84 Ursae Majoris
An Ap star with a 1.38-day brightness variation.
84 Ursae Majoris (HD 120198) is a fifth-magnitude star in the constellation Ursa Major, located about 300 light years from Earth. It is an Ap star with a strong magnetic field of 1,100 gauss and is classified as an α² Canum Venaticorum variable star, varying in brightness from magnitude 5.65 to 5.70 over a period of 1.37996 days. Its variability was discovered by Gerhard Jackisch in 1972, and it received the variable star designation CR Ursae Majoris in 1974.
- Designations
- 84 Ursae Majoris, HD 120198, CR Ursae Majoris
- Constellation
- Ursa Major
- Distance
- about 300 light years
- Apparent magnitude
- 5th magnitude (varies 5.65 to 5.70)
- Variability type
- α² Canum Venaticorum variable
- Period
- 1.37996 days
- Magnetic field
- 1,100 gauss
Lore & Background
84 Ursae Majoris is an Ap star, characterized by a strong magnetic field and peculiar abundances of certain elements. Its brightness varies between magnitudes 5.65 and 5.70 with a period of 1.37996 days, typical of α² Canum Venaticorum variables. The star lies just 70 arcseconds from LDS 2914, but that star is believed to be a background object, not physically associated.
In 1994, John Rice and William Wehlau used Doppler imaging to map the surface distribution of iron and chromium on 84 Ursae Majoris. They found that the distributions of these elements were similar across the surface, with abundances varying by a factor of 15. In the regions with the minimum chromium abundance, chromium was still about 600 times more abundant than on the Sun.
The star's angular diameter was measured in red light during 2015 and 2016 using the CHARA array, yielding a limb-darkened angular diameter in milliarcseconds.
Reader's Guide
84 Ursae Majoris is notable as a well-studied example of an Ap star with a strong magnetic field and surface chemical inhomogeneities. Its variability, discovered by Gerhard Jackisch in 1972, is typical of α² Canum Venaticorum stars, where rotation modulates the visibility of spots with enhanced element abundances. The Doppler imaging work by Rice and Wehlau in 1994 provided detailed maps of iron and chromium distributions, showing that these elements vary by a factor of 15 across the surface and that chromium is extremely overabundant relative to the Sun even in its minimum regions. The star's proximity to LDS 2914 is a line-of-sight coincidence. The angular diameter measurement from the CHARA array in 2015–2016 adds to the physical characterization of this variable star. Its variable star designation, CR Ursae Majoris, was assigned in 1974.
Did You Know?
- 84 Ursae Majoris varies in brightness from magnitude 5.65 to 5.70 over 1.37996 days.
- Its magnetic field strength is 1,100 gauss.
- Chromium abundance on its surface is at least 600 times solar in the minimum regions.
- It is located 70 arcseconds from LDS 2914, a background star.
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