Shallow Sky Object of the Month: Mirzam—A Beta (β) Cepheid Star

Original article appears in GuideStar December, 2020.

 Mirzam—A Beta (β) Cepheid Star

by Bill Pellerin, GuideStar editor

Object:  Mirzam, sometimes spelled Murzim, β CMa
Class:  β Cepheid Star
Constallation:  Canis Major (CMa)
Magnitude:  1.98
Period: 6 hours
R.A.:    6 h 22 m 42 s (2000 coordinates)
Dec:    -17 deg 57 min 21 sec
Size/Spectral:  B1
Distance:   500 ly
Optics needed: Unaided eye or small telescope

When your eye gets anywhere near Sirius, the brightest star in the sky (not counting our sun), you’ll be dazzled by it. Look again, and to the west-southwest, about 5.5 degrees away you’ll find the β star of the constellation, Mirzam. It’s bright, shining at a magnitude of 1.98 and you should be able to find it easily.

This star is a β Cepheid star (the designation, β CMa is simply coincidental with the β Cepheid category). Interestingly, this star is sometimes considered the prototype for the β Cepheid category of variable stars, but, more commonly, and easier to understand, the star β Cephei (β Cep) is considered the prototype. The General Catalog of Variable Stars (GCVS) identifies these type of stars as BCEP variables.

Do not confuse this type with Cepheid variables, whose prototype star is Delta (δ)Cep.

Characteristically, a β Cep star is low amplitude (in magnitude), short period variable. You would likely have to make measurements with a CCD imager or photometer to detect the variations in these stars. The variability of β CMa is from 1.97 to 2.01 magnitude, so measurements that can resolve magnitude differences of .01 mag are required. Typically, visual observations can resolve, at best, differences of .1 magnitude.

These changes in magnitude are caused by pulsations within the star, and scientists believe that there are three pulsations that are active simultaneously. Sometimes, the pulsations coincide in such a way as to amplify the variation in magnitude of the star. This effect is seen about every 50 days.

β Cep stars are typically much larger than our Sun with masses of 7 to 20 solar masses. Our subject star, β CMa is about 13 solar masses and has a surface temperature of about 23,000 K. This compares with the Sun’s temperature of about 6,000 K. Color and temperature are the same thing (see Wein’s Law), so this star is a blue-white in color.

One of the leading researchers on this type of stars was Otto Struve (1897-1963), who has a family heritage of observing variable stars. He was the director of the Yerkes Observatory and the founding director of the McDonald Observatory near Fort Davis, TX.

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