Bakersfield Night Sky — May 20, 2017
by Nick Strobel
The week after commencement BC faculty were working on the next step of creating the Guided Pathways System (college GPS) in a series of intensive all-day work sessions. During that week the Cassini spacecraft zipped through the tiny gap between Saturn’s cloud tops and its rings in its fourth of the Grand Finale Orbits. As it went through the plane of the rings, Cassini beamed a radio signal through the rings, so the Cassini team on Earth could see how the rings changed the radio signal. How the signal was affected will tell us about the sizes of the particles in the rings and how the ring structure changes at different distances from Saturn.
Cassini scientists are also using the radio signal to measure the mass of Saturn’s rings from how the gravity of the rings changed the velocity of Cassini. Changes in the velocity of Cassini create shifts in the frequency of the radio signal—more mass means more gravitational force that creates greater changes in the velocity and that results in greater changes in the radio frequency. We measure the changes in radio frequency and work backwards to get the mass.
Something similar, by the way, is used by police officers when they use their radar guns to measure the speed of your car. The radar gun bounces a beam of radio waves off your car and the reflected radio waves will have a change in frequency that depends on your speed. Too great a change in the frequency means you get a ticket. If you want to impress your friends at a party, you can tell them that this is an example of the “doppler effect”. (Well, you will either impress them or get another eye roll.)
Measuring the mass of the rings will help us figure out the age of the rings. A more massive measurement probably means the rings are old, up to a few billion years of age. If the rings are on the low mass end, then they’re probably just a hundred million years or so old. Figuring out the age of the rings will give us further insights into the individual differences in how the giant planets formed—why is Saturn so impressively endowed with such a gorgeous set of rings and not the other giant planets? Also, the physics of the ring system is very similar to the physics of the disk that formed the planets around our sun 4.6 billion years ago and the proto-planetary disks we see around other stars today.
Earthbound Saturn observers will see Saturn as a bright starlike yellow-white object rising at about 10 p.m. among the stars of Ophiuchus. Saturn is also now in the same direction as the bulge of the Milky Way, so the sky background behind Saturn will be significantly brighter. To the right of Saturn, about a hand width with fingers outstretched at arm’s length from your head, will be the dimmer but still bright, red-orange heart of Scorpius, the red supergiant Antares. The star chart below shows the sky at 11 p.m. tonight.
Three zodiac constellations west of Saturn is Virgo in which you’ll see the super-bright king of the planets, Jupiter. We’re now exploring that planet up close with the Juno spacecraft. The Juno spacecraft completed its sixth flyby of Jupiter yesterday in an orbit that got it to within about 2700 miles from the cloud tops.
To the left of Jupiter in Virgo is the medium-bright star, Spica. Almost directly above Spica is the brightest star north of the celestial equator, Arcturus, in the constellation Bootes. You can also find Arcturus by extending the arc of the handle part of the Big Dipper. Arcturus and Saturn will be about the same brightness with Arcturus the brighter and more orange of the two. Spica is dimmer than Arcturus because Spica is almost seven times further away than Arcturus. Spica actually emits about fifty-six times more energy than Arcturus, or about 12,100 times more energy than the sun.
To the left of Bootes is the bowtie shape of Hercules. Binoculars will help you see the fuzzy patch in the upper right part of Hercules, M13. In a telescope, you’ll see that the fuzzy patch resolves into a cluster of hundreds of thousands of stars called a globular cluster.
Later this week, in the early morning of May 22, you will see the thin waning crescent moon pass by Venus low in the east in the pre-dawn sky. The moon will be at new moon phase on May 25, so look for a beautiful waxing crescent moon low in the west shortly after sunset the evenings of May 26 to 28.
Want to see more of the stars at night and save energy? Shield your lights so that the light only goes down toward the ground. See www.darksky.org for how.
Director of the William M Thomas Planetarium at Bakersfield College
Author of the award-winning website www.astronomynotes.com