What are the types of stars?

Names of star types e.g. red dwarf and brief descriptions/explanations of them please?

Stars can be divided up and classified in a number of different ways. The three main ways are by color (more specifically by the color in which the star appears brightest), by overall brightness, and by evolutionary state. Usually we have to use at least two of these classifications to uniquely describe a star - saying a star is red is fine, but unless we know whether the star is red and bright or red and faint, we won’t know what type of star exactly we’re dealing with.

Astronomers have found that we can plot all three of these things on what’s called a Hertzsprung-Russell (or H-R) diagram, named after the two people who developed it. This diagram plots brightness against colour, and stars fall on these axes in a unique way. As stars age, they will also move around on this diagram in a unique way (which can now be predicted by theoretical models). This means that the location of a star on the H-R diagram can give us all three pieces of information - colour and brightness (somewhat obviously, since we need those to plot on the diagram), but also the evolutionary state.

Colour is often broken up into a series of spectral types, labeled with the obtuse OBAFGKM scheme - you may have heard the mnemonic “Oh Be A Fine Girl/Guy Kiss Me” to help remember the order. (I prefer the alternate “Only Bored Astronomers Find Gratification Knowing Mnemonics”, but that’s just me.) Fundamentally, this breaks the blue stars (classified with O & B) from the redder ones, which tend to hang out in K & M classifications. Our sun, for reference, is a G type star. A and F stars fill in the gaps between blue and yellow - technically this is “green” but those stars would appear white to us, so they’re often color coded white instead.

Brightness at fixed color basically tells us about the size of the star. If you have two objects that are the same colour, and one is brighter, then there must be more surface area putting out that light, and therefore the star must be larger.

Within this diagram we can draw little ovals and give names to certain classes of stars. Anything along the main diagonal line going from bright blue to dim red is a “main sequence star”. These are basically normal stars - the higher up you get, the shorter a life you get. Bright blue stars (blue supergiants and blue giants) burn through their fuel very quickly, and tend to explode into a supernova in relatively short order. Red dwarfs, on the other end of the main sequence, will burn their hydrogen much more slowly, and live for billions of years. Even further off in the bottom right corner, we’d find the brown dwarfs, which are even fainter and redder than the red dwarfs. (If you’re curious about the dwarf stars in particular, there was a post all about dwarf stars recently!)

Then we have red giants and red supergiants, which live in the top right corner of this diagram. These are stars which used to be on the main sequence, but burnt through all the hydrogen available to them in their cores. Since the balance between energy creation and gravity is changing, these stars can expand quite rapidly, eventually reaching pressures in their cores which allow them to burn helium. They will continue expanding & burning until they run out of fuel and do not have the pressure required to burn the next set of elements. They appear near the top of the diagram because they’re huge - a lot of surface area puts out a lot of light.

Down in the bottom left we find the white dwarfs - these are the remains of red giant stars that have moved on to their final state. These fall in the bottom corner because they’re incredibly small, so they don’t produce a lot of light. They are, however, incredibly hot, and glow white, which pushes them to the left.

In principle, you could split this diagram up into a grid, and find something interesting and unique about the stars that fall in each grid, but as a starting point, this kind of coarse division works pretty well.

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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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