The Sun Our Star. by Stan Owocki The Sun, Our Star

Transcription

1 The Sun Our Star by Stan Owocki This is a ball, a hot glowing one. We all live in its thrall, our very own Sun. It lights up our sky and hides in our night, masking stars that seem shy till it sinks out of sight. But it self is a star, just looks big and shiny, cause it's nowhere as far as the stars that seem tiny. It's just the right place to keep us all warm, from the coldness of space, out of which it did form. That was eons before, when a cold dusty cloud got compressed to a core within its own shroud. From gravity's power, it started to glow; with each floor of its tower, core pressure did grow. The Sun, Our Star Pushing atoms to fusefuse, turning some of their mass into power to use, like the burning of gas. The light that is made from this fusion of matter by a chain of cascade to the surface does scatter. This is the source of our sun's steady shine, just half through its course after eons of time. But when finally spent our star will then morph, puffing outward to vent from red giant to dwarf. All we know will be gone but that's far away, so be sure with each dawn to bask in sun's day. For over four billion years it's been warming our Earth, so that oceans of tears could evolve to your birth.

2 This is a ball, a hot glowing one. We all live in its thrall, our very own Sun. By far the brightest thing in our sky, the Sun looks much like a great ball of fire. Its ball-like, spherical shape results from the central, inward pull of self-gravity from its own mass. It glows (or shines) because, like fire, it is very hot, about 5500 degrees Centigrade (C). At this temperature the collisions among atoms excites them to higher energy levels, which then give off visible light as they decay back down. This is known as thermal radiation. The picture shown was taken with a special filter that highlights small differences in temperature that arise from bubbling motions (called convection ) near the Sun s surface. These motions generate magnetic fields, which in turn help support gaseous prominences above the solar surface, as seen here around the edge, or solar limb. As the Sun both lights and heats our world, we on Earth certainly do live in its thrall.

3 It lights up our sky and hides in our night, q=sunset+stars+sequence+photo&client=firefoxb-1&tbm=isch&source=iu&ictx=1&fir=c_n5kdltx2yn5m%253a%252cgobs YLyb_gmREM%252C_&usg= 7bFQoxPEG5_ijnoFGTlrzawX6Y%3D&sa=X&ved=0ahUKEwip7uT39s3ZAhVimuAKHam1BhcQ 9QEIMzAF#imgdii=uEfUsbZJj9ZUaM:&imgrc=4kCAwtRr2Rpu8M: masking stars that seem shy till it sinks out of sight. The Earth is also a ball, and as it rotates, there are times when we re on the daytime side that faces the Sun, which then lights up our sky ; then there are times we are on the dark side, when the Sun hides in our night. The Sun is so bright that just the scattered light in the Earth s atmosphere makes the daytime sky outshine all the stars that would otherwise be visible. (The moon, on the other hand, is close enough and reflects enough of the Sun s light to be visible even during the day.) So it is only after the Sun sinks out of sight, after sunset, that the stars that are always there become unmasked, and so are visible to us. The picture here is a time-lapse composite showing the time just before and after sunset, when the sun fades, and the multitude of stars appear (here clustered along the Milky Way, which represents the edge-on view of the flat disk of our galaxy).

