What makes something a moon?

 

Every now and again I ask my sisters to come up with astronomy questions that they never thought to ask anyone before and, out of a list of weird and wacky questions, there is always at least one that sparks something and leaves me thinking “you know what, I’m not sure.”

The beauty of the world today is that we have access to all the data we could want to answer these questions it is just the case of hunting out the right sources and filtering out the credible information. So I went out in search of an answer to the beautiful and simple question of,

What is a moon?

to get a more in depth definition or series of parameters other than the logical definition of a natural satellite. Here is what I found out.

A moon is a natural satellite orbiting around a planet that is itself presumably orbiting a star. 

That is it. Even the IAU (International Astronomical Union) doesn’t have an official definition other than natural satellite. That said they didn’t have one for a planet until 2006, and that on a technicality excludes all exoplanets, so maybe it is best not to ask them to define it just yet, though they do have sole control over the naming of all natural satellites and planets.

Unsurprisingly the first moon discovered was our own, though it was originally thought to be a planet; it sets the generic image in our minds for what a moon should look like. A large spherical rocky body smaller than the planet it is orbiting. And while, yes a moon must be smaller than the object it is orbiting, it does not need to be significantly large nor spherical, and though most are primarily rocky, unlike our moon, many others exhibit geological activity and even maintain dynamic atmospheres. One moon of Saturn, Rhea, is even thought to have a moon of its own.

In our solar system there is a grand total of 171 natural satellites orbiting the 8 main planetary bodies, and that number has almost doubled since 2003 alone.

Mercury – 0

Venus – 0

Earth – 1

Mars – 2

Jupiter – 66

Saturn – 62

Uranus – 27

Neptune – 13

The four outer solar system planets (Jupiter, Saturn, Uranus, and Neptune) all also have ring systems. These are set apart from moons as they are not a distinct whole object and are instead comprised of dust and ices forming a continuous band around a planetary body.

Mars and its two moons; Phobos (left) and Deimos (right)

There are also a number of these moons that would perhaps benefit from another classification, such as captured asteroids. The classic example is the moons of Mars, Phobos and Deimos, which are among the smallest in the solar system and are irregular in shape; Phobos even has a crater nearly half the width of it distorting its shape even more. Jupiter and Saturn also have a number of natural satellites that are more likely captured asteroids, orbiting at highly inclined angles from the equatorial plane and travelling along elliptical paths about the main planetary body. Jupiter has 16 such objects orbiting it, and Saturn has 9. Some of these types of body are also thought to be the remnants of a larger moon that broke up altering the orbits of the debris while not leaving the gravitationally bound system.

Perhaps, like with the planetary definition, any definitive distinction between these object and their rounder neighbors will result in a reduction of the number of classified moons in the solar system. However, until then there will continue to be confusion and disagreements between the exact definitions until it can be clearly and concisely defined, most likely by the IAU.

Pictures credit: NASA 

What’s next?

For some fantastic information of the soar system and all of its natural satellites check out

http://solarsystem.nasa.gov/planets/profile.cfm?Object=SolarSys

The moons of our solar system by Ms.Kaiter©2006-2011 is also a great site to check out

http://mail.colonial.net/~hkaiter/moons.html  

The discovery of the Galilean moons in 1610 is brilliantly explained in Galileo’s own word at

http://solarviews.com/eng/galdisc.htm

Biggest, brightest, and the most dense

The beauty of the universe beyond our orbit is that it is a universe of extremes far, far removed from the nicely scaled world around us. But to what extremes can it go; what are the limits that it encounters, because lets face it there are always limits.

My question; how big, bright, or dense can a star physically get and have we observed them?

From the LMC zooming into massive stars (R136a1 on the far right)

4,000 light years away embedded in a large optical nebula is one of the largest stars to be observed, VY Canis Majoris. It was thought to be over 2,000 times the radius of the Sun, if you placed it at the center of our solar system the Earth would be engulfed, its surface would extend out past the orbit of Saturn. Recent observations, however, estimate the radius of VY CMa at a smaller but still considerable size of 1,420±120 Rsun. This type of star is known as a Supergiant, and although overwhelmingly large in size Supergiants are relatively cool, with temperatures around 3,000K, making them appear red in color. Surveys of supergiants have revealed that the largest can range in size between 1,000 and 2,000 times that of the sun, although the upper limit is fuzzy due to the nature of the observations required to make such measurements.

The upper limit for the radius of a star is likely to be around this range as at some point the outer material of the star will become unbound and the motion of the plasma will exceed the gravitational escape velocity of the core star. This type of star also commonly undergoes continuous fluctuations where the radius can change dramatically making an absolute measurement difficult, two of these pulsating variables can be found in Sagittarius, VX and KW Sagitarii, both with a maximum radius of  ~1,500 solar radii.

These large stars are found at the end stage of their evolution the Supergiants marking the last stop before they burn out and, although bright for their stellar type, are a far cry from the brightest of them all, the bright and the young.

