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Norman Sperling
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7 Spectral Types in 1 Big Loop

© Norman Sperling, April 15, 2012
Part of a series on Educational Star Parties:
Star Parties Designed for Students (July 7, 2012)
Trading Cards for Telescopes and Celestial Objects (September 20, 2012)
Telescope Triplets (November 25, 2011)

When I teach about stars, the 7 main spectral types usually seem rather abstract. I show their different spectra, but that's hard to relate to what students actually see in a starry sky. I show Planck curves and explain how surface temperature results in color differences that you can actually notice. Star colors aren't the sharp tones of advertising signs, but you can definitely notice the tinges.

Star tinges are less than impressive to the naked eye, because starlight is so dim that it mostly triggers the black-and-white-registering rod cells in your retina. Only the 20 or so brightest stars deliver so much light that they also trigger a few color-sensitive cone cells, and those only barely.

But even a small telescope collects enough light to trigger a whole lot more cones in your retina, making the colors appear appreciably bolder. So a star party that is deliberately planned for student education should use 7 small telescopes to point at a bright star of each of the 7 spectral types, to emphasize their different colors. Arrange the scopes so a single line of viewers looks through all 7 scopes in order, either OBAFGKM or MKGFABO. After everybody has seen that, re-aim those scopes to their next targets.

Yes, A and F stars really do look white, but now you appreciate how real that is, unlike an artifact of not triggering enough cone cells.

For each spectral type, at any position of the sky, you can find examples at third magnitude or brighter.

All 7 spectral types are blatant around the Great Winter Oval:
O: Mintaka and Alnitak
B: Rigel, Bellatrix, El Nath, Alnilam, and Saiph
A: Sirius
F: Procyon
G: Capella
K: Aldebaran and Pollux
M: Betelgeuse

The Great Winter Oval has many advantages. It's accessible late in the Fall semester, late in the evening; all winter long; and just after dusk well into Spring semester. Since it straddles the equator, it's easily seen from practically everywhere that people live. Only in May, June, and July is it not available - parts of it even then.

When part of the Great Winter Oval is hidden by the Sun's glare, here are some bright alternatives:
O: zeta Ophiuchi and zeta Puppis
B: Alpheratz, Algol, Regulus, Spica, and Alkaid
A: Denebola, Alioth, Mizar, Gemma, Vega, Deneb, Altair, and Fomalhaut
F: Polaris, Algenib, and Sadr
G: the Sun, beta Corvi, Vindemiatrix, eta Bootis, eta Draconis, and beta Herculis
K: Alphard, Dubhe, Arcturus, and Kochab
M: Antares, Mira, and beta Andromedae

Decrease the number of telescopes needed, and make the contrast more vivid, by showing wide, bright, color-contrast double stars:
Algieba: K + G
Albireo: K + B
gamma Andromedae: K + B
Cor Caroli: A + F

Bigger scopes show color contrast in:
32 Eridani: G + A
h3945 Canis Majoris: K + F

Don't try to add spectral class W unless you're far enough south to see the only bright one, gamma Velorum, -47 degrees. There are only about 150 Wolf-Rayet stars known in our galaxy. No others are close enough to look brighter than 6th magnitude. The biggest bunch is around the Summer Triangle.

I'll comment more on planning star parties for student education in later postings.

The Journal of Irreproducible Results
This Book Warps Space and Time
What Your Astronomy Textbook Won't Tell You

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