Overview

Overview of Variable Stars

by Bob Nelson, Ph.D. (Prince George Centre)


Designations

1. Proper names (eg Mira, Betelgeuse, Polaris), Greek letter designation (Delta Cephei, Rho Cassiopeiae, Chi Cygni)

  • These are retained.

2. One or Two letter designation followed by constellation (Argelander):

  • R, S, T, U, V, W, X, Y, Z
  • RR, RS, RT, RU, RV, RW, RX, RY, RZ
  • SS, ST, SU, SV, SW, SX, SY, SZ
  • TT, ... ... ... ... ... ...
  • ... ... ... ... ... ...
  • ... ... ... ... ...
  • ... ... ... ...
  • ... ... ...
  • ... ...
  • ZZ
  • AA, AB, AC, ... ... ... ... ... ... ... ... ... ... ... ... AZ
  • BB, BC, ... ... ... ... ... ... ... ... ... ... ... ... BZ
  • ... ... ... [J is omitted]
  • QQ, ... ... ... ... ... ... ... ... QZ

These give 334 combinations in all. Examples: R LMi, S Cyg, T Her, V Aur, Y Per, Z Cyg, SS Cyg, TV Her, WX Cyg

3. V followed by a number and constellation

Examples: V728 Her, V969 Oph

Light Curve

  • The brightness is plotted versus time (light curve).
  • If regular, the time for one cycle is the period.
  • Periods may be anywhere from minutes to years.
  • The brightest part of the curve is the maximum; the dimmest, the minimum.
  • The difference between the minimum and maximum is the amplitude.
  • Amplitudes may vary from 0.01 to 14 magnitudes.
  • Many variables are irregular or semiregular, and there is no unique period or amplitude.

Types

  1. Eruptive

    Eruptive variables show sudden, unpredictable outbursts of light

    1. Supernovae
      • Giant star blows off outer layers.
      • Chinese observed SN 1006, 1054, 1181 AD.
      • Tycho observed SN 1572, Kepler observed SN 1604 (V843 Oph).
      • estimated 25-100/year in our gal. not observed by telescope
      • Type I (old, population II stars) - max mag: -20 (abs).
      • Type II (newer, population I stars) - max mag: -15 (abs).
      • The two light curves are distinctly different.
    2. Novae
      • binaries with white dwarf and extended (giant) star
      • Extended star periodically dumps material onto white dwarf, the latter flares up.
      • brightness rise: 7 to 16 mags over several days
      • fades to minimum over years or decades
      • Eta Carinae (1843), T CrB (1866), GK Per (1901), DQ Her (1934), V1500 Cyg (1975)
    3. Dwarf novae
      • same as above but recurring (are all novae recurring?)
      • binaries with K to M subgiant with white dwarf
      • quiescent with periodic outbursts of 2-6 mag at intervals of 10 days to several years
      • Outbursts last several days.
      • U Sco (1863, 1906, 1936, 1979, 1987, 1999, and 2010)
      • T Pyx (1890, 1902, 1920, 1944, 1966, 2011)
      • RS Oph (1898, 1933, 1958, 1967, 1985, 2006)
      • SS Cyg (per 20 - 90 days, amplitude 3-4 mag)
      • U Gem, SS Aur (shorter time scale)
      • Z Cam are similar (may be quiescent for long periods).
    4. FU Orionis
      • nova-like variables of spectral type A to F
      • exhibit occasional flareups (to 6 mag) lasting for decades
    5. Nova-like
      • Spectrum or light variations resemble that of novae.
      • MAY become novae sometime in the future.
      • R Aqr, Z And, BF Cyg
    6. S Doradus
      • very young and very massive, spectral types B to F
      • very bright (to -10 abs mag)
      • brightness variations of 1 to 3 mags
      • They periodically blow off outer layers at high speed.
      • They often exhibit P Cyg spectral lines
      • broad emission with sharp absorption at shorter wavelengths.
    7. T Tauri
      • very young stars, intermediate mass, just entering main sequence (Sp type A - K?)
      • Light is near constant most of the time but is occasionally dimmed suddenly by 0.5 to 1 mag (hours to days).
      • Dimming is thought to be circumstellar dust.
    8. R Coronae Borealis
      • Highly luminous, spectral types F, G, K, R
      • Sim to above, but sudden drop in light is 1 to 9 mags.
      • Dimming is caused by carbon dust (soot!).
      • Recovery is in 10s to 100s of days.
    9. Flare stars
      • faint red dwarfs which show extremely fast outbursts of up to several magnitudes in 1 or 2 minutes(!)
      • UV Ceti, DO Cephei, Alpha Centauri C
  2. Pulsating

