Note: This post does not have so much to do with astronomy as it does with my feelings about astronomy and how my feelings have evolved.  It's going to read very much like a personal blog post.  If this does not interest you, stop reading here.



I talked to John Huchra today.  It's probably fairly obvious that this was a rather one-sided conversation, but such is life.  I wasn't going to mention him on this blog, but I think it's more appropriate now that it is no longer an ongoing class assignment.  And I can't write an astronomy blog without talking about him, seeing as he's the reason I'm studying the subject.

I'm told I first met Dr. Huchra at a party when I was an 8th grader.  We met by chance.  He was the married to the friend of a friend of my father's.  I don't remember this meeting at all.  The first time I remember meeting him, I was asking him for a job of sorts.  My high school had an institution called project week in which we all took a week off in January to do something exciting in a field that interested us.

So in January 2008 I found myself shadowing Dr. Huchra and doing some busy work that I now doubt was actually of any relevance to his research.  It's funny; I don't actually remember much of the science part so much as sitting down and having coffee with him.  I don't even like coffee.  I have an exclusive long term relationship with tea.  I only drank the coffee because he bought it for me.  He handed me a mug, we walked down to the lobby and filled up with some sludge that claimed to be from Starbucks.  I tried to pay and he told me I could get it next time.  I can't even remember what we talked about.  That's my most vivid memory of the professor.  It's also my greatest regret—the one I haven't been able to get out of my head since last October—I never bought him coffee.

When I arrived at Caltech, I was absolutely certain I wanted to do whatever it was that he did.  Never mind that I had no idea what he actually did.  He was my mentor and I was determined to make him proud.  I'm still not entirely sure what he did.  It's a bit beyond my current knowledge base.  Anyway, this is all a bit beside the point.  I talked to John Huchra today.  I went through my SURF last summer feeling guilty that I was working in his building when he was gone.  A building that I, as an undergrad, had no right to.  I talked about that too.  It's why I've been avoiding visiting him.  I may have a SURF at the CfA again this year, though, and I had to deal with it some time.

So we talked.  And I realised that over the last term I no longer want to be him.  Impress him, yes.  Live up to his expectations, yes.  But be him, no.  If I go into his field, it will be entirely a matter of chance.  I don't know what I want to do with my life anymore.  But I do know that one day I'm going to be a real astronomer working on whatever field I find interesting.  And I have a right to work at the CfA.

As people reading this may know, we had an assignment a while back to interview an astronomer.  As we all know, astronomers are busy people.  I got one interview back, but Dr. Jan Vrtilek at the CfA was kind enough to fill out an interview when he had time.  I first met Dr. Vrtilek several years back as the father of a high school friend.  He has since transformed from "my friend's dad" to being a colleague and I had the pleasure of talking with him several times during my SURF at the CfA this past summer.  His support and advice were valuable resources.

What is the difference between an astronomer and an astrophysicist at this point in time? Which, if you have a preference, are you?

My short answer would be that there's really no difference at this point in history, and that astronomy/astrophysics is really just a branch of physics (which is why a strong background in physics is likely the optimal preparation for an astronomy/astrophysics career). But a fuller accounting may be a bit more complicated: because astronomy/astrophysics is largely not an experimental science in the usual sense (with exceptions for some areas such as laboratory astrophysics) there is an ongoing history of empirical/statistical/classification studies that can run ahead of a well-developed physical understanding; historically, such work has been associated more with the "astronomy" than the "astrophysics" aspect of the science, and some of these historical associations are still reflected in the subject areas of papers that have shown up in, say, the Astronomical Journal rather than the Astrophysical Journal. But these are small effects, and not something that I think is much worth worrying about.

