What is a Nebula? – Updated

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 (several types -Ia and II – will be described at a later date), 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 was formed in the hearts of red giants and the iron in our bloodstream was produced in the final days of a massive star’s existence, right 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!

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.

As each generation of stars further enriches the universe by spreading their life’s work as a nebula, the following generation of stars contain more of the heavier elements as there is now more available thanks to the previous generation synthesising (producing) them from hydrogen and helium over the course of their lives. Meaning 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 detectable differences in the spectra of the stars they produce.

There are three main populations of stars though I shall keep a description of each for a further post more focused on the topic.

There are several very different types of nebulae but these types will be discussed in depth in further posts.

 

Aurorae on Uranus

Hubble has for the first time spotted Aurorae on the distant ice giant Uranus. In the image below you can see the turquoise disk of the planet has a bright ‘blotch’.

Uranus' Aurorae Credit:NASA, ESA, and L. Lamy

An aurora is produced when a stream of charged particles from the solar wind (the material ejected from the Sun) collides with a planet’s magnetic field (more properly called its magnetosphere) and excites the particles within the atmosphere casing them to glow. This glow is what we observe as the aurora.

On Earth aurorae with a blue or red colour are due to excited nitrogen, whilst green or a redish brown hue is due to excited oxygen. The aurorae can dance across the sky in waves of coloured light and whilst some last for a few brief minutes others can remain active for hours depending on the conditions creating them – solar storms for example can create very powerful aurorae.

Aurorae have been observed on other planets as well, particularly Jupiter and Saturn; both of which have prominent auroral systems. Those present in Uranus’ atmosphere are considerably fainter and appear to last only for a few short minutes at a time.

These images represent the first observation of Uranus aurorae, with previous data collected directly during the Voyager 2 flyby in 1986.

These new observations should help to reveal more about Uranus’ magnetic field, which we currently know little about.

You can read more here

Image of the Week – Hubble’s Birthday Treat – 17/04/12

In celebration of the Hubble Space Telescope’s 22nd anniversary the ESA has released this truly stunning image of the star forming region 30 Doradus.

30 Doradus Credit: NASA, ESA, ESO, D. Lennon and E. Sabbi (ESA/STScI), J. Anderson, S. E. de Mink, R. van der Marel, T. Sohn, and N. Walborn (STScI), N. Bastian (Excellence Cluster, Munich), L. Bedin (INAF, Padua), E. Bressert (ESO), P. Crowther (Sheffield), A. de Koter (Amsterdam), C. Evans (UKATC/STFC, Edinburgh), A. Herrero (IAC, Tenerife), N. Langer (AifA, Bonn), I. Platais (JHU) and H. Sana (Amsterdam)

30 Doradus is better known as the Tarantula nebula and is located 170,000 light years away within the Milky Way’s largest satellite galaxy the Large Magellanic Cloud (LMC).

The image shows a region of space approximately 650 light years across with several million stars present within. Combined, the sum total of the stars’ masses shown in this image would be well over a million times the mass of our own Sun.

The stars are grouped into smaller clusters ranging in age from about 2 million – 25 million years old, whilst this may sound ancient in human terms as far as the universe is concerned even the oldest star in the region is a newcommer.

The brightest cluster is NGC 2070 being one of the youngest (between 2 and 3 million years old) and most actively starforming regions with the larger structure Astronomers find it an attractive region to study. Recently in fact, it was revealed that at the heart of the cluster (which contains upwards of half a million stars) there is a dense clump of stars designated RMC 136 where the largest stars yet discovered reside. Indeed several of these monsters are more than 100 times the mass of our own sun, truly cosmic giants.

The fierce output of the regions hot stars sculpts the regions gas and just into the fantastic arcs and bubbles we can see in the image. The fierce radiation bombardment of radiation is also exciting the gas and dust molecules of the nebula making them glow in their own right and classing the region as an emission nebula.

The image is composed of data from both Hubble and the ESO’s MPG/ESO 2.2-metre telescope and represents the one of the largest such mosaics in existence today. The data was captured by both telescopes during an observing run in October 2011.

You can read more here.

A Star’s Death Giving Life to a Monster – Recovered

3.8 billion light years away in the constellation Draco deep inside the centre of an inconspicuous galaxy, something happened at 12:57:45 on the 28^th of March 2011 that flooded the SWIFT satellite’s sensors with x-rays, and in the process sent astronomers scrambling to get a glimpse with their ground and space-based observatories.

