We lift the Lion's paw to find a bright, red variable star, a germinating planetary nebula, galaxies rarely visited, and a diversity of doubles.
Just when I think I'll never get through all the deep-sky objects in the winter sky, April arrives with a gala of galaxies. The bounty never seems to end, not even after decades of skywatching.
Years ago, I thought I had to see as many seasonal deep-sky objects as possible before they slid into the sunset. Not anymore. Comets, supernovae, and exceptional conjunctions must be acted upon quickly and with laser focus, but deep sky? If I don't see a galaxy this season, it'll be back the next. This approach has allowed me to savor what I see more.
Amateurs enjoy visiting with old favorites at the start of a new season. Some of mine are the Needle Galaxy (NGC 4565), M3, M13, and the Whirlpool (M51). But no night is complete without leaving the familiar behind and exploring new territory. This week we'll start with the bright variable star R Leonis in western Leo and wander around to see what else we can find.
Perfectly placed for viewing from nightfall to the wee hours, R Leo was the first variable star observed by famous Ohio stargazer, author, and comet discover Leslie Peltier, and it remained his favorite. I loved his book Starlight Nights, so R Leo soon became a favorite of mine, too!
R Leo is a Mira-type variable star, a pulsating red giant with a period of 312 days. By good fortune, it's peaking right now around magnitude 5.4 and bright enough to see from a dark sky with the naked eye. I like binoculars or a telescope better, so I can relish its gorgeous orange-red color. R Leo is easy to find, set in a tiny triangle of stars just 5° due west of Regulus and ½° southeast of 18 Leonis. When faintest, this stellar ruby can dip as faint as magnitude 11. If you start now, you can watch it slowly fade as we approach summer.
Once you've finished observing R Leo, perhaps even taking a minute to estimate its brightness using this AAVSO chart, nudge your scope 1° back east to one of the easiest cataclysmic variable of the spring sky, X Leonis. Like R, it's very easy to find as it practically touches the 6.6-magnitude star 21 Leonis.
Normally next to impossible to see at magnitude 17, X Leo jumps back into view every few weeks, at or near magnitude 12, putting it within the range of a 6-inch telescope. The sudden eruptions or outbursts occur without warning and irregularly. X Leo's last blast took place in late February, so we're overdue for another. Catching an outburst is like playing a game of peek-a-boo. You'll drop by 10 different nights and see nothing, but then on the 11th, there it is!
X Leo is a member of the dwarf novae class, a binary where one member is a white dwarf surrounded by a whirlpool of glowing hydrogen gas called an accretion disk. The gas fueling the disk is siphoned from the close companion star. Changes in the rate of flow of material into the disk can cause it to suddenly burn much hotter and brighter and create the outburst we see in our telescopes. Outbursts last a few days to a few weeks until the star fades back to minimum. Here's a chart to help you follow its ups and downs.
Let's return now to 18 Leonis and wend our way westward 1.5° by Bow, Arrow, and Stick to arrive at a unique object with a one of the coolest (coldest?) names, the Frosty Leo Nebula. This tiny object, the brightest part of which measures about 12″ across, looks exactly like a magnitude 11 star in my 10-inch telescope at 76×. It lies 0.9′ south-southwest of a 12.7-magnitude star. But if you apply a magnification of 200× or higher, the object expands into a bright, dense knot of nebulosity visible even on moonlit nights.
In unsteady seeing little else is visible, but when the air calms and you jack up the power to 350× and higher, amazing details await the patient eye. In my 15-inch, Frosty reveals two brighter kernels of nebulosity separated by an evanescent dark lane (where the central star is hidden) and the northern of its two ansae. The ansae mark where bipolar outflows from the restless star plow into a circumstellar shell of previously released material. I saw the one as a delicate patch of haze north-northwest of the main nebula with averted vision.
When you slide this tiny treasure into your field of view, know that you're watching an aging red giant star in the process of shedding its outer atmosphere as it downsizes to a white dwarf.
