Summer 2003

Fixed Stars

Sixty years ago Walter Baade showed that the stars in galaxies could be divided into two populations, which he called populations I and II. He did this by resolving the stars in the central region of Messier 31 and in its companions, and measuring their colors. Perhaps no other astronomer would have had the skill to make this observation with the instruments available at the time, and few would have had the insight to see its importance. Within a few years, stellar evolution studies had advanced so far that it became apparent that the populations were primarily an age classification, where the population II stars were created during the galaxy formation process. Only after the galaxy reached roughly its present form did the population I stars begin to appear.

Sixty years is not a very long time in the history of astronomy, and it seems surprising that this very fundamental distinction was recognized only so recently. Actually, that there was a difference between stars in different locations in the galaxy or with different kinematical properties, had been known for some time. Baade himself refers to the discovery by Jan Oort, twenty years earlier, that high velocity (population II) and low velocity stars had different spectral characteristics. Standard texts at the time made no secret of these differences. In the 1938 edition of Astronomy by Russell, Dugan and Stewart we find the Hertzsprung-Russell diagram (not referred to by that name) described for globular and for open clusters. Regarding globular clusters we read “The brightest stars are all red … . The stars two or three magnitudes fainter are mostly white.” The description of open clusters is even more explicit. “Among the brighter members, stars of early spectral class, belonging to the upper part of the main sequence, are always found, and in about half the clusters bright yellow and red stars (evidently giants) also occur. When these are present the brightest stars of the main sequence are of Class A … . In other cases … the brightest stars are of Class B, and there are no red giants.” Given what we now know about stellar evolution, nothing could be more suggestive of an age difference, but until Baade’s discovery and the work that followed, this was not appreciated.

Open and globular clusters are excellent examples of populations I and II, respectively, and both are abundant in the summer sky. Among your correspondent’s favorites is Messier 11, the Wild Duck Cluster, in Scutum. This is an exceptionally rich open cluster of intermediate age. The brightest stars are about the color of Vega. Messier 8, the Lagoon Nebula in Sagittarius, is much younger, and its brightest stars are quite blue, but it is rather far south for convenient observation. It should not be difficult to verify the colors of these relatively bright stars. Population II is represented by the globular clusters, of which Messier 13 in Hercules is one of the best examples. Globular clusters are generally much more distant, so that it will be difficult, without a very large telescope, to verify that the brightest stars here are yellow.

In addition to being younger, population I is richer in heavy elements than population II. This is to be expected, since the heavy elements were formed in massive stars and returned to the interstellar medium by supernova explosions. They were present in far smaller quantities when the population II stars were formed. Before star formation started, the universe was composed almost entirely of hydrogen and helium, so that the very first stars must have been quite free from heavy elements. Stars of this sort, which have yet to be found in their purest state, have been referred to as population III. It is possible that, without the heavy elements which are essential for dust formation, only very large stars could form, in which case they all will have exploded as supernovae long ago. If small stars formed, they will still be with us, but they will be very faint and hard to find. It is also likely that their atmospheres will have been contaminated by accretion from the interstellar medium, making them almost indistinguishable from ordinary stars. But the search goes on, and if such stars can be found, they will give us a valuable new insight into the early evolution of the universe.

                          Planets

During most of August Mercury will be visible, with difficulty, in the southwestern evening sky. This will be a very unfavorable apparition for northern observers.

At the beginning of the quarter Venus will still be visible in the northeastern morning sky, but it will soon disappear from view. It will pass behind the Sun in the middle of August and not reappear until October.

Mars will be visible during the entire quarter, rising in the evening hours during July and thereafter being visible during the whole night. It will reach opposition on August 28. This will be a very special opposition, since it coincides closely with perihelion. Mars will then be at its largest, 25.1" diameter, and at its brightest, magnitude -2.9. Such a perihelion opposition occurs only once every 15 or 17 years. However, the alignment this time is so exact that our distance from Mars will be less (by a very small margin) than it has been at any time during the past 100,000 years. Observers wishing to celebrate this event should be manning their telescopes at 2:52 a.m. PDT on the morning of August 27, when the separation will be 55,758,006 km. Those wishing a better view will have to wait until August 28, 2287, when Mars will be 70,000 km closer. Perihelion oppositions of Mars (or any other planet) recur each time on about the same date. At the end of August Mars is well south of the equator, so southern observers always have the best view. Observers not wishing to undertake the journey to the southern hemisphere can wait at home for a few thousand years until precession has moved the ecliptic around to a more favorable position.

Jupiter can be seen briefly, low in the western evening sky at the beginning of July. It then disappears from sight, passing the Sun on August 22 and reappearing in the eastern morning sky toward the end of September.

Saturn will appear in the eastern morning sky during July and will become more easily visible in the morning hours as the quarter progresses.

Uranus and Neptune both pass opposition during the summer quarter. Uranus is at opposition in Aquarius on August 24 with a magnitude of 5.7, so that it is just visible to the unaided eye. Neptune is at opposition in Capricornus on August 4 with a magnitude of 7.8.

Meteor Showers

The best meteor shower of the whole year, the Perseids, which reaches maximum on August 13, will be spoiled by the full Moon this time around, but there are other minor attractions for the enthusiastic meteor observer.

The Southern delta-Aquarids is the strongest of a whole series of showers reaching maximum between the middle of July and the middle of August. The maximum of this shower should be on about July 28, when the Moon will be absent. This shower, like most of the others in the series, is relatively rich in faint meteors, which makes it less spectacular than it otherwise might be. It is best observed in the morning hours.

The alpha-Capricornids, which peaks just two days later, is a very weak shower, but it is noted for the occasional appearance of very bright events, even fireballs.

The alpha-Aurigids is a relatively weak shower, said to be much in need of further observation. Over the years there have been reports of short bursts of intense activity, suggesting that closer monitoring would be rewarding. The Moon will be a few days before first quarter.

Comets

Comet 2002 V1 (NEAT) passed close to the Sun on February 18 and is now fading rapidly. It is observable only from the southern hemisphere.

I have found no recent information on Comet 2002 X5 (Kudo-Fujikawa), but I expect that it has by now faded to the point where it is no longer easily observable.

Comet 2002 Y1 (Juels-Holvorcem) is now receding from the Sun and fading. It is best observed from the southern hemisphere.

Comet 2002 T7 (LINEAR) is still quite faint, but it is a long way from perihelion, which it will reach in April of next year. By that time it may well have become an interesting naked eye object.

Eclipses

There will be no eclipses during the summer quarter. Most of us enjoyed watching the total lunar eclipse on evening of May 15. In most places the weather was favorable. For those who may have missed it, or just want more of the same, there will be an almost exact repeat on the evening of November 8.