Catch Cepheus, King of the North Pole while the Moon Moves Post-Midnight!

This image by Michael Watson of Toronto shows the beautiful and large Elephant Trunk Nebula, and the massive, red star Mu Cephei, aka Herschel’s Garnet Star, on its northern edge. The area of sky shown here covers about six by 12 times the diameter of the full moon, with north (towards Polaris) on the left. When enlarged, the dark dust in the core of the nebula resembles an elephant’s trunk. Michael’s gallery of fantastic astrophotos is here.

Hello, September Stargazers!

Here are your Astronomy Skylights for the week of September 6th, 2020 by Chris Vaughan. Feel free to pass this along to your friends and send me your comments, questions, and suggested topics. You can also follow me on Twitter as @astrogeoguy! Unless otherwise noted, all times are expressed in Eastern Time. To subscribe to these emails please click this MailChimp link.

I can bring my Digital Starlab portable inflatable planetarium to your school or other daytime or evening event, or teach a session online. Contact me through AstroGeo.ca, and we’ll tour the Universe, or the Earth’s interior, together!

The moon will be sliding sunward in the post-midnight sky this week, leaving evenings all around the world nice and dark for seeing the sights. Taking advantage of that, I’ve prepared a tour of Cepheus, the King of the North Pole, for you. Meanwhile, Jupiter and Saturn – and later, Mars – still gleam in the evening sky, and Venus does the same in the east before dawn. Here are your Skylights!

The Moon

The moon will be out of the evening sky all over the world this week – perfect for serious night-time stargazing. When it rises tonight (Sunday) at 10 pm local time, it will sit near Uranus – above the stars that form the head of Cetus (the Whale). Then the moon will linger into the morning daytime sky. On each subsequent night this week the moon will rise later and wane in phase – crossing the stars of Taurus (the Bull) from Monday to Thursday, and Gemini (the Twins) on the coming weekend.

On Thursday at 5:26 am EDT (or 9:26 Greenwich Mean Time) the moon will officially pass its last quarter phase, when it will rise after midnight and shine in the daytime sky until Friday afternoon. At last quarter, the moon is half-illuminated on its western side, towards the pre-dawn sun.

High in the southeastern pre-dawn sky on Friday morning, the waning moon will pass very close to the medium-bright star designated 1 Geminorum, which marks the toe-tip of Castor. That same morning, the moon will be positioned just a finger’s width to the right of a large, open star cluster in Gemini known as Messier 35 or the Shoe-Buckle Cluster. To see the cluster’s stars more easily, hide the bright moon just beyond the right edge of your binoculars’ field of view.

The Planets

Speedy little Mercury will be in the west-northwestern post-sunset sky this week – but the low angle of the evening ecliptic will keep it very close to the horizon for everyone living at mid-northern latitudes. If you’re game to look for it, a short viewing window starts at 8 pm local time. If you live near the equator, or in the Southern Hemisphere, Mercury will put on a great show for you during September!

Once the sky darkens after sunset, very bright, white Jupiter will pop into view low in the southeastern sky. Once the sky darkens more, dimmer, yellowish Saturn will appear, too – less than a fist’s diameter to Jupiter’s left (east). Good binoculars will reveal Jupiter’s four large Galilean moons named Io, Europa, Ganymede, and Callisto as they dance around the planet from night to night. Even a modest-sized telescope will show Jupiter’s brown equatorial belts and the famous Great Red Spot (or GRS, for short) – if the air is steady. Due to Jupiter’s 10-hour period of rotation, the GRS appears every second or third night from any given location on Earth. In the Eastern Time zone, the Great Red Spot will be crossing the planet’s disk after dusk on Tuesday and Thursday. It will also be visible starting in late evening on Monday, Wednesday, and Saturday.

From time to time, the round, black shadows cast onto Jupiter by its Galilean moons are visible in amateur telescopes as they cross (or transit) the planet’s disk for a few hours. From dusk until 9:08 pm EDT on Tuesday evening, Io’s shadow will be traveling across Jupiter. Europa’s shadow will do the same from dusk to 10:17 pm EDT on Friday evening.

Saturn is a spectacular sight in backyard telescopes. Even a small backyard telescope will show Saturn’s rings. They’ll be getting narrower every year – until they vanish for a few weeks during the spring of 2025. In the telescope, the rings are almost as almost as wide as Jupiter’s disk. See if you can see the Cassini Division. It’s the narrow, dark gap that separates Saturn’s main inner ring from its outer one.

