The Young Astronomers
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Astronomy on its own is a massive area of study, covering everything from the planets through solar systems and galaxies right out to the scales of the full Universe.
Taken alone however astronomy cannot provide the full picture of how our Universe fits together into the fascinating entity we see before us today, it is but one piece in the puzzle of understanding the cosmos (admittedly a rather large one!).
For quite a considerable time now some of the more informational posts on the blog have ventured into the more mathematical side of astrophysics, this will continue but we also hope to produce material on the aspects of particle physics and chemistry that help to underpin our working knowledge of the universe.
This new content will be produced at a level everyone one can follow and hopefully gain something from. It will also not take away from our regular astronomy news updates which will remain core to the blog.
We will also be announcing a new feature on the site within the next little while that we hope will make investigating a particular topic in astronomy much more accessible though it will take some time to get everything in order.
Stay tuned for more!
I have decided to explore a range of astro and geo engineering projects in a series of posts.
Whilst I am stepping outside my comfort zone in relation to the physics involved it is a topic for which I have a great interest and I will do my best to set out the information in as clear a way as possible and who knows we might just learn something along the way.
An example of a mega structure - A Halo Ring (specifically Installation 05)Credit: Bungie, Mircrosoft, Andrew Davis
Following an informational introduction to the series and background information, I plan to include the following mega-scale projects;
- Dyson Spheres, Rings, Swarms and variants
- Alderson Disks
- Stellar Engines
- Jupiter Brains
As well as several smaller and currently more practical projects;
- Large Scale Space Stations
- Moon Bases
- Space Elevators
- Large Scale Mass Drivers
Along with Terraforming – the adaptation of alien worlds to serve the needs of life currently present here on Earth, including humans.
Finally I aim to cover a few example s of advanced technology that could be possible in the near future in relation to astrophysics and space travel
- Artificial Gravity
- Artificial Intelligences
- Large Scale Starships
- Von-Neumann Devices
- Self-Replicating Artificial Life
As I explained in a previous post about star types there are many varieties of stars; they range from red dwarfs to blue supergiants. As I’m sure you can see these are broad groups with, in some cases, very different types of stars being lumped together under the same banner. Thankfully astrophysicists have another stellar classification system that is considerably more definitive, this is the spectral class system.
The system first splits all the stars visible in the universe into broad categories based on their colour: -
- O – Blue
- B – Blue-white stars
- A – White
- F – Yellowish white (cream)
- G – Yellow
- K – Orange
- M- Red
If you have difficulty in remembering these classes why not use the mnemonic
Oh Be A Fine GirlGuy Kiss Me
An image showing Spectral classes of main sequence stars Credit: Kieff
With the discovery of brown dwarfs, three new groups have been added to this system:
- L – Very dim red dwarfs & hot brown dwarfs
- T – Cool brown dwarfs
- Y – As of yet hypothetical cold brown dwarfs, with surface temperatures close to room temperature
These groups are also quite broad but are subdivided by adding a number after the general class. This number ranges from 0-9 and indicates roughly where a particular class lies within its broader spectral class with the system being accurate to 1/10 of a class. For example a F4 star is a Yellow white star 4 tenths between an F0 and A0 star.
Even with this subdivision there is a problem: – A main sequence red dwarf could have the same spectral class as a red supergiant. This is avoided using the final section of the classification system, a roman numeral from zero (technically the Romans didn’t use a zero but anyway 
) to seven is used to denote the general type of the star. There are also various subclasses which are not covered here.
- Type O are the largest ‘hypergiant’ stars.
- Type I are the supergiants.
- Type II are the bright giants – stars smaller than supergiants but having a higher luminosity than most ‘normal’ giant stars
- Type III are the normal giants
- Type IV are the subgiants – stars larger than the main sequence but not large enough to be classed as a true giant.
- Type V are the main sequence stars or dwarfs (not white dwarfs however).
- Type VI are the sub-dwarfs – small stars that sit below the main belt of main sequence stars
- Type VII are the white dwarfs
These when plotted produce the following diagram: -
Diagram showing the main elements of the Yerkes spectral classification. Note spectral class is along the base and absolute magnitude increases when moving up the diagram. Modifications: Peter Clark
So using the full classification system our sun or Sol is a class G2V star.