I work principally in the are of galaxy groups and clusters, with emphasis on the mechanisms of AGN feedback on intragroup and intracluster gas. The decision to work in this area was essentially 'opportunistic': when I joined the Chandra project a couple of years before launch, I had just returned to the CfA from a couple of years working at NASA Headquarters in D.C., and there was an opportunity to move from my earlier area (molecular astrophysics) into a new, potentially very exciting area related to what Chandra would do (and indeed, it turned out that Chandra has made quite remarkable contributions in this area, so it turned out very well).

I was a physics and math double major as an undergrad at Wisconsin/Madison. As my undergraduate time was drawing to completion, I know that I would need a specialization, and thought about what area would hold long-term appeal for me. Given my liking for the two astronomy courses that I'd taken, my experience with a quite unusual summer program in high school (the Summer Science program, that has been running for a half century, and is now operated as a nonprofit by its alumni: seehttp://summerscience.org), and the general excitement and promise of the field, the choice wasn't difficult. I applied to a half-dozen graduate schools, and five accepted me. These were days when travel was expensive (at least for me) and there was no internet; I chose Harvard based on rather general considerations and with far less information than a modern student would consider acceptable.

In a general sense, it's about what I expected. I never had a single conception of what my future must be ("university professor or bust"), but wanted to be active in forefront work that I found exciting, and my career has done that quite well. Although I did my thesis in active galaxies, the most exciting offer had me going to postdocs and then staff positions at Columbia, Goddard, and the CfA in molecular astrophysics and infrared astronomy. At that point, and with finances running low, I had an interesting offer from NASA Headquarters to join the Visiting Senior Scientist program (alas, it no longer exists) and become a visiting administrator for NASA's infrared and radio astrophysics programs. I had the great good fortune of landing in a team of terrific mentors and colleagues who are still now my close friends, and was able, among many duties, to work as program scientist for COBE, which I think of as one of NASA's greatest missions. I'm not sure that I'd want to do this work forever, but for two and a half years it was a tough but invaluable experience that taught me much about the management and funding of science, but also brought me into touch with a cross-section of the most interesting people and projects of that time. Just as I was finishing up, the Chandra project, a couple years before launch, was staffing its science center, and I was very pleased to get an offer to return to the CfA, in a new life as X-ray astronomer. This also gave me the chance to select a new research area, and I chose X-ray study of galaxy groups and clusters (as described in 'areas of research' above). 

Now all of this is hardly "typical", and I suppose that if it teaches a lesson it's that while planning and goals for your career path are necessary and important, what actually happens depends on the unpredictable fluctuations of opportunity. Flexibility is a virtue.

My goals have always been to "do something interesting" in the field, and I've never been committed a priori to some particular, narrowly-focused area. As a result of this approach, as well as of the practical consequences of managing a two-career family, I've had several switches of subfield. (The plus to this is a thrilling ride through a broad swath of astronomy; the downside is never having that confident settled feeling, but instead the sense of perpetually racing to catch up in the latest area.)

Given my long association with NASA projects, I suspect that I would be likely to be working in the aerospace industry.

I'll identify the two most positive aspects for me: 

(1) The intellectual excitement and challenge. I hardly have to expand much on this for you -- you're already in the middle of it. For me it has aspects of using physics I like in really cool situations on the grandest scale, a sense of contribution, however modest, to an enduring and worthwhile enterprise, and the pleasure of great variety. 

(2) The company of my colleagues. I have been very fortunate in my colleagues and friends. I try to be surrounded by people who are smarter than I am (I strategy I strongly encourage!), and who are also people of decency, humor, and integrity with whom I work on a basis of enduring trust and friendship. This matters immeasurably to the quality of life. 

As to the worst, there are the inevitable exhaustion, frustration, and uncertainty. But you are not likely to escape occasional encounters with these in any challenging profession, and there is no line of work that I can think of these days that will offer you a sure path to peace and security!

I can't offer much more here than the obvious: get a really solid physics and math background. Work with a strong sense of strategy. Find good advisors and listen to them. Find talented, reliable, congenial collaborators. 

