Of Hunters and Babies

By Irwin Horowitz, 2-01-08

	Caption: (c) European Southern Observatory

This month, I want to focus on one of the most awe-inspiring objects in the night sky, the Orion Nebula. 

When one looks at the constellation of Orion, which dominates the night sky at this time of year, the three belt stars (Alnitak, Alnilam and Mintaka) bisect his midsection in a straight line.  Above them lies Betelgeuse, a red supergiant star that is so vast that if it were placed in the center of our solar system its outer fringes would extend beyond the orbit of Mars.  Below and to the right of the belt lies brilliant Rigel, a hot blue supergiant star with a surface temperature of 11000 K, about twice as hot as our Sun.

The nebula is located within the sword that dangles under the belt of Orion.  From a dark sky site, it is readily visible to the unaided eye as a faint smudge.  Charles Messier, the 18th century French comet hunter, listed it as the 42nd object in his famous catalog (M42).  Viewing the object through a telescope barely provides the merest glimpse of its stunning beauty, as our eyes are too weak to capture the rich colors seen in long exposure images such as this fantastic mosaic acquired by the Hubble Space Telescope.

At the heart of this nebula lies a cluster dominated by four stars.  Because they are observed in the shape of a trapezoid, the cluster is known as the “Trapezium.” These stars have masses between 15x and 30x the mass of our Sun.  They are hot, blue stars that emit copious quantities of ultraviolet light.  It is the light from these stars which causes the nebula to glow as it ionizes the gas.

What is the Orion Nebula?  Over the years of studying this object, astronomers have determined that we are privileged to have a front row view of a stellar nursery.  We are literally witnessing the creation of brand new solar systems as we behold this wonder.  Some of the stars are estimated to be less than a few hundred thousand years old!  In stellar terms, that is barely past the point of having the doctor slapping a newborn on its bottom and getting its first breath of air.

The process of star formation begins with a massive cloud of very cold gas and dust.  These clouds, known as Giant Molecular Clouds (GMCs), typically contain tens or hundreds of thousands of solar masses of material.  The temperature of this material is only a few degrees above absolute zero.

Shock waves propagating through the space between stars (typically formed via supernovae explosions) push this material to the point that its own self gravity causes it to collapse and form a substantial number of proto-stellar disks.  Each disk forms because of the need to conserve angular momentum.  Over thousands of years, material at the heart of these disks coalesce to form proto-stars, and as more material is accreted onto these objects, they eventually become hot enough and dense enough for hydrogen fusion reactions to commence in their core.  This moment represents the birth of a new star.

The remaining material in the disk will continue to orbit this new star and collide to form planets.  This basic model is believed to explain the origin of our own Sun and planetary system 4.5 billion years ago.

M42 is about a dozen light years in radius.  Within that volume lie thousands of these newborn suns.  The nebula is actually part of a much larger formation known as the Orion Molecular Cloud Complex.  This cloud contains several other well known astronomical objects, including another piece of the Orion Nebula (M43), the Flame Nebula, a reflection nebula (M78) and the Horsehead Nebula.  Each of these objects is formed as a result of the interaction between the radiation emitted by the hot, massive newborn stars and the gas and dust that permeates this region of our galaxy.

While much is well understood about this object, there is still new information coming forth.  Just recently, astronomers have revised their estimated distance to the nebula from about 1500 light years to a much more precise value of 1350 light years, about 10% closer.  The result of this modification is to increase the estimated ages of the stars we observe. 

How this works is as follows: stars evolve based primarily on their initial mass.  Even though more massive stars have more hydrogen fuel available for fusion, they burn through it at a much faster pace.  Therefore, we can estimate the ages of objects like M42 by observing the masses of stars that are just finishing the core hydrogen fusing part of their lives.  After this phase, they evolve into giant and supergiant stars like Betelgeuse.  For more massive, brighter stars, this results in a younger age for the object.

Since the nebula is closer than previously thought, our estimate of the absolute brightness of the stars we see there has decreased.  The stars that are starting to evolve into giant stars are not as big and as bright as we believed, and therefore must be older than previously estimated.

This month, we will have the opportunity to view another total lunar eclipse.  Such an event was visible here for those who woke up in the early hours of the morning of August 28th.  This time, the eclipse will occur at a much more amenable time of the evening.  On the night of February 20th, the full Moon will rise around 7 p.m. MST (6 p.m. PST), shortly after the start of the partial phase of the eclipse.  Totality will commence at 8 p.m. MST and will continue until about 8:50 p.m., when the Moon begins to come out of eclipse. 

While the weather in February is much less predictable than in August, if we should have clear skies this will be an excellent opportunity to view one of these celestial events.  As an added bonus, the planet Saturn will be located only a few degrees below and to the left of the eclipsed Moon.  The bright star seen above is Regulus, the heart of Leo, the Lion.

As we note that Saturn is close to the full moon during this month’s eclipse, it means that the planet is in opposition on February 24th.  It rises around sunset and sets at sunrise and is visible all night long following Regulus across the sky.  Mars is also very prominent in Gemini, the Twins.  The remaining naked eye planets (Mercury, Venus and Jupiter) are all presently too close to the Sun to be seen at night.

The Boise Astronomical Society will hold their next regular membership meeting on Friday, February 8th at 7 p.m. in Classroom #2 of the Discovery Center of Idaho.  Our special guest this month is KIVI Channel 6 chief meteorologist Scott Dorval.  He will be telling our members about “Cloud forecasting under the cover of darkness.”