Milwaukee Astronomical Society



The Andromeda Legend

  1. To find it we start with the Great Square of Pegasus, the four stars here {trace out Great Square} that we found earlier, and Andromeda's head is the Northeast (upper left hand) corner star. The rest of Andromeda is then the figure formed by the two curved lines that radiate Northeast away from that corner {trace out Andromeda}.

  2. Now let's find the Andromeda galaxy. Start with Andromeda's head, then go to the next pair of stars, then to the next pair of stars after that (a little further apart). Follow the line of that pair up and to the right (Northwest) until you get to the next star. Look for a little fuzzy patch just to the right of that star. It sometimes helps if you don't look straight at it, but just off to one side a little bit. When you spot it, just note that you are seeing far beyond our own galaxy, 2 million light years away.

    You need a good, dark sky to see the galaxy by eye, but it is easy to find in binoculars. In fact, it looks best in a good pair of binoculars, 10x50 or bigger. It is also an easy target for the telescope. If you look hard in the telescope you might see one or two smaller fuzzy patches near Andromeda. These are satellite galaxies, little galaxies orbiting the big one! Our galaxy, the Milky Way, has satellites of its own, called the Magellanic Clouds. They can be easily seen, looking like detached portions of the Milky Way, but they can be seen only in the Southern Hemisphere.

    Andromeda in Telescope    What you're looking at

  3. You can use Andromeda to gauge how good your sky is... the clearer the sky, the fainter the magnitudes you can see. Starting with Andromeda's head at a magnitude 2, the brighter curved line is all magnitude 2 stars except the dimmer one right after her head -- that one is magnitude 3. The other curved line is all magnitude 4 stars. The star we used to find the Andromeda galaxy, that's a magnitude 5 star -- notice there is one off to either side of the galaxy. If you can see those two stars you have a really good clear, dark sky. The second pair of stars from Andromeda's head points to the 5th magnitude stars we just found. The first pair of stars points to a similar (and closer) pair of magnitude 6 stars, visible only under the very best of conditions.

  4. We'll swing the telescope real quickly over to the star at Andromeda's left foot (the Southeast one), γ (gamma) Andromedae {point out γ Andromedae}. This is a double star -- can you see the color difference between the two stars? The bright one is yellow-orange, the other is a bluish-green. This is one of the coolest looking doubles in the sky. It's a true double star -- actually it's a four-star system, the blue one is really three stars, but they're too close together for our telescope.

    Another star of interest is &upsilon (upsilon) Andromedae, a star very similar to our own sun {point out υ Andromedae}. In 1999 three planets were shown to be orbiting ε Andromedae, making this the first system of planets to be discovered outside our own. (We can't see the planets in our telescope - they were found by calculation from the motion of the star).

  5. Andromeda is a princess and she is shown chained to a rock, by her daddy the King. And it wasn't even for anything that she did, but we'll get to that in a second. If you follow the chains up to the rock {formed by λ, κ, ι, and ο Andromedae, point out the rock} right next to that little bitty star right there {point out 13 Andromedae} is where we are going to focus the telescope. When you look in the eyepiece you'll see two stars and something else... a puff of smoke, maybe. Compare the little puff to the two stars next to it. Can you see a color difference? The puff is actually blue, or blue-green. In fact is known as the Blue Snowball. It's a star like the other two that you see, but this star has blown itself apart!

    Snowball in Telescope    What you're looking at

    This is another planetary nebula, like the Ring Nebula. They are called planetary nebulae because the disk shape suggested the look of a planet to early astronomers. In fact it has nothing to do with planets at all. This is what's left of a red giant star that, about 1700 years ago, did what all red giants eventually do. When the fuel at the core runs so low that the nuclear reactions can no longer hold up the weight of the star, it all collapses in to the center, which in turn raises the temperature so high that the star blows off its outer envelope of gases, losing much of its mass. This exposes the core to outer space, or, more accurately, exposes outer space to the nuclear reactions going on at the core. The intense radiation from the burning core causes the expanding shell of gas to light up like a neon light, and voila -- the faintly glowing disk that you see here. With a larger telescope you can still see the tiny star that remains at the center of the Blue Snowball -- now a white dwarf.

    A few billion years from now, our sun will look a lot like the Blue Snowball.

  6. So how did Andromeda end up chained to a rock? It all started with her mother, the Queen Cassiopeia, whom we met a little while ago, the Big W over by Polaris.

    Cassiopeia had a reputation far and wide for her beauty, and that was not enough for her. She started going around boasting that she was more beautiful than the Nereids, the sea nymphs. When the Nereids caught wind of this they complained to Poseidon, god of the sea, who sent a huge sea monster, Cetus, to wreak havoc in the kingdom Godzilla-style.

  7. You can just see Cetus rising on the eastern horizon at this time of the year {trace out the head & body of Cetus} -- pretty terrifying, huh? Well, this is the Kraken from "Clash of the Titans", as the ancient Greeks portrayed him since they didn't have movies let alone CGI. That star right there in the middle of the monster's neck is known as Mira Stella, "The Wonderful Star", technically known as Omicron Ceti.

