What are stars?

A good question.  Are they like people?  They live.  They die.  They have populations and clusters.  Technically as Carl Sagan said “we are star-stuff.”  So people and stars are sort of similar.  But the main difference is that they are gigantic spheres of extremely hot plasma.  Okay, so not exactly the same thing, but they are still assuredly awesome.  One could even say stellar.

A star is a large amount of gas, mostly Hydrogen, and is held together by two forces that are essentially equal.  This is called hydrostatic equilibrium.  The name comes from fluid dynamics, where the only fluid, incorrectly, was thought to be water.  The first force is gravity.  On a large scale gravity has more importance and attracts this massive amount of gas together.  But with only gravity attracting the gas inwards there has to be an outwards force to create equilibrium.  This force comes from fusion.  We say fusion powers the Sun because it releases insane amounts of energy and can create a radiation pressure that works outward against the force of gravity.  It has so much energy, it ionizes the gas and is the reason why stars are made of plasma.  The Sun and in fact the whole night sky is all thanks to fusion.

Stars have many qualities which will be discussed later, but an important aspect is that just like anything else they have a structure.  Here is the Sun:

Parts of a Star

If it could talk, it would probably be bragging how hot it is (even though it’s not that hot for a star) and that it is like an onion; it has many layers.

Stars have many shells or layers.  Fusion begins with fusing Hydrogen into a shell of Helium.  In general, as fusion occurs, heavier nuclei sink towards the core or center and form these shells from convection.  The core therefore has the most energy since it takes more energy for higher level reactions to occur and it is the central area of energy production.  But going to the outside of the star it begins to cool down.  This occurs at first through radiation, the transfer of energy by light.  The light travels quickly through a Radiative zone where the light is emitted or scattered.  Then the star transfers energy with its chief method of energy transfer, convection, the transfer of energy through a fluid to balance temperature.  This occurs in the plasma of the star in the Convection zone.  On the Sun, hot fluid rises and moves out of the star, while cold fluid sinks, which is why it balances in temperature and cools down going towards the outside of the star.  Note that energy leaves a star by radiation, not convection, since energy from a star leaves with emitted light travelling through space, and space is not a fluid or solid.

The photosphere is similar to the radiative zone, but it more specifically refers to the presence of light and energy created. It represents the area where the light makes a star become opaque.  The chromosphere is similar to the convection zone and is where the colors of the Sun are mostly seen.  It technically can’t be seen except during a solar eclipse or with a light filter since the light of the photosphere over powers it.  The chromosphere also contains spicules, spikes of gas that eject out to the next part of the Sun, the corona.  This is similar to the Sun’s atmosphere and is made of plasma too.  A mystery still keeping scientists up at night is that the corona is hotter than the surface of the Sun.  But it isn’t just a little hotter.  To compare temperatures, the core is about 15-16 million K, the photosphere, closer to the surface, is about 5700 K, the chromosphere can be 5000-4400 K, and the corona is a million K.  Iron detected in the atmosphere could be the cause, but scientists are unsure.

The Sun also has a magnetic field just like the Earth.  We know it has one due to something known as the Zeeman Effect.  Atoms can produce light associated with magnetism when excited or ionized.  When this light is released lines of light at certain wavelengths can be seen, and these lines can be seen to split due to magnetism.  The magnetic field is most likely due to the movement of plasma which creates an effect similar to a dynamo, which uses motion to rotate a coiled magnet and through some awesome science this creates electricity.  On the Sun it’s a bit different, but theoretically the plasma convecting should be conductive and extremely magnetic, therefore the massive amount of rotation on the Sun  should create a magnetic field.

Many things that occur on the Sun involve this magnetic field, the following are examples.  Sunspots are relatively cool (emphasis on “relatively” since they can be 3000-4500 K) and appear dark.  The magnetic field shifts around a lot since there is so much plasma moving around.  This creates differential rotation (parts rotating differently) in the Sun.  It can get so twisted up it can become like a rubber band, curling up and piercing the outside of the Sun.  This creates a spot where the magnetic field leaves and reenters the Sun.  There is an 11 year sunspot cycle because the magnetic field of the Sun flips like the Earth’s (just a lot more often since it is humongous) and can be traced to have minima and maxima.  Solar winds are streams of charged particles shot out of the Sun which can use the energy of the corona to escape the Sun’s gravity.  These can be the cause of geomagnetic storms, which can affect satellites and technology on Earth or be a possible danger for astronauts.  Coronal Mass Ejections or CMEs are an explosion where all that coiled up magnetic force throws out material into the Corona.   Solar flares are sudden, intense variations in brightness in the Sun.  This results in massive CMEs.  Both CMEs and solar flares are caused by sunspots and are associated with solar winds, but whether CMEs and solar flares are associated is still being researched.  Some of these can be characterized as a solar prominence.  No, not how the Sun is so awesome it is important, famous, or noticeable (though, that should be an alternate definition as the Sun does do that), it is something which can cause a loop-like shape starting in the photosphere and entering out into the corona.

If the Sun could talk here it may say how loopy it is.

There are multiple measurements for stars using the Sun.  They are useful since the Sun is a fairly average star, and it acts as a good basis for comparison.  There is the Solar mass (about 2X10^30 kilograms, fitting the mass of the Earth about 30,000 times), Solar luminosity (about 4X10^26 W, an average lightbulb is 100 W…it’s fairly obvious that the Sun is brighter), Solar radius (sadly not an easy 1 million km, but it is about 700,000 km; able to fit the Earth about 110 times), and age of the Sun (currently about 4.5-5 billion years, which is really long…there is really no other way to put it)

Next is the naming of stars.  Sadly, if you are looking to “buy a star” and name it (perhaps for romantic reasons) you will be disappointed.  At least, if you expect it to mean anything.  Most stars have been cataloged and therefore buying a name for it is the same thing as pointing to a star and making up a name for it, except you’d have to pay for it.  Ancient Astronomers would generally name stars for a relationship to a constellation to make them easier to remember, but now there are catalogs and designations based on the imaging used, coordinates, and properties of the object being studied.  There is the Bayer designation, naming a star based on the constellation and brightness related to a Greek letter (so, the brightest star in the constellation Taurus would be called Alpha Tauri, but ancient Astronomers would call it Aldebaran which is sometimes used as a common name).  But now there are modern catalogs, the major ones being the Messier (as mentioned, M), New General Catalog (NGC), and Index Catalog (IC).  There are many more catalogs, but in general cataloging stars is extremely important.  They allow Astronomers to know where they are looking, what they are looking at, and how to revisit or calculate positions of an object that is being looked for.

One of the most interesting things to all this: these processes and more occur in stars in the whole universe.  As Carl Sagan said, “Billions upon billions”, referring to how the universe has billions and billions of galaxies, each having billions and billions of stars, all functioning more or less the same way.  In fact, the same processes that occur with light bulbs and power plants on Earth occur on the magnificent scale of the Universe, and that is nothing short of mind-blowing.


TL;DR — Stars are amazing.  They act as prime examples of how mechanics demonstrated on Earth can be applied on extremely large scales.  Stars develop from large clouds of Hydrogen which condense from gravity, and radiation from fusion repels this to maintain a star.  This makes magnetic fields and pretty much a giant ball of plasma.  Stars have been studied for 1000’s of years, and still have many mysteries to be found, which reveal more about the Universe as we study them.

This entry was posted in General.

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