Traveling at the speed of light it would still take us over 100,000 years to exit the Milky Way. The observable universe alone has over two trillion galaxies.
This type of size and scale is incomprehensible to us mere mortals.
But, from the small part of the vastness we have discovered, we have found many mysteries.
Some of these mysteries are them stars.
You see, these things can be so rare, so weird, or so far away that we barely know anything about them.
And I'm not talking about the boring ones like red dwarfs or white dwarfs.
I'm talking about the ones that make you go "wow, that's a damn star?"
Let me start with the most common type of star: the main-sequence star.
These are the stars that fuse hydrogen into helium in their cores, like our Sun. They come in different sizes and colors, depending on their mass and temperature.
The hottest ones are blue (O and B spectral types), then white (A), yellow (F and G), orange (K), and red (M).
These are easy to understand, because they follow a simple pattern: the more massive they are, the hotter and brighter they are.
They also live longer if they are less massive, because they use up their fuel more slowly.
But there are non main sequence stars.
These are the ones that have either run out of hydrogen in their cores, or never had enough to begin with.
They are called giants, supergiants, subgiants, subdwarfs, and dwarfs.
The supergiant Betelgeuse.
They have different spectral types, such as C for carbon stars, S for zirconium stars, D for white dwarfs, L for brown dwarfs, T for methane dwarfs, and Y for even cooler dwarfs.
These are harder to understand, because they have different processes going on in their interiors and atmospheres.
Some of them fuse heavier elements than hydrogen and helium, such as carbon, oxygen, neon, silicon, and iron.
Some of them have strong winds that blow away their outer layers.
Some of them pulsate or explode periodically.
Some of them have magnetic fields that generate flares and spots.
And some of them have planets or companions orbiting around them.
So which of these non-main-sequence stars are the least understood by astronomers?
There is no exact answer to that question, because different astronomers may have different opinions and interests.
But I'm going to give you my personal top three candidates:
Y dwarfs:
These are the coolest and dimmest stars known to exist. They have temperatures below 500 K (227 °C or 440 °F), which is colder than some planets.
They emit most of their radiation in the infrared spectrum, which is invisible to our eyes.
They are so faint that they can only be detected by powerful telescopes like the Spitzer Space Telescope or the James Webb Space Telescope
We don't know how many of them there are in the galaxy, how they form, or what their atmospheres are made of.
Wolf-Rayet stars:
These beasts are the hottest and most luminous stars known to exist.
Sporting temperatures above 25,000 K (24,727 °C or 44,540 °F), they are hotter than some supernovae.
They emit most of their radiation in the ultraviolet spectrum, which is harmful to life.
They are so bright that they can be seen from millions of light-years away. They also have powerful winds that blow away their outer layers at speeds up to 2,000 km/s (1,200 mi/s).
We don't know why they are so hot and windy, how long they live, or what triggers their final explosion.
Magnetars:
These are not technically stars, but neutron stars with extremely strong magnetic fields.
They have densities up to 10^18 kg/m^3 (10^15 times that of water), which is comparable to an atomic nucleus.
They have obscene magnetic fields up to 10^11 T (10^15 times that of Earth), which is enough to rip apart atoms and molecules.
They emit bursts of gamma rays and X-rays that can affect nearby planets and stars.
We don't know how they form, how they maintain their magnetic fields, or what causes their bursts.
These are just some examples of the types of stars that are the least understood by astronomers.
There may be others that we haven't discovered yet, or that we haven't studied enough. The universe is full of surprises, and stars are no exception.
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