Binary Star Systems and Clusters:

Define a binary star system

• Explain the difference between 4 types of binary star systems

• Describe the two types of star clusters

• Explain why star clusters are so important to astronomers

Binary Star systems are more common than you may think.  Between 50 - 70% of stars that you see in the night sky are two or more stars orbiting together.  Two are the most common system and these pairs are called binary star systems.  There are several different types of binary star systems out there and how close they are together plays a huge role in how the stars evolve and eventually die.

Binary stars are two stars orbiting a common center of mass.  One doesn't orbit the other. 

Binary pairs are classified based on their orbit.  Wide binaries are stars that have orbits that keep them spread apart from one another. These stars evolve separately, with very little impact from their companions.  They may have once contained a third star, which booted the second one to a distant orbit before being ejected completely itself.

Close binaries, evolve nearby, and do have influence over each other, sometimes even transferring mass from one star to another.  There are several types of close binaries.

Visible Binary:  

A pair of stars that are far enough apart that both can be viewed through a telescope, or even with a pair of binoculars.  5 - 10% of visible stars are visual binaries.  Mizar and Alcor, the middle star in the handle of the Big Dipper, is a visual binary.

Eclipsing binary: 

Two stars whose orbits are at an angle so that, from Earth, one passes in front of the other, causing an eclipse.  This feature is based on the line of sight rather than any particular feature of the stars.

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Spectroscopic binary:

Stars that appear close even when viewed through a telescope.  Scientists must measure the wavelengths of the light the stars emit and determine their binary nature based on features of their spectrum.  When the stars are side by side as viewed by Earth their spectra are normal, however, when one star is moving towards Earth and the other is moving away the spectra split and we see redshifted lines (star moving away) and blueshifted lines (star moving towards).

Astrometric binary: 

are stars that seem to dance around a space; that is, their companions cannot be identified but only inferred.  Maybe the companion is too dim to see, or it is hidden in the glare from the primary star.  Sirius is the most famous example of an astrometric binary.

Optical binary:  Sometimes called a double star, are not actually a binary system, they only appear to be because they are near each other in the line of sight but they do not orbit one another.

Naming binary systems:  The brighter star is classified as star A and the dimmer star is star B.  For example Sirius is actually a double star system.  The bright star you see at night is Sirius A while Sirius B is a small white dwarf star.  If both stars are the same brightness then the discoverer of the star system gives the name.

• If we can see both stars in the system and track their orbits they are called visual binary stars.

• If two stars align with the Earth so we can see one star pass in front of the other then they are called eclipsing binary stars.

• If two stars are so close together that scientists need to look at spectra to determine there are two stars, then they are called spectroscopic binary stars.

• If a pair of stars were so close together that only the wobble in the sky would show that there was a binary star system there, then these would be called astrometric binary stars.

A star cluster is a group of stars that share a common origin and are gravitationally bound for some length of time.  They are particularly useful to astronomers because they provide a way to study and model stellar evolution and ages of stars.  This is because all the stars in a cluster are approximately the same distance away from the Earth so we can compare their brightness.  Also, all the stars in the cluster formed at about the same time so we can learn about how different masses stars live.

Open clusters have loosely grouped stars that can be easily distinguished with a telescope or with the naked eye like the Pleiades cluster.  Open clusters are located in the dusty spiral arms on the plane of spiral galaxies.  Stars in an open cluster have a common origin - they formed from the same initial giant molecular cloud.  Open clusters typically contain a few hundred stars though this can vary from as low as a few dozen up to a few thousand.

Stars within an open cluster are only loosely bound by gravity.  As the cluster rotates around the galaxy it eventually disperses due to gravitational interactions with other objects in the galaxy. Whilst our Sun is likely to have initially formed in an open cluster there is now no discernable clustering with nearby stars. Open clusters are therefore usually relatively young objects. Open cluster stars belong to Population I, meaning they are young and have a higher amount of metals in their composition.

Globular clusters are almost the exact opposite of Open clusters.

Globular clusters contain several thousand to one million stars in spherical, gravitationally bound systems of tightly packed (in space terms anyway) stars.  Their stars are the oldest stars in the galaxy, some between 10-12 billion years old!  They are considered population II stars because they have very low metal and heavier elements in their atmospheres.  This is because they formed from the early material of the universe which was largely hydrogen and helium.  There hadn't been several generations of supernovas yet to create the heavier elements. There is little dust or gas in a globular cluster so no new stars are forming.

Globular clusters are not found in the gassy and dusty disk of the galaxy.  They are found in the region outside of the disk known as the halo.

An open cluster has the following properties:

• contain stars with higher levels of metals and heavier elements.

• found in the disk of the galaxy

• Contain young, hot stars