There is maybe one point at which particular care is warranted. When you're going for your PhD, your choice of advisor and research group is critical. This is a spot where problems can arise that cost you much time and frustration. You'll want to assess very carefully the project (for how workable it is and how it will place you for your future plans), the advisor (for success with earlier students), and the entire team (for the "spirit" and effectiveness of the group).

I suppose the very high level of uncertainty at various points, especially early on. There was a whole series of moves at 2 or 3 year intervals, which means being in job-search mode about half the time. It came with the uncomfortable realization that there's a fair amount of luck involved; I've gone through searches with only one offer at the end, and that makes the role of chance all too clear. I can't say too much about the importance of a really positive and supportive personal life to sustain you through the difficult spots, and to help you enjoy the smooth stretches!

Not at the moment. You have a good set of questions! If there's anything you'd like me to clarify or expand on, please just let me know.

Second Authors: Nathan, Lauren


Introduction
We know how stars are formed to a certain extent and that while they are on the main sequence, they are supported by hydrostatic equilibrium. However, when a star moves off the main sequence and can no longer support itself, what happens? We assume that at this point, the core of the sun has converted all of its mass to energy and is now undergoing gravitational collapse.

Methods
We know that the Sun generates energy throughout its lifetime at a rate of:

   

 

 

It is easy to tell that we have one molecule per cubic lambda.  So we have:

The actual value is 8 times this, for reasons I can't remember, but Nathan tells me Professor Johnson said the factor of 8 was okay to include in our calculations.  So multiplying this by the mass of a hydrogen atom and using T = the temperature of the sun's core, we get density

Which is more than twice the current maximum density of the sun's core.

Conclusions
We find that by converting 0.7% of the mass of the sun's core into energy, the sun's lifetime is roughly 10 billion years, which agrees with what scientists have predicted.  The density of the core after collapse will also be far greater than the current density of the sun's core, which is reasonable or it would not be able to support the sun post-collapse.

As some readers may know, this past summer I had the privilege of being able to work for Dr. Andy Goulding at the Smithsonian Astrophysical Observatory.  Dr. Goulding is a first year Smithsonian Research Fellow in the High Energy Astrophysics division.  He did his PhD work in AGN activity at the University of Durham, UK in the fall of 2010 before moving to Boston as a postdoc.  Working with him was a wonderful educational experience that I'd happily do over if I had the chance.  He gave me a look into the life of a researcher and how much research differs from course work.

We've vaguely kept in touch since September and he generously agreed to answer a few questions on his career for me.  Of course, given the title of this blog, the first question was obvious.  He took a lot of time in answering these questions, and I definitely learned some new things.  For example, I had no idea that there was a difference between a postdoctoral research associate and a fellow.


What is the difference between an astronomer and an astrophysicist at this point in time?  Which, if you have a preference, are you?

From a professional point of view, this is really semantics. People have degrees/PhDs in astronomy and/or astrophysics - it depends on institution. However, it is more likely that someone who is an amateur (non-PhD) is considered to be an astronomer. Classically, an astrophysicist attempts to understand and interpret the astronomical observations through application of physics. My PhD is in astrophysics, so in the strictest sense, I am an astrophysicist.

Black hole growth and galaxy evolution. Black hole physics was considered a "popular science" in the 1990s, so when I was younger it was very easy to pick up a book in the local store, and this naturally progressed from a hobby into a profession.

As an undergrad I studied Theoretical Physics - this involved particle theory, supersymmetry, advanced quantum theory and general relativity - this had little to do with astronomy, so it does not necessarily follow that your undergrad major must be your graduate major. Despite offers from several graduate schools in the UK, I decided to stay at my undergrad institution, Durham University, as it has a fantastic worldwide reputation as well as the largest astrophysics/cosmology department in the UK.

