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Petite Mercury

 Mercury is the smallest planet in our Solar System. But it is actually smaller than some if the moons in our Solar System.

 

Here is a graphic which shows Mercury's size in comparison to some of the larger moons.

 

So why is Mercury a planet even though it is smaller than Ganymede and Titan (and barely larger than Callisto)?

 

We need to define what makes an object a planet.

1.       It orbits around a central star.

2.       It falls into a well-defined category of planets: terrestrial or Jovian.

Obviously, Mercury fits criteria one. It orbits the Sun.  But so does Pluto. Why isn’t it a planet anymore?

Let’s just look at what defines a terrestrial planet. From its name, terrestrial planets are ”Earth-like”, i.e. similar to Earth. Mercury is a terrestrial planet.

Hold on, you might say. Mercury is nothing like Earth. It’s hot and rocky, has no atmosphere, and there is no liquid water on its surface. You would be correct, in that sense.  But what we mean by terrestrial planets is that they have a defined set of parameters.

1.       They are smaller than Jovian planets.

2.       They have a few or no natural satellites.

3.       Their densities (mass contained in a specific volume) are large, i.e. rocky.

So let’s compare Mercury and Pluto.

1.       They both are much smaller than Jovian planets.

2.       Mercury has no moons, while it is believed that Pluto has at least five known moons. Compare this to Neptune which has at least 14 moons, the fewest of the Jovian planets.

3.       Mercury’s density is about 5400 kg/m³and that of Pluto is 2000 kg/m³. Earth’s density is 5515 kg/m³.

So the third reason is why Pluto is not a planet. More discussion on why Pluto is not a planet will follow later on.

Another unique thing about Mercury is that it does not have an atmosphere.  Mercury might have once had an atmosphere, but because of its proximity to the Sun, the solar wind probably stripped Mercury of an atmosphere.

Come back next time and we will discuss Mercury being lopsided.

Mercury

Mercury is the closest planet to the Sun in our solar system.  It is also the smallest planet in our solar system.

Everyone knows the Sun is the largest single object in our solar system. There are eight planets with Jupiter being the largest and as previously mentioned, Mercury is the smallest.  Based on this, you would think Mercury is the 9th largest object in our solar system.  It is not.

Planets are generally spherical in shape due to rotation and gravity. Mercury is not.

Planets have an atmosphere. Mercury does not.

Mercury helped proved Einstein's General Theory of Relativity.

Mercury has a unique orbital resonance with the Sun because of its proximity to the Sun.

Stay tuned for explanations.

And before we begin, we've found Han Solo frozen in carbonite on Mercury.

Our Average Star

The Sun is an average star. It is a main sequence star, classified as G2V.

From Cornell University's Department of Astronomy

We should start by defining what we mean by Main Sequence.  In the above diagram, called the Hertzsprung-Russell diagram or H-R diagram, you can see that most of the stars that have been classified lay on the band starting in the upper left to the lower right.  The stars in the upper left are hotter, bigger, more luminous but generally younger than those in the lower right. Despite these differences, the stars and all the stars in between on this bad are similar. All these stars are fusing hydrogen in their cores; via the proton-proton chain or the C-N-O cycle*.

*The C-N-O cycle is just another way of converting four hydrogen nuclei into one helium nucleus, using carbon (C), nitrogen (N), and oxygen (O) as catalysts. This cycle is generally found in hotter stars. We will not go into detail here about the C-N-O cycle. If you wish to learn more, leave a comment below.

Generally, the hotter and bigger a main-sequence star (also called dwarf stars, not be confused with white dwarfs), the less time it spends on the main sequence. From the above diagram, the hot stars are classified as O and B, medium stars are A, F, and G, and cooler stars are K, M, L, T, and Y.  The letters were originally meant to represent the strength of their hydrogen lines with A having the strongest and O stars the weakest (L, T, and Y are relatively new classes). Now, the spectral types are divided into temperatures classes. Spectral types can be broken done further in each type. For example, the hottest A stars will be classified A0 and the coolest A9.

Why do hotter stars spend less time on the main sequence than the cooler stars? They do have more mass, therefore more hydrogen to fuse into helium. But, because of their massive size, they must fuse hydrogen much faster to overcome the stars propensity to collapse under its own weight*.

