What kinds of dwarf star are there?

What different types of dwarf star are there and could you mention a bit about each? e g. Some brown dwarf stars have liquid iron raining down on them.

“Dwarf” was originally a term used to distinguish between the two kinds of red stars in the universe - very massive, and very small. These were termed “red giants” and “red dwarfs”. The dwarf terminology gradually expanded to mean “not giant” stars of any colour, and the line between “giant” and “dwarf is somewhat poorly defined; the Sun is technically a "yellow dwarf” star.

What most people think of when they hear “dwarf star” are brown dwarf, red dwarf, and white dwarf stars. There are also a few theoretical kinds of dwarf stars, which is where black dwarfs fall. These stars are all classified based upon their colour, although confusingly these are not usually the colors they would appear to our eyes. (Brown dwarfs, for instance, would appear a deep pink - see above for 3 brown dwarfs as they would appear to us.)

Yellow and red dwarf stars are normal stars - they burn hydrogen in their cores and live on the main sequence of stellar lifetimes. Red dwarfs are smaller than our sun, only getting up to 50% the size of our sun. As a result, their surfaces are cooler, hence the colour shift towards the red. They don’t consume their hydrogen as quickly as our sun does, so even though they’re less massive and thus have less hydrogen, they still live for a much longer time than our sun will. Because red dwarfs require less matter to create, they are the easiest to make. Red dwarfs are therefore the most abundant type of star in the galaxy - our nearest stellar neighbor is a red dwarf.

Brown dwarfs are failed stars. They’re essentially massive Jupiters - large collections of gas that are not massive enough to create the pressures required to start burning hydrogen into helium. These dwarfs can be pretty cold; there was one found not too long ago that was only as warm as a cup of coffee. A brown dwarf can’t do anything except sit there and slowly radiate away its heat - it won’t ever become a fully fledged star. The iron rain you refer to was the conclusion of a study from 2006; evidence was found that at the temperatures of the star they were looking at, the iron they detected in its atmosphere should be forming liquid droplets and raining down towards the surface of the star. Further studies have found evidence for massive, Jupiter-style storms in the atmospheres of these stars. The behavior of the metals and other elements in a brown dwarf’s atmosphere will depend strongly on the temperature of the star in question. Since “brown dwarf” is a rather broad term, some of these stars will be too cold for iron rain, and some will be too warm. Of course, the presence or absence of a particular element will depend on the gas the dwarf formed out of, since the brown dwarf is not building any new elements itself.

White dwarfs are the most exciting to make. They are what is left over after a main sequence star (like our Sun) dies. The star will have gone through the red giant phase, and then shrugs off its less dense outer layers into a planetary nebula. At the end, all that is left is a hot, dense core of what was once the centre of the star in a volume about that of the Earth. They are so dense that the pressure provided by the electrons of the atoms within the star pushing against each other is what keeps them from becoming any smaller, and so hot they glow white just from trapped heat. This is the end-point of our Sun.

The black dwarf - still a theoretical object - is the name we would give to a white dwarf star which had managed to completely lose all of its heat, effectively going completely out. The length of time it takes for a white dwarf star to lose all of its heat is longer than the length of time the Universe has been around, so we don’t expect to see many of these around.

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How do Coronal Mass Ejections happen, and how do they affect us?

A Coronal Mass Ejection, or CME, is when the sun suddenly releases a lot of matter and energy from its surface, flinging it outwards into space. To understand why it would do this, we need to know a bit more about how the surface of the sun works.

The Sun is a miasma of incandescent plasma, and it rotates around its axis once every 30 days (roughly). But because the sun isn’t a solid body like the Earth, the entire Sun does not have to rotate at the same rate. If the equator of the Earth tried to rotate faster than the poles, the rocks that make up the surface of our planet would have to shear apart. But because the Sun is just gas, it can have an equator that rotates a few days faster than its poles - a gas doesn’t have the same resistance to shear as rock does.

