Open Clusters Background Information

An open cluster, sometimes called a Galactic Cluster, is a group of 10s or 100s of stars that were born from the same initial cloud of gas (mainly Hydrogen) and dust. When they are young - a few million or tens of millions of years old - these clusters contain some very large, bright stars (called O or B-type stars). The very youngest clusters (usually less than 10 million years old) often still contain the remains of the gas cloud from which the stars were born – this is seen as nebulosity.

Cluster stars are very useful as they were all formed from the same giant cloud , so they have the same chemistry, and they are all at about the same distance from us, although they are typically hundreds or thousands of light years away. By observing a group of stars in a cluster, we can assume they are all made of the same stuff, and they are all the same distance away from us – so any differences between them are really caused by their different mass.

Astronomers measure the intensity of light from the stars in a cluster through different filters with a process known as photometry and plot the colors of the stars on a color-magnitude diagram. Once a measure of how “red” or “blue” the stars are is made, more information about them can be obtained – massive stars are usually very blue and hot, intermediate mass stars (like the Sun) are yellow, and the very lowest mass stars are red and cool.

H-R Diagrams

Hertzsprung-Russell (HR) Diagrams

In the early 20th century, after investigating the effects of an object’s temperature and of the color of its radiation, scientists reasoned that there should be a relationship between the temperature of a star and its luminosity. If all stars were alike, those with the same luminosity would have equal temperature and hotter stars would be brighter than cooler ones.

In 1911, Ejnar Hertzsprung (Denmark), plotted a graph of star’s magnitudes against their color. Independently in 1913, Henry Russell (USA), constructed a plot of stars’ magnitudes against their spectral class, confirming that indeed, there did seem to be some sort of relationship between a star’s luminosity and its temperature, and the stars fell into distinct groups. Such a plot was thereafter named the Hetzsprung-Russell or H-R diagrams.

A star on a HR diagram is represented by a dot. Since a large number of stars are usually represented on a HR diagram, there are a large number of dots on the diagram, as shown below. The y axis on a HR diagram represents the star’s luminosity and the x axis represents the temperature of the star.

The main areas of a HR diagram are labelled above and are briefly described below for stars of about the same mass as our Sun (solar mass stars):

Main sequence

The main sequence is a band which stretches from the bottom right of the HR diagram up to the top left, hence it goes from cooler, dimmer stars up to brighter, hotter ones. Most stars, including our Sun spend most of their lives on the main sequence as they fuse their Hydrogen into Helium in their cores.

Red Giant Branch

Once solar mass stars have fused all their Hydrogen into Helium, they evolve off the main sequence into the Red Giants area (which astronomers call the Red Giant branch or RGB). At this point, the Hydrogen shell surrounding the core of the star begins to burn, producing even more Helium.

Red Supergiants

When the Hydrogen shell burning is finished, the shell of Helium begins fusing into heavier elements such as Carbon and Oxygen. As this happens, the star moves into the Red Supergiants region of the HR diagram.

White Dwarfs

Once all the Helium has been fused into other elements, the outer layers of the star are ejected outwards into what is known as a planetary nebula. The exposed core of the star (made up of Carbon and Oxygen) which is leftover, is a white dwarf. A white dwarf cannot keep the fusion going and gradually becomes fainter and cooler. Evolution from the red supergiant area to the white dwarf area, happens very quickly in comparison to how long the star stayed on the main sequence.
  

Photometry to Find the Color of a Star

Photometry is the measurement of the intensity or brightness of an astronomical object, such as a star or galaxy by adding up all of the light from the object. For example, a star looks like a point of light when you look at it just with your eyes but the Earth’s atmosphere smears it out into something that looks like a round blob when you use a telescope to look at it. In order to measure the total light coming from the star, we must add up all of the light from the smeared out star. Photometry is generally used to generate light curves of objects such as variable stars and supernovae, where the interest is the variation of total light energy output by the system over time. It can also be used to discover exoplanets, by measuring the intensity of a stars light over a period of time. Deviations in the light output can indicate objects in orbit around the star. These instructions explain how photometry can be carried out on groups, or clusters of stars, from images taken with different filters, in order to plot a color magnitude diagram. 

In order to plot a HR diagram, the temperature and luminosity of the stars need to be known. The simplest indication of a star’s temperature is its color. A star’s color is simply a measure of the amount of light from the star in one filter compared to another. The most common color system is B-V, which is simply an object’s magnitude as measured through the B filter, minus its magnitude as measured through the V filter. Other filters may be used however, and it is most common to compare the colors by taking the shorter (more blue) wavelength and subtracting the longer (more red) wavelength.

To find these values, you can use SalsaJ to measure the brightness of as many stars in the cluster as you can in at least two filters. You then use the CMD plotter spreadsheet, or make your own spreadsheet, to convert the values from the photometry to magnitudes. You can then subtract the magnitudes from the two colors, and plot these values against the magnitude in the longer wavelength. This plot is a color-magnitude diagram, equivalent to a H-R diagram.

Photometry of Star Clusters with SalsaJ

Download SalsaJ from the Hands On Universe Website.

Launch SalsaJ.

Download the Template CMD Plotter for Open Office or the Template CMD Plotter for Excel.

Download open cluster data. If you don’t have an observing session, you can search the public archive for open clusters from this list.

Use the screencasts below to learn how to use SalsaJ and the CMD plotter to make a color-magnitude diagram for an open cluster:

Part 1:

 

Part 2: