StarWatch: Moravian College Astronomy

JUNE 2020

JUNE STAR MAP | MOON PHASE CALENDAR | STARWATCH INDEX | NIGHT SKY NOTEBOOK

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lunar phases

1242 JUNE 7, 2020: Beyond the Rainbow: The next time that you have the opportunity to witness the majesty of a rainbow after a late afternoon storm, consider the fact that within its colorful beauty are the answers to some of astronomy’s most intriguing questions: the temperature and chemical composition of the stars that I’ll discuss here, whether a star is single or part of a multiple system, exoplanets, magnetic fields, and the acceleration of the universe, even the dark matter and energy to explain it. Newton (1642-1727) began the scientific investigation by allowing a shaft of sunlight to enter a prism producing the spectrum of colors. He also discovered that it was impossible to break the rainbow into further subcategories, but it was possible to recombine it into white light once again. His conclusion was that light was a combination of colored particles mixed “confusedly” to produce the white light that Newton observed. Advancements continued with names like Joseph Fraunhofer (1787-1826), Gustav Kirchhoff (1824-1887), and Robert Bunsen (1811-1899). Fraunhofer, a self-made German optician, discovered over 600 dark lines in the solar spectrum and cataloged them. Some of them are still used today. Working together, Bunsen and Kirchhoff developed the spectroscope, allowing the light under investigation to pass through a narrow slit before reaching the prism, thus improving the resolution of the spectrum. These investigations were coupled with the Bunsen burner to allow chemicals to be incandesced (made to glow through heating) without the interference of background colors. Kirchhoff saw that different chemicals produced unique emission lines that could identify the chemicals being luminesced, and that these emission lines were duplicated in the dark absorption lines of the sun’s spectrum. That led him to conclude that the same elements were also present in the sun. The spectral classification of stars began in the mid-19th century with two major efforts. One was to obtain a large sampling of low-resolution spectra photographically, while the other was to image the spectra of a select number of stars to a high degree of precision. At the same time, the laboratory work of Kirchhoff and others made it abundantly clear that there was a relationship between the bright emission lines being produced under controlled laboratory experiments, along with the dark absorption spectra of the stars being imaged. Classification schemes were proposed based upon the line intensities of hydrogen, but it was not until the early years of the 20th century, with the development of quantum theory (physics of the atom), that it became clear that the distribution and intensity of the absorption lines were a function of the temperature of the stars. Outer electrons of the various atoms making up the chemical components of a star were able to jump to higher orbitals by absorbing energy in discrete steps and in relationship to the temperature of the star. Electrons of a specific element absorbed energy coming from the direction of a star’s photosphere, then emitted these same energy units (quanta) in random directions, “filtering” these wavelengths to create the dark lines that were observed in an absorption spectrum. The Harvard (Observatory) classification system was shaped by Annie Jump Cannon (1863-1941) in 1901 by comparing the different line intensities of various elements. She created a temperature sequence from hot to cool, rearranging the more orderly hydrogen series to become O, B, A, F, G, K, and M stars. Cannon developed a mnemonic to remember the jumbled reordering, “Oh, Be A Fine Girl—Kiss Me,” but I prefer Oh, Becker’s Astronomy Field Guide Kills Me. As I recall, the desperate Penn State student who invented that prompt failed my course.

1243

JUNE 14, 2020: Summer Solstice Almost Over

When I was a kid and well into my late 30’s, the family would gather on Christmas Eve at my grandparent’s apartment on North 8th Street in Allentown, PA. There was always good food, camaraderie, and my grandfather’s shortwave radio tuned to a German station playing carols in the background. It never failed that sometime during the evening my Opa, with a wavering voice and tears in his eyes, would proclaim that “Tomorrow, Christmas will be over.” It always created some type of response from his daughter, my mother, that he should enjoy the moment, but in Europe, it already was tomorrow; and that’s where my grandfather was mentally, thinking about the Christmases of his youth in Solingen Wald when “tomorrow” Christmas would be truly over. In a way, that’s how I think about the summer solstice. It used to be my favorite “holiday,” but both of my grandfathers died on that day which has tempered my enthusiasm. I’ve been waiting for the summer solstice since the long shadows of Christmas, watching the sun climbing higher in the sky, excruciatingly slow at first, then most rapidly at the vernal equinox, and now again slowly as it crests at the top of its altitude wave (west of Hawaii) on Saturday, June 20, at 5:45 p.m., EDT. The seasons are one of the most misunderstood concepts held by the general public. They have little to do with Earth’s changing distance from the sun. In fact, Earth will be at aphelion, farthest from Sol on July 4 at 7 a.m. EDT, about three weeks before the mid-Atlantic experiences its highest temperatures for the year. Most people who realize that seasons are not a function of distance fall back on the Earth’s axial tilt which is correct. The axis, the imaginary line about which the Earth rotates (spins), passes through the North and South Poles. In between lies the equator. If projected into space, it forms the celestial equator. Stars, planets, the sun, and the moon, move across the sky in arcs which are concentric to the celestial equator because of Earth’s daily rotation. Earth’s axial tilt causes its orbital plane, the ecliptic, to be tilted 23.5 degrees to the celestial equator. The sun by definition must always be positioned on the plane of the ecliptic. As the Earth orbits Sol once each year, the reflection of Earth’s revolution causes the ecliptic-bound sun to travel completely around the sky, above the celestial equator (CE) from the vernal equinox to the autumnal equinox and below the CE between the autumnal and vernal equinoxes. The sun favors the Northern Hemisphere from spring to fall, shining directly above the Tropic of Cancer (23.5 degrees north latitude) on the solstice day. Because the sun is north of the equator from spring to autumn, its path across the sky is longer, rising north of east and setting north of west. At noon it transits (crosses) the south at a much higher altitude. Sunrise, sunset, and altitude are at their extremes at solstice time. The combination of a higher sun providing us with more direct energy, plus the increase in the length of time that the sun remains visible during the day, causes conditions to get warmer as the land absorbs more energy during the daylight hours than it can release over the duration of the shorter nights. Climatically, the Earth’s heat budget in the mid-Atlantic reaches equilibrium somewhere near the third week in July when our highest temperatures are generally recorded. However, without a doubt, the moment the sun reaches its apex for the year, it’s downhill until the winter solstice, when the sun regroups and reverses direction and starts its slow uphill climb once again. Yes, in just one week it will all be over. Bummer...

1244 JUNE 21, 2020: The Universe as a Time Portal: When the Austrian, Christian Doppler (1803-1853), mathematically solved the relationship between the pitch (highness or lowness) of sound due to its relative motion to an observer, little did he know that he was providing the basis for creating a time portal into our past. Sound travels in waves. You can think of them as ocean waves, but more uniform with a constant distance between wave crests (wavelengths), creating a specific sound as they move through the medium of air and are intercepted by your ear. Take a sound-emitting device like the blaring horn of a car moving towards you. The approaching wavelengths of sound are compressed in the direction of motion, with more waves reaching your ear in a given time interval (frequency). The higher the frequency, the higher the pitch that the sound will produce. Once the car passes, the wavelengths of emitted sounds are stretched, the frequency goes down and so does the pitch. The driver detects none of this because she/he is traveling with the sound source, neither approaching nor receding from it. The Doppler shift in sound is all around us. You just have to become “attuned” to listening for it which might mean mentally filtering out some of the distracting noises that are occurring around you. The same principles of the Doppler shift apply to light with the difference that electromagnetic radiation does not need a medium through which it can travel. The French physicist, Armand Fizeau (1819-1896), measured the speed of light, and also predicted (1848) that light waves should undergo a similar Doppler effect, shifting towards the blue (higher frequency) if the object were approaching and the red if receding. The methodology for measurement was a shift in the dark lines of an absorption spectrum being created by a moving source like a star, compared to laboratory experiments that were conducted when the light-emitting object was at rest. These absorption lines in the spectrum were created by specific wavelengths of light being filtered as it passed through the star’s own atmosphere. Coupled with the use of photography, a permanent record of the star’s spectrum could be obtained for laboratory analysis. Credit Sir William Huggins (1824-1910) for applying this technique to the brightest star of the night, Sirius, in 1868. Huggins noticed a blueshift in two of its spectral lines of hydrogen. Modern observations show that Sirius is moving towards us at a velocity of 3.4 miles/second. Vesto Slipher (1875-1969) of the Lowell Observatory in Flagstaff, AZ in 1912 began applying this same technique to nebulae (actually galaxies) scattered across the sky. He determined that most of them had a red shift and that they were receding from Earth at velocities far greater than any observed star. Edwin Hubble (1889-1953), building on Slipher’s work (1929), determined a direct relationship between the amount of red shift, i.e., the recessional velocity of the galaxy and the distance that the galaxy was positioned from Earth. This became known as the Hubble constant. Hubble found that the universe was expanding, he thought uniformly, with objects farther away moving much faster than nearby galaxies. Now we know the universe is accelerating from observations made by the Hubble Space Telescope. Looking at Jupiter currently visible in the morning sky allows us to see Jove the way it appeared about one half hour in the past. That is how long it takes the sun’s reflected light to reach us from Jupiter, but observing a galaxy that has been determined to be 12 billion light years away, gives us a glimpse into the distant past. The information being carried on its emitted light waves has taken 12 billion years to reach us, so we do not see the galaxy as it appears today, but rather as it was 12 billion years in the past. It reveals itself when the universe was in its infancy, when the cosmos was substantially smaller, denser, and hotter—very different from what it is today. This remarkable achievement utilizing the red shift and its relationship to distance has allowed astronomers to begin grasping the history of all histories, that of our universe, not with speculation, but by the hard evidence provided by meticulous observations.

