The spring sky gradually livens up as we move closer to the intense planetary activity coming this summer. Mars crosses paths with Saturn in the early morning, Venus returns to its perch in the evening sky, and Jupiter sails across the night from horizon to horizon. Nothing ever remains static in the heavens. Wait long enough and even the constellations change.
Mars continues across Sagittarius encountering various celestial objects including Saturn, which looks almost rooted to the spot in the constellation. This duo of planets appears in the early morning as two bright dots about 20 degrees above the southern horizon. On April 2, Mars even slips between the ringed planet and the globular cluster M22. The conjunction of the two planets, less than one and a half degrees apart, is impressive to the naked eye, but you need an optical instrument and a very dark sky to spot the third member of the group, a small ball of diffuse light less than a half degree to the south of Mars.
M22 (the 22nd object in Messier’s catalogue) is one of the closest and brightest globulars in the heavens. The some 150 globular clusters on the outskirts of our Galaxy each contain about a hundred thousand stars. These stars are among the oldest in the Universe and are held together by gravity in a sphere about a hundred light-years across.
Closer to home, on April 7, the third quarter of the Moon forms a lovely visual alignment with Mars and Saturn, passing less than two degrees from the ringed beauty.
Vivid Venus appears at nightfall throughout April. The planet starts the month rather low on the western horizon among the stars in Aries but quickly rises a few degrees in altitude later in the month and has some interesting encounters in the second half of April.
On April 17, the conjunction of Venus and the Moon at twilight creates a photogenic scene and a chance to admire a young lunar crescent just two days old. If you first spot Venus near the setting Sun, look about 5 degrees to the right, slightly closer to the horizon, to make out the thin crescent Moon.
As of April 20, Venus reaches the winter constellation Taurus, which is slowly sliding out of the night sky but not without offering one last tableau. Venus first approaches the Pleiades cluster (minimum distance of 3.5 degrees on the 25th) before slipping between this open cluster and the reddish star Aldebaran from April 26 to 28.
Unlike globulars, open clusters like the Pleiades are home to young stars that are still gravitationally bound following their common birth. About 115 million years old, the stars in this cluster will gradually disperse as result of the gravitational interactions driving their revolution around the centre of the Galaxy.
Also note that on April 18, the Moon encounters another open cluster, the Hyades. This time, our natural satellite even occults the red star Aldebaran (the Bull’s eye) for observers in Canada’s Far North.
All through the night
Jupiter rises earlier and earlier as we approach its opposition in May. In retrograde in the constellation Libra, the gas giant outshines all the stars and planets visible late at night in April.
Watch Jupiter rise on the southeastern horizon around 10 p.m. in early April, particularly on the evening of April 2 when the gibbous Moon appears just above the planet (and to its left on April 3). You can see the same configuration on April 29 at sunset (8 p.m.), except that then a beautiful full Moon rises just before Jupiter.
Just above our heads
As spring evenings grow milder, take a moment to admire a constellation found directly at the zenith this time of year: Ursa Major. Often taken for granted, the Great Bear is much more than just the Big Dipper asterism making up the animal’s hindquarters. Many galaxies and a few nebulae are found here, but practically no star cluster—other than Ursa Major itself!
By studying the movement of stars in this region of space, we know that dozens of stars, including five of the seven stars in the iconic dipper, move at the same velocity and in the same direction. Like all stars, these stars orbit the centre of our Galaxy, completing one revolution in about 250 million years. Yet the stars in this group move, in relation to the solar system, about 3 kilometres a second faster in this revolution, while approaching the centre of the Galaxy at a speed of 10 kilometres a second. This group is known as the Ursa Major Moving Group.
The movement of these stars, as well as the study of their chemical composition, leads us to believe they are siblings born together between 400 and 500 million years ago. Originally they were part of an open cluster like the Pleiades, but the cluster slowly dispersed.
You can try to visualize this movement the next time you observe Ursa Major. The two stars in the Big Dipper that aren’t part of the group are Alkaid (the left tip of the handle) and Dubhe (the upper right of the bowl). The other five stars, in the centre, drift slowly together toward the handle. Since interstellar space is mind-bogglingly vast, we won’t notice any change in the asterism for thousands of years. We still have plenty of time to admire our good old dipper.