Eric Answers

Wednesday, October 19, 2011

Hey all, as you may know I've been collecting astronomy questions from people.  Now I'm going to answer a few of them.

Jessica and choirqueer asked: "Astronomy vs. astrology vs. astrophysics.  What is the difference?"

I've combined the two questions for ease of answering.  Technically, astronomy has more to do with the qualitative or observational study of all objects not contained in the Earth's atmosphere.  Astrophysics is part of astronomy, but is focused on the applications of physics to astronomy and understanding why things are the way they are through physics.  The title of this blog comes from something an old friend of mine used to say despite the fact that he was, in fact, an astrophysicist.
Astrology is completely different in today's world although in antiquity astrology was astronomy.  Astrology is a belief that astronomical phenomena affect our lives as humans on earth and is widely regarded as unscientific.  I personally don't believe constellations and planetary motion have any effect on our lives as constellations are patterns that humans have assigned to stars which aren't even necessarily close together and planets are predictably governed by physics, but belief is very personal and I am not one to tell people they are wrong.

 LilyForest asked: "How noisy is the sun, assuming we could actually here it?"
I actually attended a colloquium at the CfA that dealt with this over the summer.  It was a fascinating topic.  Here's a video from ESA talking about the vibrational modes of the sun way more eloquently than I possibly could.
Basically, the sun produces "noise" due to its surface vibration.  However, this noise is generally not in the human audible range and also cannot reach us on earth.  Here is a clip from Stanford of the audio from 3 modes: Solar Sounds

 NastyNate (Nathaniel) asked: "How many total planets have been discovered and recorded in the universe?"
This is actually a question for my professor, who studies planets outside of our solar system.  However, since this is my blog and not his, according to this website which seems like a credible source run by a Paris Observatory scientist gives the current number a 694 planet candidates found outside of our solar system as of today.

Greg asked: "Given the universe is expanding at an accelerating rate, will the rate of expansion eventually pass the speed of light?
The expansion of the universe is a very tricky subject.  Currently, the recession velocity of galaxies due to the expansion, which is proportion to the speed of light multiplied by the redshift, can be greater than the speed of light for redshifts greater than 1.  However, this is greatly dependant on the coordinate system and reference frame.  Since we can argue that galaxies are moving apart due to expansion of the universe, the short answer is yes: the expansion for distant objects is even currently greater than the speed of light.  The coordinates in which these are moving faster than c, are not the same coordinates used in relativity so this doesn't really contradict relativity.  Presumably this is a result of the odd behaviour of comoving coordinates which are explained best in Ned Wright's tutorial.

Jogirl asked: "Why is Pluto not considered a planet anymore?  How can it be a planet one day and not the next?"
Pluto no longer fits the criteria for a planet.  According to the International Astronomical Union, the current criteria for a planet are the following:
  1. It is in orbit around the sun
  2. It has sufficient mass to have taken on a spherical shape due to self gravity
  3.  It has cleared the neighbourhood of its orbit.
Looking at 1 and 2, Pluto may be a planet.  However, it does not fill the third requirement.  Pluto has very little mass in comparison to the combined mass of numerous objects in its orbit.  In comparison, the Earth is by far the most massive object in its orbit.  Basically, Pluto failed to gravitationally bind or expel the other similarly small objects in its immediate neighbourhood and is therefore one of many similar objects in the area rather than The One Large Thing in its orbit, if that makes any sense.
One answer I can give you for your second question is that science progresses by falsification.  This means that the rules in science are constantly changing and things are being redefined in order to comply with new rules.  As we learn more about the universe we realise that some things we thought before are not the case.  For instance, we now know that the earth orbits the sun.  It's not that one day the sun orbited the earth and then it changed, but that science changed and our theory was then modified to better fit the new model.

6 Responses to Eric Answers

  1. So the thing about the Pluto thing is that in order to be a planet, most of the stuff in its orbit needs to either be cleared away by its superior gravitational qualities or Pluto needs to eat all the stuff in its orbit to become the only large object in its path.

    Right now, Pluto is a tiny fraction of the total size of all the things around it. Basically in order to be a planet it would need to be the only thing or close to the only thing, and certainly the largest thing there. Does that help?

  2. Hey cool! I knew Pluto was no longer a planet because we found other objects as massive as it, but I didn't realize it was because we found other such objects IN ITS ORBIT. That's awesome. So Pluto's orbit is more like a sparse asteroid belt in which Pluto happens to be one of the larger objects?

    This is a great post! You should do a column like this for the student paper.