Image of the large-scale structure of the Universe, showing filaments and voids within the cosmic structure. Who knows how many other civilizations might be out there? Credit: Millennium Simulation Project
Ever since Galileo pointed his telescope at Jupiter and saw moons in orbit around that planet, we began to realize we don’t occupy a central, important place in the Universe. In 2013, a study showed that we may be further out in the boondocks than we imagined. Now, a new study confirms it: we live in a void in the filamental structure of the Universe, a void that is bigger than we thought.
In 2013, a study by University of Wisconsin–Madison astronomer Amy Barger and her student Ryan Keenan showed that our Milky Way galaxy is situated in a large void in the cosmic structure. The void contains far fewer galaxies, stars, and planets than we thought. Now, a new study from University of Wisconsin student Ben Hoscheit confirms it, and at the same time eases some of the tension between different measurements of the Hubble Constant.
The void has a name; it’s called the KBC void for Keenan, Barger and the University of Hawaii’s Lennox Cowie. With a radius of about 1 billion light years, the KBC void is seven times larger than the average void, and it is the largest void we know of.
The large-scale structure of the Universe consists of filaments and clusters of normal matter separated by voids, where there is very little matter. It’s been described as “Swiss cheese-like.” The filaments themselves are made up of galaxy clusters and super-clusters, which are themselves made up of stars, gas, dust and planets. Finding out that we live in a void is interesting on its own, but its the implications it has for Hubble’s Constant that are even more interesting.
Hubble’s Constant is the rate at which objects move away from each other due to the expansion of the Universe. Dr. Brian Cox explains it in this short video.
The problem with Hubble’s Constant, is that you get a different result depending on how you measure it. Obviously, this is a problem. “No matter what technique you use, you should get the same value for the expansion rate of the universe today,” explains Ben Hoscheit, the Wisconsin student who presented his analysis of the KBC void on June 6th at a meeting of the American Astronomical Society. “Fortunately, living in a void helps resolve this tension.”
There are a couple ways of measuring the expansion rate of the Universe, known as Hubble’s Constant. One way is to use what are known as “standard candles.” Supernovae are used as standard candles because their luminosity is so well-understood. By measuring their luminosity, we can determine how far away the galaxy they reside in is.
Another way is by measuring the CMB, the Cosmic Microwave Background. The CMB is the left over energy imprint from the Big Bang, and studying it tells us the state of expansion in the Universe.
This is a map of the observable Universe from the Sloan Digital Sky Survey. Orange areas show higher density of galaxy clusters and filaments. Image: Sloan Digital Sky Survey.
The two methods can be compared. The standard candle approach measures more local distances, while the CMB approach measures large-scale distances. So how does living in a void help resolve the two?
Measurements from inside a void will be affected by the much larger amount of matter outside the void. The gravitational pull of all that matter will affect the measurements taken with the standard candle method. But that same matter, and its gravitational pull, will have no effect on the CMB method of measurement.
“One always wants to find consistency, or else there is a problem somewhere that needs to be resolved.” – Amy Barger, University of Hawaii, Dept. of Physics and Astronomy
Hoscheit’s new analysis, according to Barger, the author of the 2013 study, shows that Keenan’s first estimations of the KBC void, which is shaped like a sphere with a shell of increasing thickness made up of galaxies, stars and other matter, are not ruled out by other observational constraints.
“It is often really hard to find consistent solutions between many different observations,” says Barger, an observational cosmologist who also holds an affiliate graduate appointment at the University of Hawaii’s Department of Physics and Astronomy. “What Ben has shown is that the density profile that Keenan measured is consistent with cosmological observables. One always wants to find consistency, or else there is a problem somewhere that needs to be resolved.”
Artist’s conception of the Gaia telescope backdropped by a photograph of the Milky Way taken at the European Southern Observatory. Credit: ESA/ATG medialab; background: ESO/S. Brunier
In 2013, the European Space Agency deployed the long-awaited Gaia space observatory. As one of a handful of next-generation space observatories that will be going up before the end of the decade, this mission has spent the past few years cataloging over a billion astronomical objects. Using this data, astronomers and astrophysicists hope to create the largest and most precise 3D map of the Milky Way to date.
Though it is almost to the end of its mission, much of its earliest information is still bearing fruit. For example, using the mission’s initial data release, a team of astrophysicists from the University of Toronto managed to calculate the speed at which the Sun orbits the Milky Way. From this, they were able to obtain a precise distance estimate between our Sun and the center of the galaxy for the first time.
For some time, astronomers have been unsure as to exactly how far our Solar System is from the center of our galaxy. Much of this has to do with the fact that it is impossible to view it directly, due to a combination of factors (i.e. perspective, the size of our galaxy, and visibility barriers). As a result, since the year 2000, official estimates have varied between 7.2 and 8.8 kiloparsecs (~23,483 to 28,700 light years).
Infrared image from Spitzer Space Telescope, showing the stars at the center of the Milky Way Galaxy. Credit: NASA/JPL-Caltech/S. Stolovy (SSC/Caltech)
For the sake of their study, the team – which was led by Jason Hunt, a Dunlap Fellow at the Dunlap Institute for Astronomy & Astrophysics at the University of Toronto – combined Gaia’s initial release with data from the RAdial Velocity Experiment (RAVE). This survey, which was conducted between 2003 and 2013 by the Australian Astronomical Observatory (AAO), measured the positions, distances, radial velocities and spectra of 500,000 stars.
Over 200,000 of these stars were also observed by Gaia and information on them was included in its initial data release. As they explain in their study, which was published in the Journal of Astrophysical Letters in November 2016, they used this to examined the speeds at which these stars orbit the center of the galaxy (relative to the Sun), and in the process discovered that there was an apparent distribution in their relative velocities.
