In my solar system series, I occasionally find it useful to explain space science concepts in detail that most people are not familiar with. But there are many, far more basic ideas that never seem to get traction in the public awareness, so I will now explore some of the concepts that are often woefully misunderstood by the average person.
1. Astronomy and astrology have nothing to do with each other.
This is a massive space geek peeve that will probably never be resolved, because the two words are similar and both concern the heavens - thus people with minimal scientific background are bound to keep confusing them. I've met more than one educated professional who started talking to me about horoscopes when I mentioned an interest in astronomy. Astrology is a religious/occult practice stemming from ancient civilizations that claims human events are shaped by the positions of arbitrarily-defined constellations and the characters of gods associated with planets (e.g., Mars, Jupiter, etc). It is meaningless crap - yes, I am a Taurus, but don't tell me that's why I'm hard-headed and skeptical. Astronomy is the scientific study of objects and phenomena in space. It is the real deal, and it is awesome.
All that we know about space comes from astronomy; none of it comes from astrology. In fact, the word "astrology" is a misnomer, because there is no logic or study involved: It's just religion. The only reason it's called astrology is that in ancient times, a desire to know the future motivated people to develop practical techniques for charting stars and planets, but since then it has made no contribution to anything practical or logical.
2. Solar systems and galaxies are not the same thing.
At UC Berkeley a few years ago, I swear to you I met this girl in a bar who didn't know the difference between a solar system and a galaxy. And she wasn't a "townie" or anything - she was a student at one of the most prestigious universities in the world, studying linguistics. I already knew the misconception existed, but the fact that it goes that far up the IQ food chain suggests that it is commonplace. Once you know the difference, it's hard to grasp how anyone who has ever even heard of either could not know, but apparently it's a normal thing.
The difference between a solar system and a galaxy is the difference between a single cell in your body and you - and I'll bet you've never mistaken one for the other. A solar system is centered on one star or a handful of stars that orbit each other, and includes any planets or asteroids that orbit them. That's why it's called a solar system. Our solar system has one star, eight planets, and a bunch of smaller objects in it. A few solar systems contain stellar-mass black holes rather than active stars, but they're still solar systems.
A galaxy contains anywhere from millions to billions of solar systems. There are a number of different shapes of galaxies based on how they evolved - some are just shapeless blobs of stars, some are pancakes, some are spirals (like ours). Our galaxy, the Milky Way, has about 300 billion solar systems, although not all of them have planets - some are violent or unstable enough that all they're likely to have is asteroidal debris. Flattened galaxies like ours have "supermassive" black holes at their center, with millions of stellar masses in them.
So, to review, solar system:
Galaxy:
3. Black holes look white.
While it is true that a black hole would be black if you completely isolated it from any nearby matter and just put it in front of a bright background, that's not how things work in reality. They constantly suck in material, and are deeply encased in a thick soup of it. The matter surrounding black holes is heated so intensely that it emits X-rays, and if you were to look directly at it from relatively close with your naked eyes, it would look white - that is, before the high-energy spectra burned your eyes out. Light doesn't escape from the black hole, but it does escape from the highly-compressed matter falling into it just before it passes the point of no return, the event horizon. This is a black hole:
The reason we can know certain things are black holes rather than stars is that their gravity is typically much higher than a star, and the light coming from them is way more powerful than what a star produces - i.e., X-rays or even gamma rays, whereas light from stars is usually, mostly limited to ultraviolet.
4. A planet or moon with more mass or bigger size doesn't necessarily have higher surface gravity.
As the number of "super-Earth" planets discovered around other stars continues to rise, I've noticed that a lot of people generally believe that a planet with twice the mass of Earth has twice the surface gravity. This is not necessarily the case. Although the total gravity field caused by the planet is twice as intense, that's not the same thing as the intensity of gravity experienced on the surface. A surface can be at different depths in the planet's gravity field, so if you have a denser body, the surface will be deeper in the field than a less-dense body with the same mass, and thus have higher surface gravity. Conversely, if you spread the same mass out farther because the planet has low density, it's higher up in the field and the gravity you feel on the surface will be lower.
