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Early 2017, Evening Sky

Topic: Planets
Article Published - February 13, 2017
by Dr. Jim Lattis, Director, UW Space Place
Early Night Sky 2017 in Wisconsin

Our evening sky in the early part of the year is dominated by the familiar constellation, Orion, which is made up of bright stars and interesting objects. A brief examination reveals a fine tableau of star formation and evolution if you know what to look for.

One of the most interesting objects in the sky is the Orion Nebula, which is visible even to the naked eye in the "sword" hanging from Orion's belt. See above. Look into the southwestern sky about 8:30 pm CST (or 9:30 CDT after we go on "summer" time on 12 March) to see a chain of bright stars from left to right: Sirius, the brightest in the sky (but not as bright as Venus!), the three stars of Orion's Belt, and the bright reddish star Aldebaran. Look a bit farther to the right for the distinct cluster of stars called officially the Pleiades, but also commonly known as the Seven Sisters. Get out your binoculars to take a good look at the Pleiades and see that there are many more sister stars there than seven. Binoculars will reveal dozens, and telescopes show hundreds of stars in what astronomers call an "open cluster." All of those stars formed about the same time and relatively recently: approximately 100 million years ago, so our Sun is 50 times older than the Pleiades. These stars all lie about the same distance from us, about 450 light-years.

Now look back to Aldebaran and notice that the stars nearby it on the right also cluster, although more loosely than the Pleiades. These are the Hyades, another open cluster, but quite a bit closer than the Pleiades, only about 150 light-years away. (Aldebaran is not a member of the Hyades but is actually in the foreground, only about 60 light-years away from us.) Like the Pleiades, the Hyades stars formed at about the same time, which is estimated to be more than 600 million years ago, so closer to one-tenth the age of the Sun. More massive stars age and explode (as supernovas) faster than their less massive siblings, which is why the Pleiades has more bright, bluish, massive stars than the Hyades. Astronomers think that most stars, including our own Sun, form in groups, like open clusters, which then disperse into the more general populations of stars in our galaxy.

Where do such open clusters form? Look back to the Orion Nebula for a fine example. Your binoculars will show it to be a diffuse region with some very bright, young stars. The nebula is about 1,400 light-years from us, so appears more compact than the Pleiades or Hyades, and it has more young, massive, bright stars, some only a few million years old. And there are many more still forming, hidden deep inside the nebula but evident to astronomers who can use infrared light and millimeter-wavelength radiation to penetrate the optical fog.

The most prominent feature of the evening sky of early 2017 is the planet Venus, easily visible in the west after, and even before, sunset. It is now near maximum brightness, and a small telescope will reveal something interesting that the unaided eye misses. As Galileo discovered in about 1611 with his primitive telescope, Venus exhibits phases similar to those familiar phases of the Moon. In February and March this year, as Venus drops lower in the evening sky toward the sunset, we'll see it transition from something less than a "quarter" phase into a thin crescent. But because it is approaching us, and will eventually overtake Earth, the disk of the planet (or what we can see of it) will increase in size, so the crescent will become wider as it also becomes thinner. The diagram shows how the phases will progress in the very near future.

Venus Early 2017 in Wisconsin

From the University of Wisconsin's "Space Place"
Villager Mall, 2300 S Park Street, Madison

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