As well as helping to separate stars into classes the colour of a star also hints at its temperature. However this may appear counter intuitive: – Humans have become accustomed to blue meaning cold and red meaning hot however with stars the opposite is true – the bluest of stars are the hottest and the red varieties are much cooler. The other colours fall within these extremes (the first image above shows the main temperatures of stars ranging from cool red main sequence stars on the to the much hotter blue main sequence stars on the right).
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This seemingly strange colour pattern can be explained using the principles of electromagnetic emission: - The hotter the star the more high energy, high frequency, electromagnetic (EM) radiation it emits. A hot star will emit most of its energy in the form of X and gamma rays, what visible light it releases will be in the high energy blue region of the spectra giving its blue colour. A moderately hot star will emit visible light at a larger range of frequencies and so takes on a white appearance (white is a mixture of all the visible colours of light). An average temperature star like our sun releases more of its energy towards the lower energy end of the spectrum and so appears yellow. A ‘cool’ star emits much of its energy in the low energy end of the spectrum emitting little X or gamma rays. This means that more of the stars energy is released in the lower frequency, lower energy part of the E-M spectrum (such as radio and microwaves), this in turn means that the majority of the visible light they emit is in the red end of the visible spectrum and as such these stars appear red.
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The next paragraph relates directly to main sequence stars – those stars that are fusing hydrogen in to helium in their cores.
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The temperature of a star is not the only difference between the spectral classes. As can be seen in the above diagram an O class main-sequence star is many times the size of an M class main-sequence star. This is in part due to the mass difference between the two classes an M class main sequence star can be as low as 0.1 solar masses (10% the mass of our sun) whilst the mass of an O class star can be as much as 60 solar masses. Due to the higher mass the star needs more room to store the mass and so has a larger radius.
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There is a close relationship between the temperature of a star and its luminosity - brightness. A cool M class star emits very little energy per second and so has a low luminosity value – it is dim. A hot O class star releases a great deal of energy per second and so is bright – it has a high luminosity. However this is not the full story, an M class supergiant will be more luminous that a G class main sequence star as can be seen below. This is because despite a red star releasing less energy per square area than a yellow star, a supergiant has much more area to emit radiation over than a yellow main sequence star and thus is inherently brighter.
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All this information can be used to plot a star on the Herztsprung-Russell diagram (See below).
The H-R Diagram (Original Image Designer Unknown)
The diagram plots the spectral class (and thus the temperature), the luminosity, mass and the lifetime (for main sequence dwarfs) of stars.
To help visualise this information I have added some labels to the standard diagram (which are colour coded to their appropriate arrows).
Altered H-R Diagram Modifications: Peter Clark
All stars start their lives on the main sequence and it is here where they remain for most of their lives. After they deplete their reserves of hydrogen and swell into red giants they move of the main sequence towards the top right hand corner of the diagram. The most massive of which ascend the diagram even further and enter the horizontal branch of the supergiants. Once (or if) a star becomes a white dwarf it drops to the bottom left of the diagram where it slowly lowers further to become a dead black dwarf.
I hope this post has shed some light (pun intended 
) on the classification of stars.
You can read more generally about spectra here
Hundreds of light years away in Pegasus there lies two stars locked in a gravitational dance. One star is- as far as astronomers can make out – a blue star and the other, called LL Pegasi, is a much older carbon star nearing the end of its life, and is doing so in a spectacular fashion!
Credit: ESA/NASA & R. Sahai
This is IRAS 23166+1655, it’s a beautiful sight, and one that has sparked a great amount of interest amongst astronomers recently. You can’t see the binary system itself, as it’s shrouded by the spiral of gas and dust, causing the starlight to be completely blocked. But if observed in the near-infrared you can see the two stars, revealing what is going on at the centre.
The carbon star is dying; with every pulsation another layer of the star is thrown off, creating what is known as a pre-planetary nebula; a short period in the stars life just before the final product – a planetary nebula – is created, with the temperature having risen to 30,000 Kelvin, ionizing and lighting up the gas and leaving only the core of the star behind.
