Unless you are an early bird, February 2018 has not been a good month for viewing planets. Mars, Jupiter, and Saturn are pre-dawn objects. Mercury passed behind the Sun on February 17th (superior conjunction) and was lost in the Sun’s glare. At month’s end, Mercury can be found in evening twilight if you know exactly where to look. The only bright planet in the evening sky as this February turns to March is Venus. Because Venus is quite bright, it will not be difficult to identity low in the western sky soon after sunset. Venus will be in our evening sky from March until late October.
The sixth planet from the Sun, Uranus, can be seen with the naked eye if you’re sharp-eyed and are away from city lights. Because Uranus was discovered with the help of a telescope, most people don’t realize that it is (just barely) a naked-eye planet. We’ll have more to say about Uranus, Neptune, and Pluto in our next blog. But for now, let’s focus on one of the most familiar constellations: Orion the Hunter. Here's a nice photo. (credits at 1)
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Using the map above (credits at 2), notice that the stars Betelgeuse (BEETLE-juice) and Bellatrix (BELLA-tricks) mark Orion’s shoulders. Then south of (that is, below) Bellatrix is Rigel (RYE-jell), marking one of Orion’s feet. The other foot is shown on the map as the Greek letter kappa, but its common name is Saiph (SAY-iff). In the color photograph, you can see that Betelgeuse is reddish and Rigel is white. This tells us that the surface of Betelgeuse is cooler than our Sun’s surface, while Rigel’s surface is hotter than the Sun’s. Of the four stars, Rigel is the brightest-looking, followed by Betelgeuse, Bellatrix, and Saiph.
But which of these four stars is the most powerful light source? Based on detailed studies using the spectra of tens of thousands of stars, astronomers have concluded that Rigel is the most powerful, while Bellatrix is the least powerful of these four. Rigel puts out as much light as 50,000 Suns, while Bellatrix shines with the light of 11,000 Suns.
How about the physical sizes of these stars? Which one is the biggest? It’s Betelgeuse – its diameter is about 800 times the diameter of the Sun! The diagram below (credits at 3) compares diameters of several stars, including the Sun, Rigel, and Betelgeuse. Here’s another way to picture the immense size of Betelgeuse: imagine replacing the Sun with Betelgeuse at the center of our planetary system. Four planets – Mercury, Venus, Earth, and Mars – would all lie under the surface of Betelgeuse!
Finally, how far away are these stars? They all lie between 250 and 900 light-years away. Betelgeuse is about 500 light-years away, which means that the light entering your eye when you look at Betelgeuse left the star about when Columbus was discovering the New World. The most distant of the four stars is Rigel, which is about 900 light-years away. The light you see from Rigel left that star a few decades after the Norman conquest of England!
But these stars aren’t the most distant naked-eye objects in Orion. Have a look at the three stars that form Orion’s belt, and now the stars that mark his dagger. The middle star of the dagger (which is labeled M42 on the map above) is not a single star. It’s called the Orion Nebula, and the name M42 stands for Messier #42, the 42nd object on a list constructed by the 18th-Century French comet hunter Charles Messier.
Comets are hazy-looking objects, but from week to week they move against the background of stars. Messier compiled his list of objects that could be mistaken for comets. His idea was to help other comet-hunters avoid being distracted by these permanent fixtures among the stars. But M42 and the other objects on Messier’s list have now been carefully studied. Some of them, including M42, consist of stars surrounded by gas and dust. Other Messier objects include entire galaxies of stars like our Milky Way.
The Orion Nebula is about one degree across on the sky (about twice the diameter of the Full Moon) – our eyes see only the brightest central part. The stars within M42 are relatively young – less than a million years old. In another million years or so, much of the glowing gas and some of the stars will disperse from the nebula.
Astronomers estimate the distance to the Orion Nebula to be about 1300 light-years, so the light we’re receiving from M42 left the nebula about 700 AD, early in the Middle Ages. The photographs below (taken by the Hubble Space Telescope) (credits at 4) reveal beautiful detail in that “middle star” of Orion’s dagger.
Our next post will focus on three much nearer objects: Uranus, Neptune, and Pluto. Their distances from Earth are measured in light-hours, not light-years. Until then, happy stargazing!