    Pulsating variables periodically expand and contract, pulsating in size, temperature, and luminosity

    1. Long Period Variables ("Miras")
      • most common type of variable
      • periods from 80 to 1000 days, not abs. constant
      • amplitudes greater than 2.5 mag (average 5 mag)
      • giant stars with M-, C-, or S- type spectra
      • Chi Cyg, R Leo, R Hyd, T Her, TV Her, RS Aur, Z Cyg
    2. Semi-regular and Irregular
      • mostly red giants with poorly defined or no periods
      • Alpha Her, Rho Per (Semi Regular); Betelgeuse, Mu Cep (Irregular)
    3. Cepheids
      • important for the distance scale
      • spectral types F, G, K
      • periods from 1 to 70 days
      • amplitudes between 0.1 and 2 mag
      • absolute magnitude (median) -1.5 to -5 (highly luminous)
      • period-luminosity relation - measure period, then read absolute magnitude
      • Type I Cepheids (Delta Cephei, Polaris, Eta Aql)
      • Type II Cepheids (W Vir) - diff. period-luminosity rel
    4. RR Lyrae ("Cluster variables")
      • among the most common (4500 in Milky Way Galaxy)
      • common in globular clusters (Population II - metal poor)
      • A to F giants, 100 times more luminous than the sun
      • Periods vary from 0.2 to 1.2 days.
      • Amplitudes are less than 2 magnitudes.
    5. RV Tauri
      • Pulsating supergiants of spectral type F to K
      • Exhibit alternate shallow and deep minima (occasionally confused with eclipsing binaries)
      • Periods vary from 30 to 150 days.
      • Amplitudes vary from 3 to 4 mags.
    6. Delta Scuti
      • spectral types A2 to F5
      • periods 0.02 to 0.4 days, amplitude 0.003 to 0.04 mag
      • Light curves are highly variable.
      • Many are on (or near) the main sequence.
    7. ZZ Ceti
      • non-radially pulsating white dwarfs
      • periods 30 sec to 25 min, amplitude less than 0.2 mag
    8. Z And (symbiotic stars)
      • close binaries with cool giant and hot companion
      • Variation is caused by a combination of the cool giant's pulsation plus an interaction (mass interchange?).
      • Double system is often enclosed in nebulosity.
  3. Eclipsing

    Eclipsing variables are binary stars whose orbit is nearly edge-on and that periodically eclipse one another. Binaries may be detached (separate), semi-detached (one star fills the Roche lobe), or contact (both do).

    1. Algol type
      • Components are spherical, well separated (detached).
      • Light curve is flat between eclipses.
      • U Cep, U Sag
    2. Beta Lyrae
      • gravitationally distorted, ellipsoidal components
      • spectral types O, B, or A
      • amplitude less than 2 mag, periods greater than 1 day
    3. W Ursae Majoris
      • highly distorted dwarf components, type F, G, or K
      • exhibit continuously changing light curves
      • amplitudes less than 1 mag, periods less than 1 day
      • contact binaries

References

  • JAAVSO vol 15, #2 (75th Anniversary Ed), pp 77 - 95, and elsewhere
  • Abell, Morrison, & Wolff, Exploration of the Universe pp 435-7, 542-7
  • Sky Catalogue 2000.0, pages xvii to xx
  • Burnham's Celestial Handbook, p 88-91, and elsewhere
  • Petit, M., Variable Stars, 1982 (John Wiley & Sons, New York)
  • Hoffmeister, C., Richter, G., & Wenzel, W., Variable Stars, 1985 (Springer-Verlag, New York)
  • Sterken, C. & Jaschken, C., Light Curves of Variable Stars, 1996 (Cambridge U. Press, N.Y.)
Author: 
Anonymous
Last modified: 
Sunday, May 5, 2019 - 5:01pm