In short, our Sun moves around the center of the Milky Way at a speed of 240 km/s (149 mi/s), or 864,000 km/h (536,865 mph). Naturally, some of the more than 200,000 candidates were moving faster or slower. But for some, there was no apparent angular momentum, which they attributed to these stars being scattering onto “chaotic, halo-type orbits when they pass through the Galactic nucleus”.
As Hunt explained in Dunlap Institute press release:
“Stars with very close to zero angular momentum would have plunged towards the Galactic center where they would be strongly affected by the extreme gravitational forces present there. This would scatter them into chaotic orbits taking them far above the Galactic plane and away from the Solar neighbourhood… By measuring the velocity with which nearby stars rotate around our Galaxy with respect to the Sun, we can observe a lack of stars with a specific negative relative velocity. And because we know this dip corresponds to 0 km/sec, it tells us, in turn, how fast we are moving.”
Detection of an unusually bright X-Ray flare from Sagittarius A*, a supermassive black hole in the center of the Milky Way galaxy. Credit: NASA/CXC/Stanford/I. Zhuravleva et al.
The next step was to combine this information with proper motion calculations of Sagittarius A* – the supermassive black hole believed to be at the center of our galaxy. After correcting for its motion relative to background objects, they were able to effectively triangulate the Earth’s distance from the center of the galaxy. From this, they derived a refined distance of estimate of 7.6 to 8.2 kpc – which works out to about 24,788 to 26,745 light years.
This study builds upon previous work conducted by the study’s co-authors – Prof. Ray Calberg, the current chair of the Department of Astronomy & Astrophysics at the University of Toronto. Years ago, he and Prof. Kimmo Innanen of the Department of Physics and Astronomy at York University conducted a similar study using radial velocity measurement from 400 of the Milky Way’s stars.
But by incorporating data from the Gaia observatory, the UofT team was able to obtain a much more comprehensive data set and narrow the distance to galactic center by a significant amount. And this was based on only the initial data released by the Gaia mission. Looking ahead, Hunt anticipates that further data releases will allow his team and other astronomers to refine their calculations even more.
“Gaia’s final release in late 2017 should enable us to increase the precision of our measurement of the Sun’s velocity to within approximately one km/sec,” he said, “which in turn will significantly increase the accuracy of our measurement of our distance from the Galactic center.”
As more next-generation space telescopes and observatories are deployed, we can expect them to provide us with a wealth of new information about our Universe. And from this, we can expect that astronomers and astrophysicists will begin to shine the light on a number of unresolved cosmological questions.
M33, the Triangulum Spiral Galaxy, seen here in a 4.3 hour exposure image. Astronomers used JWST to examine a section of its south spiral arm to search out and find nearly 800 newly forming stars. Credit and copyright: John Chumack.
Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the Triangulum Galaxy, also known as Messier 33. Enjoy!
During the 18th century, famed French astronomer Charles Messier noted the presence of several “nebulous objects” in the night sky. Having originally mistaken them for comets, he began compiling a list of them so that others would not make the same mistake he did. In time, this list (known as the Messier Catalog) would come to include 100 of the most fabulous objects in the night sky.
One of these is the Triangulum Galaxy, a spiral galaxy located approximately 3 million light-years from Earth in the direction of the Triangulum constellation. As the third-largest member of the Local Group of galaxies (behind the Andromeda Galaxy and the Milky Way), it is the one of the most distant objects that can be seen with the naked eye. Much like M32, M33 is very close to Andromeda, and is believed to be a satellite of this major galaxy.
Description:
At some 3 million light years away from Earth, the Triangulum Galaxy is the third largest galaxy in our Local Group and it may be a gravitationally bound companion of the Andromeda Galaxy. Its beautiful spiral arms show multitudes of red HII regions and blue clouds of young stars. The largest of these HII regions (NGC 604) spans nearly 1500 across and is the largest so far known.
The Triangulum Galaxy (M33), taken by the Swift Gamma-Ray Burst Mission. Credit: NASA/Swift
It has a spectrum similar to the Orion Nebula – our own Milky Way’s most celebrated starbirth region. “M33 is a gigantic laboratory where you can watch dust being created in novae and supernovae, being distributed in the winds of giant stars, and being reborn in new stars,” said University of Minnesota researcher and lead author Elisha Polomski. By studying M33, “you can see the Universe in a nutshell.”
Of course, our curiousity about our neighboring galaxy has driven us to try to understand more over the years. Once Edwin Hubble set the standard with Cepheid variables, we began measuring distance by discovering about 25 of them in M33. By 2004 we were studying the red giant star branch to peer even further. As A.W. McConnachie said in a 2004 study of the galaxy:
“The absolute bolometric luminosity of the point of core helium ignition in old, metal-poor, red giant stars is of roughly constant magnitude, varying only very slightly with mass or metallicity. It can thus be used as a standard candle. This technique then allows for the determination of realistic uncertainties which reflect the quality of the luminosity function used. Finally, we apply our technique to the Local Group spiral galaxy M33 and the dwarf galaxies Andromeda I and II, and derive distance. The result for M33 is in excellent agreement with the Cepheid distances to this galaxy, and makes the possibility of a significant amount of reddening in this object unlikely.”
By 2005, astronomers had detected two water masers on either side of M33 and for the first time ever – revealed what direction it as going in. According to Andreas Brunthaler (et al), who published a study about the distance and proper motion of the galaxy in 2005:
“We measured the angular rotation and proper motion of the Triangulum Galaxy (M33) with the Very Long Baseline Array by observing two H2O masers on opposite sides of the galaxy. By comparing the angular rotation rate with the inclination and rotation speed, we obtained a distance of 730 +/- 168 kiloparsecs. This distance is consistent with the most recent Cepheid distance measurement. This distance is consistent with the most recent Cepheid distance measurement. M33 is moving with a velocity of 190 +/- 59 kilometers per second relative to the Milky Way. These measurements promise a method to determine dynamical models for the Local Group and the mass and dark-matter halos of M31, M33, and the Milky Way.”
