Deep space objects > Star clusters
Star clusters are a group of stars with a common origin and a gravitational link for a specific time. It is a useful tool for astronomers as it helps study and model stellar evolution. There are two main types of star clusters: open clusters (open) and globular clusters. Learn more about the galaxy’s star clusters in an interesting video.
Types of star clusters
Open star clusters
Open star clusters are so named because individual stars can be resolved easily. For example, the Pleiades and Hyades are so close that individual stars can be easily seen with the naked eye. They are sometimes called galactic clusters because they are located in dusty spiral arms. Stars in an open cluster have a common origin (the same initial molecular cloud was formed). Usually, the cluster contains several hundred stars (can reach several thousand).
The stars are bound by gravity, but it’s pretty weak. The cluster revolves around the galaxy and finally dissipates due to gravitational contact with stronger objects. It is believed that the Sun appeared in an open cluster, which no longer exists. Therefore, these are always young objects. A nebula is still visible in the Pleiades, hinting at recent formation.
The open clusters are filled with Population I stars — young and highly metallic. In width, cover from 2 to 20 parsecs.
Messier open star clusters
Other notable open star clusters
Globular star clusters
Globular clusters of galaxies contain from a couple of thousand to a million stars located in a spherical gravitational system. They are in a halo and represent the most ancient stars – population II (developed, but low metallicity). The clusters are so old that any star (above the G or F class) has already stepped over the main sequence. The globular cluster has little dust and gas because new stars do not form there. The density in the interior is much higher than in areas near the Sun.
In globular clusters, stars also share a common origin. But this type holds objects firmly by gravity (stars do not scatter). There are approximately 200 globular clusters in the Milky Way. Among them, you can recall 47 Toucan, M4 and Omega Centauri. Although about the latter, there are suggestions that it may be a dwarf spheroidal galaxy.
Messier globular star clusters
Other notable globular star clusters
Age of star clusters
Star clusters are incredibly valuable to astronomers because they can help determine the age of a star and track its evolution.
The stars of open clusters have a common origin, so their metallicity levels converge, which means that all members will pass through the evolutionary stages in the same way. In addition, they are located at the same distance, which also allows you to display the absolute value. If you see bright stars that stand out, then they are much lighter than their weaker neighbors.
With this information, scientists create digital charts for the clusters. They display the apparent value of V on the vertical axis relative to the digital index B – V horizontally. With spectrographic parallax, you can calibrate the chart to display the absolute value.
If we build diagrams for them, we get the bottom graph. Since they are at different distances, it is calibrated to absolute values.
A new scale is visible on the right vertical axis. “Years” is the age of the cluster. The pair in Perseus is so young that most of the stars are in the main sequence stage. The Pleiades are slightly older and do not have stars exceeding color index 0 (spectral type A0). More massive objects have already stepped over to the giant branches. M67 does not have a star hotter than a color index of 0.4. Most significant is the pivot point in the diagram, where the cluster turns off the main sequence. The lower the main sequence, the older the cluster.
Globulars are usually much older than open ones, so the color magnitude in the diagram shows more developed stars. They are also devoid of objects with a large mass. This point is illustrated below for example M55.
There is a cluster of hot stars in the main sequence above the shutdown point. They are called blue stragglers. Scientists believe that due to the high stellar densities in globular clusters, some are capable of merging. The combined mass makes the star hotter and brighter than the main mass. Star clusters are not eternal structures and they are destroyed. Examine this process in the video. Also, use the online star map to find the clusters yourself. If you can’t buy a telescope, then visit our page with a virtual model of the Milky Way galaxy or view a photo from the list of clusters.