The life, death and rebirth of stars
Inside the hearts of stars.
Essentially, stars are enormous balls of highly compressed hydrogen gas formed from vast clouds that drift throughout the Universe. As a clouds condenses under the force of its own gravity, pressure within its center increases. Increased pressure results in higher temperatures until the center becomes hot enough to trigger a thermo nuclear reaction called fusion. When fusion occurs, the short hooks of the strong nuclear force engage two hydrogen atoms and create one atom of helium. But the new helium atom weighs slightly less than two hydrogen atoms, so the excess mass is converted into a spectacular amount energy as explained by Einstein's famous equation E=m*c2. Stars fulfill the dreams imagined by ancient alchemists because they actually transmute one element into another.
- Stars are held together by gravity. Gravity tries to compress everything to the center. Thermal and radiation pressure from fusion within the star's core pushes back. Thus, a star exists in an uneasy equilibrium with gravity. Fusion tries to expand the star while gravity tries to crush it.
- Image credit: www.astronomynotes.com
Fusion also releases an outward force that pushes back and stops the cloud's compression. Thus, a star exists in an uneasy equilibrium with gravity. Fusion tries to expand the star while gravity tries to crush it.
Our sun is currently at this stage. Each second, our Sun converts 500 million metric tons of hydrogen into helium and 5 million metric tons of excess material is released as energy. This means every year, 157 trillion metric tons of solar material is converted into energy that's eventually released as solar radiation. The Earth receives about two billionths of the Sun's output, or about two quintillion watts. This is enough energy to power 100 average light bulbs for about 5 million years -- an period exceeding the time we humans have walked on Earth..
The cores of supergiant stars are even more compressed and therefore hotter than average stars like the Sun. A supergiant star is defined as one with over 10 times the material of the Sun. The hotter temperatures of supergiant stars results in faster fusion. Therefore supergiant stars burn through their hydrogen at a faster rate and shine for a shorter period of time.
Starlight's last gleaming
Over ten billion years, a typical star like the Sun will convert about 10% of its hydrogen into helium. Although it appears that the star could continue shining for another 90 billion years, this is not what happens. The helium laced core begins to contract; the outer layers expand and cool and the star takes on a redder hue. The star is now called a red giant. As the core contracts, pressure increases until it becomes hot enough to fuse helium into heavier elements. At this stage of the star's existence, it has only 100 million years remaining.
- Flimsy as a soap bubble, concentric wraithlike shells of expanding gas surround white dwarf stars in the Helix(L) and Dumbbell(R) Nebulae.
- Image credit: R. Jay GaBany
After this period has passed, the red giant star will have converted most of its material into carbon. So, the star will attempt to fuse it into iron. Problematically, the cores inside stars like the Sun lack sufficient pressure to accomplish this. So, the outward pressure of energy will decline enabling gravity to crush the core. In a flash, the core may reach temperatures of billions of degrees Celsius. The core's atoms will become crushed so closely together that the repulsive forces of their nuclei recoil and bounce outward like a rubber ball hitting a brick wall. This sends a shockwave throughout the star that expels some of the star's outer layers as a gas shell and it may trigger a brief new round of fusion. For many stars, this process will continue for a short period of time with the star appearing to pulsate, expanding and contracting over the course of a few millennia, spewing its atmosphere into space as one or more concentric shells of glowing gas illuminated by the ghostly ultraviolet glow of its exposed shrunken core.
The spent core of a star like the Sun is called a white dwarf. Its almost pure carbon- something like coal but considerably more dense. Most white dwarfs are about the size of Earth but weigh as much as our Sun. For a while, the white dwarf will glow white hot from its left-over heat but it will eventually cool to the temperature of space and become a black dwarf.
Throughout this event, some of the star's planetary family may become engulfed by the bloated red giant, some may shift into more distant orbits while others may be ejected from its solar system entirely,
When we peer across our corner of the galaxy, we see many examples of dim stars surrounded by colorful, glowing shells of gas. Their appearance resembles the shape and color of the Sun's distant planets, so they are called planetary nebulae. The closest example is the Helix
and another is called the Dumbbell
Supergiant stars, those that have 10 or more times the mass of our Sun, have a different fate than typical stars. After only shining about 10 to 15 million years (compared to over 10 billion for a typical star like our Sun) the supergiant's core will have turned into carbon and its exterior will have swollen to become a red supergiant. Betelgeuse, the bright ruddy star that forms Orion's left shoulder, is a good example.