Composite image of the Triangulum Galaxy (Messier 33), taken at Mount Lemmon Observatory. Credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona
Yes, it’s moving toward the Andromeda Galaxy, much like how Andromeda is moving towards us! In 2006, a group of astronomers announced the discovery of an eclipsing binary star in M33. As A.Z. Bonanos, the lead author of the study that detailed the discovery, said:
“We present the first direct distance determination to a detached eclipsing binary in M33, which was found by the DIRECT Project. Located in the OB 66 association, it was one of the most suitable detached eclipsing binaries found by DIRECT for distance determination, given its 4.8938 day period.”
By studying the eclipsing binary, astronomers soon knew their size, distance, temperature and absolute magnitude. But more was yet to come! In 2007, the Chandra X-ray Observatory revealed even more when a black hole nearly 16 times the mass of the Sun was revealed. The black hole, named M33 X-7, orbits a companion star which it eclipses every 3.5 days. This means the companion star must also have an incredibly large mass as well….
Yet how huge must the parent star have been to have formed a black hole in advance of its companion? As Jerome Orosz, of San Diego State University, was quoted as saying in a 2007 Chandra press release:
“This discovery raises all sorts of questions about how such a big black hole could have been formed. Massive stars can be much less extravagant than people think by hanging onto a lot more of their mass toward the end of their lives. This can have a big effect on the black holes that these stellar time-bombs make.”
Artist’s rendering of the black hole found in orbit of the large blue star in M33 . Credit: Chandra/Harvard/HST
Stellar bombs? You bet. Gigantic stellar explosions even. Although no supernovae events have been detected in the Triangulum galaxy, it certainly doesn’t lack for evidence of supernova remnants. According to a 2004 study by F. Haberl and W. Pietsch of the Max-Planck-Institute:
“We present a catalogue of 184 X-ray sources within 50′ of the nucleus of the local group spiral galaxy M 33. The catalogue is derived from an analysis of the complete set of ROSAT archival data pointed in the direction of M 33 and contains X-ray position, existence likelihood, count rates and PSPC spectral hardness ratios. To identify the sources the catalog was correlated with previous X-ray catalogues, optical and radio catalogues. In addition sources were classified according to their X-ray properties. We find seven candidates for supersoft X-ray sources, of which two may be associated with known planetary nebulae in M 33. The majority of X-ray detected supernova remnants is also detected at radio frequencies and seen in optical lines. The low overall X-ray detection rate of optically selected SNRs can probably be attributed to their expansion into interstellar matter of low density.”
Or the creation of black holes…
History of Observation:
While the Triangulum Galaxy was probably first observed by Hodierna before 1654 (back when skies were dark), it was independently rediscovered by Charles Messier, and cataloged by him on August 25, 1764. As he recorded in his notes on the occasion:
“I have discovered a nebula between the head of the northern Fish and the large Triangle, a bit distant from a star which had not been known, of sixth magnitude, of which I have determined the position; the right ascension of that star was 22d 7′ 13″, and its declination 29d 54′ 10″ north: near that star, there is another one which is the first of Triangulum, described by the letter b. Flamsteed described it in his catalog, of sixth magnitude; it is less beautiful than that of which I have given the position, and one should set it to the rank of the stars of the eighth class. The nebula is a whitish light of 15 minutes in diameter, of an almost even density, despite a bit more luminous at two third of its diameter; it doesn’t contain any star: one sees it with difficulty with an ordinary refractor of one foot.”
The location of the Triangulum Galaxy in the night sky. Credit: Wikisky
While Sir William Herschel wouldn’t publish papers on Messier’s findings, he was an astronomically curious soul and couldn’t help but study M33 intently on his own, writing:
“There is a suspicion that the nebula consists of exceedingly small stars. With this low power it has a nebulous appearance; and it vanishes when I put on the higher magnifying powers of 278 and 460.” He would continue to observe this grand galaxy again and again over the years, cataloging its various regions with their own separate numbers and keeping track of his findings: “The stars of the cluster are the smallest points imaginable. The diameter is nearly 18 minutes.”
Yet it would take a very special observer, one named Bill Parsons – the third Earl of Rosse – to become the very first to describe it as spiral. As he wrote of it:
“September 16, 1849. – New spiral: Alpha the brighter branch; Gamma faint; Delta short but pretty bright; Beta pretty distinct; Epsilon but suspected; the whole involved in a faint nebula, which probably extends past several knots which lie about it in different directions. Faint nebula seems to extend very far following: drawing taken.”
Quite the description indeed, since it would eventually lead to Rosse’s description of M33 being “…full of knots. Spiral arrangement. Two similar curves like an “S” cross in the center”, and to other astronomers discovering that these “spiral nebulae” were extra-galactic!
The location of Messier 33 in the Triangulum constellation. Credit: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)
Locating Messier 33:
While actually locating Messier 33 isn’t so difficult, seeing Messier 33 can be. Even though it is billed at nearly unaided eye magnitude, this huge, low surface brightness galaxy requires some experience with equipment and observing conditions or you may hunt forever in the right place and never find it. Let’s begin first by getting you in the proper area! First locate the Great Square of Pegasus – and its easternmost bright star, Alpha. About a hand span further east you will see the brightest star in Triangulum – Alpha.
M33 is just a couple of degrees (about 2 finger widths) west. Now, the most important part to understand is that you must use the lowest magnification possible, or you won’t be able to see the proverbial forest because of the trees. The image you see here at the top of the page is around a full degree of sky – about 1/3 the field of view of average binoculars and far larger than your average telescope eyepiece.