*This process of preventing a star from collapsing under its own gravity is called hydrostatic equilibrium. The force from the energy created in the core equals the force of gravity and keeps the star from completely collapsing. However, in some stars, gravity can overcome the force of photons pushing out. But that is for a different post.

G stars, then, are average. They have hydrogen lines that are medium in strength, stronger than K, M and O stars, but weaker than A, B, and F stars. G stars are noted for having strong Ca-II lines (doubly-ionized Calcium, Ca 2+). They fall in a surface temperature range of 5200 to 6000 K (our Sun is about 5800 K) and are yellowish in color.

Stars are also classified by luminosity. Class 0 stars are hypergiants, Class I stars are supergiants, Class II are bright giants, Class III are giants, Class IV are subgiants, Class V are main sequence or dwarfs, Class VI are subdwarfs, and Class D are white dwarfs (not the same as main sequence)*.

*White dwarfs are the final stage in the life of a lower mass main sequence star. Our Sun will end up as a white dwarf once it begins to evolve off the main sequence in about 4.5 billion years.

So our Sun is average. It is a G2V star, similar to 7.5% of all main sequence stars in the solar neighborhood (i.e. stars within 250 light-years of the Sun). But why is it weird?

1. We have planets in our solar system. Well, so do other stars. But up until the mid-1990s, we thought that we were the only planetary system.
2. The lower mass planets are closer to the Sun than the higher mass stars. From everything we’ve seen, it appears that higher mass planets are closer to their parent star compared to our planetary system. This may be a selection bias as it is easier to find larger planets closer to their parent stars than those farther away or lower mass planets close to their parent star. As technology gets better, this bias may go away.
3. In one word: YOU. You exist and as far as we know, Earth is the only celestial body that has life of any kind. The one thing we are doing now is looking for planets that may have liquid water*.

*Water is important because it is the only substance in the universe that is actually less dense as a solid than as a liquid. At 4°C, water is at its densest. If you take a glass of water and put in ice cubes, the ice floats. This is common knowledge. But scientifically, this makes no sense. Solids are denser than their liquid counterparts, and gases are the least dense. Why is this important? We know that life began in water, and if water followed the rules of phase changes, as the surface water froze, it would sink to the bottom, until the entire body of water was frozen solid. Nothing could survive in that, and life could not evolve.

We could find life around other stars with better technology, but for now, we are the only planet with life.

There are things we do know about planets around other stars.

1. Planets around O, B and in a small way, A main sequence stars, probably do not have life or at least intelligent life because those stars do not spend a lot of time on the main sequence (generally less than a billion years) which would not give life a chance to evolve.
2. Planets around M, L, T, and Y main sequence stars probably do not have intelligent life because in order to have life, liquid water is a must. Planets around these stars need to be close to the star to be warm enough, must be large enough to have a magnetic field, and probably much larger sibling planets to deflect comets or asteroids that could potentially prevent any sort of large life forms from evolving.
1. It must have the right surface temperature to allow liquid water to flow on the surface.
2. It must be large enough to have a liquid molten core to create a strong enough magnetic field to prevent radiation from its parent star from reaching the surface.
3. If there was enough material in the stellar nebula, the star probably would have been bigger.

 Next time, we will get into the planets in our solar system itself, starting with Mercury, our smallest planet.

Sunspots

The Sun goes through a phase called the solar cycle. This is a period of 11 years where the quantity of sunspots typically change over the course of the cycle.  At the beginning of the cycle, the sunspots are few in number, and begin to appear around 40° latitude, in both the northern and southern hemispheres.  Near the end of the cycle, the number of sunspots reaches a maximum. Then drops to zero. Then a new cycle begins.

However, at the beginning of the new cycle, something strange happens.  The Sun switches polarities. The North Magnetic Pole becomes the South Magnetic Pole, and vice versa.  This also happens on Earth, but takes much longer to occur, approximately hundreds of thousands of years. This will be a topic for future blog post.

The 11-year cycle is the period that describes the quantity of sunspots and other phenomena of solar activity. The Solar Magnetic Cycle is the 22-year period where the Sun flips its magnetic poles, then back again.

