The spring has come, and the onset of mild weather in most of the northern hemisphere brings forth a hunger for new celestial objects to observe. Following on from our top picks for winter stargazing, here's our selection of the best targets for spring viewing. Some of the objects to be described can be seen down as far south as 30-40 degrees South latitude.
To begin with, although this list is aimed at those with smaller telescopes, or in some cases binoculars, these objects also pay off nicely in larger scopes. Some objects should be examined using low power (about 5x per inch of aperture), others using high power (about 25-30x per inch), and others may look their best in between. For a 4-inch telescope, low power is about 20x, and high power is 100-120x (possibly up to 150x on a good night.)
The Moon and planets
The best object for viewing with a small telescope never changes – the Moon. Using low power or high power, the Moon will show more detail than any other celestial object with detail that shifts as shadows change according to the phase of the moon. The planets Jupiter, Saturn, and Mars also pay off frequent viewing when they appear in the nighttime sky.
At present Jupiter is clearly visible in the western sky early in the evening, but disappears into twilight by the middle of May. Jupiter is currently about 35 seconds of arc in diameter – enough to see the disk and the most prominent stripes at high power. Don't forget to keep track of the four Galilean moons, Io, Europa, Ganymede and Callisto. They were discovered by Galileo, and formed part of his evidence for a Sun-centered solar system. Generally, the planets require high power for the best observations, but even low power will allow you to watch the changing patterns of the Jovian moons from night to night. Also, planetary views are always better when the planet is high in the sky, and often the best time for observation is in the early morning when the thermal currents which begin near sunset subside.
In contrast, Saturn presently rises in the east at about 9 pm local daylight time, and will be high in the southeast and southern skies through spring and into early winter. The disk of the planet and the remarkable rings show clearly at fairly low power (the rings are the width of the Moon when observed at about 40x magnification). Saturn's moon Titan can also be seen in small instruments, and it is fascinating to watch its 16 day orbit around Saturn.
Using high power on a good night using a 4-inch telescope, you may be able to make out Cassini's Division – a dark gap appearing on the rings. The width of Cassini's Division is only about ten percent of the total width of the rings visible in a small scope, but can often be clearly seen owing to its large visual contrast. Again, it is much more likely that you'll see the rings well after midnight.
Unfortunately, Mars is in the daytime sky throughout the spring, and as the upcoming opposition (closest approach) in April 2014 is rather distant, so that Mars will only appear about half the size it does at closest possible approach (about 15 seconds of arc), it will not be an exciting target for a small telescope in coming months.
Comet PANSTARRS
Although I've mentioned the usual solar system targets, let's not forget Comet PANSTARRS (C/2011 L4). This comet is currently located near the westernmost star of the seat of Cassiopeia. Currently at a brightness of around sixth magnitude (the dimmest star visible with the unaided eye at a dark site), it will remain in the circumpolar skies for northern observers throughout the summer. At the time of writing (April 20), Comet PANSTARRS is located in Cassiopeia, right between the two stars at the end of the "W" that make a right angle with their neighbors. At the end of May it spends a couple of weeks within about five degrees of the North Star at about eighth magnitude. At this point it will likely be visible in a 3- or 4-inch telescope, but will only be a patch a bit brighter than the background. A computerized Go-To scope would be a big help to find its position.
Double stars
Now let's leave our solar system behind. The most exciting objects may be globular clusters, nebulae, and other galaxies, but there are some stellar systems close-by that are worth a look. A good place to start is in the Big Dipper. Technically a part of the constellation Ursa Major, the Big Dipper is arguably the best recognized pattern of stars in northern skies, although at 60 degrees North latitude it is difficult to see south of the Equator.
The star at the bend in the Big Dipper's handle is actually a double star, Mizar and Alcor. Both members of the double star system can be easily seen with the unaided eye in dark skies (they are separated by one-third of the Moon's width) and are reasonably bright (Mizar is a second magnitude star, while Alcor is a fourth magnitude star just east of Mizar.) Even in a city they are easy targets for a pair of binoculars. Once you find them, center them in the field of view of a telescope at a magnification of about 100-150x. You will find that Mizar is itself a double star, Mizar A being the brighter of the two, and Mizar B with one-fifth the brightness lying about 14 seconds of arc northeast of the brighter star. At the suggested magnification, the two components of Mizar will appear to be separated by about the width of the Moon. All three of the visible stars are hot white in color.
There is still a surprise in store, although one cannot see it in a telescope. Mizar A, Mizar B, and Alcor are each close double stars. In addition, it has recently been demonstrated that Mizar and Alcor are gravitationally bound, so that the system is a sextuple star, even though only three components are detectable without very sophisticated instruments.
A final story. When looking at the Mizar-Alcor system, about midway between the two is the third brightest star in the field of view. This star, much dimmer at eighth magnitude, is formally known as HD 116798 (catchy, eh?), but was once thought to be the first planet of the Sun not visible to the unaided eye. In 1722, one Professor Georg Liebknecht thought he saw the star moving relative to its neighbors, an error perhaps encouraged by his use of a non-achromatic telescope. He announced the discovery of a new planet, and named it Sidus Ludoviciana (Ludwig's star) in honor of his monarch, the Landgrave Ludwig of Hessen-Darmstadt. His compatriots quickly and somewhat raucously dismissed Liebknecht's discovery. We are led to believe that Ludwig was not amused.
Another prime double star to enjoy in the late spring skies is Alberio (Beta Cygni) in the constellation of Cygnus the Swan. Albireo is found at the head of the Swan, also known as the Northern Cross. It appears to be a modest third magnitude star, but even in 10x binoculars can be seen to be a double star. Alberio A is a third magnitude reddish-yellow star, separated by 34 seconds of arc from Alberio B, a sixth-magnitude blue-white star. Again, it turns out that Alberio A is a very close double star in its own right, with a separation of about 0.4 seconds of arc.
In a small telescope at medium magnification, Alberio is one of the best stars in the sky to see a dramatic difference in stellar color. The color difference is best seen by putting the telescope slightly out of focus, as it is easier to detect color on extended objects. The amount of color contrast also improves the less scattered light is in the sky, so a dark observing site is best. In the case of Albireo, however, the difference in color can be seen even from poor sites. A hint – extra magnification will help reduce the darkness of the field of view background.
Stellar clusters
Here we'll look at one of the best open star clusters and at one of the finest globular star clusters. Open clusters are loose aggregations typically containing hundreds of young stars and often surrounded by remnants of the star-generating gas and dust clouds from which they grew. Globular clusters are spherical groupings of hundreds of thousands of very old stars. Both types of clusters are gravitationally bound, and are typically packed within a 10-30 light year region of space.
One of the best open star clusters visible in the evenings at this time of year is M44, also known as the Beehive Cluster. With a visual magnitude of 3.7, it is an easy sight right in the center of the four-star asterism that includes the brightest stars in the constellation Cancer. In dark skies it is easily seen with the unaided eye as a fuzzy patch, and in any case is obvious in a pair of binoculars.
In a small telescope the Beehive lights up, with 40-50 separate stars being visible within a field of view about 1 - 1 1/2 degrees. While best seen with low power (no more than 30x, as you don't want to cut off the outskirts of the cluster), judicious use of higher powers within the cluster can make small stellar patterns come alive.
Our top springtime globular cluster is M13 in Hercules. Hercules is a large constellation, but at its center is a quadrilateral formation of third and fourth magnitude stars. The quadrilateral is about 15 degrees west of Vega, the brightest star in Lyra, and one of the brightest in the spring skies. M13, at sixth magnitude, is located just north of midway on the western side of the quadrilateral. Its visual diameter in small scopes is about 10 minutes of arc – a third the width of the Moon.
M13 is 25,400 light years distant from Earth, about 145 light years across, and contains some 300,000 stars, so the individual stars are separated on average by about 2 light years. M13's stars are much closer together in the core of the cluster, where the stars are about half a light year apart.
Just what can be seen of M13 using a small telescope depends on many factors, including experience and the size and quality of your optics. Any binoculars or scope will show that M13 is a fuzzy disc with a bright center, and this is often quite a sight when the background stars are also seen at low power. The challenge for M13, and all globular clusters, is to resolve their stellar structure, that is, to partially break the image into individual stars. The size threshold for resolving M13 is about a 4-inch telescope, although on exceptional nights individual stars can be seen with a scope as small as 70 mm (2.8 inch) aperture. Resolution requires magnification of at least 80x, and on a good night 40-50x per inch can be used.
Nebulae
The spring is short on good extended and planetary nebulae, so we will delay talking about those until looking at the best objects of the summer.
Galaxies
On the other hand, the spring is a great time for viewing galaxies for the same reason that it contains very few nebulae – the Milky Way is not in a favorable position until the wee hours of the morning. Galaxies are best seen during periods when the Moon is set, at least two hours from dawn or dusk, and preferably at an hour when they will be as high above the horizon as possible. Save for the very brightest, galaxies will require suburban or rural viewing locations. They are very difficult targets for a small scope from urban locations and even dark locations (an unlit football field or park) surrounded by a city will experience major light pollution. It is also important to let your eyes adjust in the dark for about 30 minutes before doing any serious observing.
An important technique for seeing galaxies and other objects with low surface brightness is averted vision. When using averted vision, you look a bit to the side of where an dim object ought to be. The sharpest vision is in a central region of the retina called the fovea. All of the photosensitive cells in the fovea are cones, which are responsible for our color vision, and are not very sensitive in the dark. Looking a bit away from your target shifts the target onto areas of the retina filled with rods, which are extremely sensitive to light at night. This makes it easier to see dim objects.
Let's start with the M81/M82 double galaxy. To me, the easy way to find these galaxies is to follow an arrow pointing from Phecda to Dubhe in the bowl of the Big Dipper, and then extend the arrow by the same length.
Wonderfully placed for springtime observation in the northern hemisphere, M81 can be seen on a moonless night in suburban areas with good binoculars, and both galaxies can be seen in a small telescope. Unfortunately, the pair is located very low on the northern horizon for those viewers south of the equator.
M81 is the primary member of a cluster of galaxies – the M81 cluster – that lies some 12 million light years distant. M81 shines at about magnitude 7, and its neighbor M82 can also be seen in the same field of view at about magnitude 9. M81 and M82 are separated by about 300,000 light years (about 15 percent of the distance between the Milky Way and the Andromeda Galaxy, M31.) Their apparent separation is just wider than the moon.
M81 is a large spiral galaxy, with its axis of rotation tilted so that it appears as an ellipse with the long axis being about twenty minutes of arc in length. Two of M81's spiral arms can be seen above and below the main disc in larger telescopes (a 6-inch telescope on a good rural night will show the arms using averted vision.)
M82 is an oddity – a starburst galaxy. Every few hundred million years or so, M82 approaches M81, and is torn and rearranged by the gravitational forces of M81. In the process, massive gas and dust clouds are formed and concentrated, followed by a rapid formation of many stars known as a starburst. Radio telescopes reveal ionized hydrogen bridges between the two galaxies testifying to their encounter about 200 million years ago. Quite a bit smaller than M81, it appears as a dim fat cigar about 8-10 minutes of arc in length on a good night.
Some of the object suggested for observation here are more difficult than those on our Christmas list, but are well worth the effort. It is a good idea, whenever possible, to find an object using star charts rather than the Go-To capacity offered by many modern scopes, as that way you will learn the skies, instead of just the objects. Finally, if there is one critically important tip, it is to always use clean optics. If you don't know how to safely clean telescope and binocular objectives and eyepieces, many internet sites will provide instructions (my particular favorite is by Company Seven.
A good general source for further investigating these objects is Deep-Sky.
Best of luck, and happy viewing!