However, by using the least amount of magnification with a telescope you are causing M33 to appear much smaller – allowing it to fit within eyepiece field of view range. The larger the aperture, the more light it gathers and the brighter the image will be. The next thing to understand is M33 really is low surface brightness… Light pollution, a fine haze in the sky, moonlight… All of these things will make it difficult to find. Yet, there are places left here on Earth where the Triangulum Galaxy can be seen with no optical aid at all!
Enjoy your quest for M33. You may find it your first time out and it may be years before you see it in all its glory. But when you do, we guarantee you’ll never forget! Be sure to enjoy this video of the Triangulum galaxy too, courtesy of the European Southern Observatory:
Enjoy your quest for M33. You may find it your first time out and it may be years before you see it in all its glory. But when you do, we guarantee you’ll never forget!
And here are the quick facts on M33 to help you get started:
Object Name: Messier 33 Alternative Designations: M33, NGC 598, Triangulum Galaxy, Pinwheel Galaxy Object Type: Type Sc, Spiral Galaxy Constellation: Triangulum Right Ascension: 01 : 33.9 (h:m) Declination: +30 : 39 (deg:m) Distance: 3000 (kly) Visual Brightness: 5.7 (mag) Apparent Dimension: 73×45 (arc min)
Artist's impression of The Milky Way Galaxy. Based on current estimates and exoplanet data, it is believed that there could be tens of billions of habitable planets out there. Credit: NASA
Our Milky Way is a pretty vast and highly-populated space. All told, its stars number between 100 and 400 billion, with some estimates saying that it may have as many as 1 trillion. But just where did all these stars come from? Well, as it turns out, in addition to forming many of its own and merging with other galaxies, the Milky Way may have stolen some of its stars from other galaxies.
Such is the argument made by two astronomers from Harvard-Smithsonian Center for Astrophysics. According to their study, which has been accepted for publication in the The Astrophysical Journal, they claim that roughly half of the stars that orbit at the extreme outer edge of the Milky Way were actually stolen from the nearby Sagittarius dwarf galaxy.
At one time, the Sagittarius Dwarf Elliptical Galaxy was thought to be the closest galaxy to our own (a position now held by the Canis Major dwarf galaxy). As one of several dozen dwarf galaxies that surround the Milky Way, it has orbited our galaxy several times in the past. With each passing orbit, it becomes subject to our galaxy’s strong gravity, which has the effect of pulling it apart.
A model of the tidally shredded Sagittarius dwarf galaxy wrapping around a 3-D representation of the Milky Way disk. Credit: UCLA/D.R. Law
The long-term effects of this can be seen by looking to the farthest stars in our galaxy, which consist of the eleven stars that are at a distance of about 300,000 light-years from Earth (well beyond the Milky Way’s spiral disk). According the study produced by Marion Dierickx, a graduate student at Harvard University’s Department of Astronomy, half of these stars were taken from the Sagittarius dwarf galaxy in the past.
“We see evidence for streams of stars connected to the core of the galaxy, and indicating that this dwarf galaxy passed multiple times around the Milky Way center and was ripped apart by the tidal gravitational field of the Milky Way. We are all familiar with the tide in the ocean caused by the gravitational pull of the moon, but if the moon was a much more massive object – it would have pulled the oceans apart from the Earth and we would see a stream of vapor stretched away from the Earth.”
For the sake of their study, Dierickx and Loeb ran computer models to simulate the movements of the Sagittarius dwarf over the past 8 billion years. These simulations reproduced the streams of stars stretching away from the Sagittarius dwarf galaxy to the center of our galaxy. They also varied Sagittarius’ velocity and angle of approach to see if the resulting exchanges would match current observations.
Computer-generated image showing the disc of the Milky Way (red oval) and the Sagittarius dwarf galaxy (red dot). The yellow circles represent stars that have been ripped from the Sagittarius dwarf and flung far across space. Credit: Marion Dierickx / CfA
“We attempted to match the distance and velocity data for the core of the Sagitarrius galaxy, and then compared the resulting prediction for the position and velocity of the streams of stars,” said Loeb. “The results were very encouraging for some particular set of initial conditions regarding the start of the Sagittarius galaxy journey when the universe was roughly half its present age.”
What they found was that over time, the Sagittarius dwarf lost about one-third of its stars and nine-tenths of its dark matter to the Milky Way. The end result of this was the creation of three distinct streams of stars that reach one million light-years from galactic center to the very edge of the Milky Way’s halo. Interestingly enough, one of these streams has been predicted by simulations conducted by projects like the Sloan Digital Survey.
The simulations also showed that five of Sagittarius’ stars would end becoming part of the Milky Way. What’s more, the positions and velocities of these stars coincided with five of the most distant stars in our galaxy. The other six do not appear to be from Sagittarius dwarf, and may be the result of gravitational interactions with another dwarf galaxy in the past.
“The dynamics of stars in the extended arms we predict (which is the largest Galactic structure on the sky ever predicted) can be used to measure the mass and structure of the Milky Way,” said Loeb. “The outer envelope of the Milky Way was never probed directly, because no other stream was known to extend that far.”
Computer model of the Milky Way, the Sagittarius dwarf galaxy, and the looping stream of material between the two. Credit: Tollerud, Purcell and Bullock/UC Irvine
Given the way the simulations match up with current observations, Dierickx is confident that more Sagittarius dwarf interlopers are out there, just waiting to be found. For instance, future instruments – like the Large Synoptic Survey Telescope (LSST), which is expected to begin full-survey operations by 2022 – may be able to detect the two remaining streams of stars which were predicted by the survey.
Given the time scales and the distances involved, it is rather difficult to probe our galaxy (and by extension, the Universe) to see exactly how it evolved over time. Pairing observational data with computer models, however, has been proven to test our best theories of how things came to be. In the future, thanks to improved instruments and more detailed surveys, we just might know for certain!
And sure to check out this animation of the computer simulation, which shows the effects on the Milky Way’s gravity on the Sagittarius dwarf galaxy’s stars and dark matter.
