from now, these clouds may merge with our own Milky Way Galaxy. Or, they could keep orbiting us, passing close enough to suck up hydrogen and keep their star-forming process going.

But in a few billion years, when the Andromeda Galaxy collides with our own, they may find themselves having no choice but to merge with the giant galaxy that results. One might say Andromeda regrets spitting them out, and is coming to collect them!

We have written many articles about the Magellanic Clouds for Universe Today. Here’s What is the Small Magellanic Cloud?, What is the Large Magellanic Cloud?, Stolen: Magellanic Clouds – Return to Andromeda, The Magellanic Clouds are Here for the First Time.

If you’d like more info on galaxies, check out Hubblesite’s News Releases on Galaxies, and here’s NASA’s Science Page on Galaxies.

We have also recorded an episode of Astronomy Cast about galaxies – Episode 97: Galaxies.

Sources:

What is the Biggest Star in the Universe?

What is the Biggest Star in the Universe

This article was originally published in 2008, but has been updated several times now to keep track with our advancing knowledge of the cosmos!

My six-year old daughter is a question-asking machine. We were driving home from school a couple of days ago, and she was grilling me about the nature of the Universe. One of her zingers was, “What’s the Biggest Star in the Universe”? I had an easy answer. “The Universe is a big place,” I said, “and there’s no way we can possibly know what the biggest star is”. But that’s not a real answer.

So she refined the question. “What’s the biggest star that we know of?” Of course, I was stuck in the car, and without access to the Internet. But once I got back home, and was able to do some research, I learned the answer and thought I’d share it with the rest of you But to answer it fully, some basic background information needs to be covered first. Ready?

Solar Radius and Mass:

When talking about the size of stars, it’s important to first take a look at our own Sun for a sense of scale. Our familiar star is a mighty 1.4 million km across (870,000 miles). That’s such a huge number that it’s hard to get a sense of scale. Speaking of which, the Sun also accounts for 99.9% of all the matter in our Solar System. In fact, you could fit one million planet Earths inside the Sun.

Using these values, astronomers have created the terms “solar radius” and “solar mass”, which they use to compare stars of greater or smaller size and mass to our own. A solar radius is 690,000 km (432,000 miles) and 1 solar mass is 2 x 1030 kilograms (4.3 x 1030 pounds). That’s 2 nonillion kilograms, or 2,000,000,000,000,000,000,000,000,000,000 kg.

Artist's depiction of the Morgan-Keenan spectral diagram, showing the difference between main sequence stars. Credit: Wikipedia Commons
Artist’s depiction of the Morgan-Keenan spectral diagram, showing the difference between main sequence stars. Credit: Wikipedia Commons

Another thing worth considering is the fact that our Sun is pretty small, as stars go. As a G-type main-sequence star (specifically, a G2V star), which is commonly known as a yellow dwarf, its on the smaller end of the size chart (see above). While it is certainly larger than the most common type of star – M-type, or Red Dwarfs – it is itself dwarfed (no pun!) by the likes of blue giants and other spectral classes.

Classification:

To break it all down, stars are grouped based on their essential characteristics, which can be their spectral class (i.e. color), temperature, size, and brightness. The most common method of classification is known as the Morgan–Keenan (MK) system, which classifies stars based on temperature using the letters O, B, A, F, G, K, and M, – O being the hottest and M the coolest. Each letter class is then subdivided using a numeric digit with 0 being hottest and 9 being coolest (e.g. O1 to M9 are the hottest to coldest stars).

In the MK system, a luminosity class is added using Roman numerals. These are based on the width of certain absorption lines in the star’s spectrum (which vary with the density of the atmosphere), thus distinguishing giant stars from dwarfs. Luminosity classes 0 and I apply to hyper- or supergiants; classes II, III and IV apply to bright, regular giants, and subgiants, respectively; class V is for main-sequence stars; and class VI and VII apply to subdwarfs and dwarf stars.

