Mercury Helps Reveal Our Middle-Aged Sun’s Secrets
By Judith E Braffman-Miller
About 4.56 billion years ago, our Solar System was born from the relic, shredded fragments left behind by the long-dead, nuclear-fusing furnaces of previous generations of ancient stars. Today, our Sun is a lonely sphere of roiling, glaring, mostly hydrogen gas, but it probably was not always as solitary as it is now. Indeed, our sun is thought to have been born as a member of a dense open stellar cluster–along with thousands of other sparkling sibling stars–that formed from a frigid, dense blob, embedded within one of the many giant, cold, and dark molecular clouds that float like ghosts throughout our Milky Way Galaxy. Our Sun is a middle-aged star, and at almost 5 billion years of age, it still has about 5 billion years to go before it runs out of nuclear-fusing fuel and perishes–stars, like people, do not live forever. In January 2018, a team of NASA and Massachusetts Institute of Technology (MIT) astronomers announced that they had indirectly measured our Sun’s mass-loss and other solar parameters by studying alterations in the orbit of the innermost major planet, Mercury.
The orbits of our Solar System’s eight major planets are expanding. This occurs because our Sun’s strong gravitational grip slowly weakens as our Star grows older and loses some of its mass. The new values obtained by the NASA and MIT scientists improve upon previous predictions by reducing the amount of uncertainty. This is of particular importance for calculating the rate of solar mass loss because it’s tied into the stability of G, the gravitational constant. Although G is thought to be a fixed number, whether it really is a constant remains a fundamental unanswered question in physics.
“Mercury is the perfect test object for these experiments because it is so sensitive to the gravitational effect and activity of the Sun,”commented Dr. Antonio Genova in a January 18, 2018 NASA Press Release. Dr. Genova is lead author of the study published in Nature Communications and a researcher at MIT, working at NASA’s Goddard Space Flight Center (GSFC) in Greenbelt, Maryland.
Indeed, Mercury is so sensitive to the gravitational effect and activity of our Star that it has been used to establish observational evidence for Albert Einstein’s Theory of General Relativity (1915). The first three tests, proposed by Einstein in 1915, concerned the “anomalous” precession of the perihelion (when it is closest to our Sun) of Mercury, the warping and bending of traveling light (gravitational lensing) in gravitational fields, and the gravitational redshift. Sometimes alternatively termed the Einstein shift, the gravitational redshift is the process in astrophysics by which electromagnetic radiation, originating from a source that is in a gravitational field, is reduced in frequence (redshifted) when observed in a region at a higher gravitational potential. This is a direct result of gravitational time dilation. Gravitational time dilation basically means that if someone is outside of an isolated gravitational source, the rate at which time goes by increases as the observer moves away from that source.
The precession of Mercury was already known in 1915; experiments revealing light bending that validated the predictions of General Relativity came in 1919–with increasing precision measurements performed in later tests. Astrophysical measurement of the gravitational redshift was claimed to be measured in 1925. However, measurements sufficiently sensitive to actually confirm the theory were not performed until 1954. A program of more accurate tests began in 1959, and these tested the various predictions of General Relativity with a higher degree of accuracy in the weak graviational field limit. This greatly limited possible deviations from the theory.
Like the planet Venus, Mercury circles our Star inside Earth’s orbit as an inferior planet, and it never travels further from it than 28 degrees. When Mercury is observed from Earth, this close proximity to our Sun means that the planet can only be observed when it is close to the western or eastern horizon, during the early morning or early evening. During dawn and twilight, Mercury appears as a brilliant morning or evening celestial object. In spite of this, it is frequently much more difficult to observe than Venus. Mercury displays a complete range of phases, that are like those of Venus and Earth’s Moon, as it travels in its inner orbit relative to that of our own planet. This continually occurs over the course of what is called the synodic period every 116 days.
Mercury is locked gravitationally to our Sun in a 3:2 spin-orbit resonance–and it rotates in a way that is unique in our Solar System. When Mercury is observed relative to the fixed stars, it rotates on its axis precisely three times for every two orbits it makes around our Sun. As seen from our Star, in a frame of reference that rotates with the orbital motion, Mercury seems to rotate only once every two Mercurian years. For this reason, an observer on Mercury would witness only one day every two years.
At perihelion, Mercury is only about 46,000,000 kilometers away from our Sun, while at its aphelion (when it is farthest from our Sun), it is 70,000,000 kilometers from it. Mercury’s extreme orbital eccentricity, in combination with its rotation rate of three times in two Mercurian years, causes some very bizarre things to occur. For example, at certain longitudes, an observer standing on Mercury’s surface would watch the Sun rise and then gradually grow larger and larger, as it made its long journey towards its highest point in the sky. At the same time, the myriad stars would zip three times faster in their flight across Mercury’s sky. An observer, standing on Mercury’s surface at perihelion, would see a Sun that appears more than three times larger than it does on Earth.