A small telescope will also show several of Saturn’s moons – especially its largest, brightest moon, Titan! Because Saturn’s axis of rotation is tipped about 27° from vertical (a bit more than Earth’s axis), we are seeing the top surface of its rings – and its moons can arrange themselves above, below, or to either side of the planet. During evening this week, Titan will migrate counter-clockwise around Saturn, moving from the lower left of Saturn tonight (Sunday) to the right of the planet next Sunday. (Remember that your telescope might flip the view around.)

Reddish Mars will steadily increase in disk size and brightness this month. Earth will close the distance to it until October 6. Mars will put on a great sky show this month – but we don’t have to wait until then for good views of it! I’ve already seen its southern polar cap quite clearly in my small telescope. This week, the Red Planet will be rising in the east before 9:30 pm in your local time zone. Then it will cross the sky until dawn, when it will be positioned about five fist diameters above the southwestern horizon. Nothing in the southern sky is as bright, nor as red! Mars will start a retrograde loop on Wednesday that will last through opposition in October, and end in mid-November. This week, Mars will sit within the narrow “V” of modest stars at the bottom of the constellation of Pisces (the Fishes).

Mars recently passed its northern solstice – the beginning of winter in Mars’ Northern Hemisphere – so its South Pole is now tilted towards the sun (and us). Viewed in amateur telescopes from our vantage point on Earth, Mars’ southern polar cap will appear as a bright, white spot on the red planet (although your telescope’s optics may flip Mars over).

This week’s moonless sky will be ideal for looking for the Ice Giant planets. Blue-green Uranus will rise at about 9:30 pm local time and sit a generous fist’s diameter to the lower left (or 13° to the celestial east) of much brighter Mars, in southern Aries (the Ram). Meanwhile, dim and distant Neptune is located among the stars of eastern Aquarius (the Water-Bearer) – almost four fist diameters to the right (or 38° to the celestial west) of Mars. This week, Neptune will already be in the lower southeastern sky after dusk. Then it will climb higher until about 1:30 am local time, when you’ll get your clearest view of it while it’s halfway up the southern sky.

Extremely bright Venus will rise in the east-northeast at about 3 am local time this week, and then remain visible until sunrise as it is carried higher in the eastern sky by the rotation of the Earth. Viewed in a backyard telescope, Venus will show a gibbous, half-illuminated shape. The planet will be traveling east through the modest stars of Cancer (the Crab). By the end of this week, the bright planet will sit a few finger widths to the lower right (or 2.5 degrees to the celestial south) of the big open star cluster in central Cancer known as Messier 44 or the Beehive. Watch for the lovely waning crescent moon sitting a fist’s diameter above (or 11 degrees to the celestial west) of Venus on Sunday, September 13.

Venus is shifting towards the sun – but the later sunrises at this time of year will let it shine in a dark, pre-dawn sky until early December! And while you’re up early, enjoy a view of the bright star Sirius and Orion (the Hunter) sitting well off to Venus’ right.

Cepheus the King

For observers living at mid-northern latitudes around the world, the constellations that circle the north celestial pole stay above the horizon at all times – day and night. But that doesn’t mean they are always visible. The circumpolar Big Dipper is part of the large constellation Ursa Major (the Big Bear). During evening in autumn every year, Ursa Major sits so low over the northwestern horizon that the stars of the famous dipper are often hidden behind buildings or trees. Six months later, that constellation is nearly overhead during evening – the best position for exploring its deep sky treats.

From September to December, the circumpolar constellation of Cepheus (the King) is perfectly positioned for viewing, nice and high in the northern sky during evening – and it contains some fascinating sights because it’s close to the Milky Way. Let’s explore the king of the North Pole!

In Greek mythology, Cepheus was the king of ancient Ethiopia. He was married to Queen Cassiopeia. Their daughter Andromeda was the princess who was rescued by the hero Perseus, astride his winged horse Pegasus. Each of those characters is represented by a constellation in the northeastern sky during autumn. I’ll relate their tale in a future Skylights. In Arabic tradition, Cepheus was named Al-Multaheb. In ancient China, the stars of Cepheus were divided amongst the Black Tortoise of the North (北方玄武, Běi Fāng Xuán Wǔ) and the Three Enclosures (三垣, Sān Yuán), part of the Heavenly Purple Palace that occupies the region around Polaris. Other ancient cultures incorporated Cepheus’ brightest star, Alderamin, into their own star pictures.