4 But it self is a star, just looks big and shiny, cause it's nowhere as far as the stars that seem tiny. But the Sun itself is really just a quite average star, in terms of its mass, size, and intrinsic brightness (called luminosity ). It s apparent brightness and size just seem so much greater because it is much, much closer to us. Indeed, our planet is locked by the Sun s gravity into a nearly circular orbit, taking one year to complete the circle. By looking carefully at the relative positions of stars on the sky during the course of a year, we can tell (with very precise modern instruments) that closer stars appear to shift their positions relative to the background, moredistant stars. This parallax effect is exactly analogous to the shift you see if you hold a finger at arm s length, then view it alternately with one eye or the other. If you move your finger closer, you can tell that the angle shift gets bigger. Indeed, if you measure this angle, then given the distance between your eyes, you can use simple geometry to work out the distance to your finger. For our Earth moving one radius of its orbit, called an astronomical unit (au), the parallax angle of even the nearest stars is very small, less than 1 arcsecond (1/3600 of a degree). Astronomers thus have defined a new standard unit called a parsec (short for parallax second ), as the distance for which a baseline distance of 1 au would lead to exactly 1 arcsec parallax. One parsec works out to be very far, about 200,000 au! At such a distance, our Sun s angular diameter would be 200,000 times smaller. Even the most powerful telescopes can not directly resolve such tiny angles, and so see stars as mere points of light. Moreover, since apparent brightness declines in proportion to the square of the distance (known as the inversesquare law ), a star with the same luminosity as our Sun appears (200,000) 2, or about 40 Billion times dimmer! No wonder then that the daytime Sun completely masks all the other stars in the sky.

5 It's just the right place to keep us all warm, from the coldness of space, out of which it did form. The Earth continuously loses heat by emitting infrared light into space; but at its distance to the Sun (1 au=150 million km), the heating from the Sun s light balances this loss, leading to a moderate equilibrium temperature, somewhat above the freezing temperature of water (0 C= 32 F), but well below its boiling temperature (100 C = 212 F). The dominance of liquid water is a quite unique property of our planet Earth. By comparison, Venus (and Mercury) are much hotter (nearly 500 C ~= 800 F), so any water is in a steamy vapor form, while on Mars (and all outer planets and moons, including Pluto), any water on the surface is frozen as ice or snow. So the Sun does indeed keep us comfortably warm from this intrinsic coldness of outer space, out of which it did form. The lower right picture shows the Orion nebula, a region of present-day star formation, where stars like our Sun (and also some more massive and much more luminous) are currently being born out of the cold clouds of gas and dust (the dark patches) there.

6 That was eons before, when a cold dusty cloud got compressed to a core within its own shroud. The Sun also formed from such clouds of dust and gas that were very cold, as low as about ten degrees above absolute zero (0 K = -273 C = -460 F). At such low temperatures, the outward-pushing gas pressure was so small that the self-gravity of a dense cloud started to draw the matter together into a even denser core that was embedded, or shrouded, in the overall larger cloud. All this occurred long ago, about 4.5 billion years (based on radioactive dating of the oldest meteorites). Our Sun, Earth, and whole solar system are thus all about this age of 4.5 Gigayears (Gyr).

7 From gravity's power, it started to glow; with each floor of its tower, core pressure did grow. The gravitational contraction of the central core caused it to heat up, eventually even reaching temperatures of C, comparable to the current Sun s surface. Once the surrounding cloud dissipated and so became transparent, the Sun started to glow, first as a relatively cool (3000 C) reddish star, and eventually approaching the Sun s current temperature and yellowish color. As the surface heated up, the Sun was still contracting, with its interior getting ever hotter and denser, leading to a very high core pressure and temperature (many millions of degrees!). The outward push of this high core-pressure (red arrows) eventually came to balance the weight (inward green arrows) of the tower of mass layers above it; at this point the Sun stopped contracting, and instead began tapping a new source of energy (Hydrogen fusion) from its core.

8 Pushing atoms to fuse, turning some of their mass fuse into power to use, like the burning of gas. The high core pressure and temperature smashes the Hydrogen ions (protons) together with sufficient energy that they overcome their mutual electrical repulsion, and so get close enough that the attractive, much stronger nuclear force can bind them together. As illustrated in the right panel showing the zoomed-in core, this Hydrogen fusion occurs in several steps, but eventually ends in the formation of a stable nucleus of Helium, with two protons and two neutrons. Their combined mass is slightly less (by 0.7%) than that of the original 4 protons. By Einstein s famous formula, E=mc2, this excess mass (m) is converted to a lot of energy (E), since the square of the speed of light (c2) is very large. This fusion burning of Hydrogen gas now provides the power to replenish the energy being lost by the Sun s surface.

