Starwatch Newsletter Minnesota Starwatch is a tape-recorded message describing the night sky in the Midwest, which can be called by telelphone number (612) 624-2001 It is updated montly, and is produced by the Department of Astronomy, University of Minnesota 116 Church Street SE, Minneapolis, MN 55455

After several unusual months of skies filled with bright planets, things are settling back into a more common pattern. The only really obvious planet in the evening during July is Venus, now relatively low in the west right after sunset. About mid month Venus will be positioned just above the first magnitude star Regulus. Between July 12 and July 13 those two will be passed by the new, crescent Moon. Jupiter, which was very prominent a couple of months ago. It now sets just after the Sun early in July, but will pass behind the Sun on July 20. Jupiter will reappear as a bright object in the fall morning predawn skies. This month that role in the early morning is taken by Saturn, while an alert observer with a clear view of the eastern horizon may be able to spot Mercury in the twilight just before sunrise.

Remarkably, the sun will be farther from the earth during July than at any other time of the year. That detail seemingly conflicts with the fact that July is typically the hottest month of the year in Minnesota. On the other hand, it is the winter season in the southern hemisphere. So, in places like Chile or Australia this is the coolest season. That just serves to remind us that the seasons are not set by our distance from the sun, but by the tilt of the earth's daily spin axis relative to the annual motion of the earth around the sun. That places the sun successively higher in the sky in summer and lower in the sky in winter, enhancing or diluting the sun's ability to heat the earth's surface.

Observers away from city lights in July can easily make out the Milky Way as it swipes almost overhead from north to south in the late evening. This diffuse band of light is really the collected contributions of myriads of stars concentrated in the disk-like structure of the galaxy we live in. Our Milky Way is huge, so that the light from a star would take several hundred thousand years to cross it. However, one of the great discoveries of the 20th Century was that the Milky Way galaxy is not at all alone in the universe. Closest to home, our galaxy is a member of a modest group of galaxies. The best known other member of the group is the Andromeda galaxy, which is roughly a million light years away and faintly visible to the unaided eye in the northeastern evening sky in July. The Andromeda galaxy is roughly the size of the Milky Way, but its distance from us reduces its apparent size to a couple of degrees, or several diameters of the Moon. The pull of gravity keeps these two large galaxies and several smaller ones tied together in a loose swarm. Much richer clusters of galaxies are now known to be common, however. Some of these groupings contain several thousand galaxies. The closest of those clusters is in the constellation Virgo, now visible in the southwestern sky after dark. The Moon will appear below the Virgo galaxy cluster on the evening of July 15. The Virgo cluster is a whopping 60 million light years away, but so extensive that it still extends several degrees across the sky. It is close enough that many of its brightest galaxies can be seen in small telescopes. In fact, sixteen of the famous Messier objects are galaxies in the Virgo cluster. An even richer galaxy cluster is found just to the north of Virgo in the constellation Coma Berenices. That cluster is several times farther away, so can only be seen with moderate telescope power. Recent studies have demonstrated that clusters like these are not randomly located, but are strung along vast "cosmic filaments" that extend for hundreds of millions of light years. In fact, we now know that over cosmic time clusters pull in smaller clusters with by way of their gravity, leading to violent "mergers".

Over the past couple of decades astronomers have been surprised to discovered that most of the matter in galaxy clusters is in forms other than the obvious galaxies of stars. In fact, only a few percent of all the matter in clusters, as measured by their gravity, can be accounted for by the galaxies. Several times more mass is found to be in the form of hot diffuse gas. This gas is visible from outer space with X-ray telescopes, such as the recently launched Chandra Observatory. The distribution of that gas and its temperature allows astronomers to map out the total gravitational well holding the gas and galaxies together. The biggest surprise of all is that several times more matter than exists in the hot gas is in some other, invisible form. The nature of this, so-called, "dark matter", is not understood at present. All we know for sure is that it is not "ordinary" matter of the kind that makes up stars and planets, and, yes, people. That puzzle is one of the biggest driving forces in astronomy as we enter the 21st Century.

The Minneapolis Planetarium offers a wide variety of programs for all ages. For more information, call 612/630-6150.

For those interested in the Minnesota Astronomical Society, call 651/649-4861 for information on their upcoming events.

Minnesota Starwatch is available on-line at http://www.astro.umn.edu/Outreach/pub_out.html

This has been Minnesota Starwatch, produced by the University of Minnesota Astronomy Department in cooperation with WCCO Weather Center.

Last Updated: Thu Jun 13 16:55:08 2002