Mira variable

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Mira, the prototype of the Mira variables

Mira variables

planetary nebulae and become white dwarfs
within a few million years.

Mira variables are stars massive enough that they have undergone

solar masses,[2] stars that have already lost about half their initial mass.[citation needed] 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).[3][4] 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.[5]

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,[6] a result which is consistent with previous images of individual Mira stars,[7][8][9] so there is now pressure to do realistic three-dimensional modelling of Mira stars on supercomputers.[10]

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.[11] 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.[12]

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.[13]

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.[14] 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.[15][16]

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.[17]

Visualisation of Mira type variable
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.[18]

Star
 Brightest
magnitude

 Dimmest
magnitude
 Period
(in days)
 Distance[citation needed]
(in parsecs) 		Reference
Mira 2.0 10.1 332 92+12
−9[19] 		[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[19] 		[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[19] 		[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[19] 		[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[20] 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[21] [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[22] 		[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

References

  1. ^ "1997JAVSO..25...57M Page 57". adsabs.harvard.edu. Retrieved 2023-02-23.
  2. S2CID 12395165
    . Retrieved 22 November 2020.
  3. S2CID 4262929
    .
  4. S2CID 14529151
    .
  5. S2CID 16934459
    .
  6. S2CID 30825403
    .
  7. doi:10.1086/116182
    .
  8. doi:10.1086/186069
    .
  9. doi:10.1046/j.1365-8711.1999.02512.x
    .
  10. doi:10.1051/0004-6361:20078096
    .
  11. S2CID 12805849
    .
  12. S2CID 17425157
    .
  13. S2CID 14200520
    .
  14. S2CID 16663228
    .
  15. S2CID 359940
    .
  16. Bibcode:2002JAVSO..31....2Z
    .
  17. Bibcode:1997JAVSO..25...57M
    .
  18. ^ Gaia Collaboration (2018), Gaia DR2, VizieR, retrieved 20 April 2019
  19. ^
    S2CID 18759600
    .
  20. ISBN 978-1139495226
    .
  21. S2CID 119022145
    .
  22. doi:10.1093/mnrasl/slx082
    .

External links
Retrieved from "https://en.wikipedia.org/w/index.php?title=Mira_variable&oldid=1190409441"