30,000 25,000 20,000 15,000 10,000 5,000 0
KelvinsA
AO
OF
FG
GK
KM
MB
B60 ASTRONOMY • APRIL 2021
OBSERVING BASICS
This month, we’ll talk about the relationship
between a star’s spectral class and its color. But
first, I have a question for you: Which constel-
lation has the most stars of spectral class K9? The answer
will appear later in this column.
Spectral class is the primary way stars are categorized.
There are seven major classes, based on the star’s surface
temperature. Arranged from hottest to coolest, they are
identified by the capital letters O, B, A, F, G, K, and M,
remembered by the mnemonic “Oh Be A Fine Girl/Guy
Kiss Me.” (But that’s a little outdated, so try to come up
with your own and share it with us!) Each class is further
divided into subgroups numbered from 0 to 9, ranging
from hottest to coolest. For example, spectral class K9 is
hotter than M0, but cooler than K8. A star’s spectral class
also indicates its color. O-, B-, and A-type stars appear
blue to bluish white to white. F- and G-type stars give off
faint to rich yellow hues, while K- and M-type stars
appear orange to red-orange.
The following stars, all plotted on our Star Dome map
(see page 34), run the gamut of spectral groups and are
all visible on an early April evening. Check them out,
preferably with binoculars, and note what colors you see.The color contrast between stars of differing spectral
classes is much easier to detect when they are close
enough to compare side-by-side. Pollux (Beta [β]
Geminorum) — a star of spectral class K0 — is distinctly
yellow-orange when compared to pure white Castor
(Alpha [α] Geminorum), an A1-type star. After compar-
ing these two, jump to F7-type Polaris (Alpha Ursae
Minoris). What color do you see?
For more of a challenge, compare Mizar (zeta [ζ] Ursae
Majoris) and Alkaid (eta [η] Ursae Majoris), the outer-
most stars in the handle of the Big Dipper. Type B3
Alkaid is nearly a full class hotter than type A2 Mizar.
But does it appear bluer? Four of the remaining Big
Dipper stars are spectral class A, but one is a K1 star. Find
it by study ing each with binoculars or a telescope.
Here’s a spectral class challenge in the form of a match
game. From mid-northern latitudes in April, the constel-
lation Corvus appears above the southern horizon dur-
ing early evening. Its five brightest stars form a
sail-shaped asterism called Spica’s Spanker. I’ve listed
them in the left-hand column below. In the right-hand
column are their spectral classes, rearranged from hot-
test to coolest. Check each star with binoculars or a
telescope and see if you can match it to its spectral class.
Alchiba (Alpha [α]) B8
Kraz (Beta[β]) A0
Gienah (Gamma [γ]) F0
Algorab (Delta [δ]) G5
Minkar (Epsilon [ε]) K2
Now back to our opening question: Which constella-
tion has the most stars of spectral class K9? It’s a four-way
tie between Canis Major (the Big Dog), Canis Minor (the
Little Dog), Canes Venatici (the Hunting Dogs), and
Vu lpecu la (t he Fox). All of these stars are K9 (canine)!
All right, stop the groaning. It’s April and I couldn’t
re sist a l it t le Apr i l Fool ’s f u n. To be Si r iu s — I mea n ser i-
ous — the idea for this column came from an email sent
by Hong Kong resident Michael Sloboda to Phil
Harrington and myself. He noted that an online search
turned up plenty of examples of stars of spectral classes
K0 through K7, but practically no K8 or K9 stars. Why?
Unlike the other K-type stars mentioned earlier in this
article, which are luminous giants, the typical star of
spectral class K is an orange dwarf — a main sequence
star that’s cooler, less massive, and intrinsically fainter
than the Sun. If you were to study a list of the brightest
stars, late K-type stars would be virtually nonexistent.
But look at a list of the nearest stars, and you’ll find the
occasiona l K8 or K9 star.
Questions, comments, or suggestions? Email me at
[email protected]. Next month: choosing an ideal
observing site. Clear skies!Learn the alphabet for stellar temperatures.
Color-coding
stars
Color Star Spectral Class
O Meissa (Lambda [λ] Orionis) O8
B Regulus (Alpha [α] Leonis) B7
A Denebola (Beta [β] Leonis) A3
F Procyon (Alpha [α] Canis Minoris) F5
G Capella (Alpha [α] Aurigae) G5*
K Alphard (Alpha [α] Hydrae) K3
M Betelgeuse (Alpha [α] Orionis) M2
*actually a binary system with stars of spectral classes G1 and G8.Stars are typically
categorized by
spectral class based
on their colors, which
are directly correlated
to their temperatures.
ASTRONOMY: ROEN KELLYBY GLENN CHAPLE
Glenn has been an
avid observer since
a friend showed
him Saturn through
a small backyard
scope in 1963.BROWSE THE “OBSERVING BASICS” ARCHIVE
AT http://www.Astronomy.com/ChapleSPECTRAL CLASS
SURFACE TEMPERATURE