The Sun also has a pretty intense magnetic field, which normally would start at the top pole, and travel smoothly downward to the bottom pole, as it does on the Earth. However, since the equator of the Sun is travelling a little bit faster than the poles, the magnetic field gets dragged along with the faster material, which pulls the magnetic field into a twist. After enough twisting, the magnetic field begins to form little loops that pop out of the surface. You can replicate this effect by taking a bit of string or cable and twisting it - at some point, the cable will want to make a twisted loop if you give the line some slack. These little loops tend to be associated with sunspots.

As the years go by, the magnetic field of the sun gets increasingly twisted, and these loops get bigger and more common on the surface of the sun. As the magnetic fields get increasingly tangled up in themselves, if the bases of the magnetic fields (or, in our cable analogy, some of the cable closer to your hands, not in the loop itself) touch, they snap together and create a new magnetic line, without the loop. This then leaves the loop in the lurch - but it doesn’t just hover over the surface of the sun. The ‘snapping’ together generates a lot of energy, which is all dumped into flinging the material which was trapped in the loop outwards, into the solar system.

These particles have extremely high energies, which means they leave the sun’s surface at an extremely high temperature and at an extremely fast pace. Since these can occur at any point on the Sun’s surface, (although they don’t tend to form at the exact poles, since the magnetic field doesn’t get very twisted there) and and the Sun is constantly rotating, the probability of a CME being headed straight for the Earth would be pretty low, if they shot directly out from the surface. However, CMEs are notable because they eject particles over a wide swath of space, so our odds of running into this stuff is much higher than you would expect. So what happens when they head for us?

Fortunately, the Earth’s magnetosphere takes the brunt of the blow from these particles. The magnetosphere can be thought of as a giant magnetic shield, deflecting charged particles that come our way. This protects us from most of this kind of radiation from the sun. Our magnetic field sinks into the planet at the magnetic north and south poles (close to, but not exactly the same as, the rotational north pole). This means that there’s a bit of a divot in our magnetic field, and particles can get stuck in here, and go around bombarding the atmosphere with radiation, causing the atmosphere to glow. This is what causes the Northern and Southern lights - also known as the aurora. If you’re in the far north and you hear that there’s a solar storm coming, head outside when it hits - there’s a good chance of seeing the aurora any time a CME comes our way.

Less aesthetically pleasing is the fact that CMEs can do a fair amount of damage to some of our satellites in orbit. Satellites are built to be able to handle slightly more than an average amount of radiation under normal circumstances. But if we’re getting hit with the kinds of energies that CMEs bring, even after 93 million miles, some satellites can’t handle the dosage. The constant bombardment of the satellite by charged particles can cause the satellite itself to become charged. This is very similar to becoming electrically charged by shuffling around in socks on carpet. If the satellite gains enough charge, it can short-circuit itself, which will kill the satellite, if a crucial part fails. (In space based satellites, most parts are crucial.) This kind of thing mostly affects satellites that are in very high, particularly geocentric orbits, like GPS satellites. The International Space Station is usually unaffected since it’s in a lower orbit, although in the case of strong storms, the astronauts can take shelter in more highly shielded portions of the the ISS.

On the surface of the Earth, most of the time the most noticeable part of a CME is the aurora; most of the other consequences of a coronal mass ejection just don’t make it to the surface.

That said, in 1989, a solar storm knocked out power to 6 million people living in Quebec because there was so much turbulence in the magnetic field of the Earth that it induced a current in the power lines, and overloaded a set of circuit breakers. In the face of extremely large coronal mass ejections, we can have problems on Earth. Fortunately, as long as we have telescopes observing the sun, we will always have several days warning.

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What would a human need to survive re-entry on their own?

What sort of protection would a human need to survive reentry into the atmosphere without a space ship or other vessel? Preferably with parachute and sans parachute scenarios.

By the time you finish protecting someone from re-entry, you will have basically built a little person-sized spacecraft around them. Re-entry is a technologically challenging thing to survive, and even the smallest problem can escalate quickly, as the Columbia disaster taught us only too well.

The main source of the problems with re-entry is that if you’re orbiting the earth, you’re going extremely fast. The ISS travels at just under 8 km/s, which translates into 17,224 mph, or 27,720 km/h. When landing, generally we want our sideways velocity to be as close to zero as physically possible, so we’re going to have to slow down by more than 17,000 mph.

