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The Helix Nebula is one of the most familiar nebulae in astronomy, and it’s been nicknamed the “Eye of God”. Its official designation is NGC 7293, the Helix Nebula is located inside the constellation of Aquarius. The Helix Nebula is one of the closest examples of a planetary nebula. Astronomers have estimated its distance to only be 700 light-years away.
The central star of the Helix Nebula was once a star very similar to our own Sun. As the star neared the end of its life, it expanded into a red giant and puffed away its outer layers. The central star is destined to become a white dwarf star, as it slowly cools down. It’s no longer actively fusing hydrogen, and only shines with the remaining heat from when it was once a star.
The Helix Nebula that we see today is actually just a momentary phase in the death of the star. The inner layers of gas and dust expanding away from the central star were probably released about 6,500 years ago, with the outer layer released about 12,000 years ago. We can see them because they’re illuminated by the central star. But eventually they’ll get far enough away that they’re no longer bright enough to see. From that point on we’ll just see the central white dwarf star.
Because the Helix Nebula is so close, images from the Hubble Space Telescope revealed knots of material in the expanding shells of gas and dust. There are more than 20,000 of these knots in the nebula, and they have cometlike tails stretching away from the central star.
One of the most successful mission ever sent to Mars is the Mars Exploration Rover program, with the two rovers Spirit and Opportunity. They were launched separately to Mars in 2003 and landed safely several months later. They were supposed to last about 3 months on the surface of Mars, but have now survived more than 5 years.
Spirit and Opportunity used technology developed with the Mars Pathfinder mission. They used an airbag system to land on the surface of Mars without using retrorockets to touch down gently. They also use the rover technology first used with the Sojourner rover, but instead of operating from a base, Spirit and Opportunity were designed to be completely independent, able to communicate directly back to Earth.
The purpose of the Mars Exploration Rover mission (MER) was to search the surface of Mars for evidence of past water on the surface of Mars. Spirit landed in the huge Gusev Crater on Mars, a region that could have been an ancient lake on Mars. Opportunity touched down on the other side of the planet in a region called Meridiani Planum.
Both Spirit and Opportunity are equipped with solar panels that supply electricity to let them crawl around the surface of Mars, as well as their scientific instruments that let them study the surface of Mars and its rocks. They’re also equipped with a grinding tool that lets them scrape away the outer layer of rocks and see the material underneath.
Within just a few months of arriving on Mars, both Spirit and Opportunity fulfilled their mission objectives, and discovered evidence that large quantities of water used to be on the surface of Mars. Spirit discovered hints that water had acted on a rock called Humphrey, while Opportunity found layers of sedimentary rock that would have been formed by deposits in water. Both rovers continued to find additional evidence for the presence of water.
Over the course of their mission on the surface of Mars, both rover traveled several kilometers. Spirit climbed a small mountain, and Opportunity crawled into a large crater to sample the walls for evidence of past water. And both rovers continued to perform quite well, for many years beyond their original estimate life spans.
We have written many articles about the Mars Exploration Rovers for Universe Today. Here’s an article about the troubles for the Spirit rover, and here’s an article about Martian weather.
Mars and Venus are the two terrestrial planets most similar to Earth. One orbits closer to the Sun, and one orbits more distant to the Sun. But both are visible with the unaided eye, and two of the brightest objects in the night sky.
Venus orbits at an average distance of only 108 million km from the Sun, while Mars is an average of 228 million km. Venus gets as close to Earth as 38 million km, and Mars gets as close as 55.7 million km.
In terms of size, Venus is almost a twin planet of Earth. Its diameter is 12,104 km, which is 95% the diameter of Earth. Mars is much smaller, with a diameter of only 6,792 km. And again, in terms of mass, Venus is almost Earth’s twin. It has 81% the mass of Earth, while Mars only has 10% the mass of Earth.
The climates of Mars and Venus are very different, and very different from Earth as well. Temperatures on the surface of Venus average 461 °C across the entire planet. That’s hot enough to melt lead. While the average temperature on Mars is a chilly -46 °C. This temperature difference comes from the fact that Venus is closer to the Sun, but also because it has a thick atmosphere of heat trapping carbon dioxide. The atmosphere on Venus is nearly 100 times thicker than Earth’s atmosphere at sea level, while the atmosphere on Mars is 1% the thickness of Earth.
