The constellation Orion. Credit: Matthew Spinelli NASA/APOD
Orion dominates the winter sky in the northern hemisphere. Its large size and collection of bright stars — such as Betelgeuse at the shoulder, Rigel below the belt, and the three stars in the belt — make it easy to spot, even for beginning stargazers.
So how about those stars in the belt? They’re one of the most famous asterisms in Western culture, but beyond what we see with our eyes, what are their astronomical properties?
Introduction to Orion
First, a brief word about the constellation itself. In many mythologies, the shape is seen as a human figure — and in Greek mythology, it was named after a hunter, according to a web page from the Chandra X-Ray Observatory.
There are several “reasons” in mythology for why Orion ended up in the sky. One was because he was too boastful about how many animals he could kill — so he was put there to teach humility, since he and his dogs (Canis Major and Canis Minor) chase after animals in the sky but can’t catch them. Some say he died from a scorpion bite, and other legends say he was killed by his lover Artemis accidentally, when her brother Apollo tricked her to shooting an arrow at him.
Wide angle shot of Comet Lovejoy with the constellation Orion, showing rich fields of red nebula, star clouds and dark nebula with the bright green naked eye comet. Credit and copyright: Chris Schur.
Because Orion is on the celestial equator, Chandra adds, it is easy to see all over the world: “Ancient Indians saw the figure as a king who had been shot by an arrow (represented by the stars in Orion’s belt). Ancient Egyptians thought the stars in the belt represented the resting place of the soul of the god Osiris. The Arabs saw the constellation as the figure of a giant.”
The Orion’s belt stars
The three stars in the belt are Mintaka, Alnilam and Alnitak. According to an astronomer with the National Radio Astronomy Observatory, Ronald Maddlaena, these are the meanings of the three stars: Mintaka (on the west) means “belt”, Alnilam (in center) means “belt of pearls” and Altnitak (right) means “girdle.” The three range between 800 and 1,000 light-years from Earth.
The stars “probably formed at about the same time some ten million years ago from the molecular clouds astronomers have found in Orion,” wrote Maddalena.
In this image, the submillimetre-wavelength glow of the dust clouds is overlaid on a view of the region in the more familiar visible light, from the Digitized Sky Survey 2. The large bright cloud in the upper right of the image is the well-known Orion Nebula, also called Messier 42. Credit: ESO/Digitized Sky Survey 2
Here are their properties compared to the Sun:
Mintaka: 20 times more massive and 7,000 times brighter. (Surface temperature 60,000 Fahrenheit.)
Alnilam: 20 times more massive and 18,000 times brigher. (Surface temperature 50,000 Fahrenheit.)
Alnitak: 20 times more massive and 10,000 times brighter. (Surface temperature 60,000 Fahrenheit).
To further blow your mind — these stars also have companion stars orbiting with them, so what you see from Earth with the naked eye isn’t necessarily what you always get.
Composite picture of a dark red Moon during a total lunar eclipse. Credit: NASA/ Johannes Schedler (Panther Observatory)
What makes a planet a planet? The Moon is so big compared to the Earth — roughly one-quarter our planet’s size — that occasionally you will hear our system being referred to as a “double planet”. Is this correct?
And we all remember how quickly the definition of a planet changed in 2006 when more worlds similar to Pluto were discovered. So can the Moon stay the Moon, or is the definition subject to change?
Defining a planet
First, it’s important to understand what the official definition of a “planet” is, at least according to the International Astronomical Union. In its own words, according to a vote in Prague in 2006, the union has this definition:
“A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.”
What this means is that a planet must move around the Sun (and not move around something else), that it’s massive enough to have a round shape due to gravity, and that it will swoop up any dust or debris in its orbit as it moves around the Sun.
But let’s be clear on something; the IAU definition of planet is not without controversy. There is still a strong contingent of people who say that Pluto is indeed a planet, including the principal investigator of a spacecraft (New Horizons) to examine the world: Alan Stern.
“It’s an awful definition; it’s sloppy science and it would never pass peer review,” he told the BBC in 2006. He said that the line between dwarf planets and planets is too artificial, and that the definition of a “cleared neighborhood” is muddy. The Earth alone has many asteroids that follow it — or approach or cross its orbit — not to mention the massive planet Jupiter.
UV observations from Hubble show the size of water vapor plumes coming from Europa’s south pole (NASA, ESA, and M. Kornmesser)
Definition of a ‘satellite’
The Moon is not a unique phenomenon in our Solar System, in the sense that there are other planets that have satellites around them. Jupiter and Saturn have many dozens! Referring again to the IAU, the union also said in 2006 that it does not consider Charon a dwarf planet despite its large relative size to Pluto.
But Charon’s status as a moon could change in future, the IAU acknowledged. That’s primarily because the center of gravity in the system is not inside of Pluto, but in “free space between Pluto and Charon”. This center is called the “barycenter”, technically — and in Jupiter and Saturn’s cases, for example, all the barycenters of the various moons reside “inside” the huge gas giants.
