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Merope Nebula  
The star Merope, which is surrounded by a reflection nebula. The nebula isn't the ancestral nebula, it is simply a cloud of gas that the star has stumbled into.
Image copyright R. Gendler
The life of a star after it has been born and the ancestral cloud has dispersed is called the main sequence. This is the phase that the Sun is currently in. During this phase, stars fuse hydrogen into helium to make the star glow. The amount of hydrogen fuel in each star is finite and a star will eventually use all hydrogen. When it has run out, the star will attempt to fuse heavier elements such as iron and will become an unstable red giant. This is explained more clearly in the star death section. The Sun will exhaust its hydrogen supply in approximately 5 billion years time.
 
  Hertzsprung Russell diagram  

The temperature of stars affects the brightness and colour of stars. The hottest stars are blue and are usually the brightest. One example is Sirius. Red stars are cooler and usually less brighter. Red dwarfs are fainter but red giants such as Betelgeuse are as bright as some blue stars.

As time goes on, the temperature and brightness increase to maintain the nuclear fusion at the core.

       
The Hertzsprung Russell diagram compares the temperature and brightness of all types of stars. Red stars have a lower temperature and luminosity, whereas blue stars are the opposite.
Image copyright North Arizona University
Pleiades Omega Centauri
This is a famous open cluster called the Pleiades. It is surrounded by a reflection nebula because it is a moving cluster and came into contact with the cloud thousands of years ago. The star at the bottom is Merope.     Omega Centauri is an astounding globular cluster found in southern skies. It is billions of years old and has 2 million stars. Globular clusters contain many old stars, which are the orange stars in this picture.
Image copyright R. Gendler Image copyright V. Wendel and B. Flach-Wilken

Most stars are arranged in clusters and many stars are born as a cluster inside a nebula and stay together after the cloud has gone. They stay together by being gravitationally bound. There are two main types, open and globular, examples of which can be seen in the above images.

Open clusters have between 100 and 1000 stars although many have less than 100. Nearly all of them are young at an age of a few million years old and they usually contain bright blue stars.

Globular clusters are tightly packed spherical clusters that contain very old stars. They are found at the edges of galaxies and can contain between 10 000 and 10 million stars. Altogether there are about 200 globulars in the Milky Way and thousands of open clusters.

  Rho Ophiuchi region    
     
    Rho Ophiuchi is a triple star at the centre of the blue reflection nebula at the top. The yellow nebula surrounds a massive red giant called Antares. Two globular clusters are also visible.
Image copyright R. Gendler/J. Misti/S. Mazlin
 
 
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