|Negative Image of M86 (labeled) by Bob Olson|
|M51 – Image by Bob Olson (his favourite galaxy)|
Sanjeev Sivarulrasa used a Canon 40D and an 85 mm refractor to image the Horsehead area, M45 and M13.
|Image of M13 by Sanjeev Sivarulrasa|
Richard Alexandrowich is also president of the Glengary Stargazers, a small astronomy club in Alexandria. On his way home from work at 2 am one morning he got the idea of inviting someone from the Canadian Space Agency to give a talk. After a lot of hard work, the club succeeded. The event was held on April 5 at the Best Western in Cornwall. The speaker was Tara Hillebrandt, the real time support manager for the Canadian space arms on the ISS. She talked about Canada's involvement with the ISS.
Simon Hanmer told us about NASA's new press release on its Messenger mission to Mercury. The most exciting thing in the release was at the end. Mercury is a peculiar planet. Like all rocky planets, it has a core of nickel iron. The other rocky planets have an outer part, called the crust on Earth, and stuff in between, called the mantle. Mercury's core is huge in relation to the planet's size. Two theories have been proposed. A collision early in the history of the solar system blew off the outer layers. Or since Mercury is so close to the sun, the sun blew off the outer layer during Mercury's formation.
It is important to know what the outer part of Mercury is today. Initially it looked like this was anorthosite, the same composition as the light parts of the Moon (the same composition as the original Coffee Mate). However the press release stated there are no felsic rocks on Mercury (no Coffee Mate on the surface). But there is basalt. Basalt forms the ocean basins on Earth. It is a very common rock in the outer parts of normal planets. So NASA will eventually say there is something wrong with the models explaining Mercury's large core. You heard it here first.
Deep Sky: Gary Susick's Pick
Gary also recommended a book: Atlas of the Messier Objects.
Thanks to Ann and Art Fraser for the after meeting refreshments.
Richard Alexandrowich Glengarry Stargazers
All that we see in the heavens has an evolutionary tale to tell. Objects ranging from grain-sized meteoroids to massive galaxy clusters have their stages of existenance. The birth, life, and death of stars present their stories as well. Let us skip the first two chapters of a star's life and examine the mysteries of stellar death. Nothing can be more picturesque or beautiful as a snapshot of a dying star. However, one thing is clear, not all stars meet fate in the same fashion. The key element that determines how a star will die lies in its mass. Stars about eight times heavier than our Sun explode in a catastrophic event known as a supernova. The smallest of stars, the red dwarfs disappear gradually with a whimper. And stars similar to our Sun die in a "middle-of-the-road" fashion. They undergo a series of smaller explosions, puffing off layers of their material into space. In the aftermath, a small degenerate white dwarf is left to hang in space. Only the most massive stars will yield a compact object such as a neutron star or a black hole after they go ka-boom. Visually through our telescopes observers see them as colourful, star-like, planetaries. The majority of planetary nebulae reside along the galactic plane. In fact four globulars are known to harbour these enigmatic objects. With that said, it is logical to assume that planetaries are found in all galaxies.
In 1784, English astronomer, William Herschel coined the term 'planetary” when he likened these stellar corpses to his newly-found planet, Uranus. Since then the name stuck. Astronomers love exploring these ghostly apparitions. They are a popular object for study. As more and more images of these nebulae were beamed down to Earth from the Hubble space telescope, the intensi for curiousity became more apparent.
Only decades ago it was a popular myth that all stars in the heavens went supernova. Tools like the Hubble proved this was not the case. Since the mid-1990fs the exploration of planetary nebulae went into full throttle. We know that there are hundreds out there waiting to be discovered. According to my latest information, about 1,500 have been snared. Stellar astronomers estimate that as many as 10,000 are strewn throughout the galaxy all in various stages of development. Some we will probably never see due to intervening gas and dust. Size is also a problem. Since some are so tiny and distant (in arc-seconds) it is often a challenge to find one, let alone see it.
It is much easier to spot a full-grown planetary when the envelope of material expands enough to a size we can detect. The hot central star must shed its outer layers and begin ionizing it. Astronomers dissect these objects into several wavelengths. But there are aspects about them that are poorly understood. Looking at them in optical light, we note that they come in three basic forms: round, elliptical, and butterfly shaped. It is in the final "helium flash" that a planetaries' shape is determined. The dying phase of a sun-like star begins at the end of a distended red-giant's life. Unfortunately, for Earth, our Sun will meet a similar fate. In a nutshell, these stars are beginning to exhaust their supply of fuel necessary for the continuation of thermonuclear reactions and then cast off shells of gas we see in those colourful Hubble snapshots. We must note that the star slips off the main-sequence branch and enters the realm of the red-giants.