Another caution, however: the IAU says “there has been no official recognition that the location of the barycenter is involved with the definition of a satellite.” So for now, it doesn’t have any bearing. That said, one question to consider is if the Moon’s barycenter is inside the Earth?
This Cassini raw image shows a portion of Saturn’s rings along with several moons. How many can you find? Credit: NASA/JPL/Space Science Institute
The answer right now is “yes”. But over time, that barycenter will move outside of Earth. That’s because the Moon is slowly receding from our planet at a rate of about 3.8 centimeters (1.5 inches) a year. It’ll take a long time, but eventually the center of our system’s mass will not be within our planet.
And if you read back to an IAU interview in 2006, you’ll see that at that time, the IAU defined a “double planet” as a system where both bodies meet the definition of a planet, and the barycenter is not inside either one of the objects. So for now, the Earth is a planet and the Moon a satellite — at least under IAU rules.
Some times planets just head off into the mysterious Universe all on their own, without a star to orbit. How and why do planets go rogue like this?
We’re accustomed to thinking about solar systems as places of order. All the planets orbit their parent star, everything is neatly arranged in ellipses and rings. Even the asteroid belt has division lines of dry and icy. Planets do what they’re told: orbit that star until the end of time. No Pluto, you may not go outside and play with the other planets. You’ll spend your lunch hour in detention with Haumea until we decide what we’re going to do with you for not cleaning up your play area.
Some planets just can’t be held down. They’re the Jimmy Deans, the greasers, the Marlon Brandos, the Cool Hand Lukes. They break all the laws and play by their own set of rules. They’re a rolling stone, baby. To ask them to settle down would just be to deny their nature. So instead of orbiting a star, they go rogue and fly off into the Milky Way, possibly seeking fame, fortune and adventure, but keeping to the beat of their own drummer.
A rogue planet is any planet that doesn’t orbit a star. Instead of being a member of a solar system, it orbits the Milky Way on its own. Or in the case of really deviant planets, it’s been ejected out of the Milky Way entirely. Make no mistake, this is not a small condition affecting a few planets. It’s estimated that there are billions of rogue planets out there in the Milky Way.
How does this happen? How can we get rogue planets? Is it the way they were raised? Something that happened in the way they were born? Some rogue planets started out as part of a solar system, and then something happened. Some event “kicked” them out into deep space. You could get a collision or near miss with another star, or even a black hole. As two stars pass one another, their gravitational interactions can cause all kinds of mayhem to a nice orderly orbital system. Planets can be kicked into higher or lower orbits, smashed into stars or flung out with an escape velocity that means they’ll never orbit their star again.
Planets can also escape when their star disappears. Sounds impossible? Sometimes stars go out for cigarettes and just never come back. When a massive star detonates as a supernova, the force of the explosion can eject planets at tremendous velocities away from the former star, flinging those billiard balls all over the hall. But the vast majority of rogue planets probably formed early on in their solar systems. Things were rough and chaotic back then, with planets smashing into each other with all kinds of near misses. These interactions could bully out smaller neighbors with not so much as a nod. Jupiter, I’m looking at you.
This artist’s conception illustrates what a “hot jupiter” might look like.
It’s also possible that planets could form as orphans, within a solar nebula, away from a star entirely. If a pocket of hydrogen collects together into a sphere, but it doesn’t have enough mass to actually ignite as a star, it’s another type of rogue planet. We’ll just pretend these ones were raised by Nuns.
What would it be like for these planets? Without the light from a star, these would be incredibly cold places. This isn’t just sad metaphor. The outer layers, exposed to space would be as cold as interstellar space, just a handful of degrees above absolute zero.But deep down below the surface, there would still be leftover heat from their formation, so it’s possible that life could survive down there, kept alive within a warm cocoon.
And who knows, maybe after billions of years, a rogue planet could get captured by a star again, and thawed out. It might get a second chance, or it could all end tragically, racing for pinks along the Devil’s elbow out past the Pillars of Creation. There are many ways that planets can go rogue, in fact, it’s possible that there are more starless planets in the Milky Way than there are stars.
So what do you think? Should we set sail from the Sun, and seek out adventure in the Milky Way?
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Eugene Cernan on the lunar surface, December 13, 1972. Credit: NASA.
Why explore space? It’s an expensive arena to play in, between the fuel costs and the technological challenge of operating in a hostile environment. For humans, a small mistake can quickly become fatal — something that we have seen several times in space history. And for NASA’s budget, there are projects that come in late and over budget, drawing the ire of Congress and the public.
These are some of the drawbacks. But for the rest of this article, we will focus on some of the benefits of going where few humans have gone before.
Spinoffs
Perhaps the most direct benefit comes from technologies used on Earth that were first pioneered in space exploration. This is something that all agencies talk about, but we’ll focus on the NASA Spinoff program as an example. (NASA will be used as the prime example for most of this article, but many of these cited benefits are also quoted by other space agencies.)
The program arose from NASA’s desire to showcase spinoffs at congressional budget hearings, according to its website. This began with a “Technology Utilization Program Report” in 1973, which began as a black-and-white circular and progressed to color in 1976 following public interest. Since that year, NASA has published more than 1,800 reports on spinoffs.
