Falling Leaves And Autumn Skies

The autumn sky shows the transition from the summer Milky Way, through the “watery” constellations of autumn, to the bright stars of winter. Credit: Starry Night software.

Autumn is the favorite season for many skywatchers. You can get your last look at summer stars and, if you stay up late, your first look at winter stars. Best of all, it gets dark earlier and the night temperatures are still comfortable.

Our graphic this week shows a panorama of the sky looking south around 8 p.m., just after the sky becomes fully dark. After the change to standard time this weekend, this will be the view at around 7 p.m.

Looking towards the west, at the right in the graphic, you can see the familiar constellations of summer. Sagittarius and the core of the Milky Way Galaxy are setting in the southwest, while the summer triangle: Deneb, Vega, and Altair, shines overhead.

It’s not too late to revisit some of the popular summer objects: double stars Albireo and Epsilon Lyrae, the globular clusters in Hercules and Ophiuchus, the Ring Nebula in Lyra, and the bright nebulae and clusters of Sagittarius.

Looking south, the upside down triangle of Capricornus rides high. Its rightmost star, Algedi, is a naked-eye double. Above Capricornus, just to the left of Altair, is the tiny constellation Delphinus, the dolphin, one of the few constellations that actually looks like its name. It’s worth exploring the region between Altair and Albireo, where you will find two of the finest deep sky objects: Brocchi’s Cluster, popularly called “the coat hanger,” and the Dumbbell Nebula, one of the largest and brightest planetary nebulae.

Many of the constellations in the autumn sky have watery associations. These include Capricornus (the sea goat), Delphinus (the dolphin), Aquarius (the water bearer), Pisces (the fish, plural), Piscis Australis (the southern fish, singular), and Cetus (the whale). Most of these are lacking in bright stars, with the exception of Pisces Australis which contains the first magnitude star Fomalhaut, the first star to have one of its planets directly imaged by the Hubble Space Telescope.

Although Aquarius is dim in terms of stars, it contains a number of fine deep sky objects, including the globular cluster Messier 2, and two fine planetary nebulae, the small bright Saturn Nebula snd the huge Helix Nebula. The latter is probably the planetary nebula closest to the sun, about 700 light years distant, and as a result is very large in size, almost as large as the moon. Because of its large angular size, its light is spread out over a wide area, making it very hard to see. You will need a narrow band filter on your telescope to spot it. 

Off to the east, the Square of Pegasus dominates the sky. This consists of three stars in Pegasus with the fourth corner of the square being marked by Alpheratz, the brightest star in the constellation Andromeda, which trails away to the northeast.

Right in the upper left corner of the graphic are the two largest and brightest galaxies in our neighborhood, the Andromeda Galaxy and the Triangulum Galaxy. These are located symmetrically on either side of the second pair of stars eastward from Alpheratz in Andromeda.

The Andromeda Galaxy (to the north) is large and bright. If you live in a city, you will need binoculars to see it, but sharp-eyed observers in the country, including myself, have spotted it with their unaided eyes. The Triangulum Galaxy is almost as large, but nowhere near as bright as Andromeda. It is best seen in small binoculars. Oddly, it is very hard to see in the narrow field of a telescope because its dim light is spread across such a large area.

Finally, in the northeast you can see the first of the winter stars, the bright star Capella in Auriga and the brilliant Pleiades Cluster in Taurus. Soon Orion will arrive in the east, in the words of Robert Frost:

            You know Orion always comes up sideways.

            Throwing a leg up over our fence of mountains,

            And rising on his hands, he looks in on me…

 Stay up until midnight, and you will see him, too.

 

Asterisms and Constellations

On a recent warm and humid summer night, sky transparency was very poor. Only the brighter stars punched through the water-laden atmosphere but three stars were very prominent. They formed a triangular pattern aptly called the Summer Triangle.

The Summer Triangle begins to rise in the Spring.  As seen from mid-northern latitudes in mid-May near midnight.

The Summer Triangle is an example of an asterism: a group of stars that form a recognizable pattern or shape. The Big Dipper, the Little Dipper and the Great Square of Pegasus are other examples of asterisms.

Asterisms are often confused with constellations and indeed, in ancient times, constellations were mythological figures, animals or objects that were seen in groupings of stars.

The Big Dipper asterism as seen from mid-northern latitudes in mid-May at 10:00 p.m.

