This is a perfect week to grab the binoculars and check out Orion’s (the Hunter) majestic sword, one of winter’s true astronomical treats. The sword hangs well below Orion’s distinctive three-starred belt. Unaided eyes can generally detect three patches of light in Orion’s sword, but binoculars or a telescope quickly reveal that the middle object is not a star at all, but a bright knot of glowing gas and stars known as The Orion Nebula (or the Great Nebula in Orion or Messier 42, aka M42).
The Orion Nebula is one of the brightest nebulae in the entire night sky and, at 1,400 light-years from Earth, it is one of the closest star-forming nurseries to us. It’s enormous. Under a very dark sky, the nebula can be traced over an area equivalent to four full moons!
Buried in the core of the nebula is a tight clump of stars collectively designated Theta Orionis (Orionis is Latin for “of Orion”), but better known as The Trapezium, because the brightest four stars occupy the corners of a trapezoid shape. Even a small telescope should be able to pick out this four-star asterism, but good seeing conditions and a larger aperture scope will show another two faint stars. The trapezium stars are hot young O- and B-type stars that are emitting intense amounts of ultraviolet radiation. The radiation causes the gas they are embedded in to shine brightly, by both reflecting off gas and dust as blue light and also by energizing Hydrogen gas, which is re-emitted as red light. That is why there is so much purple in colour images of the nebula.
Within the nebula, astronomers have also detected many young (about 100,000 years old) concentrations of collapsing gas called proplyds that should one day form future solar systems. These objects give us a glimpse into how our sun and planets formed.
Stargazers have long known about the stars in the nebula’s core, but detection of the nebulosity around them required the invention of telescopes in the early 1600’s. In the 1700’s, Charles Messier and Edmund Halley (both famous comet observers) noted the object in their growing catalogues of “fuzzy” objects. In 1880, Henry Draper imaged it through an 11-inch refractor telescope, making it the first deep sky object to be photographed.
In your own small telescope, you should see the bright clump of Trapezium stars surrounded by a ghostly grey shroud, complete with bright veils and dark gaps. More photons would need to be delivered to your eye before colour would be observed, so try photographing it through your telescope or with a camera/telephoto lens on a tripod. Visually, start with low magnification and enjoy the extent of the cloud before zooming in on the tight asterism. Can you see four stars, or more? Just to the upper left of M42, you’ll find M43, a separate lobe of the nebula. It surrounds the unaided-eye star nu Orionis (ν Ori).
While you’re touring the sword, look just below the nebula for a loose group of stars, 1300 light-years away, called Nair al Saif “the Bright One of the Sword”. This main star is a hot, bright star expected to explode in a supernova one day. It is surrounded by faint nebulosity, too. Astronomers believe that this star was gravitationally kicked out of the Trapezium cluster about 2.5 million years ago.
Sweeping down the sword and to the left (east) brings us to the star named Mizan Batil ath Thaalith (and d Orionis) at the tip of the sword . This magnitude 4.7 star is near the limit for visibility in moonless suburban skies. About two finger widths to its right is another star of similar brightness, named Thabit, “the endurer”.
Moving upwards towards Orion’s belt, half a finger width (30 arc-minutes, or the moon’s diameter) above the Orion Nebula, you’ll find another clump of stars dominated by c Orionis and 45 Orionis. A larger telescope, or a long-exposure photograph, reveals a bluish patch of nebulosity around them that contains darker lanes forming the shape of a figure, called the Running Man Nebula. This is another case of gas reflecting light from the two stars mentioned.
Just above the Running Man sits a loose cluster of a few dozen stars best seen in binoculars. Then we jump higher — most of the way towards Alnitak (the eastern-most belt star), to check out a beautiful little grouping of stars collectively called Sigma (σ) Orionis. What makes this a special treat is that, in a small telescope, we find four or five stars crammed together. Check it out with your telescope — trust me, it’s pretty! It’s a bit more than a finger width to the lower right of Alnitak. Let me know what you see!
