BL Telescopii

{{Short description|Star in the constellation Telescopium}}

{{Starbox begin}}

{{Starbox image

| image = 250px

| caption = A visual band light curve for BL Telescopii, plotted from data published by van Genderen (1983)

}}

{{Starbox observe

| epoch=J2000.0 (ICRS)

| ra={{RA|19|06|38.10893}}{{cite web |url =http://simbad.u-strasbg.fr/simbad/sim-id?Ident=BL+Telescopii&NbIdent=1&Radius=2&Radius.unit=arcmin&submit=submit+id |title = BL Telescopii -- Eclipsing binary of Algol type (detached) |work = SIMBAD Astronomical Database|publisher=Centre de données astronomiques de Strasbourg |accessdate = 14 July 2014}} |

| dec={{DEC|-51|25|03.2131}}

| appmag_v=7.09 - 9.08

| constell=Telescopium

}}

{{Starbox character

| class=F4Ib+M |

| b-v=

| u-b=

| variable=Algol

}}

{{Starbox astrometry

| radial_v=+98.57

| prop_mo_ra=-6.876

| prop_mo_dec=0.525

| parallax=0.6223

| p_error=0.0410

| parallax_footnote={{cite Gaia DR2|6657628225713981312}}

| dist_ly=

| dist_pc=

| absmag_v={{val|-4.5}}

}}

{{Starbox orbit

| period_unitless = {{val|778.0|ul=days}}

| k1 = {{val|19.3|0.5}}

| eccentricity = {{val|0.31|0.02}}

| inclination = ~68{{cite journal |bibcode=1974MNRAS.167..283V |title=The long period, high latitude, eclipsing system V748 Cen (= Cen X-4?) and BL Tel |journal=Monthly Notices of the Royal Astronomical Society |volume=167 |issue=2 |pages=283–298 |last1=Van Genderen |first1=A. M. |last2=Glass |first2=I. S. |last3=Feast |first3=M. W. |year=1974 |doi=10.1093/mnras/167.2.283 |doi-access= free}}

}}

{{Starbox detail

| mass =

| radius = 64

| temperature = 5,438

| luminosity = 3,225

}}

{{Starbox catalog

| names=CD−51 11917, HD 177300, HIP 93844

}}

{{Starbox reference

| Simbad = BL+Tel

}}

{{Starbox end}}

BL Telescopii is a multiple star in the constellation Telescopium. An Algol-like eclipsing binary, the star system varies between apparent magnitudes 7.09 and 9.08 in just over 778 days (2 years 48 days),{{cite web|url=http://www.aavso.org/vsx/index.php?view=detail.top&oid=36332 |title=BL Telescopii |author=Otero, Sebastian Alberto |date=31 July 2006 |work=AAVSO Website|publisher=American Association of Variable Star Observers|accessdate=14 July 2014}} which is generally too faint to be seen with the unaided eye.{{cite web|url=http://www.skyandtelescope.com/resources/darksky/3304011.html?page=1&c=y|title=The Bortle Dark-Sky Scale|last=Bortle|first=John E.|date=February 2001|work=Sky & Telescope|publisher=Sky Publishing Corporation|accessdate=14 July 2014|archive-date=31 March 2014|archive-url=https://web.archive.org/web/20140331202746/http://www.skyandtelescope.com/resources/darksky/3304011.html?page=1&c=y|url-status=dead}} This is mainly due to the system being an eclipsing binary (that is, one star passing in front of the other star and resulting in a change in brightness). The eclipse itself dims the star by two magnitudes and lasts around 104 days.

Dutch astronomer Willem Jacob Luyten noted this star to be variable in 1935. Minima were retrospectively identified in old photographic plates from 1913 and 1919, and then observed by Howarth in 1936. Initially thought to be an R Coronae Borealis variable, its true nature as an eclipsing binary became clear in the 1940s.{{cite journal | title = The supergiant eclipsing system BL Telescopii |author = Feast, M.W. |journal = Monthly Notices of the Royal Astronomical Society|year = 1967 |volume= 135 |issue = 3 |pages=287–304 |bibcode= 1967MNRAS.135..287F |doi=10.1093/mnras/135.3.287|doi-access= free }}

The primary component is a yellow supergiant, whose spectral type has been calculated as either F5Iab/b or F4Ib. It is intrinsically variable, varying in brightness by 0.02 magnitude. It has pulsations of two periods, 92.5 days and 64.8 days in length. It has been classified as a UU Herculis variable—a class of yellow supergiant with semiregular variability.{{cite journal | title =The pulsations of yellow semi-regular variables II. The F supergiant in the high-latitude binary BL Telescopii |author = Zsoldos, E. | journal =Astronomy and Astrophysics |volume = 286 |pages= 870–74 |year=1994 |bibcode =1994A&A...286..870Z}} These stars are thought to have affinities with Cepheid variables and lie near the instability strip on the Hertzsprung–Russell diagram. The secondary was identified as an M-type star from TiO (titanium oxide) absorption bands visible during the eclipses.

The BL Telescopii system lies outside the galactic plane and has a high space velocity; it is a runaway star.