On December 19th, 2013, the European Space Agency’s (ESA) Gaia spacecraft took to space with for a very ambitious mission. Over the course of its planned 5-year mission (which was recently extended), this space observatory would map over a billion stars, planets, comets, asteroids and quasars in order to create the largest and most precise 3D catalog of the Milky Way ever created.
Since that time, the ESA has made two data releases that cover the first three years of the Gaia mission. The second data release, which took on April 25th, 2018, has already proven to be a treasure trove for astronomers. In addition to the positions, distance indicators and motions of over a billion stars and celestial objects in the Milky Way Galaxy, it also contained a hidden gem – the proper motions of stars within the Large Magellanic Cloud (LMC).
Located about 200,000 light-years from Earth, the LMC has dense clouds of dust that results in it experiencing high rates of star formation. In addition, it’s central bar is warped (where the east and west ends are nearer to the Milky Way), suggesting that it was once a barred dwarf spiral galaxy who’s spiral arms were disrupted by interaction with the Small Magellanic Cloud (SMC) and the Milky Way.
For these reasons, astronomers have been hoping to derive the orbits of dwarf galaxies (and globular clusters) that revolve around the Milky Way. In so doing, they hope to learn more about how our galaxy evolved due to mergers with clusters and other galaxies. By determining the proper motions of the LMC’s stars, the Gaia mission has provided clues as to how the Milky Way and its largest satellite galaxy have interacted over time.
As you can see from the image (at top), the bar of the LMC is outlined in great detail, along with individual star-forming regions like the Tarantula Nebula (aka. 30 Doradus, which is visible just above the center of the galaxy). The image combines the total amount of radiation detected by the observatory in each pixel. The radiation measurements were then taken through different filters on the spacecraft to generate color information.
This allowed Gaia to obtain information about the total density of stars within the LMC as well as their proper motions. As you can see, the image is dominated by the brightest, most massive stars, which greatly outshine their fainter, lower-mass counterparts. The proper motions of the stars observed is represented as the texture of the image – which looks a lot like a fingerprint.
From this, scientists were able to see an imprint of the stars rotating clockwise around the center of the galaxy. Using this information, astronomers will be able to create new models on how the LMC, SMC, and Milky Way evolved together over time. This, in turn, could shed light on how galaxies like our own, formed and evolved over the course of billions of years.
As with other information contained in the first and second data releases, this latest discovery demonstrates that the Gaia mission is fulfilling its intended purpose. The third release of Gaia data is scheduled to take place in late 2020, with the final catalog being published in the 2020s. Meanwhile, an extension has already been approved for the Gaia mission, which will now remain in operation until the end of 2020 (to be confirmed at the end of this year).
And be sure to enjoy this animated view of the LMC’s rotation, courtesy of the ESA:
The European Space Agency’s (ESA) Gaia mission is an ambitious project. Having launched in December of 2013, the purpose of this space observatory has been to measure the position and distances of 1 billion objects – including stars, extra-solar planets, comets, asteroids and even quasars. From this, astronomers hope to create the most detailed 3D space catalog of the cosmos ever made.
Back in 2016, the first batch of Gaia data (based on its first 14 months in space) was released. Since then, scientists have been poring over the raw data to obtain clearer images of the neighboring stars and galaxies that were studied by the mission. The latest images to be released, based on Gaia data, included revealing pictures of the Large Magellanic Cloud (LMC), the Andromeda galaxy, and the Triangulum galaxy.
The first catalog of Gaia data consisted of information on 1.142 billion stars, including their precise position in the night sky and their respective brightness. Most of these stars are located in the Milky Way, but a good fraction were from galaxies beyond ours, which included about ten million belonging to the LMC. This satellite galaxy, located about 166 000 light-years away, has about 1/100th the mass of the Milky Way.
The two images shown above display composite data obtained by the Gaia probe. The image on the left, which was compiled by mapping the total density of stars detected by Gaia, shows the large-scale distribution of stars in the LMC. This image also delineates the extent of the LMC’s spiral arms, and is peppered with bright dots that represent faint clusters of stars.
The image on the right, on the other hand, reveals other aspects of the LMC and its stars. This image was created by mapping radiation flux in the LMC and is dominated by the brightest and most massive stars. This allows the bar of the LMC to be more clearly defined and also shows individual regions of star-formation – like 30 Doradus, which is visible just above the center of the galaxy in the picture.
