Tony's Astronomy Corner

April 2008:


Tony Martinez

Each month I will describe sights of interest in the night skies of South Carolina. These sights will be broken down into three sections; what you can see with the naked eye, with binoculars, and with a small telescope. The best time to view the night sky is at and around the times when the Moon is not visible, what is known as a New Moon; which will occur this month on April 6th. For April, your best viewing nights will be from April 1st through April 9th, and April 23h through April 30th.

The Star chart below is set for Florence, SC on April 15th at 9 pm.



One of many interesting astronomy objects are star clusters. Star clusters are large groups of stars that occupy a small area in space and the stars in a cluster are gravitationally bound to each other. There are two major types; open clusters and globular clusters.

An open star cluster is a loose grouping of stars usually containing two hundred or more stars. These clusters are usually found imbedded in the spiral arms of our galaxy, and contain very young stars. Last month I suggested that you search for the Messier objects to improve your observing skills. A total of 26 Messier objects are open star clusters. The most famous open star cluster is the Pleiades (Seven Sisters). A quick look through binoculars shows that it contains much more than seven stars.

This month is the perfect time to observe another famous open cluster, known as the Beehive Cluster, or Praesepe (Latin for manger). The Beehive is mentioned throughout history, and can be seen with the naked eye from modestly dark sky locations. It has been said that ancient sailors used it to predict weather; if the Beehive was visible, good weather was expected, and if not visible, there could be bad weather approaching. With the naked eye it is a faint smudge in the sky, but with binoculars it becomes obvious that it is a star cluster. It can also be observed through low power telescopes. Either way, as you sweep the region searching for it, it will “jump out” at you when you find it. Below is a telescopic view of the Beehive.



Presently, an easy way to locate this cluster is to draw an imaginary line between the star Regulus in Leo and the planet Mars in Gemini. Aim your binoculars halfway between, to find the Beehive Cluster. Remember, the Beehive is also M44, so you have added another Messier to your list. The star chart below will also help, but don’t try to use the constellation Cancer as a guide, since it is too faint to be of use.



One fate of an open star cluster is that the stars slowly disperse. When this occurs, the dispersed stars are known as a star association. One impressive association is found in Perseus centered near the star Mirfak. However, Perseus has set and will not return until September. So where else can you look for these associations? The Big Dipper itself! A total of five of the seven stars in the Big Dipper along with nine other nearby stars were members of a long ago open star cluster. An additional twenty-three other stars are believed to also be members of this same association. If someone on a planet 1000 light years away were looking at us, our Sun would appear to be part of this loose star cluster. The image below is a computer-generated view looking back at our Sun from the star Meissa, 1000 light years away. Meissa is located at the head of Orion the Hunter.



The second type of star cluster is a globular. The globular cluster contains very old stars, is much more densely packed than open cluster, and may contain more than a million stars. Unlike the open star clusters, globular clusters are outside of our galaxy, but closely “hovering” above and below, represented by the dots in the image below.



The most famous globular in the northern hemisphere is the Hercules Cluster, M13, which contains several hundred thousand stars. M13 will rise in the east in late April. Note: a total of 29 Messier objects are globular star clusters. As you look at the picture of M13 below, try to imagine how amazing the night sky would look if we lived on a planet of one of the stars of this cluster.



Naked Eye Sights: Remember the Pleiades, the naked eye Messier object (M45), which you now know is an open star cluster. Look to the West early evening on April 8th. The crescent Moon slowly moves toward the Pleiades; covering (occulting) some of the Pleiades stars before the Moon sets.

To test if you live in a dark sky location, using the chart shown above, try to find the Beehive Cluster with your naked eye.

Finally, say goodbye as Orion ends his winter visit this month, slowly sinking into the western horizon as April progresses.

Binocular Sights (7 to 10 power): The Moon and the Pleiades will be much more impressive if you view them through your binoculars in the early evening on April 8th.

Use the above chart, and try to find the Beehive Cluster with your binoculars.

Telescope Sights (60-100mm): Try to find the Beehive with your telescope, but be sure to start with low power. Of course anytime you have your telescope out this month be sure to check out the planet Saturn.

See you next month.