Stellar Timebombs

White dwarfs are often thought of as the quiet remains of stars that quietly cool over the aeons not troubling anyone except in the most extreme circumstances (See my post on Type Ia Supernovae for more details), though recent research indicates that many may be timebombs slowly ticking down to destruction.

An Artist's Impression of a Type Ia Supernova Credit: David A. Aguilar (CfA)

White dwarfs have a generally accepted upper mass limit of about 1.4 (termed the Chandrasekhar limit), any such star that exceeds this upper value will be unable to prevent itself undergoing gravitational collapse this creates a runaway nuclear fusion reaction within the star which promptly tears itself apart as a Type Ia Supernova. Two main theories have been put forward to create the conditions required for such an event to occur.

1. Two sub Chandrasekhar limit White dwarfs within a binary star system merge forming a single super Chandrasekhar limit White dwarf that immediately goes supernova.
2. A single White dwarf can slowly accretes mass from a non white dwarf binary partner which eventually pushes the White dwarf over the critical mass initiating the supernova.

The preferred explanation for most such events is the slow accretion scenario as stellar merger events are known to be extremely rare.  Though this scenario comes with its own set of problems, specifically proving beyond reasonable doubt that it is indeed the true picture of what is occurring.

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Observationally there should be certain signs that the explosion has been produced in this way.  The area around the supernova should contain traces of hydrogen and helium from material that the White dwarf hadn’t managed to gobble up and from its disturbed partner that would have undergone mass loss as a result of the supernova. As of yet no such feature as ever been detected, to make matters more interesting still the partner of a white dwarf that has gone supernova has never been detected even long after the fires of the explosion have faded.

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A team of researchers working on the problem think they have found an explanation for this troubling lack of supporting observations.

As a White dwarf accretes material from its binary partner it gains two ‘things’. The most obvious is of course the mass of the material being accreted, the second is perhaps less well known – angular momentum. Angular momentum is the product of an object’s angular velocity (the speed at which an object is rotating) and its moment of inertia (its resistance to changes in the rate of rotation), and line its counterpart must be conserved if no external forces are acting.

As the material being accreted is rotating around the White dwarf at considerable speeds it has a correspondingly large amount of angular momentum. As this must be conserved when the material eventually ‘falls’ onto the White dwarf this momentum is transferred to the dwarf which responds by increasing its rotation rate – like an ice skater spinning faster as they pull in their arms.

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Faster rotation rates can help to support the dwarf against the crushing force of gravity, as the rotational motion acts to hold the core up preventing the core from reaching the critical density required to initiate the catastrophic fusion reaction which would trigger the supernova.

Thus, White dwarfs can exceed the Chandrashekar limit provided they are rotating fast enough.

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Now we arrive at the most interesting point of this research.

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As White dwarfs slowly ‘spin down’ (their rotation speed slowly decreases over time) meaning that for a White dwarf that has accreted enough matter to exceed the Chandrashekar limit, but are rotating fast enough to stabilise themselves are literal time bombs ready to detonate.

Depending on their mass, initial rotation speeds and rate of rotational slow down, a super-Chandrashekar limit White dwarf may remain stable for up to a billion years after it ceases accreating matter from its companion. This allows the companion to evolve in its own right potentially into another hard to detect White dwarf (explaining why no companion stars have yet been identified) and is more than ample for any surrounding gas and dust from the accretion to dissipate explaining why the expected hydrogen and helium outer shell has been detected, they may not be there to detect!

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That means that there may me millions, if not billions of White dwarfs out there that are slowly heading towards their destruction. Perhaps as you are reading this, somewhere out in the depths of the universe, the time has come for one such White dwarf which has just been torn apart in a final blaze of light and radiation. Perhaps one day the light from that explosion will reach Earth and be observed by astronomers completing a story that has spanned eons.

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You can read more about the research here

and can access the research paper here

Image of the Week – The Lambda Centauri Chicken -24/09/11

To make up for the long gap between the recent images of the week, here is another!

Lambda Centauri Credit: ESO

The Lambda Centauri nebula is more commonly known as the Running Chicken nebula (or IC 2944) thanks to the fluke alignment of gas within.

It is an emission nebula glowing from the harsh bombardment of the ultraviolet light produced by the hot young stars that have been birthed by the nebula’s dusty clouds. The red glow indicates the familiar presence of excited hydrogen, a feature common in and around such star forming emission nebulae.