The next set of images (shown below), which were also obtained using data from the first 14 months of the Gaia mission, depict two nearby spiral galaxies – the Andromeda galaxy (M31) and its neighbor, the Triangulum galaxy (M33). The Andromeda galaxy, located 2.5 million light-years away, is the largest galaxy in our vicinity and slightly more massive than our own. It is also destined to merge with the Milky Way in roughly 4 billion years.
The Triangulum galaxy, meanwhile, is a fraction the size of the Milky Way (with an estimated fifty billion stars) and is located slightly farther from us than Andromeda – about 2.8 million light-years distant. As with the LMC images, the images on the left are based on the total density of stars and show stars of all types, while images on the right are based on the radiation flux of each galaxy and mainly show the bright end of the stellar population.
Another benefit of the images on the right is that they indicate the regions where the most intense star formation is taking place. For many years, astronomers have known that the LMC boasts a significant amount of star-forming activity, forming stars at five times the rate of the Milky Way Galaxy. Andromeda, meanwhile, has reached a point of near-inactivity in the past 2 billion years when it comes to star formation.
In comparison, the Triangulum Galaxy still shows signs of star formation, at a rate that is about four and a half times that of Andromeda. Thanks to the Gaia images, which indicate the relative rates of star formation from elevated levels of radiation flux and brightness, these differences between Andromeda, Triangulum and the LMC is illustrated quite beautifully.
What’s more, by analyzing the motions of individual stars in external galaxies like the LMC, Andromeda, or Triangulum, it will be possible to learn more about the overall rotation of stars within these galaxies. It will also be possible to determine the orbits of the galaxies themselves, which are all part of the larger structure known as the Local Group.
This region of space, which the Milky Way is part of, measures roughly 10 million light-years across and has an estimated 1.29 billion Solar masses. This, in turn, is just one of several collections of galaxies in the even larger Virgo Supercluster. Measuring how stars and galaxies orbit about these larger structures is key to determining cosmic evolution, how the Universe came to be as it is today and where it is heading.
An international team of astronomers recently attempted to do just that using the CosmicFlows surveys. These studies, which were conducted between 2011 and 2016, calculated the distance and speed of neighboring galaxies. By pairing this data with other distance estimates and data on the galaxies gravity fields, they were able to chart the motions of almost 1,400 galaxies within 100 million light years over the course of the past 13 billion years.
In the case of the LMC, another team of astronomers recently attempted to measure its orbit using a subset of data from the first Gaia release – the Tycho–Gaia Astrometric Solution (TGAS). Combined with additional parallax and proper motion data from the Hipparcos mission, the team was able to identify 29 stars in the LMC and measure their proper motion, which they then used to estimate the rotation of the galaxy.
Gaia’s observations of the LMC and the Small Magellanic Cloud (SMC) are also important when it comes to studying Cepheid and RR Lyrae variables. For years, astronomers have indicated that these stars could be used as indicators of cosmic distances for galaxies beyond our own. In addition, astronomers working at the Gaia Data Processing and Analysis Consortium (DPAC) tested this method on hundreds of LMC variable stars in order to validate data from the first release.
Astronomers are eagerly awaiting the second release of Gaia data, which is scheduled for April of 2018. This will also contain measurements on stellar distances and their motions across the sky, and is expected to reveal even more about our galaxy and its neighbors. But in the meantime, there are still plenty of revelations to be found from the first release, and scientists expect to be busy with it for many years to come.
Astronomers have finally observed something that was predicted but never seen: a stream of stars connecting the two Magellanic Clouds. In doing so, they began to unravel the mystery surrounding the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC). And that required the extraordinary power of the European Space Agency’s (ESA) Gaia Observatory to do it.
The Large and Small Magellanic Clouds (LMC and SMC) are dwarf galaxies to the Milky Way. The team of astronomers, led by a group at the University of Cambridge, focused on the clouds and on one particular type of very old star: RR Lyrae. RR Lyrae stars are pulsating stars that have been around since the early days of the Clouds. The Clouds have been difficult to study because they sprawl widely, but Gaia’s unique all-sky view has made this easier.
The Mystery: Mass
The Magellanic Clouds are a bit of a mystery. Astronomers want to know if our conventional theory of galaxy formation applies to them. To find out, they need to know when the Clouds first approached the Milky Way, and what their mass was at that time. The Cambridge team has uncovered some clues to help solve this mystery.