The agency has several goals in doing this. “Dispelling the myth of wasted taxpayer dollars” is one NASA cites, along with encouraging the public to follow space exploration and showing how American ingenuity can work in space.
There are many commercialized advances the program says it contributed to, including “memory foam” (first used for airline crash protection), magnetic resonance imaging and smoke detection. In many cases, NASA did not invent the technology itself, but just pushed it along, the agency says.
An MRI image of the lower back. Credit: NASA
But as counterpoint to NASA’s arguments, some critics argue the technology would have been developed anyway without space exploration, or that the money spent on exploration itself does not justify the spinoff.
Job creation
Another popularly cited benefit of space exploration is “job creation”, or the fact that a space agency and its network of contractors, universities and other entities help people stay employed. From time to time, NASA puts out figures concerning how many associated jobs a particular project generates, or the economic impact.
Here’s an example: in 2012, NASA administrator Charles Bolden published a blog post about the Curiosity Mars rover landing, which was picked up by the White House website. “It’s also important to remember that the $2.5 billion investment made in this project was not spent on Mars, but right here on Earth, supporting more than 7,000 jobs in at least 31 states,” he wrote.
Hazcam fisheye camera image shows Curiosity drilling into “Windjana” rock target on April 29, 2014 (Sol 615). Flattened and colorized image shows Mount Remarkable butte backdrop. Credit: NASA/JPL/Marco Di Lorenzo/Ken Kremer – kenkremer.com
But the benefit can cut in a negative way, too. NASA’s budget is allocated by Congress, which means that the amount of money it has available for employment fluctuates. There are also some programs that are highly dependent on grants, which can make stable jobs challenging in those fields. Finally, as the priorities of Congress/NASA change, jobs can evaporate with it. One example was the space shuttle’s retirement, which prompted a job loss so massive that NASA had a “transition strategy” for its employees and contractors.
It’s also unclear what constitutes a “job” under NASA parlance. Some universities have researchers working on multiple projects — NASA-related or not. Employment can also be full-time, part-time or occasional. So while “job creation” is cited as a benefit, more details about those jobs are needed to make an informed decision about how much good it does.
Education
Teaching has a high priority for NASA, so much so that it has flown astronaut educators in space. (The first one, Christa McAuliffe, died aboard the space shuttle Challenger during launch in 1986. Her backup, Barbara Morgan, was selected as an educator/mission specialist in 1998 and flew aboard STS-118 in 2007.) And to this day, astronauts regularly do in-flight conferences with students from space, ostensibly to inspire them to pursue careers in the field.
Christa McAuliffe and Barbara Morgan practice teaching from space. Credit: “The Lost Lessons”
NASA’s education office has three goals: making the workforce stronger, encouraging students to pursue STEM careers (science, technology, engineering and mathematics), and “engaging Americans in NASA’s mission.” Other space agencies also have education components to assist with requirements in their own countries. It’s also fair to say the public affairs office for NASA and other agencies play roles in education, although they also talk about topics such as missions in progress.
But it’s hard to figure out how well the education efforts translate into inspiring students, according to a National Research Council report on NASA’s primary and secondary education program in 2008. Among other criticisms, the program was cited as unstable (as it needs to change with political priorities) and there was little “rigorous evaluation” of its effectiveness. But NASA’s emphasis on science and discovery was also praised.
Anecdotally, however, many astronauts and people within NASA have spoken about being inspired by watching missions such as Apollo take place. And the same is true of people who are peripherally involved in the field, too. (A personal example: this author first became interested in space in the mid-1990s through the movie Apollo 13, which led to her watching the space shuttle program more closely.)
New Rosetta mission findings do not exclude comets as a source of water in and on the Earth’s crust but does indicate comets were a minor contribution. A four-image mosaic comprises images taken by Rosetta’s navigation camera on 7 December from a distance of 19.7 km from the centre of Comet 67P/Churyumov-Gerasimenko. (Credit: ESA/Rosetta/Navcam Imager)
Intangible benefits
Added to this host of business-like benefits, of course, are the intangibles. What sort of value can you place on better understanding the universe? Think of finding methane on Mars, or discovering an exoplanet, or constructing the International Space Station to do long-term exploration studies. Each has a cost associated with it, but with each also comes a smidgeon of knowledge we can add to the encyclopedia of the human race.
Space can also inspire art, which is something seen heavily in 2014 following the arrival of the European Space Agency Rosetta mission at Comet 67P/Churyumov–Gerasimenko. It inspired songs, short videos and many other works of art. NASA’s missions, particularly those early space explorers of the 1950s and 1960s, inspired creations from people as famous as Norman Rockwell.
There also are benefits that maybe we cannot anticipate ahead of time. The Search for Extraterrestrial Intelligence (SETI) is a network that advocates looking for life around the universe, likely because communicating with beings outside of Earth could bring us some benefit. And perhaps there is another space-related discovery just around the corner that will change our lives drastically.
