The T Cor Bor Watch continues. Have you been checking Corona Borealis overhead these clear evenings? The recurrent nova T Coronae Borealis could erupt to 2nd magnitude anytime this summer. Or fall. Or later? Astronomers are pretty sure it's preparing to blow sometime fairly soon for the first time since 1946.
What's the exact spot to watch?
Look a third of the way from Arcturus to Vega. There's Alpha Coronae Borealis, also known as Alphecca. At magnitude 2.2 it's the only moderately bright star in the delicate Northern Crown. Alphecca is easy to see through my suburban light pollution. The rest of Corona Borealis is not.
Keep an eye out a third of the way from Arcturus to Vega. Arcturus shines in the west these evenings; Vega is nearly overhead. So, turn this view about 90° clockwise to match the current evening panorama.
Is Alphecca alone? One of these days, it won't be!
The point in Corona Borealis to examine. Again, turn this 90° clockwise.
In 1866 and 1946 T Cor Bor peaked at 2nd or 3rd magnitude, roughly matching Alphecca.
Even now, as it simmers along near its normal 10th magnitude, T CrB is a pretty easy pickup using Bob's charts with a small telescope. Give it a look while it's still gathering its forces.
T Cor Bor's rise last time took just a few hours, and its peak brightness lasted only a day or so.
But for your best chance of catching it early, just look. Every night you can.
SATURDAY, AUGUST 31
■ Late these evenings as autumn approaches, Fomalhaut, the Autumn Star, makes its appearance above the southeast horizon. Its rising time will depend on where you live. Watch for it to come up two fists to the lower right of first-magnitude Saturn. By 10 or 11 p.m. you should have no trouble spotting Fomalhaut low in the southeast if you have an open view in that direction.
■ As dawn brightens on Sunday morning September 1st, look east-northeast for the hairline crescent Moon with Mercury 4° to its right, as plotted below. Binoculars will help in the brightening sky.
The thin waning crescent Moon guides the way to Mercury low in bright dawn on the mornings of August 31st and September 1st. Bring binoculars.
SUNDAY, SEPTEMBER 1
■ Look for bright Vega passing the zenith as twilight fades away, if you live in the world's mid-northern latitudes. Vega goes right through your zenith if you're at latitude 39° north (near Baltimore, Kansas City, Lake Tahoe, Sendai, Beijing, Ankara, Athens, Lisbon).
■ Another sign of the advancing season: Cassiopeia is now high in the northeast after dark, its W pattern tilting up. And below it, starry Perseus is reaching upward.
The highest part of Perseus includes the wintry Double Cluster. To find it, look back to Cassiopeia. Counting down from the top, note the third segment of the W. Continue that segment downward by twice its length, and there you are.
You're looking for what seems like a small spot of enhanced Milky Way glow. Binoculars or a finderscope will help you detect the Double Cluster even through a fair amount of light pollution. The pair are a glory in a telescope.
MONDAY, SEPTEMBER 2
■ As September begins to take hold and nights grow chilly, the Great Square of Pegasus displays itself in the east, balancing on one corner. Its stars are only 2nd and 3rd magnitude. Your fist at arm's length fits inside it.
From the Square's left corner, leftward or lower-leftward extends the backbone of the constellation Andromeda: three stars in a slightly curving line (including the corner) that are about as bright as those forming the Square.
This whole giant pattern was named "the Andromegasus Dipper" by the late Sky & Telescope columnist George Lovi. It's shaped sort of like a giant Little Dipper with an extra-big bowl. It's currently lifting its contents upward.
The actual Little Dipper, meanwhile, is tipping over leftward in the north. It's only 40% as long as the Andromegasus Dipper, and most of it is much fainter. As always, you'll find that it's oriented more than 90° counterclockwise compared to Andromegasus. It's dumping its contents out.
■ New Moon today (exact at 9:36 p.m. EDT).
TUESDAY, SEPTEMBER 3
■ The coming of September also means that Scorpius, the proudly starring highlight of the southern sky during July, is now tilting over and lying down in the southwest after dark, preparing to bed down and drift off for the season.
WEDNESDAY, SEPTEMBER 4
■ Find little Sagitta the Arrow barely a fist at arm's length above bright Altair high in the south these evenings. If your light pollution is too much, use binoculars.
Now imagine rotating the Arrow on its point a third of a turn counterclockwise. Its middle star would now rest just barely (0.4°) below M27, the big Dumbbell Nebula. At a total magnitude of 7½, the Dumbbell is a largish but subtle gray glow nearly 0.1° wide, easily seen in binoculars or a finderscope under a dark sky. In a 4- to 8-inch telescope it's a rectangle or hourglass. It's the brightest planetary nebula in the sky if you sum up all its spread-out light.
The Dumbbell Nebula, M27. This view is 0.9° wide, about the width of a 60-power field of view in a typical telescope eyepiece. North is up, east is left. The star 14 Vulpeculae is magnitude 5.6.
The star HD 189733, magnitude 7.7, is a yellow-orange K dwarf 63 light-years away from us, notable for having a hot-Jupiter exoplanet very closely orbiting it. The nebula is far in the background: about 1,360 light-years away.