Mars is the most studied planet in the Solar System (after the Earth). There have been dozens of missions sent to Mars, including orbiters and rovers. Although many missions have been lost, there have been several that have successfully orbited the planet and several that have landed on the surface. Missions have also been sent to Venus, and you might be surprised to know that the Soviets sent a series of landers called Venera that actually reached the surface of Venus and survived long enough to send back a few photographs.
Mars has two moons, Phobos and Deimos, while Venus has no moons. And neither planet has rings.
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Mars Pathfinder was NASA mission to Mars, which launched on December 4th, 1996 and landed on the surface of Mars on July 4, 1997. Unlike the missions that went before it, the Pathfinder lander was also equipped with a tiny rover called Sojourner, which could venture away from the lander, crawl around the surface of Mars and study rocks up close. It was a relatively inexpensive mission that tested out many of the technologies build into later missions, like the Mars Exploration rovers Spirit and Opportunity.
The purpose of Pathfinder was to prove that the concept of “faster, better and cheaper” missions would work. Pathfinder only cost $150 million and was developed in under 3 years. It was also sent to study the surface of Mars, including the geochemistry of the rocks, the magnetic properties of the surface and the structure of the planet’s atmosphere.
When the Pathfinder mission arrived at Mars, it entered the atmosphere and deployed a parachute. Instead of using retrorockets to land gently on the surface, however, Pathfinder used an airbag system. This allowed it to save fuel; instead of landing gently, it was dropped from an altitude of about 100 meters onto the Martian surface. It bounced several times and came to a rest before opening up like the petals of a flower. Once everything checked out, the tiny Sojourner Rover was deployed onto the surface of Mars.
The area around the Pathfinder site had many rocks, large and small, and the NASA scientists gave them unique names like “Barnacle Bill” and “Yogi”. Sojourner was able to crawl around and study these rocks up close. It was able to study the chemical makeup of the rocks, and confirmed that they formed from past volcanic activity. Over the course of the entire mission, Pathfinder and Sojourner returned 16,500 images and made millions of measurements of the Martian atmosphere.
Pathfinder stopped communicating with Earth after 83 days on the surface of Mars. Its battery was only designed to be recharged 40 times, and once its battery stopped working, the spacecraft was unable to keep its electronics heated in the cold Martian night. After it stopped communicating, NASA decided to name the lander after Carl Sagan. It became The Carl Sagan Memorial Station.
An artist's impression of how quasars may be able to construct their own galaxies. Image Credit: ESO/L. Calcada
The birth of galaxies is quite a complicated affair, and little is known about whether the supermassive black holes at the center of most galaxies formed first, or if the matter in the galaxy accreted first, and formed the black hole later. Observations of the quasar HE0450-2958, which is situated outside of a galaxy, show the quasar aiding a nearby galaxy in the formation of stars. This provides evidence for the idea that supermassive black holes can ‘build’ their own galaxies.
The quasar HE0450-2958 is an odd entity: normally, supermassive black holes – also known as quasars – form at the center of galaxies. But HE0450-2958 doesn’t appear to have any host galaxy out of which it formed. This was a novel discovery in its own right when it was made back in 2005. Here’s our original story on the quasar, Rogue Supermassive Black Hole Has No Galaxy.
The formation of the quasar still remains a mystery, but current theories suggest that it formed out of cold interstellar gas filaments that accreted over time, or was somehow ejected from its host galaxy by a strong gravitational interaction with another galaxy.
The other oddity about the object is its proximity to a companion galaxy, which it may be aiding to form stars. The companion galaxy lies directly in the sights of one of the quasar’s jets, and is forming stars at a frantic rate. A team of astronomers from France, Germany and Belgium studied the quasar and companion galaxy using the Very Large Telescope at the European Southern Observatory. The astronomers were initially looking to find an elusive host galaxy for the quasar.
The phenomenon of ‘naked quasars’ has been reported before, but each time further observations are made, a host galaxy is found for the object. Energy streaming from the quasars can obscure a faint galaxy that is hidden behind dust, so the astronomers used the VLT spectrometer and imager for the mid-infrared (VISIR). Mid-infrared observations readily detect dust clouds. They combined these observations with new images obtained from the Hubble Space Telescope in the near-infrared.
Observations of HE0450-2958, which lies 5 billion light years from Earth, confirmed that the quasar is indeed without a host galaxy, and that the energy and matter streaming out of the jets is pointed right at the companion galaxy. This scenario is ramping up star formation in that galaxy: 340 solar masses of stars a year are formed in the galaxy, one-hundred times more than for a typical galaxy in the Universe. The quasar and the galaxy are close enough that they will eventually merge, finally giving the quasar a home.
David Elbaz of the Service d’Astrophysique, who is the lead author of the paper which appeared in Astronomy & Astrophysics, said “The ‘chicken and egg’ question of whether a galaxy or its black hole comes first is one of the most debated subjects in astrophysics today. Our study suggests that supermassive black holes can trigger the formation of stars, thus ‘building’ their own host galaxies. This link could also explain why galaxies hosting larger black holes have more stars.”
‘Quasar feedback’ could be a potential explanation for how some galaxies form, and naturally the study of other systems is needed to confirm whether this scenario is unique, or a common feature in the Universe.
What accretes quietly in the night and can be a blast to observe? Try a FUor… These high accretion, high luminosity phase pre-main sequence stars may only last a few decades – but display an extreme change in magnitude and spectral type in a very short period of time. While FU Orionis may be the prototype you know about, there’s a lot more to learn and even more to observe! Step outside in the dark with me and let’s take a look…
What we know so far about FU Orionis-type stars is they flare with abrupt mass transfer from an accretion disc onto a young, low mass T Tauri-type star. In itself, this is very exciting because nearly half of T Tauri stars have circumstellar disks or protoplanetary discs. These could very well be the forerunners of planetary systems similar to our own solar system! How do we know there is a disc there? Try variablility. “Variable circumstellar extinction is pointed out as responsible for the conspicuous variations observed in the stellar continuum flux and for concomitant changes in the emission features by contrast effect. Clumpy structures, incorporating large dust grains and orbiting the star within a few tenths of AU, obscure episodically the star and, eventually, part of the inner circumstellar zone, while the bulk of the hydrogen lines emitting zone and outer low-density wind region traced by the [OI] remain unaffected.” says E. Schisano (et al), “Coherently with this scenario, the detected radial velocity changes are also explainable in terms of clumpy materials transiting and partially obscuring the star.”
While accretion rates for a FUor could range anywhere from 4 to 10 solar masses annually and its eruptions last up to a year or longer, astronomers believe their entire lifetimes only last a few decades. The proto-star itself may also be limited to undergoing an average of one to two eruptions each year. “The brightness of FUors increases by several magnitudes within one to several years. The currently favored explanation for this brightness boost is that of dramatically rising accretion from the disc material around a young star. The mechanism leading to this accretion increase is a point of debate.” says S. Pfalzner, “The induced accretion rates, the overall temporal accretion profile, the decay time, and possibly the binarity rate we obtain for encounter-induced accretion agree very well with observations of FUors. However, the rise time of one year observed in some FUors is difficult to achieve in our simulations unless the matter is stored somewhere close to the star and then released after a certain mass limit is transgressed. The severest argument against the FUors phenomenon being caused by encounters is that most FUors are found in environments of low stellar density.”
Surprisingly enough, even given the short period of time in which a FUor exists, no one has ever seen one phase out. “A cross-correlation analysis shows that FUor and FUor-like spectra are not consistent with late-type dwarfs, giants, nor embedded protostars. The cross-correlations also show that the observed FUor-like HH energy sources have spectra that are substantively similar to those of FUors.” says Thomas P. Greene (et al), “Both object groups also have similar near-infrared colors. The large line widths and double-peaked nature of the spectra of the FUor-like stars are consistent with the established accretion disk model for FUors, also consistent with their near-infrared colors. It appears that young stars with FUor-like characteristics may be more common than projected from the relatively few known classical FUors.”
