Ask An Astronomer 

Here are the answers to selected questions submitted between . Questions may have been edited for clarity or brevity. Click on a link to move directly to the answer.

• Rings around the Moon
• Duplicate Earths
• The Observable Universe
• The Interstellar Medium
• The Expansion of the Universe
• Seeing the Big Bang
• Comets and Asteroids

1. What caused the large ring around the moon I saw last night?

   So, did it look a little like this?

   Ice crystals in the upper atmosphere, even if there are too few to appear "cloudy", can refract light from the Sun or moon down to you. Because of the angles of the crystals, only light that's at exactly the right angle comes down. So all the crystals that appear, from your position, to be the same angle from the moon send some moonlight your way, and that's why it appears to be a circle!

   The phenomenon is a little bit like a rainbow, which also has a characteristic size that's related to the angles inside raindrops. In fact, I think it's true that the moon halo is split into colors, much like a rainbow. However, at night your color vision is pretty poor so it's hard to notice the colors.

2. Can there be another planet in our solar system in the same orbit as earth but on the opposite side of the sun? We would never see it because it would always be behind the sun.
   
   It turns out that such an orbit is unstable in the sense that if you put a planet (or any object) there, any small nudge would cause it to move far away from the point directly opposite Earth. It's very similar to trying to stand a pencil on its top: if the pencil isn't _exactly_ vertical, it will quickly fall over.

   A recent news item provides a nice illustration of what would happen if you tried to put such an object in the solar system.
   
   

3. What is the difference between the observable universe and the universe?
   
   This is actually a very fundamental question that philosophers, religious leaders and scientists have struggled with for ages. Unfortunately, as I'll admit up front, there is not an entirely satisfying answer. The question is a bit like the old classic `if a tree falls in the forest and no one is around does it make a sound?'. We will never be able to do an experiment to find out if the tree makes any noise - because in order to test it, we have to be in the forest, violating the premise of the experiment. The only way we can really `know' something is if we can test it. The same is true for the universe; the only universe that we know anything about is the universe that we observe or test. We can speculate that there exists a `real' Universe (with a capital U) that obeys different fundamental laws and that the observable universe is merely a projection of that Universe. Unfortunately, until we observe these properties our Universe is purely speculation. If one chooses to believe in the existence of a `real' Universe based purely on faith that's perfectly fine, but that belief can not be verified and built upon to further scientific development. In short, there is no difference between the real Universe, and the observable universe; the only things we can know are those that we can observe.
   
   

4. I have read the interstellar medium is 100 million degrees, 200 times hotter than the surface of our sun. How does this gas between stars get so hot? Why does it stay so hot for so long?
   
   First, a background on the ISM: The ISM is a gas of very low density, 1 atom per cubic centimeter. Its temperature ranges between 20 Kelvin and 20 Million Kelvin, so it can get hot.
   • How is the ISM heated?

     Supernovae explosions produce blast wave shocks that race out and heat the ISM. Young stars also heat the ISM with a similar type of shock wave called the stellar wind.

     Now, if some of this hot gas comes in contact with some cool gas, conduction will heat the cool gas. However, if you've got so few atoms per cc, you can imagine that the time scales for two of them to crash into each other are big, so this kind of heating is not very efficient.

   • Why does it stay so hot?

     Well, mostly because conduction is such a poor method for cooling. The only other method is for atoms to release some energy in the form of light, i.e. radiative cooling, and loose some temperature.

     This is a long and slow process.

   
   
   

5. According to the big bang theory all stars move outwards on the perimeter of a sphere from the center. The moving speed of this matter is much less than the speed of light. If that is correct how can telescopes pointed to the center today pick up any light emitted from stars that were formed billions of years ago when this light should have passed us long time ago? How does the red shift in the light frequency determine a distance?
   
   Your questions stems from a slight misunderstanding about how we picture the expansion of the universe. As you write, one way to think about this, is to picture stars (actually galaxies, which are just bound conglomerations of stars) attached to the perimeter of a sphere, which itself is expanding, carrying the galaxies with it. Personally I like the balloon analogy, where one pictures an inflating balloon with little ants crawling around on the surface, representing the galaxies.

   The key point in either analogy is that our three-dimensional space is represented by the (2D) SURFACE of the sphere/balloon. In this analogy it doesn't make sense to look at the center, since the center of the balloon isn't part of space. It lies somewhere in hyperspace, but our observations have nothing to say about this point. As the universe expands, the fabric of space itself is actually growing, the universe is getting larger, just like the surface area of the balloon. On average every ant on the surface is moving away from every other ant. I say "on average", because one must allow for the possibility of an ant (or a galaxy) moving relative to the underlying space, at a rate greater than the expansion itself. A real life example is M31 (Andromeda galaxy). It happens to have a large peculiar velocity, in a direction towards us, and is actually blueshifted. On average, however, all galaxies are moving away from all others.

