Star date: 02:01:99

The Hunters Mighty Nebula

The Great Nebula in Orion is a stellar birthplace. An area where stars are being born, to live out their lives after slowly separating from each other over the course of millions of years. One day, these stars too will grow old and die, releasing the materials to create some new nebula which will one day form its own stars. Many of these stars too will one day grow old and die, releasing their material, forming yet more stars. This is the process, performed over billions of years that give life to new stars.

All the things in the universe, other than hydrogen and helium (and a small amount of the lithium present in the universe today) were created from the lives and deaths of stars. Only hydrogen, helium and some lithium were created during the Big Bang. From this information, we can deduce that the oldest stars contain almost nothing except hydrogen and helium, while the younger stars are composed of significant quantities of heavier elements (hydrogen and helium being the two lightest elements).

The Great Nebula in Orion (also known as M42) is over 25 light years in diameter, and appears over a degree across as seen from the Earth. This is the same angular distance as two full Moons or two full Suns put side to side. The brightest stars in the Orion Nebula as seen from Earth are those found in a cluster near the middle of the nebula. This cluster is known as the Trapezium. They are young, hot stars, some of which are exciting the gas around them. The most massive of these stars has a mass about 40 times greater than that of our own Sun. This means that that star has a temperature in excess of 30,000 degrees c (over 60,000 f). This means that the energy output of this star is in the region of the ultraviolet. This star must also be very young, for this amount of energy output would require a tremendous fuel consumption, giving the star a very short life. Thus, we can conclude that the stars in the Trapezium did not reach the main sequence (the main part of their lives) more than a couple of million years ago.

Stars in other parts of the Orion Nebula, however, tend to show different ages. The ages in the other parts of the nebula range between 8 and 12 million years old. Compare this with our Sun, which is already 5 billion years old, and should continue to shine for another 5 billion years.

For being so beautiful and remarkable, the Orion Nebula is almost a vacuum. The nebula itself contains about 600 atoms per cubic centimeter, or about 9500 atoms per cubic inch (an area one inch by one inch by one inch). Most of interstellar space contains around 1 atom per cubic centimeter (16 atoms per cubic inch), and air at sea level which contains around 10,000,000,000,000,000,000 atoms per cubic centimeter.

Numbers this large (and small numbers as well) are often referred to by scientists using a method known as scientific notation. The number above (a 1 with 19 zeros after it) is referred to as "10 to the 19th power". The number of atoms in a cubic inch of air at sea level pressure is greater than 1.6 times 10 to the 20. This is the same as saying 160,000,000,000,000,000,000. An ounce of Silicon has 6 times ten to the 23rd (600,000,000,000,000,000,000,000) atoms.

This nebula (a wonderful sight through any good pair of binoculars or a telescope) can be seen as what appears to be the middle "star" in the sword of Orion. Look for it this week just ten degrees east of due south, and forty degrees above the horizon at 8 pm. The nebula, although faint to the naked eye, can be seen from any but the most light polluted skies. In fact, when Orion is visible (which is most of the winter) I will often check to see is a spot is suitable for observing by attempting to find the Orion Nebula with my unaided eye. If I can spot the nebula, the sky there is dark enough for observing.

Clear skies and good viewing.

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