We know how black holes form. Don’t we …?
Recently it seems the more we look into the stars and galaxies that populate this universe, objects we used to thing we had figured out quite well, it has become clear that we are not as educated in the workings of the cosmos as we once thought.
The Mystery Magnetar Credit: ESO
The basic results of star death have been thought to be rather simple. A low mass star, that is one with less than about 10 solar masses forms a white dwarf a medium mass star between 10 and 25 solar masses go supernova and leave behind a neutron star\pulsar. The high mass stars (those above 25 solar masses) also go supernova and create a black hole from their cores’. (The most massive stars are actually theorised to explode and leave nothing – the explosion is so forceful it vaporises the entire star – such an explosion is known as a pair instability supernova but that is for another day).
Astronomers using the ESO’s Very Large Telescope (VLT) have made a very interesting discovery while surveying the super star cluster Westerlund 1, that once again shows that perhaps we don’t know as much as we thought we did. Westerlund 1 is the closest super star cluster to Earth yet discovered, it lies between 3.5 and 5 kiloparsecs (or somewhere between around 11,500 and 16,500 light years if you prefer) away in the southern hemisphere constellation Ara – The Altar.
You may be wondering while the distance figure is less than accurate, unfortunately it is down to the distance being measure itself. It is so far for the more accurate parallax measuring system to be used and so other methods must be employed. These methods can give different results based on different conditions hence the rather large estimate range. Despite this the rough estimate puts the cluster at the outer edge of the Milky Way’s galactic Bar which may go someway in explaining how the cluster grew to such proportions – it contains many high mass stars including a large number of highly evolved supergiants.
The cluster can be seen in its full glory in this annotated image from the ESO’s VLT.
Westerlund 1 Credit: ESO
A larger version can be viewed here.
The labelled magnetar is the star of particular interest in this marvellous cluster. A magnetar is a type of supernova remnant; specifically a neutron star with an incredibly strong magnetic field many thousands of times more powerful than the Earth’s own magnetic field, they are very rare as only a handful have been identified in the Milky Way.
The cluster contains stars that formed in a single formation event over a short period of cosmological time between 3.5-5 million years ago. Using this age figure calculated from the rate of stellar evolution in stars of different masses – the more massive a star is the faster it dies, thus the age of the cluster can be determined by measuring the highest mass star in the cluster (this has been achieved by carefully studying binary systems which allows for the mass of the stars to be accurately measured by detecting stight changes in their orbits). This puts an upper limit on the age of the cluster because had it been any older this star to would have gone supernova.
Neutron stars as I stated above are thought to form from ‘progenitor’ stars of between around 10-25 solar masses. Based on age estimates on the cluster (as detailed above) the magnetar’s parent star weighed in at least 40 solar masses! This means it was well above the mass of a star that was thought to collapse into a stellar mass black hole. Whilst the limit is be no means exact, a star that is almost double the rough limit is very unusual and very interesting.
Whilst no one is quite sure how a magnetar forms as apposed to a ‘normal’ neutron star, it does still have the same basic structure and they are still subject to the gravitational forces that formed them and are constantly trying to crush them further into a black hole. As this has clearly not happened in this case, this single star presents a rather large problem. Some may be quick to say that the accepted mass limit for neutron stars in obviously wrong.
However, as this star seems to be the exception to the rule rather than a common occurrence, perhaps it is circumstances that are to blame, rather than a flaw in our understanding. If the parent star had been contained in a binary system its partner may have removed sufficient mass via mass transfer and accretion and in doing so lowered the mass sufficiently to avoid the total gravitational collapse of the star into a black hole.
Though this idea creates a number of questions too, where is the companion star? As far as we can currently tell the magnetar is alone with out a binary partner though it is quite possible that the force of the supernova detonation blew the pair apart, this helps to explain why the magnetar is on the outer edge of the cluster. Though a massive amount of material would have had to be removed, which makes it a difficult idea for some to accept. Perhaps one day we will know for certain, finding the star’s partner would certainly help.
Who knows the next big discovery could be just around the corner.
I leave you with this artist’s impression video of travelling through the cluster to the magnetar.
Video credit to ESO
Read more about the discovery here and more about black holes here
Image of the Week – 20/08/10 – The Galactic Volcano
Due to the website maintenance this post is a day late. I apologise for the delay.
This image is a combination of data from the Chandra X-ray ( shown in blue) observatory image and one captured in the radio section of the electromagnetic spectrum by NSF’s Very Large Array (VLA) (Shown in red and orange).
The Galactic Volcano Credit X-ray (NASA/CXC/KIPAC/N. Werner, E. Million et al); Radio (NRAO/AUI/NSF/F. Owen)
They show the galaxy M87 an Elliptical galaxy located 55 million light years away in the Northern Virgo Cluster. It contains a well studied AGN (Active Galactic Nucleus) and is one of the most ‘radio loud’ objects visible from Earth.
The below image shows only the Chandra data
Chandra Image of the Galactic Volcano Credit: X-ray (NASA/CXC/KIPAC/N. Werner, E. Million et al)
The cluster containing M37 contains lots of hot gas and dust (this can be seen in the outskirts of the Chandra only image.) Under normal circumstances this material would ‘fall’ under the influence of gravity into the galaxy, cool and form new stars.
