This is an extension to my post: – Stellar Spectral Classes Explained which can be found here. As I previously explained stars can be placed into groups based on distinguishing features in their spectra. Whilst the main groups have already been discussed there are a few special ones that I think should be given special attention.
Wolf-Rayet
NGC 6888 Credit: NASA
Spectral class W stars or Wolf-Rayet stars are spectacular sights to behold. These are high mass stars nearing the end of their lives, and beginning to loose the eternal struggle against gravity. As the star beings to die the nuclear reactions within begin to destabilise, this destabilisation will eventually cause the star to rip its self apart as a supernova explosion blasting all but the core into space at phenomenal speeds and extreme temperatures.
The star can stave of this end by blowing off some its outer layers into space, this is detectable as massive jets of material blasting off into space from a tiny point or shells of material drifting off from its parent star. This mass loss is at best a temporary restpite from the prospect of a supernova and only delays the inevitable the star. In a few short million years this stop gap measure fails to maintain the star’s stability and the unavoidable happens with the star going out with a bang.
As Wolf-Rayet stars are short term evolutions of the rare high mass stars lasting for just a few million years Wolf-Rayet stars are comparatively rare. An example can be found in the Crescent nebula (NGC 6888 – image above).
The nebula formed when the central supergiant began to ‘vent’ its upper atmosphere off to space. The nebula is classed as an emission nebula as it is emitting light of it’s own thanks to the bombardment of ultraviolet light from its parent allowing the nebula to fluoresce as it expands.
As the exact composition or each star is subtly different, along with the countless ways a star can disperse material into space no two Wolf-Rayet nebulae are the same. Indeed with the vast array of factors that influence the overall shape, colour and structure of nebulae radically different results are visible.
Take NGC 2359 for example. Despite being formed in the same way, differing interactions with the interstellar medium have produced a nebula that could not be more different. Its distinctive shape has given rise to its more common name – Thor’s Helmet.
Thor's Helmet Credit: Andrew J Dumbleton & iTelescope.Net
The Wolf-Rayet spectral class is divided into two subgroups: WC and WN. WC stars have their spectra dominated by carbon emission and WN are dominated by Nitrogen.
Supergiants
While not really a spectral class on their own, there are three supergiant stars that I think are stunning enough to get a mention here. One of the most well know supergiant stars is the hypergiant Eta Carinae.
Eta Carinae and the Homunculus Nebula Credit: Nathan Smith (University of California, Berkeley), and NASAESA
The star is a massive 100
which is close to the theoretical upper mass limit possible for any star. If a star was to be much more massive, it would tear itself apart through radiation pressure – the force produced by the star’s nuclear reactions. Eta Carinae is expected to go supernova within the next few million years. When it explodes the star will shine with many hundreds of time its normal luminosity (potentially being visible in the daytime here on Earth) while the resulting debris may even form a black hole. Whatever matter escapes the formation of the black hole will enrich the surrounding space with the heavier elements required for planet formation and which form the seeds of life.
The star is actually a binary pair with one star orbiting its much more massive partner. The pair are embedded within the Homunculus Nebula – the lobes of material in the above image that are believed to have been released during the supernova impostor event in 1841. During this event the pair brightened to a level just short of that of a real supernova. The stars survived the detonation though even well over a century later their internal structures have not yet fully recovered. This was the prototype event of its class and may be a sign that the star is approaching supernova as a similar incident was recorded in another galaxy two years before the true supernova.
The Homunculus Nebula, and by extension Eta Carinae lie around 7500 light years from Earth in the direction of the southern constellation Carina – The Ship’s Keel – within the larger Carina Nebula.
The Carina Nebula Credit: NASA, ESA, N. Smith (University of California, Berkeley), and The Hubble Heritage Team (STScI/AURA)
Eta Carinae is located within the small glowing clump half way up the image about three thumb widths in from the left hand side.
Another such hypergiant star is the Pistol star (G0.15-0.05). It is found near the heart of our galaxy – in the central bar rather than one of the spiral arms like Sol or Eta Carinae. It is one of the most luminous stars known to astronomers as it shines with the equivalent output of 4 million
The difference in luminosity is so great the Pistol star releases the same energy Sol does in a year in 20 seconds!!! (This figure is an approximation) It undergoes periodic blasts as it struggles to hold itself together (it is similar to the Eta Carinae system in terms of mass). These blasts have shed stellar material into space which can today be seen as the Pistol nebula (the bright blob at the centre of the image is the star itself).
