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Beta Lyrae variable
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<h2 id="mass-flows">Mass flows</h2> <p>These mass flows occur because one of the stars, in the course of its <a href="/facts/Stellar_evolution/Qa4vAGmn">evolution</a>, has become a giant or supergiant. Such extended stars easily lose mass, just because they are so large: gravitation at their surface is weak, so gas easily escapes (the so-called <a href="/facts/Stellar_wind/bn5J6vpx">stellar wind</a>). In close binary systems such as beta Lyrae systems, a second effect reinforces this mass loss: when a giant star swells, it may reach its <a href="/facts/Roche_limit/ho5gpNr3">Roche limit</a>, that is, a mathematical surface surrounding the two components of a binary star where matter may freely flow from one component to the other. </p><p>In binary stars the heaviest star generally is the first to evolve into a giant or supergiant. Calculations show that its mass loss then will become so large that in a comparatively very short time (less than half a million years) this star, that was once the heaviest, now becomes the lighter of the two components. Part of its mass is transferred to the companion star, the rest is lost in space. </p> <h2 id="light-curves">Light curves</h2> <p>The <a href="/facts/Light_curve/zN0wdtaR">light curves</a> of beta Lyrae variables are quite smooth: eclipses start and end so gradually that the exact moments are impossible to define. This occurs because the flow of mass between the components is so large that it envelopes the whole system in a common atmosphere. The <a href="/facts/Amplitude/aEckuXJM">amplitude</a> of the brightness variations is in most cases less than one <a href="/facts/Apparent_magnitude/7pCTF1Me"> magnitude</a>; the largest amplitude known is 2.3 magnitudes (V480 Lyrae). </p><p>The period of the brightness variations is very regular. It is determined by the revolution period of the binary - the time it takes for the two components to orbit once around each other. These periods are short, typically one or a few days. The shortest-known period is 0.29 days (QY Hydrae); the longest is 198.5 days (W Crucis). In beta Lyrae systems with periods longer than 100 days one of the components is generally a <a href="/facts/Supergiant/y7w8WEcm">supergiant</a>. </p><p>Beta Lyrae systems are sometimes[<i>quantify</i>] regarded[<i>by whom?</i>] as a subtype of the <a href="/facts/Algol_variable/4tdb6oIN">Algol variables</a>; however, their light curves differ (the <a href="/facts/Eclipse/zXeAKwyj">eclipses</a> of Algol variables are much more sharply defined). On the other hand, beta Lyrae variables look somewhat like <a href="/facts/W_Ursae_Majoris_variable/A4Iqf11n">W Ursae Majoris variables</a>; however, the latter are in general yet closer binaries (so-called <a href="/facts/Contact_binary/bLXIsCBW"> contact binaries</a>), and their component stars are mostly lighter than the beta Lyrae system components (about 1 M☉). </p> <h2 id="examples-of--lyrae-stars">Examples of β Lyrae stars</h2> <p>The prototype of the β Lyrae type <a href="/facts/Variable_star/Ac8mp2gy">variable stars</a> is <a href="/facts/%CE%92_Lyrae/hlDncrxg">β Lyrae</a>, also called Sheliak. Its variability was discovered in 1784 by <a href="/facts/John_Goodricke/BuN2Hu1A">John Goodricke</a>. </p><p>Nearly a thousand β Lyrae binaries are