If you look at the light curve provided by SWIFT, the x-ray brightness fluctuates considerably over a period of days. You get the first massive burst, then it calms, and then you get some more bursts days after the original event. This is very different from GRBs, such events usually consist of a huge burst of x-rays and then a dimmer afterglow of a whole variety of radiation before fading from view over a period of hours at the most. So if it isn’t a GRB, then what is it?

The massive bursts happen to be coming from the centre of the galaxy, lighting up the heart of the galaxy with the power of 1 trillion suns; outshining the galaxy itself 100 times over. Like most of the galaxy population, a super massive black hole (SMBH) happens to lurk here. Could it be that the black hole has woken up? Active galaxies emit a huge amount of radiation, including X-rays, right across the electromagnetic spectrum after all.

With data from various surveys – including FERMI and ROSAT – astronomers have concluded that before this event there has been no  sign of activity from the SMBH for the past 20 years at least, so for it to flare up without warning is very  unusual!

So far the most popular theory with the most evidence suggests a main sequence star with a mass equivalent to our sun’s wandered too close to the gravitational grip of the SMBH; a monster weighing in at 10⁷ solar masses. During a single pass it would have had to put up with one side of it being stretched and tugged at more than the other, until the gravitational pull was so powerful that the star started to get torn apart.

The matter from the disintegrated star has now settled to form a temporary accretion disk that provides fodder for the black hole. The material in the disk started to interact, and a mixture of friction and magnetic fields collimated the radiation into jets which we view as head-on, drowning out the host galaxy with its luminosity.

If this is indeed the case, the bursts of radiation seen with SWIFT and other observatories should cease after a period of months to just over a year. This would show that the star is slowly getting devoured or spat out from the accretion disk, until one day there will be no fodder for the black hole at all, and it will settle back down into its dormant state and probably won’t wake up again until the galaxy merges, or another star falls prey to its gravity well.

There is another theory I’ve picked up from Arxiv by Dokuchaev et al. which is rather more exotic:

Instead of a star being destroyed via accretion, something massively destructive happened to a star cluster near the centre of the galaxy… But first, let me focus on GRBs.

There are two types of Gamma Ray Bursts; short GRBS and long GRBs. So, what’s the difference?

Long GRBs are the most common. They’re likely to come from Type 2 supernovae, the type of supernova you get when a high mass star implodes, leaving only a core or a black hole behind. The insanely bright jets of gamma rays are thought to come from the poles of stars that are collapsing into black holes and last up to a few minutes.

Short GRBs are less common, and are likely to come from the merger of two neutron stars or a neutron star colliding with a stellar mass black hole. They last less than two seconds, but are still as destructive to anything that lies in their path as the Long GRBS.

The star cluster mentioned earlier would have a whole variety of stars to choose from, including stellar remnants such as the ones mentioned above. The stars with the most mass will migrate to the centre of the cluster, until eventually the gravitational pull of each star in the vicinity causes them to interact with each other rather destructively…

Neutron stars start to collide with each other and stellar mass black holes, creating plenty of Short GRBs as they go along. The many GRBs account for the repeating flares recorded by SWIFT and other observatories. In the period of two days 7% of the stars that make up the cluster collapsed into an accreting super massive black hole!

If this theory is correct the black hole won’t shut down any time soon like in the most popular theory, but will carry on for many years as it gains in mass over time from devouring the remainder of the star cluster and perhaps beyond.

However, I’m standing by the first theory  Either way there’s some very interesting speculation surrounding this amazing object!

You can read more on this event here and here (both in PDF format) and on NASA.

WISE Shows the Sky is Awash with Blazars

The latest release from NASA’s WISE mission has shown that just over 200 previously unidentified high energy objects are likely to be blazars.

Artist's Impression of an active Blazar Credit:NASA/JPL-Caltech

A blazar is a form of active galactic nucleus (AGN) – a galaxy where the central black hole is ‘feeding’ on large amounts of material resulting in the release of huge amounts of radiation including two tight very bright jets.

The angle at which the AGN is situated relative to the Earth determines which form of AGN we observe even though all are tied to the same processes.

In the case of a blazar we are looking directly down the AGN’s jets you could even say right down the barrel of the gun!

AGN at various angles; Credit: Aurore Simonnet, SSU NASA E/PO.