Star formation is evidenced further by the presence of Bok Globules – the dark black objects in the image particularly concentrated in the top right corner around the cluster of bright blue stars. These are small dense regions of gas and dust that are collapsing to form the next generation of stars.

These particular Bok Globules have received particular attention.  They are known as Thackeray’s Globules in homage to their discoverer Andrew David Thackeray, a South American astronomer who identified the globules in 1950 and famously observed by the Hubble Space Telescope.

Thackeray's Globules Credit: NASA/ESA and The Hubble Heritage Team STScI/AURA)

Eventually the stars within these globules will erode the cocoon of surrounding material and will reveal themselves to the universe.

Unfortunately, such beautiful emission nebulae are short lived in astronomical terms, lasting just a few million years before their gas has either been used to forge stars or blown out from the area by fierce stellar winds. The most massive of stars will burn out in flashes as they rapidly chew through their supply of hydrogen briefly lighting up the area again as a supernova and glowing remnant.

The main image was produced using data from the Wide Field Imager on the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory.

You can read more here.

Image of the Week – Cassini’s Fantastic Five – 22/09/2011

NASA’s Cassini Orbiter continues to send us fantastic images of Saturn, its rings and its moons. This latest image is no exception, it shows Saturn’s rings almost edge on along with five of the majestic planet’s expansive collection of moons.

Saturn's Fantastic Five Credit: NASA/JPL-Caltech/Space Science Institute

Going from the right hand side of the image and moving to the left, we have – Rhea, Mimas (a.k.a. the Death Star moon after its uncanny resemblance to the infamous space station), Enceladus, Pandora and Janus.

Although the image is black and white it was created using the green section of the electromagnetic spectrum.

Cassini will continue to produce stunning images like this for quite some time to come.

You can read more here

Image of the Week – Stars in a Twin Colour World -17/09/2011

The stunning open star cluster NGC 2100 has been captured like never before in this ESO image.

NGC 2100 Credit: ESO

The cluster is located in the fringes of the Tarantula Nebula within the Large Magellanic Cloud.

The blue regions of the image show the presence of ionised oxygen. The energy required to ionise the oxygen is supplied by the massive stars located deeper within the Tarantula Nebula, specifically within the large star cluster RMC 136. The red glow at the base of the image displays the presence of less energetic excited hydrogen marking the edge of the influence of the monster stars within RMC 136 and where smaller, cooler and less energetic stars dominate.

As NGC 2100 is a reasonably dense open cluster, it is at most a few hundred million years old, and is likely to be considerably younger. Open clusters form from the same general region of nebulosity and their stars are loosely held together by their mutual gravity. They then drift apart over time under the effect of gravitational perturbations from other objects and eventually disperse entirely with each individual star travelling on its own way through the cosmos.

You can read more here.

The Not so Hot Stars

We usually associate stars with ludicrously high temperatures, indeed the surface of the Sun is around 5780 Kelvin (of course there are many stars with much higher temperatures, and the cores of all stars are several million degrees Kelvin).

There are objects out there that are very similar to stars but have much cooler temperatures. These are the Brown dwarfs, sometimes termed failed stars they are more properly known as a class of sub-stellar object. That is a group of objects that are below the mass limit (about 0.08) to sustain hydrogen fusion – the primary energy source of stars.

Brown dwarfs occupy the lower right hand corner of the Hertzsprung–Russell diagram as shown below (with all objects falling below the magnitude +15 line being Brown dwarfs): -

The Hertzsprung-Russell Diagram Original Source Credit: Kaler, James B. Modifications: Peter Clark

There are three sub-classes of Brown dwarfs, each identified as a distinct spectral class (and no there is no easy way to remember the three): -

• L
• T
• Y

Though it should be noted that the hottest Brown dwarfs fall into the spectral class M along with the majority of Red dwarfs.

Spectral Class L

An L Class Dwarf Credit: R. Hurt/NASA

This group contains high mass, hot Brown dwarfs and the coolest Red dwarfs that are the faintest and least massive of all true stars. L class dwarfs are very dark red in colour and their emission peak lies within the infra-red region of the electromagnetic spectrum. As such they are very faint and exceptionally difficult to detect at optical wavelengths even when using a telescope as powerful as Hubble.