Our Sun, and its thousands if sparkling sibling stars, were born in a dense blob embedded within the undulating folds of a cold, giant molecular cloud. This dense blob eventually collapsed under the merciless pull of its own gravity to give birth to the new baby Sun. Within the secret depths of these vast, frigid molecular clouds, composed of gas and dust, delicate tendrils of material merge and clump together for hundreds of thousands of years. Then, squeezed tightly together by the crushing squeeze of gravity, hydrogen atoms within this dense blob suddenly and dramatically fuse. This process lights a baby star’s fire–and our baby Sun was no exception. This stellar fire will last for as long as our Sun “lives”. This is how stars are born.
Stars that are chemically alike are generally are found within the same cloud at about the same time. Star-birth has been compared to the way popcorn in a popper behaves. As the pot heats up–pop, pop, pop–bright baby stars are born.
All stars (including our Sun) are born this way–through the gravitational collapse of an especially dense blob embedded within the swirling, whirling folds of a giant, dark molecular cloud. Such ghostly, beautiful clouds float through our Milky Way Galaxy in great numbers, and they are scattered throughout the space between stars. These stellar cradles also contain the remnants of older generations of stars that perished long ago. All of the atomic elements heavier than helium are formed in the searing-hot hearts of the Universe’s stars or, alternatively, in a supernova blast heralding the demise of a massive star (supernova nucleosynthesis). Stars cook up, in their seething-hot nuclear-fusing cores, increasingly heavier and heavier atomic elements out of lighter ones (stellar nucleosynthesis). The Big Bang birth of the Universe, that is thought to have occurred almost 14 billion years ago, only produced hydrogen, helium, and traces of lithium (Big Bang nucleosynthesis). Literally, all of the heavier atomic elements–called metals by astronomers–were manufactured in the hot hearts of the stars, or in the death throes of massive stars. Star-birthing dark molecular clouds contain the “ashes” of older stars, and these freshly forged heavy atomic elements are destined to be recycled in the fires of a younger generation of bright new baby stars, that are born within the swirling folds of their natal clouds.
Today, our Sun is enjoying an active, brilliant mid-life. But, as stars go, there is nothing particularly special about it. There are eight major planets and an assortment of moons and other objects in our Sun’s family, which is located in the outer suburbs of a typical, though majestic, star-lit spiral Galaxy–our Milky Way. If it were possible to trace the history of atoms, currently residing on Earth, back about 7 billion years or so, we would likely find them scattered throughout our Milky Way. Some of these widely-dispersed atoms now are contained in a single strand of an Earthling’s genetic material (DNA), even though in ancient cosmological times they were formed deep within the hidden depths of alien stars–that have long since perished–that were dwelling in our then-youthful Galaxy.
In about 5 billion years, or so, our Sun will begin its death throes. A star of our Sun’s mass “lives” about 10 billion years. A middle-aged star, like our Sun, is still bouncy enough to blissfully burn hydrogen in its hot heart by way of the process of nuclear-fusion. When our Sun, and similar stars, finally begin to deplete their necessary supply of nuclear-fusing fuel, their looks change. They are now the stellar senior citizens of the Cosmos. Our elderly Sun will first become a swollen, monstrous, crimson red giant star, that will be sufficiently bloated to engulf first Mercury, then Venus, before it (possibly) cannibalizes Earth. At last, our Sun will finally hurl off its outer gaseous layers, while its core remains intact. All of our Sun’s material will ultimately collapse into this tiny relic body that is only about the size of our Earth. In this way, our Sun will undergo a sea-change to become a type of stellar-ghost called a white dwarf star. The new white dwarf will be encircled by a beautiful expanding shell of multicolored gas termed a planetary nebula. These beautiful objects–sometimes referred to as the “butterflies” of the Cosmos–were given this name by astronomers who originally thought that they bore a resemblance to the giant outer planets Uranus and Neptune. A white dwarf radiates away the energy of its collapse, and is generally made up of carbon and oxygen nuclei floating in a strange sea of degenerate electrons. The equation of state for degenerate matter is “soft”. This means that any contribution of additional mass to the body will only result in an even smaller white dwarf. Continuing to add more and more mass to a white dwarf will only make it shrink more and more–and for its central density to grow larger. The stellar-ghost’s radius will finally shrink to only a few thousand kilometers. Therefore, a white dwarf star, such as our Sun is doomed to become, is destined to become progressively colder with the passage of time.