Once a predoc has completed their PhD, they will generally look for a Post-doctoral position at a different institution - these come in 2 flavors: (1) research associate and (2) fellowship. A research associate position is when a group/faculty has money available to employ a post-doc to carry out their specific research and help pre-doc students within the research group. A fellowship is generally a highly sort-after monetary prize which may or may not be linked with a specific institution and allows the holder to carry out the research of their choice - this is often predicated by the research proposal which is submitted in order to win the fellowship.

At this stage, I am not really in a position to answer this question. I completed my PhD in late 2010, and moved to Harvard shortly after to begin my fellowship which I had been accepted for earlier in the year. As I have only been here for a little over 12 months, it is not really possible to answer this question. 'Typically', an astrophysicist will expect to go from grad student (3-6 years) -> post-doc associate (2-3 years) -> [post-doc associate (2-3 years) ->] post-doc fellow (2-3 years) -> [post-doc fellow (2-3 years) ->] associate professor (3-10 years) -> tenured professor (indefinite). N.b., I added the other post-doc positions as some people prefer to stay as post-docs for longer periods of time to help with their publication records to move to the next stage and/or keep their teaching duties lower.

As I said above, my career is still becoming established. As I skipped the post-doc associate stage, and gained a fellowship on my first position, my current goal (for next year) will be to win a second prize fellowship to further expand my publication record.

My Masters degree is in theoretical physics, many people with this degree become statistical analysts and/or military defense specialists.

This is quite an interesting question as I'm pretty sure that you will get a different answer from every person. Being an astrophysicist is all about 'puzzle solving', we all have some intrinsic desire to answer the questions which are the most difficult to answer, so when we answer them, or move a step closer to answering them, this is the 'best part of being an astronomer'. However, it can be worst, you can find yourself working on one project for 6 months and then finding out that you have gone down completely the wrong avenue, and you have to start over. Of course, there are certainly perks too - for example, we travel to very exotic places (a lot), telescopes ares not generally in well-populated areas so you get to travel to places like Hawaii, the Canary Islands, Australia, the Atacama desert (Chile).

Of course, get very high grades and after that, you need something on your cv that will get you noticed (e.g., a summer studentship in a department)

At this point in time, astrophysics is struggling for funding from the government due to certain projects costing significantly more than was originally budgeted for, as such further funding money which would be relatively easy to propose for 5 years ago is not forth-coming. Hence, much more time must be put into proposing for the next year's projects, and this slows down the current research. This is not necessarily 'difficult' but it is certainly frustrating.

Astronomy as an undergraduate student and astronomy research are nothing alike (as you have already seen, Eric).

Introduction
Star formation is governed by the collapse of a cloud of particles into a gravitationally bound sphere which we call a star.  The radius of the could at which this occurs is called the Jeans Length, where the gravitational force of the cloud overcomes the thermal energy causing it to expand.  Here we examine the time scale of such a collapse and also calculate the Jeans Length.

Methods
In order to determine the time it takes for this collapse to occur in terms of the mass and size of the cloud, we consider a cloud of mass M and a test particle a distance away from it.  We assume the cloud has a mass given by

 

We were recently, or not so recently—I'm very good at procrastinating—assigned a multi-part blogging to find out what it truly means to be an astronomer.  I realise as a sophomore astrophysics major that I still don't understand the specifics of what being an astronomer entails or means to me.  To quote my friend Alexa,

I just want to be one. So much. “Space” is, if you think about it, everything but Earth. When we study it, we’re pausing our narcissistic tendencies for just a moment. We’re not everything; we’re part of everything. Ignoring that is shameful.

She stated in the best way possible what attracts me to astronomy, but that still doesn't mean I know what astronomy is.  Right now I just think of astronomy as some nebulous loosely defined field of Things I Would Like To Do Because They Are Amazing, but that's not an acceptable answer to the question.  So without further delay I shall attempt to synthesise my thoughts on the topic.

