STI 2051 was part of an observing report titled "Double the Fun" on DeepSkyForum and "Lake Sonoma on 2/13 and 2/15" on my own web site (Adventures in Deep Space). I've stopped posting observing reports here because of restrictions on using labeled SDSS images. But here's the post on STI 2051...
04 31 11.5 +58 58 37
V = 11.4/12.4; Separation 10"
This unusual 10" pair was discovered by Dutch Roman Catholic priest and astronomer Johan Stein in 1908. It consists of a mag 11.1 red dwarf (Stein 2051A) and a mag 12.4 white dwarf (Stein 2051B), at a distance of only 18 light years. This HST image shows only the white dwarf component (bluish color), along with a very close but distant field star. At 10” separation, the red and white dwarf pair were very easily resolved at 200x. The brighter red dwarf component on the southwestern side had a reddish hue with careful examination. The duo has a large proper motion, but is located ~9' E of mag 8.9 HD 28176.
Stein 2051 B is the 6th nearest white dwarf to the Sun after Sirius B, Procyon B, van Maanen's star, LP 145-141 and 40 Eridani B. Remarkably in 2017 Stein 2051B was observed using the HST passing in front of a more distant star about 5,000 light-years away. The HST news blurb reads…
"Looks can be deceiving. In this Hubble Space Telescope image, the white dwarf star Stein 2051 B and the smaller star below it appear to be close neighbors. The stars, however, reside far away from each other. Stein 2051 B is 17 light-years from Earth; the other star is about 5,000 light-years away.
Astronomers made the Hubble observations of the white dwarf, the burned-out core of a normal star, and the faint background star over a two-year period. Hubble observed the dead star passing in front of the background star, deflecting its light. During the close alignment, the distant starlight appeared offset by about 2 milliarcseconds from its actual position. This deviation is so small that it is equivalent to observing an ant crawl across the surface of a quarter from 1,500 miles away. From this measurement, astronomers calculated that the white dwarf's mass is roughly 68 percent of the sun's mass.”
The results were recently published by Sahu, Anderson, Casertano, Bond et al, in a paper titled "Relativistic deflection of background starlight measures the mass of a nearby white dwarf star". Here’s the abstract:
"Gravitational deflection of starlight around the Sun during the 1919 total solar eclipse provided measurements that confirmed Einstein’s general theory of relativity. We have used the Hubble Space Telescope to measure the analogous process of astrometric microlensing caused by a nearby star, the white dwarf Stein 2051 B. As Stein 2051 B passed closely in front of a background star, the background star’s position was deflected. Measurement of this deflection at multiple epochs allowed us to determine the mass of Stein 2051 B —the sixth nearest white dwarf to the Sun—as 0.675 ± 0.051 solar masses. This mass determination provides confirmation of the physics of degenerate matter and lends support to white dwarf evolutionary theory.”