Shallow Sky Object of the Month: Plaskett's Star

Original article appears in GuideStar February, 2013.

Plaskett’s Star finder and detail charts—north is up
Star chart generated by TheSkyX © Software Bisque, Inc. All rights reserved.
Object: Plaskett’s Star
Class: Blue Star
Constellation: Monoceros
Magnitude: 6.1
R.A.: 06 h 37 m 24 s
Dec: 06 deg 08 min 07 sec
Size/Spectral: (O Class)
Distance: 6600 ly (uncertain)
Optics needed: Small telescope

Why this is interesting:

I observed this star on January 19 of this year, and it’s a real beauty. It’s the brightest star in a field of stars and shines like a steely-blue diamond in the sky. I found this star in a catalog while looking for an O class star for my ‘Observing Stellar Evolution’ Astronomical League program.

Most of us are aware of the colors of stars, the most obvious example of the difference in colors being Albireo. This orange/white star pair starkly shows the differences that exist in star colors. Color and temperature are the same thing. The hotter the star, the bluer the star, the cooler the star, the redder. It turns out that finding stars of all (OBAFGKM) colors is something of a challenge because the colors of stars are not equally distributed among the stars in the sky.

You remember that star colors are designated by a letter O being the hottest (whitest) stars and M being the coolest (reddest) stars.

Only one in a million stars is an O star so they’re far from being the most common star in the universe. The F (white) stars are about 3% of the stars; the K (orange) stars are 12% (Albireo A is a K star), and M (red) stars are 77% of the stars in the universe. Unfortunately these M stars are low luminosity (dim) so they’re difficult or impossible to see.

Plaskett’s Star is actually a pair of O stars orbiting each other with a period of just over 14 days. You will not be able to split the pair of stars with your telescope; their binary nature only reveals itself when a spectra is taken of the star. The mass of the star system is estimated at over 100 solar masses.

The star is named after John Stanley Plaskett (1865-1941) who as a Canadian astronomer studied spectroscopic binaries.

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