Date: September 19th, 2012
Title: Encore: A Tour through the Lives of the Stars
Podcaster: André Gonçalves
Link: Andre’s blog: http://astro-andregoncalves.blogspot.com/
This podcast originally aired on July 30th, 2009
Description: This podcast is about stellar evolution, how the stars are born, live and die. I will talk about the reactions in the cores of the stars and I will try to explain the effects of the mass and chemical composition in the lifetime of a star as well as its possible endings. My intention is to give to you a general picture of this, to understand our sun’s fate, and to look above at the night sky with new eyes and more knowledge.
Bio: I am André Gonçalves from Vieira do Minho, Portugal. In 2003, I looked at the eyepiece of a telescope for the first time. Then, I researched a lot about astronomy and I bought my first telescope and binoculars (I was only thirteen). Throughout 2006, I entered in a program for the best students in mathematics of the country at University of Porto (2006/2007) and I did a summer student program of two weeks at Centro de Astrofísica da Universidade do Porto (CAUP). In the following year, I made a 4.5 inch dobsonian at CAUP! Today, I am eighteen years old and I am doing my graduation in physics at University of Minho, Portugal. Now, I have three telescopes and some cameras and I make my observations and imaging whenever I can.
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Hello stargazers and curious people, I am André Gonçalves from Vieira do Minho, Portugal.
The night sky is full of stars if you live in a dark site, but do you know how they born, where they are born? Stars are born in giant clouds of interstellar gas and dust, called nebulae. One example of these star nurseries is the Great Orion Nebula, visible in Orion constellation in winter nights. Some parts of a nebula are denser than others, so matter tends to accumulate in these areas by gravity, forming a proto-star. As well as more matter accumulate, gravity gets stronger and pulls even more matter, leading to an increasing of temperature and pressure, the proto-star heats and radiates in the infrared.
When the temperature reaches about 10 million Kelvins, nuclear fusion reactions begin and the hydrogen fuses into helium in the core, which generates energy and the proto-star lightens. A star is born!
The pressure from the core to the boundaries of the star caused by the energy released in the nuclear fusion equals gravity, preventing the star to collapse. The star is in hydrostatic equilibrium.
And then what happens? Well, the lifetime of the star will depend basically on its mass and chemical composition. Then, stars with similar mass and composition should have a similar stellar evolution and a similar time of existence.
Stars with a higher percentage of light elements, can create, by nuclear fusion, progressively heavier elements. These will create heavier elements and so on, until there is not enough energy to continue the fusion (stars can not fuse iron into heavier elements); the stellar envelope simply is not massive enough to bear down enough pressure on the core. Stars with less light elements should reach this state in less time.
As I mentioned before, the lifetime of a star depends also on its mass, and this is the major factor, but why? In a massive star, gravity is very strong, thus more energy needs to be generated by nuclear fusion to keep the star in equilibrium. Consequently, a massive star will run out of fuel faster than a low-mass star.
But how long may a star live? The lifetime of a star may range from only a few millions years (for the most massive) to trillions of years (for the less massive, like a red dwarf), and the age of the universe is “only” about 13.7 billions of years. Well below a trillion!
After its birth, the star continues to fuse hydrogen into helium and its bright, size and temperature doesn’t have significant changes over the most of the star existence. It shines steadily for millions, billions or trillions of years, depending on its mass.
However, nothing will last forever and stars are no exception to the rule. A star may end in three different ways, it becomes a white dwarf, a neutron star or even a black hole!
If the star has less than eight solar masses, after it run out of hydrogen in its core, there is no power source to fight against gravity and the star starts to contract. The fusion of hydrogen continues in the outer layers but it is not enough to stop the contraction, and the temperature at the helium rich core increases. The high temperatures make the remaining hydrogen to fuse faster and the star brightens. In its core the temperature is enough to fuse helium into carbon and oxygen. The energy released heats up the outer layers and the star expands and its density becomes low, except in the core. As the star expands, its surface becomes cooler and its color turns red. Now, its size is enormous, it’s bright and it’s red – it is now a red-giant. The inner layers contract to form a white dwarf, a small but very dense star, and its outer layers continue the expansion through space creating a planetary nebula. The Ring Nebula is one of the more famous examples in the night sky. This is our sun’s fate and its diameter should reach the orbit of Venus! This will happen in 5 billion years to come, so don’t worry.
However, if the star has more than eight solar masses, its fate will be very different. We already know that a star like this will have a shorter life, but how does it die? Well, the hydrogen will eventually run out, the star contracts, the temperature rises and the fusion of helium into carbon and oxygen begins.
When the helium at the core exhausts, there is no energy enough to fight against gravity again and the star contracts once more, there is a new rise of the temperature, high enough to fuse carbon into neon and magnesium and to fuse oxygen into silicon and sulfur (calcium, chromium, argon, titanium, among others, can also be produced in these reactions) and the cycle goes on till the star has a nucleus mostly made of iron.
Since iron are one of the most tightly bound nuclei, if they are fused they do not release energy (in fact, absorbs energy), this is because more energy is required to fuse iron, than the energy released by the fusion reaction. Now, there is no energy source to stop gravity and once again the star contracts. But this time, the star is compressed at its limits and the result is a gravitational collapse and a catastrophic explosion of the massive star – an event called supernova.
These are one of the most energetic explosive events known! Its brightness may outshine an entire galaxy, before fading from view over several weeks or months! While many supernovae have been seen in nearby galaxies, they are relatively rare events in our own galaxy. The last one was seen by Kepler in 1604 and another supernova exploded in 1054 and its remnant is the Crab Nebula easily seen in amateur telescopes. As previously mentioned, the stars cannot fuse iron. So, where did the heavier elements come from? The answer is, the selenium, cooper, iodine, zinc, etc, that your body needs, were created in the supernova. And the carbon and oxygen, the main constituents of your body, were made in stars. Thought that you haven’t anything to due to with the stars? You couldn’t have been more wrong. After all, we are stardust!
When a supernova begins, the explosion heats and expels the star’s outer layers and much of its material at velocities up to 10% the speed of light, creating a shock wave that sweeps up the expanding shell of gas called a supernova remnant.
As the core of a massive star is compressed during a supernova, it collapses into a neutron star. To help picture this, you may think that gravity squashed the electrons into the nuclei, which reacted with the protons, creating neutrons and antineutrinos. The result is a super dense star composed almost entirely of neutrons. Its diameter can be as small as 20km, but a few teaspoons of it would weight about a billion tons! In addition, they have rotation periods between about a few milliseconds to 30 seconds!
Neutron stars are impressive, but if the star is really massive, it may continue to collapse into a black hole!
Now we know the lives of the stars, how they born and how they die. If it’s a low mass star it will end in a planetary nebula leaving behind a white dwarf; if it’s a massive star it will end in a supernova, creating a neutron star or a black hole.
I hope you enjoyed this podcast. Thanks for listening and happy International Year of Astronomy!
End of podcast:
365 Days of Astronomy
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