Home > Concepts, Ephemeris Program, Star Clusters > 11/21/2014 – Ephemeris – The Hyades a very important star cluster

11/21/2014 – Ephemeris – The Hyades a very important star cluster

November 21, 2014

Ephemeris for Friday, November 21st.  The sun will rise at 7:47.  It’ll be up for 9 hours and 22 minutes, setting at 5:09.   The moon, 1 day before new, will rise at 7:42 tomorrow morning.

The face of the constellation Taurus the bull looks like the letter V sideways above the rising Orion the Hunter in the east at 9 p.m.  The bright star at the tip of a letter V of stars is Aldebaran.  Look with binoculars at the letter V shape and you will see the stars of the Hyades star cluster  The Hyades is the closest star cluster to us, at about 151 light years.  And is important for that reason.  Before satellites like Hipparcos.  The Hyades were the only star cluster to be directly measured by a technique called parallax, using the radius of the earth’s orbit as one side of a surveyors enormous triangle. Its many stars at the same distance were used to determine distances of star clusters even farther away.  Additional techniques based on the distance of the Hyades allow us to measure distances to the galaxies.

Times are for the Traverse City/Interlochen area of Michigan.  They may be different for your location.

Addendum

The eastern sky at 9 p.m.

Orion and the Hyades.

Hyades and Pleiades

The Hyades (lower left) and the Pleiades (upper right). My photograph from many years ago.

Hyades v. Pleiades

H-R diagram showing the Hyades and Pleiades. Credit: European Southern Observatory.

The Hertzsprung-Russell diagram or H-R diagram plots stars by brightness on the vertical axis versus surface temperature on the horizontal axis, hot to the left to cool on the right.  The plot of stars for open or galactic star clusters, where the stars are burning hydrogen in their cores lie on a diagonal line called the main sequence.  When matching the plots of two star clusters based on apparent magnitude the main sequence plot for the more distant star cluster will be dimmer by a certain magnitude difference.  Since the brightness of and light source diminishes by the inverse square of the distance.  The difference in brightness equates to a difference in distance.

For more information on the H-R diagram check out the Wikipedia article.  It’s more than about distance.

 

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