The Hertzspirg-Russel diagram, showing the relation between star's color, AM. luminosity, and temperature. Credit: astronomy.starrynight.com
The Hertzspirg-Russel diagram, showing the relation between star’s color, AM. luminosity, and temperature. Credit: astronomy.starrynight.com

There is also the Hertzsprung-Russell diagram, which relates stellar classification to absolute magnitude (i.e. intrinsic brightness), luminosity, and surface temperature. The same classification for spectral types are used, ranging from blue and white at one end to red at the other, which is then combined with the stars Absolute Visual Magnitude (expressed as Mv) to place them on a 2-dimensional chart (see above).

On average, stars in the O-range are hotter than other classes, reaching effective temperatures of up to 30,000 K. At the same time, they are also larger and more massive, reaching sizes of over 6 and a half solar radii and up to 16 solar masses. At the lower end, K and M type stars (orange and red dwarfs) tend to be cooler (ranging from 2400 to 5700 K), measuring 0.7 to 0.96 times that of our Sun, and being anywhere from 0.08 to 0.8 as massive.

Based on the full of classification of our Sun (G2V), we can therefore say that it a main-sequence star with a temperature around 5,800K. Now consider another famous star system in our galaxy – Eta Carinae, a system containing at least two stars located around 7500 light-years away in the direction of the constellation Carina. The primary of this system is estimated to be 250 times the size of our Sun, a minimum of 120 solar masses, and a million times as bright – making it one of the biggest and brightest stars ever observed.

Eta Carinae, one of the most massive stars known. Image credit: NASA
Eta Carinae, one of the most massive stars known, located in the Carina constellation. Credit: NASA

There is some controversy over this world’s size though. Most stars blow with a solar wind, losing mass over time. But Eta Carinae is so large that it casts off 500 times the mass of the Earth every year. With so much mass lost, it’s very difficult for astronomers to accurately measure where the star ends, and its stellar wind begins. Also, it is believed that Eta Carinae will explode in the not-too-distant future, and it will be the most spectacular supernovae humans have ever seen.

In terms of sheer mass, the top spot goes to R136a1, a star located in the Large Magellanic Cloud, some 163,000 light-years away. It is believed that this star may contain as much as 315 times the mass of the Sun, which presents a conundrum to astronomers since it was believed that the largest stars could only contain 150 solar masses. The answer to this is that R136a1 was probably formed when several massive stars merged together. Needless to say, R136a1 is set to detonate as a hypernova, any day now.

In terms of large stars, Betelgeuse serves as a good (and popular) example. Located in the shoulder of Orion, this familiar red supergiant has a radius of 950-1200 times the size of the Sun, and would engulf the orbit of Jupiter if placed in our Solar System. In fact, whenever we want to put our Sun’s size into perspective, we often use Betelgeuse to do it (see below)!

Yet, even after we use this hulking Red Giant to put us in our place, we are still just scratching the surface in the game of “who’s the biggest star”. Consider WOH G64, a red supergiant star located in the Large Magellanic Cloud, approximately 168,000 light years from Earth. At 1.540 solar radii in diameter, this star is currently one of the largest in the known universe.

But there’s also RW Cephei, an orange hypergiant star in the constellation Cepheus, located 3,500 light years from Earth and measuring 1,535 solar radii in diameter. Westerlund 1-26 is also pretty huge, a red supergiant (or hypergiant) located within the Westerlund 1 super star cluster 11,500 light-years away that measures 1,530 solar radii in diameter. Meanwhile, V354 Cephei and VX Sagittarii are tied when it comes to size, with both measuring an estimated 1,520 solar radii in diameter.

The Largest Star: UY Scuti

As it stands, the title of the largest star in the Universe (that we know of) comes down to two contenders. For example, UY Scuti is currently at the top of the list. Located 9.500 light years away in the constellation Scutum, this bright red supergiant and pulsating variable star has an estimated average median radius of 1,708 solar radii – or 2.4 billion km (1.5 billion mi; 15.9 AU), thus giving it a volume 5 billion times that of the Sun.