During the main-sequence phase of its life, a sun-like star is burning hydrogen into helium. As the star begins to age, the helium is converted into carbon and oxygen. It is at this point that the fusion comes to a halt. If the mass of the core is less than 1.4 solar masses, the collapse abruptly stops and a white dwarf is born. All nuclear burning stops at the star’s core.
Over a period of several thousand years the ill fated red-giant begins to belch out material and pulsates like a Mira-type variable. The star has now become unstable. A large part of a star's mass is ejected into space in an expanding shell. These puffs of stellar debris provide evidence that the star is aging and facing a mortal chapter in its life. To understand the fate of stars like the Sun, astronomers must look back in time several hundred years. Our natural galactic laboratory comprised of several hundreds of billions of stars provides us with specimens in different stages of evolution. Having many samples to work with is a big plus for astronomers.
After the star ejects it's outer layers, it wraps itself in a dusty cocoon shrouding itself from optical light. If a young planetary begins to shine, it is only because newly formed dust particles reflect light in our direction. It will take some time before the dust can expand sufficiently to create pathways for light to reach our eyes. Meanwhile as we divert our attention to the stars core, the fusion process has converted it's heart into a lump of carbon about the size of our planet. In stars that are destined to become planetary nebulae, carbon is the ultimate fusion byproduct. There simply is not enough mass to continue fusing elements into calcium, silicon, and iron.
Since heat cannot be replenished, the tiny star begins to cool like an ember in a fireplace. Thermonuclear fusion continues to take place in the outside layers. The forces of gravity persist by squeezing hydrogen and helium, but only for a brief period. Carbon that is found in the star's atmosphere begins to settle and as it does, adds mass to the already inert carbon core. What fuel remains lies too far from the core and there is not enough mass above it to compress it to its fusion point. And then the drama begins. The final spasmodic fits of helium cooking and whatever is left of it, flings these outer layers into the interstellar medium. Each hiccup creates a new bubble of gas having an escape velocity matching that of some of our space-probes, or about 36,000 mph.
The final helium flash will determine the nebula’s artistic shape. This event will form the dust lane and luminous lobes found along the major axis. A small percentage of this new dust will mingle with the interstellar medium and in the process will begin to reflect light from the dying star. Areas close to the budding planetary will become enriched with carbon-and silicate-rich particles. The scaffolding for living organisms, those carbon-based molecules, will also compliment the interstellar medium.
Time spans for those nebulous glories are relatively short, about 10,000 years, so astronomers must capitalize on these celestial moments. It won't be too long before a planetary will expand and disperse into the vacuum of space.
We have collected many images of classes of these objects in various stages of development, but they are still shrouded in mystery. One conundrum relates to their anatomy or structure that has scientists scratching their heads. Why are the outflows asymmetrical and not round? Should not a round star produce circular debris of material? What is the mechanism that is responsible for the shapes we see in telescopes? Is it possible that the dying star has an unseen orbiting companion that will ultimately dictate the shape of the nebula? Magnetic outflows may be more complicated than we can envision. Perhaps we don't fully understand magnetohydrodynamic (MHD) models that apply to dying stars.
When astronomers examined these nebulae in ultraviolet light, they discovered gas flows that were impressively fast, much faster than their manuals had predicted. This "fast wind" slams into the slower wind that was ejected in earlier spasmodic episodes. In doing so, it clears the gas and incinerates whatever lies in its path. This creates a cavity between the star and the surrounding nebula.
|M57 – Image by Gary Boyle|
A classic visual example is the famous Smoke-Ring Nebula (M57) in Lyra. Astronomers suspect that Earthlings are gazing down the throat of a face-on cone in M57. From our vantage point, it only appears as a nebulous ring around a star. The collision process from the fast wind heats up the gas and as a consequence instruments on Earth begin to detect x-rays emanating from the nebula. Whatever gas is left is scooped up into a tenuous membrane. Beyond this rim we find the slow wind that has since decelerated from earlier episodes. Finally the gas bubble is breached as it reaches the outer edge of the slow wind. This cosmic stew then empties into the ISM. From this recipe a new generation of stars, planets, and people are created. The halos or bubble-like lobes we see in Hubble images are the end result of this process. The central star, usually dim or invisible in backyard scopes, eventually glows very hot and casts a bluish light. Intense heat from the star emits ultraviolet radiation stripping electrons from their atoms. At this time, observers begin to note the variety of hues and intricate shapes associated with these nebulae.
The challenge for amateur astronomers is to resolve some detail in these stellar corpses. Planetaries fall under two general categories, they are either bright but small, or large distended objects resembling ghostly apparitions. High power, good optics, and dark skies are a must to view these stellar denizens.
Imagine this scenario. Perhaps one day in the very distant future, we may have a ringside view of the fate that awaits our closest star, the Sun. For now, we can look into the eyepiece and marvel in the beauty of a star's death.