- First observed by the Chinese on July 4, 1054, the Crab nebula is expanding at a rate of about 2,000 kilometers per second- the cloud has grown to over ten light years in diameter since the time of its explosion!
- Image credit: R. Jay GaBany
Unlike the Sun, a supergiant star has the pressures needed to fuse carbon into iron. However, this fusion process consumes more energy than it produces. So, as its energy output declines, the delicate balance with gravity's relentless inward push is lost and gravity wins. This causes the core to collapse in a violent explosion known as a supernova.
As the star explodes, the expanding gas can be sufficiently intense to trigger fusion that creates heavy elements such as uranium. This, plus other radioactive elements created in the explosion, dumps even more energy into the gas, causing it to glow so brilliantly that it may become brighter than the combined starlight from every other star in its galaxy!
The fate of a supergiant star's core depends on its mass. For example, if the core is between 1 and 3 times the Sun's mass, then pressure from the collapse creates neutrons by slamming the core's electrons into protons while the core shrinks to the size of a city. Not surprisingly, this object is called a neutron star and a teaspoon full weighs as much as a mountain.
However, even neutrons cannot resist the pressure created by a collapsing core with more than 3 times the Sun's mass. When this core implodes, nothing can stop it. As its size decreases, its gravity massively increases and its diameter becomes a point smaller than the period at the end of this sentence. Cores with more that 3 solar masses become a black hole and nothing that falls inward, not even light itself, can escape its gravitational grip.
- On May 30, 2011, a French observer, Amédée Riou, using a modest 8-inch backyard telescope and an off the shelf camera detected the second supernova (centered) to occur over a short period of six years in the Whirlpool galaxy, also known as M51.
- Image credit: R. Jay GaBany
One of the most recent supernovae in our home galaxy occurred on July 4, 1054. We know this because it was recorded by the Chinese and it could be seen in broad daylight for 23 days. We know exactly where this star exploded because, today, we can see its remnants as the Crab Nebula
But, but we observe hundreds of supernova in neighboring and distant galaxies each year. For example, on May 30, 2011, a French observer, Amédée Riou, using a modest 8-inch backyard telescope and an off the shelf camera detected the second supernova
to occur over a short period of six years in the Whirlpool galaxy
, also known as M51. The progenitor was recognized to be a single supergiant star with between 18 and 24 solar masses.
Two stars of roughly the same mass will evolve roughly in parallel. But the more massive star will convert more of its nuclear fuel faster, become a red giant sooner and be the first become a white dwarf. So, there are many examples of double stars where one is a red giant, the other a white dwarf. So of these pairs are in orbit about each other at very close range. There are even some cases where the stars touch. Such close associations inevitably lead to flows between their atmospheres where the white dwarf robs material from its red giant companion. Over time, sufficient mass accretes to the white dwarf that it triggers a gravitational collapse and explosion that annihilates it. These types of supernova can be though of as a gigantic nuclear bomb going off in space with the bomb being the size of the Earth and it's mass about 1.4 times that of our Sun.
Recycle and rebirth
Both the expanding shells expelled in planetary nebulae and the material exploded by spectacular supernova will eventually drift from the site of their creation and merge with the tenuous gas and cosmic dust that floats in the spaces between stars. Sometimes the gas and debris from a supernova remnant will drift into a stellar nursery and mix with material in the process of forming new stars. This pollinates the new stars with the heavy elements and some of the leftover material may congeal into the star's family of planets.
Life on Earth is intimately connected to the stars. All living things on our planet progress through a cycle of birth and death and so do the stars in the sky above us. The material that composes our bodies was created long ago in giant red stars and supernova explosions. The relative abundance of chemical elements found throughout the Universe matches the abundance of atoms generated inside stars that there is little doubt red giants and supernova are the crucibles where matter is forged. The Sun is a second or third generation star therefore all the material in it and all the matter we see around us has been through one or two cycles of stellar alchemy. The presence of heavy elements found on Earth suggests there was a nearby supernova explosion shortly prior to the formation of our solar system. Science also suspects that supernova triggered the interstellar cloud from which our Sun and its planets were created to collapse.
We live in a Universe that is wondrous beyond measure! Sequestered on our tiny planet, we are fortunate to be located in the calm between two spiral arms, safely half way from the violent center of our galaxy and warmed by a reliably stable star that enabled life a chance to take hold and evolve. So, our environment does not end at the edge of our atmosphere. It extends throughout the solar system and reaches across the 100 million diameter disk of the Milky Way. We are profoundly connected to the Cosmos that surrounds us and as our understanding of the stars has improved, the apparent distance that separates them from us has decreased.