What does our Solar System really look like? If we were to somehow fly ourselves above the plane where the Sun and the planets are, what would we see in the center of the Solar System? The answer took a while for astronomers to figure out, leading to a debate between what is known as the geocentric (Earth-centered) model and the heliocentric (Sun-centered model).
The ancients understood that there were certain bright points that would appear to move among the background stars. While who exactly discovered the “naked-eye” planets (the planets you can see without a telescope) is lost in antiquity, we do know that cultures all over the world spotted them.
The ancient Greeks, for example, considered the planets to include Mercury, Venus, Mars, Jupiter and Saturn — as well as the Moon and the Sun. The Earth was in the center of it all (geocentric), with these planets revolving around it. So important did this become in culture that the days of the week were named after the gods, represented by these seven moving points of light.
All the same, not every Greek believed that the Earth was in the middle. Aristarchus of Samos, according to NASA, was the first known person to say that the Sun was in the center of the universe. He proposed this in the third century BCE. The idea never really caught on, and lay dormant (as far as we can tell) for several centuries.
Earth is at the center of this model of the universe created by Bartolomeu Velho, a Portuguese cartographer, in 1568. Credit: NASA/Bibliothèque Nationale, Paris
Because European scholars relied on Greek sources for their education, for centuries most people followed the teachings of Aristotle and Ptolemy, according to the Galileo Project at Rice University. But there were some things that didn’t make sense. For example, Mars occasionally appeared to move backward with respect to the stars before moving forward again. Ptolemy and others explained this using a system called epicycles, which had the planets moving in little circles within their greater orbits.
But by the fifteen and sixteenth centuries, astronomers in Europe were facing other problems, the project added. Eclipse tables were becoming inaccurate, sailors needed to keep track of their position when sailing out of sight of land (which led to a new method to measure longitude, based partly on accurate timepieces), and the calendar dating from the time of Julius Caesar (44 BCE) no longer was accurate in describing the equinox — a problem for officials concerned with the timing of religious holidays, primarily Easter. (The timing problem was later solved by resetting the calendar and instituting more scientifically rigorous leap years.)
While two 15th-century astronomers (Georg Peurbach and Johannes Regiomontanus) had already consulted the Greek texts for scientific errors, the project continued, it was Nicolaus Copernicus who took that understanding and applied it to astronomy. His observations would revolutionize our thinking of the world.
Retrograde motion of Mars. Image credit: NASA
Published in 1543, Copernicus’ De Revolutionibus Orbium Coelestium (On the Revolutions of the Heavenly Bodies) outlined the heliocentric universe similar to what we know today. Among his ideas, according to Encyclopedia Britannica, was that the planets’ orbits should be plotted with respect to the “fixed point” Sun, that the Earth itself is a planet that turns on an axis, and that when the axis changes directions with respect to the stars, this causes the North Pole star to change over time (which is now known as the precession of the equinoxes.)
Putting the Sun at the center of our Solar System, other astronomers began to realize, simplified the orbits for the planets. And it helped explain what was so weird about Mars. The reason it backs up in the sky is the Earth has a smaller orbit than Mars. When Earth passes by Mars in its orbit, the planet appears to go backwards. Then when Earth finishes the pass, Mars appears to move forwards again.
Other supports for heliocentrism began to emerge as well. Johannes Kepler’s rules of motions of the planets (based on work from him and Tycho Brahe) are based on the heliocentric model. And in Isaac Newton’s Principia, the scientist described how the motions happen: a force called gravity, which appears to be “inversely proportional to the square of the distance between objects”, according to the University of Wisconsin-Madison.
Artist’s conception of the Kepler Space Telescope. Credit: NASA/JPL-Caltech
Newton’s gravity theory was later supplanted by that of Albert Einstein, who in the early 20th century proposed that gravity is instead a warping of space-time by massive objects. That said, heliocentric calculations guide spacecraft in their orbits today and the model is the best way to describe how the Sun, planets and other objects move.
An artists illustration of the central engine of a Quasar. These "Quasi-stellar Objects" QSOs are now recognized as the super massive black holes at the center of emerging galaxies in the early Universe. (Photo Credit: NASA)
Imagine matter packed so densely that nothing can escape. Not a moon, not a planet and not even light. That’s what black holes are — a spot where gravity’s pull is huge, ending up being dangerous for anything that accidentally strays by. But how did black holes come to be, and why are they important? Below we have 10 facts about black holes — just a few tidbits about these fascinating objects.
Fact 1: You can’t directly see a black hole.
Because a black hole is indeed “black” — no light can escape from it — it’s impossible for us to sense the hole directly through our instruments, no matter what kind of electromagnetic radiation you use (light, X-rays, whatever.) The key is to look at the hole’s effects on the nearby environment, points out NASA. Say a star happens to get too close to the black hole, for example. The black hole naturally pulls on the star and rips it to shreds. When the matter from the star begins to bleed toward the black hole, it gets faster, gets hotter and glows brightly in X-rays.
Fact 2: Look out! Our Milky Way likely has a black hole.
A natural next question is given how dangerous a black hole is, is Earth in any imminent danger of getting swallowed? The answer is no, astronomers say, although there is probably a huge supermassive black hole lurking in the middle of our galaxy. Luckily, we’re nowhere near this monster — we are about two-thirds of the way out from the center, relative to the rest of our galaxy — but we can certainly observe its effects from afar. For example: the European Space Agency says it’s four million times more massive than our Sun, and that it’s surrounded by surprisingly hot gas.
Sagittarius A in infrared (red and yellow, from the Hubble Space Telescope) and X-ray (blue, from the Chandra space telescope). Credit: X-ray: NASA/UMass/D.Wang et al., IR: NASA/STScI
Fact 3: Dying stars create stellar black holes.
Say you have a star that’s about 20 times more massive than the Sun. Our Sun is going to end its life quietly; when its nuclear fuel burns out, it’ll slowly fade into a white dwarf. That’s not the case for far more massive stars. When those monsters run out of fuel, gravity will overwhelm the natural pressure the star maintains to keep its shape stable. When the pressure from nuclear reactions collapses, according to the Space Telescope Science Institute, gravity violently overwhelms and collapses the core and other layers are flung into space. This is called a supernova. The remaining core collapses into a singularity — a spot of infinite density and almost no volume. That’s another name for a black hole.
Fact 4: Black holes come in a range of sizes.
There are at least three types of black holes, NASA says, ranging from relative squeakers to those that dominate a galaxy’s center. Primordial black holes are the smallest kinds, and range in size from one atom’s size to a mountain’s mass. Stellar black holes, the most common type, are up to 20 times more massive than our own Sun and are likely sprinkled in the dozens within the Milky Way. And then there are the gargantuan ones in the centers of galaxies, called “supermassive black holes.” They’re each more than one million times more massive than the Sun. How these beasts formed is still being examined.
A binary black hole system, viewed from above. Credit: Bohn et al. (see http://arxiv.org/abs/1410.7775)
Fact 5: Weird time stuff happens around black holes.
This is best illustrated by one person (call them Unlucky) falling into a black hole while another person (call them Lucky) watches. From Lucky’s perspective, Unlucky’s time clock appears to be ticking slower and slower. This is in accordance with Einstein’s theory of general relativity, which (simply put) says that time is affected by how fast you go, when you’re at extreme speeds close to light. The black hole warps time and space so much that Unlucky’s time appears to be running slower. From Unlucky’s perspective, however, their clock is running normally and Lucky’s is running fast.
Fact 6: The first black hole wasn’t discovered until X-ray astronomy was used.
Cygnus X-1 was first found during balloon flights in the 1960s, but wasn’t identified as a black hole for about another decade. According to NASA, the black hole is 10 times more massive to the Sun. Nearby is a blue supergiant star that is about 20 times more massive than the Sun, which is bleeding due to the black hole and creating X-ray emissions.
Illustration of Cygnus X-1, another stellar-mass black hole located 6070 ly away. Credit: NASA/CXC/M.Weiss
Fact 7: The nearest black hole is likely not 1,600 light-years away.
An erroneous measurement of V4641 Sagitarii led to a slew of news reports a few years back saying that the nearest black hole to Earth is astoundingly close, just 1,600 light-years away. Not close enough to be considered dangerous, but way closer than thought. Further research, however, shows that the black hole is likely further away than that. Looking at the rotation of its companion star, among other factors, yielded a 2014 result of more than 20,000 light years.
Fact 8: We aren’t sure if wormholes exist.
A popular science-fiction topic concerns what happens if somebody falls into a black hole. Some people believe these objects are a sort of wormhole to other parts of the Universe, making faster-than-light travel possible. But as this Smithsonian Magazine article points out, anything is possible since we still have a lot to figure out about physics. “Since we do not yet have a theory that reliably unifies general relativity with quantum mechanics, we do not know of the entire zoo of possible spacetime structures that could accommodate wormholes,” said Abi Loeb, who is with the Harvard-Smithsonian Center for Astrophysics.
Diagram of a wormhole, or theoretical shortcut path between two locations in the universe. Credit: Wikipedia
Fact 9: Black holes are only dangerous if you get too close.
Like creatures behind a cage, it’s okay to observe a black hole if you stay away from its event horizon — think of it like the gravitational field of a planet. This zone is the point of no return, when you’re too close for any hope of rescue. But you can safely observe the black hole from outside of this arena. By extension, this means it’s likely impossible for a black hole to swallow up everything in the Universe (barring some sort of major revision to physics or understanding of our Cosmos, of course.)
Fact 10: Black holes are used all the time in science fiction.
There are so many films and movies using black holes, for example, that it’s impossible to list them all. Interstellar‘s journeys through the universe includes a close-up look at a black hole. Event Horizon explores the phenomenon of artificial black holes — something that is also discussed in the Star Trek universe. Black holes are also talked about in Battlestar: Galactica, Stargate: SG1 and many, many other space shows.
I don’t want to freak you out, but you should be aware that there’s a gigantic galaxy with twice our mass headed right for us. Naw, I’m just kidding. I totally want to freak you out. The Andromeda galaxy is going to slam head first into the Milky Way like it doesn’t even have its eyes on the road.
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Transcript
I don’t want to freak you out, but you should be aware that there’s a gigantic galaxy with twice our mass headed right for us. Naw, I’m just kidding. I totally want to freak you out. The Andromeda galaxy is going to slam head first into the Milky Way like it doesn’t even have its eyes on the road.
This collision will tear the structure of our galaxy apart. The two galaxies will coalesce into a new, larger elliptical galaxy, and nothing will ever be the same again, including your insurance premiums. There’s absolutely nothing we can do about it. It’s like those “don’t text and drive commercials” where they stop time and people get out and have a conversation about their babies and make it clear that selfish murderous teenagers are really ruining everything for all of us all the time.
The Andromeda Galaxy will collide with the Milky Way in the future. Credit: Adam Evans
And now that we know disaster is inbound, all we can do is ask WHY? Why this is even happening? Isn’t the Universe expanding, with galaxies speeding away from us in all directions? Shouldn’t Andromeda be getting further away, and not closer? What the hay, man!
Here’s the thing, the vast majority of galaxies are travelling away from us at tremendous speed. This was the big discovery by Edwin Hubble in 1929. The further away a galaxy is, the faster it’s moving away from us. The most recent calculation by NASA in 2013 put this amount at 70.4 kilometers per second per megaparsec. At a billion light-years away, the expansion of the Universe is carrying galaxies away from us at 22,000 km/s, or about 7% of the speed of light. At 100 million light-years away, that speed is only 2,200 km/s.
Which actually doesn’t seem like all that much. Is that like Millenium Falcon fast or starship Enterprise Warp 10 fast? Andromeda is only 2.5 million light-years away. Which means that the expansion of the Universe is carrying it away at only 60 kilometers per second. This is clearly not fast enough for our purposes of not getting our living room stirred into the backyard pool. As the strength of gravity between the Milky Way and Andromeda is strong enough to overcome this expansive force. It’s like there’s an invisible gravity rope connecting the two galaxies together. Dragging us to our doom. Curse you, gravity doom rope!
The Hubble Space Telescope’s extreme close-up of M31, the Andromeda Galaxy. Picture released in January 2015. Credit: NASA, ESA, J. Dalcanton, B.F. Williams, and L.C. Johnson (University of Washington), the PHAT team, and R. Gendler
Andromeda is speeding towards us at 110 kilometers per second. Without the expansion of the Universe, I’m sure it would be faster and even more horrifying! It’s the same reason why the Solar System doesn’t get torn apart. The expansion rate of the Universe is infinitesimally small at a local level. It’s only when you reach hundreds of millions of light-years does the expansion take over from gravity.
You can imagine some sweet spot, where a galaxy is falling towards us exactly as fast as it’s being carried away by the expansion of the Universe. It would remain at roughly the same distance and then we can just be friends, and they don’t have to get all up in our biz. If Andromeda starts complaining about being friend-zoned, we’ll give them what-for and begin to re-evaluate our friendship with them, because seriously, no one has time for that.
The discovery of dark energy in 1998 has made this even more complicated. Not only is the Universe expanding, but the speed of expansion is accelerating. Eventually distant galaxies will be moving faster away from us than the speed of light. Only the local galaxies, tied together by gravity will remain visible in the sky, eventually all merging together. Everything else will fall over the cosmic horizon and be lost to us forever.
This annotated artist’s conception illustrates our current understanding of the structure of the Milky Way galaxy. Image Credit: NASA
All things in the Universe are speeding away from us, it’s just that gravity is a much stronger force at local levels. This is why the Solar System holds together, and why Andromeda is moving towards us and in about 4 billion years or so, the Andromeda galaxy is going to slam into the Milky Way.
So, if by chance you only watched the first part of this video, freaked out, sold your possessions and joined some crazy silver jumpsuit doomsday cult, and are now, years later watching the conclusion… you may feel a bit foolish. However, I hope that you at least made some lifelong friendships and got to keep the jumpsuit.
Really, there’s nothing to worry about. Stars are spread so far apart that individual stars won’t actually collide with each other. Even if humanity is still around in another 4 billion years or so, which is when this will all go down. This definitely isn’t something we’ll be concerned with. It’s just like climate change. Best of luck future generations!
What do you think, will humans still be around in 4 billion years to enjoy watching the spectacle of the Milky Way and Andromeda collide?
This annotated artist's conception illustrates our current understanding of the structure of the Milky Way galaxy. Image Credit: NASA
We have a lot of crazy informal names for space sights. Sometimes they’re named after how they are shaped, like the Horsehead Nebula. Sometimes they have a name “borrowed” from their constellation, such as the Andromeda Galaxy. But what about our own galaxy, the Milky Way? Why does this band of stars across Earth’s sky have a name associated with food?
First, let’s back up a bit and talk a bit about what the Milky Way actually is. Astronomers believe it is a barred spiral galaxy — a galaxy with a spiral shape that has a line of stars across its middle, as you can see in the picture above. If you were to fly across the galaxy at the speed of light, it would take you an astounding 100,000 years.
The Milky Way is part of a collection of galaxies called the Local Group. We’re on a collision course with the most massive and largest member of that collection, which is the Andromeda Galaxy (also known as M31). The Milky Way is the second-largest galaxy, and the Triangulum Galaxy (M33) the third-largest. There are roughly 30 members of this group all told.
To get a sense of its immense size, you’ll be glad to hear the Earth is nowhere near the Milky Way’s center and its powerful, supermassive black hole. NASA says we’re roughly 165 quadrillion miles from the black hole, which is found in the direction of the constellation Sagittarius.
The magnetic field of our Milky Way Galaxy as seen by ESA’s Planck satellite. Credit: ESA and the Planck Collaboration.
As for how our galaxy got its name, it is indeed because of its milky appearance as it stretches across the sky. While spotting the galaxy’s arms is a challenge from our current light-polluted centers, if you get out to a more rural area it really begins to dominate the skies. The ancient Romans called our galaxy the Via Lactea, which literally means “The Road of Milk.”
And according to the Astronomy Picture of the Day website, the Greek word for “galaxy” also derives from the word “milk”. It’s hard to say if it was a coincidence, because the origin of both the Milky Way’s name and the Greek word for galaxy are long lost to prehistory, although some sources say that it was inspired by the Milky Way’s appearance.
It took thousands of years for us to understand the nature of what we were looking at. Back in the time of Aristotle, according to the Library of Congress, the Milky Way was believed to be the spot “where the celestial spheres came into contact with the terrestrial spheres.” Without a telescope, it was hard to say much more, but that began to change in the early 1600s.
Beautiful view of our Milky Way Galaxy. If other alien civilizations are out there, can we find them? Credit: ESO/S. Guisard
One important early observation, the library adds, was from the noted astronomer Galileo Galilei. (He’s best known for being credited for the discovery of four of Jupiter’s moons — Io, Europa, Callisto and Ganymede — which he spotted through a telescope.) In his 1610 volume Sidereus Nuncius, Galileo said his observations showed the Milky Way was not a uniform band, but had certain pockets with more star densities.
But the true nature of the galaxy eluded us for some time yet. Other early observations: the stars were a part of our Solar System (Thomas Wainwright, 1750 — a claim that was later shown as erroneous) and that the stars appeared to be denser on one side of the band than the other (William and John Herschel, in the late 1700s).
It took until the 20th century for astronomers to figure out that the Milky Way is just one of a large number of galaxies in the sky. This came, the library says, through a few steps: doing observations of distant “spiral nebulas” that showed their speeds were receding faster than the escape velocity of our own galaxy (Vesto Slipher, 1912); observations that a “nova” (temporary bright star) in Andromeda was fainter than our own galaxy (Herber Curtis, 1917); and most famously, Edwin Hubble’s observations of galaxies showing that they were very far from Earth indeed (1920ish).
The Hubble Ultra Deep Field seen in ultraviolet, visible, and infrared light. Image Credit: NASA, ESA, H. Teplitz and M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (Arizona State University), and Z. Levay (STScI)
There are in fact more galaxies out there than we could have imagined even a century ago. Using the Hubble Space Telescope, periodically astronomers have used the powerful observatory to gaze at a tiny patch of the sky.
This has produced several “deep fields” of galaxies billions of light-years away. It’s hard to estimate just how many there are “out there”, but estimates seem to say there are at least 100 billion galaxies. That’ll keep astronomers busy observing for a while.
The summertime Milky Way from Scorpius to Cygnus is broader and brighter than the winter version because we look into the direction of its center. Credit: Stephen Bockhold
The Milky Way is our home galaxy, the spot where the Earth resides. We are not anywhere near the center — NASA says we’re roughly 165 quadrillion miles from the galaxy’s black hole, for example — which demonstrates just how darn big the galaxy is. So how big is it, and how does it measure up with other neighborhood residents?
The numbers are pretty astounding. NASA estimates the galaxy at 100,000 light-years across. Since one light year is about 9.5 x 1012km, so the diameter of the Milky Way galaxy is about 9.5 x 1017 km in diameter. The thickness of the galaxy ranges depending on how close you are to the center, but it’s tens of thousands of light-years across.
Our galaxy is part of a collection known as the Local Group. Because some of these galaxies are prominent in our sky, the names tend to be familiar. The Milky Way is on a collision course with the most massive member of the group, called M31 or the Andromeda Galaxy. The Milky Way is the second-largest member, with M33 (the Triangulum Galaxy) the third-largest, NASA says. Andromeda appears much brighter in the night sky due to its size and relatively closer distance. There are about 30 members of this group.
The Andromeda Galaxy will collide with the Milky Way in the future. Credit: Adam Evans
Because we are inside the Milky Way’s arms, it appears as a band of stars (or a fuzzy white band) across the Earth’s sky. Casting a pair of binoculars or a telescope across it shows a mix of lighter areas and darker areas; the darker areas are dust that obscures any light from stars, galaxies and other bright objects behind it. From the outside, however, astronomers say the Milky Way is a barred spiral galaxy — a galaxy that has a band of stars across its center as well as the spiral shape.
If you’re looking for the center of the galaxy, gaze at the constellation Sagittarius, which is low on the summer sky horizon for most northern hemisphere residents. The constellation contains a massive radio source known as Sagittarius A*. Astronomers using the Chandra space telescope discovered why this supermassive black hole is relatively weak in X-rays: it’s because hot gas is being pulled inside the nebula, and most of it (99%) gets ejected and diffused.
Sagittarius A in infrared (red and yellow, from the Hubble Space Telescope) and X-ray (blue, from the Chandra space telescope). Credit: X-ray: NASA/UMass/D.Wang et al., IR: NASA/STScI
Based on observing globular clusters (star clusters) in the galaxy, astronomers have estimated the Milky Way’s overall age at 13.5 billion years old — just 200 million years younger than the rest of the universe.
However, scientists are beginning to think that different parts of the galaxy formed at different times. In 2012, for example, astronomers led by Jason Kalirai of the Space Telescope Science Institute pinned down the age of the Milky Way’s inner halo of stars: 11.5 billion years old. They used white dwarfs, the burned-out remnants of Sun-like stars, to make that measurement.
Kalirai’s group’s research indicates that the Milky Way formed in the following sequence: the halo (including globular star clusters and dwarf galaxies), the inner halo (whose stars were born as a result of this construction) and the outer halo (created when the Milky Way ate up nearby ancient dwarf galaxies).
Artist’s impression of the structure of the Milky Way’s halo. Credit: NASA, ESA, and A. Feild (STScI)
While we’ve been focusing on the parts of the galaxy that you can see, in reality most of its mass is made up of dark matter. NASA estimates that there is about 10 times the mass of dark matter than the visible matter in the universe. (Dark matter is a form of matter that we cannot sense with conventional telescopic instruments, except through its gravitational effect on other things such as galaxies. When masses gather in high enough concentrations, they can bend the light of other objects.)
We have written many articles about the Milky Way for Universe Today. Here’s an article about the rotation of Milky Way, and here are some facts about the Milky Way. We’ve also recorded an episode of Astronomy Cast about galaxies. Listen here, Episode 97: Galaxies.
You’ve probably heard the trope about how aliens have been watching old episodes of “I Love Lucy” and might think these are our “historical documents”. How far have our signals reached?
Television transmissions expand outward from the Earth at the speed of light, and there’s a trope in science fiction that aliens have learned everything about humans by watching our television shows. If you’re 4 light-years away, you’re see the light from the Earth as it looked 4 years ago, and some of that light includes television transmissions, as radio waves are just another form of electromagnetism – it’s all just light.
Humans began serious television service in the 1930s, and by the modern era, there were thousands of powerful transmitters pumping out electromagnetic radiation for all to see. So are aliens watching “I Love Lucy” or footage from World War II and believing it all to be part of our “Historical Documents”?
The first radio broadcasts started in the early 1900s. At the time I’m recording this video, it’s late 2014, so those transmissions have escaped into space 114 years ago. This means our transmissions have reached a sphere of stars with a radius of 114 light-years.
Are there other stars in that volume of space? Absolutely. It’s estimated that there are more than 14,000 stars within 100 light years of Earth. Most of those are tiny red dwarf stars, but there would be hundreds of sunlike stars.
As we’re discovering, almost all of those stars will have planets, many of which will be Earthlike. It’s almost certain some of those stars will have planets in the habitable zone, and could have evolved life forms, technology and television sets and were able to learn of the Stealth Haze and the Mak’Tar chant of strength.
Will the signals be powerful enough to stretch across the vast distances of space and reach another world so that many generations of aliens can hang their hopes that James Tiberius Kirk never visits their planet with his loose morals, questionably applied prime directive, irresistible charms and pants aflame with who knows what kinds of interstellar STIs?
Here’s the problem. Broadcast towers transmit their signals outward in a sphere, which falls under the inverse square law. The strength of the signal decreases massively over distance. By the time you’ve gone a few light years, the signal is almost non-existent.
The Square Kilometer Array
Aliens could build a huge receiver, like the square kilometer array being built right now, but the signals they could receive from Earth would be a billion billion billion times weaker. Very hard to pick out from the background radiation. And by Grabthar’s hammer, I assure you it’s only by focusing our transmissions and beaming them straight at another star do we stand a chance of alerting aliens of our presence. Which, like it or not, is something we’ve done. So there’s that.
We’ve really been broadcasting our existence for hundreds of millions of years. The very presence of oxygen in the atmosphere of the Earth would tell any alien with a good enough telescope that there’s life here. Aliens could tell when we invented fire, when we developed steam technology, and what kinds of cars we like to drive, just by looking at our atmosphere. So don’t worry about our transmissions, the jig is up.
What do you think? Is it a good idea to alert aliens to our presence? Should we get rid of all that oxygen in our atmosphere and keep a low profile?