The combined data shows that this is not the case with M37 however; its central supermassive black hole has other ideas for the in-falling matter. The black hole has powerful jets blasting into space these pass on some kinetic energy to the cooling gas and dust near the centre of the galaxy and through it into space at supersonic speeds as the plumes of gas visible in the combined image.
This has been compared to the recent eruption of the Icelandic volcano Eyjafjallajokull, which caused significant air travel disruption across Europe. The similarities are not the two effect on humans as M37 is far to distant to have any tangible effect on the Earth or any part of the Milky Way for that matter. In the eruption hot volcanic gasses created at the local site of the eruption threw ash particles high into the atmosphere and allowed them to travel for several thousand miles. This is not unlike the hot x-rays produced by the central black hole ‘uplifting’ the cooler material and carrying it far into space.
To conclude here is an annotated version of the image showing the location of the plumes and black hole in relation to the rest of the image.
Labelled Image of the Galactic Volcano Credit: X-ray (NASA/CXC/KIPAC/N. Werner, E. Million et al); Radio (NRAO/AUI/NSF/F. Owen)
To read more about black holes click here
To read more about this particular galaxy and black hole click here
A Bubble Blowing Black Hole
Astrophysicists have made a very interesting discovery about stellar mass black hole. By combining data collected by the ESO’s Very Large Telescope and NASA’s X-ray telescope Chandra, new information has been revealed about a microquasar.
An artist's impression of the microquasar Credit ESO
A microquasar is a type of binary star system, in which a main sequence star is losing mass to its higher mass collapsar counter part (any stellar object that has undergone significant gravitational collapse – e.g. White dwarfs (these are not common as a part of a micro quasar system), neutron stars and black holes. These microquasars exhibit some of the properties of the conventional quasars – They emit large volumes of electromagnetic radiation at a range of frequencies right from radio waves to X-rays and they possess radio jets – a highly focused stream of matter and radiation particularly visible in the range of radio waves- they also show variability in the amount of X-ray radiation they emit like conventional quasars.
Whilst microquasars have been observed for some time and around a dozen have been detected in the Milky Way alone including GRS 1915+105, discovered in 1994 by the GRANAT X-ray satellite contains the most massive stellar black hole yet detected. The monster weighs in at a huge 14 solar masses. Whilst stars can contain as much as 100 stellar masses it is important to remember that is mass is spread out over a large area whilst in a black hole it is concentrated within an area a tiny fraction the size of the average planet.
The microquasar in question is located 12 million light years from Earth in the edge of the galaxy NGC 7793. Despite being only a few time the mass of the sun the black hole is behaving in a very similar way to the radio galaxies that are the more typical quasars which contains black holes many millions of times the mass of the sun.
The black hole is emitting a jet of rapidly moving charged particles and radiation. As this jet smashes into the interstellar gas it heats it and forces it to expand. This happens is with most if not all jets but what is particularly interesting is the size of this ‘bubble’. The shock has expanded it to a size of over 1000 light years! What’s more this bubble in continuing to expand at a rate of nearly 621,000 mph!! Using the size of the bubble and the rate of expansion it has been calculated that the black hole has been active in this way for at least 200,000 years.
This microquasar has created a bubble twice as large as any previous microquasar yet discovered. This may mean that this is an exceptional case or that we have looked over similar candidates in the past. Regardless this new discovery will allow astrophysics to get a better insight into the similarities and differences between stellar black holes and the supermassive black holes found lurking in the heart’s of galaxies.
Read more about binary systems here
Read more about black holes here and here for black holes and quasars
Black holes, Doughnuts and Galaxies with Indigestion.
In this blog post I decided to focus on one of my favourites 
Quasars (which is the short name for Quasi-Stellar Objects) are highly luminous terrors that I have been fond of ever since I learnt of their existence at Galaxy Zoo. These galaxies are amazingly bright and can be seen blazing billions of light years away. So what are they?
Most galaxies have a super massive black hole lurking at their nucleus. They can be quiet things, only grabbing the odd unlucky wisp of matter and not causing too much trouble, these are the non-active ones. In a galaxy hosting a Quasar its super massive black hole is active (also known as an AGN; an Active Galactic Nucleus) and pulling dust, gas and whatever other objects it can grab along the way into a hot doughnut shaped disk – an accretion disk. As this disk swirls around the black hole awaiting its fate, the matter that it consists of causes friction and creates huge amounts of energy, causing the centre of the galaxy to get so bright that it outshines the galaxy itself many times over.
For an example of a quasar here is 3C 273:
3C 273; Credit: SDSS
This monster lurks in the constellation Virgo, at around 2 billion light years away it isn’t the farthest quasar known, in fact it is actually one of the nearest. To give you an idea of just how bright it is, it outshines our galaxy over a hundred times. You may have wondered what the fuzzy line poking out of 3C 273 is, I first thought it was a distant galaxy but it is actually the quasars jet. The energy created in the accretion disk also gets concentrated into jets of plasma at the poles of the black hole by the magnetic fields; these jets can beam out for thousands of light years and travel at near to the speed of light. And to think all of that is caused by something only the size of a solar system. 3C 273’s jet stretches through space for over 200,000 light years, that’s twice the size of our galaxy!
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