As the AGN must be lined up almost exactly with Earth for a blazar to be observed they are understandably rare compared to the other forms of AGN which have a much larger range of possible viewing angles. That being said the WISE data has the potential to reveal several thousand more.

A team using the WISE data looked at 300 objects that had previously been detected as high energy gamma-ray sources by the Fermi Space Telescope, though up to now had remained unidentified.

Using WISE the team was able to observe these gamma ray hotspots in infra-red wavelengths and showed that just over half are most likely to be blazars. WISE had also observed 50 new blazars outside those Fermi oddities along with taking observations of more than 1000 previously identified blazar candidates.

One of the project leads, Francesco Massaro has explained that there may be several thousand more as of yet unknown blazars hidden within the WISE data that could be revealed using the techniques developed for this first sample.

An image of one of the new WISE identified blazars Credit:NASA/JPL-Caltech/Kavli

You can read more here

 

NGC 2467 – Recovered

The Hubble Space telescope has given us a new insight in to the star forming region NGC 2467.

NGC 2467 Credit: NASA, ESA and Orsola De Marco (Macquarie University)

For a full size image click here

The region is a nebula composed mainly of hydrogen and it is using this hydrogen to create new stars of all masses.

Not long ago, (by astronomical terms of course!) the nebula would have been cold and dark as the newly forming stars had yet to break out of their gaseous progenitors. The most massive and thus hottest newborn stars have now blasted through the clouds of dust and gas that surrounded them, subjecting the surrounding nebula to a fierce stream of ultraviolet radiation that is eroding the larger structure and making it fluoresce in response. This fluorescence makes the nebula an emission nebula as it is emitting its own light rather than just reflecting that of the stars it contains.

Analysis of the data indicates that the majority of this radiation comes from one star. It is the bright star visible near the centre of the image.

Only the largest stars have currently emerged from their nurseries with many smaller, perhaps more sun like stars remain hidden within their denser pockets of the nebula.

NGC 2467 is like many other star forming regions including the more famous Orion Nebula. NGC 2467 is located around 13,000 light years from Earth with the Orion Nebula around 10,000 light years away.

The Nebula is located in the constellation Puppis which is completely visible to residents of the southern hemisphere (those in the northern hemisphere can observe the northern section of the constellation however it sits close to the horizon)

The image is a combination of several taken with the HST’s Wide Field Channel in several filters. The images where collected by the telescope all the way back in 2004.

Read more here

Image of the Week – Martian Dust Devil Goes for a Spin – 09/04/12

NASA’s Mars Reconnaissance Orbiter has produced a truly stunning image of a Martian dust devil using its High Resolution Imaging Science Experiment (HiRISE) instrument.

The Martian Dust Devil Credit: NASA/JPL-Caltech/UA

The Devil is about 12 miles high (though only 70 meters across) and was produced as the Sun warmed the ground in such a way that a vortex was created. This sort of weather feature is most common when the sun’s heating effect is felt most strongly and fittingly this particular event occurred just two weeks before the northern hemisphere’s summer solstice.

NASA scientists have put together this video showing the dust devil in action

You can read more about this weather phenomenon here

The International Space Station

240 miles above your head a 420 tonne satellite orbits the Earth at 17000mph. It has been there, albeit in various states of construction, for 14 years, and for the last 11 of those it has been continuously occupied.

The fantastic photo taken by Italian ESA astronaut Paolo Nespoli from a Soyuz capsule that is currently my desktop background. Source: ESA/NASA

The International Space Station is a feat of engineering like no other. Not only does it demonstrate our technical ability to construct, launch, and maintain a permanent presence in space, but also our ability to coordinate the work of five different space agencies and their operations all over the planet.

But the journey from its conception has not been an easy one, the ISS was born out of three separate national programmes: NASA’s Freedom station, proposed in the early ‘80s as a response to the Soviet space stations Mir and Salyut, the Russian (formerly Soviet) Mir-2 project designed as a replacement for the aging Mir station, and the European Columbus space station project.

Budgetary constraints brought on by post-Cold War political changes made it increasingly clear that no single national programme was going to create a fully functioning scientific outpost. Instead the suggestion to combine the three programmes into a single international one was put forward and agreed in 1993 by US Vice-President Al Gore and Russian Prime Minister Viktor Chernomyrdin.

