Editing Horizontal branch (section)

==Horizontal branch morphology==
Stars on the horizontal branch all have very similar core masses, following the helium flash.  This means that they have very similar luminosities, and on a [[Hertzsprung–Russell diagram]] plotted by visual magnitude the branch is horizontal.

The size and temperature of an HB star depends on the mass of the hydrogen envelope remaining around the helium core.  Stars with larger hydrogen envelopes are cooler.  This creates the spread of stars along the horizontal branch at constant luminosity.  The temperature variation effect is much stronger at lower [[metallicity]], so old clusters usually have more pronounced horizontal branches.<ref name="KippenhahnWeigert2012">{{cite book|author1=Rudolf Kippenhahn|author2=Alfred Weigert|author3=Achim Weiss|title=Stellar Structure and Evolution|url=https://books.google.com/books?id=wdSFB4B_pMUC&pg=PA408|date=31 October 2012|publisher=Springer Science & Business Media|isbn=978-3-642-30304-3|pages=408–}}</ref>

Although the horizontal branch is named because it consists largely of stars with approximately the same absolute magnitude across a range of temperatures, lying in a horizontal bar on a color–magnitude diagrams, the branch is far from horizontal at the blue end. The horizontal branch ends in a "blue tail" with hotter stars having lower luminosity, occasionally with a "blue hook" of extremely hot stars.  It is also not horizontal when plotted by bolometric luminosity, with hotter horizontal branch stars being less luminous than cooler ones.<ref name=yee>{{cite journal|bibcode=1994ApJ...423..248L|title=The Horizontal-Branch Stars in Globular Clusters. II. The Second Parameter Phenomenon|journal=The Astrophysical Journal|volume=423|pages=248|last1=Lee|first1=Young-Wook|last2=Demarque|first2=Pierre|last3=Zinn|first3=Robert|year=1994|doi=10.1086/173803|doi-access=free}}</ref>

The hottest horizontal-branch stars, referred to as extreme horizontal branch, have temperatures of 20,000–30,000&nbsp;K. This is far beyond what would be expected for a normal core helium burning star. Theories to explain these stars include binary interactions, and "late thermal pulses", where a thermal pulse that [[asymptotic giant branch]] (AGB) stars experience regularly, occurs after fusion has ceased and the star has entered the superwind phase.<ref>{{cite journal|doi=10.1088/2041-8205/737/2/L27|title=RAPIDLY PULSATING HOT SUBDWARFS IN ω CENTAURI: A NEW INSTABILITY STRIP ON THE EXTREME HORIZONTAL BRANCH?|journal=The Astrophysical Journal|volume=737|issue=2|pages=L27|year=2011|last1=Randall|first1=S. K.|last2=Calamida|first2=A.|last3=Fontaine|first3=G.|last4=Bono|first4=G.|last5=Brassard|first5=P.|bibcode=2011ApJ...737L..27R|doi-access=free}}</ref>  These stars are "born again" with unusual properties. Despite the bizarre-sounding process, this is expected to occur for 10% or more of post-AGB stars, although it is thought that only particularly late thermal pulses create extreme horizontal-branch stars, after the planetary nebular phase and when the central star is already cooling towards a white dwarf.<ref>{{cite journal|bibcode=2008ASPC..391....3J|title=Hydrogen-Deficient Stars: An Introduction|journal=Hydrogen-Deficient Stars|volume=391|pages=3|last1=Jeffery|first1=C. S.|year=2008}}</ref>