Universe Guide


Rho2 Arietis, 45 Arietis, HD18191, HIP13654

Rho2 Arietis is a red pulsating giant star that can be located in the constellation of Aries. Rho2 Arietis is the brightest star in Aries based on the Hipparcos 2007 apparent magnitude. The star can be seen with the naked eye, that is, you don't need a telescope/binoculars to see it.

Rho2 Arietis is the Bayer Classification for the star. HIP13654 is the reference name for the star in the Hipparcos Star Catalogue. The Id of the star in the Henry Draper catalogue is HD18191.

Rho2 Arietis has alternative name(s), RZ Ari.

Location of Rho2 Arietis

The location of the star in the galaxy 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 Rho2 Arietis, the location is 02h 55m 48.50 and +18d19`54.0 .

Proper Motion of Rho2 Arietis

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 -14.98 ± 0.18 miliarcseconds/year towards the north and -7.78 ± 0.30 miliarcseconds/year east if we saw them in the horizon.

Physical Properties (Colour, Temperature, Radius) of Rho2 Arietis

Rho2 Arietis has a spectral type of M6IIIvar. This means the star is a red giant star. The star is 7483.00000000 Parsecs from the Galactic Centre or terms of Light Years is 24406.8030315200000000s. The star has a B-V Colour Index of 1.45 which means the star's temperature has been calculated using information from Morgans @ Uni.edu at being 4,076 Kelvin.

Rho2 Arietis Radius has been calculated as being 16.31 times bigger than the Sun.The Sun's radius is 695,800km, therefore the star's radius is an estimated 11,347,927.02.km. However with the 2007 release of updated Hipparcos files, the radius is now calculated at being round 14.20. The figure is derived at by using the formula from SDSS and has been known to produce widely incorrect figures.

Rho2 Arietis Apparent and Absolute Magnitudes

Rho2 Arietis has an apparent magnitude of 5.76 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 0.30 If you used the 2007 Parallax value, you would get an absolute magnitude of 0.60. 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 Rho2 Arietis

Using the original Hipparcos data that was released in 1997, the parallax to the star was given as 8.08 which gave the calculated distance to Rho2 Arietis as 403.67 light years away from Earth or 123.76 parsecs. It would take a spaceship travelling at the speed of light, 403.67 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 9.28 which put Rho2 Arietis at a distance of 351.47 light years or 107.76 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 7,483.00 Parsecs or 24,406.80 Light Years. The Galacto-Centric Distance is the distance from the star to the Centre of the Galaxy which is Sagittarius A*.

Variable Type of Rho2 Arietis

The star is a pulsating Semi-Regular Star which are giants or supergiants of intermediate and late spectral variable type which means that its size changes over time. The Variable Type is usually named after the first star of that type to be spotted. Rho2 Arietis brightness ranges from a magnitude of 5.643 to a magnitude of 5.297 over its variable period. The smaller the magnitude, the brighter the star. Its variable/pulsating period lasts for 0.3 days (variability).

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.

Rho2 Arietis Facts

Alternative Names

Flamsteed Name45 Arietis
Flamsteed Short Name45 Ari
Short NameRZ Ari
Bayer DesignationRho2 Arietis
Hipparcos Library I.D.13654
Bonner DurchmusterungBD+17 457
Henry Draper Designation18191

Visual Facts

Star Typegiant star
Absolute Magnitude0.30 / 0.60
Visual / Apparent Magnitude5.76
Naked Eye VisibleYes - Magnitudes
Right Ascension (R.A.)02h 55m 48.50
Declination (Dec.)+18d19`54.0
Galactic Latitude-35.45 degrees
Galactic Longitude159.93 degrees
1997 Distance from Earth8.08 Parallax (milliarcseconds)
 403.67 Light Years
 123.76 Parsecs
2007 Revised Distance from Earth9.28 Parallax (milliarcseconds)
 351.47 Light Years
 107.76 Parsecs
Galacto-Centric Distance24,406.80 Light Years / 7,483.00 Parsecs
Proper Motion Dec.-14.98 ± 0.18 milliarcseconds/year
Proper Motion RA.-7.78 ± 0.30 milliarcseconds/year
B-V Index1.45
Radial Velocity47.88 ± 0.36 km/s
Spectral TypeM6IIIvar
Colour(M) Red

Variable Star Details

Variable Star ClassPulsating
Variable Star TypeSemi-Regular Star which are giants or supergiants of intermediate and late spectral
Mean Variability Period in Days0.279
Variable Magnitude Range (Brighter - Dimmer)5.297 - 5.643

Estimated Facts

Calculated Effective Temperature4,076 Kelvin

Sources and Links

SIMBAD SourceLink

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