Michelle Greear, age 12, of San Bruno, California, for her question:
Which is the largest nebula?
A nebula is a cosmic cloud out there in space and there are countless nebulae strewn through the heavens. If you know where to look, you can spot at least one bright one and several dark ones. Early telescopes revealed more of them, no two alike. Later, better telescopes revealed many more in finer detail. Astronomers classify them according to size and type.
In winter, you can see a hazy nebula near the middle star in Orion's sword. The constellation Scorpius reigns in the summer sky and near its heart, the super giant star Antares illumines a bright nebula of cosmic gases. Nearby, you can spot dark nebulae, like ragged holes torn in the glimmering Milky Way. These nebulae are vast cosmic clouds of gases and dust. The bright ones are lit up by stars, often from inside. The dark ones are not. The group of planetary nebulae is smaller, with a central star inside a ring or halo of glowing gases. Astronomers know that at least some of them are old nova or supernova stars that exploded most of their gases in a blaze of glory.
Countless huge, cloudy nebula are strewn among the 100 billion stars of our galaxy and some 370 smaller planetary nebulae have been spotted. However, our view from the earth includes merely a pie slice section of the pin wheeling galaxy. Objects on the far side are out of range. The great nebula in Orion is the largest in our skies. We see it from 1,600 light years away. In telescope pictures it looks like a bright, gauzy butterfly a celestial butterfly with a wing span of 25 light years.
Our galaxy measures 100,000 light years from side to side. And beyond it, the oceans of space are strewn with more galaxies. So far 31,350 have been counted. Sometimes all these outer galaxies are classed as nebulae. The two nearest ones shine big and bright above the southern hemisphere.
These are the Large and Small Clouds of Magellan. Scientists down in Chile are surveying them right now, especially the enormous bright Tarantual Nebula in the Large Cloud. It dwarfs the Great Nebula in Orion. Recently it was announced that the Tarantula is the largest nebula of its kind within viewing range from the earth. It is a bright, fuzzy ball with wispy edges and its width is 800 light years. It out¬shines 100,000 suns and there is enough material in its thinly strewn gases to make half a million suns. This new champion is 32 times wider and 5,000 times more massive than the Great Nebula in Orion. At its distance of 150,000 light years, it is also some 90 times farther away. Let's suppose the Tarantula Nebula replaced the Great Nebula in Orion. It would shine brighter than Venus at its brightest and light up a patch of sky 60 times wider than the full moon.
The immense Tarantula Cloud is illumined from inside by clusters of huge, brilliant stars. They are hot, young stars blue white giants and red supergiants. Astronomers suspect that they were made from materials in the great gaseous cloud. Perhaps the Tarantula Nebula is a sort of cosmic cradle where new stars are born. However, its blazing young stars are burning fast and are not expected to last a trillion years.