The team used Gaia to detect RR Lyrae stars, which allowed them to trace the extent of the LMC, something that has been difficult to do until Gaia came along. They found a low-luminosity halo around the LMC that stretched as far as 20 degrees. For the LMC to hold onto stars that far away means it would have to be much more massive than previously thought. In fact, the LMC might have as much as 10 percent of the mass that the Milky Way has.
The Arrival of the Magellanic Clouds
That helped astronomers answer the mass question, but to really understand the LMC and SMC, they needed to know when the clouds arrived at the Milky Way. But tracking the orbit of a satellite galaxy is impossible. They move so slowly that a human lifetime is a tiny blip compared to them. This makes their orbit essentially unobservable.
But astronomers were able to find the next best thing: the often predicted but never observed stellar stream, or bridge of stars, stretching between the two clouds.
A star stream forms when a satellite galaxy feels the gravitational pull of another body. In this case, the gravitational pull of the LMC allowed individual stars to leave the SMC and be pulled toward the LMC. The stars don’t leave at once, they leave individually over time, forming a stream, or bridge, between the two bodies. This action leaves a luminous tracing of their path over time.
The astronomers behind this study think that the bridge actually has two components: stars stripped from the SMC by the LMC, and stars stripped from the LMC by the Milky Way. This bridge of RR Lyrae stars helps them understand the history of the interactions between all three bodies.
A Bridge of Stars… and Gas
The most recent interaction between the Clouds was about 200 million years ago. At that time, the Clouds passed close by each other. This action formed not one, but two bridges: one of stars and one of gas. By measuring the offset between the star bridge and the gas bridge, they hope to narrow down the density of the corona of gas surrounding the Milky Way.
Mystery #2: The Milky Way’s Corona
The density of the Milky Way’s Galactic Corona is the second mystery that astronomers hope to solve using the Gaia Observatory.
The Galactic Corona is made up of ionised gas at very low density. This makes it very difficult to observe. But astronomers have been scrutinizing it intensely because they think the corona might harbor most of the missing baryonic matter. Everybody has heard of Dark Matter, the matter that makes up 95% of the matter in the universe. Dark Matter is something other than the normal matter that makes up familiar things like stars, planets, and us.
The other 5% of matter is baryonic matter, the familiar atoms that we all learn about. But we can only account for half of the 5% of baryonic matter that we think has to exist. The rest is called the missing baryonic matter, and astronomers think it’s probably in the galactic corona, but they’ve been unable to measure it.
Understanding the density of the Galactic Corona feeds back into understanding the Magellanic Clouds and their history. That’s because the bridges of stars and gas that formed between the Small and Large Magellanic Clouds initially moved at the same speed. But as they approached the Milky Way’s corona, the corona exerted drag on the stars and the gas. Because the stars are small and dense relative to the gas, they travelled through the corona with no change in their velocity.
But the gas behaved differently. The gas was largely neutral hydrogen, and very diffuse, and its encounter with the Milky Way’s corona slowed it down considerably. This created the offset between the two streams.
The team compared the current locations of the streams of gas and stars. By taking into account the density of the gas, and also how long both Clouds have been in the corona, they could then estimate the density of the corona itself.
When they did so, their results showed that the missing baryonic matter could be accounted for in the corona. Or at least a significant fraction of it could. So what’s the end result of all this work?
It looks like all this work confirms that both the Large and Small Magellanic Clouds conform to our conventional theory of galaxy formation.
Scientists have known for some time that the Milky Way Galaxy is not alone in the Universe. In addition to our galaxy being part of the Local Group – a collection of 54 galaxies and dwarf galaxies – we are also part of the larger formation known as the Virgo Supercluster. So you could say the Milky Way has a lot of neighbors.
Of these, most people consider the Andromeda Galaxy to be our closest galactic cohabitant. But in truth, Andromeda is the closest spiral galaxy, and not the closest galaxy by a long shot. This distinction falls to a formation that is actually within the Milky Way itself, a dwarf galaxy that we’ve only known about for a little over a decade.
At present, the closet known galaxy to the Milky Way is the Canis Major Dwarf Galaxy – aka. the Canis Major Overdensity. This stellar formation is about 42,000 light years from the galactic center, and a mere 25,000 light years from our Solar System. This puts it closer to us than the center of our own galaxy, which is 30,000 light years away from the Solar System.
The Canis Major Dwarf Galaxy Dwarf Galaxy is believed to contain one billion stars in all, a relatively high-percentage of which are in the Red Giant Branch phase of their lifetimes. It has a roughly elliptical shape and is thought to contain as many stars as the Sagittarius Dwarf Elliptical Galaxy, the previous contender for closest galaxy to our location in the Milky Way.
