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Nearby Galaxy Surprisingly Young
Pi_0's don't shower writes "The hubblesite is reporting that a galaxy discovered 70 years ago, I Zwicky 18, has been confirmed to be one of the youngest galaxies in the universe, at only 500 million years old. By contrast, our Milky Way, Andromeda, and most other nearby galaxies are 12 billion years old. This galaxy is the closest newly-formed galaxy, at only 45 million light years away, which has rather interesting implications for galaxy formation."
There are a number of ways to do this. Perhaps the #1 way is through what is called a
Hertzsprung-Russell diagram, named after the two astronomers who came up with this method of classification.
There are two primary aspects that allow you to plot an individual star on this diagram: Its spectral color (litterally, what color you see the star as) This can be assigned a number as a specific peak color frequency. This also can be interpreted as the temperature of the "surface" of the star. The other axis is the absolute brightness of the star. This is the number of photons you can count in a given period of time relative to how far away it is. For measuring stars in a galaxy, distance measurements can pretty much assume that all of the stars are roughly the same distance away, thus simplifying this task. Close stars will obviously be brighter, but stars like Alpha Centari, an ordinary yellow star, are apparently as bright as Betelguese, a star almost 100 times further away.
Keep in mind that only until the Hubble telescope was able to resolve the individual stars in this galaxy, this study wasn't able to happen for this particular galaxy... which is why this is now news. Really a neat project on the whole.
The point here is that young stars fall onto what is called "The Main Sequence". These are stars like our sun that are still mostly converting Hydrogen to Helium as the primary source of nuclear energy. This relationship is quite well defined, and has been observed in not only the Milky Way Galaxy, but in other galaxies where individual stars have been able to be identified.
Stars that run out of Hydrogen fall into a very different pattern. Like I mentioned with Betelguese (in the constellation Orion), it is much further away but yet just as bright. Also, its color is more Red (in a clear rural sky you can even see this reddish color), which puts it outside of the Main Sequence. That is because (according to current theory) it has run out of Hydrogen and is now in the process of turning Helium into Carbon. This group of stars, known as "Red Giants" play a huge role in determining the age of a galaxy or other group of stars.
When stars finally run out of Helium, the rest of the elemental transmutation takes place rather quickly, or the fusion simply stops. Without going through the subsequent steps, the star eventually turns into a Super Nova (if there is enough stuff in the star to produce it) and leaves behind a white dwarf or neutron star (for really big stars... black holes are yet more stuff). A white dwarf is quite dim even for its distance, but yet its surface temperature is quite hot. These give yet another very distinctive plot on the HR Diagram.
So the whole point here is that you can measure the age of a group of stars based on the relative brightness of the stars in that group. Very large stars tend to live very short lifespans because they do a very efficient job of doing the fusion. Small stars (like Wolf 359) will be still doing the hydrogen fusion 20 billion years from now. Over time (and based on considerable observation examples... not just this galaxy in the article but also concrete parallax measurements of close stars to our own as well) you will see fewer and fewer stars on the very blue end of the HR diagram and more and more Red Dwarfs, with white dwarves showing up in increasing numbers as well.
One other critical tool for measurement is also trying to determine what elements are in the spectra of the stars. Different elements show up in stars and can be measured based on if they are absorbing or emitting light from the surface of the star. Mind you, this doesn't indicate much in terms of what is happening in the center of the star, but rather what is on the surface of the star. The current assumption is that the Universe started out with mainly Hydrogen and Helium, and only very small amounts of other elements. If you find large quantities of other elements in a star (like significant quantities of Iron on the sur