However, this average estimate includes a margin of error of ± 192 solar radii, which means that it could be as large as 1900 solar radii or as small as 1516. This lower estimate places it beneath stars like as V354 Cephei and VX Sagittarii. Meanwhile, the second star on the list of the largest possible stars is NML Cygni, a semiregular variable red hypergiant located in the Cygnus constellation some 5,300 light-years from Earth.

A zoomed-in picture of the red giant star UY Scuti. Credit: Rutherford Observatory/Haktarfone
A zoomed-in picture of the red giant star UY Scuti. Credit: Rutherford Observatory/Haktarfone

Due to the location of this star within a circumstellar nebula, it is heavily obscured by dust extinction. As a result, astronomers estimate that its size could be anywhere from 1,642 to 2,775 solar radii, which means it could either be the largest star in the known Universe (with a margin of 1000 solar radii) or indeed the second largest, ranking not far behind UY Scuti.

And up until a few years ago, the title of biggest star went to VY Canis Majoris; a red hypergiant star in the Canis Major constellation, located about 5,000 light-years from Earth. Back in 2006, professor Roberta Humphrey of the University of Minnesota calculated its upper size and estimated that it could be more than 1,540 times the size of the Sun. Its average estimated mass, however, is 1420, placing it in the no. 8 spot behind V354 Cephei and VX Sagittarii.

These are the biggest star that we know of, but the Milky way probably has dozens of stars that are even larger, obscured by gas and dust so we can’t see them. But even if we cannot find these stars, it is possible to theorize about their likely size and mass. So just how big can stars get? Once again, Professor Roberta Humphreys of the University of Minnesota provided the answer.

VY Canis Majoris. The biggest known star.
Size comparison between the Sun and VY Canis Majoris, which once held the title of the largest known star in the Universe. Credit: Wikipedia Commons/Oona Räisänen

As she explained when contacted, the largest stars in the Universe are the coolest. So even though Eta Carinae is the most luminous star we know of, it’s extremely hot – 25,000 Kelvin – and therefore only 250 solar radii big. The largest stars, in contrast, will be cool supergiants. Case in point, VY Canis Majoris is only 3,500 Kelvin, and a really big star would be even cooler.

At 3,000 Kelvin, Humphreys estimates that cool supergiant would be as big as 2,600 times the size of the Sun. This is below the upper estimates for NML Cygni, but above the average estimates for both it and UY Scutii. Hence, this is the upper limit of a star (at least theoretically and based on all the information we have to date).

But as we continue to peer into the Universe with all of our instruments, and explore it up close through robotic spacecraft and crewed missions, we are sure to find new and exciting things that will confound us further!

And be sure to check out this great animation that shows the size of various objects in space, starting with our Solar System’s tiny planets and finally getting to UY Scuti. Enjoy!

We have written many articles about stars for Universe Today. Here’s The Sun, What’s the Brightest Star in the Sky Past and Future?, and What Is The Smallest Star?

Want to learn more about the birth and death of stars? We did a two part podcast at Astronomy Cast. Here’s part 1, Where Stars Come From, and here’s part 2, How Stars Die.

Fast Radio Bursts On Repeat – Aliens, Or A Rotating Neutron Star?

Very recently, a team of scientists from the Commonwealth Scientific and Industrial Research Organization (CSIRO) achieved an historic first by being able to pinpoint the source of fast radio bursts (FRBs). With the help of observatories around the world, they determined that these radio signals originated in an elliptical galaxy 6 billion light years from Earth. But as it turns out, this feat has been followed by yet another historic first.

In all previous cases where FRBs were detected, they appeared to be one-off events, lasting for mere milliseconds. However, after running the data from a recent FRB through a supercomputer, a team of scientists at McGill University in Montreal have determined that in this instance, the signal was repeating in nature. This finding has some serious implications for the astronomical community, and is also considered by some to be proof of extra-terrestrial intelligence.

FRBs have puzzled astronomers since they were first detected in 2007. This event, known as the Lorimer Burst, lasted a mere five milliseconds and appeared to be coming from a location near the Large Magellanic Cloud, billions of light years away. Since that time, a total of 16 FRBs have been detected. And in all but this one case, the duration was extremely short and was not followed up by any additional bursts.

