Globular Clusters

Compared to open clusters globular clusters are old and regularly shaped We find them outside the galactic plane in something called the galactic halo.

First of all, we know they are old because they lack metals. They also are typically red meaning that they are not newly formed as blue stars are typically young. They also are large in size and contain hundred of thousands of star in a sphere hundreds or thousands of light years in diameter.

They are spherical because of the multitude of stars which allow gravity to pull them into a sphere. Unlike open clusters with relatively few stars, which do not have a define shape. Globular clusters also formed near the time when our galaxy formed.

They were first discovered by Charles Messier in the 1700's while he was searching for comets. Many are found in the Messier catalogue along with open clusters and nebulae. All of the globular clusters are outside the plane of the Milky Way. They have also been found orbiting other galaxies similar to ours.

M13 (Messier Catalog, 13th item), also known as the Hercules Cluster (in the constellation Hercules)
Image Credit:
Martin Pugh

M72

Image Credit:

NASA, ESA, Hubble, HPOW

Globular clusters were also important in determining the size of the Milky Way. Back in the 1920's, there was a Great Debate on how big the Milky Way was The determining factor was to look at how the globular clusters orbited. the center of the galaxy, and astronomers could use that to figure out out that our galaxy was about 100,000 light-years in size.

Open Clusters

Stars may sometimes be grouped in numbers of the 100s or the 1000s. Typically, stars that formed in the same nebula and generally stuck together are loosely bound to each other in what is called an open cluster.


An open cluster is just as it sounds. The cluster of stars does not have a defined shaped but does have defined members. These are the characteristics of pen clusters.

• Open clusters consist of younger stars (i.e. bluer stars) with higher metallicities.
• Typically, we find open clusters in the disk of our Milky Way because that is where the younger stars generally are found in our galaxy.
• Open clusters contain anywhere from a few dozen to a few hundred stars and are about 20 light-years across with the main body being about 3 to 4 light years in size.

Open clusters can easily be found in the night sky. In fact, if you look at the constellation Taurus, there is a really bright cluster above the head of Taurus called the Pleiades, or the Seven Sisters.

Pleiades Open Cluster

via APOD



Another example of a open cluster is actually found in the same constellation. The Hyades are found in the head of Taurus and are near the star Aldebaran. Though Aldebaran appears to be in the cluster, it is not because of its age, size, and distance from the others.

Hyades Open Cluster

via APOD

Next time, we will talk about a different stellar cluster, globular clusters.

Novae and Type I Supernovae

Sometimes, when a white dwarf forms, it is in a binary (or multiple) star system. If the companion star is close enough, the white dwarf call pull some of the material off the companion, and accrete it onto the surface of the white dwarf.


When the stellar material hits the hot surface of the white dwarf, the material can fuse quickly and create what is called a nova. Novae are not as energetic as supernovae, but will increase the luminosity greatly. However, the material accreted is rapidly used up and the nova dies down. Nova can occur many times and do have a predictable period that can be measured.


However, if the material on the surface of the white dwarf accretes too fast, the material can increase the mass of the white dwarf to over the Chandrasekhar limit. When this happens, instead of a nova explosion, the white dwarf undergoes a catastrophic collapse. What occurs is a Type I Supernova.


Since the white dwarf mass becomes larger than 1.4 solar masses, the electron degeneracy of the white dwarf cannot overcome the gravity the white dwarf experiences. The white dwarf catastrophically collapses, allowing all the material in the white dwarf to fuse rapidly. The outward explosion from this sudden release of energy completely destroys the white dwarf and companion star. The way astronomers differentiate between Type I Supernovae and Type II Supernovae is the lack of hydrogen lines in Type I. Because the white dwarf has no remaining hydrogen and the hydrogen from the companion star is completely used up in the fusion process, we know that Type I Supernovae can only be created by the sudden collapse of a white dwarf that accretes material from a companion star.

From Wikipedia, OOCalc chart

Looking at the above image, another difference between Type I and Type II supernova is that Type I are typically much brighter in the beginning but fade much quicker.