}}{{Starbox reference
| Simbad = 21+Vul
}}{{Starbox end}}21 Vulpeculae is a single,[23] white-hued star in the northern constellation of Vulpecula. Its distance can be estimated from the annual parallax shift of {{Val|10.97|0.23|ul=mas}},[1] yielding a separation of 297 light years. The star is faintly visible to the naked eye at night, having an apparent visual magnitude of 5.19.[3] It is moving further away with a heliocentric radial velocity of about +7 km/s,[3] having come within {{Convert|74.53|pc|ly|order=flip|abbr=on|sigfig=3}} around 4.2 million years ago.[3]There is some disagreement about the class of this star. Cowley et al. (1969) listed it as an A-type subgiant star with a stellar classification of A7 IVn,[4] where the 'n' notation indicates "nebulous" lines due to rapid rotation. The luminosity class of IV suggests the star has exhausted the supply of hydrogen at its core and is evolving away from the main sequence. Slettebak (1982) classified it as an A5 IV shell star[29] and it was so regarded by Hauck & Jaschek (2000),[30] while Abt & Morrell (1995) listed it with a class of A5 Vn,[5] indicating a rapidly-rotating A-type main-sequence star. It is a Delta Scuti type variable star,[7] with a dominant pulsation period of 0.1881 days and an amplitude of 0.016 in magnitude.[20]
David and Hillenbrand (2015) found an average mass for this star of 1.61 times the mass of the Sun,[13] whereas Zorec and Royer (2012) list a much higher mass estimate of {{Val|2.36|0.03|u=solar mass}}.[14] It is about 356[13] million years old with a projected rotational velocity of 222 km/s.[20] This rapid rotation is giving the star an oblate shape with an equatorial bulge that is an estimated 16% larger than the polar radius.[38] Observations since 1997 suggest 21 Vul has an orbiting disk of gaseous material that is too equatorially confined to make it a shell star. The line strengths from this disk have been decreasing over time.[39]
References
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5. ^1 2 {{citation | title=21 VUL - A new luminous long period Delta Scuti star | last1=Garrido | first1=R. | last2=Saez | first2=M. | journal=Astronomy and Astrophysics | volume=79 | pages=347–349 | year=1979 | bibcode=1979A&A....79..347G }}
6. ^1 2 3 4 5 6 {{citation | last1=David | first1=Trevor J. | last2=Hillenbrand | first2=Lynne A. | title=The Ages of Early-Type Stars: Strömgren Photometric Methods Calibrated, Validated, Tested, and Applied to Hosts and Prospective Hosts of Directly Imaged Exoplanets | journal=The Astrophysical Journal | volume=804 | issue=2 | pages=146 | year=2015 | bibcode=2015ApJ...804..146D | doi=10.1088/0004-637X/804/2/146 | arxiv=1501.03154 | postscript=. }}
7. ^1 2 3 {{citation | title=Statistical Properties of Galactic δ Scuti Stars: Revisited | last1=Chang | first1=S.-W. | last2=Protopapas | first2=P. | last3=Kim | first3=D.-W. | last4=Byun | first4=Y.-I. | journal=The Astronomical Journal | display-authors=1 | volume=145 | issue=5 | date=2013 | bibcode=2013AJ....145..132C | doi=10.1088/0004-6256/145/5/132 | arxiv=1303.1031 | postscript=. | page=132}}
8. ^1 2 {{citation | title=The Relation between Rotational Velocities and Spectral Peculiarities among A-Type Stars | journal=Astrophysical Journal Supplement | last1=Abt | first1=Helmut A. | last2=Morrell | first2=Nidia I. | volume=99 | pages=135 | year=1995 | postscript=. | bibcode=1995ApJS...99..135A | doi=10.1086/192182 }}
9. ^1 2 {{citation | title=A study of the bright a stars. I. A catalogue of spectral classifications | journal=Astronomical Journal | last1=Cowley | first1=A. | last2=Cowley | first2=C. | last3=Jaschek | first3=M. | last4=Jaschek | first4=C. | volume=74 | pages=375 | year=1969 | bibcode=1969AJ.....74..375C | doi=10.1086/110819 }}
10. ^1 {{citation | title=Spectral types and rotational velocities of the brighter Be stars and A-F type shell stars | last1=Slettebak | first1=A. | journal=Astrophysical Journal Supplement Series | volume=50 | pages=55−83 | date=September 1982 | doi=10.1086/190820 | bibcode=1982ApJS...50...55S }}
11. ^1 2 3 4 {{citation | title=Rotational velocities of A-type stars. IV. Evolution of rotational velocities | last1=Zorec | first1=J. | last2=Royer | first2=F. | journal=Astronomy & Astrophysics | volume=537 | pages=A120 | year=2012 | doi=10.1051/0004-6361/201117691 | bibcode=2012A&A...537A.120Z | arxiv=1201.2052 | postscript=. }}
12. ^1 {{citation | title=Interferometric observations of rapidly rotating stars | last1=Belle | first1=G. T. | journal=The Astronomy and Astrophysics Review | volume=20| page=51 | year=2012 | doi=10.1007/s00159-012-0051-2 | bibcode=2012A&ARv..20...51V | arxiv=1204.2572 | postscript=. }}
13. ^1 {{citation | title=New Data on A-Type Disk Stars | last1=Abt | first1=Helmut A. | journal=The Astrophysical Journal Supplement Series | volume=174 | issue=2 | id=499–503 | date=February 2008 | doi=10.1086/522698 | bibcode=2008ApJS..174..499A }}
14. ^1 {{citation | title=A-shell stars in the Geneva system | last1=Hauck | first1=B. | last2=Jaschek | first2=C. | journal=Astronomy and Astrophysics | volume=354 | pages=157–162 | date=February 2000 | bibcode=2000A&A...354..157H | postscript=. }}