18 Ursae Majoris
18 Ursae Majoris, or e Ursae Majoris, is a single star in the northern circumpolar constellation of Ursa Major. It is faintly visible to the naked eye with an apparent visual magnitude of 4.832. The annual parallax shift measured from Earth's orbit is 27.90 mas, which provides a distance estimate of 117 light years. It is moving closer to the Sun with a heliocentric radial velocity of −16 km/s, and is an unbound and older member of the Ursa Major Moving Group.
| Observation data Epoch J2000.0 Equinox J2000.0 | |
|---|---|
| Constellation | Ursa Major |
| Right ascension | 09h 16m 11.32784s |
| Declination | +54° 01′ 18.6878″ |
| Apparent magnitude (V) | 4.832 |
| Characteristics | |
| Spectral type | A6 V |
| U−B color index | 0.087 |
| B−V color index | 0.186 |
| Variable type | δ Sct |
| Astrometry | |
| Radial velocity (Rv) | −16.13±1.30 km/s |
| Proper motion (μ) | RA: +49.43 mas/yr Dec.: +59.92 mas/yr |
| Parallax (π) | 27.90 ± 0.20 mas |
| Distance | 116.9 ± 0.8 ly (35.8 ± 0.3 pc) |
| Absolute magnitude (MV) | 2.03 |
| Details | |
| Mass | 1.72±0.02 M☉ |
| Luminosity | 12.83 L☉ |
| Surface gravity (log g) | 3.98±0.2 cgs |
| Temperature | 7,450±150 K |
| Rotational velocity (v sin i) | 158±17 km/s |
| Age | 1.05+0.10 −0.15 Gyr |
| Other designations | |
| Database references | |
| SIMBAD | data |
The stellar classification assigned to this star is A6 V, which indicates it is an A-type main-sequence star that is generating energy through hydrogen fusion at its core. The star's variability was first noticed by American astronomer Frank Schlesinger in 1914 and it has been given the variable star designation DD UMa. This is a low amplitude Delta Scuti variable with a magnitude change of around 0.4 and pulsation cycles of 9.4 and 15.0 cycles per day. It is rotating rapidly with a projected rotational velocity of about 158 km/s. This is giving the star an oblate shape with an equatorial bulge that is an estimated 6% larger than the polar radius.
18 UMA is about a billion years old with 1.72 times the mass of the Sun. It is radiating around 13 times the Sun's luminosity from its photosphere at an effective temperature of about 7,450 K.