An artists depiction of how the spectra of elements in the stars of the Milky Way reflect the importance these elements play in human life. Credit: Dana Berry/SkyWorks Digital Inc.; SDSS collaboration.
Scientist Carl Sagan said many times that “we are star stuff,” from the nitrogen in our DNA, the calcium in our teeth, and the iron in our blood.
It is well known that most of the essential elements of life are truly made in the stars. Called the “CHNOPS elements” – carbon, hydrogen, nitrogen, oxygen, phosphorous, and sulfur – these are the building blocks of all life on Earth. Astronomers have now measured of all of the CHNOPS elements in 150,000 stars across the Milky Way, the first time such a large number of stars have been analyzed for these elements.
“For the first time, we can now study the distribution of elements across our Galaxy,” says Sten Hasselquist of New Mexico State University. “The elements we measure include the atoms that make up 97% of the mass of the human body.”
Astronomers with the Sloan Digital Sky Survey made their observations with the APOGEE (Apache Point Observatory Galactic Evolution Experiment) spectrograph on the 2.5m Sloan Foundation Telescope at Apache Point Observatory in New Mexico. This instrument looks in the near-infrared to reveal signatures of different elements in the atmospheres of stars.
Quote from Carl Sage. Credit: Pinterest
While the observations were used to create a new catalog that is helping astronomers gain a new understanding of the history and structure of our galaxy, the findings also “demonstrates a clear human connection to the skies,” said the team.
While humans are 65% oxygen by mass, oxygen makes up less than 1% of the mass of all of elements in space. Stars are mostly hydrogen, but small amounts of heavier elements such as oxygen can be detected in the spectra of stars. With these new results, APOGEE has found more of these heavier elements in the inner part of the galaxy. Stars in the inner galaxy are also older, so this means more of the elements of life were synthesized earlier in the inner parts of the galaxy than in the outer parts.
So what does that mean for those of us out on the outer edges of one of the Milky Way’s spiral arms, about 25,000 light-years from the center of the galaxy?
“I think it’s hard to say what the specific implications are for when life could arise,” said team member Jon Holtzman, also from New Mexico State, in an email to Universe Today. “We measure typical abundance of CHNOPS elements at different locations, but it’s not so easy to determine at any given location the time history of the CHNOPS abundances, because it’s hard to measure ages of stars. On top of that, we don’t know what the minimum amount of CHNOPS would need to be for life to arise, especially since we don’t really know how that happens in any detail!”
Holtzman added it is likely that, if there is a minimum required abundance, that minimum was probably reached earlier in the inner parts of the Galaxy than where we are.
The team also said that while it’s fun to speculate how the composition of the inner Milky Way Galaxy might impact how life might arise, the SDSS scientists are much better at understanding the formation of stars in our Galaxy.
“These data will be useful to make progress on understanding Galactic evolution,” said team member Jon Bird of Vanderbilt University, “as more and more detailed simulations of the formation of our galaxy are being made, requiring more complex data for comparison.”
Sloan Foundation 2.5m Telescope at Apache Point Observatory. Credit: SDSS.
“It’s a great human interest story that we are now able to map the abundance of all of the major elements found in the human body across hundreds of thousands of stars in our Milky Way,” said Jennifer Johnson of The Ohio State University. “This allows us to place constraints on when and where in our galaxy life had the required elements to evolve, a sort ‘temporal Galactic habitable zone’”.
The catalog is available at the SDSS website, so take a look for yourself at the chemical abundances in our portion of the galaxy.
Hubble image of NGC 248, two nebulas located in the Small Magellanic Cloud. Credit: NASA, ESA, STScI, K. Sandstrom/SMIDGE team
The Hubble Space Telescope has revealed some amazing things over the past few decades. Over the course of its many missions, this orbiting observatory has spotted things ranging from distant stars and galaxies to an expanding Universe. And today, twenty-six years later, it is still providing us with rare glimpses of the cosmos.
For example, just in time for the holidays, Hubble has released images of two rosy, glowing nebulas in the Small Magellanic Cloud (SMC). These glowing clouds of gas and dust were spotted as part of a study known as the Small Magellanic Cloud Investigation of Dust and Gas Evolution (SMIDGE), an effort to study this neighboring galaxy in an attempt to better understand our own.
The images were taken by Hubble’s Advanced Camera for Surveys (ACS) in September 2015 and feature NGC 248 – two gaseous nebulas that were first observed by astronomer Sir John Herschel in 1834 and are situated in such a way as to appear as one. Measuring about 60 light years in length and 20 light-years in width, these nebulas are among a series of emission nebulas located in the neighboring dwarf satellite galaxy.
Small and Large Magellanic Clouds over Paranal Observatory Credit: ESO/J. Colosimo
Emission nebulas are essentially large clouds of ionized gases that emit light of various colors – in this case, bright red. The color and luminosity of NGC 248 is due to the nebulas heavy hydrogen content, and the fact that they have young, brilliant stars at the center of them. These stars emit intense radiation that heats up the hydrogen gas, causing it to emit bright red light.
As noted, the images were taken as part of the SMIDGE study, an effort on behalf of astronomers to probe the Milky Way satellite – which is located approximately 200,000 light-years away in the southern constellation Tucana – using the Hubble Space Telescope. The ultimate goal of this study is to understand how dust is different in galaxies that have a far lower supply of the heavy elements needed to create it.
In the case of the SMC, it has between one-fifth and one-tenth the amount of heavy metals as the Milky Way. In addition, its proximity to the Milky Way makes it a convenient target for astronomers who are looking to better understand the history of the earlier Universe. Essentially, most star formation in the Milky Way happened at a time when the amount of heavy elements was much lower than it is now.