So, what exactly are sunspots?  Sunspots are large blemishes that dot the surface of the Sun.  In a previous blog post, it was discussed how the Sun does not rotate uniformly, but at different rates at different latitudes.  In the course of rotation, magnetic field lines on the surface of the sun get “kinked” and leave the surface of the Sun.  Therefore, a sunspot is a pole of a giant magnet . And because there does not exist a magnetic monopole (in theory), sunspots will always come in pairs.

Sunspots appear dark on the surface of the Sun because they are cooler than the surrounding photosphere. In reality, sunspots are 4500 K* (7640°F) when the surrounding photosphere is 6000 K (10,340°F).

*Kelvin (K) is the temperature scale used in astronomy. It is based on 0 K being absolute zero (-273.15° C or -459.67° F) with absolute zero being the lowest theoretical temperature possible. Nothing actually can be 0 K.  The coldest temperature achieved on Earth is liquid helium, which is at 4 K; in space, interstellar/intergalactic space is around 2.73 K.  

The reason why sunspots are darker/cooler is that they prevent convection cells from forming below them. In The Sun’s Layers, the convection zone is directly below the photosphere of the Sun and is what transfers energy to the surface from the inner layers of the Sun.  By blocking the cells from bringing up energy, the sunspots will be cooler than the rest of the surface.

Sunspots are also below the surface of the Sun. the center of the sunspot is called the umbra, and the edges are the penumbra.  The penumbra slopes up to the surface of the Sun from the central umbra.

 

Neutrinos

 

In the last post, we discussed how energy is created in the nucleus of the Sun via the proton-proton chain. One of the remnants of converting four hydrogen nuclei into one helium atom is a particle called an electron neutrino.

 

Why is an electron neutrino weird? Well, there are a few reasons.

 

1. Neutrinos are not exactly massless, but very near so. It is believed that neutrinos could be part of what is known as dark matter. If one can get enough neutrinos, they can make a substantial dent in the amount of dark matter theorized to exist.
2. They are highly non-reactive. Right now, there are approximately 65 billion neutrinos are passing through every square centimeter of your body right now. But don’t worry, they don’t like you. On average, it would take a block of lead one-eighth of a light-year* wide to stop one neutrino.

*one light-year is the distance light travels in one year. It is approximately 5.9 trillion miles, or 25 million times the distance from the Earth to the Moon

1. There is a way to count neutrinos, but it takes a lot of water. Here is a link to a neutrino detector in Ontario, Sudbury Neutrino Observatory
2. There a three types, or flavors, of neutrinos. These three neutrino flavors are associated with the three leptons; electrons, muons, and tau particles. There are also the anti-neutrinos associated with the antileptons; positrons (anti-electrons), anti-muons, and tau anti-particles.
3. Neutrinos can change flavors if they interact with matter. Neutrino detectors only count a third of that expected by the fusion of hydrogen into helium. This was known as the Solar Neutrino Problem until it was discovered that neutrinos can change flavor.

Drunken Photons

 

The sun creates energy by fusing hydrogen into helium in the core. The process of taking four hydrogen nuclei and combining them to make one helium nucleus is a process called the proton-proton chain*.

 

*Fusing four hydrogen nuclei into one helium nucleus is called the proton-proton chain because one hydrogen nucleus is just a proton. The neat thing about a helium nucleus is that it is made up of two protons and two neutrons, and the total mass of the helium nucleus is 0.07% lighter than four protons. That extra 0.07% is converted to energy via Einstein’s famous equation, E=mc².  That may not seem like a lot of energy, but considering that there are trillions reactions occurring every second in the core, the energy adds up to quite a bit.

Gamma Ray represents energy

Fusion in the Sun

 

The energy given off by the proton-proton chain is emitted via high energy photons. These photons are able to escape the core and enter the radiation zone of the Sun.  It is here that something weird happens.

Photons collide with atoms, and give some of its energy to the atom. This lowers the energy of the photon.

Photons can also be re-emitted at higher temperatures. This causes the photon to actually gain energy.  A photon can either lose energy or gain energy. This path the photon takes is called a random path.  The best way to describe it is a drunken walk. If you have ever watched a drunk person walk, you notice that the next step he or she takes is totally at random.

Australia National Telescope Facility

 

In general, the photon emitted via the proton-proton chain is a highly energetic gamma ray. Via a series of collisions, that one photon ends up being thousands of lower energy light photons. However, it takes hundreds of thousands of years.