With every complete orbit the star makes every 800 years or so in its gravitational dance with its companion another spiral shell is formed, with the material rushing out at 50,000 km per hour!
The ionization has yet to take place to create the final planetary nebula, so how is it that you can see the pre-planetary nebula at all? If you look carefully at the spiral you’ll notice the right side is more lit up than the other, this side of the spiral happens to be the nearest to the galactic plane where the highest concentration of stars is and therefore the highest concentration of starlight. The nebula is lit up by the starlight!
The paper written on LL Pegasi by M. Morris et al has a diagram showing where the galactic plane lies in relation to the nebula, so do have a look! It’s also a very interesting paper to read
You can read more here, and the paper is here (in PDF format)!
This post is by AliceS for the Young Astronomers.
Admin note: Alice would like me to note that this was the first post she produced for the Young Astronomers, and that the content is quite old. She has however made several alterations since the post was originally released.
Hello everyone, and thanks to the Young Astronomers for allowing me aboard despite being a comparatively crumbly non professional astronomer! Now, assuming I can get WordPress to bend to my will, my first post is going to be about words in astronomy that end up not meaning quite what they should, if they don’t want to be misleading. Like all sciences, astronomy is done pretty much in the dark (sorry about that) – and sometimes names stick before we know what we’re actually talking about. Here are a few.
Revolution
When we hear of revolts and revolutions, we think of noisy coup d’etats in which the angry mob displaces the, er, other angry mob – and either things improve for the country in question or they don’t, but in any case, it’s a radical change. But the word “revolution” actually means “going round in a circle”. The Earth completes one revolution round the Sun every – you got it – year. Doesn’t seem a very revolutionary word, does it?
It came from Copernicus. His revolution was, really, the ultimate revolution in Science: the recognition that we are not at the centre of the Universe; that, rather, we revolve around the Sun. The book he wrote (which was only published just before he died, as he knew it wouldn’t be popular!) was called “De revolutionibus orbium coelestium”, or “On the revolutions of the heavenly spheres”. It was a revolution, because it called into question the dogma of the day that the entire Universe was created for us.
Handmade oil painting reproduction of The Copernican System by Andreas Cellarius, devised by Nicolaus Copernicus.
(Picture credit: this online art gallery!)
Which, incidentally, led on to . . .
Planets
The word “planet” comes from Greek, and means “wandering star”. Ancient people had no way to tell a planet from a star, except for its odd motion – moving in comparison to the rest of the stars in the sky, which of course was because all planets orbit the Sun at the same distance – and the fact that they don’t twinkle the way stars do. Both basically looked like points of light, and, with a little hard (if incorrect) thinking, might just as well be fixed to “celestial spheres”. Stars, of course, give off light of their own, not reflect their star’s light as a planet does.
In fact, the Sun and the Moon were also originally called “planets”. Gradually the word “planet” came to mean “world that is not a star” – one discovery that helped this along was Galileo’s sighting of Jupiter’s moons in 1610.
But of course the definition of “planet” has changed more than once since then. People seem to feel for some reason that Pluto has been harmed by no longer being known as a planet. In fact we now know that so many different types of objects orbit the Sun in our Solar System, we need to reclassify them somehow!
So, lots of changes as science progresses. To be fair on the ancient Greeks, they couldn’t planet to happen . . .
Planetary Nebula
This is the name for beautiful nebulae such as the Cat’s Eye Nebula. They are actually nothing to do with planets, but apparently looked like them in the 18th century when telescopes were not poweful enough to tell the difference.
The Cat's Eye nebula, a "planetary nebula" from a star too small to explode as a supernova. Credit: NASA
A planetary nebula is a much more gentle and orderly shell of gas than a supernova remnant. It is created when a small or medium star, like our own Sun, puffs off its outer layers at the end of its life. It’s often very hot, ionised gas, and is therefore an emission nebula – shining with its own light. It also contains elements such as carbon and oxygen, which are essential for forming rocks, planets, and life.
The word “nebulae”, however, does at least mean clouds. Astronomers referred to “spiral nebulae” many years ago, believing these to be beautiful spiral-shaped clouds at the same sort of distances as the stars in our Galaxy. They had no idea that these were galaxies millions of light years away from our own!
