# Is there a stellar database that indicates how long ago stars in our Galaxy formed?

There are several ways of determining the age of a star: its position in the HR diagram, the presence of a protoplanetary disk, it belonging to a cluster...

When did the stars in our Galaxy form? Do we know their age? Does there exists a database of stars for which we know the age, from which this average could be computed ? Is there a histogram of how long ago the stars in our Galaxy formed?

The amount of stars being formed as a function of time is shown for instance in Snaith et al 2014, but this doesn't directly translate into a histogram of the age of the stars currently in our galaxy, since the more massive ones that formed early on are gone.

• @Alchimista Feel free to ask that question! I fail to see what makes it more sensible, though. Jan 8 '20 at 9:43
• I am not going to argue but it is indeed what I wanted to suggest. You can average the age of lions and trees as both being living orgsnisms, sure. But what can you meaningfully extract out of it? Jan 8 '20 at 13:19
• Otherwise things can get sense if a context is given. An example might be arxiv.org/abs/1410.3829 Jan 8 '20 at 13:24
• @Alchimista "What is the average age of stars in the MW?" is in principle more easily answered than "When did the MW form?", since the former can be computed from a suitable database of stellar age estimates, while the latter has to deal with questions of assembly and mergers. Jan 13 '20 at 13:25
• @Alchimista: There was no single point at which our galaxy formed, as this is a complex process of hierarchical assembly. Additionally, star formation rates are continuous and peaked in our (as many other galaxies) around $z=2$, producing an IMF of which the most massive stars are long gone. On top of that, galaxies continue to accrete extragalactic gas from which new star-forming regions arise. OP's questions has therefore full validity, and your suggestion is oversimplified. Jan 15 '20 at 12:19

There isn't really a database as you request. Finding the ages of stars is difficult. Only one star has an accurately known age - the Sun. That comes from radioisotope dating of meteorites. For other stars we must rely on models to a greater or lesser extent and we can only estimate an age if the star has a mass or is in a phase of its evolution where things are changing rapidly enough to give some handle on how old it is. For stars at the mass of the Sun or a little bit bigger, one can use evolution in the Hertzsprung-Russell diagram. Stars become more luminous as they burn through their core hydrogen and precise measurements can give an age to about $$\pm 1$$ Gyr. These are the data that Snaith et al. (2015) use in their paper.

So what Snaith et al. do is they attempt to constrain the star formation history of our Galaxy (or at least stars in the disk of our Galaxy) by making a Galactic Chemical Evolution model that predicts how the abundances of silicon and iron change with time and compare the results of their model with the observed silicon and iron abundances in this set of solar-type stars with reasonably well-estimated ages. Iron and silicon are used because they provide differing constraints. Iron is produced during the evolution of relatively low-mass ($$1.5 - 4 M_{\odot}$$) stars that live their long(ish) lives (1-10 Gyr), become white dwarfs and then some of the white dwarfs explode as type Ia supernovae. Silicon is produced by massive stars with short lives ($$<0.1$$ Gyr) and spread promptly into the interstellar medium by core-collape type II supernovae.

A representative lot from Snaith's paper is shown below. The star forming rate is shown in plot (a) and the match of the model to the observational data (they are trying to match the solid green points here) on chemical abundance is shown in the other plots.

They test a number of models and different assumptions, but it appears to be quite a robust result (found in other studies also) that there was a burst of star formation 10-12 billion years ago, a lull at 8-9 billion years ago and then a more constant, lower rate over the last 7 billion years. The very low rate in their model over the last 2 billion years is unlikely to be true and is probably an artefact of the age uncertainties in their data points (e.g. stars with age of $$1 \pm 1$$ billion years) and we certainly see plenty of star formation in the disk of the Galaxy today.

Contrary to what you say in your question this does (almost) represent the age distribution of stars in the Galaxy. That is because the vast majority of stars (90+%) are of a solar mass or lower. Such stars have main sequence life times of 11+ billion years, such that almost every one of them that was born is still a main sequence star now.

There will be a small reduction in the numbers of the oldest stars, because a small fraction of those ($$<10$$%) will have lived and died and also because a small fraction of the oldest stars may have escaped the Galaxy entirely (due to various kinematic processes that cause their orbital speeds to change). There is also an effect whereby if you take a volume close to the Sun, the oldest stars will be under-represented because they are distributed more broadly around the disc midplane for the same reason.

But by an large, the upper left plot in the picture is roughly what we expect the age distribution of stars to be in the Galactic disk.

There are a small population of very old stars ($$12+$$ bilion years), perhaps 1%, that are distributed more widely in the Galactic halo.

In terms of asking the question in the title, there are various catalogues which include stellar ages, if you search on VizieR for the category "Ages" (in the "Astronomy" menu on the right-hand side) you will find a large number of such catalogues, but you will have to bear in mind that they focus on certain sets of objects rather than stars in general.

One example is the Geneva–Copenhagen Survey of the Solar Neighbourhood (which includes F- and G-dwarfs), which has also been used as the source for stellar ages in the XHIP (Extended Hipparcos compilation) catalogue.

Another example is the Lachaume et al. (1999) age determinations for main-sequence stars in the B9–K9 spectral type range.

Computing the age distribution of stars in the galaxy from these catalogues will require correcting for the various selection biases involved, this is not a trivial task.

I don't know about stars in general, but for giant stars, the following histogram shows the age distribution of those in our Galaxy based on their location in the Galaxy.

This means that most stars are between 7 and 8 billion years old.

• No, it doesn't. It might mean that most giants are 7-8 Gyr, but given they have to have lived lives as main sequence stars, that is hardly surprising. Then of course they die, so there might be a shortage of old ones. In short, you can tell very little about the typical ages of stars without knowing much more about stellar evolution, the mass distribution of the sample and the selection effects in its construction. Feb 20 '20 at 11:26