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  Night Sky

August Night Sky

August 2009

Going, Going… Gone?

—Charlie Christmann

When giant stars start to shrink rapidly, astronomers begin to think supernova. Though these occur throughout the universe on a regular basis, we don’t usually see them without a telescope because most occur so far away. To look up and find one visible to the naked eye would be amazing, especially if the star that goes nova is only 640 light-years (3,762 trillion miles) away from Earth.

The last supernova observed in our galaxy was Cassiopeia A in 1680. Before that, the astronomer Kepler observed a supernova in 1604 and Tycho One in 1572. A supernova occurred in the Large Magellanic Cloud, a companion galaxy to our own, in 1987.

What star are astronomers watching? Betelgeuse. Yes, the alpha star in Orion is acting up. If you are up early this August, look between 2:30 a.m. and sunrise for Orion in the eastern sky. Betelgeuse is the red-hued star on the left shoulder of Orion. Its birthplace was far away from its current location. The star’s motion shows it to be a runaway member from the Orion OB1 association found to the upper right of the belt.

Betelgeuse is a red supergiant, and one of the largest and most luminous stars known. Red supergiants are the largest stars in the universe in terms of volume, although they are not the most massive. Betelgeuse is approximately 8.5 million years old, much younger than the 4.6-billion-year-old sun; but the larger the star, the shorter its lifespan. And at nine hundred times the size of the Sun, Betelgeuse is getting old.

Betelgeuse is classified as a semi-irregular variable star, and its brightness varies over time. It also seems to vary slightly in size. But, since 1993, Betelgeuse has shrunk by fifteen percent, the equivalent of Venus’s orbit around the sun. Why? Astronomers can only guess. But they do know that supergiant stars burn out their hydrogen fuel quickly and then switch to helium, carbon, and other elements. At each step, these stars partially collapse, refueling using heavier elements. Betelgeuse is expected to fuse elements through neon, magnesium, sodium, and silicon—all the way to iron. When the iron is used up, the star is destined to explode.

Researchers believe Betelgeuse is at the collapse stage and may explode into a supernova, perhaps in our lifetime. When it does, it will most likely leave a compact neutron star about the size of a small town behind.

Since Betelgeuse is 640 light-years away, it may have already exploded, and it’s just a matter of time before we witness the colorful supernova explosion. If and when the light from this explosion reaches Earth, expect it to be bright, some think as bright as the full moon. It could cast a shadow or be visible in daylight. Though this event will be beautiful and exciting, Orion, the great hunter, will be wounded, never to look the same again.

More Sights to See

Go out two hours after sunset and check out the east-northeastern sky. Low in the northeast is Perseus with bright stars Atika low on the horizon, Algol and Mirfak above. Above Perseus is Cassiopeia looking like a big “W” in the sky. To the right of Perseus and below Cassiopeia is Andromeda. Its two bright stars are Almach and Mirach.

In the east-northeast, below Andromeda is Triangulum, with Aries below that just under the horizon. Aries’s bright star is Hamal. To the right will be Pisces, not quite fully above the horizon. Above Pisces and right of Andromeda is Pegasus. Its great square body is framed by Alpheratz on the left, Scheat on top, Markab to the right, and Algenib below. Above Scheat is Matar.

August Meteor Shower

The Perseid shower peaks on the 11th. Though the meteors seem to radiate from Perseus, look all over the night sky for meteors. Activity usually peaks after midnight. Unfortunately, a waning gibbous moon rises around midnight to partly spoil the view for the dimmer meteors.

The Planets and Moon

Mercury will be setting about forty-five minutes after sunset. Look hard to find it.

Venus will rise in the east about 2:30 in the morning and should be bright at -3.8 magnitude. The thin crescent Moon will be just above Venus on the 17th. Look for Betelgeuse to the right of Venus this month.

Mars rises about 1:45 a.m. in the east.

Jupiter reaches opposition, opposite the Sun as seen from Earth, on the 14th. It is at its closest approach to Earth, so this is a good time to break out the telescope to see the color bands and moons. There will be a Moon-Jupiter conjunction on the 6th.

Saturn will be low in the west at sunset.

The Moon is full on the 5th. This will also be a penumbral eclipse. Watch for the lower left part of the Moon to dim slightly, and then brighten. Mid-eclipse is 7:39 p.m. MDT. The Moon is new on the 20th.






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