Astrology/Astronomy
Once upon a time, these two words meant the same thing. In the days when it was essential to know when to expect floods or plant your crops, and indeed when there were no TVs or streetlights at night, people would have known the sky very well. It would make perfect sense to think, “When such-and-such a constellation rises above that hill, it’s time to plant this out”, or “Oh dear, that one. The weather will be bad soon.” Into the Middle Ages, royals employed professional astrologers. A British tabloid newspaper claimed that Dr Brian May, the Queen guitarist who is also an astronomer, has a PhD in astrology . . .
Any word ending in “-ology” (biology, geology etc) usually means science. However, as the science and the myths separated, they needed two different names. They now have pretty well nothing to do with each other – but a lot of people don’t believe me when I say that!
Nova
The word “nova” implies newness. However, a nova is a star so old that it’s no longer strictly a star. It’s a massive explosion caused by the accretion of gas onto a white dwarf. This explosion makes it look as if a new star has appeared in the sky, hence the name.
This white dwarf is pinching this gas from a nearby star, usually in a binary system; every so often, it acquires enough for fusion to start again. It has to reach about 20 million Kelvin to do this, as a white dwarf is made of extremely compressed material which contains no hydrogen fuel to fuse (otherwise it would still be a star!). In order to make this even simpler, novae are not to be confused with supernovae, although a Type I supernova can result from the same sort of process.
The Big Bang
Time and again I’ve been told almost angrily: “It doesn’t make sense. The Big Bang was an explosion, so how could it create such an ordered Universe?”
The term “Big Bang” was actually coined as a derogatory joke, byFred Hoyle, who preferred the steady state theory (that the Universe remains the same size and had no beginning). He said in the 1960’s on a radio program something along the lines of that he didn’t believe the Universe could have begun in one big bang. The name stuck!
We will never know what sort of noise it made – of course, even if we’d been around to hear it, it would have been so incredibly hot and violent that we’d have been smashed to bits. Certainly everything would have been bumping into each other a lot. There were no atoms and molecules as we know them, let alone solid objects or stars – everything was a seething plasma of atomic nuclei, electrons, and most of all radiation. It’s particles bumping into each other that make noise. But when the Big Bang occurred, any noise that occurred would have been inside it.
That’s because any explosion we think of today is nothing like the Big Bang at all. An explosion happens in one place, and its shock waves – flying shrapnel, for instance – fly out and damage their surroundings. The Big Bang didn’t have any surroundings. It’s easy to think of it as an expanding globe, with a centre and an edge. We think of the edge as rippling through something – perhaps the Earth! – at some point in time.
It sounds like it took place – in, well, a place. Somewhere we could go and visit. From there we’d see the evidence of destruction, perhaps everything rushing away . . .
That is everywhere and nowhere. The Big Bang happened right where you’re sitting. It happened across the room for you, and it happened on the other side of the Universe. It’s quite a mind-blowing thought. But it really wasn’t much like a bomb!
An artist's impression of the size of the Universe at the time of the Big Bang, then inflation, then its expansion. Credit: NASA / WMAP Science Team
It was really quite complex too, with inflation, and a period of darkness (because all the atomic nuclei and electrons were flying around in too disorderly a manner to let light through. This is what happens inside a cloud – there’s too much stuff in the way, so light bounces off everything in random directions and goes any old where. It also means it’s relatively dark).
And guess what else? It wasn’t big at all. It was small. It wasabsolutely tiny – smaller than the head of a needle – perhaps smaller than an atom! How did all this stuff in the Universe today come out of something so small? We don’t know. In fact, theoretically, such an object shouldn’t exist. It’s called a “singularity”, and it means, because it’s too small even to have a size, it must have infinite density. But we know there are black holes which are also singularities – and, really, when we look at the earlier Universe and see how much smaller and hotter it was, and when we do the mathematics, it’s the only conclusion we can come up with.
It’s not only how we began, but it’s an immense – and immensely complicated – puzzle. It’s odd to think that something so huge and important could have such a jokey, normal name. But Universes happen before words do!
Alice
AAPOD
Portal to the Universe
