Bakersfield Night Sky – March 1, 2014
By Nick Strobel
Between the time I wrote my previous column and it appeared in print, the Yutu lunar rover from China that was thought to have died re-awakened the day before Valentine's Day. The rover has not moved from where it had mechanical problems but it still has carried out some observations with its ground-probing radar, panorama camera and infrared imaging equipment. The rover and the Chang'e-3 lander, which is still operating fine, went back into sleep mode on February 23rd for two weeks during the lunar night. The Moon takes four weeks to spin once on its axis. That is also how long it takes to orbit the Earth, so the Moon keeps one side facing the Earth. It also means that each lunar day is two weeks long (yes, two weeks from sunrise to sunset) and each lunar night is two weeks long. Since the Moon has no atmosphere, the temperature during the night at the rover's & lander's location gets down to a very chilly minus 180 degrees Celsius (-292 deg F)! During the night the rover and lander are programmed to enter into sleep mode to conserve power. When the Sun rises, they can get the power they need from their solar panels. During the day the temperature at their location reaches 100 deg Celsius (+212 deg F). The Yutu rover was designed to last for three months and the Chang'e-3 lander is expected to last for a year.
The Moon rotates so slowly because of the tidal force from the Earth acting on the Moon. Tides are produced by different strengths of gravity acting on an object. The side of the Moon closest to the Earth feels a stronger gravity pull from the Earth than what the Moon's center feels and the center feels a stronger pull than the far side of the Moon. Other parts of the Moon feel intermediate amounts of gravity. All those different strengths of gravity from the Earth stretch the Moon out along the line between their centers. When the Moon was much younger, it spun more rapidly. The Moon rock flexed as the Moon spun. Rock doesn't flex instantaneously, so the Moon's tidal bulge was slightly misaligned with the Earth-Moon center-to-center line. The Earth's gravity acted on that tidal bulge trying to get the Moon lined up. The Earth's tugging on the Moon slowed down the Moon's spin rate. Eventually, the Earth's relentless gravity prevailed and the Moon now spins just once for every one time it orbits the Earth.
The tidal forces were also much stronger when the Moon was young because the Moon was much closer to the Earth. When the Moon first formed as a result of a giant impact of a Mars-sized object with the Earth about 4.5 billion years ago, the Moon was only 40,000 miles away. That is much closer than its current 240,000 miles, so the tides it felt from the Earth were much stronger. The strength of tides increases as the CUBE of the distance, so at half the distance, the tidal force is eight times as strong.
Since gravity works both ways, the Earth, of course, also feels tides from the Moon. They are primarily responsible for the rise and fall of the ocean tides on the coast (the tidal effect due to the Sun's gravity is about a third as strong as the Moon's). If the Earth's gravity pull on the Moon slowed its spin rate down, then the Moon has to do the same thing to the Earth. In fact, the Earth's rotation rate is slowly decreasing due to the Moon's gravity. When the Earth was young, one Earth day was just six hours long, or a quarter of its current length. The tides the Earth experienced from the Moon were enough to flex the solid rock surface by up to 60 meters (200 feet) during high tide. Over the next 4.6 billion years, the Moon spiralled outward from the Earth, another effect of tides. The tidal forces are now weaker and the Moon is spiralling outward at a very slow snail's pace of about 3.8 centimeters per year. In the far, far future if the Earth and Moon survive the Sun's death, the Moon will be far enough away from the Earth to take 47 of our current days to orbit the Earth and the Earth will take 47 of our current days to spin once. Both the Earth and the Moon will be tidally-locked with only one side facing each other. The Moon will remain fixed at one position in our sky just like the Earth is now for observers on the Moon.
The huge change between the day and night temperatures on the Moon illustrate the powerful effect of a moderately-thick atmosphere. Since the Earth and Moon are at the same distance from the Sun, you can see that the Earth would also experience huge swings of temperature between the day and night if we didn't have the moderating influence of an atmosphere. (I would say that such big changes between day and night would create huge winds but, of course, without an atmosphere, there wouldn't be any winds.) Without an atmosphere, the average temperature on the Earth would be about -19 deg Celsius, far below the freezing point of water. The greenhouse heating effect of the atmosphere warms the Earth to comfortably above the freezing point, so the oceans of water can remain liquid. This difference in the temperature of the Earth without an atmosphere compared to the real situation was known as long ago as the 1820s with the work of Joseph Fourier. Our knowledge of the specific processes of the greenhouse effect has been considerably refined since Fourier's time, of course.
One last piece of astronomy news that came across my computer screen I want to share with you is the high-resolution X-ray imaging of a supernova remnant called Cassiopeia A ("Cas A" for short) by NASA's new X-ray space telescope NuSTAR. Cas A is what remains after a star much more massive than the Sun exploded about 11,000 light years away. Massive stars will fuse heavier and heavier elements at their cores in their old age. When they make iron from the fusion of silicon at billions of degrees, that is the end of the nuclear fusion chain process and the star can't gain any more energy by making things heavier than iron. The relentless crush of gravity is no longer held back by the extreme heat and radiation of the nuclear fusion and the core collapses extremely quickly. The violent collapse sends out a massive shock wave outward through the layers surrounding the core and the star explodes in a supernova while the core shrinks down to become a neutron star or, in the case of the most massive stars, a black hole. Such a death is illustrated in the Black Holes show at the William M Thomas Planetarium.
The general sequence of a massive star's death as I've described above has been known for several decades. However, when astronomers have looked at the fine details with their supercomputer simulations of the explosion, their progress in understanding has been stymied by the simulated explosions fizzling out. The shock wave in the simulations would stall out. The problem with the simulations is probably that they were assuming the star interiors were more uniform with nice spherical layers of gas than the interiors of real stars. Real stars are lumpy and the inner regions slosh around before the star detonates. NuSTAR has been able to map the distribution of a radioactive isotope produced at the heart of the exploding star called titanium-44. Unlike other elements such as iron, silicon, and magnesium, the radioactive titanium-44 glows in x-rays all the time while the other elements only glow in x-rays as the shock wave moves outward through the star's upper layers. Therefore, the titanium-44 gives us a more direct look at what happened in the star's core during the explosion. The titanium-44 in Cas A is concentrated in clumps at the remnant's center like what would be the result of a sloshing star's core. Gas bubbles form and are able to blast through the shock wave and re-energize any stalled shock wave. Even more importantly, the NuSTAR images argue against the competing model of supernova explosions that uses jets to propel the blast wave. In science, whenever you have two competing models that explain the observations, it is extremely helpful to find that one observation that definitively discounts one of the models and fits the other model. The NuSTAR image of the titanium-44 seems to be such an observation. Although, it is still possible that the remaining model might also be wrong, at least we know we don't have to spend mental energy exploring the blind alley of the discounted model.
The NuSTAR team will see if their conclusion holds up when they look at other supernova remnants. Images from NuSTAR and a supercomputer simulation of the first 150 milliseconds of the explosion are available on the NuSTAR website at www.nasa.gov/nustar .
From the ground we can spot Jupiter ending its backward, retrograde motion in between the two twins of Gemini. Jupiter is the brightest star-like object in our sky until Venus rises around 4 AM. However, Jupiter will set at about 3 AM, so the two brightest planets won't share the same sky. The Moon is just one day past new phase this evening, March 1st, so look for a thin crescent Moon low in the west-southwest in the following few days. The first star chart below shows a fat Waxing Crescent Moon passing below the Pleiades star cluster on March 6th and a First Quarter Moon just above the orange-red Aldebaran at the eye of Taurus, the bull, the following night. A much brighter Waxing Gibbous Moon passes by Jupiter the nights of March 9th and 10th but Jupiter will be bright enough to see even with the glare of the Moon. The first star chart below is for 9 PM but is centered on the west-southwest sky. If you turn around toward the East shortly after 9 PM, you'll be able to see Mars rising up next to the bright star in Virgo called Spica. About three hours later Saturn will rise with the stars of Libra. The second star chart below is for the midnight view .
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