The Young Astronomers
From the monthly archives: January 2012
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.
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.
, 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.
The Eagle Nebula is one of the most well known regions in the universe having been snapped many times over the years by several telescopes including Hubble.
The latest images of the region come from the ESA’s Hershel Infrared Space Observatory and the XXM-Newton X-ray Observatory.
The Eagle Nebula seen by Hershel and XXM-Newton Credits: far-infrared: ESA/Herschel/PACS/SPIRE/Hill, Motte, HOBYS Key Programme Consortium; X-ray: ESA/XMM-Newton/EPIC/XMM-Newton-SOC/Boulanger
This image spans approximately 75 light years across the entirety of the nebula.
This image is a combination of data from both telescopes of the dense central region of the nebula. We can learn more about the information the image displays if we separate the data from each observatory, first lets have a look at the XXM-Newton X-ray data.
XXM data of the Eagle Nebula Credits: ESA/XMM-Newton/EPIC/XMM-Newton-SOC/Boulanger
Each individual dot on the image is an X-ray source with the various colours indicating the energy of the X-rays being emitted by the source, red being the lowest energy (0.3-1keV) working up through medium energy sources shown in green (1-2keV) to the highest energy sources displayed in blue (2-8keV).
The XXM was observing the area to help determine the source of the Eagle Nebula’s strong emission. One theory suggests that a hidden supernova remnant could be supplying the nebula with large quantities of energy whilst remaining obscured by the nebula’s dense cloud. To help determine if this theory is valid the XXM is scouring the area in an attempt to detect any sign of a faint X-ray emission extending from the central region. The scientists believe that if the XXM doesn’t detect any more emitting material than has already been identified by previous searches using Sptizer and Chandra this will be strong support of the hidden SNR explanation.
Now lets examine the Hershel data:
Hershel's view of the Eagle Nebula Credits: ESA/Herschel/PACS/SPIRE/Hill, Motte, HOBYS Key Programme Consortium
This displays the nebula in infra red wavelengths with 70 microns displayed in blue, 160 microns in green (both of these wavelengths were captured using filters in the PACS – Photodetector Array Camera - instrument) and finally 250 microns in red(images by SPIRE - Spectral and Photometric Imaging Receiver).
All these wavelengths are associated with very cold gas, indeed any gas displayed in blue here is just 40K above absolute zero down to that displayed in red which is a chilly 10K.
The twisted gas tendrils are still collapsing and will continue to form the next generation of stars for quite some time yet before the nebula finally disperses. Perhaps the most famous region within the nebula are the ‘Pillars of Creation’ which are in the above images which can be viewed just below the central point in the image (the eagle for which the nebula is named is located half way up the image on the left hand side, with its head pointing inwards). Indeed the Pillars are the central feature in one of the most recognisable image in all of astronomy:
The Pillars of Creation as seen by Hubble Credits: NASA/ESA/STScI, Hester & Scowen (Arizona State University)
The image was taken by Hubble in visible light using filters that isolate emission from excited hydrogen (Hα), singly ionised sulphur (SII) and doubly ionised oxygen (OIII). For scale, the tallest pillar is approximately four light years in height.
Now if we look at the same region in the infra red part of the spectrum (this time the data is provided by the ESO‘s, VLT’s ANTU telescope using the ISAAC instrument – yes that is quite a lot of acronyms), it looks completely different.
The Pillars of Creation as seen by ANTU Credits: VLT/ISAAC/McCaughrean & Andersen/AIP/ESO
At these wavelengths all but the densest regions of the Pillars are virtually transparent allowing us to gaze in wonder at the clumps of stars forming at the tips.
I leave you with this composite image, containing X-ray, visible and infra red data, enjoy.
Composite image of the Eagle Nebula Credits: far-infrared: ESA/Herschel/PACS/SPIRE/Hill, Motte, HOBYS Key Programme Consortium; ESA/XMM-Newton/EPIC/XMM-Newton-SOC/Boulanger; optical: MPG/ESO; near-infrared: VLT/ISAAC/McCaughrean & Andersen/AIP/ESO
You can read more about this fantastic collection of images here.
This truly stunning image of the Eastern Veil SNR was released at the very end of 2010 by the Issac Newton Group of Telescopes.
NGC 6992 Credit: A. Oscoz, D. López, P. Rodríguez-Gil and L. Chinarro
The nebula is located approximately 1470 light years from Earth and was produced by a detonating star that died between 5000 and 8000 years ago.
