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The Stars: The Definitive Visual Guide to the Cosmos by DK | What we've learnt.

We love learning about astronomy. There is infinite things out there and it’s, quite frankly, amazing. Stars are one of the only things we can see every day that show us we really are not on our own in this universe. Every day we see a star. And every night we see millions.


We thought it would be fun to review a book of that genre seeing as we haven’t touched on this subject in the past really.


Here’s what we’ve learnt about different types of stars since reading! Hopefully you’ll also learn something from this!




Here’s different types of stars in order of their lifespan and what they can become.






Energy created is from gravitational energy heating.
It’s not really a star yet, just lots of gases from a molecular cloud that have collapsed. It doesn’t take long t o get going, only around 100,000 years. Gravity and pressure slowly increases which makes it collapse.



T.Tauri Star



Not quite a main sequence star, but it’ll stay in this form for 100 million years, so it’s worth talking about. When a Protostar ends it’s phase and gravitational pressure is all of the stars energy is when a T.Tauri begins it’s life. There’s no nuclear fusion yet but they are as hot and actually brighter than a main sequence star (what our sun is… we’ll get into that next) They’re known for being very detrimental, with their sun spots, x-ray flares and stellar winds!



Main Sequence Star



The most common kind of star, as we said, our own sun is one as well as most the stars around us. They all vary in size, brightness and heat. But they work the same way. Hydrogen to helium and repeat.

There are a types of specific stars that come into this category.



Yellow Dwarf



They’re not exactly small, as you would gather from the name, but if you compare them to the supergiant’s and hypergiants, they’re a grain of sand.

Our sun is a yellow dwarf. They also have more of a chance of expanding into a red giant than any other dwarf star.



Orange Dwarf Star



More like a red dwarf, they are smaller and colder than yellow dwarfs but live longer.



Red Dwarf



No, not the TV show. 70% of stars are red dwarfs. Tiny stars that have low mass and are about as big as Jupiter (roughly). They are impossible to see with the naked eye and they burn very slowly. Their average life is around 10,000,000,000,000 years. So long that all of the red dwarfs in the universe are technically young as our universe is only 13.75 billion years old. If you compare a red dwarfs life to a yellow dwarfs. Our sun will only survive for another 5 billion years. So, red dwarfs live an awfully long time compared to most stars (excluding the white and black dwarfs).



Brown Dwarf



Another very small star, even smaller than the red dwarf. It has less hydrogen so they cannot sustain a fusion reaction. In laymen’s terms, it’s a failed star.



Blue Dwarf



Similar to the brown dwarf, but rather than having little hydrogen it has none. So it shrinks and eventually burns out. This is usually what happens when a red dwarf is dying before it turns into a white dwarf. They increase their radiative rate by making their surface hotter making them blue – thus the name this happens because stars increase in luminosity as they age.






Again there are different types, but mostly they work the same. These can all range in size. But they are made in the same way.

Blue Giants are very hot but usually cool to a red giant, blue giants can jump this step and become a blue supergiant, but we’ll take about that later.

When a star has run out of hydrogen it’s core fusion cannot happen. Two things can happen to a star, but it all depends on it’s size. Most stars with small masses’ simply turn slowly into a white dwarf and never get to a giant size. However, stars with large masses do not. So it expands when it’s Hydrogen shell ignites. They can become huge. But they burn everything they can and this happens very quickly, meaning their lives only last 100 or so million years. They are also cooler than the smaller stars in general. Red giants and blue giants are the common ones


Eventually they should become a white dwarf just as the other stars do. But what happens when they don’t? Usually stars like ours become a red giant until their helium core runs out and the layers drift, making a shell of gas – you may have heard of it, a planetary nebula? What’s left is the small core which would become a white dwarf normally. But, not always!






As we said, massive stars do not last long and quickly run out of fuel. Different fusions build up in the core with different components fusing together. It starts with a helium core which is mostly surrounded by cooling, expanding gas. It’s this gas that makes the star a supergiant. When silicon fusion starts, Iron is made. Which is big bad news – stress the big. Eventually the nuclear reactions happen that form lots of different elements around the, now iron, core. These supergiants are usually red supergiants, but they can be blue supergiants. But unlike the blue giants turning into red giants, they become supernovas before they become red supergiants.

Surely they can’t get bigger than this? Oh, how you are wrong.





Yes, that is a thing. UY Scuti is an example (one of the biggest stars we know of). They show huge amounts of luminosities and are very rare. There are also different classes in these but most are red. They also have a very high rate of mass loss, so do not live very long. As you can imagine, these like to go out with a bang!

When the core collapses and strips it pulls on the matter above the star. The matter falls and compresses. A shock wave moves outward and particles called neutrons are created (more energy than the sun has in it’s entire lifespan). These neutrinos are absorbed and blast out when silicon leads to iron which cannot fuse into another element. The radiation pressure drops and there’s no balance left. Everything collapses in the centre of the star. A supernova happens which creates new elements such as calcium and nickel which can go onto form new planets and stars. You know that calcium in us? That may have come from supergiant star! You never know!

If it’s really big (more than 2.1 times the mass of our sun) a supernova doesn’t happen. Rather a black hole not a neutron star.

Black holes happen to huge stars when the mass of the core collapses.  



Neutron Star



So, the supernova happens and we are left with the core of the original star. What we call, a neutron star.

When the mass is between 1.35 and 2.1 times the mass of our star a white dwarf isn’t formed, rather a supernova – which has just happened now. Particles merge and become neutrons. The core is quite literally, a neutron star. FYI – if the star is bigger than 2.1 we’ve got ourselves a black hole. Sometimes the magnetic fields are a quadrillion times stronger than the sun’s so, the rare magnetar is formed. Another form of a neutron star, and the most magnetic object in the universe. There’s also pulsars – another neutron star that emit a strong radio signal.

Neutron stars are incredibly dense. A pinch of the star would be about as heavy as the biggest mountains on earth. It’s extremely hot, spin very fast and basically a giant atom supported by gravity. They cool down by emitting radiation – similar to a white dwarf. Eventually they will just cool down and become a dark, cold ball of neutrons. Unless there is a sudden massive gain in mass – which could possibly create a black hole.



White Dwarfs



This is what usually happens in the last stages of a star’s life. About the size of earth but about half of its former mass. They are very hot. Pretty much, what is left over when a star dies. Almost all 97% of stars become this. Very sense, very bright stars. One of the hottest things in the universe. All heat is trapped, so it gets cold very very slowly, we mean slow as in, 100 billion, billion years.



Black Dwarf



What a white dwarf will eventually become. Invisible, and the coldest temperature you can get in the universe. Pretty much a white dwarf that cannot emit heat or light.




So, that’s it! The lives and kinds of stars! As you can see, we did actually learn an awful lot here! We hope you did too!


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