The Summer Sky

(MIRA image)

Massive stars, when they age, eventually reach an unstable state and explode as supernovae.  Less massive stars, like the Sun, evolve in a more gentle way.  After expanding through the giant to the red supergiant phase, they expel their outer layers, leaving the hot inner part of the star, which eventually becomes a white dwarf.  For a short time (100,000 years or less), we can see this expanding shell of gas surrounding the hot central star.  Sir William Herschel called these objects planetary nebulae because they looked somewhat like planets.  Obviously they are really quite different.  If the shell were expelled uniformly in all directions, we would always see a circular ring, because it is the edge of the shell which always looks brightest.  In fact, planetary nebulae come in many different forms, indicating that the expansion is not uniform in all directions.

One of the best known planetary nebulae is the Ring Nebula in Lyra (Messier 57).  It is relatively large (maximum diameter about 2 minutes) and rather faint, suggesting that it is in a late stage of evolution. It is visible, using a moderate sized telescope, in the evening sky in summer, and is located between Sheliak (Beta Lyrae) and Sulaphat (Gamma Lyrae).  The ring is quite elongated, suggesting that we are seeing a real ring which is inclined to the line of sight.

As noted above, this is probably not the right explanation. We are very likely looking at a shell which has different expansion velocities in different directions.  A smaller but much brighter planetary nebula is NGC 6543 in Draco.  It is located in an empty region of the sky between Delta and Zeta Draconis.  If the sky is not completely dark, the greater brightness makes for easier observing.  

Planets

Mercury can be seen low in the western sky at the end of July, but it is not readily observable from the northern hemisphere.  It will be most easily found on July 23 to 26 when it is close to Venus and Regulus.  A better opportunity to observe Mercury will occur in the middle of September, when it will be fairly high and bright in the morning sky.

Venus remains low in the evening sky all summer, moving south as the summer progresses.  Although it is approaching maximum elongation, observing conditions will remain unfavorable from the northern hemisphere.

Mars will also move into the southwestern evening twilight during the summer and will become fainter and increasingly difficult to observe.

Jupiter will be in retrograde motion during the whole summer and will reach opposition on August 9.  However, this will be an unfavorable opposition because of its far southern declination.

At the beginning of the summer, Saturn will rise about midnight, but by September it will be visible during the whole night, as it approaches opposition.  It will be stationary on August 2.  Saturn will now remain north of the equator for several years and be increasingly favorably located for northern observers.  After having missed several lunar occultations, observers in the southern part of North America will finally be able to see one on September 18.

As reported in the previous edition, Uranus and Neptune will reach opposition on July 29 and 21, respectively.  However, their far southern declinations make observation from the northern hemisphere difficult.  They are now separated by about 10 degrees and are gradually drifting apart.  Patient observers will find them close together again in the year 2162.  

Meteor Showers

This year the Perseids, everyone's favorite meteor shower, due around August 12, will find some interference from the Moon, which will be just past first quarter.  The two much weaker summer meteor showers, the S Delta Aquarids (July 28, morning hours only) and the very scarce Alpha Aurigids (August 31) will not be much bothered by the Moon.

Eclipses

There will be two eclipses in September, neither one visible from North America.  A partial solar eclipse on September 2 will be best visible from Antarctica, where it will be greater than 80 per cent. A total lunar eclipse, the last eclipse in 1997, will occur on September 16, but it will hardly be visible outside the eastern hemisphere, and not at all from North America.