The Pistol Star Credit: NASA
Our final supergiant is XX Triangulum (HD 12545). This is a red supergiant star with luminosity of around 100
, however its most interesting feature is not its size or colour but its temperature distribution – It has been revealed that one hemisphere is cooler than the other. The cooler hemisphere has a dark region like a sunspot on Sol but at a size that dwarfs Sol. Like the sunspots on our own star, magnetic fields are thought to be responsible for this unusual feature.
HD 12545 Credit to NASA
White Dwarfs.
White dwarfs are the remains of main sequence stars that have lost the majority of there atmosphere to space at the end of the red giant phase. A white dwarf is approximately the size of Earth but as they are the cores of dead stars they are incredibly dense – 1×109 kgm-3 or put differently, if we could extract a one cubic meter of a White dwarf it would ‘weigh’ one million kilograms. This extreme density is a result of confining potentially more than a solar mass of material into a comparatively tiny region of space, think of how large the Sun is compared to the Earth and you will get some idea of the compression required.
All White dwarfs must have a mass lower than about 1.5 solar masses as this is the Chandrashekar limit – if the star was any more massive the force supporting it against gravity (electron degeneracy pressure) would be overwhelmed and the star would collapse further and then detonate as a type Ia supernova.
White dwarfs are given the spectral classification D. An example of a White Dwarf is Sirius B – the small dim companion to the brightest star in the sky:
The Sirius System - Sirius B is the small dot in the lower left Credit: Credit: NASA, ESA, H. Bond (STScI), and M. Barstow (University of Leicester)
Neutron Stars and Pulsars.
Neutron stars are the high density remains of supernovae. They form from the remains of massive stars that have exceeded the Chandrashekar limit. They are composed of exotic degenerate matter and neutrons hence their name. The upper mass limit for a neutron star is approximately 3 solar masses, anything more massive would exceed the Tolman-Openhiemer-Volkof limit and collapse into a black hole (as neutron degeneracy pressure would be unable to support the star against gravity).
A pulsar is a neutron star that has retained enough angular momentum to spin rapidly. They release the majority of their energy in two beams that emanate from their poles. A pulsar can rotate as rapidly as 30 times a second and some rotate even faster than that! When the beams pass in the direction of the Earth the star’s luminosity appears to pulse giving the star there name.
Pulsars slowly slow down and so the period of one pulse cycle (the time taken for the star to rotate once on its axis) increases as the star’s velocity lowers due to drag and eventually (after an exceptionally long time) the star will stop spinning all together. The energy emitted by neutron stars is the release of thermal energy as the star cools – it is not releasing any energy by nuclear processes, this process ended when the star went supernova. Neutron stars and pulsars are usually white and so fall in the F spectral category.
Magnetars
Magnetars are neutron stars with exceptionally powerful magnetic fields. They emit large amounts of X and Gamma rays as a result of this field strength. They are also known as soft gamma repeaters (SGRs) or anomalous X-ray pulsars (AXPs) due to their tendency to emit burst of gamma or X-rays at irregular intervals.
Brown Dwarfs
Brown dwarfs now have their own post that goes into some detail.
You can read The Not so Hot Stars by clicking the link.
Sub Brown Dwarfs
Some astronomers feel that a category for ‘failed brown dwarfs’ is needed. This would mean stars that are below the mass limit for brown dwarfs (about 13 times the mass of Jupiter) but significantly above the normal mass of a planet. No such objects have yet been confirmed however spectral Class Y has been suggested, though their is some debate if such objects would be better classified as low mass Brown Dwarfs.
Planetary Nebulae
Some of the most spectacular sights in the cosmos come in the form of Planetary Nebulae. The name is a bit of a misnomer – it was first thought that planets formed from such nebulae but now we understand that they are created by the mass release of red giants as they become white dwarfs, however the name has stuck regardless. One of the most famous examples is the Ring nebula or M57.
M57 Credit: NASAESA The Hubble Heritage Team (AURA/STScI)
The Ring Nebula is located in the constellation Lyra at a distance of about 2300 light years from Earth.
Another more delicate but no less beautiful nebula is the Hourglass Nebula – MYCN18.
MYCN18 Credit: In image
The nebula is slightly tilted towards us so we are looking down through the top of the formation. The dense green glob and the centre contains the dying star.
Unfortunately planetary nebulae do no last long as the are only tenuous clouds of gas and dust illuminated by their dying parent. After a few ten thousands years the nebulae expand so far they become to diffuse to illuminate and they then disperse into space and fade from view. Thankfully they put on one heck of a show while they can!
More information about planetary nebulas and other forms of nebula including a more in depth spectral analysis will be made available through Project Nebula.
