DavidLeodis wrote:
The information would seem to imply that 24,500 light-years equates to 13 billion years? It confuses me (easily done I admit) that something that is 13 billion years old can be only 24,500 light-years away, as I'm sure I have seen it reported that many objects are very much further away in light-years yet are apparently very much less old.
When we talk about incredibly distant galaxies, they invariably have very high redshifts. We know that they are old because the spectral lines in the light that is only now reaching us from these galaxies have been shifted enormously to the red by the expansion of the universe. Often we only see these galaxies in infrared light, even if they originally emitted a lot of energetic shortwave light.
The light from globular clusters like M5 is not redshifted. The Milky Way globulars orbit the center of our galaxy, and although they are on wide orbits, their distance from us doesn't change all that much. Certainly not in our lifetimes!
But it is important to understand that we can't use redshift to determine the age of the Milky Way globular clusters. To find out how old these globulars are, astronomers measure the metallicity of the stars that belong to the clusters. Take a look at the following diagram of stars of different metallicity:
The top spectrum belongs to the Sun. As you can see from the very "squiggly line" of the solar spectrum, the Sun has lost a lot of light due to absorption by elements in its outer atmosphere. This "squiggly line" tells us that the Sun is a moderately metal-rich star. Although it certainly consists mostly of hydrogen and helium, it also contains quite a bit of of heavier elements such as oxygen, carbon, calcium, iron etcetera.
But you can see that the spectra below the spectrum of the Sun become increasingly straight. The bottom spectrum, which is almost perfectly straight, is the spectrum of a perfect "first-generation star", made of pure hydrogen and helium. Such a star has never been found. The third spectrum from the top belongs to a real star, however. This is likely a second-generation star. The gas it was made of had been "contaminated" by heavier elements from a previous generation of stars, which produced heavy elements in their cores during their lifetimes and scattered these heavy elements when they exploded as supernovas.
The stars of M5 are quite metal-poor. Their spectra are very unlike the spectrum of the Sun. Because of that, we
know that M5 is a very old cluster.
But we can also tell the age of a globular cluster by the shape of its Herzsprung-Russell diagram. A Herzsprung-Russell diagram plots the luminosity of the stars versus their color and thus temperature. The color and luminosity of a star is a function of its mass and evolution. In the diagram on the left, you can see a slightly wavy diagonal line that is blue at the upper left and red at the lower right. The stars that fall on the main sequence all fuse hydrogen to helium in their cores, but their color and luminosity depends on their mass. The brighter and bluer they are, the more massive they are.
But the most massive stars quickly evolve off the main sequence. They turn into supergiant stars of different colors. Stars of intermediate mass evolve slower, but they too will eventually use up their core hydrogen and turn into giant stars. Most of the giant stars are red or orange giants. In the diagram on the left, you can see the variously colored supergiants scattered at top and the red, orange or yellow giants below the supergiants. These giants form the so-called giant or red giant branch.
In this black and white HR diagram, you can see that the giant branch appears to be connected to the main sequence.
In a very young cluster like
NGC 2264,
only the most massive stars have even reached the main sequence. The less massive stars are located to the right of it, still contracting towards the main sequence. In young cluster
NGC 2362, the most massive star is a blue supergiant, whereas most of the other stars are still located on the main sequence.
What about the HR diagram of the Pleiades? This brilliantly blue cluster contains no bright red stars, and therefore it lacks a red giant branch. It also has no supergiant branch. But interestingly, it also has no O-type stars. Therefore the top of the main sequence is empty for the Pleiades. Were there ever any stars there in that spot, when the Pleiades were younger? Well, in my amateur opinion, the Pleiades is such a rich cluster that it is likely that it was born with at least one O-star. But if so, that star has exploded and disappeared a relatively long time ago. Astronomers agree that the Pleiades cluster is at least a hundred million years old, and perhaps several million.
Here you can see the HR diagrams for various clusters. The older the cluster is, the shorter is its main sequence. As the cluster ages, the main sequence shrinks from the top down as more and more stars turn into red giants.
This is a typical globular cluster HR diagram. The main sequence is very short, and the so-called turn-off point has "eaten its way down" to late F and early G-type stars. Only a very old cluster could have an HR diagram like this one. And because the cluster is so metal-poor (again because it is old), it has a blue horizontal branch, which is unique to very metal-poor clusters.
Astronomers use the position of the the turn-off point to calculate how old a cluster is. For M5, the estimated age is 13 billion years.
All in all, this is how we know that M5 is about 13 billion years old.
Ann