   It's also not true that this recession velocity must be less than the speed of light. Einstein's special relativity does state that nothing may travel faster than the speed of light, but this holds for objects moving with respect to an underlying reference frame. Einstein's theory says nothing about how fast space itself can expand. Two galaxies that are receding from each other at twice the speed of light due to the expansion of the underlying space, are not able to exchange any kind of information, since this information is confined to travel through the expanding space itself, at a speed no greater than the speed of light. Thus Einstein's theory is not violated in any way.

   If you think through this expanding balloon analogy in more depth, you will discover that the rate of recession between two ants must be proportional to the distance between the two. Again, this distance would not be calculated by drawing a straight line from one ant to the other, piercing the surface of the balloon, but instead by measuring the distance from one to the other along the surface of the balloon. Think the distance between Sydney and London (10562 miles / 16997 km) - what's meant is the path along the surface of the earth, not the length of a hypothetical tunnel through the center of the earth.

   So, the fact that we can pick up light emitted by galaxies billions of years ago is explained by the fact that the universe (the surface of the balloon) has expanded to an incredible size, and the photons we receive from these galaxies cannot travel outside of our three-dimensional world.

   I think the above should have also clarified why redshift is proportional to distance. As you correctly state, the redshift depends on the relative speed between the objects. By the sphere/balloon analogy you now understand that in our expanding universe the average recession rate is proportional to distance and hence the direct relation between distance and redshift - Hubble's Law.
   
   

6. If the universe is expanding and the speed at which it is expanding is certainly consistently lower than the speed of light then the light emitted by the birth of the universe must have passed us by a long time ago. How is it possible to look back and see the birth of the universe?
   
   Many people struggle with the idea that looking farther out means looking farther back, at least in the temporal sense. Presumably, you already get this point, or at least part of it. Just to make sure we're on the same page though, along with anyone who might stumble across this, let me put it this way:
   
   The light emitted by the birth of the universe comes from... well, everywhere. The early universe was a boiling sea of high energy particles. These particles were packed closely enough to keep light trapped basically right where it was sitting. In other words, the universe was opaque.
   
   However, the universe was expanding, which means the boiling sea of particles was cooling off, as all expanding substances do, and eventually, the whole shebang cooled to the point where light could escape, which it did, from every point in the universe at once.
   
   So let's imagine you're sitting in a spaceship in the middle of all of this, watching the universe become transparent. (With appropriate radiation shielding, or some sort of super-robot body. Use your imagination.) What you'd see in the first instant is all the now-free light rushing at you from the nearby part of the universe, naturally, the light that's had time to reach you. Wait five minutes longer, and you're still seeing light, just light that had to travel farther to get to you. But all this light was emitted at the same time, so you conclude that the light is five minutes old, and you're seeing something that happened five minutes ago.
   
   Hopefully, you see where this is going. Wait around a few billion years, and you're still seeing light from that same event, way back when, but which was emitted so far away that it took all of those several billion years just to reach you. So by virtue of the long unfathomable distances between, you get to look back in time.
   
   The key to your question may be in your understanding of the place in time and space from which the light was emitted. If the light had been emitted immediately, i.e. at the same instant the universe was created as a tiny little speck, then all the light would certainly have raced past us by now. But we're not looking at some cosmic firecracker, the burst of light that was produced by the explosion that started it all, in fact we can't, precisely because the universe was too bright for the light to move around for the first few hundred thousand years. (Well, too energetic, anyway. Hopefully, this makes sense.) By the time the universe was transparent, it was also very big, such that we can look out to long distances at times long ago.
   
   Which would be the whole story, if the universe had stopped expanding once it turned transparent. It did not, however, so the answer is a little more complicated (though if you made sense of the above explanation, you should be in good shape). It turns out that your second assumption--that the universe expands more slowly than light speed--is wrong.
   
   Which might sound like a joke at first, but I assure you I'm serious. Objects are constrained to move at velocities less than light speed, but space itself may stretch as fast as it wishes, over great distances. Of course, if the space between two points is stretching faster than light, then those two points are prevented from ever trading information, and might as well belong to entirely separate universes.
   
   So our universe effectively has some maximum volume, namely, that volume which is expanding away from us more slowly than light speed. It just happens that this volume still encompasses light that was emitted in the earliest years of the universes existence. Presumably, this light will eventually disappear over the edge from our perspective, never to be seen again.
   
   
   
   

7. What is the difference between comets and asteroids?

   The main difference between comets and asteroids is composition. A comet is made of mostly ice while an asteriod is composed mostly of rocky material. This is the reason comets are so much more impressive in the sky when they come in near the sun -- they're melting! They leave a long trail of vaporized water behind them in their orbit as they get close to the sun and that trail, lit up by the sun, is what we see as 'comets' in the night sky. For more information on comets and asteroids, I suggest looking at NASA's website -- they've done missions to both!

Thanks to Alex McDaniel, David Lai, Shawfeng Dong, Gabe Prochter, Ian Dobbs-Dixon, Jay Strader, Justin Harker, Karrie Gilbert, Kyle Lanclos, Laura Langland-Shula, Lynne Raschke, Marla Geha, Michael Kuhlen, Nick Konidaris and Scott Seagroves