Almost everyone in North America is familiar with the Big Dipper which is part of the figure of the Big Bear, or the constellation of Ursa Major.

The Big Dipper asterism belongs to the constellation Ursa Major (Great Bear).

The modern constellation of Ursa Major includes all stars within an area defined by the International Astronomical Union in 1930. So the star 24 Ursae Majoris "belongs" to the constellation Ursa Major even though it is not part of the figure of the bear.

Modern constellation boundaries

Some asterisms such as the Big Dipper, the Sickle of Leo, the teapot of Sagittarius and the Great Square of Pegasus have been known for a long time. All are best appreciated when viewed without optical aid because of their large angular size.

But over the years, people using binoculars and telescopes have come across other striking asterisms and some of these have become well known to amateur astronomers.

Here are some examples.

The Diamond Ring

A tight group of 7th and 8th magnitude stars with Polaris as the "solitaire". Best seen with binoculars in a dark sky or a small telescope with a low power eyepiece showing about a 1° field.

The Coathanger
RA = 19h 25m, Dec = 20° 04'

A group of fifth and sixth magnitude stars in Vulpecula appearing like an upside down coathanger to northern hemisphere observers. Use binoculars for best views.

ET Cluster
RA = 1h 19 m, Dec = 58° 17.5'

This open cluster, also known as NGC 457, is located in Cassiopeia. With a bit of imagination you can make out the figure of ET. (Hint: the two bright stars are ET's eyes). Because of its small size, a telescope is needed to make out this asterism.

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The Lure of Variable Stars

I’m a sucker for action. I love change. My favorite planet is Jupiter because of its rapid rotation, ever-changing moons, and volatile cloud features. I love watching Near Earth Asteroids and comets as they move across star fields. Recently I’ve become addicted to watching solar flares and prominences in rapid action with my solar telescope. But most of all, I love to observe variable stars.

All stars vary in brightness to some degree. Even our Sun, which seems so stable, changes its brightness as more or less of its surface is obscured by sunspots. But there are stars in the sky that undergo vast changes in brightness and color. Many are highly unpredictable in their behavior, and need years of study to uncover the mechanisms that drive them.

The Variable Zoo

The most famous are the novas and supernovas which suddenly shoot up from obscurity to prominence. Supernovas are relatively rare in our neighborhood. The last one was over 400 years ago in 1604. Novas are more common, several being observable in any given year.

Some stars appear to vary for purely mechanical reasons. These are called eclipsing binaries: two stars in a close orbit where one star eclipses the other, as regular as clockwork. Algol in the constellation of Perseus is a famous example of an eclipsing binary.

Other stars expand and contract slowly because of processes going on within them. The most common of these “pulsating variables” are long period variable stars like Mira in the constellation Cetus. Mira is larger in diameter than the orbit of Mars, and changes size, brightness, and color over a period of just under a year. It ranges over nearly six magnitudes in brightness, meaning that at its brightest, it is a hundred times brighter than when it’s at its dimmest. Another group of pulsating variables is called the Cepheids, named after the star Delta Cephei. These have much shorter periods than the Miras, ranging from 1 to 70 days, and their period is closely tied to their luminosity, which has led to their use as measuring sticks to determine the distance of globular clusters and galaxies.

Another group of variable stars is called “cataclysmic variables.” These include novas, supernovas, and the so-called “dwarf novas.” These last are the stars that interest me the most because they show the most action. My favorite is SS Cygni (TCY 3196-723-1), located close to the open cluster Messier 39. This star normally sits around twelfth magnitude, just visible in a small telescope, but every few weeks it shoots up unpredictably to about eighth magnitude. If you’re lucky enough to catch it in outburst, you can actually see it get visibly brighter. Stars like SS Cygni are actually close double stars consisting of a red dwarf and a white dwarf. The white dwarf is surrounded by a disk of gas stolen from the red dwarf which is drawn down into the white dwarf where it ignites, causing the sudden outburst in brightness.

 

Observing Variable Stars

Professional astronomers realized over a century ago that there were more variable stars in need of study than they could handle, so they enlisted the aid of amateur astronomers to monitor the brightness of a number of stars well suited to amateur observation: stars which changed in magnitude over a wide range and which took a long period to complete their cycle of brightness. For many years this work required no more than a telescope and a good set of charts, and such simple visual observations are still useful today, although nowadays amateurs have access to photoelectric photometers and CCD cameras which are capable of studying just about any star. The American Association of Variable Star Observers acts as a central clearing house for all sorts of amateur variable star observations, providing instruction, charts, and other support, and giving amateurs a simple online system for recording their observations.