The point of being a professional astronomer, in my experience, is to contribute understanding of what the universe is, how it is structured, how it came to be, and what its future might hold.  Most likely this is because the first astronomer I ever met was a professor of cosmology.  I've come to accept that he's probably the reason my main research interest tends to observational cosmology.  Of course, this is a very broad and relatively unhelpful answer to the astronomer question.  Sure, that's the intention, but how do we get there?

I think it's safe to assume that the journey to becoming a professional astronomer begins as an undergrad, or if you're very lucky, as a high school student.  I think mine was a bit of both, as I did get the opportunity as a junior to do some busy work for he of blog title fame.  But that was a week long, and although it was some exposure I doubt it's how careers in astronomy start.  Careers in astronomy, at least for a Caltech student, probably start with a SURF fellowship.  I know SURF was my first real look at what an astronomer does.  I sat alone in an office 8-9 hours a day, writing code in a language I'd never seen before to analyse data I didn't understand, and I had fun doing it.  I think that enjoyment that's what sets the astronomer apart from the average person.

Then the natural course of things is to go to graduate school.  This is where you decide What You Want To Do With Your Life.  As far as I know, you don't have to decide right away.  Unless you're in the UK in which case you need to know what you want to study before you've learnt anything about it.  At least, that is what my SURF mentor who is English tells me.  Being a grad student requires doing semi-independent research under the guidance of a faculty member who works on a similar topic.  You'll probably start to hate your field at some point during this process, but hopefully you'll get over it soon.  Next is the postdoctoral fellow.  I have no idea what a postdoc does.  Don't tell my SURF mentor that because he is one.

I believe your career options then become a) professor at an academic institution, b) research scientist some place like the SAO, or c) finance.  I'm sure there are more options, I'm just uneducated in that side of things.  The first two options strike me as pretty similar except the professor track astronomer will probably have to teach at some point or another.  This part is where you get to move on to independent research in topics that interest you.  You might find out something that only you know about, and that's a rewarding experience.  Although any work in astronomy is rewarding if it's what's truly exciting and inspirational to you.

I have given my impression of what it takes to become an astronomer.  So once again, we're back to the question of what does it mean to me to be an astronomer?  It's going to take a lot of work.  Astronomy, as it turns out, is hard.  But the work will be worthwhile because I'll be learning how the universe works, or how we think the universe works.  Maybe I'll end up amending some of that knowledge.  Who knows?  Being an astronomer means getting excited about the mysteries of space and our tiny place in it.  It means realising how small we really are in the grand scheme of things, accepting that, and moving on to understand why.  Most of all, it means that when your friend starts talking about M83 and means the band, this is all you can see.

Abstract
We would like to know how the sun is being "supported".  We assume that this mechanism is hydrostatic equilibrium, but to be sure we work through the derivation.

Introduction
We know that the sun is somehow being prevented from gravitational contraction.  Our theory is that it is supported by hydrostatic equilibrium, which means that the internal pressure provides an opposing support force.  We calculate the gravitational force on a mass shell, the pressure required to balance it, and then derive the force equation for hydrostatic equilibrium.

Methods and Results
We first assume the Sun to be a spherical gas cloud with density ρ(r).  We consider a differential mass shell of this sphere with radius r.  We recall that the volume of a sphere is ⁴/₃πr³ and that the differential volume is its derivative. Then we get a differential mass dM:

We know the equation for universal gravitation:

Here we let M be the total mass enclosed by the mass shell and m be the differential mass element.  As a result, we get the differential gravitational force to be:

We know that pressure is equal to force divided by area.  So we can say:

Now dividing by dr on both sides of the equation we arrive at the equation of hydrostatic equilibrium:

Conclusions

We have derived from simple physical laws that the equation for hydrostatic equilibrium is a plausible explanation for the way the sun is supported.  A quick search shows that we are indeed correct.  Hooray!

Abstract
Considering the angular diameter of the sun and the astronomical unit, we can estimate the radius of the sun, the AU in solar diameters, and the mass of the sun using Kepler's 3rd law.

