May 7, 2023
Sunday, May 7, 2023
It is now final exams week at Bakersfield College. On Friday will be the largest graduating class ever with a record-breaking group who will walk across the stage to get their diploma with family and friends cheering them on. There will be so many people that we'll use both sides of the stadium like we did last year. We'll be doing that for a few years at least because BC continues to grow!
The William M Thomas Planetarium is closed for the summer. Our spring shows were packed and a number of school groups came through for field trips, so I'd say we've moved beyond the pandemic!
Venus continues to shine brilliantly in the west after sunset. It is now midway between the tips of the horns of Taurus and the foot of the Castor half of Gemini. It will continue getting slowly brighter as it draws closer to us in its inside faster orbit. Through a telescope, it still appears as a gibbous phase (more than half lit up) but it'll be at quarter phase at the end of the month. Counter-intuitively, Venus is actually brighter at crescent phase than it is when we see more of its daylight side because at crescent phase, it's significantly closer to us.
Mars is now next to the head of the left side of Gemini, Pollux. The moon was at full phase on Friday, so it's now rising about three hours after sunset as a waning gibbous. It will be in between Scorpius and Ophiuchus. On the night of commencement, May 12, the moon will be at last quarter, rising after midnight, meaning no moon glow to wash out the fireworks display that will conclude the commencement celebration. By the middle of the following week, early risers will be able to see a thin waning crescent moon in the east just before sunrise.
One of the two April planetarium shows was “Black Holes”, which had another sold out performance. People love black holes! The two major astronomy news stories are nice follow ups to the Black Holes show. Both are about supermassive black holes with millions to billions of times the mass of the sun that reside at the centers of almost all galaxies, including our home galaxy, the Milky Way.
The first news story is about the search for binary supermassive black holes—two big black holes orbiting each other. We expect them to be fairly numerous because they would form as one host galaxy collides with another host galaxy—something we see happening often in galaxy clusters. As the galaxies collide and eventually merge, their central supermassive black holes should also merge into an even larger supermassive black hole.
However, we have yet to detect a binary supermassive black hole. Well, detect conclusively that is. There have been some hints but no firm evidence yet. The merging of supermassive black holes produce gravitational waves with a frequency undetectable by our current LIGO-type detectors. Also, the merging galaxies are all far away from us, so it is really, really hard to distinguish one black hole's accretion disk of material spiraling into the black hole from the accretion disk of its companion. It all just blurs together.
The most promising technique is to use high-resolution radio observations from arrays that span the globe. The radio arrays concentrate on the base of the jets that shoot out perpendicular to the accretion disks from the center of the disks. While most of the material in the accretion disk will eventually spiral into the black hole, some of the material is ejected from the system in narrow jets via a mechanism that we're still trying to figure out.
A team of radio astronomers led by Silke Britzen at the Max Planck Institute for Radio Astronomy might have found the first definite binary supermassive black hole in a flaring galaxy called OJ287. That galaxy's jet is pointed almost directly at us and it appears to change shape and brightness in a periodic fashion every 22 years. OJ287's jet also pulses with the same 22-year period. These three periodic changes fit best what we'd expect from two black holes orbiting each other. Continued observations are needed with radio arrays that span the globe and future arrays that include radio telescopes in space connected to the ones on the ground.
The second black hole story is about the supermassive black hole at the center of M87 that was the subject of the first-ever black hole image a few years ago by the globe-spanning Event Horizon Telescope. Using a different array of radio telescopes of similar size to the EHT and longer wavelengths, a team led by Ru-Sen Lu of the Shanghai Astronomical Observatory were able to image M87's jet down to where it connects with the accretion disk around the supermassive black hole. The jet appears to be a hollow, parabolic cylinder.
There also appears to be a central ridge—a spine running along the jet's center. Such spines have been seen in other galaxy jets much farther out from the galaxy's nucleus. We don't know what causes the central ridge but it's hoped that future higher resolution images will show us how they are formed.
I hope you'll be able to experience a truly dark sky filled with thousands of stars this summer!
—
Nick Strobel
Director of the William M Thomas Planetarium at Bakersfield College
Author of the award-winning website www.astronomynotes.com