    This is a very famous variable star, changing from bright to dark over the course of about a year. For four months, Mira becomes just visible to the naked eye, then for eight months it can be seen only with binoculars or a telescope. Mira was the first to be discovered of this type of variables, those that take about a year to go through the cycle, hence these are all called "Mira variables". We just happen to be lucky to see it now assuming of course that you can see it, otherwise I skip mentioning it.

  8. All of which brings us to Cepheus the King -- Cassiopeia is his queen, and Andromeda is his beautiful daughter. Cepheus is a house-shaped constellation very close to the Northern horizon at this time of the year. {Trace out Cepheus}.

  9. This constellation contains the reddest star in the sky, μ (Mu) Cephei, also called "The Garnet Star" and is located halfway between the two stars at the bottom of the house. It is a red super-giant, 1,500 times the size of the sun. It was considered the largest star known to man until just recently when 3 other stars (which you can't see without a big telescope) were measured about the same size but just barely edge it out. Placed where our sun is, the surface of μ Cephei would extend out past Jupiter.

  10. If you are ever on an expedition to the planet Mars, you might want to know that μ Cephei is the pole star for Mars. In just about another 6,000 years it will be our pole star, too. That's because our North Pole is actually moving through the sky as the earth wobbles on its axis, a lot like a spinning top does. With the axis wobbling like that, the North Pole is tracing a circle in the sky - it just happens to be passing by Polaris right now.

    It is about a degree away from our North Star now and will get about a half degree closer, then will start moving away as it continues on its circle through the sky. That circle will take it almost exactly right down the center of Cepheus, past Deneb in Cygnus, past Vega in Lyra, through Hercules leg and right by his knee, then down past the third star from the end of Draco's tail, Thuban, and back to Polaris. In fact, 3,000 years ago, Thuban was our pole star.

  11. Another star in Cepheus is of crucial importance to astronomy -- δ (Delta) Cephei. {Locate δ Cephei.} This star is a "variable", meaning the star's brightness varies over time -- in this case it varies between that of ζ (zeta) Cephei and ε (epsilon) Cephei over a period of five days. How bright is it now -- as bright as ζ, ε, or in between? We will assess again each night that we can during the week.

  12. This star was the first of its type to be discovered, hence these variable stars are called 'Cepheid' stars. Cepheid stars have gotten to just the right mass to be unstable - so the whole star is pulsating, the surface of the star is actually rising and falling, with a rhythm that is so precise you could set your watch to it. It was discovered in 1912 that this rhythm depends directly on the true brightness of the star -- the brighter the star, the longer the time between peaks. This discovery, as it happens, rocked the astronomy world. So why was this such a big deal?

    Well when a star is closer to us, it seems brighter. When it's farther away it seems dimmer, right? Well we know the true brightness of a Cepheid star, from the cycle time of its brightness. If we know the true brightness of the star, and we measure its apparent brightness, we can figure out the distance to the star. If the star is part of a cluster or a galaxy, this tells us the distance to that entire body of stars. This has been used to find the distances to globular clusters, other galaxies and even our distance from the center of our own galaxy -- 28,000 light years.

    In 1924, Edwin Hubble (yes, the telescope is named after him) used Cepheids to measure the distance to the Andromeda 'nebula' (2.3 million lightyears) and proved that it is not another solar system in formation but an 'island universe', another galaxy like our own. This was an extraordinary declaration about the structure of the universe back in 1924. Our whole system of measurement of the universe is built upon the Cepheids as our basic yardstick.

  13. Then on top of all that, if we look in the telescope we can see that bright yellow δ Cephei has a beautiful blue companion -- so this is both a variable star and a binary! The yellow star is the variable component, and once was a blue star like its companion. It is now expanding out as it goes through the end-of-life process, and is passing through an unstable phase as it does so. The pair is about a thousand light-years from us, based on the Cepheid period-brightness relationship.

    Meanwhile, back in the kingdom, we left Cetus tearing things up, and Cepheus, as the local King, is presiding over this disaster. He consulted his oracle to determine what to do, and the oracle told him that the only way to appease the angry sea god was to sacrifice his daughter, that would be Andromeda, to the sea monster. Sadly, the king chained up his daughter to the rocks by the shore to await the arrival of Cetus.

    So here we have them all -- Cepheus the King in a jam, Cassiopeia his beautiful if not terribly bright queen, Andromeda his lovely daughter chained to a rock, and here comes Cetus lumbering like Godzilla with devastation in his wake and our poor little princess in his sites! Are they all going to just sit there watching? Won't somebody DO something?

Back to the Zodiac Autumn Index On to Perseus


Your questions and comments regarding the Stargazing section are welcome. You can e-mail the author, Randy Culp for inquiries, suggestions, new ideas or just to chat.
Updated 18 July 2023