Why observe variable stars? Mainly because it’s fun! You never know from night to night what you are going to find…remember what I said about action? No special equipment is needed other than a set of star charts which plot the variable star and give the brightness of non-variable stars around it, which are used to estimate the brightness of the variable.

If you are a deep sky observer, you already have one of essential skills of a variable star observer: you know how to locate objects in the sky. It doesn’t matter how you do it. I used traditional starhopping for several years, but now I use my Orion SkyQuest XT6’s IntelliScope setting circles to locate my variables. Once you’ve located the variable, you estimate its brightness as compared to other stars on the chart, and record the time of the observation. With a little practice you can make estimates to within a tenth of a magnitude. You can then log onto the AAVSO’s web site and enter your observation. Within ten minutes it will be moved into their database of over ten million observations, and you can see your observation on a light curve along with those of hundreds of other observers around the world. What could be neater?!

Unlike most of the observations amateur astronomers make, variable star observations have a serious side. By making a numerical estimate of the brightness of a star at a particular point in time, you are logging a piece of scientific data. The AAVSO maintains records online of every observation submitted to them over the past hundred years, keeping the records available to researchers around the world.

On a typical night, I’ll observe about a dozen stars from “my” list of about sixty stars visible at different times of year.

I keep finder charts along with the AAVSO charts in plastic sleeves in a loose-leaf binder, so that everything I need is close at hand. Since you never know ahead of time how bright a variable is going to be, you need to have a complete set of charts for each star; these can be downloaded from the AAVSO web site:

The biggest challenge in finding a variable star is that you’re looking for something that may be quite bright, or may be below the magnitude limit of your telescope, totally invisible to you. So what you look for is the star field, the pattern of stars surrounding the variable. Once you’ve found the field, you then check to see how bright the variable is. You then consult your AAVSO charts to see which stars are closest in brightness to the variable. Comparison stars on the charts are marked with their brightness to the nearest tenth of a magnitude. Because a decimal point might be confused with a faint star, they are left out, so that a 9.7 magnitude star is marked “97” and a 11.4 is marked “114” on the chart. You try to find a couple of stars, one slightly brighter than the variable, one slightly fainter, and then estimate where the variable falls between them.

Equipment for variable star observing

For visual observing as I have described above, the equipment needs are very simple. There are many variable stars within range of a pair of small binoculars, and some that can be observed with the naked eye alone. On the other hand, access to a large telescope lets you follow stars that become very faint at minimum.

I have found it advantageous to use eyepieces with a wide field of view, since they show me more of the sky at any given magnification, and let me see more comparison stars without having to move the telescope about.

My current strategy is to survey “my” variables using my Celestron 6" SCT telescope. I’ve programmed the controller with the coordinates of my variables, so I can quickly move through the list. Any variables which are currently too faint to be observable with the 6”, I revisit the next night with my larger 11” Dobsonian.

Where to start?

If you’re still not sure whether variable star observing is for you, I’d recommend reading Starlight Nights by Leslie Peltier (Sky Publishing). Peltier was the finest variable star observer of the 20th century, and his book is an entertaining introduction to a wonderful man and his love of the stars. It’s probably my very favorite astronomy book.

The AAVSO web site includes everything you need to get started. It has a complete observing manual, a list of good stars to start on, and all the charts you will need, all free of charge. Visit http://www.aavso.org

I’d recommend starting on stars that are easy to find and visible all year round, such as these stars in and around the Big Dipper:

A final warning though: variable star observing is highly addictive. Variable star observers probably spend more time at the eyepiece than any other amateur astronomers because, unlike deep sky or planetary observing, they are not dependent on dark skies or steady seeing. For years I carried out regular variable star observing every clear night from the middle of a large city, even when the Moon was full. The only thing that can stop you is clouds!

If you'd like to follow along with NASA's New Horizons Mission to Pluto and the Kuiper Belt, please download our FREE Pluto Safari app.  It is available for iOS and Android mobile devices. Simulate the July 14, 2015 flyby of Pluto, get regular mission news updates, and learn the history of Pluto.

Simulation Curriculum is the leader in space science curriculum solutions and the makers of Starry Night, SkySafari and Pluto Safari. Follow the mission to Pluto with us on Twitter @SkySafariAstro, Facebook and Instagram. ​