Star birth in the southern sky
Our sun is not a first generation star. We know this because of the heavy elements we find in it … including oxygen, iron, and uranium … that had to be made in the lives and deaths of previous generations of stars. So star birth, as well as star death is an ongoing process in the universe. One of the closest and largest complexes of star birth is a familiar friend hanging high in our southern sky during the frigid nights of winter: Orion.
Figure 1 shows the southern sky seen from Tupper Lake tonight at 7 p.m. The Winter Hexagon dominates with bright Sirius marking the bottom corner and Capella, the top, near the zenith (the point directly overhead). The Milky Way’s equator arcs vertically through it as we look outward from the galactic center along the plane of the disk. Thus this region is rich with stars in all stages of their lives, star clusters and nebulae. It’s also the location of the Summer Solstice, the northernmost position of the sun’s annual trek across the sky.
The inset image shows great molecular clouds (primarily H2) in which the stars of Orion are immersed. Some of the stars outlining our familiar Hunter of the winter, were likely formed from these clouds. At the top, the Meissa (MAY-suh) Ring has a diameter of about 130 light years and is home to many forming stars and clusters of young stars. Just below the bright star Alnitak (ALL-nih-tak), is the lovely Horsehead Nebula where a dark cloud of dense gas and dust is illuminated by hydrogen gas behind. The gas glows hydrogen’s characteristic red due to being heated by the star just to the nebula’s left. To Alnitak’s left (east), Barnard’s Loop seems to surround the entire Orion complex and is estimated to be 100 — 300 light years across. It’s very faint, but can be observed with the unaided eye in very dark skies such as we have in the Adirondacks. The process that formed Barnard’s Look has not yet been determined. The Witch Head Nebula right (west) of the bright star Rigel (RYE-gel), is more than half way between us and the Orion complex. It’s thought to be part of a supernova remnant and may be actually reside near an outer boundary of the complex. The blue glow is due to dust in the nebula reflecting blue light from Rigel more efficiently than red light. It’s blue for the same reason our sky appears blue as dust and gas in our atmosphere scatter the blue light from the Sun while the red light streams along the sunbeam.
The entire Orion complex is a star-formation region. But how do stars form? Emanuel Swedenborg first proposed “The Nebular Hypothesis” in 1734 that the Solar System formed from nebulous material. This was developed by Immanuel Kant who published the idea in 1755. Pierre-Simon Laplace independently proposed a similar model in 1796. These ideas were met with interest and criticism, leading to newer models of gas condensation, planetesimal accretion and planetary capture during the 20th century. Each successive model solved problems apparent in early models and introduced new problems. However, in 1969, Soviet astronomer Victor Safronov put the better parts of all of them together in his solar nebular disk model (SNDM) that forms the basis of our current understanding.
The Hubble Space Telescope gave us the first confirmations of the theory beyond the existence of our own Solar System in images of the Orion Nebula. Starting in 1993, C. Robert O’Dell of Rice University and colleagues from other institutions, obtained images of proto-planetary disks (proplyds) in the Orion nebula in a variety of formation stages. Figure 2 shows the inner part of the Orion Nebula with images of a few of the forming solar systems.
Some of them, like the topmost inset, are seen edge-on with the inner forming star hidden by a thick disk of gas and dust that may result in planets like those in our Solar System. Others, none shown in this mosaic, are face-on disks with the forming star bright in the center of a dark disk of dust and gas. Some of the young stars illuminate the cocoon of gas and dust from which they have formed, like the bottom inset image. Yet others look more like comets with long tails than forming stars and solar systems. These may actually not survive to become star as their surrounding gas is being illuminated by a hot nearby star. These stars produce high energy radiation in the ultraviolet capable of evaporating the gas away from the forming star. Where the light and charged particles (stellar wind) from the hot star impact the star-forming cocoon, a glowing shock wave forms such as those seen in the bottom five inset images. This shock wave and penetrating light heat the gas within the cocoon making it glow brightly as seen in the bottom inset image and stream away from the forming star in a tail such as that seen in the rightmost inset image. Each of these seemingly tiny systems is the size of our Solar System and teaches us of the processes that led to its formation five billion years ago.
Proplyds have been found in other star-forming nebulae such as the Lagoon and Trifid Nebulas in Sagittarius. The proximity of the Orion Nebula, a mere 1,500 light-years away, affords us a much better view of this region than other nebulae.
Not all star formation takes place in such huge complexes, though. The Rosette Nebula, between Orion and Canis Minor, is a more isolated star-forming cloud with a star cluster, NGC 2244 in the center. The young stars have cleared the gas out of the center of the nebula by their stellar winds, giving the nebula an appearance much like a flower in high-resolution images. A quick Google search on Rosette Nebula will yield numerous images of this lovely cosmic wonder.
The volunteer astronomers at the Adirondack Public Observatory are eager to share telescopic views of the Orion, Horsehead, Rosette and other nebulae. The Roll Off Roof Observatory (RORO) is open to the public on the first and third Fridays of each month approximately one half-hour after sunset. Whether you’re an avid amateur astronomer or have never visited an observatory, come and view through our telescopes and learn about the Wilderness Above. For updates and notices, check out our website at adirondackpublicobservatory.org and our Facebook page. On our public observing days you can also call the RORO at 518-359-6317 to talk with one of our astronomers.