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:

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.

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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.

The Doppler Shift

WARNING: This post involves copious amounts of semi-difficult math. If you don’t like the realm of radicals and fractions below radicals, I suggest you stay away from the math portions of this.

Most of you know of the existence of a phenomenon called the Doppler Shift (or Doppler Effect, they’re the same thing). You know, when a train moves towards you, the sound it makes is higher, when it moves away, the sound is lower. How high or low the sound is to you is referred to as the “pitch”. There’s another version of the Doppler Shift in relation to light and astronomy. It’s called the relativistic Doppler Shift.

The relativistic Doppler Shift works the same way the regular Doppler Shift does, only it’s with colours instead of pitch. When the train moves away from you, the pitch drops because the frequency of the waves that carry sound are dropping, and therefore the pitch goes down. (For those of you who have never taken physics, a lower frequency means a lower pitch. A higher frequency means a higher pitch.) In the same way, if an object moves towards you, it appears blue because of a shorter wavelength, and if it is moving away from you, it appears red because of a longer wavelength.  In light, a smaller wavelength means you go towards the blue end of the spectrum, and a larger wavelength means you go towards the red.

Basically, everything here is relative. The relative motion of the source (i.e. some star a few dozen light years away) makes the wavelength in front of it to look relatively decreased, and the wavelength behind it to look relatively increased. And when wavelengths appear decreased or increased, the colour appears different. That’s called shifted wavelength. (Definition of shifted wavelength: How short or long the wavelength appears to be from the vantage point of the observer.)

However, the relativistic Doppler Shift can only be observed from objects moving at very great speeds a great distance away—you aren’t going to see a relativistic Doppler Shift from a car. From Earth, the only objects that have a relativistic Doppler Shift are celestial objects. Stars, deep sky objects (DSOs), distant galaxies—you can see Doppler Shifts from those. The Doppler Shift is important for figuring out just how fast a DSO is moving away or toward us. That’s why you should learn this for astronomy.

The math for the Doppler Shift is annoying and long and involves a bunch of fractions and radicals. There are five equations: the blue-shift wavelength equation, the blue-shift frequency equation, the red-shift wavelength equation, the red-shift frequency equation, and the velocity equation. Four of the five equations look incredibly similar to one another.

And there you have it. A crash course in the relativistic Doppler Shift. It’s a total of 666 words. Not too bad, eh?

And this, kids, is how the Doppler Shift works.

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TL;DR

It’s not complicated. Really.

Light and Matter

Light is both a particle and a wave. Why? Because physics says so.

Also, because the universe likes to screw with us. (Credit: Douglas Hofstadter)

Light acts both as an electromagnetic wave, and as particles called photons. For a long time, scientists thought light was just a wave, but experiments involving the photoelectric effect (this is what Einstein won his Nobel Prize for!) established that light acts like a particle too. In fact, wave-particle duality theorizes that waves sometimes act like particles and particles sometimes act like waves. We could call it an atomic example of a bipolar disorder, but that may take it too far.

After all, light is basically the only way we can truly explore the universe, since the distances are far too great for us to traverse. Assuming we could travel at the speed of light, it would take nearly a decade for a space probe to reach the nearest star and transmit information back to Earth, and it would take 50 000 years to travel to the nearest galaxy and transmit information back. Heck, human civilization hasn’t been around for 50 000 years, and it’s debatable whether we’ll survive the next one thousand without blowing ourselves up. But we can’t travel at anything close to the speed of light — the highest speed achieved by the Helios II solar probe, the fastest man-made object ever, is only a tiny fraction of the speed of light (about .00023 c) — so it would take many, many times longer.

Would it surprise you to know that astronomers use time travel on a regular basis? The distances involved in astronomy can be so astronomically huge that it takes light a very significant amount of time to reach us. So we see distant stars as they appeared thousands of years ago; they could have gone supernova in the meantime and we wouldn’t know until the light from the supernova reaches us. The most distant galaxies we can observe are seen as they would appear billions of years ago, nearly at the beginning of the universe. That’s right, light is a time machine.

It’s not just visible light, though. There’s radio waves, microwaves, infrared, ultraviolet, x-rays, and gamma rays, oh my!  But all these forms of radiation, along with our boring old visible light, are just part of the electromagnetic spectrum — all the fancy names are because scientists didn’t realize they were just naming different versions of the same thing.

The electromagnetic spectrum in all its glory.

And then there’s matter, which is stuff. You know, matter, made up of protons, neutrons, electrons, weird bosons (possibly of the Higgs variety)… It’s also mostly empty space, since atoms are basically a tiny speck of a nucleus and a whole lot of nothingness with a couple electrons zooming around in it. Yes, that means if you punch a brick wall, you are punching mostly empty space. We shouldn’t have to tell you this, but don’t try that at home. Please.

Matter can be converted into energy with perhaps the most famous physics equation of all time, E = mc^2, which is derived from Einstein’s Theory of General Relativity. Unfortunately, neither of the two astronomy geeks who write this blog understand relativity well enough to explain how exactly one would derive the equation. Suffice it to say that E = mc^2 works, and it is very relevant to stars, which release massive amounts of energy as they smash together nuclei and fuse elements into heavier elements.

Everyone knows that there are three states of matter — solid, liquid, and gas. Well, not exactly. Under the extreme conditions out there in the universe, matter can exist in all sorts of strange states, such as plasma or degenerate matter. Plasma is basically ionized gas, where electrons have been added or stripped away, turning regular atoms/molecules into ions. Degenerate matter is where plasma has been crammed into such a tiny space that it refuses to be compressed any further because all the “available” energy states fill up (Pauli Exclusion Principle). These two unusual states of matter are especially relevant to astronomy because stars are made of plasma, and white dwarfs and neutron stars are made of degenerate matter.

The white dwarf at the center of this planetary nebula (the bright dot near the center) is made of electron-degenerate matter. Science Olympians who did Astronomy in 2012 should recognize this image.

But that’s just boring old regular matter. There’s also antimatter, and dark matter.

Antimatter is really just the opposite of matter. Regular matter is made of protons and electrons, while antimatter is made of antiprotons and antielectrons; regular subatomic particles are made of quarks, while antimatter subatomic particles are made of, you guessed it, antiquarks. A word of advice: if you ever meet your antimatter counterpart, do not high-five each other, as both you and anti-you will be annihilated in a blast of energy and leave behind nothing but a giant crater. (Being science geeks, the two of us have since been distracted by trying to figure out how big said crater would be…)

As for dark matter, we only know it exists because it has effects on regular matter, particularly on the galactic scale. We have absolutely no idea what the heck it is, but we wonder, is dark matter a very MACHO type of matter or is it just a WIMP? Yes, these are actual candidates for dark matter. MACHO stands for MAssive Compact Halo Object, and WIMP for Weakly Interacting Massive Particle — if you can’t tell, astronomers like to have fun with acronyms.

I do hope this post, shall we say, “shed a little light on the matter”.

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TL;DR — Astronomers use light, as well as other non-visible forms of electromagnetic radiation, to explore the universe. It even allows us to look back in time! Many interesting objects in astronomy are made of exotic states of matter, or perhaps not even made of ordinary matter.