Mira variable

Mira, the prototype of the Mira variables

Mira variables /ˈmaɪrə/ (named for the prototype star Mira) are a class of pulsating stars characterized by very red colours, pulsation periods longer than 100 days, and amplitudes greater than one magnitude in infrared and 2.5 magnitude at visual wavelengths. They are red giants in the very late stages of stellar evolution, on the asymptotic giant branch (AGB), that will expel their outer envelopes as planetary nebulae and become white dwarfs within a few million years.

Mira variables are stars massive enough that they have undergone helium fusion in their cores but are less than two solar masses, stars that have already lost about half their initial mass. However, they can be thousands of times more luminous than the Sun due to their very large distended envelopes. They are pulsating due to the entire star expanding and contracting. This produces a change in temperature along with radius, both of which factors cause the variation in luminosity. The pulsation depends on the mass and radius of the star and there is a well-defined relationship between period and luminosity (and colour). The very large visual amplitudes are not due to large luminosity changes, but due to a shifting of energy output between infra-red and visual wavelengths as the stars change temperature during their pulsations.

Light curve of χ Cygni.

Early models of Mira stars assumed that the star remained spherically symmetric during this process (largely to keep the computer modelling simple, rather than for physical reasons). A recent survey of Mira variable stars found that 75% of the Mira stars which could be resolved using the IOTA telescope are not spherically symmetric, a result which is consistent with previous images of individual Mira stars, so there is now pressure to do realistic three-dimensional modelling of Mira stars on supercomputers.

Mira variables may be oxygen-rich or carbon-rich. Carbon-rich stars such as R Leporis arise from a narrow set of conditions that override the normal tendency for AGB stars to maintain a surplus of oxygen over carbon at their surfaces due to dredge-ups. Pulsating AGB stars such as Mira variables undergo fusion in alternating hydrogen and helium shells, which produces periodic deep convection known as dredge-ups. These dredge-ups bring carbon from the helium burning shell to the surface and would result in a carbon star. However, in stars above about 4 M_☉, hot bottom burning occurs. This is when the lower regions of the convective region are hot enough for significant CNO cycle fusion to take place which destroys much of the carbon before it can be transported to the surface. Thus more massive AGB stars do not become carbon-rich.

Mira variables are rapidly losing mass and this material often forms dust shrouds around the star. In some cases conditions are suitable for the formation of natural masers.

A small subset of Mira variables appear to change their period over time: the period increases or decreases by a substantial amount (up to a factor of three) over the course of several decades to a few centuries. This is believed to be caused by thermal pulses, where the helium shell reignites the outer hydrogen shell. This changes the structure of the star, which manifests itself as a change in period. This process is predicted to happen to all Mira variables, but the relatively short duration of thermal pulses (a few thousand years at most) over the asymptotic giant branch lifetime of the star (less than a million years), means we only see it in a few of the several thousand Mira stars known, possibly in R Hydrae. Most Mira variables do exhibit slight cycle-to-cycle changes in period, probably caused by nonlinear behaviour in the stellar envelope including deviations from spherical symmetry.

Mira variables are popular targets for amateur astronomers interested in variable star observations, because of their dramatic changes in brightness. Some Mira variables (including Mira itself) have reliable observations stretching back well over a century.

Visualisation of Mira type variable

List

The following list contains selected Mira variables. Unless otherwise noted, the given magnitudes are in the V-band, and distances are from the Gaia DR2 star catalogue.

