Distribution of starsand

Globular cluster

How far away are all these stars? Does the distribution of stars have an end?

It starts with a description of the immediate environment of stars, showing that most stars have companions. There is also a thin medium between stars.

Astronomers can also identify groups and clusters of stars. All this information is combined to define the architecture of the Milky Way galaxy.

Measuring the dimension — distance — is a great challenge.

Stars differ in absolute brightness by factors of a million or more, so a dwarf star might be 1000 times nearer than a supergiant of the same apparent brightness.

Absolute brightness or luminosity is given by L = d2F, where d is distance and F is flux or apparent brightness.

Remember that L is the true brightness of an object, or the number of photons it emits each second, while F is the brightness we measure on Earth, or the number of photons per second we collect with our telescope.

brightness is a good measure of distance if we can identify stars of the same luminosity. If luminosity is a constant, then F ∝ d-2.

. The excellent correlation between F and d means that the flux can be used to calculate distance. But if stars have a larger range in luminosity, apparent brightness is a much poorer indicator of distanced.

The range of luminosity on the H-R diagram is so large that any correlation between apparent brightness and distance has been washed out. The appearance of a star gives no useful measure of its distance.

As early as 1767, John Mitchell — who was the father of the idea of black holes — decided that there were too many alignments to be caused by chance.

He believed that the stars in each pair were close enough in space to orbit each other by gravity.

To understand how John Mitchell deduced that some stars are double,

Two types of distribution uniform and random.

Imagine that stars are distributed in only two dimensions, on a plane.

.

Uniform, Random, and Clustered Distributions of Stars.

A uniform distribution of stars would be spread out in a regular grid with equal distances between each one.

The distance from any star to its nearest neighbor is always the same. A uniform distribution may be realistic to describe the way atoms are laid out in a crystal, but it is not realistic to describe the way stars are laid out in space

A uniform distribution refers to objects that are separated by equal distances.

refers to objects that are separated by random distances. In this case, the distance between a star and its nearest neighbor star can vary quite widely. However, the average distance between stars is the same as for a uniform distribution. To see that this must be true, remember that the number of stars and the total area are unchanged, so the average spacing is unchanged too.

random distribution

Stars in the real universe show clustering.

A clustered distribution refers to objects that are separated by distances that tend to be smaller than for a random distribution. When stars are clustered on the plane of the sky, the nearest neighbors tend to be separated by small angles.

Clustering is revealed by the higher probability of any star having a neighbor at a small separation, compared to a random distribution.

An open cluster of stars is one that can contain about a dozen to a few hundred members. In addition, they are spaced loosely and are generally younger, hot stars.

Open clusters are commonly found in the disk of a galaxy.

Examples of Open Clusters:The Pleiades open cluster is a group of newly formed B-type stars. This 115 million year-old open cluster is easily visible with the naked eye. The brightest stars have a surface temperature of 30,000 Kelvin, however there are other stars that are a part of the cluster that have a variety of temperatures much cooler than these.

The Persius double-cluster is a remarkable site. This is actually a two open clusters.

Star clusters give astronomers a unique opportunity. All we know about stellar evolution come from star clusters (both open and closed). Stars in a cluster form at different times because of a variety of masses, but all contain nearly the same spectroscopic data. By careful observation, a Color-Magnitude Diagram can be created to plot the progression of evolution. A Color-Magnitude Diagram (or CMD) is really a specialized H-R diagram:

Characteristic: Open Clusters: OB Associations: Globular Clusters:

Diameter (pc):<10 30 - 200 20 - 100

Number of Stars:50- 1000 10 - 100 104 - 106

Mass (Solar):100 - 1000 100 - 1000 104 - 106

Density (Solar Mass/pc3):

0.1 - 10 <0.01 0.5 - 1000

Shape:Irregular Irregular Spherical

Color (Common):Red or Blue Blue Red

Metallicity:High High Low

Location:Disk of Galaxy Disk of Galaxy Halo of Galaxy

The organization of stars on the diagram give us the age of the cluster. Sometimes the mutual gravity of a cluster cannot hold the cluster together.

Here is a summary chart of star clusters:

globular clusters give astronomers a unique insight to how stars evolve. Globular star clusters are found orbiting the center of our galaxy in the halo.

The yellow dots are the distribution of globular clusters.They are generally comprised of old, metal poor stars - that is contain little elements heavier than helium. This means globular clusters formed a long time ago, before our galaxy contained more heavy stars. By contrast, open clusters have more heavier metals.Here are some examples of globular clusters:

The image above is of one of the most dramatic clusters around - Omega Centauri.

This Hubble Space Telescope image of M15 shows what appears to be a concentration of stars at the very center. This has led some to suggest that a black hole is at the heart of this (and others) cluster.What provides insight to this is the high concentration of white dwarfs found.

what appears to be a concentration of stars at the very center. This has led some to suggest that ablack hole is at the heart of this (and others) cluster.What provides insight to this is the high concentration of white dwarfs found.

Because globular clusters are old, it will be rare to see the CMD distribution like the one on the left. The distribution of stars will be more like the diagram on the right. Most of the stars will have already entered the Red Giant stage. This is called the Main-Sequence Turn-Off point (MSTO). The age of the cluster is determined by the stars that have already entered the MSTO stage.

Here is a summary chart of star clusters:5

Characteristic: Open Clusters: OB Associations: Globular Clusters:

Diameter (pc): <10 30 - 200 20 - 100

Number of Stars: 50- 1000 10 - 100 104 - 106

Mass (Solar): 100 - 1000 100 - 1000 104 - 106

Density (Solar Mass/pc3):

0.1 - 10 <0.01 0.5 - 1000

Shape: Irregular Irregular Spherical

Color (Common): Red or Blue Blue Red

Metallicity: High High Low

Location: Disk of Galaxy Disk of Galaxy Halo of Galaxy