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    My Journey through the Astronomical Year

    Think of this as a "companion text" to this, the main web site. Not required reading, butI hope you'll find it interesting and helpful.

Look North In August 2013 – All hail the Queen! (OK – the “W”)

Click image for larger view. (Derived from Starry Nights Pro screen shot.)

For printer friendly chart, download this.

The easily recognizable “W” of Cassiopeia (kass ee oh pee’ uh), the Queen, is well up in the northeast early on an August evening. Find it and you have a good starting point for tracing the Milky Way on south through Deneb.

When the “W” circles to a point high overhead, it will look like an “M,” of course, but that’s just part of the fun. Some people also see this asterism as forming the chair – or throne – for Cassiopeia. I like it because along with the Big Dipper, it nicely brackets the north celestial pole and provides another rough guide for finding Polaris. As the “W” rises, the Dipper plunges until it may be too close to the horizon for many to see. Both the stars of the Dipper and the stars of the “W” are 28 degrees from Polaris – roughly three fists.  When the Dipper gets on the horizon, the “W”  turns into an “M” directly above Polaris, so just measure three fists down from this “M” and you should be in the right region for finding the North Star.

Normally I do not find constellations or their associated myths too useful. Cassiopeia is an exception. Knowing the myth connected with this constellation will help you remember several important neighbors, and though we’ll meet these in the next two months, I’ll give you a “heads up” now and repeat the story when we meet the others. It goes like this:

Cepheus (King of Ethiopia)  and Cassiopeia (Queen of Ethiopia) have a beautiful daughter, Andromeda. Cassiopeia bragged so much about Andromeda’s beauty, that the sea nymphs got angry and convinced Poseidon to send a sea monster to ravage Ethiopia’s coast. To appease the monster, Cepheus and Cassiopeia  chained the poor child (Andromeda)  to a rock. But don’t worry. Perseus is nearby and comes to the rescue of the beautiful maiden, and they ride off into the sunset on Pegasus, Perseus’ flying horse! These five constellations – Cepheus, Cassiopeia, Andromeda, Perseus, and Pegasus – are all close to one another in the sky and all are visible in the fall, so we will meet them soon.

One of the bright stars of Cassiopeia is also a special aid to finding your way around the heavens, but in a more modern sense. It is part of an asterism known as the “Three Guides.”  These three bright stars are all very close to the Zero Hour Right Ascension circle in the equatorial coordinate system – the system that is roughly the celestial equivalent of latitude and longitude and is commonly used to give a permanent address to stars and other celestial objects. These three bright stars mark a great circle that goes through both celestial poles and the equinoxes and is known by the eminently forgettable name of  “equinoctial colure.”

Click image for larger view. (See note at end of post for source of this drawing.)

We’ll meet the other two stars in this asterism next month, but for now, simply take note of Beta Cassiopeia.It’s marked on our chart and is the bright star at that end of the “W” that is highest in the sky this month. Remember that this star is very near the “0” hour  circle, which you can visualize by drawing an imaginary line from Polaris through Beta Cassiopeia and eventually the south celestial pole. This line will cross the ecliptic at the equinoxes.  Of course, this helps only if you are familiar with the equatorial coordinate system! If that means nothing to you, then don’t clutter your mind with this right now.

The source for the drawing showing the equinoctial colure can be found here.

Look north in September – the king’s on the rise!

Yes, that’s Cepheus, the King – remember that Cassiopeia (the “W” ) is the Queen. Though Cepheus makes a familiar “home plate” asterism, it’s not nearly so memorable as the “W” of Cassiopeia, primarily because its stars are dimmer than those of the “W.” In fact, you might have difficulty picking it out at first, but here’s a tip: Follow the familiar “Pointers” of the Big Dipper to the North Star – then keep going. The first bright star you meet will mark the tip of the Cepheus home plate. (It’s about one fist away from Polaris – the Pointer stars are nearly three times that far in the other direction.)

Also coming up below the “W” is the “Bow” asterism that marks Perseus, who is carrying the head of Medusa, which contains the “Demon Star,” Algol. We’ll take that up next month when they’re higher in the sky and easier for all to see. Here’s a chart.

Click image for a larger version. (Developed from Starry Nights Pro screenshot.)

For a printer-friendly version of this chart, download this.

To review the connecting mythology, which helps me remember the related constellations, here’s the story in brief. Cepheus and Cassiopeia have a daughter Andromeda whose beauty makes the sea nymphs jealous. They enlist Poseidon to send a sea monster to ravage the coastline of Ethiopia, the kingdom of Cepheus and Cassiopeia. To appease the monster, the good king and queen chain Andromeda to a rock along the coast, but Perseus rescues her and together they escape on Pegasus, his flying horse. You meet Andromeda and Pegasus – the flying horse is much easier to identify as the “Great Square” – in the “look east” post this month. Also in that post we detail the “Three Guides,” three stars that mark the zero hour in the equatorial coordinate system used to give a permanent address to all stars. The first of those Three Guides is Beta Cassiopeia, visible in our northeastern sky, and so on the chart with this post.

Moving from mythology to science, Cepheus is probably best known today for a special type of star called a Cepheid variable. This is a star that changes in brightness according to a very precise time table. What’s more, it was discovered that the length of a Cepheid’s cycle – that is the amount of time it takes to grow dim and then brighten again – is directly related to its absolute magnitude. The absolute magnitude of a star is a measure of how bright it really is as opposed to how bright it appears to us. How bright it appears is, of course, related to how far away it is. That makes Cepheid variables a sort of Rosetta Stone of the skies.

It is relatively easy to time the cycle of a variable, even if the star is quite faint from our viewpoint. These cycles usually cover a few days. If you can identify the length of this cycle, you then can know the absolute magnitude of a star. And if you know its absolute magnitude, it’s a simple matter to compare that to how bright it appears to us and thus determine its distance from us.

This is a huge breakthrough. Without Cepheid variables astronomers were at a loss for determining the distance of anything more than a few hundred light years away. The distance to such”close” stars could be determined using a very common method known as parallax – that is, determining how the star appeared to change position slightly from opposite sides of the Earth’s orbit. But that change in position is extremely tiny and difficult to measure even with very close stars. With the Hipparcos satellite and computer analysis, it has been possible to use this system for stars as far as 3,000 light years. But that still is close by astronomy standards. (Keep in mind our galaxy is about 100,000 light years across.) But Cepheid variables can even be found in other galaxies. In fact, they played a huge role in proving that “spiral nebulae” were real other “island universes” – that is, other galaxies. The Hubble Space Telescope has found Cepheids out to a distance of about 100 million light years – a huge leap from the 3,000 light years we can reach with the parallax method.

There are other ways of making an educated guess at an object’s distance, and they frequently are quite complex and indirect. But the Cepheid variable has been one of the most important tools in the astronomer’s tool kit for the past century. It was in 1908 that Henrietta Swan Leavitt, a $10.50 a week “calculator” at Harvard Observatory noticed a pattern while doing tedious work cataloging stars and saw it’s importance. Though she published a paper about it, she never really received the credit she deserved during her lifetime for this breakthrough discovery.

So when you look at this “home plate” in the sky, see if you can find the fourth magnitude star, Delta Cephei – it’s not hard to spot under good conditions. (See the chart above.) When you find it, pay homage to it for the key role it has played in unlocking the secrets of the universe – for once astronomers know the distance of an object they can make all sorts of deductions about its composition, mass, and movement.

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