OBSERVATIONS OF OPEN (GALACTIC) AND GLOBULAR CLUSTERS

Background:

When giant clouds of dust and gas form, the dust becomes thick enough to block visible and ultraviolet light (short wavelengths) from stars outside the cloud. This allows the cloud to cool, by radiating away its energy in the form of infrared and radio radiation (long wavelengths) which the dust doesn't block. As the cloud cools, atoms unite to form molecules (which are also good at blocking short wavelengths and radiating away energy in the form of infrared and radio waves) and a "molecular cloud" is born. The molecular cloud continues to cool and collapse under the weak force of gravity. From the outside, these molecular clouds look like large blank areas from which no radiation (except infrared and radio) is emitted.

Parts of the cloud become very dense, and the force of gravity becomes stronger. These small pieces of the cloud collapse very rapidly and their interiors become so condensed and hot from the collapse that nuclear fusion of hydrogen into helium is triggered, and they begin to shine as stars. The radiation from the stars starts to heat the cloud from the inside (now the dust is preventing the starlight from escaping!) and it starts to break the molecules back into the atoms again. The hottest and most luminous stars (the O and B supergiants) even ionize the cloud around them to form HII (the ionized form of hydrogen) regions. These hot stars and HII regions "light up" the cloud and often provide spectacular views of it. We then see them as diffuse nebulae.

Eventually, the hot, luminous stars breakup, ionize, and disperse the cloud of dust and gas, leaving a group, or cluster of stars behind which formed from the cloud. These are called "open clusters" (or "Galactic clusters"). Open clusters usually don't contain enough stars to hold on to each other gravitationally (most of the mass of the cloud was dispersed by the luminous stars). They contain from a few dozen to a few thousand stars (usually a hundred or so). Most open clusters then break up over a hundred million years or so and the individual stars slowly orbit the Milky Way galaxy as loners, each in their own orbit (as does the Sun). Since open clusters don't contain a lot of stars, they become inconspicuous beyond a distance of about one thousand light-years. They usually appear in the telescope as a coarse collection of stars of many different brightness and colors.

Globular clusters are very different from open clusters. For starters, each globular cluster contains between 100,000 to ten million stars! The stars are all old (~10 billion years), which means that the clusters date back to the formation of the Milky Way when the Universe was much younger than today. The clusters orbit the center of the Milky Way in orbits that are tipped every which-way. This makes them look like a spherical "cloud" of clusters surrounding our galaxy. There exist about 150 globular clusters that orbit our Milky Way. The stars remain "bound" to the cluster because there are so many stars crammed in so little space that they cannot escape their collective gravitational force.

Since the clusters are so old, there are no supergiant stars left, and the brightest stars left are stars like our Sun in their red giant phase, at the end of their 10-billion year lifespan. The distances to these clusters range from a "nearby" one only 7,000 light-years away to the other side of the Milky Way about 80,000 light-years away! Because they are so far away, they appear as fuzzy blobs in the telescope, with a sprinkling of the brightest stars just visible as tiny pinpoints of light. When you look at them through the telescope, remember how far away you are looking and keep in mind that those faint stars that are barely visible are actually 1000 times more luminous than our Sun.

Objectives:

1. To observe and sketch the appearances of open and globular clusters, and understand their differences.

2. To familiarize yourself with your star atlas.

3. To familiarize yourself with observing faint objects and the magnitude system.

4. To practice finding faint objects with a telescope.

Requirements: Three detailed sketches of open clusters and two of globular clusters. Observe a total of five clusters. In your sketches and notes, include comments about their total brightness, number of stars, the brightness and color of the individual stars, shape of the cluster, etc.

Directions:

1. Set up a telescope and accurately align it with the north celestial pole.

2. Select an open cluster or globular cluster from the list below, find it in your star atlas, and find it with your telescope.

3. Select the best magnification to observe the cluster with (usually low or medium power).

4. Observe and sketch the cluster. Try to appreciate what it is and why it looks the way it does (see discussion above and your astronomy text).

Candidate Clusters:

Open Clusters                                                    Globular Clusters

M 11 (Scutum)                                                      M-13 (Hercules)

M 6 (Scorpius)                                                      M-92 (Hercules)

M 7 (Scorpius)                                                      M-22 (Sagittarius)

M 36 (Auriga)                                                        M-5 (Serpents Caput)

M 37 (Auriga)                                                        M-10 (Ophiuchus, faint!)

M 38 (Auriga)                                                        M-12 (Ophiuchus, faint!)

NGC 864 & 884 (Perseus)                                    M-2 (Aquarius)

M 103 (Casseopea)                                              M-15 (Pegasus)

M 35 (Gemini)

M45 (Taurus)

Questions: 1. Which type of cluster had the brightest stars?  What do you think this tells you about their distances?

2. Which clusters are found in the plane of the Milky Way? Which ones are found away from the Milky Way? Why?  List the constellations where you found your clusters.  Which ones are looking into the Milky Way, and which ones are looking out of it (look at where the Milky Way is shown in your star atlas to answer this).  Does this agree with what is expected for these types of clusters?

3. Sketch Hertzsprung-Russel (H-R) diagrams for a globular cluster and a young open cluster (refer to your astronomy text).  How does this tell us the ages of the clusters?

4.  What kind of stars do young open clusters have that globular clusters do not have?  Did you see any evidence for these stars in your sketches?  Which ones?  What does this have to do with their ages?

5.  What kind of cluster did the Sun come from? Where is the cluster now?

6. Which type of cluster is your favorite to view? Why?