Project Nebula – What is a nebula?
This is the informational introduction to Project Nebula and covers the generals on nebulae.
As with all sections of Project Nebula this post has been co-written by PeterC and HannahH.
A nebula is a cloud of dust and gas found within the interstellar medium filling the great voids between the stars within galaxies and star clusters.
The different types of nebula consist of different elements in different proportions. Most nebulae that have not been formed by the destruction of dying stars (i.e. SNR both types covered in this project (Ia and II), planetary nebulae and those generated by Wolf-Rayet stars), contain large amounts of hydrogen gas. These nebulae if given the right conditions to compress and heat up will form the next generation of stars.
One such star forming nebula - The Eagle Nebula Credit NASA, ESA; HST
All stars, whether they are hypergiants or red dwarfs began their lives as a nebula and rather fittingly as a star dies it returns its material to the cosmos as another nebula. This nebula is either a planetary nebula or a supernova remnant, and it is through this release of matter that the universe is provided with all the elements heavier than hydrogen and helium. This includes all the material that forms the Earth and everything on it, including humans. The oxygen we breathe is was formed in the hearts of red giants and the iron in our blood stream was formed in the final days of a massive star’s existence before it ripped itself to pieces as a supernova. It is from this that we get the saying that we are all made of star dust, we quite literally are!
As each generation of stars further enriches the universe via spreading their life’s work or heavier elements as a nebula, the following generation of stars contain more of the heavier elements as the previous generation have synthesised (created) them from hydrogen and helium. As they form from the ashes of their predecessors, and this means that each successive stellar generation contains a larger quantity of ‘metals’ – in astrophysics a metal is any element other than hydrogen and helium – this allows different populations of stars to be identified based on their metal content. This variation is due to each successive generation of star forming nebulae contain more and more dust and metals hence creating the different populations.
The Helix Planetary Nebula Credit NASA, ESA; HST Perhaps one day a new star will for from the ashes of the star that produced this lovely sight.
There are three main stellar populations.
Population III – These were the stars formed at the very beginning of the universe and if current theories are correct they where many hundreds of times more massive than the sun and are thought to have contained no metals (except for small traces of lithium as this to was formed at the start of the universe in very small quantities). Due to their high mass these stars would have quickly expended their supply of hydrogen and then went supernovae. As they where the first generation of stars they formed from the initial products from the creation of the universe, and despite their name which may be counterintuitive to some as it could be misunderstood as describing the most metal rich stars. This however is incorrect as the populations are named in order of their discovery, not their increasing metallicity. It is understood that all population III stars will have gone supernova many billions of years ago and now may only exist in the light coming from very distant galaxies which itself is billions of years old. Currently no population III stars have been detected, however many astrophysicists are currently looking for possible candidates to confirm or disprove their existence and thus too find conclusive evidence that the current theories are correct or as a way of disproving them. If any such stars are detected close to the Milky Way it would mean that they existed much more recently that thought, and thus the accepted age of the universe as around 13.5 billion years would be grossly inaccurate.
Population II stars formed from the nebulae created from the death of the population III stars. As such they contain some metals, but are still considered metal poor. Only the longer lived, low mass stars of this population are expected to still exist as the high mass stars have long since burnt out. Population II stars are most commonly found in globular clusters, and in the halos of galaxies as here the level of enrichment is lower as there is a smaller number of high mass stars and thus a smaller number of supernovae and so fewer heavy elements being distributed amongst the interstellar medium.
The globular cluster NGC 6752 Credit ESO Source - See below
After the population II stars began to die the current generation of stars began to form from their ashes. Population I stars are the most common variety in today’s universe and are considered relatively metal rich. Our sun, Sol, is a population I star but despite being considered metal rich currently Sol is only around 1.6% metals the rest being mostly hydrogen with some helium. Population I stars are found mostly in the spiral arms (which also contain some population II stars) of spiral galaxies (like Sol in the Orion arm of the Milky Way) or more generally closer to the centres of galaxies than population II stars. This graduation is because, in the more matter concentrated areas of galactic centres there are more likely to more massive stars being born and thus a higher rate of supernovae and thus a faster increase in the levels of metals in the star forming nebulae near the galactic core.
In the same way, a smaller galaxy is more likely to contain a higher proportion of population II stars than a larger galaxy as there is a smaller number of star deaths in a given tie giving a slower enrichment rate and a lower level of metallicity.
There are several very different types of nebulae but these types are discussed in depth in the next section of the project.
A nebula can hold two of the most important jobs in the universe – to create new stars from molecular hydrogen or it can spread the nuclear products of a star’s life into the wider universe enriching the surroundings and allowing for planets and life to form. It is for this reason that Hannah and myself decided to give nebulae the chance to be viewed by more people as the wonders that they truly are.
ESO Image sourced from – http://www.eso.org/public/archives/images/medium/eso0107a.jpg
Stellar Spectral Classes – Explained
As I explained in a previous post about star type there are many types of star; 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 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 Girl\Guy Kiss Me
- Image showing the various colours of the different spectral classes of main sequence stars
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 to roughly where a particular class lies within its broader spectral class, this system is 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.
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 image above shows the main temperatures of stars ranging from cool red main sequence stars on the left to the much hotter blue main sequence stars on the right).
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 viable light it releases will be in the high energy blue spectra giving its blue colour. A moderately hot star will emit viable 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 n 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.
