Leeds Astronomical Society LAS Meetings Observing Membership



NGC869/884 - Double Cluster


NGC864 and NGC884 are two open clusters, which are collectively known as the 'Double Cluster', and lie close together in the constellation of Perseus.

The double cluster is located about 7,500 light-years from Earth and is about 12.8 million years old. NGC869 has a mass of about 3,700 times that of our Sun, whilst NGC884 has a Solar mass of 2,800. Both clusters are however surrounded by a halo of stars with a combined mass of 20,000 times that of the Sun.

Each cluster is home to more than 300 blue super-giant stars, which are very hot with surface temperatures of 10-50,000 Kelvin and luminosities between 10,000 - 1,000,000 times that of the Sun. These are high-mass, short-lived, stars which have newly evolved from the main sequence of the Hertzsprung–Russell diagram.

NGC884 also features five prominent Red supergiants, which are relatively cool but large volume stars, with surface temperatures below 4,100 Kelvin and radii several hundred to a thousand times that of our Sun.

For more info. see the Wikipedia entry.





Measuring Angles

Hold your arm at full length, then close one eye & use the hand shapes shown above to measure the angular distance between the stars.

(Ain't anatomy wonderful!)


Apparent Magnitude

The apparent magnitude of a star is a measure of how bright it appears from Earth. The scale was introduced over 2,000 years ago by the Greek astronomer Hipparchus, who grouped stars into six categories. The brightest 20 or so were deemed to be 'first magnitude', slightly dimmer stars 'second magnitude', and so on until the barely visible stars were classed as 'sixth magnitude'.

Later it was recognised that our eyesight, once it has been given time to get used to darkness, has a logarithmic response. i.e. a Mag. 1 star is actually 2.512 times brighter than a Mag. 2 star, or 6.310 times brighter than a Mag. 3 star (2.512 x 2.512 = 6.310).

The six Magnitudes thus corresponds to a 2.5126 difference in brightness or 100x.

Apparent magnitude

Today the scale has now been extended, so that brighter objects can have an apparent magnitude of 0 or even negative. The brightest star Sirius, for example, has an apparent magnitude of -1.44 and the Sun is a whopping -26.74, due to it's close proximity to Earth.


Open Clusters

Open Clusters are groups of up to a few thousand stars which are loosely gravitationally bound, and which formed within molecular clouds at roughly the same time. More than 1,000 such clusters have been found within the Milky Way.

Because open clusters are situated in the galactic disc and have fewer stars than their globular counterparts, they are more prone to disruption and typically last about a few hundred million years.

Younger open clusters which still are surrounded by molecular clouds, can form emission and reflection nebula, depending on the size & luminosity of the stars. If the stars are hot enough, ultraviolet radiation causes hydrogen to ionise, emitting reddish light in an emission nebula. If the stars are slightly cooler and their light isn't sufficent to ionise the gas, it can be seen to scatter off dust particles in a reflection nebula.

Over time, the stellar winds from the stars cause the molecular cloud to dissipate, leaving behind an open cluster.

NGC869/884 - Double Cluster

For more information on open clusters see the Wikipedia entry.



Globular Clusters

Globular Clusters are spherical groups of stars that are tightly bound together by gravity, and which were largely formed early on in a galaxy's evolution. Unlike most clusters, they are found within a spherical halo around the centre of the galaxy rather than in the galactic disc and can contain hundreds of thousands of stars.

In the Milky Way there are about 150 known globular clusters, but other larger galaxies may have more. M31 - the Andomeda Galaxy has around 500, whilst large elliptical galaxies may have thousands.

In a globular cluster the density of stars increases towards the core, and the size of the cluster may have a diameter of about 100 light years.

M13 globular cluster
M13 globular cluster

Because globular clusters were formed early on in the galaxies evolution, they are today mostly comprised of old stars typically about 10 billion years old. Any brighter, hotter stars that were formed will already have used up their fuel and their life-cycle ended. Some newer stars, called 'blue stragglers' are still observed however which are thought to have arisen due to stars colliding and merging together within the cluster.

The age of the stars within the cluster can be determined by it's spectrum which show only a low abundance of elements heavier than hydrogen & helium. This indicates that globular clusters mostly consist of older 1st generation stars. It also means that rocky planets like the Eath are much less likely to have been formed due to the lack of elements heavier than helium.

The location of globular clusters in a halo away from the galactic disc means that there is very little to disturb them, so they can exist for a very long time. Some clusters in the Milky Way are however thought to have been stolen from minor galaxies when their galaxies merged with the Milky Way, early on it it's formation.

Because globular Clusters contain stars which are largely the same age & distance to Earth, the distribution of their luminosity and wavelengths can be used to determine the distance from Earth, by fitting the data to the Hertzsprung–Russell (HR) main sequence diagram.

For more information on globular clusters see the Wikipedia entry.