Systematic Differences in Spectral Classifications of G2V (Sun-like) Stars Between the Michigan Spectral Survey and Modern Surveys Relying on Keenan's Revised MK Standard Star Sequence

Or "When is a G2 Dwarf not a G2 Dwarf?"

Eric Mamajek (U. Rochester)

Last updated: 24 April 2016

Synopsis

Given that stars spectrally classified as "G2V" typically have astrophysical stellar properties very similar to that of the Sun, it is worth investigating whether there are systematic differences in classification between investigators. Here I demonstrate that there are systematic differences between the spectral classifications of two major spectral surveys that have been conducted over the past few decades: the Michigan Spectral Survey (e.g. Houk & Cowley 1975) and the recent NStars classifications by Gray, Corbally, and collaborators (e.g. Gray et al. 2001, 2003, 2006). The Michigan survey by Houk relied more closely on the spectral standard grid of Johnson & Morgan (1953) and Morgan & Keenan (1973), whereas the Nstars survey of Gray and collaborators relies more closely on the revised MK standard grid defined by Keenan later in his career (e.g. mainly Keenan & McNeil 1989). To wit: a G2V in one survey is not necessarily a G2V in another survey, and when we say a star has been classified "on the MK system", one needs to be clear about which standards one has used.

I show that stars classifed as "G2V" on the Keenan system of G-dwarf spectral standard stars (using classifications by Gray and collaborators) would actually appear to be near G4V (on average) by Houk in the Michigan Spectral Survey (although the G4 subtype was explicitly not used in the survey, so they would probably appear as a "G3/5V" "slash" type; e.g. Houk & Cowley 1975). In turn, stars classified as "G2V" by Houk in the Michigan Spectral Survey are actually "G0.5V" - on average - on the Keenan system (as classified by Gray and collaborators; e.g. Gray et al. 2001, 2003, 2006). As Houk discussed in a 1994 conference proceedings (Houk 1994), the Michigan classifications are "pretty close" to the MK system (all the more remarkable as >10^5 classifications were completed by one expert classifier!). As we show below, selecting G2V stars from the Michigan Spectral Survey catalogs is unlikely to identify many stars with properties cluster around those of the Sun (i.e. "solar twins"), but will in fact select stars that are 2.5 subtypes hotter (~200K hotter!). These systematic shifts at the >=2 subtype level are significant, and can not be simply attributed to observational and astrophysical scatter in the classifications by different authors.

While the Michigan Spectral Survey is immensely valuable as an enormous catalog of spectral classifications estimated by a single expert classifier, the spectral types among early G-type stars in the Michigan catalog will differ from those on the Keenan system (which is what the majority of astronomers now use; e.g. those recommended by Gray and collaborators; Gray & Corbally 2009).

As the Sun basically defines the G2V spectral type, and if one wants to study solar twins by studying other G2V stars, one should pay attention to these differences in "MK" classifications. As Garrison (1994) has noted, aside from a select minority of stable "anchor point" standards that have remained unchanged since Morgan, Keenan & Kellman (1943; "MK43"), there have been subtle shifts to the "MK" standard stars in the literature over the years - especially at the subtypes between where the anchor standards are well-established. Among the F/G/K dwarfs, there are only stable "anchor" standard stars at F2V, F6V, G0V, G2V, G5V, G8V, K0V, K2V, and K5V. For better or worse, between the 1940s and 1990s there were subtle shifts in the compilations of standard stars by Keenan (but also Morgan) at the intermediate subtypes. There were also numerous instances of old standard stars being introduced in the 1950s, and then later ignored/omitted in later compilations - and of course new standard stars being introduced in the 1970s-1980s that had not appeared in previous compilations.