Just how common and observable are these unusual characters? A lot more than you might think. According to Bo Reipurth (et al); “The original FUor class was defined by a small number (5-6) of pre-main sequence stars that had been observed to brighten up by 3-6 magnitudes on time scales of 1-10 years. The class has since been augmented by a comparable number of stars that have similar spectra or SEDs to the classical FUors, but that have not been observed to behave photometrically in that way. It is likely that the FUor phenomenon is recurrent, but it is not at all clear whether it is a property shared by ordinary T Tauri stars, or whether it is confined to a special minority among them. It is important that more examples be found, and found promptly, and as the result of systematic search rather than by accident as has been the case in the past. The goal would be to examine, on a regular monthly basis, all the molecular clouds within about 2 kpc that lie along the galactic plane and Gould’ s Belt for faint (or previously invisible) stars that had brightened up by a magnitude or more. It is essential that any such detections be followed up spectroscopically as soon as possible, to weed out interlopers: flare stars, cataclysmic variables, Miras, and EXors (the latter also being pre-main sequence but which unlike FUors soon return to their original brightness level, usually in a year or less). All of these objects are readily distinguishable from one another even at modest spectroscopic resolution. Such an on-going survey would serve also to follow the development of FUors.”
So let’s do the FUor dance!
IRAS 09068 641 FU Ori type object - Joe Brimacombe
According to CBET 2033 released on November 21, 2009 from the International Astronomical Union: “The discovery of a possible FU-Ori-type eruption (see Hartmann and Kenyon 1996, ARAA 34, 207) is located at R.A. = 6h09m19s.32, Decl. = -6o41’55”.4 (equinox 2000.0), and coincident with the infrared source IRAS 06068-0641. Discovered by the CRTS on Nov. 10, it has been continuously brightening from at least early 2005 (when it was mag 14.8 on unfiltered CCD images) to the present magnitude of 12.6, and may possibly brighten further. On recent images, a faint cometary reflection nebula is visible to the east. A spectrum (range 350-900 nm), taken with the SMARTS 1.5-m telescope at Cerro Tololo, on November 17, shows H-alpha in emission, all other Balmer lines and He I (at 501.5 nm) in absorption, and a very strong Ca II infrared triplet in emission, confirming it to be a young stellar object. The object lies inside a dark nebula to the south of the Mon R2 association, and is likely related to it. In addition, also inside this dark nebula, a second object at R.A. = 6h09m13s.70, Decl. = -6o43’55”.6, coincident with IRAS 06068-0643, has been varying between mag 15 and 20 over the past few years, reminiscent of UX-Ori-type objects with very deep fades. Also, this second object supports a variable cometary reflection nebula, extending to the north. The spectrum of this object also shows H-alpha and the strong Ca II infrared triplet in emission.”
Visible? Yeah. You know it. And here are the wide field results as taken by Joe Brimacombe…
IRAS 06068 641 FU Ori type widefield - Joe Brimacombe
“A smaller site of ongoing star formation in the Mon R2 molecular cloud are the objects associated with GGD 16 and 17. To the south of GGD 17, the T Tauri star Bretz 4 is probably associated with the GGD object. This star has been studied spectroscopically and was classified by as a K4 spectral type with a class 5 emission spectrum.” says Carpenter and Hodapp, “The infrared source IRS 2 is positionally coincident with Bretz 4, while the more deeply embedded IRS 1 has no optical counterpart and lies between the GGD objects. A detailed optical study showed that GGD 17 is part of a curved jet extending north of the star Bretz 4 and consisting of HH 271, and possibly also HH 273. Nebulosity close to the star shows the typical morphology of scattered light from an outflow cavity wall. The embedded infrared objects and optical reflection nebulosity in the general GGD 16-17 region is associated with 850 um emission.”
Capture a FUor… It may be the most unusual thing you’ve ever done!
Many thanks to Joe Brimacombe for the awesome images and awakening my ‘FUor’ curiousity!
This artist’s impression of a supernova shows the layers of gas ejected prior to the final deathly explosion of a massive star. Credit: NASA/Swift/Skyworks Digital/Dana Berry
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Supernovae are generally considered as fast and furious events. For the Type II, core collapse supernovae, the core implodes almost instantaneously although it takes some time for the shockwave to escape the star. As it does, the star brightens in what’s known as the “rise time” of the supernova. For most Type II supernovae, this takes about a week.