The atmosphere itself is a pretty good set of brakes- it’s a much thicker medium to go through than space, so it will slow you down, just like walking knee-deep in water is slower going than walking on land. The trouble with using the atmosphere is that you tend to exchange your speed for heat. The force of being dragged through the air is a force of friction, and as all the air particles collide with the re-entering craft, they donate a little bit of heat with each collision. Unfortunately for a poorly protected person, the atmosphere’s friction generates so much heat that the air itself turns into a 3000F (~1650C) plasma. The job of the Space Shuttle’s heat protecting tiles is to resist this intense heat (there’s something similar on the bottom of every object that has re-entered our atmosphere). When this plasma builds up, the craft is effectively cut off of communications with the ground, since all radio waves are blocked - this is known as the plasma blackout period, and lasts for a little over ten minutes. If you’re trying to protect a person from re-entry, step one is going to be to make sure that you have surrounded your person in some seriously intense insulation to keep several thousand degree plasma from roasting them to a powder.

On top of being incredibly heat-resistant, the insulation is going to have to be incredibly resistant to cold, since we’re starting in the frigid temperatures of space. For the survival of our human, it’s not enough to have a plasma-protecting layer that doesn’t crack in the insane cold of space; our person must also be kept in the very narrow range of temperatures in which we humans are comfortable. Fortunately, temperature regulation is usually built into another piece of equipment he’s going to need - a pressurized suit. Since we’re starting in space (zero air pressure), and the descent goes through a lot of extremely thin atmosphere (very little air pressure), we need our person to be cocooned in a pressurized suit to keep the gasses in his blood from boiling from the lack of atmospheric pressure.

Even with these considerations taken care of, we can’t just wrap our unlucky space-jumper in some kind of high test, pressurized, internal temperature-regulating bubble wrap and fling them out of the International Space Station. The human body is a very delicate thing and does not handle large accelerations well, and this includes spinning. If our jumper lost even a little bit of stability as he fell through the atmosphere, he could begin to tumble. The chaotic rotation of a tumble can cause strong forces - several times the force that gravity normally exerts (abbreviated 1G). A force of 6Gs for more than a few seconds can cause even seasoned pilots to black out. At the point when our pilot has blacked out, there’s almost no hope of recovering from the tumble, and if the force on the body is not reduced quickly, your chance of death increases rapidly. Tumbling was one of the major concerns with Felix Baumgartner’s jump from 24 miles up, and he did in fact tumble for some time, but managed to pull out of it - if the tumble had continued or had been harder to escape, he would have been in serious trouble.

So now, in order to be safe, we need an aerodynamically stable pressurized plasma-proof coating for our space jumper, just to survive early re-entry. This is effectively a small craft built around our person, and we’re not even close to the ground yet.

Parachutes aren’t very useful until you get reasonably close to the ground; the air needs to be thick enough to exert a strong drag force to help you slow down when it catches in the chute. You also need to be going sufficiently slowly that the air is not being heated into a several thousand degree plasma around you. But once you get down to this level, parachutes are a fantastically useful and reliable method of slowing yourself to the point where you probably will not die upon impact with the ground. Because they’re so reliable, we have attached several of these to nearly every single object we land on a surface, which includes on Mars. The Space Shuttle had a trio of parachutes deployed upon touchdown to help it slow down, and all the Gemini & Apollo class missions had parachutes deployed before splashdown. The Soyuz capsules still land this way - it’s a tried and true method.

Trying to land without a parachute is a lot harder. The space shuttle did most of its slowing down (once it made it past the plasma stage) by gliding. The shuttle was an impressive feat of engineering; once the guided portion of re-entry was over, the pilot of the shuttle manually landed a completely unpowered craft, slowing it down to a touchdown speed of ~220 mph. The runway for the shuttle is phenomenally long (about 15000 feet) and made of high traction concrete, just to give it enough time to roll to a stop.

