In astronomy, what does edge-on and face-on mean?

If you’re looking at a galaxy from the edge, it appears flat, more or less. If you’re looking down on it…well, you see the whole thing spread out in front of you.

You notice that when you see the Milky Way from earth, it’s just a relatively narrow band of stars across the sky, and that’s because you’re looking at it edge-on. If you were looking down on it, you’d see the middle of it and the spiral arms flinging around it.

I BETTER not have just answered a homework question!!!

@Val123: @Haroot asked for the case of a binary system, not a galaxy. However, the same basic idea applies.

An edge-on binary system is where our observing angle corresponds with the orbital orientation of the stars. In such a system, we see the stars eclipsing one-another at regular intervals. You can record a “light curve” that might look something like this. The valleys are where one star is in front of the other, and then again when they “reverse” position. The high plateaus are where both stars are visible. You would also observe a periodic doppler-shift of the light as the stars move towards and away from us while they circle eachother.

A face-on binary system is where we are looking directly “down” on the plane of the orbit. If our resolution is good enough, we can physically see the stars circling one another.

In reality, most systems are neither perfectly face-on or edge-on, but are inclined at some angle (usually called i). Additionally, you can’t easily tell from direct observation what the inclination angle is, unless the system is perfectly edge-on. This wikipedia article might help. An edge-on system has an inclination angle of 90 degrees, while a face-on system has an inclination of zero degrees.

Alright. I think I got it. Thanks.

@Val123 And no, it wasn’t. Just something I was confused about.

Put a lemon and a lime on a table top.

Look down on them: face-on. You’ll see the tops of the two fruits. ˚ ˚

Put your face on the edge of table and look at the fruit edge-on. You’ll see the silhouette of the outline of the two fruits. 0 0

Fine. I’m posting something I’ve been holding off on, waiting for @hannahsugs to report to me on. Here goes….

OK. My next thought was, take a plate, hold it edge on to your face. What you see then, just the edge of the plate, is very different than if you flipped it up and looked at the plate face on….but then @hannahsugs came in with how it applied to a binary star system (noted in your question…)
I read his/her post (thought about it…) and I came up with this simplistic explanation (and this is what I sent to hannahsugs, and waited to see if it was accurate…no answer so I don’t know….) So, here’s how I perceive it. Take two balls that are trailing tracers of light behind them, revolving around each other, more or less evenly. If you look down on them from above, or directly above their equal trajectories, the tracers form a circle. If you look at them from other angles, (since their trajectories aren’t exactly the same) they form other, varying patterns, such as @hannahsugs‘s graph showed, because they probably aren’t going exact circle around circle around each other. One is going faster, one is going slower, one is going up and down, the other isn’t….it’ll create different “light” patterns, depending on the angle you’re viewing them from.

D’oh! I wrote that post right before signing off my computer for a few hours. oops!

@Val123 has sorta the right idea, as far as if the stars were emitting “tracers” and seeing different patterns over time. However, I’m afraid I might have confused things with the link to the graph. In a binary star system, if the stars were emitting “tracers” of light as they moved, if you looked at them “face-on”, their paths would form too overlapping circles or ellipses. Try this website, changing the mass of the purple planet to ~150, and you have an idea of what that would look like.

If you looked at the system edge-on, you’d just see a line, with two bright “dots” moving back and forth along the line as they looked at eachother. As @Val123 and @gailcalled suggested, this is similar to taking a dinner plate or a CD and holding it flat, even with the plane of your eyes.

For a system that is somewhere between edge-on and face-on, we would simply see similar overlapping ellipses as in the face-on case, they would just be “squished” or flattened. Play with the simulation i linked to above, and try these initial inputs:
Body 1: 200 -90 0 -90 0
Body 2: 150 150 0 -80 40
Try to imagine, those could be more circular orbits, but because of an inclination angle, the appear to us to be elliptical.

The graph i showed is something different. That is a “light curve” for an edge-on binary system. It shows the total brightness of the whole system, as perceived by us, as the stars orbit eachother. It assumes that the stars are not of equal brightness. On the plateaus, from our perspective the stars are “next to” one another, so we see the full brightness from both of them. When the brighter star passes behind the dimmer star, we get the first dip in the graph, because we ONLY see the light from the dimmer star. The bright star emerges again, and we see the same brightness level as before, until the dim star passes behind the brighter star. Now we ONLY see the light from the brighter star, so there’s a dip again. Does that make more sense? The graph does NOT show position, it shows overall brightness over time.

Edge-on binary stars are very useful to astronomers. They are the only system where the inclination angle can be truly and surely known, because we can SEE the stars passing in front of one another. With systems that are inclined, we can only make an educated guess as to what the inclination angle is, or if the system is 100% face-on. When a system is edge-on, we can get true orbital velocities of the stars, which means we can get their masses, the radius of the orbit, etc. Unfortunately, as you can probably guess, edge-on or perfectly face-on binaries are rare. Random-inclination binaries are much more common. Luckily, more than ½ the stars in our galaxy seem to be in binary systems. Our sun is one of the odd-ball lonely stars. With 300 billion stars in the galaxy, at least half of them binaries, there’s some edge-on systems for us to study!

@hannahsugs THAT is VERY cool! I could spend hours playing! (Be right back)

Oh dear. I set the mass of the purple star to 1000….oops! I hope they didn’t have populated solar systems!...

Neat! I made a hydrogen atom!

Oh! I made an eye!

Oh! I created a four star system and created a traffic jam!

Oh crap! Don’t give purple a mass of 150 and a position of 142, and the yellow a mass of 50 and DON’T make both of their velocities a 10!......... DON’T DO IT!

@Haroot give it 4 stars. You can see a better example of the wave thing he had going on above.

Shoot…I can’t get them to stop getting into head on collisions. I wanted to check something, but I am God and I’ve screwed up my binary star system. I have the power to keep recreating them, but I can’t figure out how to reset them so they orbit, instead of crash….what would a good default be?

Also, IS there such a thing as a 4 star system?...?

@hannahsugs I found the original settings, so I could check what I wanted to see.

@Haroot take the link with it’s original settings of:

Yellow: Mass=200 / All three positions at 0/ velocity at -1.

Purple: Mass=10 / Position X=142, next two positions at zero / Velocity=140

Now set the velocity of Purple at 80…you can see how it makes the yellow star wobble, which creates the waves whch the instruments will read as going up and down, or side to side, or towards us and away from us (all which create the dopple shifts)

(Or, set it at 50, tell your girlfriend this is for her, go away for 4 minutes, look again and viola!) It gets better

Coolest link ever. Thanks!!!