The NSF's Arecibo Observatory, which is located in Puerto Rico, is the world largest radio telescope. Credit: NAIC
The NSF’s Arecibo Observatory, which is located in Puerto Rico, is the world largest radio telescope. Credit: NAIC

Because of their short duration and one-off nature, many scientists have reasoned that FRBs must be the result of cataclysmic events – such as a star going supernova or a neutron star collapsing into a black hole. However, after sifting through data obtained by the Arecibo radio telescope in Puerto Rico, a team of students from McGill University – led by PhD student Paul Scholz – determined that an FRB detected in 2012 did not conform to this pattern.

In an article published in Nature, Scholz and his associates describe how this particular signal – FRB 121102 – was followed by several bursts with properties that were consistent with the original signal. Running the data which was gathered in May and June through a supercomputer at the McGill High Performance Computing Center, they determined that FRB 121102 had emitted a total of 10 new bursts after its initial detection.

This would seem to indicate that FRBs have more than just one cause, which presents some rather interesting possibilities. As Paul Scholz told Universe Today via email:

“All previous Fast Radio Bursts have only been one-time events, so a lot of explanations for them have involved a cataclysmic event that destroys the source of the bursts, such as a neutron star collapsing into a black hole. Our discovery of repeating bursts from FRB 121102 shows that the source cannot have been destroyed and it must have been due to a phenomenon that can repeat, such as bright pulses from a rotating neutron star.”

The Parkes Telescope in New South Wales, Australia. Credit: Roger Ressmeyer/Corbis
The Parkes Telescope in New South Wales, Australia. Credit: Roger Ressmeyer/Corbis

Another possibility which is making the rounds is that this signal is not natural in origin. Since their discovery, FRBs and other “transient signals” – i.e. seemingly random and temporary signals – from the Universe have been the subject of speculation. As would be expected, there have been some who have suggested that they might be the long sought-after proof that extra-terrestrial civilizations exist.

For example, in 1967, after receiving a strange reading from a radio array in a Cambridge field, astrophysicist Jocelyn Bell Burnell and her team considered the possibility that what they were seeing was an alien message. This would later be shown to be incorrect – it was, in fact, the first discovery of a pulsar. However, the possibility these signals are alien in origin has remained fixed in the public (and scientific) imagination.

This has certainly been the case since the discovery of FRBs. In an article published by New Scientists in April of 2015 – titled “Cosmic Radio Plays An Alien Tune” – writer and astrophysicist Sarah Scoles explores the possibility of whether or not the strange regularity of some FRBs that appeared to be coming from within the Milky Way could be seen as evidence of alien intelligence.

However, the likelihood that these signals are being sent by extra-terrestrials is quite low. For one, FRBs are not an effective way to send a message. As Dr. Maura McLaughlin of West Virginia University – who was part of the first FRB discovery –  has explained, it takes a lot of energy to make a signal that spreads across lots of frequencies (which is a distinguishing feature of FRBs).

Scientists have been exploring the possibility that radio bursts
For decades, scientists have been exploring the possibility that radio bursts are signals from alien civilizations. Credit: AdamBurn/DeviantArt

And if these bursts came from outside of our galaxy, which certainly seems to be the case, they would have to be incredibly energetic to get this far. As Dr. McLaughlin explained to Universe Today via email:

“The total amount of power required to produce just one FRB pulse is as much as the Sun produces in a month! Although we might expect extraterrestrial civilizations to send short-duration signals, sending a signal over the very wide radio bandwidths over which FRBs are detected would require an improbably immense amount of energy. We expect that extraterrestrial civilizations would transmit over a very narrow range of radio frequencies, much like a radio station on Earth. 

But regardless of whether these signals are natural or extra-terrestrial in origin, they do present some rather exciting possibilities for astronomical research and our knowledge of the Universe. Moving forward, Scholz and his team hope to identify the galaxy where the radio bursts originated, and plans to use test out some recently-developed techniques in the process.

