More Things that go BANG! in the night
Here's what we know so far. The Universe is expanding. It is the space between everything that is expanding not galaxies rocketing away from each other. We know how fast distant objects (galaxies, quasars, etc.) are receding from us but, since we don't know how far away those objects are, we can't tell how old the Universe is. We need a "standard candle" that we can look at in distant galaxies so we can figure out how far away they are.Stars blow up! Well, not all stars blow up. Sol (that's our sun's name) will probably never blow up so you can safely plan you next vacation without that worry. When a star blows up it is WAY brighter than all other stars and so it will suddenly appear in the sky as a "new" star.
The Latin word for "new" is Nova. Scientists of all types like to use Latin words. This is mostly because, until very recently (the last fifty years or so), every student in every "good" school and university in Europe and the "New World" (like the US) was required to learn Latin. This is mostly because, until the last few hundred years, all schools and universities were run by the Catholic church and the Catholic church did all it's business in Latin. This makes lots of sense. There are lots of languages in the world and it is very difficult to have to translate documents from language to language. But with Latin, all the educated people in the Western World (Europe, North and South America) could read the same document in Latin. Until the last few hundred years, all scientific works, written in the west, were written in Latin.
There are several ways that stars can go Nova. Remember that stars are just big balls of gas (mostly Hydrogen). Just like in a hot air balloon, when you heat a gas it expands. Stars are always playing a balance game between gravity that wants to crunch the gas together and the gas wanting to expand because it is hot. In the center of a star, gravity is crushing Hydrogen nuclei (protons and some neutrons) so hard that they "stick together" and "fuse" to form Helium. A Helium nucleus is 2 protons and 2 neutrons and it is slightly lighter (less massive) than 4 Hydrogen nuclei. This tiny difference in mass gets turned into energy by the formula E=mc² . This energy keeps the star gas hot and gradually "leaks" out of the star and we see it as light (and heat and x-rays and radio waves etc.).
Big stars "burn" their Hydrogen quickly and get a central core with lots of Helium. Well, big stars have enough gravity pressure to smash Helium together and get Beryllium. Now Beryllium is a fair amount less massive than 2 Helium atoms and that mass gets converted to lots more energy so the star gets really hot and it expands. Eventually it runs out of Helium and starts smashing Beryllium together to get Carbon and Oxygen and Oxygen together to get Sulfur and all of these together with the little Hydrogen and Helium that is left and the star burns hotter and hotter and get bigger and bigger until suddenly (and I do mean suddenly!) the whole core of the star becomes Iron. Iron stops the fusing. Iron can fuse together but the atoms it makes are all slightly heavier (more massive) than the Iron atoms used so, instead of making energy, fusing Iron absorbs energy. All the heat at the core of the star stops and all that expanded gas of the star's atmosphere has nothing to hold it up. Gravity takes over. The outer atmosphere of the star starts falling back to the core faster and faster and all of it smashes down on the ball of Iron all at the same time and all that gravitational energy crushes these atoms together with the Iron and BOOM!!! The star explodes into a huge ball of glowing atoms from Hydrogen to Uranium. It's where the stuff you are made from came from. You are star stuff.
REALLY big stars, maybe 40 times our sun's mass, don't even go BOOM! They go away! The super massive star core and all that in-rushing gas in such a small space creates an object that has an escape velocity (the speed you would have to go to get off) greater than the speed of light! This we call a Black Hole.
Most stars aren't big enough to "go Nova" this way but most stars are binaries - two stars revolving about a common center of gravity. They can have an interesting way of going BOOM!
Smallish suns, like Sol, eventually fuse up most of their Hydrogen and start fusing Helium. When they do this, the star expands very large and becomes a Red Giant just like the big stars mentioned above but these stars don't have enough gravitational pressure to fuse atoms all the way to a total Iron core. They slowly cool down as they run low on fuel until they become small, hot, dense White Dwarf stars. This is what will happen to Sol way, way in the future.
In a binary system, one star will probably be slightly more massive than the other and, hence, will become a White Dwarf first. When the second star eventually becomes a Red Giant its outer atmosphere gets close enough to the White Dwarf companion. The White Dwarf starts gravitationally sucking the Red Giants atmosphere and when enough enough of it collect on the White Dwarf's surface, BOOM! the White Dwarf goes Nova! This type of nova is called a Type 1A. It's pretty easy to tell a type 1A nova from the Fraunhofer lines in its spectral (remember them?). Type 1A nova have a wonderful trait. There is an exact relationship between how bright it is and how fast it dims. So if you see a type 1A nova, (and you can see these all the way across the Universe), all you have to do is time how fast it dims, look up on charts to see how bright it really is, measure how bright it looks and BAM! you can figure out how far away it is!
There you have it. We have a "standard candle" that we can use to figure out how far away any galaxy (with a type 1A nova) is. Astronomers have plugged these measurements into the Hubble Constant equation and now tell us that the Universe is 13.7 billion years old! Kind of makes you want to send it a birthday card, doesn't it?