Comments on the Sun as the G2V standard star

The Sun is the G2V "anchor" standard according Garrison (1994), i.e. its disk-averaged spectrum provides one of the immovable spectral types that provides the backbone of the MK spectral classification system going back to the 1950s. Unfortunately the MKK 1943 atlas did not explicitly list a G2V standard. The Sun was listed as one of three G2V standards by Johnson & Morgan (1953), however it is the only standard to survive to the modern day through Keenan's revisions (both HR 483 and 16 Cyg A were reclassified by Keenan to G1V and G1.5Vb)! See notes on G2V "standards" at http://www.pas.rochester.edu/~emamajek/spt/G2V.txt. The Sun was listed as the only G2V standard in Morgan's 1965 and 1971 papers, and is listed as a "dagger" standard in the Morgan & Keenan (1973) review. Keenan & McNeil (1976) list "Sun (moon)" as their G2V standard, and Keenan (1980) calls the Sun the "defining zero point" for G2V. Keenan later repeated listed the Sun as a G2V standard star in his 1983, 1985, and 1988 standard star compilations. Keenan (1991) observationally looked for variations in the Sun's spectral type, but was (unsurprisingly) unable to detect any variations - and concluded that aside from rare white-light flares, the Sun is essentially solid as a G2V standard star. Gray et al.'s NStars survey program adopts spectra of the Sun (via Jupiter IV & Moon) as a G2V standard.

Note that some confusion regarding classification of G dwarfs may stem from the fact that Houk adopted Beta Com as a G2V standard in the Michigan spectral survey. Unfortunately, Beta Com was considered a G0V "dagger" standard in Morgan & Keenan (1973), but has only been used sparingly as a G0V standard by Keenan (only in 1976 atlas) and Gray (digital atlas and Nstars standard). Unfortunately, Gray (1989) later adopted Beta Com as a primary F9.5V standard star! Given its inglorious history, it may be better if Beta Com is never used as a spectral standard star again (indeed, if a G0V standard is needed - Beta CVn is the undisputed anchor; Garrison 1994). From this disagreement alone, we should not be surprised to see systematic differences in the classifications of early G-type stars at the 2-subtype level between Houk's Michigan classifications and those relying on Keenan's standards (e.g. Gray and collaborators).

Comparison of Gray+ and Houk Classifications of "G2V" stars

As an example, here I compare the classifications of Gray et al. (2001, 2003, 2006) vs. those of Houk (Michigan Spectral Survey, as compiled in the Hipparcos catalog). Gray et al. (2001, 2003, 2006) report spectral classifications for 372, 664, and 1676 stars, respectively. Of those, 53 are classified as "G2V" or a minor variant (e.g. "G2V+", "G2V Fe-1.0"). Of these 53, some 36 are "pure" G2V stars, whereas 16 are variants. I queried all of these stars through the Michigan Catalogue of HD stars Volumes 1-5 (Houk and collaborators 1975, 1978, 1982, 1988, 1999). Of the 53 stars classified as G2V or variant by Gray and collaborators, 35 of these had Houk classifications. A comparison of the types is shown in Table 1.

