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  Night Sky

Enjoy the starry night skies
Be a considerate neighbor. Reduce nighttime glare. Shield your outside lights downward. Let the stars light up the night.

April 2014 night sky

—Charlie Christmann

Bigger than super

What is bigger than a “supergiant” star? Try “hypergiant!” Astronomers have just found a rare and unbelievably large one.

As we look in the night sky, we see many stars of varying size and brightness. As perceived from Earth, neither of these parameters matter; we cannot tell by looking how far away a star is, nor can we tell how large it is. To give some size comparisons, we use the size of our sun as a reference. The sun is 864,900 miles in diameter; a million Earths could fit inside it.

Sirius may be the brightest star in our sky, but it is far from the largest. It is very bright to begin with, shining 24.5 times brighter than the sun at a distance of 8.6 light-years—relatively close—and about 1.7 times bigger. That does not even make the supergiant category.

The most famous supergiant is Betelgeuse, the big orange shoulder star in Orion. It may not look as bright as Sirius, after all, it is further away at six hundred light-years away and 950 times the diameter of the sun. Betelgeuse is an old star and could go nova at any time. Just slightly smaller is Antares, the red heart of Scorpius. That star is about 470 light-years away and 880 times the size of the sun. If placed in our solar system, either of these stars outer edges would extend beyond Mars.

Even these supergiants are small compared to the hypergiants. Fortunately, none are very close to us, as they normally explode to create black holes when they exhaust their fuel. VY Canis Majoris, 1,400 times the size of the sun, sits 3,900 light-years away. At that distance and 270,000 times as bright as the sun, it is below naked eye brightness at a magnitude of 7. Recent observations seem to indicate the brightness has been fading since its discovery in the 1850s.

Theory tells us that stars should not be able to grow larger than 1,500 times the sun. Seems nature had other ideas. VX Sagittarii’s size varies over time, categorized as a hypergiant pulsating variable star. Though it is a bit irregular, it pulsates, expanding and contracting over a period of 732 days. At its largest, it is more than 1,500 times the size of the sun. But UY Scuti tops the list of the largest stars in the night sky, coming in at 1,700 times the sun. In our solar system, it would engulf Jupiter. It resides 7,800 light-years away and shines 340,000 times brighter than the sun. UY Scuti is thought to be an old dying star that left the main sequence a few million years ago. Its exact future is unclear but will almost certainly involve a supernova explosion.

A recent discovery, at 1,300 times the size of the sun, is a rare yellow hypergiant named HR 5171. It is one of the ten largest stars found so far. Observations by the European Very Large Telescope Interferometer show that this star has a very close, but smaller, binary partner in a 1,300-day orbit. So close in fact that the two stars are touching, making the system look like a lopsided peanut. Yellow hypergiants are very rare, with only a dozen or so known in our galaxy. They are the biggest and brightest stars known and are at a stage of their lives when they are unstable and changing rapidly. Due to this instability, yellow hypergiants expel material outwards, forming a large, extended atmosphere around the star. Despite its great distance, nearly 12,000 light-years from Earth, the object can just about be seen with a keen eye, or ideally, with binoculars. Look in the southern part of the constellation Centaurus, to the left of Crux, near Centaurus A.

Lunar Eclipse Timetable

Penumbra first contact: 10:52 p.m. (MDT) April 14
Partial eclipse begins: 11:58 p.m. April 14
Totality begins: 1:06 a.m. April 15
Full Moon: 1:42 a.m. April 15
Mid eclipse: 1:46 a.m. April 15
Totality ends: 2:25 a.m. April 15
Partial eclipse ends: 3:33 a.m. April 15
Penumbral eclipse ends: 4:39 a.m. April 15

Yes, there is a partial solar eclipse on April 29; unfortunately, it is only visible in the southern hemisphere.

April 2014 Night Sky Chart

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