Image credit: NASA, The Hubble Heritage Team, STScI, AURA
If you look at the night sky with a telescope, you will notice fuzzy patches among the stars. Some of them are shapeless blobs, others are round. These are star clusters, the giant groups of stars held together by gravity.
Galileo was the first to resolve the fuzzy blobs, that we now call OPEN CLUSTERS, into individual stars in the beginning of the 17th century. Open clusters are relatively young objects. They contain thousands of loosely bound stars that were born at the same time in a giant cloud of gas and dust. Open clusters break apart some million years after formation. Our Sun was born in such cluster too. It is now difficult to tell which stars ones belonged to the same stellar family as our own Sun, but astronomers keep searching*.
* more on the topic in our blog post Where are the Sun’s siblings?
Another type of clusters, the spherical-shaped GLOBULAR CLUSTERS, are way more interesting. These are ancient objects, probably some of the first things to form in the Universe. The 18th century astronomer Charles Messier was the first to resolve individual stars in a globular cluster M4 in the constellation Scorpius. Globular clusters in the Milky Way Galaxy are as old as the Galaxy itself. In fact, they helped the astronomers to determine the age of our Milky Way! Globular clusters contain millions of stars of various ages separated by a distance of less than 1 light year. To compare, the nearest star to the Sun is 4.2 light years away. There are 150 known globular clusters in our Galaxy and probably up to 50 more still waiting to be discovered. Other galaxies have them too!
Why study globular clusters
Globular clusters are not just incredibly beautiful objects to look through a telescope at. They are very useful for astronomers too as they teach us about the history of our Universe and, in particular, about how stellar evolution “works”.
Also, they might be good places to look for intermediate mass black holes, the hypothetical objects with masses between 100 and 1000 solar masses. The only two kinds of black holes known at the moment are stellar size and supermassive black holes. Nobody knows if there are in-betweeners. But if there are, globular clusters might have the right conditions to create these cosmic monsters.*
Lifespan of globular clusters
Globular clusters are extremely old. But although these space dinosaurs live for a VERY long time, they can eventually fall apart too! The two processes responsible for this are evaporation and dynamic friction.
Just like the liquids evaporate when the “loose” fast molecules escape from the surface, the stars on the outskirts of a globular cluster can leave stellar home. These stars are not as tightly bound as those deep within the cluster and the fastest of them (those with a speed exceeding the escape velocity) can break free from their group. The cluster evaporates!
It turns out that small and not very massive globular clusters evaporate the fastest.
Dynamic friction, or, more specifically Chandrasekhar dynamic friction (after the Nobel laureate Chandrasekhar who discovered it), is a drag force that causes globular clusters to spiral towards the centre of the Galaxy. As a cluster orbits the centre of the Galaxy, it “tugs” on individual stars it passes by and deflects their trajectories. So that if the stars were uniformly scattered before the cluster passage, now they are slightly piled up right behind it. The collective gravitational pull from these stars slows the cluster down! For more technical details please refer to the Introduction to Astrophysics tutorial on dynamic friction.
In 10 thousand billion years all the clusters we know in the Milky Way will either evaporate or fall to the centre of the Galaxy. Let’s observe them, study them and just admire them while they are there!