The nebula is the visible portion of the much larger Cygnus Loop and is divided into several arcs, with the image above showing part of the eastern section. Since it’s formation the remnant has expanded to a size that makes it appear to have a diameter around 6 times that of the full moon, or 36 times it’s area when viewed in the night sky. This translates to roughly 50 light years in physical diameter.
The loop is one of the brightest features in the X-ray skyscape as viewed from Earth. The nebula contains large quantities of hydrogen, sulphur and doubly ionised oxygen (OIII) each of which have been picked up in the filters used by the Newton Telescopes. They are displayed in the image as red, blue and green respectively.
The classification name given to this section is NGC 6992 of the nebula, and the Eastern section is also happens the brightest region of the loop.
The nebula was first observed by William Hershel in September 1784.
As the nebula is part of the Cygnus loop it can be viewed in the constellation Cygnus and is most spectacular when viewed through an OIII filter.
The Western Veil Nebula Credit: Nick Howes
You can read more here.
This post has been produced by VanessaG for the Young Astronomers
Discovery, with her maiden flight on 30 August 1984, with STS-41D. On the ascent she carried more than 41,000 lbs of cargo which was a record at the time. This cargo was mainly science experiments to study the effects of microgravity. Discovery was also the first shuttle to retrieve a satellite and bring it back to Earth. In 1985 Discovery was the first shuttle to fly four missions in one year.
Discovery as seen from the ISS during STS-128 Credit: NASA
On STS-51D the first sitting member of the US congress blasted off into orbit, Jake Garn, the republican senator of Utah. During the landing she suffered a blown front tire and subsequent brake damage. This then meant that all further flights for five years were directed to land at Edwards Air Force Base, California until nose wheel steering was introduced and brakes improved.
After the Challenger and Columbia disasters it was Discovery who was called upon to return the US to space again and regain their independence. The return after Columbia, STS-114 under the leadership of Eileen Collins; who earlier on STS-63 was the first female pilot. This mission was also the first to do a back flip on approach to the ISS so that the station crew could photograph the underside of the shuttle which then could be studied to check for damage. This was also the first time a repair had been made to a spacecraft while in orbit, the EVA crew removing two protruding spacers in the thermal shielding.
In April 1990, on STS-31, Discovery released the Hubble Space Telescope. This was also the highest ever flown by a shuttle at 380miles. And the ‘scope is still in use twenty years later and continues to provide valuable insights into the beginnings of the universe.
STS-60, February 1994, was the first co operative mission between the then enemies of Russia and the US. This laid the foundations for international cooperation which is one of the fundamental aspects of the International Space Station. With a Russian cosmonaut flying about the American shuttle Discovery. Discovery’s next flight, STS-63 was the first mission to be piloted by a woman, Eileen Collins, who laid further foundations into international cooperation as she piloted Discovery to within 40ft of MIR. Correcting the final approach for the first shuttle docking with the Russian space station.
Discovery has seen many other significant events in international development, the first spacewalk by an African-American, the last shuttle to visit MIR. The oldest astronaut, John Glen on board STS-95 who at the time of the flight was seventy seven, and still is the oldest person to ever fly in space. Discovery also celebrated the 100th shuttle fight on board mission STS-92. And on STS-120 lead by commander Pamela Melroy met Peggy Whitson, commander of Expedition 16 on board the ISS in 2007. This not only was not only the first time the ISS has been commanded by a woman but the first time two female commanders met in space.
In her 26 year lifetime Discovery has achieved many great things in the world. Not only advancing science but also cooperation and technology that you will use everyday. In total 180 people have travelled on board Discovery and a total of 150 million miles have been travelled in orbit.
Discovery Launches to Begin STS-128 Credit: NASA/Jerry Cannon, George Roberts
NASA’s Mars Reconnaissance Orbiter (MRO), has used its High Resolution Imaging Science Experiment (or HiRISE – and yes I know what you are thinking, and yes they did do that deliberately), to capture this amazing image of the Santa Maria Crater, including the rover Opportunity sitting on the crater’s edge.
The Santa Maria Crater as Seen by the MRO's HiRISE Camera Credit: NASA/JPL/University of Arizona
The astounding detail of the image also shows the tracks of the rover on the left hand side of the image.
The small blob indicated by the arrow is the rover itself.
The image was captured on the first of March 2011 which corresponds to Opportunity’s 2,524th Martian day of operation on the Red planet.
AAPOD
Portal to the Universe