Name	SpT(Gray)	Ref.	Spt(Houk)	numeric type	Qual.Flag.	Notes
HD 1320	G2V	Gray06	G5V	5	1
HD 2071	G2V	Gray06	G8IV+(F)	8	1	R
HD 20619	G2V	Gray03	G5V	5	1
HD 25874	G2V	Gray06	G2V	2	1
HD 35041	G2V_Fe-0.4_CH-0.5	Gray06	G6V	6	1
HD 37962	G2V	Gray06	G5V	5	2
HD 38858	G2V	Gray03	G3V	3	1	B
HD 45289	G2V	Gray06	G5V	5	1	L
HD 51929	G2V_Fe-1.4_CH-0.7	Gray06	F8/G0V	-1	2
HD 59711	G2V	Gray06	G5V	5	1
HD 59967	G2V	Gray06	G3V	3	1	B
HD 66653	G2V	Gray06	G5V	5	2
HD 75519	G2V	Gray06	G5V	5	3
HD 76151	G2V	Gray01	G3/5V	4	2	BL
HD 76932	G2V_Fe-1.8_CH-1	Gray06	F7/8IV/V	-2.5	2	B
HD 78429	G2V	Gray06	G5V	5	2
HD 88084	G2V	Gray06	G5V	5	2
HD 89707	G2V_Fe-1.5_CH-0.8	Gray06	G1V	1	1
HD 95521	G2V	Gray06	G5V	5	1
HD 98553	G2V_Fe-0.7	Gray06	G2/3V	2.5	2
HD 102365	G2V	Gray06	G3/5V	3.5	1	B
HD 103743	G2V	Gray06	G3V	3	3	R
HD 104982	G2V	Gray06	G5V	5	3
HD 106489	G2V	Gray06	G5V	5	2
HD 108309	G2V	Gray06	G3/5V	4	1	LB
HD 145809	G2V_Fe-0.7	Gray06	G3V	3	1
HD 145825	G2V	Gray06	G1V	1	1
HD 146233	G2V	Gray03	G5V	5	1	BL
HD 153075	G2V_Fe-0.8_CH-0.5	Gray06	G0/2V	1	1
HD 177758	G2V_Fe-1.0_CH-0.6	Gray06	G1V	1	1
HD 195564	G2V	Gray03	G6V	6	1	B
HD 196141	G2V	Gray06	G3V	3	1
HD 199288	G2V_Fe-1.0	Gray06	G0V	0	1	L
HD 202457	G2V	Gray06	G5V	5	1
HD 210918	G2V	Gray06	G5V	5	1	LB

Here is a statistical comparison of the Gray et al. "G2V" stars with Houk's types:

Sample	#	SpT(Houk,median)	SpT(Houk,mean)	s.e.m.	rms
Gray: "G2V" only, no variants	26	G5.0	G4.4	0.3	1.4
Gray: all G2Vs & variants	35	G5.0	G3.6	0.4	2.2
Gray: all G2Vs. Houk: qual=1	23	G4.0	G3.7	0.4	2.0

So the Gray et al. G2V stars classified using Keenan's standards are basically G4Vs on Houk's system. Remarkably, only 1 of Gray's G2V stars was similarly classified G2V by Houk (HD 25874). The lack of agreement is stunning given that both Houk and Gray et al. explicitly use the Sun as a spectral standard (Houk lists Uranus, Gray NStar's WWW page lists spectra of the Moon and Jupiter IV).

This begs the question: What are the G2V stars in Houk's catalog on the Keenan/Gray system?

To answer this, I cross-compared Houk's G2V stars with classifications by Gray et al. The 5 Michigan volumes contain a total of 1106 stars classified as " G2V ", where I have chosen to ignore variants and "slash" types. I found 44 stars classified by Gray and collaborators (which rely on the Keenan & McNeil 1989 grid and the MK "anchor" standards compiled by Garrison 1994).

Name	SpT(Houk)	SpT(Gray)	numeric type
HD 105328	G2V	G0V	0
HD 114853	G2V	G1.5V	1.5
HD 117618	G2V	G0V	0
HD 119638	G2V	F8.5V	-1.5
HD 125881	G2V	G0-V	-0.25
HD 1273	G2V	G5V_Fe-1.2_CH-0.9	5
HD 128620	G2V	G2V	2
HD 134060	G2V	G0V_Fe+0.4	0
HD 134331	G2V	G0+V	0.25
HD 134664	G2V	G1.5V	1.5
HD 1388	G2V	G0V	0
HD 143846	G2V	G0V	0
HD 14802	G2V	G0V	0
HD 15064	G2V	G1V	1
HD 153631	G2V	G0V	0
HD 155918	G2V	G2V_Fe-1.0_CH-0.6	2
HD 158630	G2V	G0V	0
HD 165401	G2V	G0V	0
HD 177409	G2V	G1V_CH-0.4	1
HD 180409	G2V	F9V	-1
HD 183216	G2V	F8.5V_Fe+0.4	-1.5
HD 189340	G2V	F9V	-1
HD 202628	G2V	G1.5V	1.5
HD 205905	G2V	G1V	1
HD 206301	G2V	G1_IV	1
HD 20807	G2V	G0V	0
HD 209779	G2V	G1V	1
HD 217877	G2V	G0-V	-0.25
HD 221420	G2V	G2_IV-V	2
HD 222669	G2V	G1V	1
HD 25874	G2V	G2V	2
HD 31527	G2V	G0V	0
HD 33093	G2V	G0_IV	0
HD 33095	G2V	G1V	1
HD 38973	G2V	G0V	0
HD 4307	G2V	G0V	0
HD 45184	G2V	G1.5V	1.5
HD 56662	G2V	G0V	0
HD 64096	G2V	G0V	0
HD 67458	G2V	G0V	0
HD 68978	G2V	G0.5V	0.5
HD 73744	G2V	G5V_Fe-1.3_CH-0.9	5
HD 8076	G2V	G1V	1
HD 84991	G2V	G0-V	-0.25

