Tuesday, October 11, 2016

Supernovae PART 1: Introduction

The Milky Way is full of stars, gas and dust. The bright white dot in the centre of the photo below is Jupiter. The red laser from one of the three observatories in the photo points directly at the heart of our galaxy.

ESO/Y.Beletsky;Wikipedia

A false-colour image of the Sun taken in the
extreme ultraviolet range of the electromagnetic
spectrum taken by NASA/SDO (AIA)
   
 
The photo above of the Milky Way shows us only a small percentage of all stars in the night sky above us. Most of them are invisible to our naked eye from Earth, even those within our own Milky Way.

Stars, like our Sun (right), are gigantic spherical nuclear fusion reactions.

For most of a star's life, the outward blast of nuclear fusion is balanced by the force of gravity squeezing down on the star and it burns steadily. That balance eventually ends when the star runs out of fuel, and when it does it can (but not always!) produce the most violent explosion observed in the universe, a supernova. The Crab Nebula, below, is a six light-year wide remnant of a star that once existed about 6500 light-years away. It exploded in 1054. Not visible is a stellar remnant called a neutron star in the centre. Its powerful magnetic field whips up electrons in the cloud around it almost to the speed of light. They emit the eerie bluish glow.


The image of the Crab Nebula is a mo taken by the Hubble SpaceTelescope. The various glowing colours are different excited ionized elements blown out in the blast.

On Earth, we observe a distant supernova as the sudden appearance of a bright "new" star that fades over a period of weeks or months. A massive star can explode and then disappear into a black hole. Or it can leave absolutely no trace whatsoever of its existence. Often, a remnant remains after the original star dies, composed of the strangest densest matter known. By exploring how stars live and die, we are exploring how matter works at its limits and beyond its limits.

Like us, stars have life cycles. They are born, they change throughout their lives, and eventually they die. While some stars simply fade away, others go out spectacularly as supernovae. The science behind star death is evolving quickly thanks to new robotic high-resolution telescopes that can quickly cover large regions of the night sky and which can recognize even very minute changes in stellar luminosity. Supernovae happen suddenly and fade quickly. Now astronomers can catch one as it unfolds. By continuously scanning across hundreds of galaxies every 30 minutes, the Kepler Space Telescope designed primarily to detect extrasolar planets, for example, can also catch the first minutes of a supernova. In 2011 it caught the initial shockwaves of two brilliant distant supernovae, as two massive red supergiant stars exploded in separate incidents. Watch this brief animation of a supernova shockwave flash. In reality the flash lasts for about an hour. The animation, based on Kepler's observations, was created by NASA's Ames Research Center.


Supernovae are not only fascinating in themselves. They are essential to the creation of all the planets, moons, asteroids and life in the universe. While stars fuse together elements from neon to nickel in their cores, elements with nuclei larger than nickel can only be created in the intense furnace of a supernova explosion. By understanding the physics of supernovae explosions, physicists can understand how they seeded the universe with these heavy elements over time. Using mathematical computer modeling and observational data, they are discovering a surprising myriad of ways in which stars violently end their lives, some of which test the limits of current theory. As the most energetic events in the universe, they offer clues about how the universe is evolving over time.

STAR FORMATION

All stars start out the same basic way. A star's life begins when a cloud of dust and gas in space collapses under its own gravitational attraction. These clouds are called nebulae. A nebula consists of various molecules, neutral atoms of hydrogen and helium, and ionized gases. It can be vast, up to millions of light-years in diameter.


This iconic composite photograph taken by the Hubble Space Telescope is of the "Pillars of Creation," part of the Eagle Nebula. These pillars, composed mostly of molecular hydrogen gas and dust, are star nurseries.


This mosaic photograph of stunning nebulae shows off the Spitzer Space Telescope's capability. These infrared images were used for its 12th Anniversary calendar. As beautiful as they are, nebulae don't last.

Nebulae are transient structures in the universe. The Eagle Nebula, which is about 7000 light-years from Earth (shown below), may not even exist anymore. You can see the Pillars of Creation in the bright white center of the much larger Eagle Nebula below.


This is a three-colour mosaic image of Eagle Nebula taken by the Wide-Field Imager at La Silla Observatory in Chile.

The Spitzer Space Telescope recently imaged a rapidly expanding cloud of hot dust in this region of space. It might be the signature of an intense shockwave produced by a nearby supernova. Knowing the velocity of the shock wave, researchers estimated it will reach the nebula in about 1000 years. We will be able to witness that carnage from Earth about 1000 years from now, not because we will be seeing it in real time but because we are seeing the nebula from 7000 light-years away. When we look up at space, we are looking back in time. We see the Pillars as they were 7000 years ago. If the hot cloud is indeed a shockwave, the nebula was actually destroyed 6000 years ago.