Imagine you have two planets with identical masses: One is made entirely of iron, the other entirely of silicate rock. Both gravity fields will be the same because the mass is the same, but the iron planet will be a lot smaller than the rock planet because it's much denser. So the surface of the iron planet will be deeper in the same gravity well, and thus the force you experience on that surface will be greater even though the planet is smaller and has the same mass. This means that a planet with twice the mass of Earth might only have, say 1.2 g surface gravity rather than 2 g if it's less dense than Earth. In other words, some super-Earths could be habitable for humans despite having what sounds like a lot more mass.
We can actually see this principle at work in the solar system: Jupiter's moon Ganymede (the object on the left in the two images above) is bigger than Mercury and nearly twice the size of the Moon with more than double the Moon's mass, but has lower surface gravity than the Moon because so much of its mass is dominated by water ice as opposed to rock. You would feel slightly lighter walking around on this gigantic, massive satellite than you would on our own much smaller, much less massive Moon. The same principle applies to planets: To know surface gravity, you have to know both the mass and the density.
5. There is no such thing as "zero gravity."
Gravity continues forever in all directions, decreasing with distance but never reaching zero. Every location in a gravity field has an "escape velocity" whereby you move fast enough against the force of gravity that it will never be enough to completely turn you around, but it will always have an effect on your speed and trajectory, although increasingly minute. If the only object in the universe were a baseball, and it was a trillion miles away from you, and you began at rest relative to it, you and it would eventually come together no matter how small the gravity you both have. The reason things get so complicated is that there are competing gravity fields, and things move in various directions relative to them. So, for instance, in going to the Moon, there is a point where Earth's gravity becomes less intense than the Moon's, and that doesn't mean Earth's has ended - just that where you happen to be, the Moon's gravity overcomes Earth's gravity in the opposite direction.
6. Gravity at the International Space Station is only a little less than on Earth.
Here is another thing people badly misunderstand about being in space, particularly low Earth orbit: Astronauts on the space station don't float around because gravity is too weak at their location to pull them down - in fact, if you built a stationary tower from the Earth's surface up to the height of the station, you'd only weigh a little less than on the ground. No floating. The astronauts on the station are floating around because they're falling - they're moving sideways at such tremendous speed that they move down at the same rate the Earth's surface curves beneath them, so they're falling in a circle. That's what orbit is.
Even if you were as far away from Earth as the Moon, if you were completely stationary relative to Earth you would still weigh dozens of milligrams - low enough to float around, but things would still descend to the floor, ever so slowly. So the most accurate term for what is happening is "freefall." Even the term "microgravity" is inaccurate when describing conditions on the space station because, as stated, at the altitudes involved, gravity is not much smaller than on the surface. "Weightlessness" is technically correct, but is more a description of what the astronauts experience rather than why (because they're in freefall).
7. Your head will not explode in vacuum.
People take the term "explosive decompression" a little too literally: If you suddenly went from sea level air pressure to vacuum, your blood vessels could burst, causing your skin to bulge and fill with blood, but the goofy exploding-watermelon-head scenes in some science fiction movies are just not going to happen. If you were to survive and get back into a pressurized environment quickly enough, your body would be covered in bruises and your eyes would be completely bloodshot, but you wouldn't necessarily have suffered any strokes, embolisms, or other critical internal damage.
8. There are no sounds in space, nor other things caused by air, and sounds are muted in lower air pressure.
Most people "know" and acknowledge that there are no sounds in space when you insist on saying so, but they still find it irksome whenever that fact is depicted in media - they expect sound, and when it's not there, they complain about it. That's why movie and TV studios that cater to idiocy - which is to say, nearly the entire industry - usually have sounds in space whenever they make scifi-flavored content. They also aren't comfortable with all the other consequences of vacuum: Things like, there's no reason to make a long, swooping bank in order to turn around - you can just spin the spacecraft 180 degrees.