There is excellent agreement among the luminosity types, with only 3 of 44 (7%) of the Houk luminosity class V G2 stars being considered either luminosity class "IV" or "IV-V" by Gray and collaborators. Unfortunately the agreement in the subtypes is not as good. Only 3 of the 44 stars agree in temperature subtype (HD 128620, 155918, 25874).

Sample	#	SpT(Gray,median)	SpT(Gray,mean)	s.e.m.	rms
Houk: "G2V" only, no variants	44	G0.0	G0.6	0.2	1.3

After clipping two significant outliers (HD 1273, HD 73744; both of which were classified as very metal poor F5V stars by Gray), the mean Gray type for the "Houk G2V" stars is G0.4, with s.e.m. +-0.14 and rms scatter 0.9 subtype.

Hence, a star classified as "G2V" by Houk in the Michigan survey would be classified as approximately "G0.5V" by Gray and collaborators using the Keenan & McNeil (1989) standard star grid. Hence, the "G2V" stars in the Michigan survey will probably be systematically hotter than the Sun and solar twins by ~200K (on average; see e.g. SpT vs. Teff scale of Pecaut & Mamajek 2013).

Houk's G Dwarf Standard Stars

Here is a comparison between the F7-K0-type dwarf spectral standards adopted by Houk in the "University of Michigan Catalogue of Two-Dimensional Spectral Types for the HD Stars: Volume 1" (Houk & Cowley 1975) and those from Keenan, and other recent classifications by Gray and collaborators (using the MK standard grid largely finalized by Keenan in the 1980s).

Name	SpT(Houk)	ref	Spt(Keenan)	SpT(Gray)
iot Psc (HR 8969)	F7V	JM53	...	F7V [G89,G01]
bet Vir (HR 4540)	F9V [F8V]	MK73 [JM53]	F9V [MK73]	F8.5IV-V [G01]
HD 27836	G1V	MK73	G1V [MK73]	G1V [G89], G0V(e) [G01]
bet Com (HR 4983)*	G2V	...	G0V [MK73,K76], F9.5V [K89]	G0V [G01]
Sun (Uranus)	G2V	MK73	G2V [MK73,K76,K80,K83,K85]	G2V [G09]
85 Peg (HR 9088)*	G3/5V	...	G5Vb Fe-2 [K88,K89]	G5Vb Fe-2 [G01], G5V Fe-1 [G03]
kap Cet (HR 996)	G5V	JM53	G5V [K83,K89]	G5V [G01]
xi Boo (HR 5544)	G8V	JM53	...	G7V [G03]
54 Psc (HR 166)	K0V	JM53	K0V [K80,K83], K0.5V [K89]	K0V [G03]