“Next we would like to localize the source of the bursts to identify the galaxy that they are coming from,” he said. “This will let us know about the environment around the source. To do this, we need to use radio interferometry to get a precise enough sky location. But, to do this we need to detect a burst while we are looking at the source with such a radio telescope array. Since the source is not always bursting we will have to wait until we get a detection of a burst while we are looking with radio interferometry. So, if we’re patient, eventually we should be able to pinpoint the galaxy that the bursts are coming from.”

In the end, we may find that rapid burst radio waves are a more common occurrence than we thought. In all likelihood, they are being regularly emitted by rare and powerful stellar objects, ones which we’ve only begun to notice. As for the other possibility? Well, we’re not saying it’s aliens, but we’re quite sure others will be!

Credit: History.com/memegenerator.com
Credit: History.com/memegenerator.com

Further Reading: McGill University

Gamma Ray Bursts Limit The Habitability of Certain Galaxies, Says Study

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.

10 Interesting Facts About the Milky Way

Viewed from above, we can now see that our gaze takes across the Perseus Arm (toward the constellation Cygnus), parts of the Sagittarius and Scutum-Centaurus arms (toward the constellations Scutum, Sagittarius and Ophiuchus) and across the central bar. Interstellar dust obscures much of the center of the galaxy. Credit: NASA et. all with additions by the author.

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).

The Spiral Galaxy ESO 510-13 is warped similar to our own. Credit: NASA/Hubble Heritage Team (STScI / AURA), C. Conselice (U. Wisconsin / STScI/ NASA
The warp of Spiral Galaxy ESO 510-13 is similar to that of our own. Credit: NASA/Hubble

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).

These images taken by the Spitzer Space Telescope show the dust and gas concentrations around a supernova. Credit: NASA/JPL-Caltech
These images taken by the Spitzer Space Telescope show dust and gas concentrations around a distant supernova. Credit: NASA/JPL-Caltech

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.

Artist's concept of Sagittarius A, the supermassive black hole at the center of our galaxy. Credit: NASA/JPL
Artist’s concept of Sagittarius A, the supermassive black hole at the center of our galaxy. Credit: NASA/JPL-Caltech

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.

A mosaic of telescopic images showing the galaxies of the Virgo Supercluster. Credit: NASA/Rogelio Bernal Andreo
A mosaic of telescopic images showing the galaxies of the Virgo Supercluster. Credit: NASA/Rogelio Bernal Andreo

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!

We have written many interesting articles about the Milky Way for Universe Today. Here’s 10 Interesting Facts about the Milky Way, How Big is the Milky Way?, What is the Closest Galaxy to the Milky Way?, and How Many Stars Are There in the Milky Way?

For many more facts about the Milky Way, visit the Guide to Space, listen to the Astronomy Cast episode on the Milky Way, or visit the Students for the Exploration and Development of Space at seds.org.

Whittling Away At SN1987A

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.”

Original Story Source: Astronomers dissect the aftermath of a Supernova – International Centre for Radio Astronomy Research News Release.

‘Cosmic Zombie’ Star Triggered This Explosion In Nearby Galaxy

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.

Source: NASA Jet Propulsion Laboratory

Forging Stars – Peering Into Starbirth and Death

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!

Original Story Source: ESO Image Release.

Our Galactic Neighbors Shine In New Ultraviolet Pictures

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.

The Small Magellanic Cloud as seen by Swift's Ultraviolet/Optical Telescope. This composite of 656 separate pictures has a cumulative exposure time of 1.8 days. Credit: NASA/Swift/S. Immler (Goddard) and M. Siegel (Penn State)
The Small Magellanic Cloud as seen by Swift’s Ultraviolet/Optical Telescope. This composite of 656 separate pictures has a cumulative exposure time of 1.8 days. Credit: NASA/Swift/S. Immler (Goddard) and M. Siegel (Penn State)

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.)

Source: NASA

Rare Eclipsing Binary Stars Provide Refined Measurements in the Universe

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.

Read another recent article about studies that used eclipsing binaries to study the Light-travel-time Effect

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.

Sources: University of Hawaii, ESO