Methods
Applying basic trigonometric identities and taking the astronomical unit a to be the distances from us to the closest point of the sun to us (i.e., the centre of the circle we see from earth), we can see that:

Multiplying through by a we get a value for the radius of the sun.  It is clear from here that if we divide a by twice the solar radius we can easily determine the answer to the second part of our question.  Finally, we have Kepler's 3rd law:

Where P is the period of the earth and G = 6.7 x 10^-8 dyne-cm²/g².  From here we can solve for the mass of the sun.

Results
Solving the first equation using a = 1.5 x 10¹³ cm we get the radius of the sun equal to 6.545 x 10¹⁰ cm which is very close to the actual value of 6.955 x 10¹⁰7 s.  Dividing, we get 1 AU = 114.6 solar diameters.  Then, solving for the mass of the sun in Kepler's 3rd law with P = 3.154 x 10⁷ s, we have the mass of the sun equal to 2.007 x 10³³ g which is a surprisingly accurate number.

by Eric S. Mukherjee, Nathan Baskin, and I forget who else (sorry).


Abstract
In this problem we consider the how the temperature of the sun affects the temperature of the earth.  This is possible to estimate by assuming both the sun and the earth to behave like perfect blackbodies.

Introduction
Assuming the Earth has constant surface temperature and that it behaves like a blackbody, we can estimate the surface temperature using the energy emitted by the sun.  We also assume the sun to be a perfect blackbody.  Under these assumptions we can find the surface temperature of the Earth by knowing the temperature of the sun, the radius of the sun, the mass of the sun, the mass of the earth^*, and the radius of the earth.

Methods
We start with the equation for flux at the surface of a blackbody (σ is the Stefan-Boltzmann constant):

From this we derive the luminosity of the sun by multiplying through by the surface area:

Then the flux of the sun at the surface of the earth is (where a is the astronomical unit):

If we consider the area of the earth through which the flux passes, it is the circle of area π R_⊕².  Multiplying through by this quantity we get the power input to the earth from the sun.  We then realise that this is necessarily equal to the power output of the earth due to energy conservation which, at the surface of the earth, is equal to σT⁴π R_⊕².   Thus we have an equation of the form:

Using this equation with R_☉= 695,500 km and T_☉= 5778 K, we get T_⊕ = 279 K.

Conclusions
This temperature that we calculate is around 5.5°C which sounds reasonable for an earth without accounting for atmospheric greenhouse effects and allowing for the temperature at the poles.  The true average temperature of the earth is around 16°C but that is measured with the warming effect of the atmosphere.  The sun is not a perfect blackbody which also contributes to the difference between our calculation and the true value.

Acknowledgements
I'd like to thank the entire Ay 20 class and teachers for collective brainpower due to the fact that I can't remember who exactly worked on this problem and I'm sure we drew from the knowledge of many people in the room.  I'd also like to thank the superior computational power of Wolfram Alpha for bringing to my attention that there exponents matter when calculating ratios and that the temperature of the earth is most definitely not 1270 K.



*Note: It has been brought to my attention by Professor Johnson that the masses of the earth and sun do not actually factor into this calculation at all unless we need them to derive some of our other known constants.

As my readers may remember, a few weeks back I posted about the properties of AGN and how they affect their host galaxies.  One of these ways I listed was star formation rate.  Actually, a bit less than two weeks ago an article was reprinted from UCSD by ScienceDaily that AGN do not stop star formation as previously thought.

Prior research showed a correlation between the presence of AGN and the lack of star formation in galaxies.  This new study claims that this was a function of observational bias.  Older, more massive galaxies are easier to detect, and are also the ones with decreased star formation rate.  This study finds AGN in all types of galaxies including those in which stars are still being formed.

Read the full article here.

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.

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

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.

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.

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.

Radius of Earth

A few days back, Jackie asked:

by: Eric S. Mukherjee, Nathan Baskin

and