* = Houk's "revised standards" - she states in notes "We do not suggest that other classifiers use the types we have assigned as these are only based one plate. Others may well come to other conclusions." References: JM53 = Johnson & Morgan (1953), MK73 = Morgan & Keenan (1973), K76 = Keenan & McNeil (1976), K80 = Keenan & Pitts (1980), K83 = Keenan (1983), K85 = Keenan (1985), K89 = Keenan & McNeil (1989), K88 = Keenan & Yorka (1988), G89 = Gray (1989), G01 = Gray et al. (2001), G09 = Gray (2009; "A Digital Spectral Classifciation Atlas"). A few notes are in order. Bet Vir: Houk adopted this as the F8V standard following Johnson & Morgan (1953), but halfway through classifying Volume 1, Morgan & Keenan (1973) published their revised MK73 standard list, and made beta Vir a F9V instead (which Houk then adopted). Bet Com: Obviously the biggest difference, which Houk adopting G2V (same as the Sun), while Keenan first considered it G0V, then F9.5V (1989)! Xi Boo: despite being listed as a G8V standard in the 1943 MKK atlas and again in Johnson & Morgan (1953) - was inexplicably never listed later by Keenan in any of his 1970s-1990s compilations. Gray et al. (2003) list Xi Boo as one subtype earlier (G7V) than the G8V "anchor" standard 61 UMa (Garrison 1994; i.e. its type has been consistently G8V since the 1943 MKK atlas).

So there are some subtle differences between the classifications of the G-type dwarf standards adopted by Houk and those used by Gray and collaborators (mostly relying on Keenan & McNeil 1989).

A Comparison of B-V Colors of "G2V" Stars

A star classified as "G2V" by Gray and collaborators (using Keenan's standards) would normally be classified near "G4V" (or typically within +-2 subtypes rms) in Houk's Michigan Spectral catalog. This likely explains why the mean B-V color of G2V stars in the Hipparcos catalog (w/i 75 pc, with S/N(parallax) > 8) which are mostly classified by Houk are so blue ( = 0.617; sem=0.003, rms=0.032; N=223) compared to the color of the Sun (B-V=0.653; Ramirez et al. 2012) and the mean color of Gray-classified G2V stars (=0.647; sem=0.003, rms=0.035), and why the mean color of Hipparcos G4V stars (mostly classified by Houk) is so similar to that of the Sun and Gray G2V stars: ((G4V) = 0.663 ; sem=0.013, rms=0.034; N=13).

What are the Best Standards for AFGK Dwarfs that Represent the Modern MK System?

After the author has 1) been "burned" by taking spectra of spectral "standards" that have apparently shifted in the literature (e.g. Beta Com), and 2) mistakenly assuming that spectral types listed in one survey are on the same system as those in other surveys, I decided several years ago to compile some notes on the MK standard stars, and come to some conclusions on which standard stars had the best "pedigree", and make those notes public. This was also done in parallel with a taking spectra of many standard stars using the CTIO 1.5-m telescope (some results reported in Pecaut, Mamajek, Bubar 2012 and Pecaut & Mamajek 2013). After conducting this literature exercise over the years, I can highly recommend to the reader that they do not adopt spectral standards blindly from compendia like e.g. Garcia (1989). My notes are compiled, rather unceremoniously, in ASCII text format at: http://www.pas.rochester.edu/~emamajek/spt/. Eventually I hope to make a more respectable website for them, but for now they are probably sufficient as is for anyone really interested in the subject. In the same files I've also compiled my estimates of the best stellar parameters (e.g. Teff, colors, etc.) for the various spectral types.

Here I list what I believe are the "best" dwarf standards for the AFGK-type stars - i.e. those that have the best pedigrees and appear to represent the long-term structure of the MK classification system. This table includes the "anchor" standards listed by Garrison (1994), but includes additional stars in the intermediate subtypes. The stars in italics may be somewhat controversial as standards, or have limited pedigrees, but appear to be the best available (however their status as "best" may change).