Other than gliding, the technology to control a landing is only now being developed. SpaceX is working on what they call the Grasshopper engine, which is meant to be able to do vertical takeoffs and landings, and just a few days ago managed to take off, lean over to one side, hover, and then safely land again. So presumably, if a person’s structured protective re-entry gear came with rockets on the bottom of it, it could control the descent and slow them down automatically to a smooth landing. This is probably a lot kinder of a landing than the Mars Pathfinder method of landing, which is to have downward facing rockets slow you most of the way down, and then surround yourself in airbags and bounce to a stop. (Here again we have the problem of the human body being a lot more delicate than the Mars rovers - the Mars rovers hit the ground going slightly over 50 mph, which caused accelerations on the rovers themselves of over 50Gs - not particularly good for a human body.)

It’s by no means a simple task to make it back down to the ground from Earth orbit. We may someday have the materials and the technology to protect someone from the many forces involved, but at the moment, it would be a plunge to certain death.

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Astroquizzical has reached 100 followers!  Thanks to all of you for making it happen, both old and new!  It couldn’t happen without you. 
 Please keep your questions coming!  To submit questions, you can do so through  the ask page ,  the sidebar  on the main page (both anonymous-friendly),  twitter  or  facebook !   
 If you have been enjoying Astroquizzical, please share it with your friends, and I encourage all of you (and your friends!) to pass along even the idlest of space questions that come to your minds. 
 -Your grateful neighborhood astrophysicist

Astroquizzical has reached 100 followers!  Thanks to all of you for making it happen, both old and new!  It couldn’t happen without you.

Please keep your questions coming!  To submit questions, you can do so through the ask pagethe sidebar on the main page (both anonymous-friendly), twitter or facebook!  

If you have been enjoying Astroquizzical, please share it with your friends, and I encourage all of you (and your friends!) to pass along even the idlest of space questions that come to your minds.

-Your grateful neighborhood astrophysicist

Does our galaxy orbit anything?

The Moon orbits the Earth, the Earth orbits the Sun, the Sun orbits the center of the Milky Way, but does our galaxy orbit anything?

Our galaxy does indeed! The Milky Way is one of two large galaxies that make up what’s called the Local Group, which contains some fifty-odd galaxies. The other large galaxy involved is Andromeda, our closest galactic neighbor; our galaxy and Andromeda are slowly orbiting each other. The rest of the Local Group are mostly small things, like the Large or Small Magellanic Clouds, which are gravitationally tied to either the Milky Way or Andromeda, and orbit the larger galaxy to which they’re bound. Andromeda weighs in somewhere between 700 billion solar masses and a trillion solar masses. This is approximately the same mass as our own Milky Way, which is also usually considered to have about a trillion solar masses worth of stuff hanging around.

If you want to figure out how any two objects are going to orbit each other, you want to know their mass, how far apart they are, and how fast they’re moving relative to each other. With this information, you can determine what path the two objects will take relative to each other. The main thing we need to be concerned with right now is the mass. The masses of your two objects - in this case, the Milky Way and Andromeda - determine the point around which both objects will orbit. This is called the centre of mass, and is defined as the point in space that has an equal distribution of mass around it. For a system like the Sun and the Earth, the Sun contains almost all the mass - the mass of the Earth being so far away doesn’t really change the center of mass very much. The Earth only pulls the Earth-Sun center of mass the tiniest bit away from the centre of mass of the Sun itself. But if the two objects are close to each other in mass and widely separated, the center of mass is actually between the two, in empty space! This is the case for Pluto and its moon Charon, which are close enough in mass that they both technically orbit a point above the surface of Pluto.

The same is true of the Milky Way and Andromeda. They’re both extremely massive objects and very far away from each other, but they’re gravitationally tied to each other. Since neither can escape the gravitational pull of the other, they are in orbit around a point somewhere near the middle of the space between the Milky Way and Andromeda. This point - the center of mass between the two - is where our galaxy and Andromeda will eventually collide. (Not to worry, it’ll be another 3-4 billion years.)

You could scale up your question - does the Local Group as a whole orbit anything? And it does - the Local Group is part of the Virgo Supercluster, which in turn is in motion relative to even larger structures in the universe. Nothing is truly at rest, after all.

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