Back To CourseBasics of Astronomy
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I've seen quite a few movies related to space and science fiction, and it seems that with every new movie the walls and chairs of the theater tremble more and more with louder and louder explosions of spaceships, planets, and stars. Special effects, not good concepts, seem to win out in movies these days. But I'm no Roger Ebert. What I can rate and appreciate is that Hollywood - no surprise here - gets things wrong quite a lot.
If you were in space, the explosion of a supernova would be as loud as the sound of…silence. You heard me right: silence. Despite the massive and violent nature of it all, the supernova explosion is silent. That's because space is a near vacuum, and sound waves can't travel through a vacuum. But you can definitely see one, and the way supernovae are observed helps to classify their different types. That classification is what this lesson will be all about.
A humongous stellar explosion, a supernova, may be brighter than an entire galaxy for a brief time. It may be silent, but it sure is bright! The light that travels away from a supernova can be read by specialized detectors here on Earth to produce a spectrum, which for this lesson's purposes can be remembered as an arrangement of colors in order of wavelength.
A spectrum may have added or missing lines of colors that clue astronomers into the different compositions of matter from which the lights came or passed through. And by 'composition,' I mean the stuff the star is made of. It's like a barcode on a product label that tells a machine what the product actually is. In our case, when a massive star is days away from a supernova explosion, it is composed of a lot of different things, from hydrogen at the very outer layers to iron at its very core.
Most stars that are eight or more times the mass of our sun die as a Type II Supernova. A Type II Supernova is a supernova that is classified as having hydrogen lines in its spectra that are made by the explosion of a very large star. The hydrogen lines come from the hydrogen-rich outer layers of the star as the star explodes.
But there are other kinds of supernovae. A Type I Supernova is a kind of supernova with no hydrogen lines in its spectrum. How can this occur if I just said that the outer layers of the star are rich in hydrogen? Well, there are two ways. The first is a supernova explosion that results from the collapse of a white dwarf, termed a Type Ia Supernova. A white dwarf is a remnant of a star that wasn't massive enough to force the ignition of carbon fusion for energy.
To understand how a Type Ia Supernova occurs, you also need to know a term called the Chandrasekhar Limit. The Chandrasekhar Limit is equal to about 1.4 solar masses, and it tells us that all stable white dwarfs must be smaller than 1.4 solar masses. This means stars more massive than 1.4 solar masses can only become white dwarfs if they shed mass as they evolve, and we know this does occur and helps to explain why medium mass stars upwards of eight solar masses can become white dwarfs.
This loss of mass can be imagined as a star basically uncoating itself of many layers of clothing, like a person would when they come inside from the cold. But if a white dwarf is in a binary system - a system where two stars circle one another around a common center of mass - it can exceed this limit, thereby leading to its collapse. This happens through mass transfer, where the white dwarf gains mass from the other star in the system.
Unlike massive stars that have a useless iron-rich core, a white dwarf has usable fuel by way of its carbon-oxygen interior. It's just that it wasn't large enough prior to mass transfer to actually ignite it, but as the white dwarf begins to collapse following mass transfer, its temperature and density increase and the core begins to fuse and explodes thereafter.
It's like one really big person putting their many layers of heavy clothing onto a much smaller person, one that can barely handle them to begin with because they're so small. This leads to their crushing collapse. Or you can think of a white dwarf as a pressure mine that explodes when too much weight is put on it as its previously inert explosive material is lit by a fuse that's triggered to ignite by the newfound weight.
In either case, the explosion is so violent that at one point, the Type Ia Supernova can be brighter than a Type II Supernova. The reason a Type Ia Supernova has no hydrogen lines is because a white dwarf has negligible amounts of hydrogen.
Less commonly, a Type Ib Supernova might happen in a binary system. A Type Ib Sueprnova is an explosion of a massive star after it has lost its outer layers of hydrogen and developed an iron core. Since the star lost its hydrogen prior to the explosion, we expect no hydrogen lines to occur. The way this supernova occurs is quite simple.
In a binary system, the more massive star will lose its outer layers, which are rich in hydrogen, to the other star in the system by transferring its matter. The remaining parts of the more massive star develop a useless iron core, a dead end for the star, leading to its subsequent collapse and explosion, as per the lesson on supernova, how massive stars explode. In essence, a Type Ib Supernova is like a Type II Supernova, but one that no longer has any hydrogen.
As a side note, more recently astronomers have recognized a Type Ic Supernova, a supernova devoid of hydrogen and helium in its spectrum. There is still some debate in the scientific community as to how different Type Ib and Type Ic supernovae truly are, but we'll just leave it at that for this lesson.
Since there are so many different types, I'll offer you my own way of remembering the differences between these supernovae. Type II Supernovae are our default supernovae: they have everything, including those hydrogen lines. Using basic math, to get from two to one, you have to subtract one from two, though one thing we subtract from a Type II to get to a Type I is hydrogen, so Type I Supernovae are missing hydrogen, but the reasons differ for a Type Ia versus a Type Ib Supernova.
A star destined to be a Type Ib is bigger than a type Ia and loses hydrogen to its companion star. Because it's so big, Type Ib wants to be like a Type II Supernova, and therefore dies in a similar way. Using that, we know Type Ia is smaller than the bigger Type Ib and it wants to become bigger like its big brother by siphoning off matter from its companion star. But it's too small to handle all that mass and collapses while trying to carry all that extra weight.
I'm going to try to lift a bit of weight off of your shoulders and summarize everything as concisely as possible so you don't have to remember too many extraneous details. A supernova is a humongous stellar explosion. A Type II Supernova is a supernova that is classified as having hydrogen lines in its spectra that are made by the explosion of a very large star.
This is different from a Type I Supernova, a kind of supernova with no hydrogen lines in its spectrum. More specifically, a supernova explosion that results from the collapse of a white dwarf is termed a Type Ia Supernova, but an explosion of a massive star after it has lost its outer layers of hydrogen and developed an iron core is known as a Type Ib Supernova.
The last important thing I need you to remember is the Chandrasekhar Limit. The Chandrasekhar Limit is equal to about 1.4 solar masses, and it tells us that all stable white dwarfs must be smaller than 1.4 solar masses. However, if in a binary star system a white dwarf exceeds this limit through mass transfer, it will explode and a Type Ia Supernova will be the end result.
Once you are done with this lesson, you should be able to:
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Back To CourseBasics of Astronomy
28 chapters | 325 lessons