Whale's Tail

Note: There will be no December 17 post. Enjoy this post or browse my older posts. I'll be back with a new post on December 24.

The brightest star in the autumn constellation Cetus the Whale is known by two traditional names: Deneb Kaitos (DENN-ebb KYE-tohs) and Diphda (DIFF-duh). The former is Arabic for whale’s tail, as the star marks the position of the celestial sea creature’s tail. The latter is more commonly used by amateur observers, and it’s from an Arabic phrase meaning second frog. The “first frog” is the nearby--and noticeably brighter--star Fomalhaut in Piscis Australis the Southern Fish.

Thar she blows!

1) About an hour after your local sunset time, face south. If you don’t know the cardinal directions at your location and you don’t have a compass, make note of where the sun sets on the horizon. That spot is approximately west. Stand with your right shoulder to the west, and you’ll be facing approximately south.

Looking south to the Great Square and Cetus
Star maps created with Your Sky

2) First, locate the Great Square asterism (star pattern) in Pegasus, high in the southern sky, on or near the meridian.

3) Now, let’s starhop. Using the two easternmost stars in the Square as pointers, draw an imaginary line between them and extend it towards the southern horizon. Traveling a little more than twice the distance between the two pointer stars, you’ll come to the bright star Diphda.

Can you discern a golden hue in the star’s light? Diphda is an orange giant star, nearly 150 times as luminous as our Sun. Of course, from our perspective nearly 100 light years away, it’s just another twinkle light in the autumn night sky. After all, one light year is nearly six trillion miles! So we can be forgiven if we think our Sun has more star power.

4) Reacquaint yourself with the “first frog,” Fomalhaut, to compare it to Diphda in color and brightness. Use the two westernmost stars in the Great Square as pointers this time. Draw an imaginary line between them and extend it towards the southern horizon. Traveling a little more than three times the distance between the two pointer stars, you’ll come to bright Fomalhaut, which will be closer to the southern horizon than Diphda.

Since Fomalhaut is a white star, try comparing the two, which may enhance Diphda’s subtle golden color. Fomalhaut’s brightness as seen from Earth, also known as apparent magnitude, is about two and a half times greater than Diphda’s, so you should discern a difference.

Astronomy Essential: The Milky Way is a barred spiral galaxy.

Our home galaxy was first determined to be a pinwheel-shaped spiral galaxy by radio astronomers in the 1950s, who began the on-going process of creating detailed maps of our galaxy’s structure. The curved “arms” of spiral galaxies--which radiate out from a dense galactic core--are regions of active star formation, which is why they are detectable in a variety of wavelengths and able to be mapped.

In 2005, new surveys of the galaxy in infrared light--conducted with the Spitzer Space Telescope--revealed a dense, bar-like congregation of stars cutting across the galaxy’s center. Extraterrestrial observers in other galaxies, positioned so as to have a face-on telescopic view of the Milky Way, would immediately recognize it as a barred spiral type.

Mother and Child

we remember Stephanie Keese, 1980-2008


Some deep-sky objects never lose their allure, even for veteran observers who have gazed at them hundreds or thousands of times through binoculars or telescopes. One of these is the magnificent Whirlpool Galaxy, 31 million light years away.

This spectacle of the northern sky is a showstopper at public star parties and observatory events. No one can forget his or her first telescopic glimpse of two galaxies interacting, one large and one small. It’s an iconic pairing I like to call “Mother and Child.”

Although I don’t normally venture outside of our own Milky Way galaxy in this blog, I will occasionally make an exception. This is one of those times.




The Whirlpool Galaxy is also known as M51, shorthand for Messier 51. This is its designation in the deep-sky catalog of the famed 18th century French comet-hunter Charles Messier (pronounced MESS ee yay). Technically, M51 refers to the large spiral galaxy only; the small, irregular companion galaxy has its own catalog number, NGC 5195. But in practice, most of us refer to these interacting galaxies collectively as M51, since they’re something of a package deal.

Messier himself discovered the ‘mother,’ and the ‘child’ was discovered by an astronomer pal of his. However, because of the limited resolving power of their telescopes, they thought the objects were nebulae (pronounced NEBB-you-lee; plural of nebula), clouds of gas and dust.

The 19th century English astronomer Lord Rosse discovered M51’s spiral nature through his whopping, six-foot-diameter “Leviathan” telescope in Ireland. It was the first time spiral structure had been recognized in a celestial object. But it was the great 20th century American astronomer Edwin Hubble who ultimately determined that many of the objects called “nebulae” (including M51) which were thought to be emerging solar systems were in fact galaxies--immense star systems lying beyond the Milky Way.

Restored Leviathan at Birr Castle, Ireland
Courtesy of Birr Scientific and Heritage Foundation

A dark site away from urban and suburban lighting is required to see the galactic twofer in all its glory. This is an ideal time of year to view this object, as it’s high in the sky. You can spot the Whirlpool Galaxy using binoculars, but you’ll need a telescope to see detail. Its proximity to the Big Dipper makes it fairly easy to locate.

1) You’ll need to face north, so if you don’t know the cardinal directions at your location and you don’t have a compass, make note of where the sun sets on the horizon. That spot is approximately west. Stand with your left shoulder to the west, and you’ll be facing approximately north.

2) Wait at least one hour after sunset to begin observing, so that twilight’s over and your sky’s good and dark. On Friday, May 23, you’ll have a couple hours of dark before the Moon rises. For the rest of the observing week, the Moon will rise even later each subsequent night.

3) Facing north, locate the Big Dipper, the distinctive seven-star asterism (recognizable star pattern) that looks like a giant, long-handled saucepan. It will be due north and somewhere between the zenith, the point directly above your head, and the northern horizon. The saucepan will be oriented upside-down. Point your binoculars or telescope at the point marked on the star map below, just off Alkaid (pronounced AL-kayd), the final star in the saucepan’s handle.

Chart created with Your Sky

Although just off the handle of the Big Dipper, which is part of the constellation Ursa Major the Great Bear, the Whirlpool Galaxy is actually in the lesser-known constellation of Canes Venatici the Hunting Dogs (pronounced KAY-neez vee-NATT-uh-sigh).

4) In binoculars and with good sky conditions, you should see an elongated fuzzy blob with two bright cores. A telescope will help you separate the blob into two distinct objects. In an eight-inch-diameter or larger reflector, you should clearly see spiral structure in the large galaxy. A large-aperture telescope and good sky conditions are needed to see “the Bridge,” the arm of the spiral galaxy that appears to reach out and touch the little galaxy.

In reality, the little galaxy is passing behind the large galaxy and has been doing so for hundreds of millions of years. But the gravitational pull of this galactic glide-by has disturbed and excited gas clouds in the spiral galaxy’s arms, setting off maelstroms of star formation. In the Hubble Space Telescope image below--the most detailed ever taken of M51--the new stars can be seen as bright blue clusters sprinkled along the sweeping arms.

It seems the child in this family portrait is quite precocious.