Delta (δ) Cephei (Σ I 58) (H V 4) HIP: 110991 SAO: 34508
RA: 22h 29.2m Dec: +58° 25′
Magnitude (AC): 4.2, 6.1
Separation (AC): 40.6″
Position Angle (AC): 191° (WDS 2012)
Distance: 983 Light Years
Spectral Classification: F5, B7
Status: “C” is variable and a spectroscopic binary
Greg and I have roamed all over Cepheus and somehow both of us missed the chance to write about this colorful gem. Not only is it one of the more colorful doubles in the sky, but the primary is also one of the most famous. My first good look at it was in a Meade AR-5 (127mm, f/9.3) and my 90mm Orion f/10.1 on a chilly, moonless night. This is a beautiful pair of widely separated stars — in fact, the companion described here, “C”, actually lies 12,000 astronomical units from the primary!
First, the colors.
In the two scopes mentioned above, the primary is a rich yellow with a tinge of red to it, and “C” is a pronounced blue color, leaning a bit toward white. Haas has them as “citrus orange and royal blue,” and considers them a showcase double – which they certainly are.
I was eager to get another look at this pair on the next clear night. The weather being fickle once again, it was another five nights before I got the chance. This time I was using a 105mm Antares f/14.3 and a 60mm f/16.7. Maybe it was the scopes, maybe something in the atmosphere, or maybe just me, but I noticed the red tint was more noticeable in the primary and there seemed to be less white in “C”. Both times my eye was drawn to an interesting trio of eight and ninth magnitude stars to the west which contributed a little something extra to the scene.
Now, for the famous part.
Surprisingly, Delta Ceph has never been graced with a name, but it certainly deserves one. It has given the name of the constellation it resides in to an entire class of variable stars, Cepheids, which are used to determine stellar and galactic distances. The luminosity of these stars is directly related to their periods of variability, which are very consistent. Delta Ceph ranges in magnitude from 3.5 to 4.3 over a period of 5 days, 8 hours, 47 minutes, and 32 seconds, and it’s 2000 times more luminous than our sun. It’s classified as a super-giant, as well it should be, with a diameter forty times that of the sun.
So how do you use a star to measure distances? Surprisingly, it’s not all that complicated.
The period of variability of a Cepheid provides the luminosity, which is the intrinsic brightness of the star measured in relation to our sun. The longer the period of variability is, the more luminous the star. With the star’s luminosity pinned down, its visual magnitude as we view it from earth provides us with its distance. Like a 100 watt light bulb seen at a distance, the star’s visual brightness as seen from earth is related to how far away the light source is. Based on this characteristic, Cepheids, like lighthouses and 100 watt bulbs, can be used to provide very reliable distance measurements. We owe the discovery of this luminosity-distance relationship to Henrietta Levitt, which was established in 1912 based on her observations of Cepheids in the Small Magellanic Cloud. A good discussion of her role can be found in Marcia Bartusiaks’ The Day we Found the Universe or in Timothy Ferris’s The Red Limit.
Because they are very luminous, Cepheids can be seen in other galaxies with large telescopes. Once identified, they can then be used to measure the distance to the galaxy in which they are found. It was Edmund Hubble‘s discovery in 1923 of a Cepheid in the Andromeda “nebula” (M31) which, when combined with additional observations throughout 1924, allowed him to establish that it lies far beyond the Milky Way. That put to rest an early twentieth century debate as to whether Andromeda was a nebulous cloud residing in our galaxy or whether it was a galaxy residing outside of our own, thus providing astronomers with the first definitive indication that an entire universe existed beyond the Milky Way. At first, telescope size limited the detection of Cepheids to nearby galaxies, but the Hubble scope and earth-bound telescopes using adaptive optics have been able to extend the distance at which Cepheids can be detected far beyond the earlier limitations. Other methods are also now used to determine galactic distances, but Cepheids are the foundation on which most all methods rest.
So when you’re sitting there at the eyepiece of your telescope enjoying the rich yellow and bright blue colors of Delta (δ), think of it as a window into the true dimensions of our universe.
And getting back to the lack of a name, I’ve always felt that Henrietta Levitt deserves more recognition for her work on Cepheids. So, I lean toward Levittania as a good name for Delta Ceph.
Observations of Delta Cephei were made on September 28th and October 3rd, 2010, under dark, moonless, relatively transparent skies with seeing wavering between pretty poor and pretty awful.
Information on Delta Ceph is from Jim Kaler’s site; information on Cepheids comes from the two books mentioned above.
(WDS data updated 6/22/2014)