1245 JUNE 28, 2020: The Dark Side of the Universe: As a kid growing up in the 50’s with a burgeoning interest in all things celestial, I remember how simple the universe seemed to be. What you saw and recorded through telescopes and their associated attachments was basically what you got. Unknown to my elementary school brain and its concrete construct of “my cosmos” was a concern among professionals about where the universe was headed; Big Bang, expansion forever, Steady State, and oscillating universes were all in play. No one ever dreamed of a universe that would accelerate forever. One of the first concrete pieces of evidence about the universe’s evolutionary track was discovered by Fred Zwicky (1898-1974) of Caltech, observing in the 1930s the 330-million light years distant Coma Cluster of galaxies found in the vernal constellation of Coma Berenices. It was a very mature system, spherical in shape, composed of at least 1000 galaxies that were gravitationally bound and have been that way since the very early universe. The light from these galaxies allowed Zwicky to obtain an approximate mass of the system, but when the actual movements of its individual members were analyzed, their orbital motions were well beyond the escape velocity of the visible system. In short, within a few hundred million years of its formation, the individual galaxies of the Coma Cluster should have gone rouge and become “rootless wanderers” in the vast ocean of the cosmos. Something else which possessed gravity, but was unseen, was holding the Coma Cluster together. Zwicky coined the term dark (cold) matter. Some 80 years later, dark matter remains one of the cutting-edge enigmas of the universe. It has gravity; it doesn’t cluster in tight packets like black holes, but is more like a general force that adds to the mass of galaxies and clusters of galaxies in astonishing amounts of between two and 100 times the mass of the visible matter that is observed. Some quantum physicists have gone so far as to speculate that dark matter may be some other dimension “bleeding” into our four-dimensional space-time continuum. We will never understand it. WOW! It gets pretty crazy out there in mathematics land, but its predictions have an excellent track record of success. The universe is a place where our five senses continue to betray our sensibilities. Gravity is a property of matter, but does gravity need matter to exist? Ponder that thought tonight as you try to fall asleep. Astronomers now feel that the universe contains about five to six times the amount of dark matter above and beyond the baryonic matter that we can see, feel, and touch. Currently, the statistics stand at five percent baryonic matter and 27 percent dark matter comprising the universe. Any astute reader has just realized that I have only accounted for 32 percent of everything. What about the remaining 68 percent? That’s the dark energy component about which we know even less. The orbiting stars in galaxies provide more evidence that dark matter exists. In the solar system where objects revolve around a star which contains the bulk of that system’s mass, 99.98 percent, planets farther from the sun orbit at a slower pace. Mercury orbits faster than the Earth which orbits faster than Jupiter, etc. This is called Keplerian motion named after the famed German astronomer and mathematician, Johannes Kepler (1571-1630), who showed that planets orbit the sun in ellipses (ovals), and move faster when closer and slower when farther from Sol. Stars in a galaxy orbit differently. In spiral galaxies like our Milky Way, stars in the galactic plane farther from the center, feel more and more internal mass pulling upon them and revolve either faster or similar to a rigid disk. As the visible matter thins farther from the center, stars in this region should begin to feel the more concentrated pull of the galaxy’s internal mass, and their orbital velocities should decline analogous to a Keplerian system; but this doesn’t happen. Orbital velocities continue to increase reacting to an invisible gravitational force existing within the galaxy which astrophysicists, for want of a better term, call dark matter. The American astronomer, Vera Rubin (1928-2016), of the Carnegie Institution of Washington (D.C.) gets the credit here. There are other proofs like gravitational lensing, and how matter originally clumped to form galaxies in the very early universe, but you already get the picture. The universe is a strange and mysterious place, where what we see is only five percent of the total landscape of reality.