So what are astronomers to make of supernova 2008iy that had an unprecedented rise time of at least 400 days?
From the time it was discovered, SN 2008iy was an oddball. When its spectra was analyzed, it was placed in the rare IIn subclass. This subclass is reserved for supernovae that feature narrow emission lines. Most supernovae have broad emission lines, if they even have emission lines at all.
To learn more about the history of this unusual case, astronomers at the University of California, Berkeley turned to archival images from the Palomar Quest survey. They searched images of the region to trace back the supernova as far as July of 2007, before which, the star was too faint to appear in images. Thus, the supernova brightening started at least that early and continued until late October of 2008 giving it a rise time at least four times as long as any previously discovered supernova.
The main clue to explain this mystery stemmed from the unusual emission lines. Generally, stars and supernovae are characterized by their absorption spectra which are caused when relatively cool gas stands between a hotter source and our detection. To generate emission lines, there must been a relatively dense medium being excited by the supernova. Furthermore, the fact that the lines were narrow implied that it was fairly motionless.
Together, this pointed to the progenitor undergoing a heightened period of mass loss prior to the detonation. The idea is such that the progenitor had shed large amounts of material. When the supernova occurred, this shell initially obscured the event. But as the ejecta from the supernova overtook the relatively stationary earlier shells, the brighter material slowly seeped out giving rise to the 400 day rise time.
While all stars undergo a period of mass loss in their post main sequence life, such a dense shell would be uncommon. To explain this, the authors turned to a type of star known as a Luminous Blue Variable. These stars are typically near the theoretical limit for the mass of a star (150 times the mass of the sun). Due to their extreme mass, they have strong stellar winds which periodically blow off large amounts of material that could create shells similar to those necessary for SN 2008iy. Unfortunately, this event was so distant that it could not be resolved to search for such a nebula. Even the host galaxy proved difficult to distinguish due to its faintness, although it is believed to be an irregular dwarf galaxy. Eta Carinae is one such luminous blue variable star. If perhaps one day soon it decides to turn into a supernova, it too will unfold in slow-motion.
Venus. Image Credit: NASA/courtesy of nasaimages.org
Here’s a question: what color is Venus? With the unaided eye, Venus just looks like a very bright star in the sky. But spacecraft have sent back images of the cloud tops of Venus, and some have even returned images from the surface of Venus.
If you could actually fly out to Venus and look at it with your own eyes, you wouldn’t see much more than a bright white-yellowish ball with no features. You wouldn’t actually be able to see any of the cloud features that you can see in photographs of Venus. That’s because those photos are taken using different wavelengths of light, where differences in the cloud layers are visible. For example, the photo that accompanies this story was captured in the ultraviolet spectrum.
Although the atmosphere of Venus is almost entirely made up of carbon dioxide, the clouds that obscure our view to the surface are made of sulfur dioxide. These are opaque to visible light, and so we can’t see through them to the surface of Venus. These clouds actually rain droplets of sulfuric acid.
Surface of Venus by Venera.
If you could get down beneath the cloud tops of Venus, you wouldn’t be able to see much either. That’s because the clouds are so thick that most of the light from the Sun is blocked before it reaches the surface. You would see a dim landscape, like you might see at twilight. The surface of the planet is littered with brownish-red volcanic rocks. The bright red color you see in the Soviet Venera images of Venus have been brightened to show more surface detail.
So, what color is Venus? Yellowish-white.
We’ve written several articles about the color of the planets for Universe Today. Here’s an article about the color of Mercury, and here’s an article about the color of Pluto.
Credit: Robert Kaufman's image of Tarantula and Orion spectra (used with permission)
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Amateur astronomers are a unique species worthy of their own reality TV show. Their craftsmanship, resourcefulness, dedication, and passion is simply amazing. Many professional astronomers rely heavily on amateurs for quick spot checks, discovery followups, collaboration on research projects, the diverse locations of their telescopes and their ability/willingness to put in long hours of observation. So what is spectroscopy, and what do the amateur astronomers get up to?