9 The light that is made from this fusion of matter by a chain of cascade to the surface does scatter. The high-energy gamma-ray light emitted from this fusion power in the core cannot directly escape, because the overlying layers are very dense and so very opaque. So instead this gamma-ray energy scatters as a chaotic random walk toward to the surface, in a cascading chain that gradually degrades the high-energy gamma rays in the core to the lower-energy, visible light we see from the Sun s surface.

10 This is the source of our sun's steady shine, now just half through its course after eons of time. The continuous generation of energy by Hydrogen fusion in the Sun s core replenishes the energy lost by its shine from the surface. So while the Sun originally got hot from gravitational contraction from its natal cloud, it has been kept stable and hot for the past 4.5 billion years by this Hydrogen fusion. If you compute the amount of energy generated by converting the 10% of the Sun s core mass that is hot enough to drive fusion, using the energy conversion efficiency of 0.7%, it turns out that it is sufficient to power the Sun s luminosity for about 10 billion years. Thus at a current age of about 4.5 Gyr, the Sun is a middle age star, about half-way though its 10 Gyr H-burning lifetime. Because it s the longest lasting phase of a star, 90% of the stars in the sky are currently undergoing Hydrogen fusion, dubbed their main sequence phase. Main-sequence lifetimes scale with the ratio of mass to luminosity, and can be just a few million years for the most massive stars, with luminosity up to a million Suns.

11 But when finally spent our star will then morph, puffing outward to vent from Red Giant to dwarf. When the core of Hydrogen has all been burned into Helium, it contracts and gets even hotter. But this then ignites fusion in a thin shell of Hydrogen at the outer edge of this hot core. Such shell burning is actually so vigorous that it generates even more energy than during the core burning phase. This will cause the Sun to expand into an even brighter Red Giant, increasing in size by about a factor hundred, with a luminosity that is up to a thousand times that of the present-day Sun! The red color results from the fact that this very large surface will be cooler, about 3000 K, roughly half the surface temperature of the current Sun. Eventually, this outward puffing of the star will get so severe that the entire stellar envelope will be lost into space, forming a planetary nebula that looks a bit like a planet (but is actually much larger and very different). Once this nebula dissipates, only the hot, dense remnant core will remain, leaving a white dwarf star, with a size about 1% that of the present Sun, roughly the size of our Earth. White dwarfs are thus the final end-state of stars like our Sun. They don t have any fusion, but because they are so small, they re not very luminous, and so take a very long time (tens ofs billions years) to cool down from their initially quite hot surface temperature (>10,000 K).

12 All we know will be gone but that's far away, so be sure with each dawn to bask in Sun's day. But long before this white-dwarf stage, the high luminosity and huge size of the Red Giant phase will likely burn up the Earth, or at least make it a completely uninhabitable cinder. But this is still far way, about another 5.5 billion years. So for now do indeed continue to enjoy basking in the Sun s warm shine, with each dawn of each day.

13 For over four billion years it's been warming our Earth, so that oceans of tears could evolve to your birth. The stability of the Sun s fusion-powered luminosity, combined with the stability of the Earth s orbit at 1 au, have provided just the right warming to keep the Earth s oceans liquid for most of the 4.5 Billion years since the Sun and Earth formed. Over time the minerals dissolved in this salty ocean water provided the raw material to form the first cells, which gradually combined to form multi-cell animals, along with plants that through photosynthesis use the energy from Sun s light to make the food that provides the raw power for life. But such evolution is very slow; it took nearly the full 4.5 Gyr to reach the level of complexity to make large animals and, eventually, humans like us. Thus the stability of the Sun s warming of the Earth was essential to letting life form and evolve to us, from the salty tears of the ocean that sparked the first cells, to the salty fluid of the womb that bore our own bodies.