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Theta Sagittae, 17 Sagittae, HD191570, HIP99352, HR7705

Theta Sagittae is a blue to white subgiant star that can be located in the constellation of Sagitta. The description is based on the spectral class. The star can not be seen by the naked eye, you need a telescope to see it. The star has an estimated age of 1.90 Billion of Years but could be as young as 1.50 to 2.20 according to Hipparcos.

Theta Sagittae's Alternative Names

Theta Sagittae (The Sge) is the Bayer Classification for the star. The Id of the star in the Yale Bright Star Catalogue is HR7705. HIP99352 is the reference name for the star in the Hipparcos Star Catalogue. The Id of the star in the Henry Draper catalogue is HD191570.

Flamsteed designations are named after the creator, Sir John Flamsteed. Sir John numbered the stars in the constellation with a number and the latin name, this star's Flamsteed designation is 17 Sagittae with it shortened to 17 Sge.

BD number is the number that the star was filed under in the Durchmusterung or Bonner Durchmusterung, a star catalogue that was put together by the Bonn Observatory between 1859 to 1903. The star's BD Number is BD+20 4453.

More details on star alternative names can be found at Star Names .

Location of Theta Sagittae

The location of the star in the night sky is determined by the Right Ascension (R.A.) and Declination (Dec.), these are equivalent to the Longitude and Latitude on the Earth. The Right Ascension is how far expressed in time (hh:mm:ss) the star is along the celestial equator. If the R.A. is positive then its eastwards. The Declination is how far north or south the star is compared to the celestial equator and is expressed in degrees. For Theta Sagittae, the location is 20h 09m 56.61 and +20° 54` 53.2 .

Proper Motion of Theta Sagittae

All stars like planets orbit round a central spot, in the case of planets, its the central star such as the Sun. In the case of a star, its the galactic centre. The constellations that we see today will be different than they were 50,000 years ago or 50,000 years from now. Proper Motion details the movements of these stars and are measured in milliarcseconds. The star is moving 98.43 ± 0.58 miliarcseconds/year towards the north and 59.68 ± 0.89 miliarcseconds/year east if we saw them in the horizon. The Radial Velocity, that is the speed at which the star is moving away/towards us is -43.10000 km/s with an error of about 0.60 km/s .

Theta Sagittae Luminosity

Luminosity is the amount of energy that a star pumps out and its relative to the amount that our star, the Sun gives out. The figure of 4.25 that I have given is based on the value in the Simbad Hipparcos Extended Catalogue at the University of Strasbourg from 2012.

Physical Properties (Colour, Temperature, Age) of Theta Sagittae

Theta Sagittae has a spectral type of F5IV. This means the star is a blue to white subgiant star. The star is 7,378.00 Parsecs from the Galactic Centre or terms of Light Years is 24,064.33 s. The star has a B-V Colour Index of 0.39 which means the star's temperature has been calculated using information from Morgans @ Uni.edu at being 6,775 Kelvin.

Theta Sagittae Radius has been calculated as being 1.52 times bigger than the Sun.The Sun's radius is 695,800km, therefore the star's radius is an estimated 1,060,565.32.km. If you need the diameter of the star, you just need to multiple the radius by 2. However with the 2007 release of updated Hipparcos files, the radius is now calculated at being round 1.53. The figure is derived at by using the formula from SDSS and has been known to produce widely incorrect figures. The star's Iron Abundance is -0.18 with an error value of 0.08 Fe/H with the Sun has a value of 1 to put it into context.

The stars age according to Hipparcos data files put the star at an age of about 1.90 Billion years old but could be between 1.50 and 2.20 Billion years old. In comparison, the Sun's age is about 4.6 Billion Years Old.

Theta Sagittae Apparent and Absolute Magnitudes

Theta Sagittae has an apparent magnitude of 6.51 which is how bright we see the star from Earth. Apparent Magnitude is also known as Visual Magnitude. If you used the 1997 Parallax value, you would get an absolute magnitude of 3.24 If you used the 2007 Parallax value, you would get an absolute magnitude of 3.23. Magnitude, whether it be apparent/visual or absolute magnitude is measured by a number, the smaller the number, the brighter the Star is. Our own Sun is the brightest star and therefore has the lowest of all magnitudes, -26.74. A faint star will have a high number.

Distance to Theta Sagittae

Using the original Hipparcos data that was released in 1997, the parallax to the star was given as 22.22 which gave the calculated distance to Theta Sagittae as 146.79 light years away from Earth or 45.00 parsecs. It would take a spaceship travelling at the speed of light, 146.79 years to get there. We don't have the technology or spaceship that can carry people over that distance yet.

In 2007, Hipparcos data was revised with a new parallax of 22.10 which put Theta Sagittae at a distance of 147.59 light years or 45.25 parsecs. It should not be taken as though the star is moving closer or further away from us. It is purely that the distance was recalculated.

The star's Galacto-Centric Distance is 7,378.00 Parsecs or 24,064.33 Light Years. The Galacto-Centric Distance is the distance from the star to the Centre of the Galaxy which is Sagittarius A*.

Source of Information

The source of the information if it has a Hip I.D. is from Simbad, the Hipparcos data library based at the University at Strasbourg, France. Hipparcos was a E.S.A. satellite operation launched in 1989 for four years. The items in red are values that I've calculated so they could well be wrong. Information regarding Metallicity and/or Mass is from the E.U. Exoplanets. The information was obtained as of 12th Feb 2017.

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Theta Sagittae Facts

Visual Facts


Primary / Proper / Traditional NameTheta Sagittae
Alternative NamesThe Sge, HD 191570, HIP 99352, HR 7705, 17 Sagittae, 17 Sge, BD+20 4453
Spectral TypeF5IV
Multiple Star SystemYes
Star TypeSubgiant Star
Colour blue to white
GalaxyMilky Way
ConstellationSagitta
Age1.90 Billion Years Old
Age Range1.50 - 2.20 Billion Years Old
Absolute Magnitude3.24 / 3.23
Visual / Apparent Magnitude6.51
Naked Eye VisibleRequires a 7x50 Binoculars - Magnitudes
Right Ascension (R.A.)20h 09m 56.61
Declination (Dec.)+20° 54` 53.2
Galactic Latitude-6.69 degrees
Galactic Longitude60.57 degrees
1997 Distance from Earth22.22 Parallax (milliarcseconds)
 146.79 Light Years
 45.00 Parsecs
2007 Distance from Earth22.10 Parallax (milliarcseconds)
 147.59 Light Years
 45.25 Parsecs
Galacto-Centric Distance24,064.33 Light Years / 7,378.00 Parsecs
Proper Motion Dec.98.43 ± 0.58 milliarcseconds/year
Proper Motion RA.59.68 ± 0.89 milliarcseconds/year
B-V Index0.39
Radial Velocity-43.10 ± 0.60 km/s
Iron Abundance-0.18 ± 0.08 Fe/H
Eccentricity0.17
Semi-Major Axis6733.00
Stellar Luminosity (Lsun)4.25

Companions (Multi-Star and Exoplanets) Facts


Exoplanet CountNone/Unaware

Estimated Facts


Calculated Effective Temperature6,775 Kelvin

Sources and Links


SIMBAD SourceLink

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