You could make a banked turn for no damn reason, if you wanted to, but it would be a massive waste of fuel. Also, there would be no need to orient a spacecraft a certain way if you were making a turn, because its trajectory is acted upon by a rocket plume, not control surfaces interacting with air. All that matters is where the rocket is pointed, not the orientation of surfaces. The whole spacecraft could be flat as a pancake and facing the direction of travel rather than being edge-on, and there would be no untoward "drag."
Even less well-known, and thus less acknowledged in entertainment, is the fact that low air pressure rather than pure vacuum has muted sounds. So on the surface of Mars, which is slightly more pressurized than vacuum, you might hear something - but it's a very quiet place. They never show that properly in movies and TV. If you want to get a sense of it, watch the scene in Total Recall where Arnold Schwarzenegger has been thrown out on to the surface and his eyes are bulging out from decompression (not a very accurate portrayal of that either), but put your fingers in your ears - on Mars, you would hear things mostly through vibrations in the ground traveling through your suit rather than the air, and you'd have to be relatively close to hear anything at all because the sound would dissipate rapidly.
9. Space is not empty.
Just relatively empty. It's actually a constant swarm of particles and energy, although very diffuse - out between the stars, you might run into only a few atoms and subatomic particles per cubic meter. But there's still stuff there, and in Earth orbit it's so dense it's practically a soup compared to what you find in interstellar space. Astronauts describe how things that have been in space - like EVA suits - smell a bit like gunpowder when they're brought back inside. You won't see anything out there, but enough particle collisions are happening to make some slight chemical changes happen on the surfaces of things. And, in fact, sometimes you do see something because particles collide with your retina and set off the rods and cones, causing little flashes of light. If astronauts were to encounter a lethal radiation storm, that's what they would see - the air around them would look like it's sparkling, even though it isn't: It would just be the chemicals in their eyes being triggered by constant particle collisions.
10. However, space looks pitch black.
In other words, it's not a purple or blue nebular cloudscape like so many scifi shows depict it - just plain fathomless black. Those beautiful Hubble images showing colorful gas clouds and starscapes are not what your naked eye would see if you were floating at those locations: You would just see the stars as multi-colored dots of identical sizes in an infinitely deep black medium - no flare effects, no twinkling (because, again, there's no air), just dots. And as for nebulae, you wouldn't see them at all - the dust in them obscures stars: They would just be dim or seemingly empty patches. Hubble has to focus on those locations for hours or days at a time to collect enough light to make a meaningful picture. This is not space:
Even those pictures from Earth of the Milky Way are based on sustained exposures, not what you'd see just looking up. With your naked eye, the Milky Way is just a particularly dense strip of stars - you can't see the gas and dust and structure of it. Those nebulae which would be bright enough to see with the naked eye are not these brilliant assortment of colors used in the most famous NASA imagery - they're dull red or brown. So if you want to imagine what famous Hubble imagery would look like to the naked eye, turn everything but the stars and areas immediately around the stars pitch black, get rid of the crossed flare effects from the stars, and dimly illuminate the gas right around each star. Voila, there you have it. It's dark out there. Here's what stars actually look like in space:
11. Stars are not on fire.
Fire is a chemical process where oxygen atoms meet up with other atoms and produce different compounds out of fuel and air. Stars are too hot for fire. The things you see on the Sun are not flames - the material inside them is not changing chemically to any significant degree. It's just very hot, and is being thrown around by changes in the solar magnetic field. Fire may occur in limited ways in the atomic gases in the atmospheres of very cool stars, but it would actually be dark against the brilliance of the light coming from the fusion occurring below.