Silicate dust within the nebula is abundantly coated in grains of pure, crystalline water ice, the reason for the "frosty" reference. Frosty Leo is a protoplanetary nebula and affords amateurs and professionals alike a rare glimpse of a key transition in a Sun-like star's life.
OK, time to rest the eyes. How about a few double stars? Two degrees west-southwest of Frosty Leo you'll run into Xi (ξ) Leonis, the first of four 5th- and 6th-magnitude stars arranged in a zigzag about 3° long. Three of them are doubles!
We'll pass Xi by and continue south to 6 Leonis, an easy pair of magnitudes 6 and 9 separated by 37″ in position angle (P.A.) 75° (secondary east-northeast of primary). I doubt you'll shout "Wow!" but I found the sight a pleasant one requiring no effort — much appreciated after working my averted vision muscles on old Frosty! Any scope will split it.
From here we continue south to Omega (ω) Leonis for some real binary excitement. This pair of stars, magnitudes 5.7 and 7.3, are so close together they seem to spark with electricity. The comprehensive Washington Double Star Catalog gives their 2016 separation as 0.9″ in P.A. 112° (east-southeast). In my 15-inch at 357× they were two tiny, jittery disks parted by the narrowest slice of sky — a positively wonderful sight. In good seeing, a 6-inch should part these two suns. Look now because their separation will shrink to just 0.5″ in the coming years. They were last so close in 1990. In 2018 they're physically farthest apart at 44.5 au, or just 5 au further than Pluto's distance from the Sun.
Our double-star soiree ends with 3 Leonis, located 50′ south-southeast of Omega. This is an easy double star as far as separation goes — 25″ in P.A. 80° (east-northeast) — but the large difference in brightness between the magnitude-6.0, red-hued primary star and its 10.5-magnitude secondary makes this a dramatic pair.
We're almost done with our walk through this nondescript yet rich cache of Leonian gems, but no one gets out of the lion's lair without grabbing a few galaxies. A few that might pique your interest are bunched in small groups near Xi (ξ) Leonis and 23 Leonis. Since they're off the beaten track you may never have made their acquaintance.
Just a degree north-northwest of 21 Leonis, you'll run into a several faint, fluffy patches, the brightest of which is the magnitude-12.6 barred spiral NGC 3020. I detected some mottling and a slightly brighter elongated center. Deep photos show numerous blue knots within the disk beyond the nucleus, no doubt regions of star formation. NGC 3020 is joined by the fainter, nearly edge-on spiral galaxy NGC 3024 (magnitude 13.0), NGC 3016 (magnitude 12.9), and NGC 3019 (magnitude 15.2). I saw all but the last in my 15-inch at a magnification of 142×. Although the four appear to be related, NGC 3020 and 3024 are much closer — about 65 million light-years — while the other two are some 415 million light-years distant. Try to imagine them at different depths in space when you dial them up.
For more faint fuzzies, point your scope ½° northwest of 6 Leonis for a gander at the trio of NGC 2911, a easy magnitude-11.5 lenticular galaxy that forms a skinny right triangle with fainter NGC 2914 (magnitude 13.2) and NGC 2919 (magnitude 12.9).
Our final stop is a compact threesome of NGC 2872-4, better known as Arp 307 from astronomer Halton Arp's Atlas of Peculiar Galaxies. At 64× the three members blended together into a hazy blob, but upping the magnification to 245× separated them into an attractive trio. The brightest at magnitude 11.9 is NGC 2872. It's paired closely with NGC 2874, a pretty spiral with a tilt similar to the Andromeda Galaxy, and NGC 2873, a much fainter object. NCG 2874 may make you do a double-take. With direct vision it's small, bright, and round, but averted vision reveals the faint outer disk, completely altering the galaxy's appearance.
There are many, many more galaxies in the region. Grab an atlas and use your new familiarity with this middle-of-nowhere locale as a waypoint to further deep sky explorations.