The next paragraph relates directly to main sequence stars – those stars that are fusing hydrogen in to helium in their cores.
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.
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.
All this information can be used to plot a star on the Herztsprung-Russell diagram (See below)
The Hertzsprung–Russell diagram From: - http://cse.ssl.berkeley.edu/bmendez/ay10/2002/notes/pics/bt2lf1509_a.jpg (Original designer unknown)
The diagram plots the spectral class (and thus the temperature), the luminosity, mass and life time (for main sequence dwarfs) of stars.
To help visualise this I have added some labels to the standard diagram (These are colour coded to their appropriate arrows)
The Hertzsprung-Russell Diagram (Annotated)
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.
There are some special cases but that is for another post.
What is a Black Hole?
Please note an audio version of this post will be made available shortly
We have all, at some time or another heard of black holes, but what exactly is a black hole and why is it ‘black’?
As I described in my post about star types, most stars grow slowly into massive red giants as they run out of hydrogen. Most then collapse heating up in their final years. This collapse is halted quickly however as most stars don’t have enough mass to create a gravitational pull strong enough to overcome electron degeneracy pressure thus preventing further collapse. In simple terms this is the force that prevents electrons sticking to the protons in the nucleus of the atom. The star can’t release any more energy and it slowly loses its outer layers to space leaving a cooling white dwarf the size of Earth.
More massive stars (those with more than 10 solar masses) continue gravitational collapse past electron degeneracy. This means that in the final moments of a massive star’s life it actually fuses electrons and protons together to form neutrons. This process releases massive amounts of energy which overcomes the gravity and the star rips itself apart in a type II supernovae. The core of the star remains as a small, dense ball of neutrons – a neutron star.
However some stars are so massive that they are even capable of overcoming the force that prevents neutrons from fusing – known as neutron degeneracy. This is far stronger than electron degeneracy and marks the point of no return; once this occurs nothing can stop the gravitational collapse. The mass required for a star to overcome neutron degeneracy is a stellar remnant (that is the remains of the supernova) of about 3.5 solar masses. This is known as the Tolman-Oppenheimer-Volkoff limit, anything more massive will collapse under its own gravity indefinitely. This means that all of the remnant’s mass is concentrated in a tiny area (in compassion to it size). This in turn creates a very small region of space with a massive gravity, and answers the second question – ‘why are black holes black?.
The reason being, a black hole has such a large gravity not even light can escape from its pull. The hole itself is known as the event horizon and it is truly the point of no return. Once past the event horizon nothing, not even light can return. As a weird side note: because light can travel into a black hole you could still see the universe outside if you passed into an event horizon but you would never be seen again.
An artist's impression of a black hole with accretion disk Credit to NASA
As there are many kinds of stars and galaxies (as shown in previous posts by Hannah and myself) there are also several kinds of black holes. They are divided into several broad groups based on mass. As the mass of a black hole increases so to does its size.
First is the smallest variety: – Micro black holes are thought to have been caused when the big bang caused the super compression of tiny amounts of matter. Micro black holes have never been observed as they are believed to have event horizons of around a few micrometers – one micrometer is one millionth of a meter.
The next class up is the ‘typical’ black hole – these are stellar black holes, which form when massive stars undergo gravitation collapse as described above. A stellar black can have an event horizon of around 15 miles (24 kilometres) – tiny compared to the size of the original star – larger stars will produce larger black holes due to the increase in mass.
Intermediate-mass black holes – are found in some globular clusters (groups of stars with galaxies). They are several times the mass of stellar black holes but are tiny compared to the final group.
Supermassive black holes – These are the ‘monsters’ found at the centre of galaxies. Sometimes called Active Galactic Nuclei or AGN. The black hole’s accretion disk (the debris orbiting and ‘falling’ into the black hole) creates a massive amount of energy which creates enough light in some cases to allow the AGN to outshine their host galaxy! A supermassive black hole also has a relationship with its host galaxy – it is now believed that the black hole at the centre of a galaxy actually affects its development. Also in those supermassive black holes currently detected another pattern has emerged. A supermassive black hole contains around 1/1000 of its galaxy’s mass. This may sound like a tiny fraction, but bearing in mind that a galaxy weighs several billions of solar masses and is around one hundred thousand light years across compared to a supermassive black hole which has several million solar masses however this is concentrated into around a few million kilometres; the correlation staggering. You can learn more about AGNs here – http://ya.astroleague.org/?p=288 a wonderful post by my colleague HannahH
Black holes can also be split into those that spin and those that don’t. Both types occur at all mass levels.
Perhaps another darker (if you will forgive the pun) aspect of black holes is their ability to grow. Any material that approaches the black hole will be drawn into its event horizon. Once past the event horizon it becomes part of the black hole, which in turn means that the hole has gained mass, its gravity well has increased and it has expanded (slightly).
An artist's impression of a feeding black hole Credit: NASA
This ‘feeding’ behaviour of black holes can be incredibly useful. As the material nears the event horizon it is drawn into an accretion disk. This is a disk of matter that orbits the event horizon, its inner edge is constantly being eroded as matter ‘falls’ into the hole. As the material nears its doom it heats up and emits electromagnetic radiation over a large range from radio to visible and even highly energetic gamma rays. These emissions allow the accretion disks to be detected and by extension the black holes at their centres, in a way the final moments of this matter allows us to detect black holes so their loss is our gain!
I hope you have enjoyed reading this brief article on black holes and I hope you have learned something useful from it