For those of us who have been using the three bright stars of the Summer Triangle to navigate the sky, Cepheus is easy to find. Just face southwest and extend the line joining Altair to Deneb upward by about two fist diameters. I prefer to view him by facing north – in which case he’ll be located about two fist diameters directly above Polaris at 10 pm local time.

In form, Cepheus is little more than a triangle affixed to one edge of a square – like a crooked house with a tall pointed roof. The peak of the roof mark the king’s feet, and the stars forming the base of the house mark his head and shoulders. Because he circles the pole, you might see him right-way up, upside-down, or sideways – depending on the time of year and time of night. This month, he’ll be head-up and feet-down during evening – but the “house shape” will be roof-down.

The brighter stars of Cepheus span or 21° (about two fist widths) of sky (from his toes to his crown) by 9° (the width of his hips and shoulders) – but the constellation’s territory is twice both of those measurements. It extends from Polaris southward – between Cassiopeia and Draco (the Dragon) – to the borders of Cygnus (the Swan) and Lacerta (the Lizard). Viewed while facing north, the bright, W-shaped constellation Cassiopeia is to the king’s right (celestial west), the curved neck of the dragon is on his left (east). Bright Deneb is high over his left shoulder, and the lizard is creeping downwards towards his right shoulder – although he seems to fending it off with an upraised staff. Some star maps connect a pair of stars to his left shoulder – as if his arm is extended.

Cepheus’ brightest star is magnitude 2.45 Alderamin, from the Arabic expression for “the Right Arm”. (Since he’s facing you, his right is on your left.) Bright enough to see even in light-polluted skies, Alderamin is a white, A-class star located about 49 light-years from our solar system. It’s somewhat hotter than our sun and about twice the size. It also appears to have a very high rotation rate – 12 hours versus our sun’s 27 days!

Eta (η) Cephei, the medium-bright star sitting about four finger widths to the left of Alderamin, marks the king’s elbow. A dimmer star designated as Theta (θ) Cephei sits two finger widths to the lower left of Eta. It marks the king’s right hand.

Starting back at Alderamin, look a bit less than a fist’s diameter below, and slightly to the right of it, for a moderately bright star named Alfirk (“the Flock”) or Beta (β) Cephei. Alfirk, which marks the king’s hip, is a pulsating variable star located 690 light-years away from us! This hot, blue-white star is more than ten times the mass of our sun, and has at least two companion stars that are too close-in to see visually. The star varies slightly in brightness every 4 hours and 34 minutes, due to the effects of iron enrichment in the star’s interior.

Errai (“the Shepherd”), also designated Gamma (γ) Cephei, sits a generous fist diameter to the lower right of Alfirk – at the “peak of the house’s roof”. It’s an orange, K-class star that shines as brightly as Alfirk – although it is only 44 light-years away from us. This star, which marks the king’s feet, is a binary star system that includes a faint, red dwarf star. In 1988 Canadian astronomers detected the signal of an extra-solar planet around Errai. It would have been the first one confirmed – but their data wasn’t definitive enough. That honour went to the star 51 Pegasi a few years later. Newer, better data confirmed Errai’s planet in 2002, now named Tadmor – the ancient Semitic name for the city of Palmyra in Syria.

The star Iota (i) Cephei completes the triangle in lower Cepheus. That orange-tinted, K-class star shines about as brightly as Errai below it and Alfirk to its left. A fist’s diameter above Iota, and a generous palm’s width to the upper right of Alderamin, is Zeta (ζ) Cephei. It’s another orange giant star that shines about the same brightness as the other corner stars, magnitude 3.35.

A little star named Epsilon (ε) Cephei sits just a finger’s width above Zeta. It shines at a modest magnitude 4.2, about half as bright as Zeta. But the really interesting star is the one located about two finger widths to Zeta’s lower right. That’s Delta (δ) Cephei. Delta Cephei varies in brightness by more than a factor of two every 5 days and 9 hours. At its peak, it’s as bright as Zeta. At its minimum, it’s as bright as Epsilon. So you can tell at a glance which part of its cycle it’s in!

