The Hyades (Melotte 25 / Caldwell 41)

Constellation
RA / DEC
Diameter in '
Magnitude
Number of stars
Brightest star (magnitude)
Distance in parsecs (3.26Ly)

Details Sketch:
Date / Time
Observing Location
Seeing / Transparency
Telescope
Eye-piece
Magnification / Field of View '
: Taurus
: 04 27.0 / +16 00
: 329
: 0.5
: 380
: 4
: 40


: 27/12/08 / 21.30
: Landgraaf
: 3 / 3
: Celestron 12x60
: -
: 12 / 300
Hyades

Observing report

On december 27th 2008 I observed the Hyades with my 12x60 Celestron binoculars, mounted on the SkyWindow. The field of view (FOV) that these binoculars provide is about 5 degrees (300') and the Hyades are just a little to wide, to fit into FOV. But although I didn't see the whole cluster in one FOV, the first impression of this cluster was simply stunning.

The brilliant deep orange Aldebaran, which actually isn't a member of the Hyades but a foreground star, dominates the field of view, together with three bright yellow companions. These are three of the four yellow giants of the Hyades: Delta, Gamma, and Theta-1. The fourth is Epsilon (which didn't fit in the same FOV). There were many different chains of stars and small asterisms (triangles and other shapes) visible. The northern and southern half's of the cluster seemed to be more or less divided by a dark empty zone, with almost no stars.

Beside Aldebaran and the three yellow stars, there were two other stars that drew my attention. First of all the brilliant white Theta-2 Tauri, with magnitude 3.3 the brightest star, the Lucida of the Hyades. It makes a beautiful double with the Theta-1, one of the yellow giants. To the south of this nice couple I detected the other prominent star, the reddish M-type variable V1146 (or 84 Tauri).

In total I counted about between 75 and 85 stars, but I'm sure with my 300mm Dobson I could double or triple that number easily. According to Archinal and Hynes (Star Clusters, Willmann and Bell) the Hyades contain about 380 stars. This makes it a rich cluster. I could not detect any nebulosity of unresolved stars or of any other kind.

Notes

In the period I observed the Hyades I was experimenting with creating my own Hertzsprung-Russell (or color-magnitude) diagrams in Excel. I am interested in stellar evolution so I wanted to see how I could combine my visual deepsky observations with the theory of stellar evolution. I constructed two different diagrams. Below you will find Aldebaran plotted on a classic HR-diagram with the temperature / color on the X- axis and the absolute magnitude on the y-axis. As you can see, Aldebaran is a cool giant, with a temperature of only 3500 to 4000 degrees Kelvin and an absolute magnitude of -0.87. The spectral type is K5III. Visually it is deep orange.

Stacks Image 464

The data for Aldebaran is taken from "Voyager", which is based on the Tycho, Hipparcos and Hubble Guide Star Catalogue.

For the HR diagram of the Hyades, an open cluster, I took a different approach. I wanted to create a color-magnitude diagram. On the X-axis you see the color index B-V and for the Y-axis I used the apparent visual magnitude (V). The data was extracted from the WEBDA database. For the Hyades there were more than 20 different UBV measurements. I chose the data from "The Hyades and Coma Berenice Star Cluster" by Johnson and Knuckles (09-1955, Astrophysical Journal). The reference number in WEBDA is 14 and there were 189 stars measured. After I plotted these stars in my excel-diagram, I immediately noticed the four stars to the top-right of the main sequence. These are the four yellow giants, which I observed when making my sketch. With a little research I was able to determine which one was which. At the top of the main sequence stars I identified theta-2 Tauri, the Lucida of the cluster. Identification of single stars is done in a few steps:

  1. I click on the dot of a single star in my excel-diagram, and I can see both parameters, B-V and V;
  2. with the help of these parameters I can identify the star in my WEBDA-data list, in other words I now know the star number within the open cluster, for instance "37";
  3. in WEBDA I go to the star level within the cluster page, and type in the number "37" to retrieve the individual star data;
  4. among other things, you get a few identifiers like Hipparcos, SAO, and HD etc. Once I have this I go to the SIMBAD database,
  5. type in the identifier and get all the info on the star, including all the different names of the star.

This method looks complicated, but it isn't. You can identify a few individual stars within minutes. I would like to make a last remark. On most HR diagrams of the Hyades, you see a few white dwarfs to the lower left of the main sequence. In the measurement I used, they probably didn't include these white dwarfs. That's why they are not plotted.
Stacks Image 468