Tarf (β Cancri)

Tarf, designated as Beta Cancri, is the brightest star in the zodiacal constellation of Cancer, the celestial crab. It also hosts an exoplanet which is 7.8 times more massive than Jupiter

Key Facts & Summary

  • Tarf is located at around 290 light-years away from our Solar System.
  • Tarf is orbited by a companion star, designated as WDS J08165+0911, thus it is a binary star system. An exoplanet, designated as Beta Cancri b, also orbits this star.
  • Tarf is an orange K-type giant star of spectral type K4III Ba1.
  • This means that it is also a Barium star, a type of cool giant showing enhanced abundances of barium in its spectrum.
  • This star is also slightly variable, having an apparent magnitude that varies from magnitude 3.50 to 3.58. Tarf’s absolute magnitude has been recorded at -1.218.
  • It is unknown how massive Tarf is, however, its radius has been estimated at around 61 solar radii, thus this giant star is 122 times bigger than our Sun.
  • Tarf is also a very energetic star, being 871 times brighter than our Sun.
  • The surface gravity on this star has been estimated at around 1.5 cgs.
  • Tarf is quite a cool star, much cooler than our Sun, having surface average temperatures that reach 3,990 K.
  • This star has a radial velocity of 22.94 km / 14.25 mi per second, and a rotational velocity of 6.9 km / 4.2 mi per second.
  • Its companion star is a red dwarf of the fourteenth magnitude.
  • Tarf’s exact age is currently unknown.
  • The constellation of Cancer is the second dimmest out of the 12 zodiacal constellations. Tarf represents a part of the celestial crab’s body, and together with Asellus Australis, they are the only stars brighter than magnitude four.
  • Cancer is one of the first 48 Greek constellations, first listed by the famous astronomer, Ptolemy, in his 2nd century Almagest. The best time of the year to observe Tarf, and the other stars and deep-sky objects located in Cancer, is during March when the constellation is the most prominent in the sky.

β Cancri

Beta Cancri is known as Tarf, or Altarf, which is the star’s traditional name. This name is of Arabic origin and may translate to “end” or “edge”, referencing the star’s position in its constellation, as it marks one of the celestial legs of the crab.

The WGSN approved the named Tarf in mid-2016, for the star designated as Beta Cancri. In the Washington Double Star Catalogue, Tarf has the designation WDS J08165+0911A.

Since Tarf is the brightest star in its constellation, it should bear the designated of Alpha, and not Beta, however, not all stars are designated based on their brightness

Formation

Tarf / Beta Cancri formed around millions of years ago from an interstellar medium rich in gas and dust. Gravity pulled the swirling gas and dust together and resulted in the brightest star in the zodiacal constellation of Cancer, known as Tarf. The exact age of this star is currently unknown.

Distance, Size, and Mass

Tarf is located at around 290 light-years / 89 parsecs away from our Solar System. It is bright enough to be visible to the naked eye, under the right conditions.

Tarf’s mass is currently unknown, however, its radius has been estimated at around 61 solar radii, thus it has around 6,100% of our Sun’s radius. This means that Tarf is around 122 times bigger than our Sun.

Other Characteristics

Tarf is an orange K-type giant star of spectral type K4III Ba1. This means that it is also a Barium star, a type of cool giant showing enhanced abundances of barium in its spectrum.

This star is also slightly variable, having an apparent magnitude that varies from magnitude 3.50 to 3.58. Tarf’s absolute magnitude has been recorded at -1.218.

Tarf is a very energetic star, being 871 times brighter than our Sun. The surface gravity on this star has been estimated at around 1.5 cgs, and it is quite a cool star, much cooler than our Sun, having surface average temperatures that reach 3,990 K.

This star has a radial velocity of 22.94 km / 14.25 mi per second, and a rotational velocity of 6.9 km / 4.2 mi per second.

Stellar System

Tarf / Beta Cancri is a binary star system, consisting of the orange K-type giant known as Tarf, and a faint 14th magnitude red dwarf star. From its angular distance of 29 arcseconds, the companion star’s star distance from Beta Cancri is around 2,600 AU, with an orbital period of around 76,000 years. The companion star is designated as WDS J08165+0911.

Tarf is also orbited by an exoplanet, which was discovered in 2014. This massive planet has a minimum mass of around 7.8 times that of Jupiter, and an orbital period of 705 days.

Location

Tarf / Beta Cancri is the brightest star in the zodiacal constellation of Cancer, the celestial crab. It marks one of the celestial crab’s legs. Though it is the brightest star in its constellation, it’s fainter brother Asellus Australis / Delta Cancri, which is the second-brightest star in the constellation, is much more famous than Tarf.

This is because Asellus Australis marks the famous open star cluster Praesepe, also known as the Beehive Cluster, or Messier 44. In antiquity, Messier 44 was used as a weather gauge, if Asellus Australis was hidden by clouds, the wind would come from the south.

Asellus Australis is also used as a guide in finding one of the reddest stars in the sky, X Cancri. The constellation of Cancer is one of the first 48 Greek constellations, listed by Ptolemy in his 2nd century Almagest.

Cancer is now part of the zodiac family of constellations, being the second faintest in the sky, and it is the 31st largest constellation in the sky out of the 88 modern constellations.

There are many interesting stars and deep-sky objects in the zodiacal constellation of Cancer, such as Asellus Australis, Tarf, X Cancri, YBP 1194, the Messier 67 open cluster, Messier 44, the QSO J0842+1835 quasar, or the OJ 287 BL Lacertae objects, among many others.

The best time to view Tarf / Beta Cancri, the other stars and deep-sky objects in the constellation of Cancer, is during the month of March when the constellation is the most prominent in the sky.

The Future

Since Tarf’s exact age and mass aren’t known, it is quite difficult to predict its future. If the star has less than 8 solar masses than it can’t end its life in a supernova explosion. Thus, the other alternative is that it will become a very dense and hot white dwarf.

Did you know?

  • If the New Horizons Probe would be headed for Tarf at a speed of around 33,000 m.p.h, it would reach the giant star in around 6 million years.
  • Asellus Australis was involved in the first recorded occultation by Jupiter around 240 BC. This was observed by the famous Greek astronomer Ptolemy, in Alexandria.
  • Asellus Australis marks the radiant of the Delta Cancrids meteor showers, which is in turn associated with the zodiacal constellation of Cancer itself.

Sources:

  1. Wikipedia
  2. Universeguide
  3. Openexoplanetcatalogue
  4. Theskylive

Image Sources:

  1. https://vignette.wikia.nocookie.net/startrek/images/a/a4/Beta_Lankal_primary.jpg/revision/latest/top-crop/width/360/height/450?cb=20140118211513
  2. https://www.universeguide.com/mobile/constellations/stars/cancerlocationaltarf.png
  3. https://upload.wikimedia.org/wikipedia/commons/thumb/b/b8/55_Cancri_B.jpg/1200px-55_Cancri_B.jpg
  4. https://www.universeguide.com/mobile/constellations/exoplanets/altarf.png
  5. https://skyandtelescope.org/wp-content/uploads/Cancer-with-Leo-and-Gemini_Greek-1.jpg
  6. https://theskylive.com/sky/stars/finder-charts/tarf-beta-cancri-finder-chart.png?c=1586784933
  7. https://www.astronomytrek.com/wp-content/uploads/2017/06/The-Beehive-Cluster-M44.jpeg