Sky Tour - Under the Dipper's HandleBy: Brian Ventrudo May 10, 2017
The stars in and around the Big Dipper serve as excellent guides to many sights in the deep sky. On this part of the celestial sphere, far above the plane of the Milky Way galaxy, there are few star clusters and nebulae. But there are galaxies, dozens of them, accessible with a small telescope along with a handful of interesting foreground stars. In this sky tour, let's take a look at some deep-sky highlights under the handle of the Big Dipper.
Let's begin with the most famous galaxy in the area south and west of the Dipper's handle, the Whirlpool Galaxy, Messier 51, which lies in the constellation Canes Venatici. This showpiece object is just 3° southwest of the star Alkaid at the tip of the handle. An elegant face-on spiral, M51 is about 1/3 the diameter of our own galaxy and lies at a distance of about 23 million light years. And it's not alone: the galaxy interacts with a much smaller neighbor NGC 5195, and the interaction has triggered an intense round of star formation, especially in the rich spiral arms of M51. In images of the galaxy, you can see evidence of this activity in the numerous pink emission nebulae and blue-white clots of new stars. Images also show a bridge of stars that appears to connect the two galaxies.
As with most spirals, the core of M51 outshines the fainter spiral arms. It's the core you can see in a pair of binoculars or a telescope. In light-polluted skies, you will only see the core, no matter how big your telescope, because the fainter spiral arms are overwhelmed by the artificially brightened sky. In a 3-inch telescope or larger, the core of NGC 5195 is also visible.
Urban stargazers often look to M51, see a couple of dim smudges, and wonder what all the fuss is about. But in dark sky, with a careful gaze and averted vision, the galaxy is transformed into a glorious (but still dim) spiral swirl in an 8" or larger telescope. Keen-eyed observers can see spiral structure with smaller scopes in dry and very dark sky.
Look south and west of the Whirlpool galaxy to see the two brightest stars under the handle of the Big Dipper, the 3rd-magnitude star CorCaroli and 4th-magnitude Chara in the constellation Canes Venatici. CorCaroli (the "Heart of Charles") was named by Edmund Halley after the martyred English King Charles I. It is a pretty double star, easily split in a small telescope even at 30-40x. The blue-white primary shines at magnitude 2.9; the fainter yellow companion is magnitude 5.6 and lies at an apparent distance of 19" from the primary. The pair is about 110 light years away.
The brighter of the two components of CorCaroli is formally called α (alpha) Canes Venaticorum. It's an unusual type of variable star with peculiar concentrations of silicon, mercury, and europium in its atmosphere. It also has an intense magnetic field, more than 5,000x stronger than Earth's, that creates enormous "starspots", similar to our Sun's sunspots. The starspots move in and out of view as the star rotates causing variability in overall brightness.
Some 6° northwest of CorCaroli, you'll find the unmistakable dull-red glow of the cool red-giant star Y Canes Venaticorum. You'll need a pair of binoculars or a finderscope to see it: it's just beyond the reach of the unaided eye. Also called "La Superba", this cool variable star has an atmosphere rich in carbon compounds that block and scatter all but red and infrared light from shining through. This makes the star far redder than nearly any other star in the sky. It's one of the most famous examples of a carbon star.
You can see La Superba in binoculars, but more aperture helps you see its color; nearly any telescope will show the deep red color of this star. Try defocusing your telescope slightly to stimulate the color-detecting cone cells in your eye.
The carbon in the atmosphere of La Superba was likely produced by nuclear fusion of helium in the star's core. Deep zones of convection dredged up the carbon into the atmosphere. The star is only about three times the mass of the Sun, but it has swollen to an enormous size. La Superba has begun to eject its outer layers and appears ready to form a new planetary nebula in the next several thousand years.
The star lies at a distance of about 1,000 light years. See an image of La Superba at this link.
CorCaroli lies at the right-angle of an isosceles triangle with itself and Chara at two vertices, and the spiral galaxy M63 at the third vertex. The galaxy is clearly oval-shaped in even the smallest telescope. A 4" scope at 60-80x shows the bright galactic nucleus of M63 surrounded by an unresolved oval running southeast to northwest. An inner oval may also be apparent. This is no illusion: M63 is known to have tight and orderly inner spiral arms, while the outer arms are looser and haphazard, as if they are somehow escaping the gravitational pull of the inner galaxy. The outer stars are in fact flying fast enough to escape the galaxy if it were not for the invisible presence of dark matter to hold it all together.
In very dark sky, some hints of the outer spiral arms become visible in a 6" or larger scope. But it takes effort and patience to see them. Use averted vision to get brief glimpses, then, if you are inclined, record what you see in a simple sketch. In time, your sketch will build up into a fuller picture of the galaxy's structure.
M63 spans about 60,000 light years and lies 37 million light years from Earth.
Look 1.5° northeast of the midpoint between CorCaroli and Chara. There you'll find M94, another face-on spiral that in long-exposure photographs looks like the clouds of a hurricane seen from above. M94 recently (10 million years ago) experienced a violent event that ejected millions of solar masses of material from its nucleus.
In a small scope, M94 reveals a bright nucleus and diffuse circular disk. As with most galaxies, more aperture and magnification help, as does clear dark sky. With a scope larger than 6-8 inches, in perfect sky, you may see a faint ring around M94. This is a region of strong star formation. It gives the galaxy the appearance of a bone-white eye staring back at you.
Article and photos © Agena AstroProducts, 2017. Reproduction without permission prohibited.
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