known: the latest edition of the <a href="http://www.sai.msu.su/gcvs/gcvs">General Catalogue of Variable Stars</a> (2003) lists 835 of them (2.2% of all variable stars). Data for the ten brightest β Lyrae variables are given below. (See also the <a href="/facts/List_of_known_variable_stars/zc293sRP">list of known variable stars</a>.) </p> <table><tbody><tr><th>star</th><th>type<a class="footnote-ref" id="fnref:1" href="#fn:1"><sup>1</sup></a></th><th>period (days)</th><th>maximum<a href="/facts/Apparent_magnitude/7pCTF1Me">magnitude</a></th><th>minimummagnitude</th><th><a href="/facts/Stellar_classification/dYrIuK9H">spectrum</a></th><th>distance(<a href="/facts/Lightyear/IGc5NdQb">lightyears</a>)</th></tr><tr><td><a href="/facts/Zeta_Andromedae/ovsDtL0l">ζ And</a></td><td>EB/GS/RS</td><td>17.7695</td><td>3.92</td><td>4.14</td><td>K1II-III</td><td>181</td></tr><tr><td><a href="/facts/DV_Aquarii/31B1e4CM">DV Aqr</a></td><td>EB</td><td>1.575529</td><td>5.89</td><td>6,25</td><td>A9V</td><td>280</td></tr><tr><td><a href="/facts/UW_Canis_Majoris/bYMBXce8">UW CMa</a></td><td>~EB/KE</td><td>4.393407</td><td>4.84</td><td>5.33</td><td>O7Ia:fp+OB</td><td>~3000</td></tr><tr><td><a href="/facts/Tau_Canis_Majoris/RQYpyIt5">τ CMa</a></td><td>EB</td><td>1.28</td><td>4.32</td><td>4.37</td><td>O9Ib</td><td>~3000</td></tr><tr><td><a href="/facts/Beta_Lyrae/hlDncrxg">β Lyr</a>(prototype)</td><td>EB</td><td>12.913834</td><td>3.25</td><td>4.36</td><td>B8II-IIIep</td><td>880</td></tr><tr><td><a href="/facts/TU_Muscae/a9EbAkau">TU Mus</a></td><td>EB/KE</td><td>1.3</td><td>8.17</td><td>8.75</td><td>O7.5V + O9.5V</td><td>15500</td></tr><tr><td><a href="/facts/Delta_Pictoris/MER967V1">δ Pic</a></td><td>~EB/D</td><td>1.672541</td><td>4.65</td><td>4.90</td><td>B3III+O9V</td><td>1700</td></tr><tr><td><a href="/facts/V_Puppis/CAvqfFx3">V Pup</a></td><td>EB/SD</td><td>1.4544859</td><td>4.35</td><td>4.92</td><td>B1Vp+B3:</td><td>1200</td></tr><tr><td><a href="/facts/PU_Puppis/7ee0dI93">PU Pup</a></td><td>EB</td><td>2.57895</td><td>4.69</td><td>4.75</td><td>B9</td><td>550</td></tr><tr><td><a href="/facts/Upsilon_Sagittarii/7pk8JeBP">υ Sgr</a></td><td>EB/GS</td><td>137.939</td><td>4.53</td><td>4.61</td><td>B8pI:+O9V ? (or F2p?)</td><td>~1700</td></tr><tr><td><a href="/facts/Mu1_Scorpii/bxbADrgI">μ1 Sco</a></td><td>EB/SD</td><td>1.44626907</td><td>2.94</td><td>3.22</td><td>B1.5V+B6.5V</td><td>800</td></tr><tr><td><a href="/facts/Pi_Scorpii/5JSJsUKd">π Sco</a></td><td>EB</td><td>1.57</td><td>2.82</td><td>2.85</td><td>B1V+B2V</td><td>460</td></tr><tr><td><a href="/facts/HD_40372/Nw3GPwfz">HD 40372</a></td><td>EB/DSCTC</td><td>2.74050</td><td>5.88</td><td>5.92</td><td>A5m</td><td>350</td></tr><tr><td><a href="/facts/CX_CMa/SuqvnQrP">CX CMa</a></td><td>EB</td><td></td><td>9.9</td><td>10.7</td><td>B5V</td><td></td></tr></tbody></table> <h2 id="external-links">External links</h2> <ul><li><a href="https://ogle.astrouw.edu.pl/atlas/EB.html">OGLE Atlas of Variable Star Light Curves - β Lyrae variables</a></li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>EB = Beta Lyrae variable D = detached binary DSCTC = Delta Scuti variable GS = at least one giant or supergiant component KE = contact binary RS = RS Canum Venaticorum variable SD = semi-detached binary <a href="/wiki/Delta_Scuti_variable" target="_blank">/wiki/Delta_Scuti_variable</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> </ol>