In three dimensions

In the seventh entry of February’s “Top 10 space stories of 2022,” the four images of the brightening of a star, caused by the possible microlensing of a black hole, triggered my love of 3D. I looked at the images as stereo pairs and noticed that there was some parallax, making the stars seem to be at varying differences. With further inspection, I determined that it was their actual movement over time that was the cause of the apparent parallax. I made a stereo anaglyph of two of the images, one from August 2011 and the other from August 2017. It appears to show the direction of movement for each star, and I was wondering if this helps in determining the distances and orbits within our galaxy for these stars and perhaps that of the black hole itself? — David Copley, Grand Junction, CO

Senior Editor Alison Klesman responds:

In their July 6, 2022, Astrophysical Journal paper, Kailash Sahu and his colleagues do state that all the stars in the field are expected to move somewhat over the course of six years, thanks to their own proper motions. So, they had to take this into effect when measuring the offset of the lensed star. The authors also noted that while they took proper motion into account, they ignored the overall parallax of the stars. This is because all the stars (including the lensed star) are in the same area of the galactic bulge, so their parallax should be the same. The parallax for these stars is expected to be small (less than 2 milliarcseconds).

You are absolutely correct that measuring such motion helps when determining information about these stars and the intervening black hole. By knowing how much of the lensed star’s apparent deflection was due to proper motion, the researchers then separated out the effects of the lensing to more accurately pin down the mass of the rogue black hole.