Where stars are born
Glenn K. Roberts lives in Stratford, P.E.I., and has been an avid amateur astronomer since he was a small child. His column, Atlantic Skies, appears every two weeks.
Most people, when they think of space, probably think of it as an endless void, empty of anything but the stars and occasionally meteors, asteroids, and comets.
However, this area is anything but empty, although its individual components can be (and often are) separated by huge distances.
Referred to by astronomers as the interstellar medium, it is composed of vast quantities of microscopic dust particles and various gases that comprise our galaxy. Where the dust and gases collect in dense concentrations, they become stellar nurseries where stars, like our own sun are formed.
After a star has formed, vast amounts of this nursery medium often remain unused and simply float around in the neighbourhood of the nearby star. With the aid of telescopes, and sometimes with the naked eye and binoculars, we can see these patches of unused interstellar medium, or patches which are referred to as nebula (singular) or nebulae (plural).
There are three basic types of nebulae: emission nebulae, reflection nebulae, and dark nebulae. Though the nebulae visible to us here on Earth lie with our own Milky Way Galaxy, they are also found in areas where stars are forming within other distant galaxies.
Emission nebulae emit their own light. They are found near hot, luminous stars that emit huge amounts of ultraviolet radiation, which causes the nearby interstellar gas to become ionized. Through a process called recombination, they emit visible-light (often reddish in colour).
A reflection nebula does not generate light of its own, as it is an area where light from a nearby star is scattered and reflected by the dust particles in the surrounding interstellar medium. Because the surrounding medium has not undergone ionization, the colour of the reflected light often has a bluish tint.
Dark nebulae are areas where the concentration of dust particles is so dense it blocks any visible light. Before this was understood, astronomers thought such areas were actually holes in the Milky Way Galaxy, where no stars existed. The black, starless patches you see with your naked eye or binoculars in the Milky Way Galaxy are, in fact, dark nebulae.
Examples of all three nebulae types (emission, reflection and dark) can be seen in the
Orion Nebula, the middle star of the sword hanging from the middle star in Orion's belt in the constellation of Orion, the hunter. This nebula is visible in binoculars as a bright, fuzzy, concentration of light surrounding the bright star Alnitak. Though you will need a good-sized telescope to see the actual detail of the three nebulae types, photos of the Orion Nebula online or in astronomy books will show them in detail.
Probably the most famous example of a dark nebula, the Horsehead Nebula (it actually does resemble a horse's head), is in the Orion Nebula. Another excellent (and extremely beautiful) example of a reflection nebula is that surrounding the Pleiades (The Seven Sisters) open-star cluster visible between the constellations of Taurus, the bull and Aries, the ram. Like Orion, these two constellations are best viewed on winter evenings when they are high in the sky.
THIS WEEK'S SKY
• Mercury (magnitude -1.5) should be visible now in the early evening sky, low above the western horizon near the sunset point, about an hour after the sun has disappeared, with Venus (magnitude -4.3 ) visible to the upper left. This is the last month for Venus as an evening sky object, as it heads towards inferior conjunction with the sun on June 3, before emerging as our "morning star" later in the month.
• Jupiter (magnitude -2.3), Saturn (magnitude +0.5), and Mars (magnitude +0.3) remain pre-dawn objects in the southeastern sky. Jupiter and Saturn rise almost in tandem around midnight (halfway between sunset and sunrise), with Mars loitering abed for another two hours. All three planets fade from view with the light of the rising sun. Watch the waning moon slip beneath the three planets on the mornings of May 11-15.
Until next week, clear skies.