By the way, the odd name Dumbbell Nebula was bestowed by John Herschel in 1828. He was referring to the exercise weights we still call dumbbells, but in his day they were sometimes made by connecting two heavy bells top-to-top by a short, thick rod. The bells were minus their clappers, so they were dumb bells. I saw an example once in a vintage image from a gym. It really did look like the nebula's hourglass shape.
A much more recent name for M27, more accurate to modern eyes, is the Applecore Nebula. The earliest use of this name that I find in print using Google Books is from 1997.
THURSDAY, SEPTEMBER 5
■ Starting about 15 or 20 minutes after sunset, look very low a little left of west for Venus and the two-day-old crescent Moon, as shown below. They're roughly 7° apart at dusk for North America. Binoculars help bring them out through bright twilight. They set before twilight ends.
As the afterglow of sunset starts to fade early Thursday evening, try for the thin crescent Moon with Venus to its right very low in the west-southwest by west.
And with binoculars, can you still make out Spica?
FRIDAY, SEPTEMBER 6
■ Now the thickening crescent Moon shines about 18° (nearly two fists) left or upper left of Venus in bright twilight. And Spica is now partway between them, as shown above.
■ Then several hours later as dawn in getting under way on Saturday the 7th, Mercury poses low over much fainter Regulus, as shown below.
You can catch Mercury pretty easily now, though it's still low as dawn brightens. Much harder is Regulus below it. This morning they're 2½° apart and closing fast. On Monday morning the 9th they'll be in conjunction, 0.5° apart.
SATURDAY, SEPTEMBER 7
■ A winter preview: Step out before the first light of dawn this week, and the sky displays the same starry panorama it will at dinnertime around New Year's. Orion is striding up in the southeast, with Aldebaran and then the Pleiades high above it. Sirius sparkles far down below Orion. The Gemini twins are lying on their sides well up in the east.
■ Mercury-Spica conjunction: Then stay up until dawn is well under way, and with an open view low to the east-northeast by east, catch Mercury on the rise. Use optical aid to try to pick up Spica now in conjunction with it, 0.5° to Mercury's right (for the time of dawn in North America). They'll both fit in the same low-power field of a small or medium-size telescope.
SUNDAY, SEPTEMBER 8
■ At nightfall, the crescent Moon shines low in the southwest. Look almost two fists upper left of it for orange Antares.
Two thirds of the way from the Moon to Antares is the near-vertical row of three stars marking the head of Scorpius. The brightest of these three is the middle one, Delta Sco. It's not all that much fainter than Antares.
THIS WEEK'S PLANET ROUNDUP
Mercury is up in pretty good view during early dawn. Watch it brighten fast this week, from fairly faint to obvious. Look low in the east-northeast starting about 50 or 60 minutes before sunrise. On the morning of August 31st Mercury is still only magnitude +0.8, but by just a week later, September 7th, it's mag –0.7. That's four times as bright! 1
Venus, magnitude –3.8, is very low above the west horizon in bright twilight. It's becoming very slightly more visible week by week. Look for it just left of due west starting 20 or 30 minutes after sunset. Much will depend on the clarity of your air. Humid means hazy.
Mars and Jupiter (magnitudes +0.7 and –2.3, respectively, in Taurus) continue pulling apart from each other in the early-morning sky. Watch for bright Jupiter to rise in the east-northeast around midnight daylight-saving time. Mars, much fainter, follows nearly an hour later, glowing to Jupiter's lower left. On Saturday morning August 31st, Jupiter and Mars are 8° apart. By a week later, the 31st, they've widened to 12° apart and Mars has edged into Gemini.
Mars forms a triangle with the horntips of Taurus, Beta and Zeta Tauri. Watch the triangle change shape daily as Mars moves eastward against the stars.
By the start of dawn each morning the two planets are very high toward the east-southeast, with Mars still to Jupiter's lower left. Mars-like Aldebaran shines to Jupiter's right or upper right. Week by week, Mars is starting to outshine Aldebaran.
Jupiter on August 20th. South here is up. The darkest piece of the belts runs narrowly to the celestial west ("preceding") from the Red Spot Hollow.
Saturn (magnitude +0.6, south of the Circlet of Pisces) is at opposition this week: on the night of September 7th. Look for it glowing in the east-southeast as soon as the stars begin to come out. It's lower right of the Great Square of Pegasus, which is balancing on one corner. The Square's upper-right edge points diagonally down toward Saturn, two fists at arm's length away.
Saturn climbs higher through the evening. It shines highest in south after midnight, through less of our blurry atmosphere for better telescopic resolution.
We see Saturn's rings nearly edge-on this year. They cast their black shadow southward (upward here) onto the globe.
Uranus (magnitude 5.7, in western Taurus) is some 20° west of Jupiter in the early morning hours. You'll need a good finder chart to identify it among surrounding faint stars.
Neptune (tougher at magnitude 7.8, near the Circlet of Pisces) is 12° east of Saturn.