Ground-based image of the Small Magellanic Cloud. showing the area of the SMIDGE survey and the position of NGC 248. Credit: NASA/ESA/Hubble/Digitized Sky Survey 2
According to Dr. Karin Sandstrom, a professor from the University of California and the principle investigator of SMIDGE, studying the SMC’s can tell us much about neighboring galaxies, but also about the evolution of the Milky Way. “It is important for understanding the history of our own galaxy, too,” he said. “Dust is a really critical part of how a galaxy works, how it forms stars.”
In addition to the stunning images, the SMIDGE team and the Space Telescope Science Institute have also produced a video that shows the location of NGC 248 in the southern sky. As you can see, the video begins with a ground-based view of the night sky (from the southern hemisphere) and then zooms in on the Small Magellanic Cloud, emphasizing the field where NGC 249 appears.
Check out the video below, and have yourselves a Merry Christmas and some Happy Holidays!
Detection of an unusually bright X-Ray flare from Sagittarius A*, a supermassive black hole in the center of the Milky Way galaxy. Credit: NASA/CXC/Stanford/I. Zhuravleva et al.
In 1971, English astronomers Donald Lynden-Bell and Martin Rees hypothesized that a supermassive black hole (SMBH) resides at the center of our Milky Way Galaxy. This was based on their work with radio galaxies, which showed that the massive amounts of energy radiated by these objects was due to gas and matter being accreted onto a black hole at their center.
By 1974, the first evidence for this SMBH was found when astronomers detected a massive radio source coming from the center of our galaxy. This region, which they named Sagittarius A*, is over 10 million times as massive as our own Sun. Since its discovery, astronomers have found evidence that there are supermassive black holes at the centers of most spiral and elliptical galaxies in the observable Universe.
Description:
Supermassive black holes (SMBH) are distinct from lower-mass black holes in a number of ways. For starters, since SMBH have a much higher mass than smaller black holes, they also have a lower average density. This is due to the fact that with all spherical objects, volume is directly proportional to the cube of the radius, while the minimum density of a black hole is inversely proportional to the square of the mass.
In addition, the tidal forces in the vicinity of the event horizon are significantly weaker for massive black holes. As with density, the tidal force on a body at the event horizon is inversely proportional to the square of the mass. As such, an object would not experience significant tidal force until it was very deep into the black hole.
Formation:
How SMBHs are formed remains the subject of much scholarly debate. Astrophysicists largely believe that they are the result of black hole mergers and the accretion of matter. But where the “seeds” (i.e. progenitors) of these black holes came from is where disagreement occurs. Currently, the most obvious hypothesis is that they are the remnants of several massive stars that exploded, which were formed by the accretion of matter in the galactic center.
Another theory is that before the first stars formed in our galaxy, a large gas cloud collapsed into a “qausi-star” that became unstable to radial perturbations. It then turned into a black hole of about 20 Solar Masses without the need for a supernova explosion. Over time, it rapidly accreted mass in order to become an intermediate, and then supermassive, black hole.
In yet another model, a dense stellar cluster experienced core-collapse as the as a result of velocity dispersion in its core, which happened at relativistic speeds due to negative heat capacity. Last, there is the theory that primordial black holes may have been produced directly by external pressure immediately after the Big Bang. These and other theories remain theoretical for the time being.
Sagittarius A*:
Multiple lines of evidence point towards the existence of a SMBH at the center of our galaxy. While no direct observations have been made of Sagittarius A*, its presence has been inferred from the influence it has on surrounding objects. The most notable of these is S2, a star that flows an elliptical orbit around the Sagittarius A* radio source.
S2 has an orbital period of 15.2 years and reaches a minimal distance of 18 billion km (11.18 billion mi, 120 AU) from the center of the central object. Only a supermassive object could account for this, since no other cause can be discerned. And from the orbital parameters of S2, astronomers have been able to produce estimates on the size and mass of the object.
For instance, S2s motions have led astronomers to calculated that the object at the center of its orbit must have no less than 4.1 million Solar Masses (8.2 × 10³³ metric tons; 9.04 × 10³³ US tons). Furthermore, the radius of this object would have to be less than 120 AU, otherwise S2 would collide with it.
As Reinhard Genzel, the team leader from the Max-Planck-Institute for Extraterrestrial Physics said:
“Undoubtedly the most spectacular aspect of our long term study is that it has delivered what is now considered to be the best empirical evidence that supermassive black holes do really exist. The stellar orbits in the Galactic Centre show that the central mass concentration of four million solar masses must be a black hole, beyond any reasonable doubt.”
Another indication of Sagittarius A*s presence came on January 5th, 2015, when NASA reported a record-breaking X-ray flare coming from the center of our galaxy. Based on readings from the Chandra X-ray Observatory, they reported emissions that were 400 times brighter than usual. These were thought to be the result of an asteroid falling into the black hole, or by the entanglement of magnetic field lines within the gas flowing into it.
Other Galaxies:
Astronomers have also found evidence of SMBHs at the center of other galaxies within the Local Group and beyond. These include the nearby Andromeda Galaxy (M31) and elliptical galaxy M32, and the distant spiral galaxy NGC 4395. This is based on the fact that stars and gas clouds near the center of these galaxies show an observable increase in velocity.
Another indication is Active Galactic Nuclei (AGN), where massive bursts of radio, microwave, infrared, optical, ultra-violet (UV), X-ray and gamma ray wavebands are periodically detected coming from the regions of cold matter (gas and dust) at the center of larger galaxies. While the radiation is not coming from the black holes themselves, the influence such a massive object would have on surrounding matter is believed to be the cause.
In short, gas and dust form accretion disks at the center of galaxies that orbit supermassive black holes, gradually feeding them matter. The incredible force of gravity in this region compresses the disk’s material until it reaches millions of degrees kelvin, generating bright radiation and electromagnetic energy. A corona of hot material forms above the accretion disc as well, and can scatter photons up to X-ray energies.