In the end, our Star will likely morph into an object called a black dwarf. Black dwarf stars are still considered to be hypothetical objects because it is generally thought that none exist in our Cosmos–yet. This is because it takes hundreds of billions of years for a white dwarf to finally cool off to the black dwarf stage, and our Cosmos is a mere 13.7 billion years old. The cooling white dwarf will first emit yellow light, then red light, as it continues to draw from the old star’s still-existing reservoir of thermal energy. Its atomic nuclei will finally be violently squeezed together as tightly as it is physically possible. At this stage, no further collapse is possible. The relic body is doomed to keep growing progressively cooler, and cooler, and cooler, until it becomes precisely the same temperature as the extraordinarily frigid space between stars, where it dwells. A black dwarf emits no light whatsoever. In the last phase of stellar evolution, as a carbon-oxygen-rich black dwarf, the ghost of our dead Sun will continue to haunt the Milky Way. Perhaps, at some time during its long journey, it will meet up with another cold, dark, enormous molecular cloud, very similar to the one from which it was born. Perhaps, if this happens, our Sun will again become a part of the great and beautiful process that will give rise to a new baby star–with all of its wonderful possibilities.
Mercury Helps Reveal Our Middle-Aged Sun’s Secrets
The study conducted by NASA and MIT astronomers started by improving Mercury’s charted ephemeris–which is a chart of the planet’s position in Earth’s sky as time passes. In order to do this, the scientists drew on radio tracking data that monitored the location of NASA’s Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) spacecraft while the mission was still active. This robotic spacecraft made three flybys of Mercury in 2008 and 2009 and orbited the planet from March 2011 through April 2015. The scientists studied the data backward, and analyzed subtle alterations in Mercury’s movement, as a way of learning about our Sun and how its physical parameters effect the planet’s orbit.
Astronomers have studied Mercury’s motion for centuries, paying special attention to its perihelion. Observations conducted long ago show that the perihelion changes over time (precession). Although the gravitational pull of other planets in our Solar System account for most of Mercury’s precession, they fail to explain all of it.
The second-largest contribution to Mercury’s precession comes from Spacetime warpage around our Sun. This is the result of our Star’s own gravity, which is explained in Einstein’s General Theory of Relativity. The success of General Relativity in explaining most of Mercury’s remaining precession is what finally persuaded astronomers that Einstein’s theory was correct.
Other, considerably smaller contributions to Mercury’s precession, are attributed to our Sun’s internal structure and dynamics. For example, one of the smaller contributions is our Star’s oblateness. This is a measure of how much our Sun bulges at the middle. In order to envision this, imagine the inflated tummy–that looks like an inner tube–around the waist of an inactive glutton. This shape, of course, differs from that of a perfect sphere. The scientists obtained an improved estimate of our Star’s oblateness that proved to be consistent with other types of studies.
The astronomers were also able to separate some of the solar parameters from the relativistic effects. This is something that had not been accomplished by previous studies that were based on ephemeris data. The researchers then went on to develop a novel technique that could simultaneously both estimate and integrate the orbits of both Mercury and MESSENGER. The new technique resulted in a comprehensive solution that included quantities linked to relativistic effects, as well as the evolution of our Sun’s interior.
“We’re addressing long-standing and very important questions both in fundamental physics and solar science by using a planetary-science approach. By coming at these problems from a different perspective, we can gain more confidence in the numbers… and can learn more about the interplay between the Sun and the planets,” commented Dr. Erwan Mazarico in the January 18, 2018 NASA Press Release. Dr. Mazarico is a geophysicist at GSFC.
The scientists’ new estimate of the rate of solar mass loss is important because it represents one of the first times this value has been constrained based on observations instead of theoretical calculations. From the theoretical studies, scientists previously predicted a solar mass loss rate of one-tenth of a percent of the solar mass over a time span of 10 billion years. This length of time is sufficient to reduce a star’s gravitational tug and permit the orbits of the planets to spread outward by approximately half an ince–or 1.5 centimeters every year per astronomical unit (AU). One AU is equal to the average distance between Earth and Sun which is about 93,000,000 miles.
This newly acquired value is slightly lower than previous predictions but has less uncertainty–which made it possible for the scientists to improve the stability of G by a factor of 10, when compared to values derived from studies of the motion of Earth’s Moon.
Study co-author, Dr. Maria Zuber, noted in the January 18, 2018 NASA Press Release that “The study demonstrates how making measurements of planetary orbit changes throughout the Solar System opens the possibility of future discoveries about the nature of the Sun and planets, and indeed, about the basic workings of the Universe.” Dr. Zuber is vice president for research at MIT.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various newspapers, journals, and magazines. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.
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