12. Most of a "gas giant" is not gas.
When you compress a gas enough at high temperature, it turns into something called a supercritical fluid, which is neither gas nor liquid but shares properties of both. Supercritical fluids completely fill the volume around them like a gas, but can act as a solvent like a liquid. So-called "gas giants" do have gaseous atmospheres in their highest layers, but most of their bulk consists of supercritical fluid phases of hydrogen and helium. But SCFs are not limited to gas giants - even solid planets can have them if they're hot enough and have dense enough atmospheres. The atmosphere of Venus, for instance, turns supercritical below 65 km altitude, so its surface has more in common with the bottom of the ocean than a surface beneath a gaseous atmosphere.
13. Asteroid belts have practically nothing in them.
You may be wowed to hear about the zillions and zillions of objects that populate this solar system's Main Asteroid Belt, and get an image in your mind - as apparently George Lucas had - that it's some kind of obstacle course of rocks floating around and colliding all the time. But when you put those numbers into the context of the sheer volume of space in which they move, you realize that actually it's the exact opposite. You would have to put a significant amount of effort into finding and reaching something in an asteroid belt. If you were just floating there looking with your naked eye, all you would see in the sky are stars and planets, nothing else. If you flew around the belt randomly for years, you would still see nothing else, although you might get a few dings from flecks of dust.
What Hollywood shows you:
What you would actually see, smack in the densest part of the Main Belt - none of which are asteroids (shopped from an Apollo photo):
Let's put it this way: The near-Earth space through which the space station and satellites orbit is practically packed solid with debris and orbital equipment compared to the asteroid belt. You would have to be traveling at a substantial fraction of the speed of light around the Main Belt - not just through it - to face the same collision dangers that astronauts face moving around at orbital speeds a few hundred kilometers above the Earth's surface. This is why space probes to the asteroid belt, like Dawn, have to have their paths carefully planned: If Star Wars were reality, we could just send probes out randomly and they'd fly past all sorts of interesting objects just by chance. But that's never going to happen: You could send a million probes out on random trajectories into the belt and statistically not one of them would ever get close enough to a sizable object to get detailed imagery.
14. The Big Bang wasn't a bang, and it didn't start at the first instant of the universe.
At first the universe was just a very dense, compact "soup" that expanded relatively slowly, but then it reached a critical point where it suddenly expanded very, very fast. Nothing was "pushing" the matter outward - space itself was expanding and pulling it along. So the term "Big Bang" was just a bit of theatrics. It inflated, not exploded - and contrary to what most people think, it wasn't thought to have been at the first instant.
15. Rockets don't need a surface to push against. (Suggested by Gooserock)
Many people see rockets taking off from the ground and make the inaccurate conclusion that a rocket needs a surface to push against in order to get going. That's not really the case, and it doesn't actually even help to have such a surface. The one and only thing causing a rocket to move is the fact that it's spewing plasma out the back - once that plasma leaves the rocket nozzle, it has no further effect on the rocket. The fact that it hits the ground after leaving is no help, and in fact is actually a problem, because without designing channels beneath the launch pad for the plasma to escape into, it would just blow back up at the rocket and blow it up. We've all heard "every action has a reaction," and that's basically all that rockets do - they spit plasma out the back because doing so causes the rest of the rocket to build up forward momentum.
16. There is no "Earthrise" seen from the lunar surface. (Suggested by pucklady)
What people may not know about the famous "Earthrise" images and films from the Apollo missions is that these were taken from lunar orbit, and the apparent motion of Earth relative to the lunar horizon is only occurring because the Apollo spacecraft is moving. In fact, since the Moon is tidally locked to Earth, and thus only ever shows one face to it, Earth doesn't really change position in the Near Side lunar sky, and is never visible from the Far Side. To change Earth's position in the sky from the surface, you would have to travel to another region. Astronomers hope to eventually use this fact to their advantage by having observatories on the Far Side that would be completely shielded from the radio noise of Earth.
9:24 PM PT: Some have expressed reservations about the Big Bang - namely, that it's a separate topic from a subsequent inflationary epoch. This is actually somewhat the point - people mistake inflation for the Big Bang, and think they were one event.