Ask An Astronomer
9 February 2003
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.
What caused the large ring around the moon I saw last night?
So, did it look a little like
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.
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.
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.
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
- 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.
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
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.
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
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
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
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.
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
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Last modified: Tue Feb 11 17:06:23 PST 2003