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01. Why do the stars in Orion look so different from each other?

Looking at Orion is more than just looking at an area that is easy to recognize in the night sky. Orion is seething with activity and illustrates a clear and concise picture, of how stars are formed. It gives us the ability to compare different types of stars and most importantly, it’s right next door to Earth, astronomically speaking.

The interest in Orion is currently at frenzy level, astronomers have always been interested in Orion because it is only 450 parsecs (1500 light years) from Earth. As viewed from ground based telescopes, Orion has twice the angular diameter of the full moon, around 1 degree. Known as the saucepan, Orion has a most distinctive and easy to find star pattern, located in the same spiral arm of our Galaxy as the Sun. Orion is named after the Hunter of Greek mythology.

If what we can see of Orion is considered exciting, that pales under the stark reality of what lies in the same region, that we can not see. To understand more about the differences in Orion, you must establish that there are differences between two sets of stars, the visible and the non-visible.

The image above shows the distinctive blue/white colour of Rigel and the cool red supergiant Betelgeuse. At the center of the nebula is a cluster of four stars called the Trapezium. The brightest star in the Trapezium, known as Theta 1 Orionis C, is a very hot 39,000 Kelvin, and is the source of most of the UV radiation, which causes the nebula to glow. Below left, shows the four stars glowing brilliantly at the bottom left edge of the photo.

The infrared vision of the Hubble Space Telescope’s Near Infrared Camera and Multi-Object Spectrometer (NICMOS) is illustrating an Orion that few people outside of astronomers ever see.

Thanks to Hubble more people are being introduced to astronomy with such magnificent images as these on the left. Stars in Orion looking different now takes on a whole new meaning.

Initially looking at the Orion Nebula in the top image, is just like looking at a single point of light against a dark background. Only under the magnification of a telescope can you see that point, is actually a mass of gaseous cloud, which contains four very bright prominent stars, named the Trapezium.

The next visual difference comes from the Hubble Space Telescope (HST) orbiting high above the Earth’s distorting atmosphere, where a totally different picture emerges (see image on the left, above). This shows Orion in a previously unimaginable view, with clearly differing nebulosity. If that was not spectacular enough, along comes Hubbles NICMOS (Near Infrared Camera and Multi-Object Spectrometer). This now lets us ’see’ with the aid of technology where our human eyes can not.

The universe now opens up to close scrutiny and we move a lot closer to understanding the violent patterns of starbirth. We get to see images that our forebear’s could not have imagined. We can be privy to this fascinating stellar maternity ward, watching the contractions and birth of the stars. Quite a few of which are very similar to our own Sun. By observing the differences and similarities of Orion we can better understand the evolutionary process of our own Earth and the role of our species, in it.

From the visible perspective, the most obvious things an observer of Orion can tell are the relative size, position and colour of the stars. Studying the colour of stars allows us to determining its temperature, which in turn is related to it’s mass.

Temperature determines a star’s colour. Red stars are cooler, around 3,000 kelvins (K), while blue stars are hotter and can have temperatures over 30,000 K. Our own yellow Sun’s temperature is around 5,800 K. (A typical star).

Differences in star colours are shown in the photo on the left of the constellation Orion. Made using a “star trail step-focus” technique. This is a time exposure method used to create star trails in altered steps.

At the bottom right, the large cool red supergiant Betelgeuse stands out from the other, hotter, bluish stars composing the body of the Orion constellation. Bright Rigel, a massive blue star, is at the upper left.

In the center of the handle of the saucepan is a bright star, which is actually the Orion Nebula . In it, is a quartet of hot stars, which are known as the Trapezium, from their trapezoidal shape.

The hottest of the Trapezium stars has a temperature of 30,000 Kelvin, and a luminosity of 300,000 times that of our sun and a mass 40 times greater than the mass of our sun.

So in essence as viewed from earth with the naked eye, the middle star in the handle of the saucepan appears as one white star, it is smaller in size than both the bluish Rigel to the left and the reddish Betelgeuse on the right of the image above. The dark background of space also leads the human eye to see these most luminous stars as very bright, even though the stars are actually at different distances to us, and are different sizes and temperatures. We see the Orion constellation as a flat two-dimensional image, when in fact it really is a multidimensional portion of the universe, aligned conveniently for us to view.

Seeing is precisely what twentieth century man has done. With the aid of technology, we have gone beyond the visible into the realm of the invisible and unlocked the very nature of the construction of the universe. We are now looking at the building blocks of creation itself, and there is no better place for earthlings to begin, than in Orion.

Star formation is currently propagating through the general vicinity of the Orion Nebula. Stars formed in Orion’s belt 8 million years ago and in the Trapezium 2 million years ago. Now protostars (young stars) are forming in the Kleinman-Low Nebula and in the future, dense cores beyond the K-L Nebula will collapse. We are really taking a look back in time as we study the different stars in the Orion constellation.

Star formation reproduces via a chain reaction. In a simplified cycle, young hot stars blow gas outward, then expanding gas compresses molecular cloud dense cores causing these cores to collapse to form new young hot stars. This cycle repeats right throughout the molecular cloud that constitutes the stellar nursery. All this is invisible to human eyes.

Assistance from the cameras on board the HST however permits us to view the invisible spectacle of starbirth, for example look at this Side-by-side Optical (left) and Near-IR (right) views of the central core of the Orion Nebula. It is only through the use of modern technology, supercomputers, space travel and mans continuing thirst for knowledge that we can now witness starbirth first hand.

Stellar siblings are not all the same, the stars of a star cluster, like those found in Orion vary due to the mass of the cloud. The composition of the gas and dust, the amount of turbulence, and the disturbances from nearby bright stars are all factors. Some stars are smaller than our Sun, a few are much bigger and more massive. Our Sun is considered to be a typical star. Stars are born single and many are born as members of double, triple, or quadruple star systems. Orion lets us look across the whole range of differing stars in the one location.

The brightest object in the image to the left is a massive star called BN (Becklin-Neugebauer). In order to see through the dust, you must look at near-infrared wavelengths (about 1500 nanometers).

The Kleinmann-Low Nebula (or KL) is a cluster of stars, presumably even younger than the Trapezium stars, which is still embedded so deeply in the gas and dust out of which it formed that it is only visible at infrared and radio wavelengths. In fact, these objects are so young that they may be still evolving to the point of becoming true hydrogen burning stars. (Main-sequence stars).

As we go deeper and deeper into the Orion constellation, uncovering different nebula, unmasking new stars, watching others die, the differences become staggering. The sheer volume of numbers is overwhelming, yet we have to this point traveled remarkably well. The next part of mans journey via radio waves and other media makes the future unbelievably exciting.

The stars in Orion do look different, and with each new day our knowledge and admiration grows. No doubt the stars of Orion and beyond will continue to challenge and absorb our minds for eons to come.


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