In addition to the dwarf galaxy itself, a long filament of stars is visible trailing behind it. This complex, ringlike structure – which is sometimes referred to as the Monoceros Ring – wraps around the galaxy three times. The stream was first discovered in the early 21st century by astronomers conducting the Sloan Digital Sky Survey (SDSS).
It was in the course of investigating this ring of stars, and a closely spaced group of globular clusters similar to those associated with the Sagittarius Dwarf Elliptical Galaxy, that the Canis Major Dwarf Galaxy was first discovered. The current theory is that this galaxy was accreted (or swallowed up) by the Milky Way Galaxy.
Other globular clusters that orbit the center of our Milky Way as a satellite – i.e. NGC 1851, NGC 1904, NGC 2298 and NGC 2808 – are thought to have been part of the Canis Major Dwarf Galaxy before its accretion. It also has associated open clusters, which are thought to have formed as a result of the dwarf galaxy’s gravity perturbing material in the galactic disk and stimulating star formation.
Prior to its discovery, astronomers believed that the Sagittarius Dwarf Galaxy was the closest galactic formation to our own. At 70,000 light years from Earth, this galaxy was determined in 1994 to be closer to us than the Large Magellanic Cloud (LMC), the irregular dwarf galaxy that is located 180,000 light years from Earth, and which previously held the title of the closest galaxy to the Milky Way.
All of that changed in 2003 when The Canis Major Dwarf Galaxy was discovered by the Two Micron All-Sky Survey (2MASS). This collaborative astronomical mission, which took place between 1997 and 2001, relied on data obtained by the Mt. Hopkins Observatory in Arizona (for the Northern Hemisphere) and the Cerro Tololo Inter-American Observatory in Chile (for the southern hemisphere).
From this data, astronomers were able to conduct a survey of 70% of the sky, detecting about 5,700 celestial sources of infrared radiation. Infrared astronomy takes advantage of advances in astronomy that see more of the Universe, since infrared light is not blocked by gas and dust to the same extent as visible light.
Because of this technique, the astronomers were able to detect a very significant over-density of class M giant stars in a part of the sky occupied by the Canis Major constellation, along with several other related structures composed of this type of star, two of which form broad, faint arcs (as seen in the image close to the top).
The prevalence of M-class stars is what made the formation easy to detect. These cool, “Red Dwarfs” are not very luminous compared to other classes of stars, and cannot even be seen with the naked eye. However, they shine very brightly in the infrared, and appeared in great numbers.
The discovery of this galaxy, and subsequent analysis of the stars associated with it, has provided some support for the current theory that galaxies may grow in size by swallowing their smaller neighbors. The Milky Way became the size it is now by eating up other galaxies like Canis Major, and it continues to do so today. And since stars from the Canis Major Dwarf Galaxy are technically already part of the Milky Way, it is by definition the nearest galaxy to us.
As already noted, it was the Sagittarius Dwarf Elliptical Galaxy that held the position of closest galaxy to our own prior to 2003. At 75,000 light years away. This dwarf galaxy, which consists of four globular clusters that measure some 10,000 light-years in diameter, was discovered in 1994. Prior to that, the Large Magellanic Cloud was thought to be our closest neighbor.
The Andromeda Galaxy (M31) is the closest spiral galaxy to us, and though it’s gravitationally bound to the Milky Way, it’s not the closest galaxy by far – being 2 million light years away. Andromeda is currently approaching our galaxy at a speed of about 110 kilometers per second. In roughly 4 billion years, the Andromeda Galaxy is expected to merge with out own, forming a single, super-galaxy.
Future of the Canis Major Dwarf Galaxy:
Astronomers also believe that the Canis Major Dwarf Galaxy is in the process of being pulled apart by the gravitational field of the more massive Milky Way Galaxy. The main body of the galaxy is already extremely degraded, a process which will continue as it travels around and through our Galaxy.
In time, the accretion process will likely culminate with the Canis Major Dwarf Galaxy merging entirely with the Milky Way, thus depositing its 1 billion stars to the 200 t0 400 billion that are already part of our galaxy.
For more information, check out this article from the Spitzer Space Telescope‘s website about the galaxies that are closest to the Milky Way Galaxy. And here is a video by the same author on the subject.
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.
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.
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.
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.
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).
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.
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!
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.
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.
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.
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.
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.
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.
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!
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.
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.”
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 Scientistsin 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).
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!
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.”