Although there is not an absolute limit set on the radius of a star there is a physical limit in their mass. This is called the Eddington Limit, marking the delicate balance between radiation pressure from fusion at the core and gravity collapsing the mass inwards. It is named after the British physicist Aurthur Eddington who, in 1919, proved Einstein’s theory of relativity by showing that light can be bent by gravity. This limit was thought to be at around 150 times the mass of the Sun, an unimaginable 2.97x10³²kg. Stars around this limit are called hypergiants, with the most massive discovered to date breaking this boundary by over and additional 100 solar masses. This hypergiant lies at the heart of the Tarantula Nebula in the Large Magellanic Cloud and is known as R136a1, with its mass smashing the stellar scales at 265 times the mass of the Sun.

This Eddington Limit also defines the boundary for the maximum luminosity that a star can reach. Luminosity is the measure of the total amount energy emitted by the star, measured in Watts. The Sun’s luminosity is equivalent to 6.41x10²⁴ 60W light bulbs (that is 641 thousand million billion light bulbs). Hypergiants are young massive bright stars, and along with being the most massive R136a1 is also the most luminous out shining the sun by a whopping 8.7 million times.

Though it is true that “The flame that burns twice as bright, burns half as long” and although our sun has already lived for over 4.5 billion years R136a1 and other hypergiants have the shortest time to grace us with their presence as they burn off their fuel at an alarming rate, around one solar mass every 20,000 years, giving them lifetimes of just a few million years.

These stars at the end of their lives will most likely supernova and go out in flame and glory for all to see leaving behind a dense central core that then collapses under its own gravity, so much so that it forces protons and electrons to combine to make neutrons forming a “Neutron star”. Neutron stars have so much mass packed into such a small volume that one teaspoonful would weigh a billion tons. The densest object discovered thus far is PSR J1614-2230, a rotating neutron star called a pulsar, and is equivalent of to Suns being squashed into a sphere the size of a small city, 1.97 times the mass of the sun with a radius of just 13km.

low-mass infant stars coexisting with young massive stars

Each of these objects represents the remarkable limit of nature; from gigantic stars the size of a solar system to the youngest and brightest routinely burning of suns.

The universe has so many weird and wacky things to offer us breaking its limits each time we look up; we just need to make sure we keep looking. 

What's next?

The following are a few links that I used along the way.

Radius –

http://arxiv.org/pdf/1203.5194v1.pdf

Mass – 

http://www.skyandtelescope.com/news/98927839.html

http://www.bbc.co.uk/news/science-environment-10707416 

Density –

http://arxiv.org/abs/1010.5790

http://www.nrao.edu/pr/2010/bigns/

PICTURES FROM: BBC, Wikipedia, ESO and ESA

Exoplanet atmospheres using only the 1000 most common words in English

Here is my attempt at explaining what I do and how we do it.

Space is big, really big; you can't imagine just how crazy big it is, and empty don't forget empty. Sure there is lots of matter making up stars grouped together in their hundreds of hundreds, but that is nothing next to how much empty space there really is. So imagine what it is like to look at one of these stars and know that there is another world out there filling just a small but important part of that empty space. 

It is these big balls of rock, which grip our attention and light up the world inside our heads.

When you look up at the night sky and gaze at the crazy number of stars you can see, just think; half of those stars have big round balls of rock rushing around them and some of those will have skies of their own.

But how do we find out what their skies are made of and if the air is good or bad?

To look at the air round these big round balls of rock we need to look at the star they are moving around.

We point a big space box at the star and see how the light we see changes as the big ball of rock and air passes between the star and us. The big ball of rock will block some of the light but some will go through the air around it and if you look at the light in all different colors how much light being blocked will change for different parts of the air. This change in how much light we see is only very small but using big space boxes makes it easier, more so if you are looking for water around other worlds as the water in our air gets in the way when we look from the ground.

We can then build up a picture of the air around these strange new worlds from the change in light we see over all colors and see if water is joined by other parts of the air that make the big round balls of rock a nice place to visit, or not as is the case most of the time.

What we do is hard but we are always one small step closer to understanding our world and how we got here.

How much do you think you can write with this thing without it stopping you, are the top 1000 words really that good? Give it a go yourself and find out.

http://splasho.com/upgoer5/

Personally I could have done with the word planet on that list but big ball of rock and sky seemed to work well enough.

#UpGoerFive

The top image is from XKCD #1071

Humanities ego vs. faith

Are we alone in the Universe, the Galaxy?

The honest answer to that question is, I don’t know. And that would be the answer you would get from anyone in the world right now. However, there are two major opinions; yes, or no.

The greatest egotistical argument of humanity is that we are in fact alone, that we are the one and only ruling civilization in the universe, without having stepped out of the front yard.

Imagine the galaxy as an estate of houses, each street spiraling out like arms, our solar system just one house on one of those streets, the Earth and all on it occupying the equivalent of the closet under the stairs. 

How can we know for certain that on the next street across there isn’t a house with an occupied closet or one on the other side of the estate with rooms filled with life. Sure we have reached out into the galaxy we can see a large number of the houses around us and get a good idea about what the rooms might be like but probes like Voyager and Pioneer are still mere ants teetering on the top of the white picket fence out front, and the sparks of life haven’t quite reached out yet.

The bible defines what I have as faith; “The substance of things hoped for, in the presence of things unseen”. I have faith that another closet out there is filled like ours instead of filled with dust busters and old wrapping paper.

So instead of letting you ego become so large that you head won’t fit out the closet door to even look just think, either way what you have is faith, and question you need to ask is what it is you are hoping for.