Their spectra show the presence of atomic alkali metals such as Sodium and Potassium (these ionise at low temperatures further demonstrating the low temperatures) and metal hydrides – a metal ion bound to a negative hydrogen ion for example Magnesium hydride – MgH. The defining spectral feature of the class is the presence of deep absorptions due to  Titanium(III) oxide (TiO) and Vanadium (III) oxide (VO).

Due to the nature of supergiant stars, L class giants and supergiants cannot form in isolation or under normal conditions experienced by evolving stars. In rare cases they may be produced through stellar collisions such as V838 Monocerotis (assuming the outburst event was indeed a stellar merger).

Spectral Class T

Class T contains only Brown dwarfs, no star regardless of how small or unusual the conditions can occupy this spectral class. Such Brown dwarfs have surface temperatures of between 700 and 1300 kelvin which is in and around the temperature of a wood fire.

They are even dimmer than their L class counterparts and similarly have peak emission in the infra-red, emitting virtually no visible light. Though they would appear to glow dimly with a purplemagenta tinge due to their chemical composition that absorbs much of the little green light they emit (Brown dwarfs that aren’t brown – go figure!).

T class Brown dwarfs are sometimes reffered to as Methane (CH₄) dwarfs as their spectra are dominated by absorptions related to Methane.

An artist's impression of a T class Brown Dwarf Credit: R. Hurt/NASA

Spectrum for Gliese 229B Credit: Oppenheimer et al.

Spectral  Class Y

These represent the coolest Brown dwarfs. Their surfaces have temperatures below 600 kelvin, and they should have detectable differences in their spectra compared to class T dwarfs. had I been writing this at the same time last month that would be all their was to say on this class but that was then this is now.

NASA’s Wide Infrared Survey Explorer – WISE – has thus far detected 100 new brown dwarfs, 6 of these fall into the Y spectral class and are located within 40 light years of our own sun.

One of these in particular in especially interesting.

An Artist's Impression of a Spectral Class Y Dwarf Credit: NASA/JPL-Caltech

WISE 1828+2650 is the coolest brown dwarf ever detected at just 298 Kelvin – 25 degrees Celsius - which is ten degrees below human body temperature; it is in the direction of the constellation Lyra.

WISE 1828+2650 Credit: NASA/JPL-Caltech/UCLA

The tiny unassuming green speck at the centre of this image is WISE 1828+2650. It may look insignificant in this image but if this same area was viewed in visible rather than infra-red that green blob would be completely undetectable.

Scientists have barely scratched the surface of the WISE data which covers one and a half sweeps of the sky captured between January 2010 and February 2011 before the spacecraft depleted its cryogenic coolant and was placed in indefinite  hibernation. The team of astronomers working on the data are very excited by the fact that there could be many more such dwarfs hidden in the data, perhaps some even cooler.

You can read more on this new discovery here.

This post is part of the Young Astronomers Databank Project.

Image of the Week – Stellar Birth-pangs – 4/09/2011

We are still here! I apologise for the lack of posts recently but going back to school has naturally taken the majority of my time, though fear not normal service will resume now.

To begin lets have a look at a much delayed Image of the Week (this week’s will be up shortly as well).

Like human babies, newborn stars announce there arrival to universe. Babies do so by crying, stars by creating bi-polar jets that put on a fantastic display.

These jets can interact with the interstellar medium (ISM), exciting and shocking material as they blast through space. This causes clumps of the ISM to heat up and emit light, and these are called Herbig- Haro (HH) objects after the first scientists to study them in detail - George Herbig and Guillermo Haro.

The Hubble Space Telescope has been used to obtain a series of high quality images of a selection of these objects which you can see in the fantastic compound image below.

HH 47, HH 34 and HH 2 Credit: NASA, ESA, and P. Hartigan (Rice University)

Such features are dynamic and can change over the space of several years. I would have liked to have a video embedded into this post but WordPress has conspired against me so I will have to make do with a link.

The video displays images taken of HH 47 over the past 14 years using Hubble’s Advanced Camera for Surveys and Wide Field Planetary Camera 2.

HH 47 is located within about 1350 light years from Earth in the direction of the southern constellation Vela – The Sails.

You can read more here.