A Refresher Note On Space, Time and Light-speed

I find it is always useful to mentally refresh myself about how space works because it can seem counter-intuitive. ALL electromagnetic (EM) radiation travels at light speed. This includes visible light, gamma rays, X-rays, radio waves, etc. Distant supernovae we observe from Earth exploded hundreds of millions up to billions of years ago. The most distant supernova observed so far (by Hubble) is called UDD10Wil and it is about 10 billion light-years away. This means its star exploded 10 billion years ago when the universe was only about 4 billion years old. The environment then and the star itself were probably very different from our modern universe today. When the James Webb Space Telescope starts operating in 2018, astronomers will be able to routinely observe the supernovae of the very first stars to form just hundreds of millions of years after the Big Bang.

Supernova shockwaves, although they travel very fast, about 40,000 km/h, they do not approach light-speed. A supernova shockwave was captured during its first minutes for the first time this year. Just before all or most of the star blows apart, a shockwave starting in the center of the star reaches its surface and expands outward into space, accompanied by a flash of light. It works much like a mechanical shockwave such as thunder rumbling through Earth's atmosphere, except that a supernova shockwave travels through the much more disperse and often ionized gases of interstellar space. To compare medium densities, Earth's atmosphere contains about 3 x 1018 molecules per cubic centimeter (cm3), while interstellar gas contains just 1 atom per cm3. The densest nebula contains about 10,000 molecules per cm3. What makes the shockwave visible are gamma photons accompanying the breakout flash, speeding out of the stars collapsing core and out through the star's surface (traveling at light-speed). By the time one of Earth's telescopes "sees" the distant EM flash, very short wavelength (invisible) gamma photons have stretched into intense visible light.

Why and when does a nebula collapse into a star? A gas/dust cloud exists in a delicate state of hydrostatic equilibrium. It is balanced between two opposing forces: Internal (outward) pressure is exerted by the thermal (colliding) motion of the molecules and atoms themselves and by the interactions between (repulsive) magnetic fields. Ionized gases in the cloud are charged objects. As they move about randomly in the cloud they create magnetic fields, which exert outward pressure. Meanwhile, the cloud particles experience the attractive (inward) force of gravity between them. A balance between these forces is reached, the cloud remains stable, until an outside force applied to the cloud disturbs it. Even a relatively mild disturbance from a gravitational collapse into a star nearby can trigger a local collapse. Regions of dust and gas can also simply collapse under their own increasing density. A typical nebular cloud can be a very productive birthing centre. Averaging about 100 light-years across and containing up to 6 million solar masses (M) of matter, a single nebula can give birth to millions of new stars.

A typical nebular cloud consists mostly of hydrogen. Depending on when and where a nebula exists, it will also contain traces of other larger atoms, molecules and different ionized gases as well, blown in from past supernovae. All the material of our solar system originally collapsed from a nebular nursery cloud much like the Eagle Nebula.

The parts of a nebula that are active star nurseries tend to be dense cold darkly opaque molecular clouds. In these regions, hydrogen exists as an H2 molecule rather than as an ionized atomic gas (a proton). Here, thermal (outward) pressure exerted by the cold hydrogen is minimal, which gives gravitational pressure an edge. It makes local collapses into denser regions possible. Gas and dust collapse inward to an increasingly dense local core that eventually evolves into a dense spinning mass called a protostar. It spins because all the angular momenta of the particles are conserved.

A typical protostar forming within a dense molecular cloud is illustrated left. Two bipolar stellar jets of material are likely the result of interactions between powerful magnetic fields wrapping around the forming star. They spin the material round and round and eject it from the star's magnetic poles.

In 2004, NASA directly glimpsed a protostar (V1647) for the first time. Still swathed in its birth blanket of gas and dust, it was actively accreting mass, a process animated in this brief video clip.



The protostar continues to accrete gas and dust from the surrounding cloud for millions of years until it has enough mass to evolve into a pre-main-sequence star. When it reaches this point it has gained its final mass. The star's mass will determine how it will then evolve over millions to trillions of years to come, as a main-sequence star. It will also determine whether or not it will eventually explode as a supernova. The larger the initial mass of dense gas and dust, the larger the star will be. Because stars vary widely in mass, what defines the birth of a main-sequence star is its core temperature rather than its final mass. Fusion is a very temperature-sensitive reaction. When the core is hot enough to trigger the fusion of hydrogen into helium, the star proper is born and it begins to shine brightly. More massive stars fuse hydrogen at a higher rate than smaller stars do. They go through their fuel faster, which means they don't last as long. The most massive stars last for just a few million years while the smallest ones last for trillions of years.

Next, we will start with the life cycles of the least massive stars - brown dwarf stars, red dwarf stars and small to mid-sized stars like our Sun - and see how they end up. Will our Sun explode into a supernova one day?

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