Delta Cephei is a giant, pulsating supergiant star located about 900 light-years from our sun. Its variability occurs because it has consumed its core hydrogen, and it is entering old age. There are many of these stars in the sky – including Polaris! They are now known as Cepheid variable stars. In 1912, Henrietta Swan Leavett of Harvard discovered that the time they take to cycle in brightness (their period) relates directly to their maximum brightness. Knowing that, she and other astronomers have been able to use them as “Standard Candles” to measure distances in the Universe, as follows: Find a Cepheid-type variable, measure its period, calculate its expected maximum brightness, and compare how bright it is with how bright it looks. (It’s similar to estimating how far away a motorcycle is at night by how bright its headlight appears.) Cepheid variables have been used to estimate the size of our Milky Way, and to work out the distances of other galaxies, like the Andromeda Galaxy.

Our last stop in the tour of Cepheus’ stars is Mu (μ) Cephei, also known as Herschel’s Garnet Star. It’s one of my favorite objects. It’s located midway between Alderamin and Zeta Cephei, and just above the line connecting those two stars. You can see it with unaided eyes in a dark sky. And that’s amazing because this star is a whopping 2400 light-years away from us! (Actually, we’re not that sure about the distance – it could be farther.)

Mu Cephei is visible at such a distance because it is huge – about 1400 times the size of our sun. If it traded places with our star, all the planets out to Jupiter would be inside the star! What makes it truly amazing to look at is the colour. It’s among the reddest bright stars in the sky. It’s an M-class star nearing the end of its life – similar to the much closer-to-Earth Betelgeuse in Orion (the Hunter) – and will one day explode in a supernova.

I’ll devote future story to the deep sky objects in Cepheus – but you will certainly see rich star fields, star clusters, and nebulas if you sweep the constellation with binoculars or a backyard telescope. For example, Mu Cephei is on the northern edge of a great cloud of pink hydrogen gas known as the Elephant’s Trunk Nebula. It’s four times the width of the full moon, and it will extend upward from Mu Cephei during September evenings. Other smaller nebulas include the Wizard Nebula (NGC 7380) near Delta Cephei, and the Iris Nebula (NGC 7023) near Alfirk. There are open star clusters between Zeta and Delta (NGC 7261), two finger widths to the upper left of Eta Cephei (NGC 6939), and in the centre of the large square, near the king’s heart (NGC 7160).

As a final thought – Cepheus is truly the king of the North Pole. Errai will replace Polaris as Earth’s pole star around 4000 AD. Alfirk will take that honour around 6500 AD. And then Alderamin will take over in 7500 AD! Let me know how your visit with the king goes!

Public Astro-Themed Events

Every Monday evening, York University’s Allan I. Carswell Observatory runs an online star party – broadcasting views from four telescopes/cameras, answering viewer questions, and taking requests! Details are here. Their in-person Wednesday night viewing has been converted to online via the observatory Youtube channel, where they offer free online viewing through their rooftop telescopes, including their new 1-metre telescope! Details are here.

My Insider’s Guide to the Galaxy webcasts with Jenna Hinds of RASC National will return on Tuesday, September 8, when we’ll cover astronomy for beginners – how the stars and planets move, what star magnitude means, how to read a sky chart, and more. And we’ll give you a peek at the best sights to see the fall. Details and the schedule are here. In the meantime, join Jenna and John Reid on alternate Thursdays at 3:30 pm EDT as they run through the RASC’s Explore the Universe certificate.

On Saturday, September 12, from 8 to 11 pm, University of Toronto’s Dunlap Institute will hold their free, giant annual star party – online! This year, the focus will be on planets. Astronomers will be live streaming views through their telescopes, and panelists (including me) will cover indigenous astronomy, Mars exploration, seeing satellites, and much more. And there will be prizes! Details are here.

The Canadian organization Discover the Universe is offering astronomy broadcasts via their website here, and their YouTube channel here.

On many evenings, the University of Toronto’s Dunlap Institute is delivering live broadcasts. The streams can be watched live, or later on their YouTube channel here.

The Perimeter Institute in Waterloo, Ontario has a library of videos from their past public lectures. Their Lectures on Demand page is here.

Keep looking up, and enjoy the sky when you do. I love questions and requests. Send me some!