The first component, the Russian Zarya cargo block originally intended for the Mir-2 station was launched in 1998, and since then the station has expanded, first with the addition of connecting and services modules such as NASA’s Unity and RKA’s Zvezda, and later with more specialist modules such as ESA’s Columbus laboratory and the Cupola observation module, the largest window in space. In total the ISS consists of fifteen pressurised modules, with one more, Russian research laboratory Nauka still to be added. They comprise laboratories, docks and airlocks, and living areas, and their combined volume is just less than 1,000 cubic metres.

That all of these modules fitted together perfectly is a success story in itself. Many had not been built when the first pieces were launched, and for most their mating in orbit was the first time they were put together. Though there have been a few minor problems, they have always been resolved quickly, and at no point has the station ever had to be evacuated.

The station’s unique conditions have allowed a large variety of experiments to be performed, many of which would be impossible on Earth. Research is being done into how structures such as crystals and organic cells form and develop outside the influence of the Earth’s gravity. NASA is also taking the opportunity to do closer studies on the effects of prolonged exposure to microgravity on astronauts and the possible implications on future manned missions to the Moon, asteroids, or Mars.

Until the end of the shuttle program in August of last year, crew and supplies were transported by a variety of means including the space shuttles, and the Soyuz and Progress spacecraft. The 6-man Soyuz craft operated by Roscosmos, the Russian Federal Space Agency, is now the only method of sending new crews to the station.

Each contributory nation retains ownership of and responsibility for the components that it added. This responsibility extends to the disposal of the station when it reaches the end of its operational life, which the current time frame places somewhere in the 2020s, depending on whether and for how long its decommissioning is postponed after the initial 2020 date. Given the huge amount of money that has been invested in the station as well as the later than expected completion date, it is very likely that the ISS’s operational life will be extended some way beyond that deadline. By that point it is also expected that commercial space ventures will play a much larger role in the life and upkeep of the station, so they too may play some role in its end-of-life decisions.

Stellar Newborns Kick Up a Fuss

The Orion nebula is the closest region of large scale star formation to Earth sitting just 1340 light years from where you are reading this post.

The nebula is in the process of birthing the next generation of stars, with many still cocooned within the clouds from which they are forming, from peering eyes. Well that’s in the visible spectrum at least. Using infra red observations we can looks through the obsuring dust as if it isn’t there at all.

This is exactly what astronomers using the Sptizer and Hershel Space telescopes have done to produce this gorgeous image:

The Orion Nebula in I-R Credit: NASA/ESA/JPL-Caltech/IRAM

The rainbow effect is due to the combination of different sets of observations through different filters. by combining the individual images the compound image can reveal the nebula in stunning detail with each colour displaying a different wavelength of I-R radiation. Using two telescopes also has advantages, as Sptizer is designed to observe at shorter wavelengths than Hershel and so by combining the two sets of data astronomers can get a more complete view of what is going on.

In this case the data revealed something very unusual indeed. Several of the young protostars have been flickering wildly, with their brightness fluctuating by as much as 20% in just a few weeks. Based on the cool temperatures of the material involved, the fluctuations had to occur far from the hot regions near the growing star, but such material should be far enough away from the star to spend years or even centuries in a slow decaying orbit before accreating onto the star’s surface.

Currently the explanation for how such a process could be so drastically accelerated is still up for debate though there are several suggestions. The other material may not be evenly distributed around the star, with some regions being more densely occupied than others. That may allow some of the denser clumps or filaments to collide with an inner, warmer shell of material causing the flare ups. It could also be caused by material piling up at the edge of the inner disk and so casting a shadow on the outer disk.

You can read more about this image here and here

Image of the Week – Hubble Spies A Glittering Jewel

The NASAESA Hubble Space Telescope has obtained the highest quality image of the globular cluster Messier 9 (M9) ever produced.

Messier 9 as seen by Hubble Credit: NASAESA

This glorious sphere of stars is far too faint to be detected by the human eye, yet Hubble can resolve it as upwards of a 1/4 of a million individual glistening stars.

M9 sits towards the centre of our own galaxy, and yet whilst relaivly close by in the grand scale of the universe it is still 25,000 light years from Earth.

The stars within M9 are twice the age of our own sun and are metal poor as a result – as they formed at a time when the cosmos was still largely deprived of the heavier elements like iron, oxygen and nickel.

The cluster was first discovered in 1764 by the French astronomer Charles Messier and was included as object 9 on his list of astronomical objects (hence its name!).

The image above covers and area of sky roughly equal to a pin head held at arms reach,a true testiment to the power of Hubble.

You can read more here