Spec. Type	Standard Star	References	Notes
A0 V	alf Lyr	MK43,JM53,MK78,K85,G87,G94,96	G87 & G96 assign "Va" lum class
A0 V	gam UMa	MK43,JM53,MK73,G87,G94,G96	G87 & G96 assign "Van" lum class, "n"=high vsini
A0 V	HD 71155	MK43,JM53,GG87,G03	GG87 & G03 assign "Va" lum class
A1 V	HD 10939	G87	G87 assigns "Va" lum class
A1 V	48 Cet	G87	G87 assigns "Va" lum class
A2 Vn	HD 56405	GG87	GG87 assigns "Va" lum class, "n"=high vsini
A2 V	HD 88955	G87,G06	G06 assigns "Va" lum class
A3 V	alf PsA	MK43,JM53,MK73,GG89,G94,G96	GG89 & G96 assign "Va" lum class
A3 V	bet Leo	MK43,JM53,GG89,G96	GG89 & G96 assign "Va" lum class
A4 V	del Leo	JM53	But GG89 calls "A4IV", G03 calls "A5IV(n)"
A5 V	HD 23194	GG89,G01
A5 Vn	HD 23886	MK78,K85,GG89,G01	"n" = high vsini
A6 V	bet Pic	GG89(non-standard)
A7 Vn	21 LMi	JM53,GG89,G03	Praesepe
A7 Vn	alf Aql	MK43,GG89,G96,G03
A8 V	HD 158352	PM13
A9 V	HD 73450	PM13	Praesepe
A9 Vn	44 Cet	GG89	"n" = high vsini
F0 V	HD 23585	MK73,MK78,K85,G89,GG89,G01	Pleiad
F1 V	37 UMa	M53,C69,C74,GG89	But G03 says "F2V"
F2 V	78 UMa	MK43,JM53,MK73,MK78,K85,G89,G94	UMa cluster
F3 V	HD 26015	M65,MK73,MK78,K85,G89,G01	Hyad
F4 V	HD 27561	M65,PMB12	Hyad
F5 V	HD 27524	M65,MK73,MK78,K85,G89,G01	Hyad
F6 V	pi03 Ori	MK43,JM53,M65,M71,MK73,G89,G94	G01 calls "F6V-IV"
F7 V	tet Per	JM53,G01	none of the F7V "standards" have great pedigrees
F8 V	HD 27808	MK73,MK78,K85,G89,G01	Hyad
F9 V	HD 10647	K88,K99,G03
G0 V	bet CVn	MK43,JM53,M65,MK73,K76,K85,K85,G94,G01,G03
G2 V	Sun	JM53,M65,M71,MK73,K76,K80,K85,K91,G94
G2 V	18 Sco	K88,G03	"solar twin"
G3 V	16 Cyg B	K85,K88,K89,G03,G06
G5 V	kap01 Cet	MK43,JM53,M65,K76,K85,K88, K89,G94,K99
G7 V	HD 111395	M53,G03	not a Keenan standard
G8 V	61 UMa	MK43,JM53,M65,K76,K85,K88, K89,G94,G03
K0 V	sig Dra	MK43,JM53,MK73,KM76,K85,K88, K89,G94
K1 V	107 Psc	MK43,JM53,M53,K88,K89,K99,G06
K2 V	eps Eri	MK43,JM53,M65,KM76,K85,G94, K99,G03
K3 V	HD 219134	MK43,JM53,K83,K85,K88,K89,G03
K3 V	HD 16160	KM76,K89,K99,G03
K4 V	HD 216803	K85,K89	Fomalhaut B. G06 calls "K4+Vk"
K5 V	61 Cyg A	MK43,JM53,M65,MK73,KM76,K80, K85,K88,K89,G94,KHM91,H94,H02
K5 V	HD 36003	K89,K99,G03,PM13	G06 calls "K5-V"
K6 V	HD 120467	K88,K89	G06 calls "K5.5V(k)"
K7 V	61 Cyg B	JM53,MK73,KM76,K80,K88,K89,KHM91,H02
K7 V	HD 157881	JM53,H94,H02,G03,PM13
K8 V	HIP 111288	G06,PM13	G06 appends "k" (Ca K emission)
K9 V	HIP 3261	G06,PM13

