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  Night Sky

Night Sky

The Milky Way Galaxy as seen from Earth. Credit: ESO/S. Brunier,

October 2009 Night Sky

—Charlie Christmann, Signpost

Home, Sweet Home

—Charlie Christmann, Signpost

Throughout the course of human history, man has been awed, frightened, mystified, and confused by the sky. Things are no different today. Take our home galaxy, the Milky Way, as an example. It’s big, it’s close, and it surrounds us. You would have thought we knew it fairly well, but we are learning new things all the time.

In the beginning, after we decided Earth was nothing special in the universe, we thought, perhaps, the Milky Way was. At one time, it was thought all of the stuff in the sky was a part of our galaxy. Yet, we were delighted to discover the Milky Way is just one of billions of observable galaxies we can see with modern telescopes.

After observation in the infrared and microwave wavelengths were possible, we discovered that our home galaxy was a spiral shape like many others we could observe in the heavens. Using that data, it appeared that the galaxy had four major arms wrapping around the central bulge of stars in the center: Norma, Scutum-Centaurus, Sagittarius, and Perseus Arms. These arms form the disk and are the major star-forming regions in the outer galaxy. When astronomers looked closely, it was discovered that we live in a minor arm, the Orion Arm, tucked between the Sagittarius and Perseus Arms.

Our home galaxy consists of an estimated two hundred to four hundred billion stars and has a huge central bulge in the center about which all the arms rotate. The oldest known star in the galaxy is 13.2 billion years old, nearly as old as the 13.8 billion-year-old universe itself. A central bar-shaped region of stars is believed to be centered on the bulge. The major arms of the galaxy originate at the ends of this bar. If you look in the direction of Sagittarius, you are looking toward the center of our galaxy. The diameter of the Milky Way is thought to be around 100,000 light years. In our local area of the disk, the thickness is around one thousand light years.

The Orion Arm is just a short arc of stars only 3,500 light years across and about ten thousand light years in length. Earth and our local star, the Sun, are located about the center of the arc on the inside edge. That places us about 26,000 light years (152,841,000,000,000,000 miles) from the center, which is all well and good, because we now know about the massive monster that lurks there called a black hole.

Also embedded in the Orion Arm with us, in the direction we are traveling at about 600,000 miles per hour, is Deneb, almost centered in the arm some 1,400 light years ahead of us. On the outer edge ahead of us and just above the galactic plane is Polaris, the North Star, 430 light years away. Trailing us is the big red star Betelgeuse, in the constellation Orion, 640 light years away. Then there is Rigel, the catty-corner foot from Betelgeuse, even closer to the outer edge of the Orion Arm, about 770 light years behind us. If you are looking at Orion’s belt in the night sky, you are seeing stars on the outer edge of the Orion Arm just past Rigel, centered about nine hundred light years away.

Looking closer to our home star, we can see we sit inside a huge bubble, called the Local Bubble. We know of several bubbles around us, one, Loop 1 Bubble is colliding with our local bubble. While the average density of the Milky Way Galaxy is about 0.5 atoms per cubic centimeter, the local bubble only has 0.05 atoms per cubic centimeter. Our bubble is the result of a supernova that exploded within the past two to four million years, clearing out much of the gas that normally resides in the galaxy. Geminga, a pulsar in the constellation Gemini, is thought to be what remains of the nova.

The Sun just happens to be located in a slightly denser part of the bubble called the “local fluff,” a cloud of gas about thirty light years across. It might worry some that the temperature of this cloud is 10,800°F; however, the density is very low at 0.1 atoms per cubic centimeter, so we are not going to burn up. The sun should exit the fluff in another ten thousand to twenty thousand years. The cloud is an outflow originating in the star-forming region called the Scorpius-Centaurus Association, in the direction of Scorpius, Lupus, Centaurus, and Crux. Other resident stars of the Local Fluff are Alpha Centauri, Altair, Arcturus, Fomalhaut, and Vega.

In just the last few years, new observations have hinted that the Milky Way Galaxy is much larger than thought, and may consist of just two arms. As instruments improve and we look harder at our home galaxy, I’m sure we will discover many more surprises.

The Stars and the Moon

Mercury is low in the east before sunrise this month. Look about fifty minutes before sunrise on the 6th. This is when the planet will be at its greatest elongation (highest in the sky) from the sun. An hour before sunrise on the 8th, you will find Mercury almost touching Saturn. Venus will be dancing brightly above the planet pair.

Venus is bright in the east before sunrise. On the 16th, find Venus, Saturn, and the crescent moon hanging around together.

Mars is also a morning star high in the southeast. On the 12th, look for a Moon-Mars conjunction an hour before sunrise. Follow the line connecting the Moon and Mars toward the horizon. There you will find closely spaced (highest to lowest) Venus, Saturn, and Mercury low in the morning sky.

Jupiter shines bright in the south after sunset. The Moon and Jupiter have a conjunction on the 26th.

Saturn is low in the sky before sunrise. The official Moon-Saturn conjunction is on the 13th. The best time to look is an hour before sunrise.

The Moon will be full on the 4th and new on the 18th.

Be very careful on the evening of the 31st. The night may be filled with all manner of strange sights and apparitions!

Stargazers unite for the annual Placitas star party

On Saturday, October 24, Las Placitas Association and the Albuquerque Astronomical Society will co-host the annual star party to promote dark skies in the Placitas community.

The party will begin around sunset, which occurs this year at 6:22 p.m. Just like the last several years, this event will be held at the Homestead Village Shopping Center (Merc), with telescopes set up immediately east of the parking lot along the gravel road. A waxing crescent moon will set around midnight and telescope volunteers may even show you the general area where Apollo 11 landed on the Sea of Tranquility just over forty years ago. Jupiter and its moons will be another showpiece object high in the sky.

Come and stay as long as you can, and don’t forget to ask lots of questions. For additional information, call Shannon Mann at 771-0126.







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