Star	Brightest magnitude	Dimmest magnitude	Period (in days)	Distance (in parsecs)	Reference
Mira	2.0	10.1	332	92+12 −9	[1]
Chi Cygni	3.3	14.2	408	180+45 −30	[2]
R Hydrae	3.5	10.9	380	224+56 −37	[3]
R Carinae	3.9	10.5	307	387+81 −57	[4]
R Leonis	4.4	11.3	310	71+5 −4	[5]
S Carinae	4.5	9.9	149	497+22 −20	[6]
R Cassiopeiae	4.7	13.5	430	187+9 −8	[7]
R Horologii	4.7	14.3	408	313+40 −32	[8]
U Orionis	4.8	13.0	377	216+19 −16	[9]
RR Scorpii	5.0	12.4	281	277+18 −16	[10]
R Serpentis	5.2	14.4	356	285+26 −22	[11]
T Cephei	5.2	11.3	388	176+13 −12	[12]
R Aquarii	5.2	12.4	387	320+31 −26	[13]
R Centauri	5.3	11.8	502	385+159 −87	[14]
RR Sagittarii	5.4	14	336	386+48 −38	[15]
R Trianguli	5.4	12.6	267	933+353 −201	[16]
S Sculptoris	5.5	13.6	367	1078+1137 −366	[17]
R Aquilae	5.5	12.0	271	238+27 −22	[18]
R Leporis	5.5	11.7	445	419+15 −14	[19]
W Hydrae	5.6	9.6	390	164+25 −19	[20]
R Andromedae	5.8	15.2	409	242+30 −24	[21]
S Coronae Borealis	5.8	14.1	360	431+60 −47	[22]
U Cygni	5.9	12.1	463	767+34 −31	[23]
X Ophiuchi	5.9	8.6	338	215+15 −13	[24]
RS Scorpii	6.0	13.0	319	709+306 −164	[25]
RT Sagittarii	6.0	14.1	306	575+48 −41	[26]
RU Sagittarii	6.0	13.8	240	1592+1009 −445	[27]
RT Cygni	6.0	13.1	190	888+47 −43	[28]
R Geminorum	6.0	14.0	370	1514+1055 −441	[29]
S Gruis	6.0	15.0	402	671+109 −82	[30]
V Monocerotis	6.0	13.9	341	426+50 −41	[31]
R Cancri	6.1	11.9	357	226+32 −25	[32]
R Virginis	6.1	12.1	146	530+28 −25	[33]
R Cygni	6.1	14.4	426	674+47 −41	[34]
R Boötis	6.2	13.1	223	702+60 −52	[35]
T Normae	6.2	13.6	244	1116+168 −129	[36]
R Leonis Minoris	6.3	13.2	372	347+653 −137	[37]
S Virginis	6.3	13.2	375	729+273 −156	[38]
R Reticuli	6.4	14.2	281	1553+350 −241	[39]
S Herculis	6.4	13.8	304	477+27 −24	[40]
U Herculis	6.4	13.4	404	572+53 −45	[41]
R Octantis	6.4	13.2	407	504+46 −39	[42]
S Pictoris	6.5	14.0	422	574+74 −59	[43]
R Ursae Majoris	6.5	13.7	302	489+54 −44	[44]
R Canum Venaticorum	6.5	12.9	329	661+65 −54	[45]
R Normae	6.5	12.8	496	581+10000 −360	[46]
T Ursae Majoris	6.6	13.5	257	1337+218 −164	[47]
R Aurigae	6.7	13.9	458	227+21 −17	[48]
RU Herculis	6.7	14.3	486	511+53 −44	[49]
R Draconis	6.7	13.2	246	662+58 −49	[50]
V Coronae Borealis	6.9	12.6	358	843+43 −39	[51]
T Cassiopeiae	6.9	13.0	445	374+37 −31	[52]
R Pegasi	6.9	13.8	378	353+35 −29	[53]
V Cassiopeiae	6.9	13.4	229	298+15 −14	[54]
T Pavonis	7.0	14.4	244	1606+340 −239	[55]
RS Virginis	7.0	14.6	354	616+81 −64	[56]
Z Cygni	7.1	14.7	264	654+36 −33	[57]
S Orionis	7.2	13.1	434	538+120 −83	[58]
T Draconis	7.2	13.5	422	783+48 −43	[59]
UV Aurigae	7.3	10.9	394	1107+83 −72	[60]
W Aquilae	7.3	14.3	490	321+22 −20	[61]
S Cephei	7.4	12.9	487	531+23 −21	[62]
R Fornacis	7.5	13.0	386	633+44 −38	[63]
RZ Pegasi	7.6	13.6	437	1117+88 −76	[64]
RT Aquilae	7.6	14.5	327	352+24 −21	[65]
V Cygni	7.7	13.9	421	458+36 −31	[66]
RR Aquilae	7.8	14.5	395	318+33 −28	[67]
S Boötis	7.8	13.8	271	2589+552 −387	[68]
WX Cygni	8.8	13.2	410	1126+86 −75	[69]
W Draconis	8.9	15.4	279	6057+4469 −1805	[70]
R Capricorni	8.9	14.9	343	1407+178 −142	[71]
UX Cygni	9.0	17.0	569	5669+10000 −2760	[72]
LL Pegasi	9.6 K	11.6 K	696	1300	[73]
TY Cassiopeiae	10.1	19.0	645	1328+502 −286	[74]
IK Tauri	10.8	16.5	470	285+36 −29	[75]
CW Leonis	11.0 R	14.8 R	640	95+22 −15	[76]
TX Camelopardalis	11.6 B	17.7 B	557	333+42 −33	[77]
LP Andromedae	15.1	17.3	614	400+68 −51	[78]

See also

• Long period variable
• Semiregular variable star

External links

• OGLE Atlas of Variable Star Light Curves - Mira Variables