Valentine’s Day Diamonds
A beautiful diamond sparkles in the southern sky during winter evenings. It’s Sirius, the brightest star in the constellation of Canis Major (the Big Dog). The constellation is located to the lower left (southeast) of Orion (the Hunter). In fact, his three-starred belt points directly towards Sirius! Another dog, Canis Minor (the Little Dog) sits three fist diameters to the left of Orion. It, too, has a bright star, named Procyon. The two dogs are assisting Orion in his hunt. (There is actually a dim constellation called Monoceros (the Unicorn) sitting between the two dogs, but its faint stars are barely visible from the city.)
The big dog sits lower in the sky and is oriented so that he looks like he’s standing up on his hind legs, begging Orion for a treat. Sirius marks the location of his throat — perhaps it’s a diamond dogtag! Sirius is the brightest star in the heavens (other than our sun) and its name means “scorching” in Greek. Some people call Sirius the “Dog Star”, and J. K. Rowling used its name for Harry Potter’s uncle, the animagus. Its brilliance is mostly due to its proximity to us. At only 8.7 light-years away, it is among the closest stars to Earth.
Sirius is famous for dramatic flashes of sparkling colour. Visible with unaided eyes, the colours are enhanced in binoculars or a telescope. They come from an excessive amount of twinkling due to the star’s low elevation in the sky combined with its brightness. The light from stars lower in the sky is passing through more of our atmosphere to reach us. Sirius itself is fairly ordinary. It is a few times the mass of our Sun and has a surface temperature of about 9,900° giving it a bluish colour. It is also moving towards us at about 27,000 km per hour. Fast, but don’t stand around waiting for it to arrive!
A very good telescope and a steady atmosphere can reveal Sirius’ tiny white dwarf companion star, officially named Sirius B. Astronomers have nick-named it “The Pup”, but I prefer “The Flea”. The Flea may be small but it’s mighty! Its temperature is about 25,000°!
(Above: A Hubble Space Telescope image of Sirius. Its tiny companion star Sirius B, aka The Pup, is the tiny dot at lower left.)
Procyon is another neighbour — only 11.5 light-years away from Earth — making it the eighth brightest star in the heavens. Its name comes from the Greek προκύον (prokyon), meaning “before the dog”, because it rises before Sirius. It’s comparable in size to Sirius, but is slightly warmer in colour.
Take your Valentine outside on the next clear evening and show them these celestial diamonds!
The Moon and Planets
The moon spends this week largely absent from the evening sky. On Monday and Tuesday, you might glimpse its slim old crescent low in the western sky just before sunrise. Mid-week, the moon disappears beside the sun — reaching its new moon phase on Thursday afternoon. This new moon will also generate a partial solar eclipse that is only visible from the daylit portions of Antarctica (about 2/3 of the continent) and southern South America.
On Friday, the slim young crescent moon will return to the evening sky, visible low over the western horizon for a short time after sunset. On the weekend the moon’s “Cheshire Cat’s smile” will hang prettily in the western early evening sky.
Towards the end of this week, you can start looking for bright Venus very low over the western horizon for half an hour after sunset. The best chance to see it falls between about 6 and 6:30 pm local time. On Friday, the very young crescent moon will sit only two finger widths to the upper left of our sister planet. Venus is just beginning a long stay in the evening sky that will kick into high gear this spring.
The rest of the bright planets are still hanging out in the eastern pre-dawn sky. Extremely bright Jupiter rises first, shortly after 1 am local time, so it reaches an elevation of about three fist diameters above the southern horizon by sunrise. You can’t miss it. Dimmer, reddish Mars is sitting about two fist diameters to the lower left of Jupiter this week, but it’s slowly moving downwards towards Saturn. Those two planets rise about 2:45 am and 4:30 am local time respectively.
Asteroid/minor planet fans can hunt down Vesta and the dwarf planet Ceres this week. Vesta is located only 5.5° (a palm’s width) to the upper left of Mars. Brighter Ceres is in Cancer (the Crab) in the eastern evening sky. Since the stars of Cancer are so dim, look for Ceres roughly midway between the bright stars Algenubi, which is the nose of Leo (the Lion) and Pollux, the lower of the twin stars in Gemini. Ceres remains visible all night long. You’ll need binoculars or small telescopes to see these two objects.
Astronomy Skylights for this week (from February 11th, 2018) by Chris Vaughan. Keep looking up to enjoy the sky! I love getting questions so, if you have any, send me a note.