Absorption spectroscopy is the study of the color and light spectrum of stars and galaxies. We all love our Hubble photos and pretty astro-photographs, however most of the real research and science comes from observing the light spectrum.
Robin Leadbeater’s LHIRESIII Spectrograph
Astronomers look at emission lines and absorption lines in the spectra to determine the make up of stars, nebulas and galaxies. Dopler effects, orbital behavior, elements of stars, even atmospheres can be determined by observing these absorption and emission lines. Scientists believe that a carbon dioxide absorption spectrum line signature in the spectrum of a star with a transiting exo-planet could eventually be the most exciting discovery – a possible indicator of extra-terrestrial life.
Why are amateurs interested?
I asked Ken Harrison the moderator of the Yahoo group – Astronomical Spectroscopy, why amateurs would be interested in absorption spectra?
“I see it as the “last frontier” for amateur astronomers. When you’ve taken the 100th image of the Orion nebulae – what do you do next?? It’s challenging, interesting and can give some scientific value to your work. Amateurs have successfully recorded the spectra of nova before the professionals and complimented other variable star work with observations of the changing spectral emissions of stars showing their Doppler shifts and atmospheric changes.”
Ken specializes in the spectra of Wolf-Rayet stars and is currently writing a book on amateur spectroscopy. Ken has been building his own spectrographs since 1992 and has used a variety of devices ranging from a simple star analyzer on a digital SLR camera to a sophisticated guided spectrograph.
A spectrograph allows light to pass through a narrow slit where it is then split into it’s spectra by passing through some sort of diffraction grating, before being captured on a CCD. The plate scale of the CCD then comes into play as angstroms per pixel instead of the usual (astrometric measure) arc/secs per pixel.
Rob Kaufman recently captured a Nova outburst Nova Scuti 2009 (V496 SCT) between the trees and clouds from his back yard.
Credit: Rob Kaufman's spectrogram of Nova Scuti 2009 (V496 SCT) outburst
Italian amateur Fulvio Mete has achieved a spectrographic separation of tight binary Beta Aurigea. The double Ha absorption line is easily identifiable in his image taken with a 14inch Celestron. Some of the world’s best telescopes are unable to separate Beta Aurigea optically, so being able to do a spectrographic separation with a back yard telescope is a significant achievement.
Fulvio Mete's spectrogram of Beta Aurigae
Perhaps there is no finer example of the quality of the spectroscopy done by amateurs than the current citizen science project on the eclipse of binary Epsilon Aurigae. Robin Leadbeater from Three Hills Observatory, a team member/contributor to the Citizen Sky project and avid amateur astronomer, has documented the changing spectra of Epsilon Aurigae, in particular monitoring the changing KI (neutral potassium) 7699 absorption line during the early stages of the ingress.
Robin Leadbeater's Spectrogram of KI 7699 absorption line in Epsilon Aurigae eclipse.
The eclipse happens every 27 years and this eclipse will be the first to be fully documented with advanced spectroscopy – clearly alot of that will be performed by skillful amateurs.
So what equipment do I need?
Ken Harrison comments that the equipment required is not necessarily expensive and it is a lot of fun.
“Luckily with the filter gratings available at reasonable prices (Star Analyser, Rainbow Optics etc) interested amateurs can start using their current equipment with minimal cost and outlay. Freeware programs like IRIS (C Buil) and VSpec (V Desnoux) allow the detailed analysis of spectra to be done without all the mathematics or detailed physics. As experience grows so do the questions. What do those absorption features mean? Why does this spectrum look completely different from that spectrum? How can I get benn resolution? Yes, it has its learning curve like any new adventure, but there are many others who have trodden the road before and only too willing to assist – To boldly go where few amateurs have gone before – Spectroscopy!!!”
The contribution of amateurs across all forms of astronomy is significant, and spectroscopy is no exception. If you want more information join one of the Yahoo groups or major amateur astronomy forums as they all have discussion groups with experienced people who are keen to help you get started.
Special thanks to Ken Harrison, Robin Leadbeater, Rob Kaufman, Fulvio Mete and Dale Mais for your photos and insight!