References: MK43 = Morgan, Keenan, & Kellman (1943), JM53 = Johnson & Morgan (1953), M65 = Morgan & Hiltner (1965), C69 = Cowley et al. (1969), M71 = Morgan, Hiltner, Garrison (1971), MK73 = Morgan & Keenan (1973), C74 = Cowley & Fraquelli (1974), K76 = Keenan & McNeil (1976), MK78 = Morgan, Abt, Tapscott (1978), K80 = Keenan & Pitts (1980), K83 = Keenan (1983), K85 = Keenan (1985), K88 = Keenan & Yorka (1988), K89 = Keenan & McNeil (1989), K99 = Keenan & Newsom (1999, The Revised Catalog of MK Spectra Types for the Cooler Stars; Keenan's final list), G89 = Gray (1989), GG89 = Gray & Garrison (1989), K91 = Keenan (1991), KHM91 = Kirkpatrick, Henry, & McCarthy (1991), G94 = Garrison (1994; "Anchor standards" ;1994ASPC...60....3G), H94 = Henry et al. (1994), G96 = Garrison & Beattie (1996; "The Brightest Stars"; Observer's Handbook), G01 = Gray et al. (2001), H02 = Henry et al. (2002), G03 = Gray et al. (2003), PMB12 = Pecaut, Mamajek, Bubar (2012), PM13 = Pecaut & Mamajek (2013). Note that not all of the references specifically mention the star as a standard star.

I am still undecided on which G1V, G4V, G6V, and G9V standards are best (or if any of them are adequate), as their use has been somewhat inconsistent in the literature.

Recommendation and Future Work

Given:
1) the shear number of Houk classifications, and their obvious utility for various purposes (e.g. characterizing new debris disk and exoplanet host stars, reddening studies, etc.), but also
2) the modern day acceptance of Keenan's 1980s-era standard star grid defining the F9V-K5V spectral sequence (e.g. Keenan & McNeil 1989; see Gray & Corbally 2009),
then
it may be worth constructing a look-up table between the Houk and Keenan systems so that samples of classified stars may be more meaningfully intercompared (and differences not simply attributed to error or astrophysical scatter).

In closing, the early-G stars in the Houk Michigan Spectral Survey volumes are not technically on the MK system as realized by Keenan's later compilations of G-dwarf standards which are now in common use (e.g. Keenan & McNeil 1989), but they appear to be systematically different at the ~+-2 subtype level in some places! G2V stars classified using Keenan spectral standard stars are not comparable to the stars classified as "G2V" by Houk. This obviously needs to be taken into account when investigators use spectral types for inferring effective temperatures, intrinsic colors, estimating reddenings and extinctions, etc.

If there are questions about which of the standard stars are probably the best modern-day representation of the MK system, I suggest that you consult my notes on the MK standard stars: http://www.pas.rochester.edu/~emamajek/spt/. I have attempted to assess which standards are best (i.e. have been repeatably used by expert classifiers and listed in compilations), and those which are worst (i.e. those whose types which have changed - sometimes repeatably - over the decades according to expert classifiers). Gray & Corbally (2009; "Stellar Spectral Classification") have reproduced tables of MK standards in Appendix A of their book, however the compilation is not complete (missing especially many Keenan standards), some of the choices of standards are not necessarily the best available (after a thorough literature review), and the compilation is missing many M standards adopted by Kirkpatrick & Henry. That said, Gray & Corbally (2009) is a great resource for understanding spectral classification and a great distillation of the past several decades of literature on the subject.

Acknowledgements: I thank Omar Lopez-Cruz, Giannina Dalle Mese, Miguel Chavez, Richard Gray, Chris Corbally, and Mark Pecaut for discussions over the past several years on spectral classification.