The interaction between the SMBH rotating magnetic field and the accretion disk also creates powerful magnetic jets that fire material above and below the black hole at relativistic speeds (i.e. at a significant fraction of the speed of light). These jets can extend for hundreds of thousands of light-years, and are a second potential source of observed radiation.
When the Andromeda Galaxy merges with our own in a few billion years, the supermassive black hole that is at its center will merge with our own, producing a much more massive and powerful one. This interaction is likely to kick several stars out of our combined galaxy (producing rogue stars), and is also likely to cause our galactic nucleus (which is currently inactive) to become active one again.
The study of black holes is still in its infancy. And what we have learned over the past few decades alone has been both exciting and awe-inspiring. Whether they are lower-mass or supermassive, black holes are an integral part of our Universe and play an active role in its evolution.
Who knows what we will find as we peer deeper into the Universe? Perhaps some day we the technology, and sheer audacity, will exist so that we might attempt to peak beneath the veil of an event horizon. Can you imagine that happening?
The night sky above the Danish 1.54-metre telescope at ESO's La Silla Observatory. The Magellanic Clouds are visible to the right of the central bar of the Milky Way. Credit: ESO/Z. Bardon
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.
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)
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.
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 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, 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!
Whirlpool Galaxy M51 (NGC 5194). Credit: Hubble Heritage Team (STScI/AURA)
N. Scoville (Caltech)
On a clear night, you can make out the band of the Milky Way in the night sky. For millennia, astronomers looked upon it in awe, slowly coming to the realization that our Sun was merely one of billions of stars in the galaxy. Over time, as our instruments and methods improved, we came to realize that the Milky Way itself was merely one of billions of galaxies that make up the Universe.
Thanks to the discovery of Relativity and the speed of light, we have also come to understand that when we look through space, we are also looking back in time. By seeing an object 1 billion light-years away, we are also seeing how that object looked 1 billion years ago. This “time machine” effect has allowed astronomers to study how galaxies came to be (i.e. galactic evolution).
The process in which galaxies form and evolve is characterized by steady growth over time, which began shortly after the Big Bang. This process, and the eventual fate of galaxies, remain the subject of intense fascination, and is still fraught with its share of mysteries.
Illustration of the depth by which Hubble imaged galaxies in prior Deep Field initiatives, in units of the Age of the Universe. Credit: NASA and A. Feild (STScI)
Galaxy Formation:
The current scientific consensus is that all matter in the Universe was created roughly 13.8 billion years ago during an event known as the Big Bang. At this time, all matter was compacted into a very small ball with infinite density and intense heat called a Singularity. Suddenly, the Singularity began expanding, and the Universe as we know it began.
After rapidly expanding and cooling, all matter was almost uniform in distribution. Over the course of the several billion years that followed, the slightly denser regions of the Universe began to become gravitationally attracted to each other. They therefore grew even denser, forming gas clouds and large clumps of matter.
These clumps became primordial galaxies, as the clouds of hydrogen gas within the proto-galaxies underwent gravitational collapse to become the first stars. Some of these early objects were small, and became tiny dwarf galaxies, while others were much larger and became the familiar spiral shapes, like our own Milky Way.
Galactic Mergers:
Once formed, these galaxies evolved together in larger galactic structures called groups, clusters and superclusters. Over time, galaxies were attracted to one another by the force of their gravity, and collided together in a series of mergers. The outcome of these mergers depends on the mass of the galaxies in the collision.
Small galaxies are torn apart by larger galaxies and added to the mass of larger galaxies. Our own Milky Way recently devoured a few dwarf galaxies, turning them into streams of stars that orbit the galactic core. But when large galaxies of similar size come together, they become giant elliptical galaxies.
When this happens, the delicate spiral structure is lost, and the merged galaxies become large and elliptical. Elliptical galaxies are some of the largest galaxies ever observed. Another consequence of these mergers is that the supermassive black holes (SMBH) at their centers become even larger.
Not all mergers will result in elliptical galaxies, mind you. But all mergers result in a change in the structure of the merged galaxies. For example, it is believed that the Milky Way is experiencing a minor merger event with the nearby Magellanic Clouds; and in recent years, it has been determined that the Canis Major dwarf galaxy has merged with our own.
While mergers are seen as violent events, actual collisions are not expected to happen between star systems, given the vast distances between stars. However, mergers can result in gravitational shock waves, which are capable of triggering the formation of new stars. This is what is predicted to happen when our own Milky Way galaxy merges with the Andromeda galaxy in about 4 billion years time.
Galactic Death:
Ultimately, galaxies cease forming stars once they deplete their supply of cold gas and dust. As the supply runs out, star forming slows over the course of billions of years until it ceases entirely. However, ongoing mergers will ensure that fresh stars, gas and dust are deposited in older galaxies, thus prolonging their lives.
At present, it is believed that our galaxy has used up most of its hydrogen, and star formation will slow down until the supply is depleted. Stars like our Sun can only last for 10 billion years or so; but the smallest, coolest red dwarfs can last for a few trillion years. However, thanks to the presence of dwarf galaxies and our impending merger with Andromeda, our galaxy could exist even longer.
However, all galaxies in this vicinity of the Universe will eventually become gravitationally bound to each other and merge into a giant elliptical galaxy. Astronomers have seen examples of these sorts of “fossil galaxies”, a good of which is Messier 49 – a supermassive elliptical galaxy.
These galaxies have used up all their reserves of star forming gas, and all that’s left are the longer lasting stars. Eventually, over vast lengths of time, those stars will wink out one after the other, until the whole thing is the background temperature of the Universe.
After our galaxy merges with Andromeda, and goes on to merge with all other nearby galaxies in the local group, we can expect that it too will undergo a similar fate. And so, galaxy evolution has been occurring over billions of years, and it will continue to happen for the foreseeable future.
This illustration shows a stage in the predicted merger between our Milky Way galaxy and the neighboring Andromeda galaxy, as it will unfold over the next several billion years. In this image, representing Earth's night sky in 3.75 billion years, Andromeda (left) fills the field of view and begins to distort the Milky Way with tidal pull. (Credit: NASA; ESA; Z. Levay and R. van der Marel, STScI; T. Hallas; and A. Mellinger)
We don’t want to scare you, but our own Milky Way is on a collision course with Andromeda, the closest spiral galaxy to our own. At some point during the next few billion years, our galaxy and Andromeda – which also happen to be the two largest galaxies in the Local Group – are going to come together, and with catastrophic consequences.
Stars will be thrown out of the galaxy, others will be destroyed as they crash into the merging supermassive black holes. And the delicate spiral structure of both galaxies will be destroyed as they become a single, giant, elliptical galaxy. But as cataclysmic as this sounds, this sort of process is actually a natural part of galactic evolution.
Astronomers have know about this impending collision for some time. This is based on the direction and speed of our galaxy and Andromeda’s. But more importantly, when astronomers look out into the Universe, they see galaxy collisions happening on a regular basis.
The Antennae galaxies, a pair of interacting galaxies located 45 – 65 million light years from Earth. Credit: Hubble / ESA
Gravitational Collisions:
Galaxies are held together by mutual gravity and orbit around a common center. Interactions between galaxies is quite common, especially between giant and satellite galaxies. This is often the result of a galaxies drifting too close to one another, to the point where the gravity of the satellite galaxy will attract one of the giant galaxy’s primary spiral arms.
In other cases, the path of the satellite galaxy may cause it to intersect with the giant galaxy. Collisions may lead to mergers, assuming that neither galaxy has enough momentum to keep going after the collision has taken place. If one of the colliding galaxies is much larger than the other, it will remain largely intact and retain its shape, while the smaller galaxy will be stripped apart and become part of the larger galaxy.
Such collisions are relatively common, and Andromeda is believed to have collided with at least one other galaxy in the past. Several dwarf galaxies (such as the Sagittarius Dwarf Spheroidal Galaxy) are currently colliding with the Milky Way and merging with it.
However, the word collision is a bit of a misnomer, since the extremely tenuous distribution of matter in galaxies means that actual collisions between stars or planets is extremely unlikely.
Image obtained by the Hubble Space Telescope and the Keck-II telescope, showing a collision that took place billions of years ago. Credit: ESO/NASA/ESA/W. M. Keck Observatory
Andromeda–Milky Way Collision:
In 1929, Edwin Hubble revealed observational evidence which showed that distant galaxies were moving away from the Milky Way. This led him to create Hubble’s Law, which states that a galaxy’s distance and velocity can be determined by measuring its redshift – i.e. a phenomena where an object’s light is shifted toward the red end of the spectrum when it is moving away.
However, spectrographic measurements performed on the light coming from Andromeda showed that its light was shifted towards the blue end of the spectrum (aka. blueshift). This indicated that unlike most galaxies that have been observed since the early 20th century, Andromeda is moving towards us.
In 2012, researchers determined that a collision between the Milky Way and the Andromeda Galaxy was sure to happen, based on Hubble data that tracked the motions of Andromeda from 2002 to 2010. Based on measurements of its blueshift, it is estimated that Andromeda is approaching our galaxy at a rate of about 110 km/second (68 mi/s).
At this rate, it will likely collide with the Milky Way in around 4 billion years. These studies also suggest that M33, the Triangulum Galaxy – the third largest and brightest galaxy of the Local Group – will participate in this event as well. In all likelihood, it will end up in orbit around the Milky Way and Andromeda, then collide with the merger remnant at a later date.
Images from Hubble’s ACS in 2004 and 2005 show four examples of interacting galaxies (at various stages in the process) far away from Earth. Credit: NASA/ESA/J. Lotz, STScI/M. Davis, University of California, Berkeley/A. Koekemoer, STScI.
Consequences:
In a galaxy collision, large galaxies absorb smaller galaxies entirely, tearing them apart and incorporating their stars. But when the galaxies are similar in size – like the Milky Way and Andromeda – the close encounter destroys the spiral structure entirely. The two groups of stars eventually become a giant elliptical galaxy with no discernible spiral structure.
Such interactions can also trigger a small amount of star formation. When the galaxies collide, it causes vast clouds of hydrogen to collect and become compressed, which can trigger a series of gravitational collapses. A galaxy collision also causes a galaxy to age prematurely, since much of its gas is converted into stars.
After this period of rampant star formation, galaxies run out of fuel. The youngest hottest stars detonate as supernovae, and all that’s left are the older, cooler red stars with much longer lives. This is why giant elliptical galaxies, the results of galaxy collisions, have so many old red stars and very little active star formation.
Despite the Andromeda Galaxy containing about 1 trillion stars and the Milky Way containing about 300 billion, the chance of even two stars colliding is negligible because of the huge distances between them. However, both galaxies contain central supermassive black holes, which will converge near the center of the newly-formed galaxy.
Two galaxies colliding in the Corvus constellation. Credit: B. Whitmore (STScI), F. Schweizer (DTM),
This black hole merger will cause orbital energy to be transferred to stars, which will be moved to higher orbits over the course of millions of years. When the two black holes come within a light year of one another, they will emit gravitational waves that will radiate further orbital energy, until they merge completely.
Gas taken up by the combined black hole could create a luminous quasar or an active nucleus to form at the center of the galaxy. And last, the effects of a black hole merger could also kick stars out of the larger galaxy, resulting in hypervelocity rogue stars that could even carry their planets with them.
Today, it is understood that galactic collisions are a common feature in our Universe. Astronomy now frequently simulate them on computers, which realistically simulate the physics involved – including gravitational forces, gas dissipation phenomena, star formation, and feedback.
And be sure to check out this video of the impending galactic collision, courtesy of NASA:
