"This is the way I wanna die. Torn apart by angry fans who want me to play a different song." -Regina Spektor
You're familiar with the classic picture of a black hole: a dark, dense region at the center from which no light can escape, surrounded by an accretion disk of matter that constantly feeds it, shooting off relativistic jets in either direction.
(Image credit: University of Warwick, retrieved from here.)
This is a pretty accurate picture of active black holes. But most black holes aren't active, and of the ones that are, they aren't active most of the time!
Most people think of black holes as marauders, gobbling up whatever poor stars happen to get in their way. You very likely have a picture of a black hole as though it behaves like a great cosmic vacuum cleaner, sucking up anything that dares get too close to it.
I can't fault you for thinking that; this is a genuine NASA video, and the picture that some very smart people have been painting for you for a long time. But that isn't quite how the Universe works.
So, how does it work? When any object falls in close to a black hole, it experiences different forces on different parts of the object. We call these forces tidal forces, because they're the same types of gravitational forces that cause the tides we experience here on Earth!
(Image credit: Barger and Olsson.)
Only, in the vicinity of a black hole, the tidal forces are much stronger than we experience on Earth. They are, in fact, much stronger than Jupiter's innermost moon, Io, experiences, and those forces are powerful enough to constantly tear Io apart, making it the only volcanically active moon in the Solar System!
No, when you get close to a black hole, you get stretched at either end so severely, and compressed in the middle so thinly, we call the process spaghettification, one of the greatest astrophysics words ever invented!
But "falling in" to a black hole, like illustrated above, practically never happens! Space is simply too big, and even for supermassive black holes -- like the multi-million-solar-mass behemoth at the Milky Way's center -- the event horizon is too small. Most stars and objects that pass nearby to a black hole simply do what all other objects in the Universe do.
Remember that space is huge, and that getting within a paltry 0.001 light years of our galaxy's supermassive black hole won't even disrupt the passing star, much less "vacuum it up," as you might have thought.
"But what if the star does get close enough," you ask, "then what happens?"
Note how, first, the star gets completely ripped apart by these intense tidal forces! But rather than acting like a vacuum cleaner and sucking it all up, most of the mass from this star doesn't get devoured at all; quite to the contrary, most of it gets ejected back out into the space around the black hole! It's only a small fraction of the original that gets swallowed, but that's totally sufficient to take a quiet, supermassive black hole, and bring it back to life!
And we know this, because we just observed a super distant galaxy -- more than 2 billion light years distant -- just become ultra bright thanks to its supermassive black hole sneaking a bite out of an unlucky passerby! Let's take a before-and-after look.
The above images, from GALEX (in the Ultraviolet) and Pan-STARRS (in the visible/IR), show this distant galaxy shortly before it started snacking on its newly accreted material. The images are low-resolution because GALEX and Pan-STARRS focus on grabbing very wide fields-of-view; when you're looking for very rare occurrences like this, you need to grab as much of the deep sky as possible!
So, that was 2009. But the next year...
The galaxy has brightened by a factor of around 350 in the Ultraviolet, and the visible/IR image has turned much bluer, an indication of the extraordinarily high energies being belched out by this suddenly noisy galaxy!
Taking a look at the before-and-after images together, you can really see the difference.
But don't be fooled by the vacuum cleaner description; it's not eating the entire thing that ran into it! This is, in fact, something that we may see happening for much smaller black holes that are much closer to us; the nearby galaxy Messier 83 just had a very similar outburst from a much smaller black hole!
Black holes aren't giant leviathans, devouring anything that comes nearby, but nor are they dainty, steady nibblers on objects that orbit. Rather, black holes are wild, violent and inevitable, tearing anything that dares approach too closely into shreds, but coming away with a snack-sized meal whose first bite makes quite an impression!
Now, if you'll excuse me, all this black hole talk has made me hungry! Where did I put the spaghetti...
"A journey is a person in itself; no two are alike. And all plans, safeguards, policing, and coercion are fruitless. We find that after years of struggle that we do not take a trip; a trip takes us." -John Steinbeck
Here on Earth, we all get to enjoy the delight of being located in an extremely fortuitous place in our Solar System. Not just today, mind you, but billions of years ago, when the Solar System's planets were first forming!
Located close enough to our Sun, when the Earth first formed, like our neighbors Mercury, Venus and Mars, we were chock full of heavy elements. Not just the carbons, nitrogens and oxygens that abound on all the known worlds, but important ones much higher up on the periodic table, including silicon, sulphur, iron, nickel, tin, lead, and even the radioactive uranium!
This might not sound so special to you, but our world would be a lot more boring if we had formed farther from the Sun. See for yourself:
Jupiter and Saturn aren't simply less dense than Earth is because they retained all that excess hydrogen that our wimpy gravitational field couldn't hold. Although that's true, they're also made out of elements that are intrinsically less dense!
We can identify where an object found anywhere in the Solar System -- including meteorites that fall to Earth -- simply by examining what they're made out of.
One of the most spectacular applications of this knowledge came when we first journeyed to the Moon. For the first time, we'd be able to analyze rocks from the Moon and their chemical composition! What we found was simultaneously profound and the most boring thing imaginable.
(Image credit: NASA / Apollo 11, retrieved from here.)
Because the Moon is made out of the same stuff that the Earth is!
Before you start saying, "Duhhhhh," as some of you are wont to do, let's remember that it didn't have to be this way. We need only look at our nearest moon-ed neighbor.
Because Mars' two moons, Phobos and Deimos, are not made out of the same stuff Mars is! They formed significantly farther out in the Solar System, originating in the asteroid belt. Only a chance encounter with another body (probably Jupiter) flung them in to the inner Solar System, where they were gravitationally captured by the red planet!
If it happens for Mars from the asteroid belt, you may be wondering about that other, even bigger belt in our Solar System, and what the possibilities might be from there.
I refer, of course, to the Kuiper Belt, the band of leftover proto-planetesimals from the formation of our Solar System. These small, icy objects are only about a third the density of Earth, but are more dense than the gas giants that lie interior to them.
If the asteroids could be flung towards the inner, rocky worlds due to the gravitational influence from Jupiter, it stands to reason that these Kuiper Belt objects could similarly be flung inwards thanks to Neptune.
(Image credit: source unknown, retrieved from clowder.net.)
There are some dead giveaways that your object isn't from the same part of the Solar System as its initial planet is. There's the elemental composition / density argument, of course, but simply via random chance, 50% of these objects that get captured will wind up revolving around their planet the wrong way. One obvious guy like this is Neptune's largest moon, Triton.
Triton is maybe the easiest one; he's so large that if he were still in the Kuiper belt, he'd be the largest object there, dwarfing (burn!) both Pluto and Eris!
But there are others, elsewhere, that don't quite look like they belong. And a Saturnian mystery may, in fact, be on the cusp of being solved thanks to this idea.
This is Iapetus, one of Saturn's moons, looking like it always does: like it came out on the wrong side of a trip through the mud. Iapetus does all the moon-like things correctly: it revolves the right way around Saturn, it's got the right density for its spot in the Solar System, its surface is even made of the same elements -- as far as we can tell -- as it ought to be.
Except for that muddy mess that discolors one of its face. What's going on here? It turns out Iapetus isn't alone.
(Illustration credit: AP / NASA, retrieved from here.)
A giant, diffuse outer ring, well beyond the Saturnian rings you're used to, pollutes Iapetus' orbit. As the tidally-locked moon speeds around Saturn, these grains from the ring smack into Iapetus, discoloring it like billions of bugs on a windshield.
The question, of course, is where did this ring come from? Because it isn't Saturn. The answer is much more fun than that!
Say hello to Phoebe, Saturn's very suspicious moon, located in the same vicinity as both this outermost ring and Iapetus. Phoebe is full of craters, a different color and elemental composition than the other moons, and -- this is a big and -- it revolves around Saturn the wrong way!
In other words, this outsider came all the way from the Kuiper Belt to become a moon of Saturn! And the journey was no picnic for Phoebe, either.
Those craters on its surface are from a lifetime of bombardment! The once-spherical Phoebe has lost a lot of its mass, and that's almost certainly where the material that makes up both the outermost ring and the diffuse discoloring of Iapetus comes from!
Let this be a lesson to all of you: if you want to be adopted by another planet, make sure you orbit the same direction as everything else! In the meantime, know that objects from the asteroid belt or Kuiper Belt could come in at any time, and could even become our planet's next additional moon!
"They will see us waving from such great heights
'Come down now,' they'll say.
But everything looks perfect from far away
'Come down now,' but we'll stay." -The Postal Service
Whether you're under urban, city skies, where only a few dozen stars are visible on a clear night, or beneath some of the darkest skies on Earth, the Universe is out there, and you can get started discovering it, right now, for yourself. You can have yourself, as Bishop Allen would sing,
When the Moon sets and the neighbors turn off all their lights, I can get skies that are darker than purely urban skies: maybe all the way down to (lower is better) a seven on the Bortle dark-sky scale.
But even with only the most major constellations and asterisms visible, there's still plenty to see, even in the city, if you're interested in exploring the night sky. Darker skies will only net you more (and fainter, dimmer, and more diffuse) objects, of course, so you'll want equipment that will be versatile. Normally, people just starting to take an interest in it have a large number of things they want, some of which they don't even realize they want. What are they?
You want something relatively inexpensive, because no one wants to blow a lot of money on something you're not even sure you'll like.
You want something that's quick easy to set up and take down, so that you'll use it often, even late at night, when you're tired and/or unmotivated.
You want something where the optics are high-quality, because you want to see out into the Universe, not a scratchy, blurry haze.
You want something with a lot of light-gathering power, capable of seeing as much as you can for the skies that you have.
And, let's face it, you want something that's durable, because you never take as good care of your stuff as you wish you did.
So, what would I recommend to a beginning skywatcher? I gave some general advice once before, but let me share with you the best astronomy present I've ever, personally, bought for myself.
This is my preferred tool for checking out the night sky, no matter where I am: a pair of Celestron Skymaster, 20x80mm binoculars. These are versatile -- both for people who do and don't wear glasses -- and they meet all of the criteria above.
They're cheap, costing only about $100 if you shop around.
You'll also need a (not included) tripod. The crummy $40 one I bought at Circuit City five years ago is just fine for this, but this is really the upper limit (in weight) that a cheap tripod like this can handle.
Some lying liars will tell you that you can do just fine without them, but they're no longer with us because they've burned to a crisp from the fires that engulfed them, originating in their pants. Why do you think it has a built-in tripod mount?!
The 80mm number in that title refers to the diameter of each light-gathering lens, which means these binoculars (when you use both eyes) have the equivalent light gathering power to a 4.5" telescope!
The optics are multi-coated and the telescope is water/weather resistant, which is very good, but not quite the best. If you want to pay a lot more, you can go for fully-coated optics, and/or a waterproof pair, but for this price point you're not going to find better.
Finally, setup takes only about 10 minutes, including focusing. The lone exception is that for some pairs (mine wasn't one of them), collimation is a problem, which requires you to either fix them yourself (with an eyeglasses screwdriver), send them in to Celestron, or take them into a camera shop. If you do have this problem, it's a one-time-only fix.
I like these binoculars much better than a telescope for quick setup and takedown, and they really shine -- especially for beginning skywatchers -- for a few reasons. First off, they have a much wider field of view, meaning that when you see an object in the sky, it's much easier for a novice to locate it through binoculars than through a telescope. Second, it's much easier to get to know your night sky, to just move around and explore. Poke around your favorite constellations and bright stars and see what's there. You can see roughly 30-100 times as many stars through a pair of binoculars like this than you can with your naked eye, no matter what the viewing conditions are. The deep-sky objects, ideal for viewing under skies with less light pollution, can be found with these binoculars, too. (The free, downloadable software Stellarium is invaluable for finding what is where, and when.)
And finally, the 20x magnification (what that first number means in binocular-speak) is enough to see some amazing things! The last time I took them out was Sunday night, which was unseasonably warm and clear. Yes, I got to see the color of some of my favorite stars, as well as the pink/red disk of Mars. But there were a few highlights that I wanted to share.
Saturn, bright and yellow in the night sky, has rings that are actually distinctly visible through these binoculars! I wasn't entirely sure I'd be able to see them, but when I had it centered in the field of view and truly focused the binoculars properly, the nearly edge-on rings came into crisp view, and it was my first time seeing them with my own, personal equipment!
As far as stars go, whenever the Big Dipper is prominent, you owe it to yourself to check out the second star from the end of the handle: Mizar. It's not only a binary star system, with the bright star Alcor also present, but there are many other, fainter stars there as well! Even with my lousy, fairly urban skies, I was very clearly able to see the third brightest star in there, which marked only the second time in my life I'd ever seen it, and again, the first time I'd found it on my own.
And last of all, there's the brightest object that isn't named the Moon in our current night sky.
Venus! Currently in its crescent phase, you'll be able to watch Venus' crescent progressively shrink and shrink over the coming 5 weeks, until in early June it actually transits in front of the Sun, for the last time until 2117!
So if you've got an interest in the night sky, but not a lot of time, money, and not even necessarily good skies, there are still some amazing sights just waiting for you. The question is what are you waiting for; the Universe is yours to explore!
"And what I wanted to do was, I wanted to explore problems and areas where we didn't have answers. In fact, where we didn't even know the right questions to ask." -Donald Johanson
You can learn an awful lot about the Universe by asking it different questions than you asked about it previously. If all you ever used were your own senses, there would be an awful lot to learn, but you would be severely limited.
Even from the highest mountaintops, for example, you'd never be able to distinguish whether the Earth was round like a sphere or flat as a pancake, if all you used were your eyes. But by looking at the Earth on a larger scale than you could achieve otherwise, its roundness becomes both apparent and indisputable.
The same thing applies to the Universe, both on large scales and small. If you want to know what the overall structure is of the Universe, you have to look at it on the largest scales. Looking at individual galaxies or even large clusters of galaxies won't get you there at all; if you want to know what your Universe looks like, you need to look at it on the largest and grandest of all scales, spanning billions of light years in all directions.
In the above image, still showing just a fraction of the Universe scanned and measured by the Sloan Digital Sky Survey, each pixel represents an entire galaxy. By measuring how galaxies cluster and clump together -- how they are distributed throughout the Universe -- we can determine what it takes to create a Universe that looks like ours.
What we learn, as you can go through in detail, is that the structure of the Universe requires that there be a type of matter in it that does not collide with either normal matter or with photons, that outnumbers our (normal) matter by a factor of five- or six-to-one, that don't respond to either electric or magnetic fields, and that... frustratingly, can not be any of the known particles in the Universe!
This would be a very, very big problem under one condition:
If the known particles and laws of physics explained all of the observed phenomena in the Universe.
In other words, if there's no new physics out there (beyond the standard model), then there's no need for any new particles out there, and so, why would there be any dark matter? There simply wouldn't be a strong motivation, not from an elementary physics standpoint.
And yet the opposite of that is also true: if there is physics out there that isn't explained by the standard model, then there must be new types of particles out there! And if there are new particles out there, there are good candidates for this dark matter. You've probably heard of some of the speculations that abound:
For example, if there's a symmetry of nature known as Supersymmetry (or SUSY, for short), then there ought to be twice as many fundamental particles as the ones we currently know about. Moreover, the lightest one is a perfect candidate for dark matter! Until we know what this particle's properties are, however, we don't know exactly what predictions to make as far as particle-particle interactions go.
While dark matter may or may not be supersymmetric in nature (many argue that SUSY may not even exist), this last part -- that until we know what dark matter's particle properties are, we don't know what predictions to make for dark matter's interactions -- is generally true. But there are plenty of other ideas. Two more speculative ones, first, and then two definitive ones.
The electromagnetic, weak, and strong nuclear forces could all unify at some high energy, in what's called a Grand Unified Theory, or GUT. One of the universal consequences of GUTs is that they all predict that protons will decay, and so that's one of the things we look for. In many variants of GUTs, there are candidates for dark matter that emerge naturally.
Same case for extra dimensions; they may or may not exist, but if they do, then there are plenty of new particles and interactions that certainly exist, and one (or some) of them may make excellent dark matter candidates.
But those -- supersymmetry, grand unification, and extra dimensions -- are speculative ideas, and may not describe our Universe. But there are two observations that we have already made in the Standard Model that already cannot be explained by the particles and interactions we know today. This means there are new particles out there, yet undiscovered, that could easily solve the dark matter problem.
For one, neutrinos have mass! According to the Standard Model, there should only be one type of neutrino -- a left-handed one -- and they should have zero mass. But this is not the case!
They are observed to have non-zero mass. In fact, all three types of neutrinos have non-zero mass, meaning there is new physics and there are new particles out there! Right-handed (or sterile) neutrinos could very easily make up the dark matter; we are searching for them as you read this! But perhaps the new physics that explains neutrinos isn't also what explains dark matter. There's another problem.
(Image credit: Normal Rockwell, retrieved from here.)
There are a couple of fundamental symmetries of nature that, at least in everyday life, seem pretty obvious. One is that the laws of physics in a mirror -- where left and right are reversed -- are the same as our normal laws of physics. (We call that Parity, or P-symmetry.) Another is that matter and anti-matter obey the same laws of physics. (We call that Charge Conjugation, or C-symmetry.) Most laws of physics that you know, like gravity and electromagnetism, always obey these symmetries.
According to the standard model, they have to; it's coded into the physics. But these symmetries don't exist for the nuclear (weak and strong) forces in the standard model. If I took something like a muon, reflected it in the mirror (applying P-symmetry), and replaced that image with an anti-muon (applying C-symmetry), I'd be testing whether the combination of CP-symmetry was a good one or not.
If it were a good symmetry, then if all the muons decayed with one orientation, all the anti-muons would decay with that specific, mirrored orientation. But they don't, and so that CP-symmetry is violated. This is good for the Universe, because CP-violation is one of the necessary things to make more matter than anti-matter. But if it happens for an interaction like this -- the Weak nuclear interaction -- then it stands to reason that it should also happen for the strong nuclear force.
The same reason this unicycle toy doesn't tip over: there must be some sort of extra, hidden weight that provides extra balance, or in the particle's case, crushes the amount of strong CP violation. Theoretically, the standard model allows you to violate both C and P together here, but we've looked, and to something like one part in a billion, we don't see any. So something -- and this means there's new physics -- has got to be forbidding it!
There's definitely physics in this world that's beyond the standard model, there's definitely more to neutrinos than we know, and there's definitely something stopping CP violation from occurring in the strong interactions. There may also be extra dimensions, grand unification, supersymmetry, or something even more exotic or surprising. But all of these possibilities require new particles, many of which make good dark matter candidates, and all of which have unknown particle parameters.
When you combine this information with our astrophysical knowledge of dark matter, you can see why I prefer the approach of using the astrophysics to try and reconstruct/determine some of the particle properties of dark matter, and try to guide us as to what we should look for. (No, really, I sometimes research that!)
We've got lots of options and lots of searches going, but there's so much we don't know about it at this point! Cross-sections, masses, reaction rates, lifetimes, etc., they're all mysteries at this point. We may not know what dark matter is, exactly, but we've got lots of strong possibilities for what it could be, and some hints that simply can't be ignored. We're desperately trying to be able to detect it directly, and solve this mystery once and for all. Welcome to the cutting edge!
"Through that last dark cloud is a dying star... And when it explodes, it will be reborn. You will bloom... and I will live." -The Fountain
I want to start off by letting you all know that I, myself, do not have any children of my own. I have taught children, adolescents and adults for nearly a full generation now in varying capacities, and while each learner is different, there's one science fact that universally seems to shatter each and every one of them.
The fact that the Sun, our Sun, the bringer of warmth, light, energy, and the sustaining force of all life on this planet, isn't going to shine forever. Quite to the contrary, someday, the Sun will die in a fiery, catastrophic explosion, one which will quite possibly obliterate our entire planet, and then eventually cease to shine at all.
Being faced with not only our own mortality, but the demise of literally everything we've ever encountered throughout the entire history of our world is a philosophical and existential challenge for even well-adjusted adults. But I was a bit taken aback when I received this question from one of my most loyal readers:
I need a good explanation for a third grader, whose Mom tells me is deeply concerned, that the sun will blow up.
I sympathize with parents in this position, because on the one hand, you want to tell the truth to your children. You want to expose them to our most accurate understanding of reality, to have them learn and appreciate knowledge, science, and using their minds.
But you not only want to do that with kindness, compassion, and optimism, you also don't want your kid having night terrors and years of therapy because you told them the gory details of, literally, the end of the world.
There is, perhaps, a wrong way to go about this. As the comedian Louis C.K. once said,
She started crying immediately, crying bitter tears for the death of all humanity... and now she knows all of those things: she's gonna die, everybody she knows is gonna die, they're gonna be dead for a very long time, and then the sun's gonna explode. She learned that all in 12 seconds, at the age of seven.
That's one approach, but maybe not the one I would choose if I were going to put some thought into it. You see, there's a remarkable story to be told, and if I were in elementary school, it just might be the most wonderful thing I had ever heard at that point in my life. Here's what I would tell a child.
The Sun that you know, the brightest thing in the sky, is no more special than any other star in the sky. Even during the day, there are thousands of stars in the sky. You'd be able to see them, too, except that our star, the Sun, is so close to us that its brightness makes all the other stars invisible, except at night.
These stars, each and every one of them, live much, much longer than anything on Earth has ever lived. While some plants and animals can live for thousands of years, the stars all live, burning brightly, for millions, billions, or even trillions of years.
That's a very, very long time! But it isn't forever, and believe it or not, we're lucky that it isn't forever.
Because if the stars never died, if they never exploded, and if they never blew up, we wouldn't be here, talking to each other, right now. And I'm so glad that we are, because you get to learn one of the most amazing secrets about life, and I get to teach it to you.
The secret is that practically everything that makes up you, me, and the entire planet -- the tiniest parts of everything we've ever known -- they were all made inside a star.
But it's too hot for you and me inside a star. In order to make Earth, and you, and me, all the good things that the stars make need to get out, so they can make something new. And how does that happen?
The old insides from those stars make planets, like Earth, and -- because we're very lucky -- some of those insides make up us, too.
The stars of the past died so that you could be here, and someday, a long time from now, our Sun will return the favor, and help make more new planets, new worlds, and new chances for life.
So yes, the Sun will blow up, someday, but when it does, that's the greatest gift any star can ever hope to give to the Universe. After all, it took billions of stars giving that gift already in order to make you. And you know what?
"Some painters transform the Sun into a yellow spot, others transform a yellow spot into the Sun." -Pablo Picasso
But the Sun will not always be a bright spot. Though it has shone for billions of years already, and will continue to shine for billions of years more, it's currently doing this by burning its hydrogen fuel -- through the fires of nuclear fusion -- into the heavier element of helium.
But after about 10-12 billion years total, all the hydrogen that can be burned in the core will be used up! (There will still be hydrogen in the outer layers; a star our size is not fully convective.) At this point, the core's temperature will increase until the heavier elements can be burned: helium into carbon, nitrogen, and oxygen, perhaps even then into neon. The Sun's outer layers will expand, as our star becomes a red giant.
To get to this point takes many billions of years, and is the fate of all stars between about 40% and 800% the mass of our Sun. But when that internal fuel is exhausted -- when the burn-able helium is used up -- what comes next? If the star were more massive, the core could collapse and we'd get a supernova, but our Sun, like most stars, is nowhere close to that.
The next step, and this only takes about 10,000 years, a blink-of-an-eye in the lifetime of a star, starts with the red giant ejecting the outer layers of the giant hydrogen envelope.
Meanwhile, on the surface of the star's core, a shell of hydrogen continues to burn, one of the last stages of fusion in a star like this. As the outer hydrogen layers continue to burn off and undergo ejection (through a process known as mass loss), the interior of the star continues to heat up.
As this process goes on, high-velocity winds are produced (like, over 100,000 miles-per-hour), shocking and shaping the gas. When it's all done, we get a planetary nebula from the outer layers, and a contracted white dwarf star at the center.
But I skipped the most interesting part! Before you get a white dwarf, before you get the hot, ionized planetary nebula, you go through this process of blowing off these outer layers with high-velocity winds. You wind up with dusty regions where the star is not yet hot enough to ionize the gas, collimated jets which can shine through the dust, and periodic, ring-like layers from the outward-moving gas.
With just 10,000 years, wouldn't it be something to catch one of these stars in the act?
What you're seeing is known as a preplanetary nebula! (Or, a protoplanetary nebula, for those of you who promise not to get it confused with a proto-planetary disk, which forms at the beginning of a star's life, not the end!) This was the Hubble Space Telescope's first view of the Egg Nebula, the most striking example of a pre-planetary nebula I've ever seen, and the first one ever discovered, less than 40 years ago.
The dense cocoon of dust shrouds the central star and hides it from our view: the star is not yet hot enough to ionize all that gas. However, this "cocoon" is asymmetrical, and in regions where the dust is thinner, light escapes, producing this four-beacon effect. We can learn even more about it by looking in the infrared, which measures warm gas.
While the hottest part is in blue, the most interesting part of the above image is the red signature, which indicates hydrogen gas! Because we can't see the star, we know that the hydrogen isn't fully ionized; it can block the light. But it is warm, and as the star continues to heat up towards the magic number of 30,000 K, eventually the light will be of high enough (UV) energies to ionize that gas and reveal the star inside.
One of the things that's interesting is that the light coming from the Egg Nebula is polarized, as this 2003 false-color image, sensitive to different polarizations, shows.
Different orientations of reflected starlight produce different results through a polarizing filter, and hence the difference in color shown in this image. The white areas are indicative of very dusty areas, where the light gets reflected/scattered many times, and hence doesn't come out with a single, mostly uniform polarization.
But that was then. Released earlier today is this latest, composite image taken with Hubble's WFC3 camera. The view is spectacular, and really gives you an appreciation for just how monstrous this dark, dusty disc around the dying star is.
The two twin beacons shining out look like four floodlights in the fog, and for good reason: these are four weak spots in an extremely dusty region of space that allow the starlight to escape! The region is so obscured that we don't even know whether there's one giant star on its own, or whether it has a binary companion in there with it. Have a look at the full-scale image of the Egg Nebula, some 3,000 light years away.
But don't forget that Hubble's an amazing thing. Even though there aren't many preplanetary nebulae, you might wonder if we've ever caught one where we've had the good fortune to look down perpendicular to the dusty disk, instead of at some unfortunate angle. Well, have I unearthed a find for you. Say hello to IRAS 23166+1655 around the star LL Pegasi, and its magnificent, dusty spiral.
Take a closer look in there; that's not a series of concentric spheres/circles, those are really spirals!
With an 800-year period estimated for this binary system, you can literally count the age of the preplanetary nebula by counting the rings in the spiral structure! In a few thousand years, all of this dusty structure will be gone, and all we'll be left with is a plain, ionized planetary nebula, with a white dwarf at the center. But for right now, you're looking at the last living stages of what was once a Sun-like star, ending its Red Giant phase and becoming a planetary nebula.
Welcome to your Universe, where it provides you with snapshots of the transition in action!
"Our planet is a lonely speck in the great enveloping cosmic dark. In our obscurity - in all this vastness - there is no hint that help will come from elsewhere to save us from ourselves. It is up to us." -Carl Sagan
Here on our planet, this is the one day that we take out of the year to stop and appreciate just how amazing the natural world really is, and how fortunate we are to have the Earth that we have. A wonderful but sad reminder of how fragile it is, and how quickly and easily we can affect it, comes through John Prine's great song,
Here on our planet, there are countless ways to celebrate what we have. But what if you weren't here on Earth; what if you were a distant space traveler, headed towards our Solar System for the first time?
You'd see a rather unspectacular, whitish star. It would appear bright only because you were close to it. Even from our nearest star, Alpha Centauri, the Sun would only be the 5th brightest star in the night sky. If you knew the proper techniques, you could tell that there were gas giant planets around it, and -- if your tools were excellent -- some smaller, inner rocky worlds, too.
But unless you journeyed into the Oort Cloud, and then in past the Kuiper Belt, only at this point would it be easy for you to see Earth.
(Image credit: NASA / JPL.)
And even then, from 6 billion kilometers (4 billion miles) away, we'd appear as no more than a speck of dirt flying through the interplanetary dust.
But as you came in closer, you'd be able to see more and better details. That is, of course, if you knew to look for Earth. There are plenty of tempting distractions.
From distant Saturn, our world is visible, seen here poking out from in between the rings. A close-up view shows that you can not only see that our planet is round in shape, but has a large moon, visible off of our upper-left limb.
If you came in all the way to the closest planet to the Sun, Mercury, your view would be different, but even more spectacular.
Our Earth, the largest disk in the photo, appears to be in either a full or nearly-full phase at all times from Mercury's vantage point, along with our Moon. Slightly more distant would be our view of Earth from Mars, which we were lucky enough to capture back in 2003 for the very first time, and in color to boot!
Seen from an outer planet, the Earth will run through all the phases, from new to crescent to gibbous to full. Our blue color, caused by our atmosphere and our oceans, surrounds patches of green and brown, where our continents poke out above the watery surface. Overlaid over all of it is the white clouds, which paint a transient covering above our world.
This is much more apparent, of course, the closer you get. When the Voyager 1 spacecraft was first leaving Earth on its journey, it snapped this far superior view than the one we can get from Mars.
In fact, Voyager 1 and 2, in 1977, became the first spacecrafts to ever photograph the full Earth and Moon in the same picture.
But it's nothing compared to the first human view of the entire Earth, seen in December of 1968 by the Apollo 8 astronauts. As they emerged from behind the night side of the Moon -- the first humans ever to do so -- this was the sight that greeted them.
This very hi-res image (click for it), taken just a few months after the BP Oil Spill, is perhaps a great example of how invisible the damage we can do to the Earth is. From even our best views from space, the Earth appears nothing if not pristine and magnificent, as this cropped section of the Gulf of Mexico shows.
So enjoy your Earth Day today in whatever way you choose. Remember that it's the only home we have, and that it's our job to take care of it, to clean up after ourselves, and to leave it in better shape than we found it. No matter where in the Universe you are, make the most of it!
"April is the cruellest month, breeding
Lilacs out of the dead land, mixing
Memory and desire, stirring
Dull roots with spring rain." -T. S. Eliot
Of course you all know the refrain, "April showers bring May flowers," but there's one April shower that brings fireballs instead: the Lyrids!
(Image credit: John Chumack, retrieved from Bob King.)
Like all meteor showers, the Lyrids come from a comet's dust trail that forms a great ellipse with respect to our Solar System. Once per year, the Earth, in its orbit around the Sun, passes through this dusty debris. When this happens, the Earth, moving at over 10,000 miles-per-hour with respect to these dust grains, cause the dust to vaporize in a fiery plunge as they collide with our atmosphere.
(Image credit: Gehrz et al. 2006, retrieved from Michael Kelley.)
Every year, right around April 22nd, this meteor shower peaks, delivering anywhere from 10 to 100 meteors per hour visible beneath dark skies.
"Why is there so much variability," you ask? Well...
(Image credit: ESA. Courtesy of MPAe, Lindau, retrieved from space.com.)
These dusty debris trails that fill our Solar System all come from comets. When a comet passes close enough to the Sun, it spews off gas, dust and ions, leaving an elliptical trail in roughly the same orbit as the comet itself. But comets spend most of their time far away from the Sun, so that these big bursts of debris only get emitted at rare intervals.
Are we going to pass through one of these big bursts, or are we going to spend this year passing through a lull in the debris trail? Like every year, we never know until we get there. The Lyrid meteor shower is a particularly tough one to predict, because the comet responsible for it, Comet Thatcher (after the 19th Century astronomer, not the 20th Century Iron Lady), is not only perpendicular to the plane of our Solar System, it also has a disturbingly long period of 415 years, meaning it won't be back until 2276!
But the comet isn't the interesting thing: the dusty debris which brings us the meteor shower is! The peak of the meteor shower, which is the best time to observe it, should occur close to midnight in North America during the night of April 21st / morning of April 22nd, making it the perfect way to usher in Earth Day!
No matter where on Earth you are, here's where you want to look.
The bright star Vega, the fifth brightest star in the night sky (and #2 in the Northern Hemisphere), should be high enough in the sky by 10 PM to easily identify it. (Vega features prominently in my summer sky tutorial, here.)
There's a small (but prominent; visible even in most cities) parallelogram nearby, just slightly closer to the horizon. Combined with Vega, that's how you can easily identify the constellation Lyra. The meteor shower should originate just a few degrees away, to the upper right of the parallelogram. But don't look directly at that spot, and don't use a telescope! The meteors originate from there, but you'll see them streaking away from that point, in random directions!
(Image credit: Wally Pacholka from the 2001 Leonids, retrieved here.)
Over the course of an hour, you should see anywhere from 10 to 100 meteors, depending on how good this year's Lyrids are. You'll have the added bonus of a moonless sky; the waxing crescent will be so minuscule that it will have completely set by time the constellation Lyra is visible. The meteors themselves are worth the price of admission, but every once in a while, the Lyrids give the gift of a true fireball: a meteor so bright it outshines the entire sky combined, lasts for a few seconds, and can even cast prominent shadows.
(Image credit: Baltimore Sun, retrieved from here.)
No promises, of course, as it's impossible to predict these things, but if I've got clear skies, you can bet I won't miss the opportunity to spend some time enjoying the wonders of the night!
For those of you who want real-time updates on the Lyrid meteor shower, including reports from around the world as they come in, well, what's the point of running the best Science news service on the web if you can't make that happen? So follow the Meteor Showers & Comets trap and stay on top of it. However you do it, make sure you enjoy the show Saturday night and into Sunday morning, and know that my eyes and millions of others will be gazing upwards with you!
"Science progresses best when observations force us to alter our preconceptions." -Vera Rubin
I want you to think about the Universe. The whole thing; about everything that physically exists, both visible and invisible, about the laws of nature that they obey, and about your place in it.
It's a daunting, terrifying, and simultaneously beautiful and wondrous thing, isn't it?
After all, we spend our entire lives on one rocky world, that's just one of many planets orbiting our Sun, which is just one star among hundreds of billions in our Milky Way galaxy, which is just one galaxy among hundreds of billions that make up our observable Universe.
Yes, we've learned an awful lot about what's out there and our place in it. As best as we can tell, we've learned what the fundamental laws are that govern everything in it, too!
(Image credit: Mark Garlick / SPL, retrieved from the BBC.)
As far as gravitation goes, Einstein's theory of general relativity explains everything from how matter and energy bend starlight to why clocks run slow in strong gravitational fields to how the Universe expands as it ages. It is arguably the most well-tested and vetted scientific theory of all time, and every single one of its predictions that has ever been precision-tested has been verified to be spot-on.
On the other hand, we've got the standard model of elementary particles and interactions, which explains everything known to exist in the Universe, and all the other (nuclear and electromagnetic) forces that they experience. This, also, is arguably the most well-tested and vetted scientific theory of all time.
And you would think that if our understanding of things were perfect, if we knew all about the structure of the Universe, the matter in it, and the laws of physics that it obeyed, we'd be able to explain everything. Why? Because all you'd have to do is start out with some set of initial conditions -- immediately following the Big Bang -- for all the particles in the Universe, apply those laws of nature that we know, and see what it turns into over time! It's a hard problem, but in theory, it should be not only possible to simulate, it should give us a sample Universe that looks just like the one we have today.
But this doesn't happen. In fact, this doesn't happen at all. This picture I painted for you above is all true, on the one hand, but we also know that it isn't the whole story. There are other things going on that we don't fully understand.
Here, as best as I can present the full history in a single blog post, is the whole story.
As we come forward from the event of the Big Bang, our Universe expands, cools, while the entire time experiencing the irresistible force of gravity. Over time, a number of extremely important events happen, including, in chronological order:
the formation of the first atomic nuclei,
the formation of the first neutral atoms,
the formation of stars, galaxies, clusters, and large-scale structure,
and how the Universe expands over its entire history.
If we know what's fundamentally in the Universe and the physical laws that everything obeys, we'll arrive at quantitative predictions for all of these things, including:
what nuclei form and when in the early Universe,
what the radiation from the last-scattering-surface, when the first neutral atoms are formed, looks like in great detail,
what the structure of the Universe, from large scales down to small scales, looks like both today and at any moment in the Universe's past,
and how the scale, size, and number of objects in the observable Universe have evolved over its history.
We have made observations measuring all of these things, quantitatively, extremely well. Here's what we've learned.
What we consider to be normal matter, that is, stuff made up of atoms, is highly constrained by a variety of measurements. Before any stars formed, the nuclear furnace of the very early Universe fused the first protons and neutrons together in very specific ratios, depending on how much matter and how many photons there were at the time.
What our measurements tell us, and they've been verified directly, is exactly how much normal matter there is in the Universe. This number is incredibly tightly constrained to be -- in terms that might be familiar to you -- about 0.262 protons + neutrons per cubic meter. There could be 0.28, or 0.24, or some other number in that range, but there really couldn't be more or less than that; our observations are too solid.
After that, the Universe continues to expand and cool, until eventually the photons in the Universe -- which outnumber the nuclei by more than a billion-to-one -- lose enough energy that neutral atoms can form without immediately being blasted apart.
When these neutral atoms finally form, the photons are free to travel, uninhibited, in whatever direction they happened to be moving last. Billions of years later, that leftover glow from the Big Bang -- those photons -- are still around, but they've continued to cool, and are now in the microwave portion of the electromagnetic spectrum. First observed in the 1960s, we've now not only measured this Cosmic Microwave Background, we've measured the tiny temperature fluctuations -- microKelvin-scale fluctuations -- that exist in it.
These temperature fluctuations, and the magnitudes, correlations and scales on which they appear, can give us an incredible amount of information about the Universe. In particular, one of the things they can tell us is what the ratio of total matter in the Universe is to the ratio of normal matter. We would see a very particular pattern if that number were 100%, and the pattern we do see looks nothing like that.
The necessary ratio is about 5:1, meaning that only about 20% of the matter in the Universe can be normal matter. This doesn't tell us anything what this other 80% is. From the Cosmic Microwave Background alone, we only know that it exerts a gravitational influence like normal matter, but it doesn't interact with electromagnetic radiation (photons) like normal matter does.
You can also imagine that we've got something wrong about the laws of gravity; that there's some modification we can make to it to mimic this effect that we can re-create by putting in dark matter. We don't know what sort of modification could do that (we haven't successfully found one, yet), but it is conceivable that we've just got the laws of gravity wrong. If a modified theory of gravity could explain the fluctuations of in the Microwave Background without any dark matter at all, that would be incredibly interesting.
But if there really is dark matter, it could be something light, like a neutrino, or something very heavy, like a theorized WIMP. It could be something fast-moving, with a lot of kinetic energy, or it could be something slow-moving, with practically none. We just know that all of the matter can't be the normal stuff we're used to, and that we've come to expect. But we can learn more about it by simulating how structure -- stars, galaxies, clusters, and large-scale structure -- forms in the Universe.
Because the types of structures you get out -- including what types of galaxies, clusters, gas clouds, etc. -- exist at all times in the Universe's history. These differences don't show up in the Cosmic Microwave Background, but they do show up in the structures that form in the Universe.
What we do is take a look at the galaxies that form in the Universe and see how they cluster together: how far away from a galaxy do I have to look before I see a second galaxy? How early in the Universe do large galaxies and clusters form? How quickly do the first stars and galaxies form? And what can we learn about the matter in the Universe from this?
Because if the dark matter -- which doesn't interact with light or normal matter -- has lots of kinetic energy, it will delay the formation of stars, galaxies, and clusters. If the dark matter has some but not too much, it makes it easier to form clusters, but still hard to form stars and galaxies early on. If the dark matter has virtually none, we should form stars and galaxies early. Also, the more dark matter there is (relative to normal matter), the more smooth the correlations will be between galaxies on different scale, while the less dark matter there is means that the differences in correlations between different scales will be very stark.
The reason for this is that early on, when clouds of normal matter starts to contract beneath the force of gravity, the radiation pressure increases, causing the atoms to "bounce back" on certain scales. But dark matter, being invisible to photons, wouldn't do this. So if we see how big these "bouncing features" are, known as baryon acoustic oscillations, we can learn whether there's dark matter or not, and -- if it's there -- what its properties are. The thing we construct, if we want to see this, is just as powerful as the graph of the fluctuations in the microwave background, a couple of images above. It's the much lesser-known but equally important Matter Power Spectrum, shown below.
As you can clearly see, we do see these "bouncing" features, as those are the wiggles in the curve, above. But they're small bounces, consistent with 20% of the matter being "normal" matter and the vast majority of it being smooth, "dark" matter. Again, you might wonder if there isn't some way we could modify gravity to account for this type of measurement, rather than introducing dark matter. We haven't found one yet, but if such a modification were found, it would be awfully compelling. But we'd have to find a modification that works for both the matter power spectrum and the cosmic microwave background, the way that a Universe where 80% of the matter is dark matter works for both.
This is from the structure data on large scales; we can also look on small scales, and see whether small clouds of gas, in-between us and very distant, bright objects from the early Universe, are thoroughly gravitationally collapsed or not; we look at the Lyman-alpha forest for this.
These intervening, ultra-distant clouds of hydrogen gas teach us that, if there is dark matter, it must have very little kinetic energy. So this tells us that either the dark matter was born somewhat cold, without very much kinetic energy, or it's very massive, so that the heat from the early Universe wouldn't have much of an effect on the speed it was moving millions of years later on. In other words, as much as we can define a temperature for dark matter, assuming it exists, it's on the cold side.
But we also need to explain the smaller-scale structures that we have today, and examine in gory detail. This means when we look at galaxy clusters, they, too, should be made of 80% dark matter and 20% normal matter. The dark matter should exist in a big, diffuse halo around the galaxies and the clusters. The normal matter should be in a couple of different forms: the stars, which are extremely dense, collapsed objects, and the gas, diffuse (but denser than the dark matter) and in clouds, populating the interstellar and intergalactic medium. Under normal circumstances, the matter -- normal and dark -- is all held together, gravitationally. But every once in a while, these clusters merge together, resulting in a collision and a cosmic smash-up.
The dark matter from the two clusters should pass right through one another, because dark matter doesn't collide with normal matter or photons, as should the stars within the galaxies. (The stars not colliding is because the cluster collision is like firing two guns loaded with bird-shot at one another from 30 yards away: every single pellet should miss.) But the diffuse gas should heat up when they collide, radiating energy away in the X-ray (shown in pink) and losing momentum. In the Bullet Cluster, above, that's exactly what we see.
Ditto for the Musket Ball Cluster, a slightly older collision than the Bullet Cluster, that's just recently analyzed. But others are more complicated; cluster Abell 520, for example, below, appears to have too much gravity associated with a location that ought to have only normal matter and not dark matter.
If we look at the individual components, you can see where the galaxies are (which is also where the dark matter ought to be), as well as the X-rays, which tell us where the gas is, you'd expect the lensing data -- which is sensitive to the mass (and hence, dark matter) -- to reflect that.
Instead, we see evidence for the gas creating a large amount of lensing, which shouldn't be. So, perhaps something funny is going on here. Maybe this is evidence in favor of modified gravity and against dark matter, as some contend. Or, perhaps, there's an explanation consistent with dark matter, and we simply have an unusual mass distribution in this type of smash-up.
But we can go to even smaller scales, and look at individual galaxies on their own. Because around every single galaxy, there should be a huge dark matter halo, comprising approximately 80% of the mass of the galaxy, but much larger and more diffuse than the galaxy itself.
Whereas a spiral galaxy like the Milky Way might have a disc 100,000 light-years in diameter, its dark matter halo is expected to extend for a few million light-years! It's incredibly diffuse because it doesn't interact with photons or normal matter, and so has no way to lose momentum and form very dense structures like normal matter can.
What we don't yet have any information about, however, is whether dark matter interacts with itself in some way. Different simulations give very different results, for example, as to what the density of one of these halos ought to look like.
If the dark matter is cold and doesn't interact with itself, it should have either an NFW or a Moore-type profile, above. But if it is allowed to thermalize with itself, it would make an isothermal profile. In other words, the density doesn't continue to increase as you get close to the core of a dark matter halo that's isothermal.
Why a dark matter halo would be isothermal isn't certain. Dark matter could be self-interacting, it could exhibit some sort of exclusion rule, it could be subject to a new, dark-matter-specific force, or something else that we haven't thought of yet. Or, of course, it could simply not exist, and the laws of gravity that we know could simply need modification. On galactic scales, this is where MOND, the theory of Modified Newtonian Dynamics, really shines.
While the NFW and Moore profiles -- the ones that come from the simplest models of Cold Dark Matter -- don't really match up with the observed rotation curves very well, MOND fits individual galaxies perfectly. The isothermal halos do a better job, but lack a compelling theoretical explanation. If we only based our understanding of the "missing mass" problem -- whether there was extra, "dark" matter, or whether there was a flaw in our theory of gravity -- on individual galaxies, I would likely side with the MOND-ian explanation.
So when you see a recent headline like Serious blow to dark matter theories?, you already have a hint that they're looking at individual galaxies. Let's see what this is about.
A paper released just two days ago took a look at stars relatively close to our solar neighborhood, and looked for evidence of this inner distribution of mass from the theoretical dark matter halo. You'll notice, looking a couple of images up, that only the simplest, completely collision-less models of Cold Dark Matter give that large effect in the cores of dark matter halos.
Indeed, the simple (NFW and Moore) halo profiles are highly disfavored, as many studies before have shown. Although this is interesting, because it demonstrates their insufficiency on these small scales in a new way.
So you ask yourself, do these small-scale studies, the ones that favor modified gravity, allow us to get away with a Universe without dark matter in explaining large-scale structure, the Lyman-alpha forest, the fluctuations in the cosmic microwave background, or the matter power spectrum of the Universe? The answers, at this point, are no, no, no, and no. Definitively. Which doesn't mean that dark matter is a definite yes, and that modifying gravity is a definite no. It just means that I know exactly what the relative successes and remaining challenges are for each of these options. It's why I unequivocally state that modern cosmology overwhelmingly favors dark matter over modified gravity. But I also know -- and freely admit -- exactly what it will take to change my scientific opinion of which one is the leading theory. And you're free to believe whatever it is you like, of course, but there are very good reasons why the modifications to gravity that one can make to have gravity succeed so well without dark matter on galactic scales fail to address the other observations without also including dark matter.
And we know what it isn't: it isn't baryonic (normal matter), it isn't black holes, it isn't photons, it isn't fast-moving, hot stuff, and it probably isn't simple, standard, cold and non-interacting stuff either, like most WIMP-type theories hope for.
I think it's likely to be something more complicated than the leading theories of today. Which isn't to say that I think I know exactly what dark matter is or how to find it. I'm even sympathetic to certain degrees of skepticism expressed on that account; I don't think I would claim to be 100% certain that dark matter is right and our theories of gravity are also right until we can verify dark matter's existence more directly. But, if you want to reject dark matter, there's a whole host of things you'll need to explain some other way. Don't completely ignore large-scale structure and the need to address it; that's a surefire way to fail to earn my respect, and the respect of every cosmologist who studies it.
And that's, as best as I can express it in a single blog post, the whole story on dark matter. I'm sure there are plenty of comments; let the fireworks begin!
